Looking forward

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

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

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

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

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

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

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

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

The ten most important questions for astrobiology: Number 6

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

Can life spread through space itself?

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

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

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

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

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

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

Voyager to the stars

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

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

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

The ten most important questions for astrobiology: Number 5

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

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

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

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

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

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

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

Catching the rays

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

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

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

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

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

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

The ten most important questions for astrobiology: Number 4

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

How does life originate?


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

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

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

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

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

Arsenic and old lace

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

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

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

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

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