Life, Unbounded

Pastures New

2011-07-05T08:51:00.000-04:00

The dearth of recent posts is about to come to an end. Life, Unbounded now has a new home at Scientific American's crispy fresh blog network! This has been a while in the making and creates a whole new platform and audience for our discussions on the search for origins in the universe. Scientific American (with the incredible efforts of Bora Zivkovic) has put together an amazing network of bloggers to inform, enlighten, entertain, and amuse the world. It's rather humbling that Life, Unbounded gets to be a part of that, and it's going to be tremendous fun.

This site will remain however, as an archive and dormant outpost. The "new" blog will simply continue the content and style at a different address. Come along and see.

Nematodes, Mars, and Moons

2011-06-04T12:06:00.000-04:00

There's been a lot of interesting stuff going on recently, many items worthy of more detailed posts that will come in the future. The discovery of multi-cellular life in the Earth's subsurface is a huge one. Several species of nematode, including a new one adapted to extreme environments, have been identified in rock fracture water at kilometer depths in a South African gold mine. These are the same environments that are already known to host some remarkable microbial life, including the lonesome D. Audaxviator that relies on natural radioactivity to generate the chemical energy it requires. The tiny nematodes feast on such single-celled organisms and have been sealed off from other subsurface and surface environments for at least 10,000 years. Suddenly the prospects for "complex" life in planetary subsurfaces seem a whole lot better; especially on Mars.

Which brings us to an exceedingly interesting new result from the study of the formation rate of solar system planets. Mars in particular may have formed fast and early, in a mere 2 to 4 million years or so, as told by the hafnium-182 isotopic clock in martian meteorite samples. This places it into a very different category of object than the Earth. In effect it was and still is a large planetary embryo, even though we've all been calling it a small planet. Embryos have not undergone major collision with other similar bodies. By contrast, the Moon-forming impact that likely occurred on our homeworld (although modern reanalysis of lunar rock water content hints at possible complications) was part of the agglomeration process 4.5 billion years ago that puts Earth into a different section of the planetary zoo.

How fast different planets assemble is something we don't always remember to think about, but there are important implications. For example, if Mars did form this fast it would have coincided with a timespan in the young solar system where elements such as highly radioactive aluminum 26 would still be abundant (having been left behind from earlier supernova events that helped generate the very nebula out of which we formed). A young Mars could have been extra hot with internal radioactive decay, opening up the possibility of extensive interior melting and setting up a very different geophysical pathway compared to the Earth. This in turn could influence the nature of subsurface environments today, and their potential occupants.

Finally, on another related topic. An interesting paper appeared recently that employed a large ensemble of gravitational simulations to explore just how rare Earth-Moon type systems might be for planets in habitable zones around Sun-like stars. The Moon is a big satellite, and as such contributes significantly to the dynamical evolution of our planet - from its day-length to the gravitational tides that sweep across us every twelve hours. It also plays a role in the spin-axis stability of our planet (where our poles point) and therefore the long-term climate variation. It's a tricky problem to study for sure, but the results are intriguing. The kinds of giant impacts that can create Earth-Moon type systems may occur at rates of around 1 in 12 systems (with a range of possibly as many as 1 in 4, or as few as 1 in 45). This isn't hugely frequent, but in a galaxy with potentially millions of Earth-analogs that certainly adds up.

All three of these discoveries or studies are noteworthy because they help crack the door open a little further into our fast developing picture of not only our particular context as a habitable planet, but the opportunities for life elsewhere. Worms, isotopes, and collisions, it's a great mix.

Wanderers?

2011-05-22T20:03:00.000-04:00

Busy week, the week that was. Catch me on NPR's All Things Considered offering up some thoughts on the current SETI situation. Also my posts have been slowed as I get into a final stretch on a new book project for a general audience. If you like astronomy, cosmology, black holes and astrobiology you're in for a treat - I hope. Hitting the stores in 2012, I'll be saying more about this over time.

As for right now, well.....

There's a certain poetry to the astronomical news that's been hitting the media about planets that may be doing precisely as their name implies (πλανήτης in case your Greek isn't as rusty as mine). Painstaking monitoring of about 50 million stars in our galaxy by the MOA and OGLE gravitational microlensing surveys have revealed a very substantial population of Jupiter-sized objects that orbit at least 10 times further from their stars than the Earth does the Sun, and may not even be bound to stars at all. Sumi et al. report these results in Nature, along with a nice commentary by Wambsganss.

Although only ten such candidates are actually detected the statistical implication (since lensing events are so incredibly rare) is that there are twice as many of these planetary bodies than normal main-sequence (hydrogen burning) stars in our main galactic terrain. That is a lot. Even more interestingly Sumi et al. claim that most of these objects, perhaps 75%, may be unbound from any parent stellar systems and are true wanderers. The basis for this argument comes from existing constraints on long orbit exoplanets. These are obtained from projects trying to directly image such worlds. Bottom line is that the imaging efforts around stars do not see as many planets as the microlensing results would imply, hence these objects have gotta be out in interstellar space.

Personally I think there is every reason to believe that there really is a huge population of free-floating planets out there. A couple of years ago various researchers came up with a framework for explaining exoplanetary orbital architectures that requires episodes of intense planet-planet gravitational interaction within a system. One consequence; lots of planets ejected away from their birth places. My colleague Kristen Menou and I dabbled in this to investigate the predictions for planet imaging surveys. It was fun. Many planetary systems may be born in configurations that are inherently dynamically unstable, chaotic. Flinging worlds to the void is a great way to 'cool' the system down.

So, apart from the wow-factor of rogue/free/unbound planets, there is real reason to chase and confirm this result. It could be a pivotal clue that tells us not only how most planetary system achieve their configurations, but also confirms that planet formation itself is an efficient process. You have to make planets a plenty around stars in order for this to all work. The alternative is also fascinating - perhaps these worlds form via a route more akin to that of stars and brown-dwarfs, another feather in the cap of gravitational accretion.

Flippin' Planets

2011-05-15T19:00:00.000-04:00

A year ago it started to become clear that a significant number of exoplanetary systems harbored retrograde worlds. These planets are seen on small orbits that have the opposite sense to the spins of the stars that host them. This is a peculiar and puzzling arrangement. We've all been brought up to think of planets and stars forming out of the same proto-stellar disk of material and that angular momentum is an almost sacrosanct quantity - can't fiddle around with that. Getting planets to go in the opposite direction is quite unsettling.

Clearly the universe is having none of that silly narrow-minded thinking. However, the question has been how to produce these retrograde worlds; hot Jupiters zipping the "wrong" way around their parent stars in closely circular orbits. A new paper by Naoz and colleagues presents some intriguing and thorough calculations about a couple of phenomena that could do the trick.

They consider pairs of large, gas giant sized, planets that start out in decently large prograde orbits. For example, the inner planet may orbit at 6 astronomical units (AU) from the star, the outer planet at about 60 AU. The trick is to follow what happens if those initial orbits are highly misaligned. In other words if the orbital planes of the two planets are more than 50 degrees inclined with respect to each other. In this type of configuration long term gravitational nudging between the planets - particularly the outer one on the inner one - can result in severe changes in the ellipticity or eccentricity of the inner planet's orbit, as well as its orientation. Over a span of a few hundred thousand years the orbit can not only shift from being nearly circular to being nearly "needle like" (highly elliptical) but its orientation can change dramatically. In the most extreme instances it can quite literally flip, shifting all the way through a 90 degree inclination to more than 90 degrees - in effect going retrograde.

This flip can occur during an eccentricity spike, which I think gives us some physical insight to what happens. At the far point (apastron) of a super high eccentricity orbit the planet is going to be almost hanging in space, slowing to a near standstill before plunging back star-wards. If already on a highly inclined orbit (close to that 90 degrees) then all it will take is a little tug from the outer planet in the right direction to flip the orbit over. It's a bit like that moment of indecision teetering at the top of a snowy hill with skis attached to feet. Left to take the piste, right to go to the bar.

At the same time, an extremely elliptical orbit will bring the planet zooming in from several AU to mere tenths of an AU from the star as the planet screeches through its periastron approach. Naoz et al have computed the strength of star-planet tidal forces and the associated energy dissipation and show that the inner planet can be yanked into a tight circular orbit around the star in an incredibly brief period. A little too close to the honey pot and you get trapped. Once this happens the inner planet is totally decoupled from the outer planet, which simply cannot get its sticky gravitational mitts on the inner world any more. The end result is a close orbiting retrograde hot Jupiter.

Remarkably Naoz et al.'s models even seem to produce about the right fraction of systems in which this will happen. The only hitch is how to set up the initial configuration of planets, particularly with the outer planet on such a large orbit and with such a large mis-alignment between their orbital planes. Luckily an earlier epoch of strong planet-planet gravitational scattering from within the zone of planet formation might just do the trick.

Planetary systems are just so incredibly diverse. And, once again, we find ourselves gazing at our own system and wondering just how far we can or should take the Copernican Principle. This boring little solar system of ours may yet turn out to be a little on the special side.

55 Cancri e: Small or Big?

2011-05-02T22:00:00.000-04:00

Detecting and characterizing exoplanets remains an extremely challenging occupation. A very good example of this has been the recent detection of the transit of a planet around a star in the nearby stellar system 55 Cancri. This is an interesting place. It contains a binary star consisting of a G-dwarf and an M-dwarf separated by at least 1000 astronomical units. At only 41 light years away the G-dwarf is visible to the naked human eye and, conveniently, harbors at least 5 planets.

These worlds had been detected using radial velocity measurements, with the first detections back in 1997. A recent reanalysis of the notoriously tricky radial velocity data (made even more so with possible mean-motion orbital resonances in the system) by Dawson & Fabrycky suggested that the innermost planet "e" was actually on a much tighter orbit than had previously been thought - with the incredibly short orbital period of 0.74 days. This also suggested it was rather lighter than thought, a super-Earth of at least 8 Earth masses.

This was intriguing because the probability of a transit increases with decreasing orbital radius. In this case it went up by a factor of three to about 33%. This has prompted a number of groups to go look for the transit again, after earlier attempts came up empty. Lo and behold the first reported transit of 55 Cancri e appeared from Winn et al. on April 27th 2011. Using the visible light telescope of the Canadian MOST mission they spotted a transit right on queue. Hopefully Dawson and Fabrycky had some champagne at the ready.

Analyzing the depth of the transit curve (a tiddly drop in starlight of about 0.02%) Winn and colleagues came up with a planetary radius of approximately 1.6 times the size of the Earth. Combined with a mass of almost 9 times that of the Earth this implies a remarkable density of about 11 grams per cubic centimeter. That's almost as dense as lead, and a clear indicator that this is an almost pure rock-iron composition.

At least that would be the case, except on May 2nd 2011 another paper appeared by Demory and colleagues that uses the Spitzer infrared telescope to detect the very same transiting planet. With this different telescope and different waveband, a transit depth is found that indicates 55 Cancri e is about 2.1 times the radius of the Earth. The statistics are good enough that this is a "3-sigma" difference from the MOST detection, indicating that this difference in measured radius would only occur by random error about 0.3% of the time.

So, with a radius of 2.1 times that of the Earth this devious planet would have to contain a significant amount of volatiles and be much less dense. Possible compositions would involve a rocky-iron Earth-like core surrounded by either 20% by mass of water or 0.1% by mass hydrogen and helium. The problem is that 55 Cancri e is so close to its parent G-dwarf star that it should have an effective temperature of about 2000 Kelvin - all over. Hardly an environment for anything that could be called a "volatile".

Its quite a conundrum. Demory et al. perform their analyses very carefully. The most likely explanation - and one that Demory and colleagues offer up - is that 55 Cancri e harbors an extended atmosphere. Perhaps carbon monoxide or carbon dioxide as a super heated "exosphere" could produce an anomalous infrared surface that makes the planet look fatter than it is. That would likely indicate some interesting compositional properties if the planet is indeed as dense as the optical data suggest. Winn et al. investigate the constraints on an atmosphere from their data. It's very interesting. The odds seem slim. The only option appears that if such an atmosphere exists it is being constantly replenished by perhaps geophysical activity on the planet itself. The current status is therefore open to question.

There is hope however. Because the star 55 Cancri shines so brightly in our skies it provides a marvelous flood of photons and the potential for extremely high quality astronomical observations. Unlike many of the mysterious new worlds we merely catch glimpses of, this one may well reveal its secrets to us in due course. Exoplanetary science continues to grow in richness.

As I write this 55 Cancri is up in the western sky of the Eastern seaboard of the United States, amongst the constellation of Cancer, the Crab. Perhaps you should run out in case you can spot it - you never know, you might just catch a transit.

SETI Lost and Found

2011-04-28T11:24:00.001-04:00

The announcement that the primary instrument in the Search for Extraterrestrial Intelligence (SETI), the Allen Array, is going offline for lack of money is a sobering reminder of the challenges faced by high risk science. It seems to be the result of a confluence of cuts and declines in both federal funding of the observatory site from the National Science Foundation and the drastic belt-tightening of a virtually insolvent state of California.

Others have written about all the great science that the Allen Array can be used for in addition to hunting for artificial signals. Paul Gilster at Centauri Dreams gives a pitch perfect discussion of this. I think it's also important to remember that SETI has helped pioneer voluntary distributed computing with their SETI@Home project. This began in 1999, the veritable stone age of what we might now call "crowdsourcing". Next time you fire up your iPhone or Android for information on local hot-dog stands you should think about that. By most standards SETI has produced very significant spin-offs along the way since it began in modern form in the early 1960's.

What about the root motivation for SETI though? Is there simply insufficient enthusiasm among scientists and the public to sustain an effort like this, even if it's to the tune of a just few million dollars a year? [A note to cynical misers; American Idol rakes in over $1 billion each year for its various interested parties. You could finish building the Allen Array and fund SETI in perpetuity for significantly less than that].

I feel that SETI has always been a hard sell. Spend an hour talking to one of its key proponents like Jill Tarter and one comes away utterly convinced that this is a vital thing for humanity to pursue. But over the following days and weeks then in all honesty doubt does tend to creep back in. This doubt is multi-faceted. There is the high-risk nature of the endeavor, the risk being the perception that as remarkable as a detection would be, the odds of actually getting it are so very small. Then there is the problem that there are so many unconstrained parameters involved, which just exacerbates the risk assessment question.

Things are changing though. Twenty years ago there was essentially no evidence that planets existed around any star other than our Sun. Sure, it would have been pretty bizarre and unsettling if ours were the only planetary system in a galaxy of 200 billion stars, but we just didn't know. Now we are in the happy situation of being able to argue about exactly how many small rocky worlds there should be, and even how many of them might be terrestrial analogs. The general answer is "lots". This really does change a big piece of the SETI equation as it is employed. Suddenly there is not only a way to improve the estimates of the number of potential targets, but to actually identify those targets. Hence the plans that were in place to use the Allen Array to monitor Kepler planets.

The more difficult questions are those that revolve around some of the implicit (and even explicit) assumptions in SETI. These are to do with the nature of "intelligent" or "technological" life, and the presumed or hoped for motivations of other worlds, other species. It's a quagmire. The problem is that we just don't know, not even a teeny bit, whether the example of humanity is a reasonable template or not. Even arguing about the equally sophisticated but non-technological evolution of dolphins or ants ignores the fact that all of life on Earth is the product of intimate co-evolution across 4 billion years. Similarly fraught are arguments about the longevity and placement of civilizations in cosmic time. The risks for success or failure with SETI are almost impossible to compute, one way or the other.

No. I think the real argument for SETI is that no-one can say whether or not there is another species in our galaxy sending out recognizable signals, and that is precisely why we should be listening.

Long before we had modern astrophysics humans looked out into the universe eyes wide open. There was certainly motivation to understand those objects or phenomena that we had already discovered. But there was also motivation to simply gather knowledge by finding, well, by finding whatever was out there. Modern SETI as performed by the Allen Array is well set up to capture all manner of natural signals as well as those of artificial origin. The transient phenomena of the universe represent one of the next great challenges for astronomy. Projects like the $400 million Large Scale Synoptic Telescope are aiming for precisely this regime. Part of the motivation is to simply discover things that we could not have found before.

Regardless of what it finds, or doesn't, the Allen Array needs to keep operating or else we lose out on the beautiful mysteries waiting for us out there in the cosmos. Let's go help it.

Three Billion Years B.C.

2011-04-19T15:02:00.000-04:00

The Earth is still forming. Every year our planet accumulates another 40 million kilograms of material, mostly in the form of microscopic interplanetary dust. More sporadically the planet is also hit by larger bodies. Hundred meter diameter asteroids or cometary lumps arrive on average every thousand years, kilometer-sized civilization manglers arrive roughly every million years. This had been going on since the Earth coagulated from the material of the proto-planetary disk around a baby Sun 4.54 billion years ago.

As we turn back the cosmic clock the rate of accumulation of material increases. The pockmarked lunar surface has served as a proxy for reconstructing the history of asteroidal and cometary impact on the Earth. Without an atmosphere or significant geophysical activity the Moon has an excellent memory of impacts, while the Earth had eroded and resurfaced itself in continual reinvention. This record has indicated that during a period between about 4.1 and 3.8 billion years ago the Earth must have been subject to a particularly brutal pummeling. A substantial fraction of the outer shell of our planet could have been laid down during what has become known as the Late Heavy Bombardment.
It's a fascinating time in the history of our world. The first indications that microbial life might have been at work come not so very long after this quite cataclysmic episode ended.

The reason for this infall of material seems likely to be connected to a period of dynamical evolution in the outer planets. Models suggest that both Neptune and Uranus could have migrated outwards and dug into a rich belt of outer, Kuiper or trans-Neptunian objects. Many of those distant small bodies would have been pushed into orbital paths that would eventually lead to passage through the inner solar system and collision with the Earth. At the same time, Jupiter and Saturn would have migrated inwards and could have scattered material from the asteroid belt onto inbound trajectories. Once the dynamical reorganization of the giant planets was finished the Late Heavy Bombardment would have tailed off. A settling planet Earth then gave rise to the tentative steps of biochemistry and single-celled organisms.

Or so we thought. New evidence is emerging from the terrestrial rock record that the Earth actually continued to be pounded by very significant impacts from 3.8 billion years ago all the way up to around 2.5 billion years ago. "Life Killer" type asteroid impacts seem to have happened roughly every 40 million years during this timespan, rather than every 500 million years as had previously been thought.

So what gives? Where did these chunks of material come from? W. Bottke and colleagues have studied the gravitational dynamics of the teenage solar system and suggest that a now-depleted inner belt of material between Mars and Jupiter could have been scattered onto an inclined set of orbits - out of the plane of the planets. This population would then slowly "leak" into Earth-crossing paths, thereby greatly extending the tail of the Late Heavy Bombardment over another billion years or so. The leftovers of these bodies are still there, known as the Hungaria asteroids.

It all looks to fit rather well. The dynamics are believable, and provide a mechanism for the impacts that littered the planet with the molten globs of rock that geologists find in layers of ancient strata. There's just one teensy question. What are the implications for the evolution of life on Earth? While evidence of microbe-built structures like stromatolites from 3.5 to 3.8 billion years ago remain a little controversial, the presence of a diverse planet-wide biosphere is pretty incontrovertible in the 3 to 2.5 billion year ago span. Apparently microbial life not only dealt with continual destructive asteroid impacts but really did rather well for itself.

This raises another intriguing issue. As W. Bottke and colleagues point out, this prolonged period of heavy impacts does effectively stop around 2.5 billion years ago. That is suspiciously coincident with the first signs of a rising oxygen content in the Earth's atmosphere (the "Great Oxidation Event"), and the eventual emergence of multi-cellular life somewhere around 1.6 to 2 billion years ago. Is there a connection? Could the continual accumulation of planetary material have held back the full-on evolutionary party of early life? It's highly speculative, but one is tempted to think that this might be further evidence for the incredible resilience of life and its near-relentless nature once it becomes entrenched on a planet.

Rotten Eggs

2011-04-11T21:41:00.000-04:00

Posts here have been a bit more threadbare than usual. Mea culpa. Can be blamed on a number of projects, including a fun writing one that you will be able to find in the May 7th issue of New Scientist magazine as their "Instant Expert" piece on Astrobiology - a lavishly illustrated 8 page spread. Hats off to the editorial and graphics staff there.

It's also been a hard time to summon the courage to move on after the kind of dreadful, gray news coming recently from NASA as missions are abandoned left, right, and center. Not good. The trickle down from this kind of mass slaughter is going to be significant. NASA's entire budget is a miniscule 0.5% or so of the federal budget of the USA, and the projects being cut are an even smaller fraction of that. Between this and the shutdown of the Tevatron it would seem that the shining city upon a hill is getting a little dilapidated, when it comes to fundamental science.

Onto better stuff. Hydrogen sulphide, in fact. An interesting result appeared recently in the Proceedings of the National Academies, by Parker et al. that re-analyzes the products of an experiment performed half a century ago. Back in the early 1950's Stanley Miller and Harold Urey performed a number of experiments on the chemistry of a mix of 'raw' ingredients of water, methane, ammonia and hydrogen gas. By subjecting this gaseous stew to electrical discharges they attempted to reproduce conditions that might have existed on a very young planet Earth. The basic idea was to see if the rudimentary molecules for life, the prebiotic organics, might arise. Some indeed did, a smattering of amino acids for example, along with a lot of chemical muck. There were big uncertainties though, not least of which was the true composition of the young Earth's atmosphere.

Quite recently some of Miller's later experiments came to light, by way of the analysis of sealed glass vials stored in his lab from the 1950's. Modern chemical analysis techniques are far more sensitive and precise than those that had been available to Miller, and it became clear that many more amino acids had formed inside his experiments than he had realized at the time.

Now Parker and colleagues have taken a look at a particular batch of vials from 1958 where Miller included hydrogen sulphide (H2S) in his starter mix. He had never reported the results. Hydrogen sulphide is a widespread compound, from volcanoes on Earth to extraterrestrial environments. Remarkably, these experiments seem to have produced the highest yields of amino compounds in any such conditions. The finger points to the presence of sulfur. A whiff of that could be truly magic for building bio-molecules on a primordial world.

Quite incredibly, the mix of amino acids is also a very close match to that of a number of carbonaceous chondrite meteorites. Given that hydrogen sulphide is certainly found in these meteorites the indication is that the same kind of chemistry could have taken place off-world in the proto-planetary environment. So, that wonderfully malodorous substance hydrogen sulphide could provide a critical boost to the abiotic chemical synthesis of some of life's building blocks - both here on Earth, and further afield.

Thank goodness Stanley Miller didn't like to throw things away.

Paradox Earth: III

2011-04-04T09:40:00.000-04:00

Understanding the climate and overall environment of the very young Earth continues to be an extremely tricky business. Previous posts on several issues (I, II) surrounding the so-called Faint Young Sun paradox have discussed some of the sticking points. In a nutshell; 4 billion years ago the Sun was about 30% fainter than it is today, a direct consequence of the fundamentals of stellar evolution. So the puzzle is that as far as we can tell the surface environment harbored liquid water, yet today's atmospheric composition would have resulted in a vastly colder climate. Boosts to greenhouse gases might solve the problem, but it remains at the hairy edge of plausibility.

Now a new study by Court and Sephton casts an even murkier pall over the problem, literally. We have high confidence (from the record of lunar cratering, as well as the orbital evolution of the outer planets) that some 4.1 to 3.8 billion years ago the Earth was subjected to period of sustained impact over about 100 million years by asteroidal-type material. The so-called Late Heavy Bombardment (LHB) was quite a pounding. It likely provided the major constituents of the juvenile Earth's outer layers. Court and Sephton have studied the effect of the sand-grain sized components of material that may have poured into the Earth's atmosphere as micrometeorites during this era. Atmospheric friction as these tiny particles raced into the upper atmosphere produces high temperatures and the grains ablate, releasing sulfur dioxide - among other gases.

Sulfur dioxide is great for making particulates in a planetary atmosphere. This increases reflectivity, and can dramatically lower the solar radiation reaching the surface. Net result; planet cools. During the LHB roughly 20 million tonnes of sulfur dioxide a year may have been dumped into the atmosphere by this flux of tiny meteorites. That's equivalent to having a massive volcano erupt into the stratosphere every year for a hundred million years. The problem of keeping the Earth warm is greatly exacerbated. Court and Sephton also point out that Mars would have received a significant flux of these sulfur-bearing micrometeorites, seemingly creating an even bigger problem for an early temperate martian climate.

There are still a lot of questions. Was the sulfur content of these particles really as high as claimed? Do we really know the rate at which such tiny grains hit the Earth? Could the atmospheric chemistry of the young Earth have mitigated the production of sulfate aerosols?

Understanding what happened on the young Earth is a major issue. It seems for every solution to keeping the planetary surface warm there is an opposing mechanism that will plunge it into deep freeze. Yet the evidence remains for the presence of substantial liquid surface water during at least the tail end of the LHB and likely much earlier. Clearly somewhere we're missing a piece of the equation, or perhaps several pieces. Being able to study the deep geological history of Mars could help enormously, since it would allow us to separate out some of the planet-specific mechanisms at play. It may also be time to think a little more radically. Putting aside the mineralogical evidence for an early aqueous environment then perhaps a deep-frozen young Earth offers some advantage for the subsequently rapid emergence of life?

JWST Launch Brought Forward to 2012

2011-03-31T23:57:00.000-04:00

Time for fun. Here on the east coast of the United States the planet has clawed its way into night again, but in a few short hours we'll be entering the Gregorian date of April 1st. Fools day. The wonderful thing about the very best April fools jokes is that as absurd as they are in retrospect, they succeed in tricking us because they sound or appear almost real. It's the same thing that makes that noble institution The Onion so incredibly funny. "Queen To Run Marathon", "Harvesting The Spaghetti Trees of Italy", "No Taxes in 2012". Told with a straight face they can just make us pause for an instant. That is interesting, these jokes are an opportunity to examine just what is ludicrous versus what might actually be a plausible extrapolation.

So, life in the universe, exoplanets, all fair game for April 1st. What would be clearly a hoax headline and what would be on the borders of acceptability?

"Star system found to harbor 18 planets, including 3 in habitable zone."

"Cassini Mission Spots Cruise Liner on Titan"

"Alien Microbes Found In Cheese: apparently taste 'really nice' "

"Journal of Discombobulation Publishes Theory: scientists baffled but impressed"

"Bacterium That Plays Music"

"Exoplanet Covered in Checkerboard"

"Life in Hydrothermal Vent Carries Gambling Gene"

"New Telescope to Search For Signs of Ruminants"

"Government Files Reveal Canals on Mars Were Real"

"New Horizons Probe Cannot Find Pluto"

"NASA to send astronauts to Neptune"

"Ancient Plants Used Sound to Communicate: fossil records tell story"

"Parallel Universe Detected in Parallel Universe"

"Kepler Mission Discovers Planets Made of Steel"

"Helium Loving Microbes Discovered"

"Europa Cracks Open: it's full of shrimp"

"SETI Hears Alpha Centauri Customer Service Menu"

and the last, best one:

"Over 1,500 Planets Discovered Since 1995: other Earths surely out there"

Carbonaceous Cotton Candy

2011-03-28T21:52:00.000-04:00

A typical proto-star is surrounded for a few tens of millions of years by a great disk of nebular material. One percent of the mass of this disk is initially microscopic dust, most likely produced in the atmospheric outflows of earlier generations of elderly stars. The other ninety-nine percent is gas, the same mix of gas we see in the great nebula scattered throughout the galaxy. From this orbiting plate of sauce both the central star grows, and the planets coalesce. While there are many hurdles yet to overcome in our understanding of planet formation, one of the trickiest occurs right at the start of this process.

