Discussion and news about the modern effort to understand the nature of life on Earth, finding planets around other stars, and the search for life elsewhere in the universe

Tuesday, July 5, 2011

Pastures New









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.

Saturday, June 4, 2011

Nematodes, Mars, and Moons

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.

Sunday, May 22, 2011

Wanderers?

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.

Sunday, May 15, 2011

Flippin' Planets

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.

Monday, May 2, 2011

55 Cancri e: Small or Big?

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.

Thursday, April 28, 2011

SETI Lost and Found

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.

Tuesday, April 19, 2011

Three Billion Years B.C.

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.  

Monday, April 11, 2011

Rotten Eggs

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.

Monday, April 4, 2011

Paradox Earth: III

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?

Thursday, March 31, 2011

JWST Launch Brought Forward to 2012

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"

Monday, March 28, 2011

Carbonaceous Cotton Candy

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]

Thursday, March 24, 2011

The End is not the End

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.

Monday, March 21, 2011

Multiple intelligence test

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.

Tuesday, March 15, 2011

Springtime on Enceladus

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.

Monday, March 14, 2011

The Romans go to Jupiter

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.

Sunday, March 6, 2011

There's Something About Meteorites

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.

Saturday, March 5, 2011

The Evolution of Planet Earth

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.

Monday, February 28, 2011

A whiff of blue sulfur

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.

Thursday, February 24, 2011

Make me a planet

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?

Monday, February 21, 2011

The X factor

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?




Tuesday, February 15, 2011

Oceans in the night

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.