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.

Oasis Earth

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

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

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

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

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

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

Moons

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

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

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

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

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

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

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

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

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

It's full of Neptunes...

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

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

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

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

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

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

It's full of planets...

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

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

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

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

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

Tick tock

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

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

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

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

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

Astrobiology: The Questions

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