How exactly the microscopic dust grains and gas-phase matter in a proto-planetary disk go from this state to even a tiny crumb of rocky, icy material is a topic of intense debate. We actually have a bit more confidence in what happens once there are meter-sized chunks of stuff flying around than we do in this earliest stage. Observations of proto-stellar systems and laboratory experiments here on Earth have suggested that the first agglomerations of solid material were probably extremely "fluffy" aggregates of the tiniest particles. Now a recent study of the structures in carbonaceous chondrite meteorites seems to shed further light on this primordial stage in planet building.

A new paper by Bland et al. in Nature Geosciences demonstrates the incredible utility of modern microscopic techniques. In this case the backscatter of electrons reveals previously hidden details about the crystalline texture of the meteorite - otherwise impossible to get at owing to the fragile and complex nature of this class of object. In a nutshell, they examine the alignment of microscopic dust grain particles that are coating what are known as chondrules inside the meteorite. Chondrules are some of the most primitive (i.e. oldest) solids from a young planetary system. Whatever they picked up in their travels, and how they picked it up, provides a unique fossil record of conditions.

The outcome is that the very first solids that formed in our solar system were indeed likely to be extremely "fluffy" or porous, with some 85% of their volume just empty space. The Bland et al. results indicate that the chondrules underwent a large amount of "rolling" and even shocking by pressure waves. In effect a turbulent environment acted to compact the initially fluffy, cotton-candy like materials into denser states. Random rollings and collisions naturally produces closely spherical bodies.

The world beneath our feet may well have begun as sticky cosmic fluff.

[But probably not pink]

The End is not the End

2011-03-24T09:59:00.000-04:00

A couple of weeks ago a slightly provocative, but intriguing paper started doing the rounds. Its title "Transit surveys for Earths in the habitable zones of white dwarfs". The author, Eric Agol, makes a careful and thorough study of the potential characteristics of Earth-type planets orbiting close enough to white dwarf stars to meet the usual rudimentary criteria for habitability (i.e. liquid surface water).

White dwarfs, the remains of the stellar cores of roughly solar-mass stars, are tremendously compact objects supported by electron degeneracy pressure - the direct manifestation of quantum mechanical exclusion, close packed electrons don't like their wave-functions overlapping. As Agol points out, a typical white dwarf is about the same size as the Earth. This would result in a doozy of a planetary transit signature (white dwarf on, white dwarf off). He then goes on to figure out what the orbital configurations would need to be around white dwarfs of varying ages and temperatures for a planet to hit the "habitable" mark.

White dwarfs are low luminosity - they're just very small - so this zone is about 0.005 AU to about 0.02 AU for a range of parameters, and lasts for at least 3 billion years as the dwarfs cool off. Planets this close in to dwarfs will have orbital periods on the order of about 10 hours. So the odds of catching transits, which are going to be extremely deep as the planets block out most of the light, are really good. Rather neatly, since the white dwarfs are so dense, such close planets will be unable to raise a tidal bulge on the star and so their orbits are likely to remain stable over long timescales.

The catch is that these planets may or may not exist, and if they do they may have very uninteresting compositions. On its way to becoming a white dwarf a solar type star will inflate its outer atmosphere all the way out to about 1 AU. This is probably the ultimate fate of the Earth, to be engulfed by the star that has nurtured it for the previous 10 billion years. Even planets outside this puffed up stellar envelope may get destroyed as tidal effects perturb their orbits, some estimates suggest that even 3 AU is not a safe distance. Furthermore, as the star loses as much as 50% of its mass before ending up as a white dwarf the fundamental dynamics of any outer planets is changed. Orbits expand outwards and the mutual Hill radii, or range of influence of planets increases. This can lead to planet-planet scattering events that rearrange the entire planetary architecture.

Despite all this, as Agol points out, we do know that planetary bodies can exist even around neutron stars - the pulsar planets. Other observations also suggest disks of material, possibly containing planetary sized bodies, around stellar remnants. Just because one batch of planets gets destroyed doesn't mean that material can't get recycled into forming "new" planets. So there may be pathways for nature to rebuild or rearrange planets to put them right in the habitable zone of a white dwarf. Whether they would have compositions that include either water or young radioactive elements (vital for maintaining internal heat and hence geophysical activity) is rather a taller order to satisfy.

Nonetheless, if we've learned anything from exoplanetary science it's that nature is going to surprise us at every turn. Spotting worlds around white dwarfs would be immensely cool, er, temperate, and would undoubtedly yield a host of insights to the nature of planet formation in general - even if these are dry and inert lumps of hand-me-down material.

Multiple intelligence test

2011-03-21T20:06:00.039-04:00

This will sound like it's off topic, but it's not. Really. Even if it rambles. Some very intriguing discussion has been taking place recently on the apparent discovery that Neanderthal's were making highly sophisticated use of fire during their heyday some 400,000 to 30,000 years ago. This included a bit of home-spun low-oxygen chemistry in manufacturing sticky pitch to help with building better tools. Given the undeniably spotty nature of the data then it seems plausible that this species had plenty more tricks up its metaphorical sleeve.

There is something extremely spooky about all of this. We know that there was once more than one distinct hominid species walking around on Earth. It seems increasingly likely that they all had good, thinking, brains. Whatever happened to eradicate, or conceivably subsume, a species like Neanderthal we may never know. Nagging suspicions include the distinct possibility that we modern humans, or rather our Cro-Magnon ancestors might have had a hand in it. We're certainly still adept at genocidal behavior.

I think this has special relevance to discussions of 'intelligent' life in the universe. It's possibly of critical importance. One angle that people take in trying to predict the likelihood of intelligence in the cosmos is the 'Rare Earth' hypothesis. This has cropped up before, so I won't go into detail here. This is really based on the notion that here we are, the sole "intelligent" life on the planet, and many distinct phenomena have to be just-so for that to have happened. A similar argument applies to any physiologically complex life. But let's turn the clock back to 35,000 BC. Now there's a world with at least 2 intelligent, but distinct, species of hominids walking around. It might be wrong to think that this was a freakish moment in Earth history. That would presume that our current status is an end-point, an equilibrium. It's no more so than the world of H. Neanderthal, Cro-Magnon (us), Denisova Hominin, and who knows who else. Yes, you can still make similar Rare Earth arguments for 35,000 BC, but eventually it has to be hard to deny that Earth was generating "intelligent" species with some amount of abandon - it'd be easier to assume that this isn't such a delicate phenomenon after all.

So the question I think this raises is whether we're missing something important about the nature of the rise of "intelligence" (as in technology, tool making, abstract thinking) on a planet. This impacts how we might search for it in the universe (from SETI to sniffing for signs of industry in planetary atmospheres), and whether it's likely to be looking around itself (listening, traveling, building signposts).

One question is: if we had today another intelligent species on Earth would we have the same level of curiosity for finding intelligence in the cosmos? It might just seem that much more mundane. Are intelligent worlds quiet and introspective because they just don't care?

Another, more sinister possibility is that multiple intelligent species can co-exist only for so long. Eventually resources become limited enough to force survival of the fittest and they annihilate each other. You might well say that this can happen for a single species just as readily. But imagine for a moment. This is another species we're talking about. What would you do if it was us or the dolphins, seriously? Whatever morality might exist will be worn pretty thin when it's your species on the line. This could lead to a curious resolution to the Fermi Paradox. The paradox is: given the age of the galaxy then if intelligent life is not incredibly rare should it not have spread enough for us to have already come across it? Perhaps intelligent life does occur in abundance. So much so that it usually crops up in several versions on a single planet, whereupon inter-species conflict wipes it all out again. Paradox solved.

What about us then? Perhaps the awful truth is that while we survived and Neanderthal's didn't, we weren't the smart ones.

Springtime on Enceladus

2011-03-15T09:35:00.001-04:00

What a difference seven years can make. Before 2004 Saturn's moon Enceladus was just another of the 61 significant natural satellites in this system. Yes, it was exceptionally reflective, its snowy white surface pretty much the highest reflectivity of any body in the solar system. Yes, it appeared to have a particularly youthful, less cratered surface, as seen by Voyager 2's brief incursion. This was an intriguing but incomplete suggestion of geophysical activity. But overall there really wasn't anything that suggested it would be more than another of the beautifully individual large moons around the great ringed world.

Then along comes Cassini. Not only did Enceladus show clear signs of a complex and geophysically active (or is that cryophysically active?) surface but it was spewing what seemed to be geysers of icy water particles out into the cold space of the Saturnian system. Scanning towards its southern polar region revealed that the great 'tiger-stripe' fissures were significantly hotter than their surroundings - although still frigid by our terrestrial standards.

Enceladus is an active, albeit tiny, world. Later flybys and flythroughs of the plumes of water have revealed the presence of salts, ammonia, simple hydrocarbons and even dust. The presence of these things suggests that somewhere inside Enceladus there is liquid water in contact with rock. Whether there is a global subsurface ocean or localized lakes is still unclear. At the southern pole then deep fissures are venting some of this pressurized water out to space. What's keeping the interior of Enceladus warm is unknown. Tidal flexure resulting from interaction with the moon Dione and Saturn's great gravitational field could provide some heating at present, but not enough. The radioactive decay of elements within a rocky core might be a significant heat provider, but the apparent localization towards the southern pole may suggest some internal lopsidedness.

It's incredible that this tiny world, just over 300 miles across and 4.5 billion years old, is still stirring. Now, the latest results from Cassini have put a better limit on just how much cooking Enceladus is doing. It is pumping out about 16 Gigawatts of thermal energy, equivalent to almost three times as much as all of Yellowstone National Park here on Earth. Since present tidal heating could at most only account for 1-2 Gigawatts this is very firm evidence that either Enceladus is still releasing pent-up energy from an earlier epoch where the moon orbits and tides were different, or that unexpectedly high
radiogenic heating is the primary energy source. Either option increases the odds of a substantial subsurface liquid water ocean.

The notion that long-term/short-term variations in moon orbits might be responsible is particularly intriguing. The idea here is that the orbits of Enceladus and its neighbor Dione may experience temporary variations that result in short episodes of intense tidal flexure on Enceladus. The thermal energy then takes time to escape through the icy crust - squeezing out as we see it, through places like the polar tiger stripes. This seems to be supported by the claimed high level of argon gas in the plumes. Argon in a planetary environment comes from radioactive decay of potassium-40. If Enceladus had been venting steadily for more than about 10 million years we would expect it to have already lost its argon. The simplest explanation is that the venting of material is episodic.

What does this mean for Enceladus as a potential harbor for life? It's unclear. If Enceladus freezes up solid in-between heating episodes then that could be a tough deal for organisms that somehow inhabit a subsurface environment. If on the other hand it just simmers down to an extended internal winter before the next summer in a few hundred thousand, or million years, then pockets of water could perhaps sustain hibernating life. Maybe Enceladus is like a perennial bulb, budding and flowering every spring, before withering and overwintering again until woken by gravity's warming embrace.

The Romans go to Jupiter

2011-03-14T22:56:00.000-04:00

With so much attention focused on the extraordinary progress being made in exoplanet searches it can be easy to forget that there is still much that we do not know about the planets in our own solar system. A great example is our very own gravity lord, Jupiter.

There is much to learn about Jupiter's internal composition. For example, we don't actually have a very good idea of the water content of its atmosphere. When the Galileo mission dropped its probe into the abyssal gloom of the Jovian clouds for a 7 hour long plunge (for which only the first hour maintained communication, as expected) it found things much drier than anyone had suspected. Although this data barely penetrated 0.3% of the way into Jupiter, the absence of much water was and still is a bit of a puzzle. It is quite likely that the probe simply entered a region with few water vapor clouds, perhaps a product of severe downdrafts in the atmosphere, but it's critically important to understand on a global scale. In essence the water content of Jupiter offers a probe of the formation pathway for the planet. Jupiter should have formed with the same kind of oxygen to hydrogen (and hence water) ratio as was in the proto-planetary disk of material surrounding our young Sun 4.5 Gyr ago. A real deviation from this would indicate either some mechanism of sequestration or something funny about Jupiter's formation history.

Then there is Jupiter's powerful magnetic field. Some 20,000 time stronger at the poles than Earth's own field it profoundly effects the environment within the entire jovian system. Intense particle radiation rains down on moons like Europa, influencing surface chemistry and appearance. Within the polar regions of Jupiter itself then the great aurora make Jupiter glow in the ultraviolet and pump out radio emission. Understanding the geodynamo inside Jupiter as well as its external manifestations are high up on the list for exoplanetary science.

Another visit to Jupiter is long overdue. Quick flybys by missions like New Horizons, on its way to Pluto and the Kuiper Belt, offer tantalizing glimpses of the system but we want to get up close again.

Sitting in a clean room in Denver is humanity's next voyager to the gas giant. Juno - the Roman goddess, wife to Jupiter, among other dodgy attributes - is due to launch in August this year. It's a terrific example of a well focused mission with some very specific goals. After a 5 year journey it will enter into an elliptical polar orbit around Jupiter, complete about 32 of these orbits and then be sent to a gaseous doom within the atmosphere. While it flies from pole to pole it will probe Jupiter with a suite of instruments that include a microwave radiometer to peer deep into the upper atmosphere, optical and infrared imagers, an ultraviolet imager/spectrograph, particle detectors, and a magnetometer. By carefully observing Doppler effects the entire spacecraft will also serve as a gravitational plumb-bob - feeling out the internal mass distribution of the giant planet. To protect against the intense radiation at Jupiter then Juno also hides it's electronics away in a shielded vault - a first for this kind of mission and likely a useful pathfinder for later attempts to explore this environment.

Remarkably this is all being done using solar power. Juno is the first deep-space probe to manage this - sunlight is a good 30 times fainter out around Jupiter than it is at the Earth. Advances in solar cell technology aren't just for humans and Juno's three panels give the craft a great three-winged shape, and total span of over 60 feet.

There's a good chance that Juno will reveal much to us about Jupiter's deep interior structure and atmospheric dynamics. This will serve as an extremely important datum for our models of gas giant planets in general. While every new planet found around a distant star is something to celebrate, our own planetary system has a vast amount to still teach us, and Juno is likely to unearth some surprises.

There's Something About Meteorites

2011-03-06T21:30:00.000-05:00

Yikes. This is a post I wasn't going to write. Claims of microbial fossils in carbonaceous chondrite meteorites in a slightly dodgy feeling journal with a peculiar angle on publicity and the all-to-laughable 'news' reporting of certain media networks really is a sticky package. I can feel my blood pressure rising.

However, a couple of conversations had me thinking about this a little more, and after reading some rather sharp and apparently hurried public criticisms of said work (can people really not be bothered to spell someone's name correctly?) I felt there was perhaps something to discuss and add after all.

The paper ruffling some indignant feathers is "Fossils of Cyanobacteria in CI1 Carbonaceous Meteorites: Implications to Life on Comets, Europa, and Enceladus", by Richard B. Hoover. The bottom line is that Hoover (a long established and respected NASA scientist) is claiming to find evidence for fossil-like remains of microbial life inside carbon-rich, highly friable meteorites, based largely on electron microscopy investigations. He then goes on to speculate that the original organisms were a) extraterrestrial in origin, b) perhaps originally inside comets, and c) these could all be related to species that would find a home in the ice on Europa or Enceladus - right down to exhibiting pigmentation consistent with those locales. Wow. That's a lot to take in.

I'm going to first ignore the rather non-mainstream nature of the 'Journal of Cosmology' that this paper appears in, as well as their odd call for open-source commentary. That's really a whole other discussion that I'll pick up later on. However, as I'll explain below, this work didn't just pop out of a vacuum. The paper itself is chock-a-block in some respects, less so in others. As some of the critiques have pointed out, there are big and serious questions about terrestrial contamination of the meteorite samples (most have been in storage for decades). Also there are the funky compositions Hoover finds for the variety of bizarre looking microscopic structures that are his principle evidence of 'life'. Some of these structures are indeed spooky and intriguing - filamentous threads just a few microns in length, seemingly composed of a core and a sheath. Structural similarities to known microbial forms are certainly suggestive. Elemental compositions are probed, but without extremely clear calibration (again, as pointed out by some of the critiques), and are decidedly odd.

My take on it all requires a little disclosure. About five years ago I was invited to a SPIE meeting in San Diego that Richard Hoover was chairing - a respectable enough venue. I guess I arrived as an interested observer. Over a few days we heard about a variety of research into astrochemistry, terrestrial microbiology and paleo-microbiology, and Hoover's findings in both extreme Earth environments and meteorites. As a skilled microscopist he had some great stuff to show off. Bacteria in permafrost emerging from spore states and yes, the bizarre looking forms from deep inside carbon rich meteorites. There was a lot of discussion about contamination and the like. The off-kilter elemental composition of the microscopic meteorite structures wasn't consistent with contamination, nor was the fact that some of these meteorites utterly disintegrate at the slightest contact with water - suggesting that their storage might not have been so haphazard after all. There was a lot of head scratching.

My problem was the same then as it is now. I had no counterpoint, no idea what the insides of a carbon-rich meteorite should look like at a microscopic level if never touched by living organisms. Heck, for all I knew these weird and fascinating structures were perfectly reasonable consequences of non-biological zero-g chemistry, but there was no calibration for that. And I'm not sure there is any calibration now. Fossils of microbial life on Earth are also pretty tough to find and study, so that isn't a great help in this case. Hoover clearly felt then, as he does now, that he was finding something genuinely important and interesting in the meteorites. Maybe he is. Sure there are flaws in the way the data is presented, and the claims are stretched beyond the comfort zone, but I think it's still worth understanding what's going on. The catch-22 is that until someone from the outside takes this work seriously enough to perform that counter-investigation I don't think anyone is going to pay too much attention. The new tarnish doesn't help.

I feel my ire directed towards the Journal of Cosmology. For those of us trying very hard to develop a still emergent 'inter-discipline' like astrobiology, free from past nonsense and wish-fulfillment speculation, the kind of sensationalist, half-baked, awkward promotional tactics they are employing is poison. This is not how science should get reported, it's based on a fantasy about the nature of discovery. Yes, every so often a study comes along that blows us all away. Game-changers happen. But they almost never, ever happen like this. Talk about shooting yourself in the foot.

It's too bad - meteorites are really, really fascinating.

The Evolution of Planet Earth

2011-03-05T17:11:00.000-05:00

About 425 million years ago something quite extraordinary happened to this small rocky planet. A new type of living structure began to cover the surface of its dry landmasses. For a distant observer then across the plains of the supercontinent Gondwana a peculiar green pigmentation would have appeared.

This was the dawn of vascular plants. A critical physiological characteristic of these lifeforms are the stomata - the pores on leaves that take in carbon dioxide, push out oxygen, and enable transpiration, the release of water vapor into the atmosphere. Transpiration is key to the uptake of water from the ground and as part of the cooling mechanism for plants.

How many stomata a plant needs is a function of species and growing environment. Some remarkable new research now indicates that we may be witnessing a fundamental shift in this parameter as global carbon-dioxide levels rise, as they are incontrovertibly doing. A pair of papers by Lammertsma and de Boer and colleagues in the Proceedings of the National Academies detail an extensive study of a diversity of plants in Florida. By tracking both changes in plant stomata over the past few decades as well as comparing modern plants to preserved samples from over a century ago, they find a clear trend. Today's plant life in Florida, and potentially elsewhere, have about 34% fewer stomata than they did more than a century in the past.

Cause and effect is tricky to understand. However, these data appear consistent with the rise in global CO2 over a hundred years. Higher CO2 concentration and plants need to breathe less, so they cut back the number of stomata. But this also impacts the rate at which they transpire, profoundly effecting the uptake of surface water and its release into the atmosphere. This process plays a central role in the Earth's hydrological cycle - the conveyor belt of water evaporation, precipitation and movement that links rivers, lakes, and oceans to the atmosphere. So what might happen if plants send less water vapor skywards? Although at first this would increase the reservoir of surface water it would also reduce atmospheric water content. Less water in the atmosphere and less precipitation. Dry regions might get drier. The entire cycle of freshwater in a region would alter. The upshot - adding CO2 to the atmosphere may actually help dry it out.

Plants are both adapting to changing environmental conditions, and effectively adapting the environmental conditions to their modified physiology. The Earth is evolving. Little wonder that debate is continuing on whether or not we are now within an episode of mass extinction - the end of the Holocene. A recent study of species diversity seems to suggest that we may not be quite there yet, at least in comparison to ancient events, but things are not particularly rosy either.

All of this brings me back to a theme that has come up before in these posts. Questions of planetary habitability and biological stability are extremely slippery. While some broad stroke assessments of whether a planet might be in a 'habitable zone' around its parent star can serve a purpose, I'm becoming less and less convinced that this is a very productive avenue of investigation. Witness the hoopla over recent planet detections. Yes, we absolutely want to learn more about how climate operates in different planetary configurations, and how geophysics and chemistry may effect the near surface environment of a world, but the equation that takes us to 'habitability' is a very murky one.

Perhaps ironically the very kinds of changes that are being wrought on the Earth by humans might also offer some vital clues about what we should be sniffing for on other worlds. The sliding of ecosystems towards some new equilibrium, or even extinction, might actually be far more informative than a situation of perfect balance. Obviously this is looking quite a way into the future, but it could be critical to sort out some of these issues now since they will help determine whether we build instruments and telescopes to operate for decades or just years. The kinds of planetary environments we are so eager to find are also those that may require lifetimes to understand. Shifting the paradigm from discovery to study will require careful planning.

A whiff of blue sulfur

2011-02-28T16:10:00.000-05:00

A lot of observations, especially those relating to the deep history of a planet, tend to become received wisdom after a time. This is not to say that they are wrong, but it's certainly true that the light of new investigations can modify what we thought to be right.

A good example of something that might just cause some hiccups is the recent discovery that the molecular forms taken up by most terrestrial sulfur may be very different in the planetary interior than closer to the surface. A new work by Pokrovski and Dubrovinsky together with a discussion by Manning in last week's Science indicates that the forms of sulfur in our planetary innards should include a significant amount of a triple-sulfur molecular ion known as the trisulfur anion (S3-).

The part of the planet they probed is usually about 100 kilometers below us. Given its inaccessible nature they used a diamond anvil and some clever techniques to both re-create the pressure-temperature environment (about 10,000 atmospheres and a few hundred degrees) and to examine the molecular structures that form. The trisulfur anion reared its head. Previously it was generally considered that sulfate and hydrogen sulphide would be the dominant high-temperature sulfur compounds popping out on the surface of a young, geophysically active planet. It's the reactions of these compounds with atmospheric oxygen that has provided one of the key tools by which we've measured its geological abundance - since oxygen alters the isotopic ratios of sulfur atoms in mineral deposits. Throwing more trisulfur into the mix may require a revision of what we think oxygen levels were 2-4 billion years ago. This could have significant ramifications for phenomena that we relate to oxygenation, including the rise of multi-cellular life.

There is another, more poetic side to this result. Not only do the very hot geofluids under scrutiny help transport and deposit gold on our planet, the presence of trisulfur suggests a similarity to another precious resource. Lapis lazuli gets its vivid blue coloration from the shuffling of electrons related to the trisulfur anion. As Manning notes, if trisulfur is abundant at depth then much of the world beneath us could be a rather hot, but appealing ultramarine.

Make me a planet

2011-02-24T13:03:00.000-05:00

Planet formation. Not so long ago one might have said that we had some pretty good ideas about how it worked. None of them were perfect for sure, but the general feeling was that somewhere in amongst the various studies we were edging towards a reasonable physical model. On the one side was core accretion - the coalescence of solids from a proto-planetary disk with giant planets slurping up gas after passing a critical core size and rocky planets coming along a little later. On the other was gravitational instability - density patterns in proto-planetary disks reaching critical points of instability where gravity would collapse gas to giant spherical blobs that could then hoover up solids to build a core. Later evaporation might produce ice-giants and the rocky planets would still form by direct accretion. A combination of these mechanisms seemed increasingly likely.

Now in the past couple of weeks a slew of new results and reconsidered old results seem to be calling for some serious rethinking of what we know about planet formation. The recent Kepler data release indicates a propensity for highly packed planetary systems - if you can build it, it will be there. It also demonstrates that Neptune-class worlds are incredibly common (actually confirming earlier microlensing results). It also further confirms an apparent 'pile-up' or discontinuity in very short period planets. Around 3 day orbital periods then something is going on between planets and their parent stars to hold them back from being tidally hauled to death in stellar atmospheres. New ultra-high fidelity imaging of young planetary systems confirms earlier sightings that proto-planetary disks of gas and dust can be far from symmetric or simple. A simple interpretation of the lop-sided annuli of circumstellar material is that we're watching as giant proto-planets scoop up matter during their multi-decade orbits. The extent of the disturbances is intriguing.

Imaging of giant planets on very long orbits of 100 astronomical units, as in the Formalhaut system, are an immense and surprising challenge to planet formation models - especially if the orbits are not highly elliptical, which could be explained by dynamical scattering from an inner origin. And finally, there is the revised talk of an outer giant in our own solar system, perhaps 1 to 4 times the mass of Jupiter. If such a world, currently nicknamed Tyche, were to exist lurking beyond about 2,000 astronomical units (orbital periods of more than 90,000 years) then our solar system would have likely formed as a highly lopsided star-planet binary.

Where do we go from here? It's quite a challenge. There was already a long list of yet-to-be-fully-solved issues, from orbital inclinations and retrograde planets to orbital migration and tidal evolution. An inherent difficulty is the vast parameter space involved. Even with a single coherent model for planet formation then every individual system will evolve at the whim of non-linear dynamics and stochastic or random processes. It seems quite likely that we will end up having to adopt some type of classification scheme to just sort the wheat from the chaff. The big question is what classification scheme has the most physical meaning?

The one aspect that I personally glean from the new wealth of data is that it is almost overwhelmingly convincing that if there is an opportunity for planets to form then they will, with a vengeance. If even pulsars can host objects likely re-coalesced from post-supernova debris then there are some pretty potent mechanisms at play. For me this suggests that a fertile ground for investigation might be to flip the question around to ask exactly what the tipping point is? Do all stars initially form planets? Or is there a critical level of element abundance, dynamical environment, or radiation environment below which there cannot be planet formation of any kind? How many of those twinkling objects in the night sky are genuinely barren and alone?

The X factor

2011-02-21T11:04:00.004-05:00

This past week the Sun underwent an X-class solar flare and subsequent coronal mass ejection event. It was notable for a number of reasons. The Sun is slowly emerging from a minimum of activity - part of its roughly 11 year cycle of magnetic disturbance - and this was the most significant event of the past 4 years where Earth was in the line of fire for millions of tons of protons squirting out into interplanetary space. Flares come in C, M and X classes - only M's and X's have the potential to cause significant impact on Earth's upper atmosphere and geomagnetic system. It was also notable for the serious media attention it received.

