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.

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?

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"

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]

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.

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.

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.