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.

Extinction

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

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

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

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

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

Slippery Planets

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

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

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

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

The ten most important questions for astrobiology: Number 10

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

Is there intelligent life elsewhere in the universe?

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

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

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

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

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

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

The good news is that barely 16 years ago we didn't know for sure that any planets existed around other normal stars. How strange that appears now. It seems almost absurd that we were ever cautious about whether the universe made planets efficiently. Now we are on the cusp of finding out just how many small rocky, even Earth-like, ones there are across the galaxy. This is arguably the biggest single advance towards dealing with question 10 in a real, non-speculative, fashion since Galileo and Copernicus opened the floodgates of astronomical reason. This is one thing that makes modern astrobiology such a compelling and exciting field. The fog is lifting on a scientific path that will eventually lead us to a census of worlds, and ultimately of their contents. Slowly we will push out further from Earth, and in doing so will further and further narrow the options for intelligent life.
 
Finally, one cannot help but feel in the most non-scientific way imaginable, that it would be so bleak and awful for such an extraordinary phenomenon as ourselves to be alone in the void that we must strive our greatest to find out the truth. Even if the ultimate answer is that yes, we are a singular event, then it would still seem that the mere act of looking, the scientific and technological effort required, would surely play some role in ensuring that we do not vanish into the gaping maw of evolutionary extinction. If we just 'are', with no rhyme or reason, then it behooves us to revel in our existence, embracing the cosmos as the very fiber of our being that it is.

The ten most important questions for astrobiology: Number 9

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

Is there microbial life elsewhere in the universe?

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

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

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

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

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

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

The ten most important questions for astrobiology: Number 8

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

Is there a relationship between interstellar chemistry and life?

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

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

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

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

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

The ten most important questions for astrobiology: Number 7

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

Is the Earth unusually suitable for complex life?

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

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

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

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


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

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