In part this is probably due to a big, thorough, and thoroughly scary report that the National Academy of Sciences put together back in 2008. 'Severe Space Weather Events – Understanding Societal and Economic Impacts' is actually fascinating reading, but it also pulls no punches. Modern human civilization has built itself an infrastructure that is acutely vulnerable to geomagnetic interference. Yes, satellites can get knocked out by solar storms. Yes, GPS can be disrupted for significant periods (aren't you glad that worldwide air travel is now utterly reliant on global positioning?). But the real double-whammy comes from the trillion watt electrical currents induced in the Earth's atmosphere and conductive surface and subsurface structures. We already know what this can do. Knock out eastern Canada's power grid for 9 hours in 1989. Melt huge electrical transformers in New Jersey. A hundred million dollars of damage in a couple of hours. And that was mild. Back in 1859 the so-called 'Carrington Event' was an upper X-class solar belch that caused large parts of the United States new telegraph system to overload and catch fire. Vivid aurora were seen in the skies as far south as Cuba. Campers thought dawn had come in the Rockies.

The National Academy report soberly estimates that another big X-class event hitting the Earth full on could cause $1-$2 trillion in basic infrastructure damage. Now that's a deficit. Little wonder that those in communications, power supply, and a host of other industries are watching the Sun very closely. We've built a lot of new systems in the past 11 years that have not yet been tested by solar storms.

All of this is close to home, but it may offer some important insight to a seemingly perennial topic in the search for life elsewhere. Much attention is focused on planets around low mass stars - over 70% of all stars are less than 1/2 the mass of our Sun. Lower stellar mass and smaller radius means that radial velocity and transit planet surveys can reach down to lower mass planets. Greater numbers also up the odds of planet-hosting systems in our immediate galactic neighborhood, far better for detailed study. These stars have extremely long hydrogen-fusing lifetimes, into the trillions of years for an object 1/10th the mass of our Sun. The drawbacks are that planets in the small and narrow habitable zones of such stars are both likely to be slow-rotating, tidally locked worlds, and subject to the excessive crankiness of this stellar class.

Energy transport within the lowest mass stars is almost entirely via convection. In other words, low mass stars are bubbling, seething spheres of plasma that more or less turn themselves inside out on a regular basis. Our own Sun by comparison is very static in its deep interior, energy being carried solely by photons bouncing their way up through its bulk. Low-mass stars flare like crazy for at least the first 1-2 billion years of their lives, and potentially much longer. Whether Earth-type planets in close orbits can remain habitable has long been a topic of discussion. At the crudest level it comes down to whether a planet can hold onto its atmosphere in the face of stellar onslaught. It seems that even a modest planetary magnetic field can go a long way to preventing atmospheric erosion.

Our current predicament points to another issue though. It's awfully hypothetical, but perhaps not as much as it was a few months ago, before Kepler confirmed the planetary richness of our galaxy. Could technological life develop on a world pounded by geomagnetic disturbances? I know, big jump here. Usually I'm discussing how appallingly earth-centric we are about the nature of life, but for once let's allow some leeway. It seems that figuring out how to exploit the flow of electrons could be severely hampered for anything but the smallest types of apparatus (planet of the iPods?). Radio-wave communication might be an enormous challenge. The radiation environment in low to high orbit could be severe - and even with a planetary magnetic field then atmospheric density variations due to flare energy input would make spacecraft stability an ongoing headache. Humans did pretty well at surviving long, long before voltaic cells - albeit as a more agrarian species. On a planet kept under electromagnetic siege there would be little to be gained by moving technology in an electrical direction. Could there be intelligent and advanced life out there that just doesn't bother with cell-phones, GPS, or planetary radar because it's too much hard work?

Oceans in the night

2011-02-15T21:00:00.000-05:00

Talk of planets, planet-like moons, and the origins of terrestrial water tends to lead to all sorts of visions of nice moist worlds and warm tropical beaches. Or perhaps that's just me. It feels like it's been a long winter. This kind of bias is extremely persistent. Even when we talk about the potential for sub-surface oceans on moons like Europa or Ganymede it can be very hard to overcome the sense that these are secondary, and second-class, environments. The truth is actually rather surprising.

For quite some time planetary scientists have studied the possible interior environments of a wide range of solar system bodies. Much can be done with purely theoretical models that seek to determine the appropriate hydrostatic balance between an object's own gravity and its internal pressure forces - be they from gaseous, liquid, or solid states of matter. Thermal energy from formation, and critically from radiogenic heating (radioactive decay of natural isotopes), all play a role. Throw in a few actual datapoints, measurements of places like Europa or Titan, and these models get much better calibrated. The intriguing thing is that one can play around with compositions and the internal layering of material in a planet-like body to find the best looking fit. As a consequence the nature and extent of any subsurface zones of liquid water can be estimated.

Asking for liquid water is a bit like asking for 'a coffee' in Starbucks. Is that a demi-latte-mocha-skim-sweet-n-low, or hot water with caffeine in it? It's extremely unlikely for water anywhere in a planetary body to be pure. Water is a fabulous polar solvent, and can absorb astonishing quantities of other things. Throw in ammonia by the bucket load and while you'd not want it in that cappuccino you still have liquid water that might be microbial ambrosia. Adding solutes can also dramatically lower water's freezing point. Stuffing in 30% Ammonia by weight can get a freezing point below 200 Kelvin (-100 F). This opens up many avenues.

Allowing for a number of variables it is possible to evaluate the likely size of subsurface water oceans in our solar system. The numbers start to get interesting. Estimates vary but here's a sampling: Europa, 2x Earths ocean volume, Ganymede and Callisto each about 1/2 Earth's ocean volume, Titan possibly 10x Earth's ocean volume, Triton 2x Earth's ocean volume. None of these numbers are particularly optimistic or pessimistic, but from these bodies alone there could readily be 10 to 16 times more liquid water slurping around off-Earth than on it.

Things get even funkier when we start to consider what might be going on beneath the surface of Trans-Neptunian Objects - those distant cold objects of which Pluto is the prototype. Factoring in estimates of their number, their history of formation, and radiogenic heating then some claims suggest that these distant dark worlds could harbor more liquid water than all the rest of the solar system. Part of the trick is that a thick layer of tens of kilometers of frozen water, methane, nitrogen and so on actually provides great insulation against thermal loss to the vacuum of space.

Some caution is advised though. Clearly many, if not all, of these environments may operate with far less energy flux - thermal or chemical - than Earth-bound oceanic systems. Some might well drop below the mean levels required to sustain any kind of deep dark biosphere. But if we insist that liquid water, polluted or otherwise, is a key ingredient for life then there is far more real estate in the solar exurbs than in our neighborhood.

Oasis Earth

2011-02-10T21:29:00.000-05:00

Water is such an integral part of life on this planet that it's surprising we don't really know where it all came from. The more we've figured out about the Earth's origins and the formation of the solar system the trickier it's all got. Part of the problem is that the Earth must have formed in a region of the proto-planetary disk surrounding our baby star well within the so-called 'snow line'. Picture a thick glop of gas and dust shaped a bit like a squashed donut in orbit around a proto-sun. It's a big structure, extending out to perhaps 100 astronomical units where it thins down to almost nothing. The increasingly hot proto-sun in the middle heats up material around it, but the great donut is a pretty effective sunshade. The gas is also pressing ever inwards and as it compresses it warms up. The end result is that in the central plane, a horizontal slice through the chunky disk, it is warm towards the proto-sun and cooler and cooler the further out you go.

Somewhere, perhaps at around 4 astronomical units out, the temperature drops below 170 Kelvin (-103 Celsius). For water this represents a transition. It's the temperature at which water molecules in a vacuum will begin to stick together at an exponential rate. It's water's sublimation point. In the proto-planetary disk then water pours out of the gas phase, freezing into solids and making the snow line. Little wonder that the giant planets and moons have such an enormous water content. For little Earth, forming around 1 astronomical unit there's nary a drop in sight.

Clearly Earth had its thirst quenched somehow. The precise nature of when and how it acquired water-bearing material has long been a puzzle. An intriguing new result in Nature Geoscience by Greenwood et al, along with a nice review by F. Robert seems to provide an enormous and juicy clue. Remarkably this comes not from some fancy new mission or astronomical observation but from applying new geochemical analysis to rocks brought back by the Apollo astronauts. Ion micro probe measurements of the mineral apatite reveal not only that the Moon has a lot more water than we thought ('lot' being a relative term), that it played a role in the Moon's early geophysical life, and that most of it seemingly has a different origin to terrestrial water.

Deuterium, the heavier isotope of hydrogen, is twice as abundant in lunar water than water on Earth. The ratio of deuterium to hydrogen has long been a fingerprint of where compounds, including water, come from in the solar system. More deuterium indicates a colder chemical origin. Cometary water has a lot more deuterium than water in meteorites. Earth's oceans look a lot more like water from volatile rich meteorites and asteroids than they do the chilly water from comets. But the Moon...Well, the Moon turns out to have water that is more consistent with a cometary origin. Pounding the Moon with comets shortly after it formed could do the trick. However, this leaves a problem. Up to this point a best bet for most of Earth's water had been the deposition of material by carbonaceous chondrite type rocks, sometime following the formation of the Moon. Either the Earth dodged the thousands or millions of comets that painted the Moon or the Moon dodged the water bearing rocks that buried the Earth.

What a predicament. How can two bodies so intimately linked, one formed from the detritus of a great collision with the other, avoid having the same cocktail flavor? One possible solution, articulated by Robert, is that both Moon and Earth got moistened by comets first. Then Earth got hit by perhaps one great object that sailed past the Moon and splatted the Earth with water containing far less deuterium - diluting the terrestrial mix.

While the discussion is not over it does raise a point of acute astrobiological interest. If this scenario is correct then that final event must have provided a very significant fraction of the Earth's water. Therefore we owe much of the past 4 billion years of gloriously damp climate to that singular moment. What if it had never happened? How habitable would Earth have been? Despite our models of forming planets that seem to indicate gaining water is quite common, there is still tremendous chance involved. Oases may indeed be hit or miss.

Moons

2011-02-06T12:57:00.002-05:00

In the continued wake of the Kepler results that indicate a likely wealth of planets in our galaxy I thought I'd post a rather more personal note about an intimately related area that I think in many respects parallels where exoplanetary science was in the early 1990's.

Our solar system plays host to an extraordinary array of natural satellites, or moons. Many of these are entirely comparable in size, composition, and even chemical and geophysical activity to bona-fide planets. The only real difference is that these worlds reside deeper in the orbital hierarchy. Nine regular satellites in our system have diameters greater than 1500 km, the largest (Ganymede and Titan) are over 5000 km in diameter - larger than the planet Mercury. Io around Jupiter has extensive and active silicate and sulfur-rich volcanism. Titan has a frigid atmosphere that is somewhat denser than the Earth's, and a diverse and global hydrocarbon cycle from gas to liquid to solid. Many moons have signs of active and quiescent cryo-volcanism - from Enceladus to Triton and Europa. They also show good evidence for subsurface liquid water oceans that readily exceed the total volume of Earth's oceans. It is little wonder than many of the current concepts for future solar system exploration missions focus on these objects - they are tremendously interesting.

It is also true that our models of how moon systems form are even less well developed than our models of planet formation. It seems that moons around giant planets probably form out of circumplanetary disks of gas and dust much like a scaled-down version of planet formation itself, but there are many caveats. There's another sneaky truth; simulations of forming planetary systems are not typically set up in ways that allow us to track satellite formation, capture, or loss (embarrassed cough). In this sense we're even further behind than the equivalent situation for planets two decades ago.

Intriguingly though the prospects for detecting moons around exoplanets may not be too bad. It may even be on a par with the situation in 1994, on the cusp of the first radial velocity exoplanet discoveries. Lurking already in the bounty of Kepler data there could be evidence for exomoons as transit duration and transit timing variations. Moons make their planets wobble just as planets make their stars wobble by offsetting the system center-of-mass.

There are some new rules though. Stellar tides can be very bad for moons. The same forces that operate to eventually bring a planet into spin-synchronicity or tidal lock with a star also perturb satellite orbits and can pump their orbital ellipticity to a point where the moon just sails off. Additionally, once a planet becomes tidally-locked to its star then there are in fact no stable moon orbits and any such objects will over time spiral inwards due to moon-planet tides. The upshot of all this is that within about 0.6 astronomical units of a solar-mass star then in all but the youngest systems you might not expect to find any moons - assuming of course that they formed in the first place. So this recent Kepler data release of planets within about 0.5 AU of their stars may not be the ideal place to look. Kepler release 3.0 may be another story when we begin to confirm planets on longer orbits.

My own interest in exomoons was in part stimulated by what is perhaps the modern classic paper on the subject, by Williams, Kasting and Wade in 1997. By Jim Kasting's own admission the inspiration for this paper titled 'Habitable moons around extrasolar giant planets' came from a viewing of a certain episode of a certain sci-fi franchise depicting a place called Endor. It's a lovely paper. A key point in it was that gravitational tides in moon systems due to moon-moon interactions could be pivotal in dissipating enough energy to make up for a moon being well outside the classical habitable zone of a star. Instead of stellar heating you'd have more geophysical heating. In 2005 I attempted a bit of a followup of my own and with some funding from NASA made a small study of the potential for 'habitable' moons around the then known exoplanets. The idea was simple, we knew the stellar input for these planets and any moons they might have, so what kind of tidal forces would be needed to push them to temperatures that could sustain liquid surface water? I was surprised to find that it could all work out pretty well. Although there are several caveats then tidal heating in a plausible range could effectively double the size of the habitable zone in these systems if we were willing to consider moons as well as planets. The lovely thing about it all was that the energy for this all geophysical warmth came from the spin and (ultimately) orbital energy of the giant planet. Life powered by angular momentum? Perhaps so.

Five years later and I was sitting on a tediously long flight watching a movie about blue-skinned aliens romping around on a lush tropical moon orbiting a gas giant planet in the Alpha Centauri system. It occurred to me how funny it was that two epic Hollywood productions framed the interim works on exomoons, obviously we should listen to scriptwriters more often. It also occurred to me that exomoons might just be ready to fully emerge from the astrophysical subconscious. A few recent publications seem to have confirmed that.

We may talk about finding the first 'Earth-like' planet (once we figure out what that actually means). What if we're more likely to find an 'Earth-like' moon around an ice or gas giant? The odds quite conceivably favor such a situation. There may be a few million rocky planets in habitable zones in the galaxy, but there could be as many or more rocky, watery moons in the extended habitable zones around giant worlds.

I'm not for a moment suggesting that we divert attention from hunting exoplanets. I also hope that some of the pioneers who devoted themselves prior to 1995 to what was seen as a fringe pursuit are the ones to find that Earth-twin, they deserve to. However, if we find barren world after barren world it will be time to turn our gaze on those strange and fantastic places that are held in thrall of giant planets.

It's full of Neptunes...

2011-02-03T09:38:00.000-05:00

Among the multitude of delicious Kepler results to digest from yesterday was an estimate of the frequency of occurrence of particular planet categories. Now the full report by Borucki et al. is available we can take a closer look.

By allowing for the known geometric effects of transit detections, sensitivity effects from stellar brightness, observation time and transit frequency and models of noise and false positive rates then the authors take a careful stab at computing the true population numbers for planets. It's not unlike being shown a single snapshot of part of a forest that also happens to be shrouded in fog and having to guess how many trees there really are. With some logic and statistics you can probably make a pretty good estimate.

The results are intriguing. For the range of stellar types in the Kepler data (mostly normal F, G, and K stars that range from a bit less massive to a bit more massive than the Sun) then it is estimated that about 6% of all such stars harbor 'Earth-sized' planets less than 1.25 times the radius of Earth within orbits of 0.5 astronomical units - or half the size of Earth's actual orbit. This orbital cut-off is simply due to the fact that Kepler has not been observing for long enough to find planets further out - yet.

Slightly larger planets, so-called 'Super-Earths' up to 2 times the size of our homeworld are similarly numerous and occur around about 7% of all such stars. Jupiter sized planets, between 6 and 15 times the girth of Earth should be present around about 4% of stars. Again, on orbits within 0.5 astronomical units.

Remarkably, the most numerous planets are those in the 'Neptune' size range, between 2 and 6 times Earth-radius. About 17% of all such stars should play host to these hefty worlds. This is to my mind a clear and excellent challenge for our theories and models of planet formation. Whatever schemes we come up with had better reproduce this kind of population distribution.

Then there is one other tantalizing feature. Borucki et al. subdivide these results into bins according to orbital radii. The trend for all planetary sizes is remarkably flat. What does that mean? It means that if one were to extrapolate these results to larger orbital radii, to the planets yet to emerge as Kepler continues its long hard stare, we might expect very similar results for all those worlds in the magical zone that is equivalent to where our own Earth orbits its G-dwarf star. In a galaxy of 200 billion stars this would imply a few million such circumstances. Of course it's not really magical, that's just our prejudice. Nonetheless whether it's scientific or not, we would perhaps all dream more interesting dreams if we knew that small rocky worlds orbited other Suns just the way we do.

It's full of planets...

2011-02-02T14:12:00.001-05:00

All the signposts were there but as always in science the proof is in the pudding. Today the Kepler mission made its second major data release and served up a massive dose of sugar and carbohydrates. With 1,235 good planet candidates, 90% or more of which are likely to pan out as the real deal then things are looking awfully rich in the hunt for those small rocky worlds that could resemble the Earth. Indeed, 68 of the new Kepler finds are less than about 1.25 times the radius of the Earth and 54 are orbiting in their stellar habitable zones. Of those latter worlds then one is 0.9 times the size of Earth, and four are less than twice the size, the rest are rather bigger - all the way up to gas giants.

There's a good chance that if you look down the street you'll see an astronomer running along chanting 'here we go, here we go'. It's such an outpouring of data that its impact is likely to continue well into the years to come. As long, that is, as societies continue to support the scientific effort. An excellent series of discussions on the state of play for exoplanetary science can be found in the recent posts by Lee Billings on boingboing, and he nails the pros and cons of these exhibitions of success. Hype can be good, but it's a tricky business.

The big Kepler list is fabulous but some of the details are even more fascinating. In this week's Nature the Kepler team also report on a remarkable system Kepler-11 in which no less than 6 planets appear to be transiting the parent star. The analysis by Lissauer et al. shows what you can do with this kind of data. Single transiting planets yield no direct handle on planet mass, only radius. Multiple transiting planets can provide a wealth of information on the orbital dynamics of a system and constraints on masses as the planets tug at each other, together with estimates of orbital ellipticity. Kepler-11 is a highly, even ridiculously 'packed' system. The 5 inner planets all have orbital periods between 10 and 47 days. Since Kepler-11 is a G-dwarf star like the Sun this is as if 5 planets orbited within Mercury's territory. Yet in dynamical terms the system appears to be quite stable, and should remain so for at least the next few hundred million years.

At least four of the planets seem to be less than 10 times the mass of the Earth, with the smallest at around 4 Earth masses. They all also appear to have substantial atmospheric envelopes on the basis of their densities. The likely components of this gas range from hydrogen to water vapor or 'steam' dominated. Kepler-11 presents a fascinating test case for models of planet formation. Most are probably lacking. The arrangement of these worlds, their apparent compositions, and their uncomplicated orbits suggests that as they formed they were coaxed and settled by a significant amount of gas or small rocky objects that helped smooth things out - a bit like a dynamical muffler. This is not simple though, and almost certainly not a universal rule.

It's all enough to give one heartburn. The great news is that Kepler is confirming that planets are extremely numerous. Time will tell exactly how many 'Earth-type' worlds there are, that is still open for bets. The bad news, a bit like getting the bill after such extravagance, is that we are really, really going to have to work hard on our models of planet formation.

Tick tock

2011-01-30T11:55:00.000-05:00

As those of us in the northern hemisphere of this small rocky planet contend with the winter nights and days it can feel like our internal clocks get a little out of whack. However, we and many other organisms actually have an extraordinarily robust built in timing mechanism that carries us through a roughly 24 hour cycle. Birds do it, bees do it, even educated C. Elegans do it. The circadian rhythm is something that may be a global property of terrestrial life. Regardless of sunlight then living things tend to operate on a daily routine, from rest to activity, and from high to low metabolic activity.

The exact biochemical origins of this internal clock have been somewhat elusive. In last week's Nature two new works by O'Neill et al. shed some more light on the subject. A possibility has been that a transcription/translation feedback loop governing expression of certain 'clock' genes played a role in setting the 24 hour timer in organisms. O'Neill and colleagues seem to have found good evidence that there are additional, possibly superior, 'time-keeping' processes at play. In essence these are chemical 'oscillators' that behave like a well-tuned pendulum. Intriguingly this type of mechanism was already known to operate in the ancient cyano-bacteria. In tandem then perhaps both the purely chemical and gene mechanisms act like a self-correcting clock, keeping life to a consistent 24 hour timetable. The genetic coding for the chemical clock seems likely to be shared amongst organisms like ourselves and ancient bacteria.

This is all very interesting. However, it also raises a number of questions that I've not seen discussed in detail in these or related experiments. 24 hours is the rotation period of the modern Earth. The Earth-Moon system has been in constant dynamical evolution since the formation of the Moon about 4.53 billion years ago following a massive proto-planet collision. At present the gravitational tides due to the Moon are dissipating energy at a rate of a few Terawatts and slowing the Earth's rotation by about a couple of milliseconds a century. Other variations, like changing ice-caps, solar tides, even tectonic shifts tend to obscure this slowdown on short timescales but over millions of years there is little doubt that the Earth's spin has been slowing. At the same time angular momentum conservation means that the Moon is receding from us at a few centimeters a year - a fact confirmed by laser ranging.

The upshot is that it's quite possible that 4 billion years ago the Earth's daylength was only 12 hours. Geological evidence is scarce to non-existent that far back, but studies of material deposited on what were once tidal shorelines indicate that around 600 million years ago the day length was certainly more like 22 hours, and the slowdown should have been more extreme in the further past. So the intriguing question to ask is how the biochemical clocks, be they the genetic or chemical variety, adjust over the millenia to that shift? Or, to be provocative, is there some way we could use our understanding of the evolution of these mechanisms to independently test the physical changes to Earth rotation over hundreds of millions to billions of years?

Celestial mechanics probed by paleogenetics? That sure sounds like fun.

Astrobiology: The Questions

2011-01-26T08:46:00.000-05:00

As a followup to the recent series of 'ten most important questions for astrobiology' (a highly biased, personal and incomplete take on what may matter the most right now in the search for life beyond the confines of the Earth) I thought I'd compile those posts into one easy-to-read-in-the-bathroom file.
This hyperlinked PDF is the result, with thanks to OpenOffice for handling links and curses to Microsoft Mac Office 2011 for vividly demonstrating the struggles a technological civilization must overcome to reach for the stars.

Extinction

2011-01-25T09:54:00.002-05:00

A theme that reoccurs in these pages is to do with the notion of extreme events in planetary environments or in organism populations. I've talked before about whether we're more likely to spot terrestrial type planets and biospheres in states of flux rather than cozy stability, or indeed whether cozy stability is just a fleeting illusion based on our biased worldview of modern earth.

Among the various characteristics of life on this planet are episodes or periods where life has undergone dramatic down-sizing. Minor and mass extinction events pepper the fossil record, and indeed define the labels we assign to the great periods of Earth history. One of the grand-daddies of all such events occurred in the late Permian about 250 million years ago. An astonishing 95% of global marine life (at least of the larger variety) was extinguished, along with something like 70% of surface life. Imagine visiting a zoo of the late Permian, a couple of trilobite relatives and some insects, maybe a fern. Exactly what caused this massive die-off has been, and still is, a matter of intense debate. Many have long pointed the finger at places like the 'traps' (step-like terrain from ancient volcanic basalt flows) in what is now Siberia as a site of enormous geophysical activity that could have profoundly altered the planetary surface and marine environment. Covering a couple of million square kilometers this active region could easily have a global impact.

A key clue as to why the Siberian Traps could have profoundly affected the planet has been the suggestion that this volcanic region would have ignited massive coal deposits. As these burned they would have dumped colossal amounts of ash, carbon dioxide and other combustion byproducts like sulfuric acid, and even methane into the atmosphere. Now a new work in Nature Geoscience by Grasby et al. describes the discovery of precisely the kind of ash deposits in the rock record of far northern Canada that would have been produced by the Siberian volcanoes. Not only that, but the nature of this ash is very similar to that produced by modern industrial coal use, suggesting that toxic slurry would have been pouring into the late Permian marine environment.

What is particularly interesting to my mind, which harks back to earlier posts, is that obviously the coal deposits that the Siberian lava ignited were themselves the product of a much earlier era of rich plant life - perhaps during the Carboniferous period some 300-360 million years ago. At the risk of greatly oversimplifying things it nonetheless seems that the exuberant growth of plant life, together with circumstances that led to burial and fossilization as coal, some 50 million years before the late Permian helped set a time-bomb for future organisms. I think we (astronomers, exoplanetary scientists) tend to ignore this kind of factor when we discuss planetary habitability. Extinction events are sometimes seen as random or disconnected from the deeper planetary history. Did it really matter what happened a hundred million years earlier if an asteroid comes plunging in or a super-volcano erupts? For the late Permian it looks like it did matter.

For discussions of the long-term habitability of planets this brings in a whole new piece to contend with. If life itself can actually lay the foundations for disaster tens to hundreds of millions of years down the line then we're dealing with a much more interconnected and potentially chaotic type of system than perhaps we thought. As always, stepping back from our incredibly narrow worldview of the modern Earth, is critical.

Slippery Planets

2011-01-20T09:35:00.001-05:00

Oh Gliese 581g, where art thou? This small planet, about 3 times the mass of the Earth, orbiting within the habitable zone of a red dwarf star a mere 20 light years from us, has been the closest thing yet to a world that we might recognize as a true family member. Announced last year by Vogt et al. it emerged from exquisite spectroscopy data taken over periods of 11 and 4 years with two specialized instruments. It was a hugely exciting discovery, something that we'd all been anticipating as the first of many more such worlds to come.

Soon after this result then further independent analysis by Pepe et al. using additional data seemed to suggest that little GL 581g and its relatively weak signature in the Doppler shifted spectra of this star might just be a figment of overambitious data modeling. More recently an even more sophisticated analysis by Gregory, employing a Bayesian, or probabilistic, approach has also suggested a low likelihood that GL 581g exists. Instead of 6 inner planets orbiting this star there may only be 5, with the small rocky world gone in a puff of statistics. Intriguingly Gregory's analysis also suggests that one of the two datasets, taken with a different spectrograph, may have some as of yet poorly understood systematic problems and a worse accuracy than assumed. This can throw the complex modeling of multiple planets off the scent.

As always. more and better data will ultimately resolve the issue. GL 581g may or may not be gone. Regardless, it is still only a matter of time before similar, real, planets show up elsewhere. This whole event does however raise some very interesting issues that are well worth a little examination. Finding planets with a time-series of ultra-high precision Doppler spectroscopy is not easy. The influence of every planet in a system is simultaneously present, shifting the center-of-mass of the system and therefore the instantaneous orbital velocity of the star about that point. One planet in a circular orbit causes a nice simple sinusoidal variation in the observed stellar velocity. One planet in an elliptical orbit causes a skewed, distorted sinusoidal variation. Two planets cause a superimposed set of variations, with different periods and phases and skewness. By the time there are 5 or 6 planets then the shape of this stellar velocity curve is determined by more than two dozen parameters. Data is never perfect, noise and systematics abound, finding the best model fit to reveal the planets is a very, very significant computational challenge. Whatever objects you think are there must also obey Kepler's laws and form a dynamically stable system. Testing all of that requires care, patience, and even a degree of luck.

The interesting thing is that as our data get better, increasingly sensitive and with longer and longer coverage, the more we are going to run into these issues. We want to find multiple planets in systems. We want to find the low mass guys. But at the same time the more complex the parameter space becomes the more 'local minima' will exist in probability space - the more potentially deceiving 'best fits' that lure us into thinking we have new planets. Over time the sheer bulk of data will offer a resolution, and our tools of analysis will get further refined. However, the next year or two may well be a little bumpy. It should be fun.

The ten most important questions for astrobiology: Number 10

2011-01-18T08:49:00.000-05:00

The final piece in this brief summary of what the most pressing questions are for astrobiology effectively brings us full circle. It is what ultimately motivates us, what we really want to know, perhaps even what we really need to know as a species. It is a question that humans have pondered for at least 2,000 years

Is there intelligent life elsewhere in the universe?

One of my favorite quotes in the long history of this question is attributable to the Greek philosopher Metrodorus of Chios in the 4th century BC, who was of the school of Democritus. Undoubtedly mangled over the centuries it nonetheless brilliantly summarizes a key part of the discussion:

"To consider the Earth as the only populated world in infinite space is as absurd as to assert that in an entire field of millet, only one grain will grow"

The universe may not be infinite but it is extremely large, containing as many as 10 to the power of 23 to 10 to the power of 24 stars, and perhaps equally as many planets of all varieties, as well as their moons and the rocky detritus of formation. It has existed for some 13 billion years and the first generations of stars formed almost as long ago and have been forming ever since. When faced with this enormity it is very hard to not imagine that there just has to be a place somewhere else where creatures like us, or at least with our capacity for self-awareness and curiosity, exist. For ten to hundreds of thousands of years Homo Sapiens, along with our genetic cousins, have thought, invented, drawn, painted, talked, sung, wondered, explored, built, rebuilt, modified, investigated, fought, loved, hated, struggled, enjoyed, and survived. What a remarkable thing that is. In the grand scope of life on Earth we are still just a fleeting quirk of evolution, but we have just begun. The dinosaurs, the insects, the fish, and the plants are shining examples of the potential longevity of complex organisms. Nothing really says we can't exist for another hundred million years if we are clever enough, and if that's correct then it seems more inevitable for the same to happen elsewhere.

If there is other recognizable intelligence out there amongst the stars how do we find it? In various posts I have discussed some of the issues, including whether it is even a safe thing to make this search. In very practical terms there are a few options. First, we can listen for the electromagnetic signs of life, whether serendipitous glimmers of structured communication or a specific, targeted communique designed to trigger the interest of other civilizations. Second, we can look for anything that smacks of the 'unnatural', whether it is a bizarre artifact that eclipses light from a distant star, or evidence of atmospheric chemistry that smells like industrial pollution, or even deliberate 'terraforming'. Third, we can send our machines, or eventually ourselves, out into the universe to explore and to carry the message that we would like to make contact. Finally, we can make ourselves sufficiently noisy and intriguing that at some point someone else, or their machines, show up on our doorstep. Though it is of course quite likely that they'd get here a few thousand years later, long after we had lost interest.

Searching the skies for signals has not yet had any real success. Although the techniques have become better and better I don't think anyone would argue that there is data that looks even slightly suggestive of an artificial origin (except just possibly, perhaps, maybe, that infamous WOW! signal). It's a very tough challenge, and it is clear that enterprises such as SETI have a long way to go before exhausting all possibilities. I think the second option may actually be an interesting one, particularly the notion of finding chemical fingerprints of a planet dealing with either pollutants or deliberate geo-engineering. We do seem to be edging towards the telescopic capabilities required to sniff at Earth-type planets, with JWST or next-gen giant instruments on the ground, so can we equate any unusual environmental parameters with intelligent inhabitants? It would be a hugely tricky task, but it may be a lot easier than the other avenues of investigation.

There are also many arguments that seek to convince that we are a rare thing, and we have discussed those before. Some are extreme but even mild variants suggest a big problem for our attempts to answer question 10. Suppose that intelligent life comprehensible to us does indeed occur across the universe, and has done so for most of the past 13 billion years, but it's a bit thin on the ground. In cosmic terms even one such species per galaxy still fills the universe with something like a hundred billion of these brainy civilizations. The problem for those intelligences is that there is on average a colossal gulf of millions of light years between them and anyone else. Barring physical laws that allow the circumvention of inconveniences like the finite and absolute limit of light speed, as well as the passage of time, then they will remain in splendid isolation. It's sobering. However, the flip side is that equally good arguments can be made for intelligent, or at least complex, life to be common. While interstellar space is still a huge gulf, it is nothing compared to intergalactic space and the equation shifts quite dramatically. So much so in fact that issues such as Fermi's Paradox then raise their heads. No wonder that scientists keep coming back to question number 10, it's full of juicy points to argue endlessly over.

The good news is that barely 16 years ago we didn't know for sure that any planets existed around other normal stars. How strange that appears now. It seems almost absurd that we were ever cautious about whether the universe made planets efficiently. Now we are on the cusp of finding out just how many small rocky, even Earth-like, ones there are across the galaxy. This is arguably the biggest single advance towards dealing with question 10 in a real, non-speculative, fashion since Galileo and Copernicus opened the floodgates of astronomical reason. This is one thing that makes modern astrobiology such a compelling and exciting field. The fog is lifting on a scientific path that will eventually lead us to a census of worlds, and ultimately of their contents. Slowly we will push out further from Earth, and in doing so will further and further narrow the options for intelligent life.

Finally, one cannot help but feel in the most non-scientific way imaginable, that it would be so bleak and awful for such an extraordinary phenomenon as ourselves to be alone in the void that we must strive our greatest to find out the truth. Even if the ultimate answer is that yes, we are a singular event, then it would still seem that the mere act of looking, the scientific and technological effort required, would surely play some role in ensuring that we do not vanish into the gaping maw of evolutionary extinction. If we just 'are', with no rhyme or reason, then it behooves us to revel in our existence, embracing the cosmos as the very fiber of our being that it is.

The ten most important questions for astrobiology: Number 9

2011-01-17T12:38:00.000-05:00

Now we're down to the last two questions of the ten most important questions for astrobiology, a highly biased and personal overview of the primary motivations and challenges for this still emerging scientific 'interdiscipline'. Number 9 is big, not quite the biggest, but hugely important and rather remarkably getting ever closer to the realm of the answerable.

Is there microbial life elsewhere in the universe?

As has been discussed before in these pages, microbial life is the dominant form of life on Earth. It represents the majority of the total planetary biomass. It represents the greatest genetic diversity of living organisms. It represents the oldest continuous lineage of life on the planet. It represents the most flexible, adaptable, and metabolically versatile life on the planet - inhabiting the greatest range of niches. It represents the component of the biosphere that has, throughout Earth's history, been pivotal in establishing, and maintaining the bio-geochemical cycles and feedback systems that help govern atmospheric, marine, and surface chemistry and the global energy budget. It was here long before the likes of us, and will be here long after the likes of us.

So how do we find out whether this ancient and persistent mode of life exists beyond the confines of the Earth? Microbial life is one of the primary targets of solar system exploration and studies. When the Viking landers arrived on Mars in 1977 they performed carefully designed tests on the response of martian soil to being 'fed', as a means to see whether microbial life would perk up and show its presence. The results are still a matter of debate. Initially something indeed seem to be going on, even vigorously. Later analysis swayed opinion to consider this a result of the quirks of martian soil chemistry. Very recently then the possibility that the chemistry owes something to microbial life in the first place has been raised. With convincing geological evidence now in place that indicates Mars' on and off again acquaintance with surface aqueous environments, NASA's new rover Curiosity will carry with it a full instrument package dedicated to finding signs of extinct or even extant microbial life. European rover plans are also going to target such studies.

The likely subsurface oceans on Europa, as well as places like Ganymede, are an intriguing potential habitat for microbes like chemoautotrophs, among others. Similarly the liquid water zones that seem to be lurking inside Enceladus are prime locales. In fact, the more we know about terrestrial microbes, from bacteria to archaea, the more places begin to look attractive, even the mid-zones of the Venusian atmosphere and the porous interiors of comets and asteroids. Even if any organisms we find turn out to be relatives, transplanted around the solar system as lithopanspermia, it will be a stunning piece of evidence for the persistence and adaptability of microbial life.

Further afield, to small rocky exoplanets, the near-term goal of making rudimentary measurements of atmospheric chemical abundances is directly motivated by the relationship of out-of-equilibrium chemistry to life, and to the planetary engineering skills of microbes in particular. Indeed, making a planet long-term suitable for life in general may, in chicken-and-egg fashion, require the geo-engineering of microbial organisms. Life, much like its internal chemistry, can self-catalyze.

No, detection of atmospheric oxygen, carbon dioxide, even co-existing methane will not tell us precisely what is going on in these distant worlds, but the simplest answer would be that a microbial-type biosphere is present. Let's imagine that we find a nice rocky planet somewhere within 100 light years that is conveniently transiting its low-mass parent star so that we can see the atmospheric oxygen, water vapor, and a couple of other interesting molecular species. The trick will be to keep looking. Even a tidally-locked 'eyeball Earth' is likely to exhibit variations in environment with time. Populations of organisms are dynamic, stellar input is never perfectly constant and the responses of a biosphere to change are as much a fingerprint as is a single snapshot. Lurking in such data might be the clues to let us see whether a microbial world, like our own, exists or not.

From what we have learned about life on Earth then if we find no evidence for microbial organisms either in our solar system or beyond, it will dramatically alter not just the odds of finding life anywhere in the universe, but also our ideas and models for how life originates. For many of us this would be a disquieting prospect, and so we hold our breath to see whether the bacteria and archaea are indeed just local examples of a cosmic phenomenon.

The ten most important questions for astrobiology: Number 8

2011-01-10T09:26:00.000-05:00

This next question is much broader than it may at first sound. It is an issue that has hovered around in the background for quite some time without too much progress, but that may be starting to change. Question number 8:

Is there a relationship between interstellar chemistry and life?

There are a number of layers to this question and to potential avenues in answering it. First, as with anything in this universe most of what's out there in the great beyond is simply hydrogen and helium (curse you Big Bang and expanding universe!). Hydrogen chemistry, while surprising complex, doesn't produce a big toolkit, and helium as we all know is mostly chemically inert. The formation of molecular hydrogen and molecular hydrogen ions does nonetheless seem to play a central role in cooling processes in the young universe that help gravity collapse matter to make the first stars. However, the kind of chemistry we're really interested in is that which takes the traces of heavy elements produced by those stars and their descendants and makes complex molecular species.

Rather remarkably, of the 150 plus interstellar molecules that we have thus far identified, and there are many more to go, then the great majority involve carbon. Carbon chemistry - organic chemistry - seems to be ubiquitous in the universe. We can even find it in the accretion disks around super-massive black holes. Why is this so? Carbon is just the right kind of atom to make all sorts of chemical links to other atoms, including nice single, double and triple covalent bonds. Particularly handy is its ability to covalently bond to itself, creating almost limitless chains or polymers. Carbon-based structures like polycyclic aromatic hydrocarbons built from benzene rings are also particularly tough and can handle a wide range of interstellar radiation environments, from hot to cold.

We, and all the life we know of, are of course based on carbon chemistry. Planets and moons in our solar system have a lot of carbon chemistry on their surfaces. Asteroids and comets can be positive smorgasbords of rich organic molecules. The circumstellar and proto-planetary disks of nascent star systems reveal, to put it crudely, a ton of carbon chemistry going on. It's a carbon, carbon world after all.

So, do we have an answer to question 8? Does interstellar chemistry have a direct bearing on the chemistry that ends up on planetary surfaces? Well, this is still open to debate. Chemistry in interstellar space, or even thicker nebula, is a long-winded, low temperature process. In the clumps of nebula or molecular clouds that end up making new stars and planets things speed up, but conditions may also undo the ancient mix of molecules brewing across a galaxy. Does it matter? Some evidence suggests that when low-mass stars are forming they may not emit enough ultraviolet light to crack open the tough interstellar molecules that would otherwise set in motion whole chemical chains. So unless interstellar space provided a rich starter mix then things might reach a bottleneck in terms of early chemistry. Of course the surface of a young rocky planet is likely a hot, rough, and chemically productive place - but perhaps the initial chemical wash it receives does actually matter, possibly even for dictating the chirality or handedness of biological molecules. Can old interstellar molecules make it through the chaos of a forming solar system and end up on young planetary surfaces? We don't know, but isotopic evidence suggests that at least some meteorites here on Earth could contain molecules that formed in the fearsome chill of interstellar space rather than more locally.

The complete answer to number 8 is therefore still elusive. New astronomical instruments like Herschel and ALMA will help open up the chemical universe further, and tucked away in a number of labs are remarkable new experiments that seek to reproduce interstellar carbon chemistry and learn more about its pathways and efficiencies. What there can be no doubt about is that the predominant chemistry of the universe as a whole is organic, it just remains to be seen which bits link the most directly to life on a planet.

The ten most important questions for astrobiology: Number 7

2011-01-04T09:13:00.000-05:00

This next question is one that really gets people riled up. With good reason. It cuts to the heart of critical issues surrounding the ways in which we define life, the nature of evolution, certain philosophical prejudices, and even our tendency to use statistics inappropriately. So, question number 7:

Is the Earth unusually suitable for complex life?

Now, I've deliberately phrased this question in this particular way in order to better illustrate some of the factors involved in addressing it. The words 'unusually' and 'complex', and even 'suitable', can all trigger lengthy debate. In and of itself the question should be straightforward - a simple rephrase would be 'are there many planets like the Earth in the universe?', but that leaves open the meaning of 'like the Earth', which generally ends up translated as 'with creatures like us running around'.

A few years back the debate over this kind of question got pretty heated with the book 'Rare Earth' by Ward and Brownlee. It tried to make the case that well, yes, the Earth is unusually suited for complex life, and that most other planets out there in the cosmos might be ok for microbes, but not the likes of us. A fair amount of the argument is based around a list of critical requirements - things like plate tectonics, the Moon, the right elemental mix and so on - get one of these wrong and, the authors contended, multi-cellular, walking talking life just doesn't have a chance at ever occurring. The simplest criticism of this type of reasoning is that there needs to be an explicit, quantifiable, connection between these phenomena and the rather horribly unknown biochemical probability space for apes like us showing up - and such connections are thin on the ground. Jim Kasting gave an excellent and thorough rejoinder to Rare Earth in a 2001 review that you can find amongst his papers. In essence Kasting made a convincing case that almost every negative phenomena invoked by Ward and Brownlee could be found to be, if not a positive for complex life, certainly not a showstopper, and vice-versa.

Intriguingly some of these arguments also to parallel discussions of the anthropic principle, although the fundamental requirements for complex life on a planet are much harder to pin down than, for example, the requirement for large, resonant, nuclear cross-sections in order to produce carbon in the universe. One suspects that there may not be anything quite as cut or dry as that in determining whether 'simple' life transitions to 'complex' life, especially when evidence here on Earth points towards microbial life having made multiple experiments with multi-cellular organization - in an on again, off again fashion.

It is also a question that bumps up against philosophical/statistical concepts of a priori and a posteriori statements. A genuine a priori statement is one that is without question (all sheep are born), while an a posteriori statement relies on interpretation of observation (most sheep have four legs). The hypothesis that the Earth is indeed unusually suited for complex life is both a bit of a priori (we are here, therefore Earth is suitable for complex life) and bit of a posteriori (the Earth is the only example of a planet suitable for complex life, and therefore may be unusual). The alternative hypothesis, that Earth is not unusually suited to complex life, suffers from the same problem, all of which boils down to having a sample size of one. Even if a Rare Earth type argument could point to a smoking gun - something utterly, undeniably critical for complex life that was also utterly, undeniably going to be unusual, it would always be questionable because of the sample size of one.

It would be wonderful to be able to answer this question with experiment. Imagine we could build hundreds of copies of the Earth, but alter something in each case. In dramatic fashion we might demand that the collisional formation of the Moon never occurred, or that terrestrial volcanism shut down after a mere billion years or so. Among the (slightly) more subtle variants we might add or remove one or two mass extinction events during the planet's history, or tweak ocean acidity during the pre-Cambrian by a pH or so, or even just remove a single seemingly unimportant species. We might find that complex multi-cellular life is acutely sensitive to us changing anything, but we might also find that convergent evolution is more than just for biochemical structures, and that complex life literally fights tooth and claw for existence once the wheels are set in motion. Obviously the universe may have performed this experiment for us already, the challenge is to find and examine those other worlds, and that is going to take us a long time at this point.

So, the answer to question 7? Go canvas the universe - and that's what most astrobiologists are aiming for.

Looking forward

2010-12-27T17:28:00.000-05:00

As our planet, a slowly crystallizing 6,300 kilometer radius ball of silicates and carbonates with a healthy dose of metals, comes streaking up on the same orbital patch it passed 12 months ago, it seems appropriate to ponder some of what might be heading our way in the future. Perhaps not by the next time we're this side of the Sun, but in the orbits to come.

There are undoubtedly many goodies on the near horizon for the study of exoplanets. A far better grasp on the number of Earth-sized worlds out there in the galaxy is on its way, thanks to both the Kepler mission and the ongoing efforts of other surveys using radial velocity, transit, and gravitational microlensing measurements. Thus far everything points towards a universe positively filled with small rocky worlds, and undoubtedly many will fulfill several of the basic, albeit biased, requirements for Earth-style climates and surface environments. On the tail of this will be the possibility of rudimentary measurements of atmospheric composition for a few select worlds, most likely orbiting low-mass stars in our neighborhood.

All is good, but these are items that have been written about ad nauseam, in these pages as well as elsewhere. Surely barreling towards our orbital solstice at 30 kilometers a second we can afford a little more reach?

In the next decade or so we should see the advent of the largest optical telescopes ever built by humans. With mirror diameters in the range of 30 meters and advanced adaptive optics, calming and helping eliminate the distortions of the Earth's atmosphere, these goliaths will not only open up exoplanetary studies but also the study of our own solar system. For the first time ever we will be able to make daily (if not by the minute) observations of places like Jupiter's icy Europa with spatial resolution of just a few tens of kilometers, combined with exquisite optical and infra-red spectroscopy. With spatial fidelity more than ten times better than the Hubble Space Telescope and the James Webb Space Telescope these monumental observatories will allow us a god-like reach across our solar system. It is impossible to know exactly what we will see. Seasonal changes on Titan will be visible - and followed across decades - as will surface changes on Io, Europa, Ganymede, and even Enceladus around Saturn. Maybe we will witness rare but revealing phenomena. Perhaps one day Europa will crack, a chasm ten kilometers deep forming as the immense stresses of the Jovian gravitational tide wins out, and a watery soup will spill forth. As it sublimates and freezes, shutting this tear like all those scars before it, perhaps we will see not just the chemical signatures of a deep biosphere but strange organisms themselves, flopping to the surface in waves. Each freezing out but giving up their latent heat to the next, all reaching for photons and oxygen ions, favorite foods from the last harvest a thousand years before.

If Enceladus is indeed the rebuilt child of Saturn's rings, themselves the product of a long-since shredded moon, perhaps we will watch as it continues to twist and crack, shaking itself to some new equilibrium, at the same time providing a warm oasis for life far from the Sun. Even the cold dark Uranian and Neptunian systems will be far more accessible to us, and perhaps cryo-volcanically active places like Triton have unexpected goodies yet to be revealed.

Eventually Mars' daily globe will adorn our screensavers, a view not afforded by orbiting instruments, their cameras designed to probe only tiny patches. We will watch as yet another sandstorm rises and engulfs the planet, and marvel at the ephemeral delicacy of the icy environment. Perhaps here too we will enjoy watching the overlays that depict the migrations and flows of a newly discovered subsurface martian biosphere, probed and followed by fleets of wandering robotic avatars.

As comets venture through their perihelion, lighting our Earthly skies, these giant instruments will yield views previously the exclusive province of high-octane spacecraft flybys. The pocked and crumbly nuclei, some wet and spewing sublimated gases into space, others discrete but laden with rich organic chemistry, will all be routinely charted and examined for critical clues to the nature of solar system assembly and possibly the chemical mix that our planet started with some 4 billion years ago.

If there is one future wish, as we tumble into another stellar orbit, then it would be for such fidelity of cosmic vision, as well as the fidelity of terrestrial vision to recognize our common humanity. As the very stuff of stars, alive in an immense and ancient universe it would be a dreadful tragedy if we squander our moment in blindness.

The ten most important questions for astrobiology: Number 6

2010-12-18T12:42:00.000-05:00

This next question for astrobiology has a fairly long history, going back at least 1500 years or so, with occasional flurries of interest over the past two hundred years. Interestingly, for every controversial idea put forward to address it there are also many sober and reasonable ideas. Question number 6 is:

Can life spread through space itself?

Like the other questions this one requires some further qualification. The typical context is the idea that life originates, or is incubated, on or in planetary bodies. Subsequent events lead to the transport of viable organisms, or at the very least viable biochemical 'seeds', across the gulf of space - either within a single planetary system or between distinct stellar systems. The term most generally used is 'panspermia', coined by the ancient Greeks and meaning 'all seed', more modern studies often refer to 'lithopanspermia', meaning the transport of life-containing material from planetary lithospheres.

It is a very interesting idea. Remarkably, the more we learn about the resilience of life here on Earth, from super-tough microbes to multi-cellular organisms like tartigrades, the more plausible it seems that organisms could in principle survive long-term exposure to interplanetary, and just possibly interstellar, space. This is especially true if they are tucked away inside big chunks of rock or ice, shielded from disruptive electromagnetic and particle radiation. The rub, and there always is one, is that the physical processes that might loft life away from a cozy planetary surface or enable their descent onto a new planetary body are pretty awfully violent - from asteroid impact induced spallation of lithospheric material (big collison=ejected planetary rocks), to hyper-velocity re-entry through a planetary atmosphere. This is such a make-or-break problem that tests have been undertaken, both in the lab and with space-born experiments to scope out how extreme pressures and temperatures weed out organisms. So far nothing suggests that the violent circumstances initiating or ending panspermia are complete deal-breakers.

Then there are issues of actual transportation within a planetary system, how long does it take and how often can a piece of planet A arrive on planet B? Detailed gravitational simulacra of our own solar system provide some insight. Earth-Mars/Mars-Earth exchange (as we know from meteoritic evidence) occurs, and can be on reasonably 'quick' timescales of a few years to a few tens of thousands of years. Even channels that carry material from, for example Earth to Europa, exist - but the efficiency is very low. Out of roughly 100 million bits of ejecta from suitable asteroid impacts on the Earth perhaps only 100 might eventually intersect Europa. So the orbital architecture of a planetary system plays a dual role - determining the rate of ejecta-producing impacts on planetary bodies, and determining the transport efficiencies of these lumps between worlds.

Then there is interstellar panspermia. Solar winds may be able to both accelerate and decelerate tiny crumbs of dust or ices carrying microbial organisms, or at least DNA fragments. A speck is propelled from one star and then if it intersects the stellar wind of another may be slowed down to drift inwards to the planetary zone. Microscopic material can sink down into a planetary atmosphere, as we see here on Earth with high-altitude interplanetary dust particles. The biggest challenge is for anything of any remote biological function to survive the interstellar environment for the huge lengths of time required.

Which gets to the last, and most speculative and abused piece of this. If life, in varied and splendid forms, is common in the universe then wouldn't at least some of it have evolved to exploit the greater terrain offered by migrating through space? Not by building machinery, but either by natural selection or just sheer biological grit. Given the apparent propensity for lithospheric material to get exchanged between planets in our solar system and the potential diversity of planetary systems out there, then it seems plausible that somewhere is a place that could be analogous to one of our terrestrial archipelagos. This Indonesia of planets could offer the kind of back and forth that would, over time, select for organisms best suited to space travel - a little evolutionary honing before they set off into the galaxy.

So the answer to Number 6 remains 'possibly', and by exploring our own solar system there is both a chance of finding our relatives and finding the misplaced chunks of planets that might give us some more clues.

Voyager to the stars

2010-12-15T09:49:00.000-05:00

Thirty-three years ago Voyager 1 was launched from Cape Canaveral. It now appears to have crossed into the true interface between the particle radiation from our Sun and that of the great gulf of interstellar space - as it travels through the heliosheath. In this region, 10.8 billion miles or 16 light hours from us, the solar wind is being turned sideways, blowing against the as-of-yet unsampled interstellar medium as we orbit around the galaxy. Eventually Voyager will cross the absolute edge of this zone of balance, through the heliopause and out into the galaxy proper.

Despite its age, Voyager is true to its name. Most of its monitoring instruments still function. It senses the ambient particle environment by registering the impact of charged particles. As their energy appears to decrease we can tell that their forward velocity is dropping, the Sun's influence now too weak to punch any further. Its 3.7 meter diameter radio dish still picks up instructions at about 16 bits/second, and relays back microwave telemetry at a typical 160 bits/second, boosting to 1.4 kilobits/second for plasma-wave data. It is overseen by 3 doubly redundant processors (6 total) that by today's standards are barely recognizable as such. These are wonderfully tough relics, ancestors with 16 and 18 bits compared to the 64-bit architectures of today's most modest laptop. The three radioisotope thermoelectric generators now generate only about 270 watts of power, compared to 470 watts in 1977 when their plutonium-238 contents were freshly minted - possibly the only rational purpose for an advanced civilization to dabble in such toxic and dangerous materials. It is a brilliantly conceived and engineered machine, something for humans to be proud of.

Together with Voyager 2 (traveling at a slower rate, still a mere 8.8 billion miles from Earth), and Pioneers 10 and 11, these amazing craft represent our first tiny steps into the interstellar void. These small specks of cosmically rare elements are for now the only outward physical sign of our presence in the universe. It is quite possible that long after we humans have become extinct, or evolved to something new, or just simply moved on, these devices will continue to serve as monuments to our curiosity and eagerness to join with the cosmos. The odds of their intersecting anything else in the universe are extremely small, it is a fair bet that they will still drift through space long after all the stars have gone out, in about 100 trillion years time. It is a sobering and fantastic thought that despite our all-to-human problems, our daily lack of perspective, we have nonetheless found on occasion the clarity to produce evidence of our existence that may live forever.

The ten most important questions for astrobiology: Number 5

2010-12-13T09:15:00.002-05:00

The next question relates, as they all do, to previous questions. It may not represent where cutting edge progress is being made in astrobiology, whether it be in microbiology or exoplanetary science, but it's a perennial query. I've lost count of how many times it's been raised after a few drinks and left hanging there casting a slightly embarrassing shadow over the conversation, so here's Number 5:

Could there be life in the universe operating on entirely different principles?

So what exactly does this mean? Typically the definition of 'life' here is pretty broad and a bit fuzzy. It ranges from stuff we'd recognize by its actions (that rock just ate my sister!) to stuff we'd have to kick ourselves about later (that cloud not only looks like a rabbit...). As for 'principles', well this too has a broad range of meaning.

At the simplest end we're talking about fiddling with terrestrial-style biochemistry. But to operate with different principles there have to be some fairly radical alterations - more than just substituting arsenic for phosphorus or silicon for carbon, assuming any of that can work. A key factor has to be the capacity to store information in a way that is both robust and yet readily accessible. Our biochemistry uses complex polymers and a great array of support structures and molecules, from histone spools to enzymes. Various array-like, or quasi-periodic molecular forms occur in other parts of nature, in clay lattices and crystals. The read-write issue is a tough one though, to read out something like DNA, to convert that information into function, requires unzipping the molecule - a dangerous exercise. Equally, the process of writing to that storage device can be both very subtle (the filter of natural selection over short to long timescales) or quite brutal (viral RNA insertion). As we look at the enormous complexity of our own biochemistry it becomes increasingly hard to blithely suggest a plausible alternative - which is not to say there couldn't be one, but it seems like an almost insurmountable challenge to invent one with just theory. The closest we can get may well be our own version of artificial life, the push and pull of electrons in semiconductors.

But what if the different principle is really different? Here we firmly enter the realm of science fiction where 'beings of pure energy' serves as a useful scriptwriter cop-out. All that we have in this case is speculation. For example, is something like a galaxy alive? They involve mutual interactions of smaller structures (stars, molecular clouds, globular clusters), large scale connective processes (density waves, magnetic fields), they have speciation, they avoid reproduction as we know it by simply being eternal, they feed off smaller structures and on occasion each other, they call out to their brethren like leviathans in the deep - their booming voices carried across the cosmos as waves of gravity. They are also full of microbial life, most stellar cells carrying a swarm of simple chemoautotrophs, little spherical bodies of rock or gas processing atoms into complex molecules - fed by the mitochondrial-like warmth of nucleosynthesis.

It makes for a good story, but therein lies the problem with this type of speculation. The story is far more compelling than anything we might realistically stand to test or learn from even if it were true.

So the answer to question 5 at the moment seems to be 'possibly, but we really have no good idea what it would be'.

Catching the rays

2010-12-06T09:52:00.000-05:00

It's sometimes very hard to choose between topics - from spectrally featureless super-Earth atmospheric light transmission suggesting a possible water vapor composition, to electrically signaling bacteria crossing the line to inter species cooperation. Then along comes an item that just scuppers even those wonderful stories.

Harvesting solar photons for direct bio-chemical processes has been the province of microbial life for at least 3.5 billion years - since sticky colonies of organisms like cyanobacteria sat in the shallow waters of Archaean Earth. Later endosymbiosis allowed plant life to pick up the trick, and make use of the hundred and seventy petawatts of solar energy flooding the planet. Now it appears that at least one species of insect has perhaps also learned to capture photons as a key resource.

This remarkable talent belongs to the Oriental hornet - Vespa orientalis. It had been known that this insect generated electric currents through various structures in its body, and it had been speculated that this related to thermal regulation. Now a new study by Plotkin et al. has shown that on the hornet's abdomen is a remarkable optical 'trap' - an arrangement of skin cuticle ridges about the size of the wavelengths of ultra-violet to optical light that serves to trap incoming photons. Light is repeatedly reflected within layers which increases the odds of it intersecting a special yellow pigment xanthopterin - tuned to selectively absorb photons. In doing so it can generate minute electrical currents within the cells - which could then power bio-chemistry. Plotkin and colleagues have gone as far as constructing a rudimentary organic solar cell in the lab, using xanthopterin, to generate electricity with about a 0.34% efficiency (compared to about 10% for the best artificial solar cells).

The full chain of the process has not yet been disentangled, however the hornet's activity is strongly correlated with the intensity of ambient ultraviolet light, and it seems that there is most metabolic activity within these optically-tuned pigmented parts of their bodies. It may still be that thermal regulation is the driving force behind the evolution of this capacity, however many things point towards Orientalis as being a genuinely solar-augmented organism.

This raises a number of intriguing points. Hybrid energy production is a useful talent, allowing greater efficiency and flexibility. Solar radiation is also the bulk energy resource on the planet - offering many orders of magnitude more power than chemical or thermal geophysical sources. So why don't we all do it? Obviously in an indirect fashion we do. Humans for example can produce vitamin-D by absorbing UV photons in our skin. Reptiles can do the same but they mainly need the direct thermal energy of sunlight to help maintain their internal temperatures. But actually generating fundamental power - electrical voltages that can be exploited for biochemistry - is another matter. There is probably a down-side, whether in terms of the biological expense of the necessary structures and chemistry, or in terms of the lifestyle that could make best use of a little solar electrical jolt - however that is all rather specific to terrestrial environments and the history of life here.

One wonders whether life elsewhere, in different circumstances, might have found better reason to walk around with a photon capturing carapace, opening up a whole range of interesting possibilities...

The ten most important questions for astrobiology: Number 4

2010-12-04T08:47:00.000-05:00

Another big question, but this one is at the core of astrobiology. It also follows on quite nicely from the hubbub of bacteria seemingly utilizing arsenic rather than phosphorus (emphasis on seemingly).
As with the other questions there are some qualifications for Number 4:

How does life originate?

Strictly speaking the slightly modified version of this question that is really under the microscope is: How did life originate on Earth? However astrobiology has to set its sights high because the origin of life here may or may not represent the same type of origin as life out there. It's also true that one of the biggest scientific motivations for astrobiology is that in order to understand our own origins we desperately need a proper context. One planet with life, even if hugely complex and with a 3-4 billion year history, is still just one sample of a phenomenon. Seeing how this phenomenon operates elsewhere in the universe would help answer all those nagging issues of probability, bio-chemical uniqueness, convergent evolution, and so on.

All signs point towards the existence of microbial life on Earth somewhere in the time-slot of 3.5 to 3.8 billion years ago - mainly from the fingerprints of ancient stromatolite rock structures, products of bacteria and archaea colonies. We really don't know how far back it all goes, owing to the dearth of unaltered continental crust from these times. We especially don't really know what was going on during and before the Late Heavy Bombardment - a period of massive pounding by asteroidal and cometary material about 4.1 to 3.8 billion years ago. It probably also doesn't make sense to be seeking a single 'moment' at which life sprung up and started tweeting. As a phenomenon of self-organizing, information carrying, reproducing, eating, pooping stuff, life in truth represents many interleaved networks and systems.

The bottom line is that right now we have no single clean answer to this question. Many, many hypotheses exist. These range from having life delivered pre-formed from other planets or star systems (panspermia) - which rather avoids the question - to an array of so-called 'pre-biotic' chemical scenarios that lead to the basic biotic molecules that we see today, or at least variants thereof. These include origins in atmospheric chemistry, deep sea vents, clay lattices, and polycyclic aromatic hydrocarbons to name but a few. Then there are ideas that seek to tackle a possible transition from molecules to real, recognizable bio-structures - like cell membranes and catalyzing molecules. Coming from the other direction people have studied the potential ancestral forms of modern DNA. Since RNA molecules can serve as both information storage and enzymes (a role generally taken by proteins in modern biochemistry) then perhaps there was an 'RNA-world' before there was a DNA-world, a simpler, much more fluid and information-loose biology that eventually hit upon the more robust 'hard-drive' data storage offered by the DNA molecular structure.

In short - it's a tough problem. No one suggestion has yet had the ring of completeness, and it may be that there simply is no single mechanism, but rather a parallel series of critical steps and components that have to have a few tens of millions of years to cook the right pie. The environment of the kitchen probably plays a big role. Small rocky planets, with a nice rich chemical wash, and both stellar and geophysical energy input may well provide one of the most conducive environments in the universe...as far as we can tell.

An intriguing avenue of investigation may be coming around the horizon with the advent of artificial biological organisms - this type of reverse-engineering might just provide some unexpected clues. Similarly, as we better understand microbial and viral ecology there may be pointers to fundamental rules that haven't quite come into focus yet. In truth, it sure would be handy to have another biosphere to compare against...

Arsenic and old lace

2010-12-02T13:11:00.000-05:00

Arsenic is an insidious element. With 5 outer valence electrons the arsenic atom is chemically similar to the biologically critical element phosphorus, but only in crude terms. Life depends extensively on phosphorus - it helps form the molecular backbone of DNA, it is part of molecules like Adenosine triphosphate (ATP) that serves as a vital rechargeable chemical battery within all living cells, as well as many other biologically vital roles. Arsenic on the other hand can weasel its way in, waving its valence electrons in a come-hither fashion, and getting the best seat in the house. The problem is that once an organism takes in arsenic, replacing some of its phosphorus, it typically begins to malfunction - arsenic is is a fatter atom and biochemistry is a sensitive thing. There is good reason why arsenic has long been a poison of choice for nefarious human dealings.

As is often the case though, microbial life has an exception to the rule. There was a flurry of buzz earlier this week about an astrobiology press-release, rumors of alien organisms were rife. Now the embargo is lifted we can reveal what the fuss was about. Wolfe-Simon and colleagues in a paper in Science this week present some remarkable discoveries made by tinkering with a particularly hardy strain of bacterium (actually a proteobacterium, part of a very diverse group). The bacterium Halomonadaceae GFAJ-1 is a halophile (salt-lover), found in the alkaline and hyper-saline Mono Lake in California. Many such bacteria are known to be highly tolerant of toxins, such as arsenic, but now it turns out this one is far more than merely tolerant.

While carefully culturing organisms from Mono Lake in the lab Wolfe-Simon et al. gradually removed phosphates from the food supply, replacing them with arsenic compounds tagged with radio-isotopes to enable them to be tracked into the bacterial cells. One bacterium, GFAJ-1 grew faster than anyone else. Not only did GFAJ-1 survive it swapped out its phosphorus for arsenic - in cell structures, and most remarkably, in DNA itself. It appears to be nature's own Transformer. It's pretty amazing. How does this organism keep a viable bio-chemical system running with the same molecular structures but with the fat arsenic atoms replacing phosphorus? It remains to be seen whether strict one-to-one replacement is really occurring, it's also the case that phosphates are still required by this organism, albeit in small amounts.

The notion that life might use arsenic instead of phosphorus has cropped up before. In particular its been wafted around as a plausible case of 'shadow life', a parallel type of life with a separate but contemporaneous origin to phosphorus using life. That's certainly an intriguing idea, although how and why GFAJ-1 would have switched over to phosphorus yet still retained the capacity to use arsenic is unclear.

For astrobiology it definitely offers encouragement that some of the seemingly hostile chemical environments in our solar system - Martian perchlorates for tea anyone? - may be ok for some very specialized 'niche' lifestyles. However in the broader context I think there's a hitch for talk of arsenic based life on other planets. Phosphorus itself is made by hefty stars over about 15 solar masses and in terms of cosmic abundance it is relatively scarce compared to other major bio-chemically important elements. It is roughly 1000 times less abundant than carbon. Nonetheless, in terrestrial life it is much more concentrated, only a factor of 30 or so less abundant than the carbon in your ham sandwich. So there is a bit of a bottleneck for life in terms of having enough phosphorus around - it wouldn't take much of a deficit of phosphorus in a forming planetary system to leave things barren. Now, if we wanted to postulate planets of arsenic-based life the problem is even more acute since arsenic is 1000 times less abundant than phosphorus - it's really a trace by comparison. Despite some of the breathless discussions already going on; the chances of arsenic-based planet-wide biospheres....slim to nonexistent.

The ten most important questions for astrobiology: Number 3

2010-11-29T23:50:00.000-05:00

It would be easy to argue that the next question is too big, to broad, and too unanswerable to be in the list. Nonetheless it's a question that keeps cropping up and is also representative of the bigger and deeper issues that astrobiology must ultimately tackle. So, question number 3:

Is this universe particularly suited for the phenomenon of life?

Because this is a bit ambiguous there are some necessary qualifications. First, implicit in the way I've phrased this question is the idea that 'this universe' is either one of many or that it represents an instance of a phenomenon that somehow reoccurs. Second, and this is really just an expansion of the question, it suggests that life requires a particular range of physical laws and/or 'contents' in a universe to happen. Third, this question hits the button to sound the anthropic principle klaxon, opening a Pandora's box of weak and strong arguments about the timing of life, its privileged position and even its necessity for the existence of the universe in the first place.

It can be quite a morass, so I'll try to stick to the easy bits. The second item above can also be rephrased as a question about what the minimum requirements are for life to show up at some point, and whether some physical laws are more or less important. If we stick with carbon-based life then there's a ready laundry list, not least of which is the need for carbon in the first place. Carbon comes from the triple-alpha process in stellar nucleosynthesis. Intriguingly this set of nuclear reactions hinges critically on resonances between nuclear energy levels, allowing beryllium and helium nuclei to gracefully flop into forming a new carbon-12 nuclei. Without that coincidence then carbon would be little abundant in the universe, rather than the 4th most abundant element in the universe - after hydrogen, helium and oxygen. This resonance hinges in turn on the value of the fine-structure constant, describing the universal strength of electromagnetic interaction. Change this constant by a few percent and among the consequences you can kiss carbon goodbye.

Even if you feel you can do without carbon - preferring some alternative life chemistry - then a different fine structure constant also messes up things like covalent bonds in molecules, at least compared to the way they are around us. Before even getting to details like this if the universe had a different overall composition, or differing gravitational constant, or differing CP violation, then it could be radically different from the outset - perhaps never making any structures beyond individual atoms. In brief, there are a lot of things that seem to have to fall into place for something like us to come along.

Because we feel (rightly) incredibly uncomfortable with any notion we might be special to the universe (the universe clearly being special to us) then things get sticky, and answering question 3 hits a wall....unless we get to the first qualification I give above: the universe is one of many, whether contemporaneous (multiverses) or one of an endless succession (cyclical). An infinite number of universes, or quantum realities, are like the infinite number of monkeys with typewriters - somewhere amongst them will be the ones that can produce life like us. Much like the realization that carbon production requires something special - and indeed how Fred Hoyle came up with the physics for it - it may be that a version of the anthropic principle is actually telling us that there must be multiple universes, or else things are even more bizarre.

Astrobiology can't answer 3 yet, at least not in any satisfactory way. But finding out more about life in the universe, carbon-based or otherwise, could just possibly nudge us in the right direction.

The ten most important questions for astrobiology: Number 2

2010-11-26T16:52:00.001-05:00

Question number 2 is close to home: Is there life elsewhere in our solar system?

For all the talk of exoplanets and habitable worlds out amongst the stars, our own humble solar system probably remains the most likely place for us to find the first convincing evidence for non-terrestrial organisms. Not only are there plausible environments for life to survive, they're close enough that we might hope to gather in-situ data. Mars is a great example, and for a mix of historical and practical reasons is still our main target for exobiological investigation. Evidence for past and present aqueous environments, as well as ongoing atmospheric chemical skulduggery - albeit contested and curiously correlated with some funky geophysical locations - reinforce Mars as a critical target for astrobiology. The extraordinary high-def mapping of the martian surface by recent missions is key to helping us home in on places that might offer the best clues, and provide targets for upcoming missions like Curiosity, nee Mars Science Laboratory.

A sub-question that Mars raises, with its likely diverse climate history, is whether we're looking for fossils or living breathing organisms. In that context the real question at hand is perhaps better stated as whether there has ever been life elsewhere in the solar system. This opens up a whole slew of intriguing environments (while not excluding the possibility of extant life in any of them). The list includes: comet nuclei, big asteroids or dwarf planets, and moons like Europa, Ganymede, Titan, and Enceladus.

Cometary nuclei are more diverse than expected, the recent Hartley 2 flyby providing spectacular evidence of this. We already knew these bodies to be rich in organic chemistry and while finding signs of life within them may be pushing it a bit, they actually hit all the buttons for 'follow the water' and 'follow the chemistry' type strategies. As we move further out in the solar system then the typical volatile content (water and hydrocarbons) of bodies tends to increase - a characteristic left from the proto-planetary disk temperature structure 4 billion years ago. The great moons of Jupiter and Saturn offer an extraordinary range of environments, from potentially vast subsurface oceans of liquid water (albeit likely full of noxious solubles) within Europa or Ganymede, to possible 'micro-environments' in Enceladus, to the novel and little probed cryo-chemistry of Titan's surface. A long way from the Sun these potential habitats can have a major energy input from phenomena like tidal heating, itself ultimately derived from nothing other than angular momentum.

Then there are nooks and crannies, some not so small, that may surprise us. The upper atmosphere of Venus can be positively temperate at the right altitude, and here on Earth it's increasingly apparent that microbial life among the clouds may be significant and substantial. Venus might not be a place to overlook after all.

The rub, and there is always one, is that getting to these places is tough. Then getting to the right spot, whether in a chasm on Mars or beneath ten kilometers of ice on Europa, is even tougher. Then there is the thorny issue of finding evidence of life if it's sparse. There are no magical CSI-like probes that conveniently produce the answers, and the problem of forward-contamination is ever-present. Nonetheless, question number 2 remains both very important and very much within our grasp, especially if we get past our space-exploration hangovers and recognize the critical role that our great wilderness in the sky can play in humanity's future.

The ten most important questions for astrobiology: Number 1

2010-11-24T09:22:00.001-05:00

As our year approaches its end, with the planet slithering ever closer to where it was 12 months ago and towards its closest approach to the Sun on January 3rd, it seems like a good time to indulge in a highly biased and incomplete bit of rumination. There have been many tremendous discoveries and advances this past year, each inching us closer to tackling some of the core questions about life in the universe. Nonetheless, lots of big and important questions remain. Some approach the philosophical, some are quite narrow, but they're all interesting, and many overlap greatly. I thought I'd do a series of posts on the ones I particularly like, you may agree or disagree with the choices - a reasonable and necessary part of the process - but hopefully they will stimulate.

Number 1 is relatively non-challenging: Are there other planets like the Earth?

We're much, much closer to answering this than we were a year ago. As surveys for exoplanets increase in sensitivity, sample size, and sampling time it appears that small rocky worlds are essentially ubiquitous. With statistical estimates that at least 1 in 4 Sun like stars harbor Earth-sized planets within their habitable zones (extrapolating from shorter period objects) there is good reason to believe that Earth mass planets in comparable orbits are definitely out there. Kepler will help nail the rates to the wall. It will remain a tough question to tackle at a deeper level though. 'Like the Earth' is a bit ambiguous. Mass and orbit are one small piece. How many of these worlds have comparable chemistry, geophysics and climate? A good bet is that there are numerous cousin planets, recognizable but nonetheless a bit alien. That may of course be just fine. We're awfully biased about ourselves, often without recognizing the fact. One of my favorite mantras is that the modern Earth is not typical of our homeworld throughout its history. The suitability of the Earth today for life is a poor template to use.

So, if we treat 'like' as a broad qualification then the answer is almost certainly yes, and in another 12 months we'll have even better evidence supporting this.

Our leaky galaxy

2010-11-19T09:15:00.000-05:00

A while back I wrote about the leakiness of our solar system - how significant amounts of material in the outer regions around the Sun could have really come from other stellar systems. Then I got on a bit of a jag about planets in other galaxies and the impending collision (well, one has to take a long-term view) with the Andromeda galaxy. Then lo and behold, like a fabulous new ice-cream flavor, someone turns up a new planet in the Milkyway that almost certainly came from another galaxy.

HIP 13044b is a giant world a little more massive than Jupiter orbiting a red giant star every 16 days, reported by Setiawan et al. in Science. Detected via the tell-tale velocity wobble of its host planet it might not garner much attention except for the fact that this system is part of the so-called Helmi star stream, some 2000 light years from us. The Helmi stream is the remnant of a dwarf galaxy that dove into the Milkyway sometime between 6 and 9 billion years ago, becoming shredded by galactic tidal forces.

Clearly the original host galaxy for this system was pretty puny, and one might argue that it was never truly distinct from the Milkyway - merely a part of its halo entourage, like some loose hair or slightly wayward limbs that would inevitably get subsumed. Nonetheless, it has some quite striking implications. The host star has an extremely low heavy element content - about a hundredth that of the Sun. This is the least element rich star yet found to host any type of planet. So the fact that it has at least one chunky world orbiting it is very interesting since lower heavy element abundance in stars correlates with a lower probability of giant planets. It's also an old and doddery star, well past its prime and has swollen in size as a result - possibly engulfing even shorter orbit planets, and possibly shedding some element-rich outer atmosphere.

Taken altogether it's another tick mark for planet formation being both a universal phenomenon and apparently a quite efficient one. As we learn more about the stellar populations in our galaxy we see just how messy they really are. Satellite galaxies can dive through the galactic plane, not just shedding their stars but also pulling out bona-fide Milkyway residents in their gravitational wakes. There may well be stars and planets that formed snug in our galactic disk that are now arcing over us, dragged tens of thousands of light years away from home. Remarkably, the opportunity to study planets with truly extragalactic origins may be all around.

The grains of Mars

2010-11-15T21:31:00.000-05:00

We know that something is afoot on Mars. Apart from the surprising and intriguing measurements of atmospheric methane plumes there is clearly something funky about the chemistry of the Martian atmosphere that cleanses it of this organic gas in very short order. Various ideas have been put forward, including the possibility that there is some important effect of the dust that is often lofted skywards in the great Martian duststorms.

Now a new work here on Earth suggests that both our planet and Mars may share something in dusty atmospheric chemistry. Shaheen et al. report in the Proceedings of the National Academy of Sciences that they have painstakingly identified a set of chemical reactions that occur between ozone, water, and carbon dioxide on an incredibly thin layer on the surface of atmospheric dust. The smoking gun is an enhancement of the oxygen 17 isotope in carbonate material on the dust grains compared to other natural carbonate sources. Isotopic fractionation points towards a different chemical history, involving the formation of hydrogen peroxide on the dust grain surfaces.

Remarkably, a similar isotopic excess was known to lurk in one of the famed Martian meteorites from the Allan Hills finds in Antarctica. While this doesn't necessarily prove a common chemical mechanism it is very suggestive that the type of grain surface chemistry that occurs in the Earth's atmosphere may happen on Mars too. Dust-grain surfaces offer a fast way for chemistry to occur by capturing reactants into a solid/liquid layer. As Shaheen et al. point out, the unusual carbonate deposits in this meteorite that have provoked so much intrigue as potential micro-fossils could possibly just be aerosol produced carbonates - we just didn't know about this mechanism for their formation.

Dust grain chemistry is not just confined to planetary atmospheres. Interstellar and interplanetary dust (admittedly far more microscopic than most terrestrial or Martian dust) plays a critical role in astrochemistry - accelerating reactions that might otherwise take an eternity, and engaging in all manner of cycles for molecular chemistry in the relatively warm and wet environment of protoplanetary disks. Given that it's perhaps not so surprising that it should play an important role in the soupy airways of rocky planets. Interesting questions remain - does this chemistry offer clues to Mars' ability to scrub itself clean of methane, and can it provide another window into paleo-climate both on Mars and the Earth?

From Andromeda with Love

2010-11-11T10:17:00.000-05:00

The galactic theme in the context of planets and life is an interesting one. Take our own particular circumstances. As unappealingly non-Copernican as it is there is no doubt that the Milkyway galaxy today is 'special'. This should not be confused with any notion that special galaxy=special humans, since it's really totally unclear that the astrophysical specialness of the galaxy has significant bearing on the likelihood of us sitting here picking our teeth. Nonetheless, the scientific method being what it is we need to pay attention to any and all observations with as little bias as possible - so asking the question of what a 'special' galaxy might mean for life is ok, just don't get too carried away.

First of all the Milkyway galaxy is big. As spiral galaxies go it's in the upper echelons of diameter and mass. In the relatively nearby universe it and our nearest big galaxy Andromeda are the sumo's in the room. This immediately makes it somewhat unusual, the great majority of galaxies in the observable universe are smaller. The relationship to Andromeda is also very particular. In effect the Milkyway and Andromeda are a binary pair, our mutual distortion of spacetime is resulting in us barreling together at about 80 miles a second. In about 3 billion years these two galaxies will begin a ponderous collision lasting for perhaps 100 million years or so. It will be a soft type of collision - individual stars are so tiny compared to the distances between them that they themselves are unlikely to collide, but the great masses of gas and dust in the two galaxies will smack together - triggering the formation of new stars and planetary systems.

Some dynamical models suggest that our solar system could be flung further away from the center of the merging galaxies, others indicate it could end up thrown towards the newly forming stellar core of a future Goliath galaxy. Does any of this matter for life? For us the answer may be moot. In about only 1 billion years the Sun will have grown luminous enough that the temperate climate we enjoy on the Earth may be long gone. In 3 billion years it may be luminous enough that Mars, if not utterly dried out and devoid of atmosphere by then, could sustain 'habitable' temperatures. Depending on where the vagaries of gravitational dynamics take the solar system as Andromeda comes lumbering through we might end up surrounded by the pop and crackle of supernova as the collision-induced formation of new massive stars gets underway. All in all it doesn't look too good. But for other places, solar systems that we see forming today, it could be a very different story.

Imagine a terrestrial world just beginning to form right now. By the time Andromeda is ploughing through our galaxy in 3-4 billion years, merging and settling, there could be a species sitting there writing a blog about what a 'special' time and place they exist in. They might marvel at how fortuitous it was for their sun to be thrown further out from the new galactic center - away from all those nasty supernova. They might stare in awe at the myriad new planetary systems being formed as the gas from these cousin galaxies collides, a golden era for new worlds. They would doubtless write papers on how galactic mergers might be signposts to highly habitable regions of the universe - unlike all those boring isolated galaxies out there. It sounds like they're getting carried away, but really they're not doing anything we don't already. Getting perspective is one of the most difficult things to do, especially from our tiny mote of existence.

The Galaxy is Not Enough

2010-11-05T12:38:00.000-04:00

Megalomania can sometimes be good. Well, with a lot of emphasis on the 'sometimes'. Here we are as a species, scooping up detections of planets around other stars at a ferocious rate. Our galaxy is however a big place, and we're going to be sifting through its two hundred billion or so stars for a very, very long time. Eventually though, presuming that we remain aware of the deeper meanings of the word civilization, we might find ourselves not content to be just parochially curious. We talk about life in the universe but the Milkyway is merely one of a couple hundred billion galaxies in the observable cosmos. Can we seek out planets in other galaxies?

Even now the answer is a surprising 'almost'. A paper by Ingrosso et al. about a year ago in the Monthly Notices of the Royal Astronomical Society, discusses in detail the possibility of seeing evidence for planetary mass objects in gravitational microlensing events occurring in the Andromeda galaxy (M31) - a mere 2.5 million light years away across our backyard. As with lensing detection of planets in our own galaxy the mechanism is that a foreground star with planets (in this case within Andromeda) acts as a gravitational lens on the light from a background star (also in Andromeda), dramatically magnifying it as the two systems drift into close alignment from our point-of-view. At the distance of Andromeda the best we can do is see 'pixel-lensing', which is a variation in the number of photons coming from individual pixels in our digital cameras, each of which contains the light from many indistinguishable stars - too close together on the sky to be separated. During magnification the tiny lensing effect of planets can also be greatly enhanced, adding characteristic spikes and wiggles to the way the light varies during the alignment.

By carefully modeling what one might expect to see, Ingrosso et al. demonstrate that there is a pretty good chance that in a few percent of Andromeda lensing events you should find the signs of Jupiter-scale planets if they are there. They even go so far as to offer support for the detection of a 6 Jupiter mass planet in a rather funky lensing event seen in Andromeda by An et al. with the gripping name of PA-99-N2. As with all micro-lensing studies there are huge challenges owing to the amount of data needed to spot events and to fully characterize them as they play out over days to weeks. The pay-off is an incredible statistical cleanliness and sensitivity.

Probing for planets in Andromeda may therefore be fruitful. What of further afield? There's the rub. The same technique might be applied to more distant galaxies, but it will require bigger telescopes, capable of fitting more pixels across the faces of these stellar swarms. It could be worth it though. Suppose we were able to evaluate the planetary populations (albeit statistically) in a representative chunk of intergalactic space. That would be an incredibly interesting set of numbers. It would tell us what galaxy types - elliptical to spiral - are most fertile grounds for other worlds, and take us closer to answering the question of the true capacity of this universe for life.

Necropanspermia

2010-11-02T11:17:00.000-04:00

It's a fitting title for a few days past Halloween. Sugar withdrawal and the odor of gently scorching pumpkin fresh in our minds. This term - necropanspermia - is a new one to me, and comes from a thorough and carefully put together paper that popped up this morning from Paul Wesson. Although it is ostensibly a review of the ideas of panspermia (and a nice balanced one at that), it also gets to some rather interesting points.

The transfer of planetary - potentially life-carrying - material between stellar systems seems only likely for tiny particles. Dust grains of a few to a few tens of microns across are the ultimate solar sailors. The radiation pressure of photons from one star accelerates these minute pieces across interstellar space. Routes that lead into new solar systems result in the gentle deceleration of the dust as it plunges into the photon cloud of a star, and the possibility exists of being swept into the atmosphere of any handy planets - just as happens here on Earth. The question is what, if any, viable organisms can hold up over millions of years of interstellar transport - subject to cosmic rays and high-energy photons - to re-plant on a new world?

Wesson's discussion takes a rather interesting direction. He argues both that viral material may be prevalent, and that this type of fragmented DNA/RNA may be quite sufficient to help 'seed' life in new environments. One motivation for suggesting this is the conclusion that most transferred organisms are dead-on-arrival, there just doesn't seem to be a way that intact DNA and/or cells are going to survive their trans-galactic journeys. Another motivation is simple physics - a microscopic dust grain might barely have space for one poorly protected bacterium, but it could easily harbor a hundred much smaller viruses.

The hypothesis that there was an ancient 'viral world' here on the Earth - complete with unique viral genes that help replication - has been around for a while. While even viruses might not arrive intact after interstellar transit, this might be far less of a problem, since they play fast and loose with their molecules in the first place. It's an interesting twist to the long and checkered history of the idea of panspermia - perhaps we should be rushing cosmic dust particles from high in our atmosphere off to the labs for some quick looks for viral DNA?

1 in 4

2010-10-28T15:36:00.000-04:00

Having sweated over my previous post I'm hesitant to knock it off the top slot, but this is too good to pass up. Appearing in the 29th October issue of Science is a wonderful paper by Howard et al. The bottom line is that they have made as careful a statistical study of Doppler detected planets as seems practical at this time. The results of monitoring 166 relatively nearby normal stars for five years indicates that an astonishing 1 in 4 such stars may harbor small Earth-mass planets on close orbits.

It's important to be very clear, the planets actually detected in this survey are rocky objects a few times the mass of the Earth, and they are emphatically not in the classically defined habitable zone of these stars - orbiting within 0.25 AU (a quarter of the distance of the Earth from the Sun). However, the extrapolation to true Earth-mass objects is pretty likely to be robust. It'd better be, this claim is effectively saying that there are tens of billions of such planets in our galaxy and that they outnumber giant worlds in close orbits.

This presents a big challenge to certain aspects of how we think planets form. Most current models suggest that there should in fact be a deficit in small rocky worlds in these close-in orbits, since the processes of orbital migration or runaway planet growth tend to thin this population drastically. Something is afoot - and it may also indicate that those holy grail worlds - the Earths in the habitable zone - are far more numerous than we had dared to hope. Indeed, the authors of this paper suggest that Earth-mass planets orbiting at 1AU could be more numerous than their close-orbit cousins.

Howard et al. end their paper by evaluating the implications for Kepler results. It's awfully promising - as many as 260 Earth sized (1 to 2 times Earth radius) planets with 50 day or less orbital periods should be coming our way, and who knows what in the habitable zone of Kepler stars...

Megavirus vs Bicosoecid

2010-10-28T10:59:00.000-04:00

It would make an excellent B-movie (if that's not an oxymoron). Giant virus attacks! Come see the battle for supremacy in technicolor 3D-surround sound!

Viruses lurk at the hairy edge of what we generally consider to be 'living' things. Small infectious structures, they replicate only by digging into a host's intracellular environment and hitching a ride. The smallest are barely 10 nanometers across - the size of a wavelength of ultraviolet light - and coded for by only a few thousand nucleotide base-pairs (compared to the more than 3 billion that code a human). Vast numbers of viruses exist in nature. In marine environments there can be 250 million individuals per milliliter of water, most of them so-called bacteriophages - targeting microbial hosts. Viruses play fast and loose with genetic material, clipping, swapping, dropping and incorporating at a mind-boggling rate. There is no doubt that they have played a critical role in the molecular evolution of life on Earth over the past 4 billion or so years. Our human genome contains millions of fragments of viral DNA - accumulated by our distant, distant ancestors and ourselves. Viruses bring into question even the very notion of 'species', we are all molecular ragdolls, a button sewn on here, a piece of thread incorporated there.

A newly discovered virus steps up the ante (and adds to the zoo of such things). Infecting a single-celled marine organism known as Cafeteria roenbergensis (good name, apparently it's a voracious eater) this virus is a monster. Its genetic makeup is 730,000 base-pairs, with 500 regions that look like bona-fide genes, many likely involved in making protein structures - something most viruses don't bother with. Like all viruses it's not a cellular organism, but here it is doing much of what regular living things do. It not only seems to have genes that could help make cell membranes, it even seems to have stolen genes directly from bacteria.

The line between this remarkable structure and 'life' is thin indeed and I think makes it very clear that there is more of a continuum than any dramatic 'jump' between complex molecules and what we clearly recognize as organisms. We have barely scratched the surface in our understanding of the relentless activity of the microscopic world. Intriguingly this Megavirus infects one of the major predators of the marine microbial environment. In that context it's hitching a ride with an organism that hoovers up both bacteria and viruses as food. What an excellent smorgasbord of genetic material it must see, one cannot help but wonder if that opportunity has helped it build its own massive library.

In the quest for life in the cosmos, and particularly as we continue to poke around in our own solar system, we need to think carefully about what we are looking for. Fragments of DNA from a formerly aqueous environment on Mars - should they ever be found - would likely offer a window into this borderworld between molecules and self-contained organisms. It's going to be messy.

Scale-free mobility

2010-10-25T12:40:00.000-04:00

Some ideas feel like an itch that you can't quite scratch because it's hard to pin down, but you really want to. A recent paper by Young et al. in Nature Physics (and available here), along with a commentary by Brockmann feels a little bit like that - at least when seen through the soupy goggles of astrobiology. On the surface this paper appears pretty innocent. It studies the nature of human mobility. With the rise of data-logging; from global positioning, cell-phone records, and the colossal airline databases of 3 billion annual travelers there is unprecedented material for studying exactly how we move around the planet. Not surprisingly we exhibit particular statistical patterns. For both distances traveled and 'rest times' (how long we linger over a cup of coffee or beneath the umbrella of some tropical beach) the distributions are inverse power laws. In plain language this means that we are more likely to travel short distances and to spend little time in any one place during travel.

There's nothing that sounds particularly revolutionary about that. But it gets more interesting. As Young et al. discuss, the particular shape of the statistics appears to be modeled rather well with a form of 'preferential return'. In other words although we may explore - making new journeys - we are more likely to travel to locations that we already know. Not only that but the more locations that we already know the less likely we are to explore new ones at all. This naturally produces a so-called scale-free distribution - if you go out to any particular pub then there will be another pub that you are twice as likely to visit, and another one that is half as likely to have your patronage. The Young et al. model boils down to assigning a probability for any given 'step' in an individual's travels that results in exploration (a new location). They give this as the number of prior travel steps to the inverse-power of....0.2.

Why 0.2 ? Nobody knows, but this describes the way human populations explore the world - individuals will deviate from this pattern, but en-masse this is what we do. It's a deceptively simple, but extremely provocative result. Does this same kind of probabilistic scaling apply to other species? Do other species have a different power law value than 0.2? What are the implications for the occupation of niches by organisms on a planetary scale and throughout the history of life? This implicit tendency for exploration to slow down in favor of re-visiting known locations makes me wonder about a recent discovery on the global East-West divide in marine microbial genetics. While environmental differences are the most obvious root bio-physical cause (particularly the differing soluble phosphorus amounts in the Atlantic vs. Pacific oceans), does this give rise to a similar 'preferential return' scaling?

Then, to return to a popular theme - is there something to be learnt from this model for the questions of life exploring the cosmos beyond a planetary homeworld? On the face of it the more you explore the more likely you are to slow down and just keep circulating amongst the places you know. Clearly the present data represents humans who are not, we presume, exploring for survival or vital resources - lattes don't count. Nonetheless it is reasonable to hypothesize that humans behave this way because there is some circumstantial evolutionary advantage to it. The hard-to-pin-down-itch is that one suspects there is something deeper - perhaps universal - in all this, waiting to be ferreted out.

Blowing hot and cold

2010-10-22T09:37:00.000-04:00

Understanding the structure, dynamics, and chemistry of planetary atmospheres is key to exoplanetary science. It's sobering then that as of now it is still an enormous challenge to even model the atmospheres of planets in our own solar system. Despite great advances a variety of trickery has to be employed to simulate something like the Jovian atmosphere, such as pretending that it has a very different soupiness and energy transport in order to overcome computational demands. Modeling the atmospheres of gas giant exoplanets is even more in its infancy. An intriguing result in the past week has come from Crossfield et al. and their study of how we see the infrared light varying in the planetary system of Upsilon Andromedae. Their Spitzer space telescope phase photometry on Ups And reveals the glow emitted by the innermost, roughly Jupiter sized, planet around this F dwarf star (about 1.3 times the mass of the Sun).

The planet orbits very tightly, every 4.6 days, and is expected to have been evolved by tidal interaction with the star to a state of spin-orbit-synchronicity - in other words, in the simplest case, its day will equal its year and there will be permanent day/night sides. This sets the planet up for an extreme case of thermal disparity. We'd expect hot atmosphere from the dayside to flow to the cold night half of the planet - in doing so there might be great jet-stream like structures, and the hottest point of the planet might get shifted along in the direction of these winds. Something like this seems to be happening on Ups And b, but to an extent that is truly puzzling. As it zips around in its orbit the glow of its hot atmosphere betrays the temperature distribution and is seen in the varying number of infrared photons collected by Spitzer. It's not in synch with the planet orbit - or more specifically it is systematically offset or phase shifted by almost 90 degrees. In other words the hottest side of the planet is almost at right angles to the direction of the star. On the Earth this would be a bit like saying the hottest time of day is at sunset instead of noon.

It's a puzzle. Some amount of offset might be expected, driven by the strong hot to cold winds, but this is extreme. There are various possibilities - maybe the stellar heating is reaching to greater depths in the planetary atmosphere than expected and altering the fundamental dynamics. Perhaps the winds are so strong that they are going supersonic, forming great shock waves that pile energy up on this side of the planet. It's a tough call - even models of these hot Jupiter-like planets disagree on such things, and none of them predict exactly what we see on Ups And b. The good thing about this result is that it challenges the modelers to really sort out what works and what doesn't - advances will be made.

Crossfield et al. also end their paper with an interesting fact. This system of Ups And is actually too bright for the upcoming James Webb Space Telescope (JWST) to observe at shorter wavelengths - its sensitive instruments would simply be saturated with photons. They further point out that a small space telescope dedicated to studying the phase curves of nearby hot-Jupiter systems might just provide the data needed to crack the problems of these extraordinary regimes of planetary atmospherics. This is a sentiment that could also apply to the hunt for terrestrial-type exoplanets - especially those that transit stars much closer than the distant Kepler objects - we need a dedicated all-sky survey to find the targets for powerhouse instruments like JWST.

Cosmic minds

2010-10-19T09:56:00.000-04:00

In recent months there has been more discussion around about the idea that 'advanced' life in the universe might employ artificial intelligences to do the dirty work of exploring. Indeed, AI might just push off on its own accord and could, it is presumed, be made effectively immortal. These patient devices could then withstand the enormous times and extreme environments involved in crossing interstellar, or even intergalactic, space. In part these ideas are an attempt to narrow the search parameters for our efforts to spot signs of intelligent life in the universe - as nicely discussed by Paul Gilster. There is something intriguing about these notions, but they also raise some other questions. I thought I'd try to stir things up a bit.

Without getting too deeply into the meaning of AI (Turing tests and the like), it's interesting to consider the history of attempts to build such things here on Earth. In the late 1950's there was a huge push to construct an AI, everyone felt it was only a decade or so away, governments dumped money into research - to little obvious avail except for general advances in computing. In the early 1980's there was another bout of enthusiasm and billions of dollars got funneled into AI work. Most of the focus was on the idea of expert systems - the idea that much of intelligence stems from the ability to assimilate and utilize large amounts of data; knowledge was the fuel for AI. Another bubble has happened more recently with the notion that true AI cannot be some disembodied piece of code, it needs a robotic avatar in the real world. Incredible things have been done in the name of AI. The fundamentals of modern programming have to some extent been reshaped by this thinking, and we indeed now experience the rudiments of AI every time Amazon or Netflix gleefully present us with 'suggestions' for what we might want to spend our money on. However, the true AI that we all imagine is nowhere yet to be seen.

Indeed, there are even deeper questions and doubts about the whole idea of a physical machine that can mimic a living mind. Roger Penrose in his dense but fascinating 480 page argument - The Emperor's New Mind - made a pretty sobering case that quantum mechanical processes are central to our type of intelligence. Of course this was written prior to gains in quantum computation, but its thesis has not yet been disproved.

So where am I going with all this? I think there are two important, although speculative, points that come from our baby-steps in AI. The first is the idea that if we keep working at it then one day an AI will 'appear', like a new iPad or model of car. What if that conceit is just wrong? It certainly appears incorrect so far. It may be that AI is something that emerges very, very gradually. So gradually in fact that we barely realize it's happening, just like the processes of natural selection and evolution. As we upload more and more of ourselves - our knowledge - onto Google or Facebook we may actually be pushing the process along, but we could have a thousand years or more to go. The second point is related. What if Penrose is correct, and only quantum processes have access to the right amount and type of computing power for intelligence? True AI may well be indistinguishable from biology.

What this could mean is that there will never be AIs in the universe that can be distinguished from their biological parents (I know, shades of Battlestar Galactica, but fiction is often prescient). There could be 'dumb' AI explorers - complex machines of limited flexibility - but there will never be true AI exploration, because those AI would likely be subject to the same problems that we have with interstellar distances and timescales. Thus, if we want to try to define a more targeted set of search criteria for 'intelligent' life in the universe (which is certainly not a bad idea), there are probably two options. One set of life is going to be just like us - whether it's AI or not. The other is going to be extremely machine-like, not super-Turing-test-smart, but well programmed and equipped with a big knowledge base. So the question is, what would exo-Google be doing out there in the universe?....

Through the halo

2010-10-18T09:36:00.000-04:00

The technology for detecting planets continues to advance. A new paper by Sascha and colleagues contains the first direct imaging data of a planet obtained with an emerging optical technology called an apodized phase plate. Interestingly this builds on a set of ideas that have also been kicking around for a while in microscopy - the techniques for peering into the infinitesimal can also help us peer into the infinite.

The classic difficulty in looking directly at exoplanets is that they always appear extraordinarily close to their parent stars on the sky. Stars are millions to billions of times brighter than planets (depending on the precise wavelength of observation), and the wave nature of light always results in the blurring, or diffraction, of light as it passes through any kind of optical instrument. So the starlight ends up swamping out the feeble emission or reflection from any orbiting planets. Getting rid of the starlight is an entire field of research unto itself. Methods such as coronagraphy, and the rather ominous sounding nulling interferometry all seek to remove or suppress the light of the star, leaving behind the glimmer of any planets. It's been likened to trying to spot fireflies in the glare of a searchlight when you're thousands of miles away - that's not a bad analogy.

The apodized phase plate is a nifty bit of trickery. An earlier paper in 2007 by Kenworthy et al. describes the application to astronomy. In a nutshell the plate (combined with a coronagraph - a disk directly blocking some of the light of the star in the image) is a transparent chunk of material - in this case zinc selenide which is highly transparent in infrared wavelengths - that is modified to introduce a complex spatial pattern of phase changes to incoming light. Ok, so that's not so much a nutshell as a mouthful of walnut. Imagine a watery surface with light reflecting from it. You've undoubtedly seen this happen - wonderfully varied patterns of light and dark occur in the reflected light, the stuff of a summers day, shimmering on the hulls of boats, or off the dirty dishes in a kitchen sink. What you're observing are the phase shifts in the light - tiny time delays because a watery surface is not perfectly flat. If the phase shifts are out of sync light cancels out, if they're in sync it adds together - and we see dark and light regions in the reflected images.

The phase plate exploits the same physics, but in a careful, mathematically controlled way. Its 'rippled' surface adds time delays in pre-determined locations across the beam of light from a distant stellar system. These are cleverly designed so that on one side of the final image the light of the star gets blanked out (nulled), but the light of any planets remains. It's a tough challenge to get this to work. You also have to remove as much of the other sources of blurriness - like that induced by Earth's own atmosphere - as possible. Sascha et al. seem to have accomplished this, re-detecting a giant planet around the young star beta Pictoris, on an orbit of a mere 7 astronomical units (that's between Jupiter and Saturn if it were our solar system).

It's a terrific step. There are certainly many side effects of such trickery - a narrow waveband, imagery of only one side of the star, calibration challenges - but together with many other emerging techniques it does seem that we're well on our way to doing even richer exoplanetary science.

We salute you...

2010-10-13T13:14:00.000-04:00

Bacteria. Asexual lords and masters of the planet. An extraordinary and fascinating piece of work has popped up that indicates a form of bacterial networking that inches this sentiment even further along from the figurative to the literal. In a study of the bacterium S. oneidensis (an organism capable of actually reducing, or 'breathing', heavy metals) El-Naggar and colleagues find that when stressed these microscopic lifeforms can grow so-called bacterial nano-wires. These incredibly thin protrusions - really stalks of protein - exhibit electrical conductivity. This work builds on earlier studies that also hinted as this property.
They're no piece of copper when it comes to transporting electrons, but they seem to be on a par with semi-conductor materials.

The experimental work is quite wonderful, and shows that colonies of S. oneidensis may actually link themselves together in a remarkable type of electrical grid. Why do they do this, are they just engaged in some form of microbial Facebook? The answer may be one of survival. The respiration of S. oneidensis is acutely dependent on the ability to off-load unwanted electrons - performing chemical reduction on anything able to accept the electrons. If a single individual can't dump its electrons it dies. So, if you can send out nano-wires and make an electrical connection with someone else you can pass off your particles - and if that individual can't accept them it can simply re-route the current further along the network, until eventually it gets slurped up. It's an incredible ability, the colony comes to the rescue of the few, and nobody has to get out of their armchair.

As the researchers point out, beyond enabling the group to respire and survive, bacterial nano-wires open up a whole new avenue of fast communication - much speedier than chemical signaling. One cannot help but see a parallel between this situation and the web of neurons and electrical synapses lurking between our own ears. The more we learn about life on Earth, the more blurred the line becomes between 'simple' and 'complex' life, and the more archaic that classification appears.

Shaken, stirred, shaken, stirred

2010-10-11T16:21:00.000-04:00

Water never ceases to amaze. Two protons, one oxygen nucleus, 10 electrons. A simple molecular configuration, two sides of a triangle, an obtuse angle of 104.45 degrees and a tendency for the electrons to huddle close to the oxygen by about a factor of ten - leading to an electrical dipole. As basic as this structure is it lends itself to an incredible array of situations. From a physical building block of planets to an extraordinarily solvent to an integral piece of terrestrial biochemistry, there is excellent reason why many astrobiologists extoll the virtues of 'following the water' in the search for life in the universe. Despite this, our deeper understanding of exactly what it is that makes water so very, very special is still surprisingly limited.

An intriguing new study by Rao, Garrett-Roe & Hamm in the Journal of Physical Chemistry (B), appears to offer a clue or two. By applying modeling techniques usually reserved for the study of complex and dynamic systems, they investigated what might be going on in liquid water on a moment-by-moment basis. The dipolar nature of water molecules means that weak electrostatic bonds (hydrogen bonds) are readily made and broken between structures. So in a crowd of water molecules all manner of temporary arrangements can be made as they jostle around. This new study indicates that there may be two main flavors of these arrangements - one a rather 'blobby' mass of several molecules, and the other a more regular, crystalline arrangement. These structures are exceedingly fleeting - breaking up and re-assembling many times a second at room temperature. The result is, and this is a very crude phrasing, a bit like a 3D piece of velcro that is constantly morphing into different shapes.

So, all jolly nice, but why do we care? Imagine throwing some other molecules into this sticky nest. Perhaps some carbonates, some amino acids, even some proteins. The shape-shifting water structures can provide the perfect chemical incubator - from catalyzing reactions to determining structural forms of complex organic molecules. This cuts to the heart of one of the long running discussions that crops up in the search for life. Why couldn't you have biology that uses something other than water? Why not methane, or hydrogen peroxide, or ammonia?

The answer may now be clearer - as far as anyone knows these other solvent-like molecules do not exhibit this same type of behavior - so it may be that they offer far fewer, if any, pathways for complex chemistry. Water may be far more intertwined in the processes of life than we had suspected.

Voyagers

2010-10-06T08:56:00.000-04:00

Overcoming preconceptions and received wisdom is central to making progress in science, and to be quite honest in pretty much anything else as well. I was reminded of this after a somewhat doleful conversation following last week's burst of exoplanetary adrenaline. It went along the lines of 'even if GL 581g was full of intelligent and occasionally amusing aliens, we're just not a spacefaring race and we'll never get to meet them'. Most of this statement is certainly true, but I was struck by the glum expression of certitude about our Earth-bound nature.

A while ago National Geographic published one of their terrific graphic illustrations that summarized 50 years of human space exploration. I can't do it justice here, so go take a look. The incredible thing is just how much space exploration we've actually tried (and often succeeded at). One target is particularly evocative, and that is Mars. I think it's fair to say that going to Mars has always been far less about politics than some other destinations. Mars looms big in our imaginations, the red planet, awfully familiar, yet awfully different. There is incredible poignancy in the list of missions to Mars. A majority have been failures, years of effort and extraordinary technological know-how thrown to the sacrificial plinth of the void. Yet those that succeeded have genuinely transformed both our understanding of this other world, and transformed our relationship to space exploration.

Here's the list, starting in 1960: Marsnik 1 (failed), Marsnik 2 (failed), Sputnik 22 (failed), Mars 1 (failed), Sputnik 24 (failed), Mariner 3 (failed), Mariner 4 (flyby), Zond 2 (failed), Mariner 6 (flyby), Mariner 7 (flyby), Mars 1969A (failed), Mars 1969B (failed), Mariner 8 (failed), Cosmos 419 (failed), Mariner 9 (orbit), Mars 2 (orbit), Mars 3 (lander), Mars 4 (failed), Mars 5 (orbit), Mars 6 (failed), Mars 7 (failed), Viking 1 (orbit/lander), Viking 2 (orbit/lander), Phobos 1 (failed), Phobos 2 (failed), Mars Observer (failed), Mars Global Surveyor (orbit), Mars 96 (failed), Mars Pathfinder (rover), Nozomi (failed), Mars Climate Orbiter (failed), Mars Polar Lander (failed), 2001 Mars Odyssey (orbit), Mars Express (orbit), Beagle 2 (failed), Spirit (rover), Opportunity (rover), Mars Reconnaissance Orbiter (orbit), Phoenix (lander).

Each of these launches, each chunk of alloy and package of electronics, was made to reach across interplanetary space. There was nothing glum about this. Bottles cast into the currents full of tentative human optimism and love and care. All the hallmarks of a space faring species negotiating its first steps. All for a minuscule fraction of resources across the years compared to wars, financial crises, pharmaceuticals, and political shenanigans. To my mind we are already a space faring species, we just haven't quite realized it yet.

This has a direct bearing on our search for life in the universe . Even as the next generations of giant telescopes and advanced optics are being built on terra firma, the ultimate goal has to be placing instruments in space - away from atmosphere, unstable environments, and with room to stretch out. Whether it's an occulting optic on a 200,000 mile virtual optical bench, or an array of interferometric mirrors, the high mountaintop of space remains where we need to go if we ever want to truly study, even map, another Earth-type planet or a related species.

A distant cousin

2010-10-01T09:24:00.000-04:00

It seems worth following up on the previous post with a little more on the planet GL 581g. With media attention temporarily swirling around this announcement and numerous opinions being offered it can be a little difficult to locate the core truths. Now that Vogt et al.'s actual scientific report is live we can see the basis for their public commentary. It's a very nice piece of work. It's also a remarkably, and refreshingly, chatty report - not something that scientific papers are particularly known for. The incredibly tricky and slippery nature of extracting planet detections from radial velocity (or Doppler 'wobble') data on the star is nicely laid out. Boy did they have to work hard on this. 11 years of data, including some from a 2nd instrument. Although there were over 200 discrete measurements of the star's motion these were, of course, spaced across a decade in time. This kind of sparse sampling - a necessary evil given the nature of telescope time allocation, weather, and competition - presents many challenges when you're looking for numerous overlaid time-varying signals.

Nonetheless, they pulled out the best solutions they could, checking against gravitational simulations of the system to make sure these answers resulted in a real, stable, system of planets and observing the star for signs of luminosity variation - sunspots and the like - that could dupe us into seeing things. All seems good. It is interesting too that they present an age for the system of 4.3 billion years, based on spectral analysis of the star - rather younger than the 7-12 billion years previous measurements had given, and the basis of some of my previous comments. Dating stars is tricky, so I'll pause on any extrapolations from that.

All of which brings us back to asking whether this really is Earth 2.0 as so many headlines have been suggesting. It's not, is the simple answer. Two big tick boxes get filled in - close to Earth mass and in the 'habitable' zone of a normal star. This alone does not make for tropical islands, lush forests, or highway systems. The real reason for being excited about this planet is that despite being extraordinarily alien, it nonetheless exhibits characteristics that place it firmly as a distant cousin. Imagine you were a hugely pampered but not overly prejudiced western explorer in the 1500's, and that no one from your neck of the woods had ever set foot beyond Lisbon. Setting off across the oceans you arrive at Papua New Guinea. You would immediately recognize other humans, however they would be engaged in complex and utterly alien work and social customs, like nothing you'd seen or experienced before. Their completely different lifestyle would confound you - but it would be obvious that you shared essential biology and characteristics. I think that's a fair analogy here. GL 581g and Earth are distant relatives, products of a universal set of mechanisms that build planets. The key is that GL 581g is the least distant relative we've come across so far.

A habitable planet

2010-09-29T17:05:00.000-04:00

So much for predictions. Now the press embargo is lifted we can take a look at what's going on around a small, nondescript, red dwarf star 20 light years away in the constellation of Libra. This star, Gliese 581 had been known to harbor 4 planets, including a couple of super-earths (less than 10 Earth masses) lurking at the very edges of the so-called orbital 'habitable zone'. Now the Lick-Carnegie Exoplanet Survey has crunched through 11 years of radial velocity spectroscopy or 'wobble' data on this star, and are announcing 2 additional planets, including a 3-4 Earth mass world smack bang in the middle of the habitable zone - GL 581 g.

This is the first world that clearly ticks off two key boxes in the laundry list for habitable planets - it's very close in mass to the Earth, and sits at a perfect distance from its parent star to stand a chance of having a temperate surface. It's a wonderful and thrilling discovery, and I'll confess to going out and staring in the direction of Libra last night - although sadly Gliese 581 is too faint to see directly with our beady human eyes. The relative ease (relative being the operative word, it took the might of the Keck observatory to pin this down) of finding this world and its sisters is a potent indicator that planets like this are quite common.

With a 37 day orbit (putting it about 0.15 AU from the 1/3rd solar mass star) there's a good chance that GL 581 g is tidally locked - with a permanent day and night side, although it's by no means clear that tidal locking is inevitable. This poses significant questions about any climate on the planetary surface - something astronomers and planetary scientists have been worrying about for a while for this kind of scenario. A thick enough atmosphere and thermal transport could help even out the drastic day/night temperature difference and keep things stable.

It's a long way from being Earth-2.0 though. The star is small, an M-dwarf, about 100 times less luminous than our Sun and strongly skewed to emitting photons in the infrared. This is also an ancient system, somewhere between 7 and 12 billion years old. GL 581 g is an old, old world bathed in red light. Although larger rocky planets than the Earth should have a more vigorous geophysical history - and the attendant chemical cycling that seems so critical for life - they too cool off with age and eventually suffer from stagnation. Whether GL 581 g has a significant water component or not is also something that we'll have to wait patiently to find out - one, or two generations of astronomical instrumentation in the future.

It's an alien place for sure. But to even be able to discuss these issues in the context of an actual, real, planet only 20 light years away is only a hairs breadth away from revolutionary - welcome to the coming age of exoplanetary science!

Jovian attraction

2010-09-27T09:50:00.000-04:00

A beautifully bright object has been hanging in the sky the past few nights. Big enough to distinguish itself from the stars by not twinkling, the planet Jupiter is closer to the Earth at the moment than it has been since the 1960's. This past Friday I found myself peering up through the canyons of Manhattan, and there it was, brilliant enough to outshine the up-lit urban canopy, muscling aside the landing lights of jetliners and helicopters, second only to the post-harvest Moon. There was something enthralling about it. This tiny disk was so distinctly alien. In my mind's eye I superimposed the great gas giant, king of worlds, two and half times as massive as all the other planets in our system put together. Its vast jet streams and storms, its incredible family of 63 moons, the potent magnetic field, the great plasma torus as Io burrows through a tight orbit. There it was, a place in the sky, a tiny condensed blob of mass. It struck me just how vivid this perspective was on the yawning gulf of interplanetary terrain. Gravity does an incredible job at packing matter down to a small volume, our solar system is indeed mostly empty void, but for these extraordinary cusps of curved space.

The scientist's curse is that any semblance of poetry often gives way to curiosity about numbers. As far and as small as Jupiter appeared in the sky it was surely doing more than just bouncing photons into my eyes. All that mass, now a mere 592 million kilometers away, shouldn't I be able to feel the Jovian lure?

It turns out not by much. If Jupiter was at the zenith then it pulls at us with a gravitational acceleration a few hundred millionths that of the Earth. At first I was disappointed, it had felt so much more out there on the street. It's all a matter of relating though. The Empire State Building is a 365 thousand ton chunk of rock and steel, that's about 730 million pounds. When Jupiter sweeps close, and rises into the night, it reaches out and makes this iconic tower weigh about 26 pounds less than normal. That's not much, but 26 pounds is something I can envisage - it's a chunk of cornerstone, maybe a small floodlight. Not so much that anyone would notice, but there nonetheless.

Of course, the lunar and solar tides sweep across us all the time and do far more - but they lack the charisma of King Jove, our planetary gravity lord. If some distant race were to be monitoring our Sun, seeking the tell-tale dips and wobbles as it is pulled at by the planets, then they too would most likely first see the presence of this gas giant. In their catalog of exoplanets Sol b would be the entry. If anyone bothered with this nondescript system it might eventually gain a Sol c, another gas giant orbiting a little further out. Would anything compel them to keep looking, to seek those small inner worlds that might, or might not, be there? Looking in from the outside is very different from looking out from the inside at a bright object in the September sky.

The Martian Methane Chronicles

2010-09-22T11:11:00.000-04:00

Since the firm detection of methane in Mars' atmosphere announced in 2009 (confirming earlier, more ambiguous results) it's been tough waiting for the next steps, impatience abounds. Methane is one of those compounds that has a predominantly biological origin on the Earth. Both here and on Mars, simple models of atmospheric chemistry suggest that methane molecules shouldn't last for very long - so if you see them then something is actively putting them into the air. Originally the models indicated that on Mars methane might last for as much as a couple hundred Earth years. The big shocker was that it was getting wiped out in less than one Earth year - showing large seasonal dependency.

We still don't understand either this result, or what's producing the methane (future measurements of isotopic compositions - heavy vs. light carbon and hydrogen - may indicate if biology is involved, since life usually prefers light nuclei). Some beautiful new results by Fonti & Marzo, using Mars Global Surveyor data, add much needed detail, but also add further layers to the mystery.

Their extraordinary map (click on the image) shows the atmospheric distribution of methane during the Martian fall, three Martian (6 Earth) years ago. Where does the methane hover over? It lurks around both regions sculpted by past volcanism (Tharsis and Elysium) and - rather provocatively - around a region where we know there are large amounts of subsurface water ice (Arabia). This is wonderful news - there's definitely a connection with the most recently active geophysical sites, the same sort of places that could produce the kind of warm, chemically rich, subsurface environments loved by the types of microbial life we are familiar with. Whether the methane itself is geophysically produced, or comes from extinct or extant life, we now know that there are places on Mars where conditions have been pretty juicy.

The hitch is that by the end of Martian winter most of this methane vanishes. Something is very efficiently scrubbing the atmosphere on Mars. Prime candidates are wind driven particulates and tough oxidizers like perchlorates (which are good for making fireworks, no, honestly) that may sweep through the skies. But come the relative warmth of spring and summer, the methane reappears, either released from frozen deposits or, just possibly, maybe, tantalizingly, the result of ongoing biological activity - the blooms of Mars. Are we witnessing the first signs of a very alien ecological system? If biology is responsible then what does this cleaning system imply for the lifestyle of organisms? In effect the methane excreta is tidied up before it can pollute, like some efficient recycling program. Is there anything equivalent here on Earth?

Pushing up daisies

2010-09-21T10:51:00.000-04:00

Life is opportunistic. About a 150 million years ago flowering plants did not exist on the Earth, today we are positively tripping over the things, so what happened? While there are many factors involved, a particularly interesting one has come up for recent discussion - and relates to a previous post on these pages.

A couple of weeks ago Bond & Scott published a paper in the rather wonderfully named scientific journal 'New Phytologist' that discusses how flowering planets, or angiosperms, spread during the Cretaceous some 65 to 145 million years ago. The novel aspect to this work is the suggestion that critically during this period, because of an elevated atmospheric oxygen level compared to today - perhaps to 25% rather than our paltry 21% by volume, surface fires were much more pervasive. I talked about this general phenomenon a while back.

So, the picture goes like this. Wildfires (well, I guess every fire was 'wild' during the Cretaceous) would have been significantly more frequent with higher atmospheric oxygen. This would have posed a significant challenge to surface plant life. Long-lived and slow growing species, like larger conifer trees - which have an ancient lineage - would have a hard time regenerating their populations fast enough. Imagine a cosy little spot, a fire rips through, everything burnt to a crisp. Seeds arrive, new plants grow, but sure enough another fire comes tearing across the land. Only those plants that had grown fast enough to mature and dump out the next round of seeds (carried off by wind and newly minted mammals and birds) would stand a chance at producing another generation. It's a vicious cycle, the fast growing angiosperms (one presumes helped along by insect and animal pollination) not only outrun the fire cycle, but they quickly produce the next round of fuel.

The upshot is that flowering plants don't get as much competition for resources from the previously dominant types of vegetation - in essence the weedy daisies win the day. The evidence for all this combustible carnage lurks in the remarkable charcoal deposits, and charcoal fossils from this geological period.

It's another example of the incredibly intertwined nature of life on a planet, and another great example of the constant 'what ifs' of evolution. Would an Earth that had always kept a low oxygen level have ended up with flowering plants - and the particular effect this implies on continental albedo and biosignatures?

Impending discovery

2010-09-16T13:51:00.000-04:00

You really have to hand it to some people. As if it weren't bold enough to go around looking for planets around other stars, and trying to figure out things like the putative climates on as-of-yet-undiscovered terrestrial type worlds (guilty), someone has to go and tell us when the first genuinely Earth-like planet will be discovered. Doing anything in early May 2011? Block out your diary.

Arbesman and Laughlin posted a few days back a rather provocative paper that includes the word 'Scientometric' in the title. [I had to look this up, since I first assumed this was lifted from an episode of the Simpsons. Apparently it's the science of measuring and analyzing science, well, there you are. Much is to do with the impact particular scientific works have on a field]. The paper, to be fair, employs some reasonable statistical methods to try to see what the trend line is for the rate of exoplanet discovery versus planetary properties. In this case the authors devise a metric for planetary habitability based - albeit in a fairly sophisticated way - on planet mass and distance from parent star, and then employ a favorite statistical trick called 'boot-strapping'. Bootstrapping is one of those methods that you resort to when you really have a limited idea of what it is that your sample is actually sampling - and when there is noisy data.

The results of their analysis suggest that early next year (the month of May) we might expect Doppler searches for exoplanets to turn up something in the Earth-mass range and in the liquid-water orbital zone around a star. Transit searches don't fare so well, with no convergence on a date, tough luck Kepler - although the reason is that there just aren't enough transiting planets yet discovered in the right orbital range to allow for a meaningful prediction. It's an admirably immodest prediction. They even refer to the so-called Hawthorne Effect - whereby their publicizing this result may in fact influence the race to achieve what they predict - although they suggest that astronomers will be too busy struggling with the challenges of the task to care much.

What I think is most interesting is not this result itself (if they're right they get to brag, if they're wrong no-one will really care), but that these - genuinely smart - guys went to the effort. That alone I think suggests that the scientific community is sensing convergence on the quarry. The caveat to all this is, at the risk of sounding pessimistic, that planets meeting this particular habitability metric could still be dry, barren, atmosphere-less rocks. Finding such worlds would indeed suggest that several boxes can be checked in the search for Earth-2, but it's not the end of the story.

The smell of disaster

2010-09-09T10:08:00.000-04:00

Here's the beginning of a radical thought. We expend a lot of scientific muscle on figuring out what can make conditions on small rocky planets as 'Earth-like' as possible: the long running theme of 'habitability' as defined by liquid surface water, temperate climate, organic chemistry, stability. It's fair enough, we have to start somewhere in our search for other worlds with life, we need to stack the deck towards something that we stand a chance of recognizing. However, this is only part of the template, the bit that is convenient and tidy.

If there is one inevitable thing about life it is that particular variants, species, modes of existence, are all prone to extinction. A new work by Drake & Griffen in Nature this week makes this point rather succinctly. They show, by subjecting water flea populations to a series of unfortunate events, how the population dynamics of a species can fundamentally shift due to environmental changes. Fluctuations in population numbers occur even in stable environments, but the character and size of these fluctuations changes in degrading environments, and beyond a certain point there is no recovery. Long before it all goes down the tube there are clear statistical indicators that things are not well - population sizes drop as the tree begins to fall.

Extinction is a fact of life. When we consider what the signatures are of life on exoplanets - chemical fingerprints in an atmosphere, selective photon absorption by bio-molecular structures (red edges), seasonal variations in albedo - we typically assume things are cozy on the ground. Given that (big) extinction events seem to happen pretty fast, one would think that the odds favor us observing a planet in the calm between these periods. However, we need to be careful about convenient truths.

We are most likely to be able to sniff out the signs of life on a terrestrial-type planet when it's in full swing. Suppose a world is having a particularly fertile episode, chock-a-block with organisms, but not a stable situation. It's prime for collapse. Relative populations will swing high and swing low. At the high point for some, a planet may show the greatest bio-signatures, and make itself far more tasty for our prying telescopic eyes. Without running the numbers it's impossible to give a precise answer, but it would seem that the odds will be shifted. We may be most likely to find not the signs of normality, but the signs of a system approaching some kind of biological collapse - just like the stock market, it's all about the fluctuations.

Another way to think about this is that success breeds vulnerability. If one measure of evolutionary success is the ability to occupy as many niches as possible, with as big a population as possible, then there are many terrestrial examples. This success comes with a price though. That level of dominance may alter the global environment itself (either chemically, or in terms of restraint on other species). If something degrades, or intra-species population dynamics get out of hand, the whole system will head south.

So, the radical thought. It may be that if/when we begin to see signs of biospheres on distant worlds around other suns, these will be places on the verge of collapse, overrun by certain types of life, populations fluctuating to unsustainable heights - albeit perhaps over timescales of hundreds or thousands of years. Our limited detection sensitivity will catch only the outliers.

It's not all doom and gloom though. If a long term goal of exoplanetary science and astrobiology is to place the Earth in proper context, and to potentially help ourselves, then watching disaster unfold elsewhere may be most helpful.

Crusty jigsaw

2010-08-31T11:57:00.000-04:00

Imagine a planet composed of liquids and gases. Close to its surface, the radiation of energy into the deep chill of space results in the crystallization of material. Varying compositions result in different solids forming, the vagaries of phase changes and atomic lattice arrangement produce a huge array of forms. Gravity keeps all this crystalline stuff tightly packed, a thin sheath around the planetary sphere, cracking here and there, floating and bobbling on top of the liquid interior. This is, of course, the nature of the Earth, and the presumed nature of any substantial rocky world with radiogenic internal heating, or enough latent heat of formation combined with youth.

It's quite sobering to be reminded that our picture of how all this crusty stuff operates, how the great plates of rock shift and slide around, is still very, very new. Fifty years ago and the idea of plate tectonics was only just taking proper shape. Fast forward and we now talk about the inevitability of plate tectonics on super-Earth's - rocky planets several times the mass of ours. We care about this because it seems that active plate tectonics and volcanism play a critical role in the long-term regulation of the Earth's climate - forcing surface temperatures into the regime where liquid water can exist.

It's intriguing therefore to see that our understanding of the physics behind plate tectonics is still a matter of intense debate and study. A couple of new results bubbled up during the summer. One is the claim of a new understanding of how the Earth's crust shifts and wiggles - based on essentially the same physics that explains how objects move through viscous fluids. In this picture then the way the planetary crust moves is very much a function of that crust itself, a bit like how your bobsled run is determined to a great extent by the mass and slipperiness of your ride, not just by the ice underneath. This runs against many previous models, where deep interior processes in the liquid part of the planet effectively determine what you see up top. Another work, employing state-of-the-art computer simulation has made recent claims to tie the deep ebb and flow of the Earth's interior to the frosty bump and drift of the outer plates. Rather nicely, this grand simulacrum also suggests that precisely how all the surface cracks and gaps, the faults and fault zones, fit together plays a critical role in determining how the overall plate tectonics of the planet operate.

Just like a fiendish jigsaw, you can't see the big picture until you know how all the small pieces go together. Perhaps not surprisingly, our crusty surface is a hugely non-linear system, hard to predict ab initio. This should raise some concerns for making claims about the nature of plate tectonics on distant exoplanets, but it also indicates a possible opportunity. It may well be that there are distinct types of crustal activity that can occur on rocky planets, from the kind of plate motions that we see on the modern Earth to more fractured styles, or more global styles. At some level these will all link into climate, history, and chemistry. Detecting the presence of atmospheric gases associated with geochemical processes - such as sulfur dioxide - could actually help tell us about the arrangement of continents (or not) on these distant worlds.

Stepping stones

2010-08-28T11:33:00.000-04:00

Maybe it's the flurry of new planets this week, or something else, but the subject of interstellar exploration has been bouncing around more than usual. A discussion that sometimes crops up when talking to others engaged in exoplanetary science is firmly in the speculative, but intriguing, category. It goes like this; let's suppose we find a terrestrial-type planet around a relatively nearby star (read less than 30 light years away), perhaps even around one of the Alpha Centauri members. Let's further suppose that - possibly with the James Webb Space Telescope, or a next-gen ground-based super 'scope - we garner evidence for an atmosphere and several big chemical clues that there could readily be a biosphere on this world. What do we do next?

There are somewhat mundane answers - build better instruments, get better statistics - that may be the most realistic, but there's also that nagging idea that the next thing to do would be to find a way to study such a planet up close. If enough coffee has been consumed then it's a matter of finding a handy Tony Stark, willing to sink hundreds of billions into a robotic interstellar probe, on a long-shot for glory. There's a problem though, unless you intend a very long round trip, how do you get the information back? While we are now pretty good at picking up signals from distant spacecraft - even from Voyager 2 at over 100 AU from the Earth - getting data back from a few light years is going to be hugely difficult. The required transmitter power, as well as interstellar scintillation, is a major hurdle.

A solution, that has cropped up in various guises, even in the idea of von Neumann probes, and the interplanetary internet, is that you don't just send one probe. Rather, you send a chain of probes - pearls on a string - capable of communicating between themselves even if not individually directly back to Earth. It would take a long time, but as the furthest end of the chain crept towards a target stellar system we'd have ongoing feedback, the continuous relay of data as we crept through interstellar space. It might be optimal to build the biggest receiver and transmitter at the outermost practical limits of our solar system - the equivalent of an internet 'backbone' - with a clear line back to Earth. So how many probes would you need to get to somewhere like Alpha Centauri?

This system is about 278,000 astronomical units (AU) away. If we optimistically think we could build probes capable of to-and-fro communication over a few hundred AU then we're talking about a thousand or more devices. This sounds awfully challenging, but remember that we (as some hypothetical sublimely patient species) don't expect probe-1 to reach Alpha Centauri for a few tens of thousands of years. We only have to launch every ten years or so. Even if each probe cost 10 billion dollars (allowing for lowered cost after the first few models) that's peanuts over this timescale. In the meantime we have an ever extending tendril out into interstellar space. Being an innovative species we would undoubtedly think of ever more wonderful things to add to the probes, increasing the scientific return.

Powering transmitters and receivers, as well as sizing their antennae or dishes, is still a problem. Given the timescale to reach the target star then even radioisotopes are going to peter out (fission reactors are a no-go, the fuel burns out too fast). Chemical energy might actually be the best option; a store of redox components, mix them periodically and recharge the batteries, the ultimate fuel-cell.

All over-caffeinated speculation. But if we ever get serious about stepping beyond, then making sure we don't drop the signal is going to be a very real issue.

Here we go...

2010-08-24T12:52:00.000-04:00

The danger with spending time on a careful post (see below) is that something hits the waves while you're at it. Take a normal G-dwarf star, stare at it with an ultra-high-precision spectrograph, wait, and find perhaps as many as 7 planets. This is the incredible announcement for the system of HD 10180 coming out of the European Southern Observatory.

5 definite 'Neptune' class worlds. One possible 'Saturn' and one itty-bitty 1.4 Earth mass object lurking on the hairy statistical edge of confirmation...

Not only that, but mostly close to circular orbits from almost on top of the star out to a few astronomical units.

It's a beautiful bit of astronomy, it's also a real eye-opener to the potential richness of planetary systems...there's planets in them there hills.....

Seeing red

2010-08-24T12:11:00.000-04:00

Seventy percent of all energy consumed by life on Earth is in the form of solar photons. Interestingly this consumption is by a minority of organisms, those exploiting the mechanisms of photosynthesis. It amounts to a globally averaged energy intake of some 100 terawatts, but even this is peanuts compared to the solar input at the Earth's surface, which is about 90 petawatts. By comparison, all available geophysical energy - thermal and chemical - is a paltry 30 terawatts.

If you want energy, then photons are the way to go. Catching and transducing photons into chemical energy in biological systems is accomplished via the extraordinary chlorophyll pigments. In oxygen producing organisms four types of chlorophyll had been known, each tuned to slightly different wavelengths. For example, chlorophyll 'a' grabs photons at around 465 nanometers (bluish visible light) and at around 665 nanometers (reddish visible light) - leaving behind the familiar green photons that we enjoy in our foliage. The other, rarer, chlorophylls absorb at similar wavelengths. Now, in a neat paper that appeared in Science last week, Chen et al. have identified a fifth chlorophyll 'f' - extracted from organisms lurking in modern day stromatolite formations.

Why get excited about this? Chlorophyll 'f' does something not seen before in photosynthesis - it slurps up photons from the near-infrared. At about 706 nanometers we're into a regime just beyond the visible. Obviously other pigments and structures can, and do, absorb photons in this regime, but only 'f' is known to actually use the photons to split water molecules - the key step in oxygenic photosynthesis.

To my mind this raises a number of fascinating connections to questions of life on Earth and beyond. It suddenly connects the dots to earlier indications of organisms exploiting infra-red photons around deep-ocean hydrothermal vent systems - or at least offers a molecular solution. It also, and here I'm heading out on a limb, might connect to an issue that's long bothered me. We've talked before about how our Sun was as much as 30% fainter three or four billion years ago. With this faintness comes a small shift in the peak output of a star's spectrum (being a blackbody to first order) towards the red. For the young Sun this would have only been a few tens of nanometers - but it would have meant a slightly better flux of these near infrared photons. In addition, a different atmospheric composition and chemistry on a youthful Earth could have altered the typical range of photons making it down to the surface. It seems not unreasonable to suspect that chlorophyll 'f' could have given ancient microbial life a leg-up over the competition. Indeed, did chlorophyll 'f' come along first?

It's pretty startling evidence of nature's capacity to find molecular machinery to exploit even low energy infra-red photons. Now picture a world around a low-mass star, with a drastically redder spectrum. Chlorophyll 'f' points the way to how organisms there might manage all the great tricks of photosynthesis that we see here on Earth.

Protein universe

2010-08-21T14:11:00.000-04:00

A couple months ago a rather stunning paper slipped into the journal Nature. It presents a sophisticated investigation of how quickly the genetic codes for proteins are evolving - across the 3.5 billion years or so of life on Earth.

The idea is that the specific genetic codes, the sequences of amino acids, that describe proteins - the workhorses of molecular biology - cannot withstand big changes, otherwise the complex structures formed will just not do their job properly. Swap in a different amino acid somewhere in a chain of a thousand and you'll no longer fold this big molecule up into the right shape, and you've gone from a 1/2 inch wrench to a pair of tweezers.

Over time though small changes can, and do, occur. As long as the final outcome permits the same job to be done by the protein, all is well. Now, let's suppose that a whole clutch of modern organisms share a common ancestor (something we've touched on before in these pages). We should be able to see just how different the protein coding has become since that time, and we should be able to tell whether this type of gentle evolution has stopped or not.

Povolotskaya and Kondrashov apply to proteins exactly the same methodology that Edwin Hubble did to the measurement of the expansion of the universe. They look to see how fast the coding is changing as a function of how different those proteins are - just as Hubble looked at recession velocities versus the physical distance between galaxies. What they find is that after about 3.5 billion years the protein universe here on Earth is still, slowly, diverging and expanding - it's not yet reached a true optimal state. They also point out that while 98% of locations in a protein sequence can't deal with quick tampering (change an amino acid there and the whole thing ceases to work), over billions of years you could more or less re-write the code for a given protein and get the same function. That's a bit like changing Hamlet by one word every new print run, until you have a totally different script, but the same outcome.

So, what says all this for the nature of life in the universe? These proteins plays roles in things like metabolic processes that have remained unaltered for billions of years - solutions for how life extracts energy from its environment that are pretty close to optimal. Yet here we see a universe of slowly diverging, expanding, molecular structures - the very fabric of the biological cosmos on Earth. To my mind this might present a huge challenge to the notion of convergent evolution - the idea that there are a limited number of molecular or physiological solutions that life can use. Take a different planet, with a biosphere a couple billion years old. The stately evolution of its protein universe would almost certainly have taken a path unlike that here, exploring this vast multi-parameter space of molecular structures along alien paths. It both supports the notion of life as a potentially extraordinarily robust phenomenon, and as a hugely diverse one.

The panspermia paradox

2010-08-19T10:51:00.000-04:00

A brief hiatus and a lot of stuff happens - the oldest rocks on Earth, 3 million year old human tools, and future directions for astronomy. So, ignore all of that and bring focus to bear on a persistent idea. The notion of panspermia - the transferral of viable organisms between planets, and even between star systems and further.

There is no doubt that planetary surface material is continually being shipped around between rocky planets and moons in our solar system. Ejected by asteroidal or cometary impacts, chunks of stuff follow a range of orbital trajectories that result in both eventual return to their origins or transferral to the surfaces of other worlds. Increasing evidence suggests that a variety of (typically microbial) organisms could be carried along, surviving both the extremes of pressure and acceleration, as well as exposure to thousands to millions of years of interplanetary space. There is a real possibility for life to both cross-infect, and even to be 'seeded' from planet or moon to planet or moon.

Enthusiasts for panspermia go further, and have been known to invoke this as a mechanism for galaxy-wide dispersal of life - taking one rare occurrence of life and spreading it. There is however a factor that to my knowledge is rarely considered, and that is natural selection. You or I, fluffy bunnies, and daffodils are all unlikely candidates for interplanetary or interstellar transferral. The sequence of events involved in panspermia will weed out all but the toughest or most suited organisms. So, let's suppose that galactic panspermia has been going on for the past ten billion years or so - what do we end up with?

Although it's a complex problem, it seems likely that life driven by cosmic dispersal will end up being completely dominated by the super-hardy, spore-forming, radiation resistant, rock-eating (endolithic) type of critters. There will be no advantage to a particularly diverse gene pool. Billions of years of galactic transferral will have whittled it down to only the most indelicate and non-fussy microbes - super efficient, super persistent, and ubiquitous - the galactic top dogs.

Now, we might argue that there are many organisms on Earth that could fit the bill, and could be the links to these ancestral interlopers. The problem, and the potential paradox, is that if galactic panspermia is real then the type of life it will evolve should be everywhere. There would be stuff on the Moon, Mars, Europa, Ganymede, Titan, Enceladus. Every nook and cranny in our solar system would be a veritable paradise for these ultra-tough lifeforms. It may be too early to rule this out, but if life is sparse in our neighborhood then it would seem to argue strongly against the possibility of Galactic panspermia.

Summer reading

2010-07-31T08:18:00.000-04:00

It's not the usual post. A conversation about interstellar travel reminded me of a short, whimsical, piece of fiction I'd noodled with a while back. Nothing to do with the search for life in the universe, but much to do with the nature of long-distance travel, human enterprise, and that cosmic force; irony. So here goes, a little reading for the celestial beach chair.

Progress
All personnel. This is Mother, attention please. As you know we have now entered the Alpha Centauri AB system. The Santa Maria is adjusting for final insertion into a wide survey orbit. All navtechs and astros are to report to workstations and are reminded to follow United Federation survey protocols. Reactors three and four will begin shutdown as plasma drives are brought offline. Personnel generations six and seven are reminded that it is critically important to assist final generations with adjustment from interstellar environment. Counselors will be available to mediate. All children are to continue school on the normal schedule.

All personnel. This is Mother, attention please. Initial survey indicates two terrestrial mass inner planets around Alpha Centauri A, one with significantly non-equilibrium atmosphere, indicating potential biosphere. Gas giant b harbours two large moons, inner moon appears oceanic with high specular reflection. Access to Santa Maria spin axis recreational arenas and hydroponic fields will be limited, personnel are encouraged to remain on outer decks for g-acclimation. Colonization refresher courses will be available on deck nine. Exceptions will be made for centenarians and older.

All personnel. This is Mother, attention please. The Santa Maria will switch internal clocks to local frame in twelve hours. Elapsed journey time will then be 1,015 Earth years, eight Earth months and three Earth days precisely. Wide band modulated transmissions have been detected from terrestrial planet c, precise nature of transmissions has not yet been established. All anthropol personnel are to report to workstations and follow contact protocols. Counselors will be available on decks seven through ten.

All personnel. This is Mother, attention please. Contact has been established with planet c. Personnel of the United Federation interstellar cruiser Vixen have established a colony there during the past ten years. Colonists report that a quantum-entanglement drive was developed during year 950 of Santa Maria journey time. Vixen departure from Earth was during year 980 of Santa Maria journey time. Counselors will be available on all decks until further notice. Anti-depressants will be made available following consultation with assigned medical personnel.

All personnel. This is Mother, attention please. Arrival two hours ago of the United Federation interstellar scout-class Sabre was responsible for strong electromagnetic pulse. All primary systems are fully functional, secondary systems will be back online shortly. The singularity worm-drive onboard the Sabre was developed during year 995 of Santa Maria journey time. The Sabre is now deploying its colonist transports to gas giant b ocean moon. Personnel are advised to join counseling groups on all decks.

All personnel. This is Mother, attention please. The results of the Santa Maria referendum on proposition 1A have been tabulated. Santa Maria will be refurbished with a singularity worm-drive and upgraded habitat resources. Cryochambers will be added to deck five for optional suspension and all interior living areas will be resurfaced and modernised. Generations six through eight will disembark and join Vixen and Sabre colonists on planet c and the gas giant b ocean moon. Remaining generations will retrain for the journey to Barnard’s Star, anticipated to take only fifteen years. Congratulations as you begin this historic voyage as the next colonists to another star. Legal counselors are available on decks two through five to assist in waiver statements with regard to the United Federation’s limited liability for your first arrival status at Barnard’s Star.

Planetary Prometheus

2010-07-27T04:48:00.000-04:00

Imagine, if you will, a planet with atmosphere, oceans, rocks and life. On this planet then most chemical reactions are either slow and geophysical, or quick and biological but very localized. There is however an exception. Because of the particular nature of this world there is the ever-present potential for a type of chemical reaction that is not only fierce and destructive, but self-propagating. Once triggered it can spread across hundreds, even thousands, of square miles. It preferentially attacks and transforms living material - leaving behind a fragile deposit, stripped of most biomatter. It can only stop by either exhausting the supply of fresh reactants, or when its chemical energy is sucked away by an un-reactive medium.

This is a tricky planet. It forever teeters on the edge of letting this chemical storm get a grip, but its climate and varied topography help to confine outbreaks. The very compounds responsible are themselves critical ingredients for much of the life on this world, and cannot be eliminated. Indeed, the reaction itself serves a number of key roles in stabilizing populations, cycling elements between air, ground, and oceans, and is ancient enough to have been incorporated into the survival strategies of large numbers of species.

Imagine we could visit this world. Entering orbit we would scan it with our telescopes. Curiously, at any given time, we would observe tens of thousands of these intense chemical maelstroms dotted across the globe. Their signatures would be quite distinct, and we might be quite astonished that life existed in such a perilous environment.

Of course, this is no hypothetical planet. It is the Earth. The chemical reaction we know as fire is a strange and intriguing, and often overlooked, aspect of life here. The young Earth of 3 billion years ago, with little or no oxygen in its atmosphere, or flammable biomatter, would have probably only seen fire in volcanic settings. Somewhere along the line, maybe a billion or two years later, with enough free oxygen, perhaps some dried up mat of plant life on a tidal shore was the first victim of arson - possibly a result of lightning. Today, fires cover the globe. Satellite imagery, or remote sensing, tells the story. The image in the upper left shows thousands of fires scattered across south-central Africa, seen by the MODIS instrument on NASA's Aqua satellite. Many of these have been set by humans, following an ancient pattern of land-use. Humans have learnt to exploit this chemical fragility.

I think we tend to underestimate the chemical reactivity of our homeworld. Fire is an excellent example - it's so familiar to us that we (well, I) even have to pause to remember that it's something chemical, fiercely exothermic. It raises a number of interesting questions. Is a phenomenon like fire simply a consequence of the kind of chemical reactivity needed for a planet to harbor life? Life on Earth needs a lot of reduction-oxidation pathways. Can you propel a biosphere to the kind of richness we see today without taking this walk on the wild side - risking destruction for the chance to make hay with oxygen?

Evidence suggests that, for example, around 270 million years ago atmospheric oxygen levels were significantly higher than today - and that fire was much more frequent on a global scale. More oxygen and it becomes hard to avoid burning all flammable materials, clearly there could be a feedback mechanism at play - complicated by geography and climate. Just how fire-prone can a planet become before it wipes out its surface biosphere?

Anyone got any marshmallows?

Exploding moss/Aquatic Mars

2010-07-26T06:25:00.000-04:00

Trawling the scientific news and literature can be an obsession - there's just so much incredible stuff being discovered all the time. It's also hard to choose favorite items, whatever ability to prioritize that evolution has endowed humans with, it's not always very effective. So, with that by way of an excuse, there were two utterly unrelated items that popped up last week that I cannot choose between - your vote.

The first involves something I'd never thought of before. Terrestrial moss (yes moss), produces spores to propagate, since it is a flowerless and seedless plant. Spores are tiny capsules a few tens of micrometers across and are perfect for being dispersed in the air. The puzzle has been that when the wind blows on Earth there is usually a small layer of atmosphere, about 10 cm wide, that serves as an interface between the planet surface and the free-flowing air. If you want to be carried off by the wind you need to get above this layer that's a result of friction, fluid dynamics, and topography. Most mosses lurk no more than about a centimeter above the ground - so how do they get their spores into the mossy equivalent of the jet-stream?

A paper by Whitaker & Edwards in Science examines the problem, and with ultra-high speed photography they find that some mosses use pressurized, explosive, capsules of spores to launch them upwards with g-forces of as much as 30,000. Although this is impressive, as lilliputians it results in a roughly 30 mile-an-hour Vespa-like launch velocity. The puzzle is that even this shouldn't get the spores high enough. The new study provides the answer - the exploding capsules are such that they produce a spore-laden vortex ring that burrows its way up to the magical 10 cm altitude. It's very much like a smoke ring, a complex structure that maintains integrity. In effect, natural selection has endowed moss with both a knowledge of the bigger picture of terrestrial atmospheric science, and some fluid mechanics that Boeing would envy. It's a further reminder that even 'simple' life is seldom just that.

One then wonders if a planet like Mars once had spore launching organisms littering its surface. A new paper by Morris et al. fills in one of the blanks. With all the geological evidence that Mars was, at least for a time, a wet place there has remained a bit of a conundrum. The simplest way for Mars to have been more temperate was for it to harbor a thicker, carbon-dioxide rich, atmosphere - with a strong greenhouse effect. If that had ever happened then there should be massive amounts of carbonate rich rocks laying around from that era. Until now the evidence for this had been scant. By studying the walls of Gusev crater with the Spirit rover Morris et al. get a look at the geological history - and they have now identified significant, and ancient, carbonate deposits from about 3.5 billion years ago. Checkmark the box.

It's not the end of the story, but it adds significantly to the evidence for a warm, wet, young Mars. The next question is how long did it retain this climate - long enough for life?

On the iceberg

2010-07-21T06:18:00.000-04:00

Although incredible progress has been made in the search for exoplanets, we're still teetering on the peak of the cloud enshrouded mountain, with little real knowledge of what lies beneath. It's instructive to take a look at where things are at today. The first image here (go on, click on it) is a plot of estimated planetary mass (typically a lower limit owing to the nature of the detection techniques used) versus the estimated distance of the stellar parent. A total of 448 planets are pulled out of the excellent exoplanet catalog. Not bad going for 15 years of effort - although it is sobering to realize that our galaxy alone almost certainly contains more than a hundred billion objects we'd term planets - and likely many more. So far the majority of our detected planetary swarm is within about 100 parsecs - about 330 light years - of the Sun. A keen eye will spot that the lowest mass planets, a few hundredths of the mass of Jupiter - the unit used on the Y-axis - also lurk preferentially in some of the closer systems. This is an artifact of the choices made in investigating systems and the nature of planet detection methods.

This 2nd plot (minus a couple of systems) reveals some more of that skewness. The low mass planets at the bottom also correlate with stars that have a slightly lower average mass than those higher up in the plot. Lower mass stars are more strongly perturbed by the gravitational tug of orbiting planets, so the telltale signs of small planets are easier to find - but they're also fainter, and so they are better targets when they're not too far away.

This lower left corner is one of the most intriguing places to go looking for planets. An astonishing 70% of all stars in our Galaxy are less than half the mass of the Sun. There are at least 300 such objects within 10 parsecs of us. They're the real dwarfs, some are a thousand times fainter than the Sun, but they can burn their nuclear fuel for a trillion years. If planets - at least the smaller rocky ones - can form efficiently around these stars, then most worlds in our galaxy, and indeed the universe as a whole, will be bathed in their reddish light. The next few years should reveal more about the closest examples, and their alien environments.

Altogether now

2010-07-14T06:21:00.000-04:00

The issue of multi-cellular life is very, very interesting. A number of years ago Carl Zimmer wrote a very nice piece discussing how, when, and why microbial life on Earth got to acting in a multi-cellular mode. Interestingly, the organism he talks about is the self-same alga - Volvox - that was the subject of the genetic detective work I mentioned in the last post. What I find so fascinating about all this (hence the sudden flurry of posts), are the broader implications for life elsewhere in the universe. This is also motivation for the kind of hare-brained social gaming experiment I suggested last time.

In astrobiology there is ongoing debate about just how unusual, or not, so-called 'complex' life might be out there in the cosmic deeps. A lot of the discussion got rounded up in Ward & Brownlee's book Rare Earth, where they tried to argue the case for our homeworld, and its biosphere, being particularly special. I couldn't help but feel at the time, and more so now, that this line of reasoning was overly mired in the increasingly archaic view that evolution is this rather inflexible progression towards 'complex' life. I think it underestimates the sheer opportunistic side of biology. We also get so blinkered by the historical and sociological context of the development of evolutionary biology that we come to see 'complex' life as a veritable house of cards. One wrong step, one asteroid too many, one poor mutation, and a planet will be forever bereft of multi-cellular life.

As Zimmer discusses, the transition between single-celled, microbial organisms and multi-cellular life has happened multiple times across different phyla during the Earth's history. It occurs as and when the advantages of being 'big', outweigh the advantages of being 'small' - jolly old natural selection. Now, here's the next bit of wide-eyed speculation [and yes, I know, countless sci-fi tales got there first...]. Does multi-cellularity necessarily mean physical connection? While there is certainly a lot of physics that can be exploited by getting together on a microscopic scale (even quantum entanglement in photosynthesis), there may equally be advantages to remote connection. And, while I really don't want to go all starry eyed here, an informationally nutritious medium like the Internet, could provide just the kind of connectivity such life would require - hooking together the individually sophisticated human microbes. Is the internet part and parcel of the evolution of life on Earth? Well, I might not go quite that far, but could a mechanism of long-distance, but intimate, connection occur in a purely biological system?

Obviously if you're James Cameron and you live on an allegorical moon called Pandora then the answer is yes. But for astrobiology it may not be crazy to relax the rules for what is meant by 'complex' life. The next question would be whether the planet-wide signatures of dispersed multi-cellular life would look any different than those from the types of life we know about.

Microbes..who, us?

2010-07-13T05:37:00.000-04:00

Stewing in heat-waves, rain, and who knows what else. Summertime in the northern hemisphere of the planet is a good time to wallow in a bit of speculative thinking. A couple of posts ago I talked about a fabulous piece of work on what could be a 2.1 billion year old example of emerging multicellular life here on Earth. Hot on the heels of this is another study of the origins and mechanisms of multi-cellular organisms. The paper, by Prochnik and collaborators, examines genetic evidence for the history of multi-cellular behavior in a type of algae.

The bottom line is that the algae seems to have very, very similar genetic building blocks to its most similar single-celled, microbial relatives. Specifically, the protein toolset doesn't alter much between the single guys and the multi-cellular descendants. What this means is that it wasn't some giant leap to move to a multi-cellular, or communal, existence - at least for these species. To use the terminology of the study, these organisms are not particularly innovative - with a bit of an evolutionary shrug of 'oh what the heck' they segued into a new multi-cellular mode of life. Of course I exaggerate, but it does seem that this was not some earth-shattering, Charlton Heston type, moment in ancient biology. The same tools can be used to fashion a new type of life.

Which brings me to the summer speculation. Here we all sit gazing at our screens, our little brains and hands pawing at the pixels and keys. Each of us is quite uni-cellular in that sense, self-contained, sending out feelers and sensors, pulling in environmental information. The medium that we are immersed in may be a virtual one, packets of electrons whizzing to and fro across the world, but it's arguably just as good as a nice bit of pond water, or agar gel.

Then something happens. Perhaps it's an earthquake, a political event, a particularly enthralling YouTube video of cats licking themselves. Whatever it is, it can serve to motivate a sudden shift of mode for thousands, if not millions of individuals. Instead of solitary action we can form a mob, whether by just pinging the same page, sharing on Facebook, emailing everyone we know, or actually hopping on a bus and going to a demonstration. The internet has made this type of behavior far, far, easier. Each of the cells may not think they're part of a larger organism, but by most standards they are. Social gaming is another excellent example - 'help me grow my carrots by clicking here' - if that isn't inter-cellular communication to perform function then I don't know what it is.

So, here's the proposal, for all you bored but brilliant programmers. How about making a social game that is actually a cunningly disguised attempt to build a virtual multi-cellular lifeform? All of us single-celled organisms will just use our familiar toolsets to participate, but in doing so we will create and evolve something new, much bigger than the sum of the parts. The point of this - other than lowering office productivity - would be to perform a scientifically useful experiment to learn about the types of networks and 'global' mechanisms involved in shifting from uni-cellular life to multi-cellular. We might, just might, gain insight to this transition...

[oh, and I'm sure someone else has thought about this before...]

Our leaky solar system

2010-07-09T06:53:00.000-04:00

We've all grown up with the notion that we're a long way from the nearest stars. This isolationist mentality extends to the way we think about all the stuff in our solar system; it's ours, all ours! However, our Sun almost certainly formed together with many other stars from the same cloud of molecular hydrogen, with the usual sprinkling of heavier elements and interstellar dust. Long since dispersed by the inexorable slewing of material as it lumbers around the Galaxy - every 210 million years or so at our placement from the galactic center - our stellar brothers and sisters may be well scattered by now.

There was a time though, some 4-6 billion years ago, when other stars, likely with their complements of dusty planet-forming material, could have swept by. The absolute outermost reaches of our local gravitational well are populated by the cold residue of planet formation, the objects we know as cometary nuclei. This postulated fuzz of stuff is the Oort cloud, and should extend outwards from us to as much as 100,000 astronomical units distance - almost halfway to the closest stars of Alpha Centauri.

An intriguing new work by Levinson et al in this weeks Science re-opens an investigation into the precise origin of the Oort cloud. From what we know about long-period comets, sailing in from this distant population, there appear to be some serious discrepancies about the likely number of objects out there in our deepest reaches. Their basic argument is that there are too many objects in the Oort cloud to be accounted for by their origin around the giant planets - from where they were gravitationally flung outwards during planet formation 4.5-5 billion years ago.

The rather shocking alternative, carefully studied with state-of-the-art computer simulations, is that as much as 90% of the Oort cloud has actually been gravitationally captured from other stars, during close passes billions of years ago. Equally, we've shed plenty of icy lumps that are now stuck with different stars - somewhere along a common path around the galaxy.

With a wave of the wand our solar system is less of an isolated specimen than a separated sibling, carrying along the familial baggage of our stellar birth cluster. This means that some of the long period cometary bodies that spew their guts as they dip towards the Sun are truly alien - born around another star altogether. As a small habitable planet, we get the occasional wash from this material, and the occasional collision. Whatever the chemistry and isotopic mix is of these ancient objects, it's slowly getting mixed into the inner solar system. Remarkably this means that there is a pathway that could conceivably take a molecule formed around a giant planet in another star system and eventually deposit it in your Pina Colada. Anyone for another round?

Original multicellular life

2010-07-02T05:54:00.000-04:00

We are cellular clumps. Great big, sticky, masses of about ten trillion cells, plus our microbial passengers. Although they all operate in unison, our cells are each quite self-contained. Membranes hold together our 3.3 billion base-pairs of information packaged up tight, and all manner of other molecular structures that allow the cell to hum away like a tiny machine. On the other end of the spectrum are the microbes, where an entire living organism is encapsulated in one single cell.

Somewhere in-between is a fascinating and tricky regime. Most microbial life - bacteria and the ancient archaea - live in colonies, typically of many, many different species. They communicate, they glue together in biofilms, they both support and rely upon each other. It is so very tempting to look at this and see how life on Earth went from these single-celled characters to multi-cellular grandeur. It's just a matter of joining the dots - literally.

Fossil evidence for when and how multi-cellular life on Earth first reared up has been awfully tough to come by though. Small squishy things do not take well to billions of years of mineralization. It does seem to have coincided rather broadly with the time at which Earth's atmosphere began to load up with significant amounts of oxygen about 2.4 billion years ago - courtesy of photosynthetic organisms like cyanobacteria. Available oxygen, while toxic to much of microbial life, opens up a whole new, energetically favorable, chemical network - a big tipping point towards large and complex lifeforms. A fabulous paper appeared this week in Nature, by El Abani and collaborators, along with a nice opinion piece by Donoghue & Antcliffe, describing perhaps the closest thing yet to our multi-cellular ancestors - hot on the tail of the oxygenating Earth.

In this paper they report fossil remains of curious looking, bumpy, swirly, scallopy characters as big as 12 centimeters, from what is now Gabon. At 2.1 billion years old they land on the cusp of atmospheric transformation. Carbon and sulfur isotopes seal the deal on these being biological in origin (see my other post). Everything about these objects screams cellular colony. Does that make them multi-cellular? Well, incredibly, the patterns in these fossils strongly suggest that the type of cell-to-cell communication that is the hallmark of creatures like ourselves was at play.

So here we are, meet your great-to-the-power-of-nine-aunt. In the search for life elsewhere in the universe we often come up against the question of whether 'complex', multicellular life is going to be rare or not. Certainly its longevity on planets probably comes down to fine details of environmental stability. However, if microbial life can gentle slither into this mode of operation - like our friends from Gabon seem to be doing - I think it ups the odds that for at least part of a biosphere's history, multi-cellular type life will be around. Perhaps we should think of life like us more as a desert bloom - it may happen only briefly when conditions are right, but the potential is always there.

Favorite nuclear flavors

2010-06-28T18:51:00.000-04:00

One of the signposts of life - and something that will likely be exploited in the study of potential Martian critters - is isotopic fractionation. The general wisdom is that life prefers light isotopes. Given the choice between hydrogen and deuterium (always present due to its construction 13.7 billion years ago during primordial nucleosynthesis) living things will tend to choose the lighter hydrogen. The same is true for carbon-12 and carbon-13, the light guy wins and is preferentially incorporated into the chemical doings of cells. When the extraordinary plumes of atmospheric methane were confirmed on Mars a couple years ago it was clear that the next step would be to try to pin down the isotopic composition of the gas. Here on Earth one can quite readily distinguish methane produced by bacteria or archaea and that produced by abiotic processes (most is biological), by tasting the isotopic composition.

So why is this so, why does life like particular isotopes. Ask a biologist and they will typically say that it's all about enzymes. Processes inside cells that work with enzymes - fabulous catalysts - happen faster with light isotopes, and bingo, you naturally sieve out the heavy guys and leave them in the dust. At least that's the line I've heard again and again. I was fascinated then when I stumbled across a very interesting work by Calsciotti. In this and subsequent papers the discovery of cellular processes involving nitrogen are described that actually prefer the heavy isotope nitrogen-15 rather than the lighter nitrogen-14. It appears that in some cases the heavier atoms provide a more energetically lucrative reaction route, et voila, like the fickle thing it is, life spits out the lighter variants in favor of something meatier.

The light isotope rule is not even that. Biochemistry again demonstrates that after a few billion years of fine-tuning it can exploit just about any trick in the book to get ahead. What I find amazing is that only about 0.4% of nitrogen on the Earth is the weightier 15, yet here we have organisms that will make use of the better chemical throughput of that isotope. One's first reaction is 'why bother?'. Obviously it may just be an unintended consequence of the chemical network, but I think the odds are good that somewhere along the line that tiny advantage will have been important. Here's the big question though, would a different biosphere - a martian one perhaps - with a presumably different evolutionary history, make all the same biochemical choices? To throw a final spin on this; Mars has a higher nitrogen-15 abundance than the Earth, a relative enrichment of about 60%. How much would this environment have swayed things?

Planetary fumigation

2010-06-23T06:21:00.000-04:00

Where do you go on Mars to look for life ? Some pretty big decisions are looming for NASA's next Mars rover - the plutonium powered, SUV-scale beast known rather delicately as Curiosity. The choice of landing sites is currently down to four, and much depends on clues to the past history of these regions. Some show signs of a watery era, with the possibility of rich sedimentary rock - the kind of places where the detritus from an ancient martian biosphere would have ended up. Curiosity will be armed with an array of instruments, some capable of analyzing the detailed organic chemistry being trampled underfoot.

There has been considerable debate in the community about where to land, how to avoid chemical and even biological contamination of where you land, and exactly what instruments to carry - given the very real limitations to hauling every gram of gear to Mars. There's no doubt that Curiosity represents the next great stage in exploring another planet, and looking for the signs of extinct, or even extant, life. We'd like to get it as right as possible.

Thinking about all this raises something else, that gets less attention. There is a supposition that if Mars once had a watery surface environment - even if only fleeting - and some kind of biosphere, then there must surely be traces left. Obviously the geochemical alteration of the surface is likely to be noticeable, but why do we assume it is hard to erase the signs of life from a planet ? It may be an Earth-centric bias.
What would it take to erase life from the Earth, and perhaps also any sign that it was ever there ?

For everything on the surface then we can imagine burning, exploding, acidifying, and irradiating until we had just an amorphous, crispy, residue that would be entirely sterile and uninteresting. However, then there's the subsurface - kilometers deep rock, oceanic sediments and porous material, isolated subterranean water. All of this is chock full of slow-living microbial life. A solution would be to 'go Venus', melt the surface and keep up strong volcanism for billions of years - heat and pressure over time will transform just about anything. Time alone might do it. If you could wait another five billion years or so (at which point the Sun will have engulfed the Earth anyway) then perhaps enough of the molecular handprint of life will have fizzled away to be unrecognizable. In short, without a cataclysm then it's hard to imagine a way to remove all traces of past or present life on Earth.

But what about Mars ? If a planet never gets a fully entrenched, deeply embedded biosphere does it retain the same stain ? Suppose for a while, in the distant past, Mars did have a wetter and more obviously nurturing surface environment. Perhaps microbial life got going across a number of optimal regions, but was then cut off - as if life on Earth had stopped 3 billion years ago. It seems it would be much easier for time and chemistry, not to mention volcanism and asteroid impact, to lay down the white-out. It sounds pessimistic, but it's really more about tackling the fundamental problems of molecular paleontology - you need to find an understanding of what might be blocking your view in order to reconstruct what was once there...

Swings and roundabouts

2010-06-21T06:09:00.000-04:00

The hunt for terrestrial-type planets is hugely challenging, although steady progress is being made. Last week NASA's Kepler mission finally began to release data on some of its candidate planet-hosting stars. In a bit of controversial, but to some extent understandable maneuvering, the Kepler team has kept what are probably the juiciest systems to themselves. The New York Times had an excellent piece on this tale. Regardless of what data any of us get our sticky paws on there is still a big hurdle to deal with. Kepler is seeking the tiny dips in starlight when planets transit their stellar hosts, but this alone is not enough to either confirm with absolute certainty the presence of a planet, or to determine the mass and detailed orbital characteristics of new worlds. The next stage involves painstaking followup with ground-based telescopes equipped with exquisite spectrometers, seeking the wispy signs of Doppler shifting starlight due to the gravitational offsets induced by planets.

It's quite a bottleneck. It's also pushing the limits of experimental sensitivity as we seek planets equivalent to the Earth - where the stellar 'wobble' can be mere centimeters per second, the speed of a royal wave. Kepler hasn't even got to those candidates yet, with orbits close to 12 months in length we need the next several years to witness multiple transits. There is no doubt that it will take a great amount of scientific cooperation and effort to finally nail down the number of Earth-equivalent worlds that Kepler sees.

Even then, what we'll end up with is really just (although 'just' is relative) a well defined estimate of how many such planets should exist galaxy-wide - together with some idea of their typical compositions and orbits. Bottom line is that the newspaper banner in three or four years will read something like '15 %' - this being the fraction of normal stars hosting small rocky worlds. Is that worth the $600 million for Kepler, plus many more millions for all the followup work ? Such is the nature of modern fundamental science. The Large Hadron Collider costs well in excess of $6 billion, that's about a billion per physics question that it might help answer. The overall cost of the Human Genome Project was about $3 billion - roughly a dollar a base-pair.

I'd argue that by comparison Kepler is positively efficient, but it does raise an interesting question. Just how much are we as a species willing to invest in the search to find other worlds and to seek out signs of other life in the universe ? It's not a cheap enterprise. For me though it is no less important than understanding fundamental physics, or our genetic blueprints. In fact I'd stick my neck out and suggest that unless we find new worlds, new biospheres, we will never have a way to place the Earth and ourselves in proper scientific context. The answer to our origins will come from seeing the bigger picture, and that is surely cheap at any price.

Seven years in deep space

2010-06-15T14:34:00.000-04:00

The falcon (Hayabusa) has landed. A few days ago the Japanese space agency's extraordinary craft re-entered the Earth's atmosphere and successfully deposited its sample return capsule in the Australian outback. This was an incredible technological feat, and apart from Apollo and Luna samples from the Moon and the solar particle and cometary dust return missions of Genesis and Stardust, the only other time a spacecraft has made it back to the homeworld possibly carrying scraps of pristine extraterrestrial material. Pristine is the key here. Hayabusa gently bounced off the asteroid Itokawa back in 2005, and may have captured grains of this ancient body that traces an elliptical orbit looping inside that of the Earth before reaching some 70% further out.

Thanks to Hayabusa we know that Itokawa is an extraordinary 'rubble pile' half a kilometer long - not a solid body but rather a loose collection of rocks from tiny pebbles to much larger. Weakly held together by its own gravity this is an amazing snapshot of one episode of planet formation; the agglomeration of solids. Figuring out the precise chemical constituents of a body like this could yield clues to the pathways by which material coalesces in the early stages of a baby star system. It could also add a big piece to the jigsaw puzzle of carbon chemistry in our solar system, and ultimately the origins of terrestrial chemistry.

There's another aspect to Hayabusa that is perhaps even grander. The image of this cleverly fabricated robot burning up across the night sky evokes some powerful emotions. Here is one of our pioneer voyagers of the deeper universe that lies all about us. A persistent machine, nurtured and nursed through a variety of problems by its smart operators. Seven years on it returns, carrying - we hope - a precious sample that will expand our view of nature. Seven years is a long time these days, Hayabusa has come back to a different world. This is a glimpse of our future in the solar system. The meteor-like streaks of returning probes, and eventually astronauts, lighting our skies. New mariners, returning to harbor, bringing exotica that change everything, just as they find a world changed by time.