The ten most important questions for astrobiology: Number 3

It would be easy to argue that the next question is too big, to broad, and too unanswerable to be in the list. Nonetheless it's a question that keeps cropping up and is also representative of the bigger and deeper issues that astrobiology must ultimately tackle. So, question number 3:

Is this universe particularly suited for the phenomenon of life?

Because this is a bit ambiguous there are some necessary qualifications. First, implicit in the way I've phrased this question is the idea that 'this universe' is either one of many or that it represents an instance of a phenomenon that somehow reoccurs. Second, and this is really just an expansion of the question, it suggests that life requires a particular range of physical laws and/or 'contents' in a universe to happen. Third, this question hits the button to sound the anthropic principle klaxon, opening a Pandora's box of weak and strong arguments about the timing of life, its privileged position and even its necessity for the existence of the universe in the first place.

It can be quite a morass, so I'll try to stick to the easy bits. The second item above can also be rephrased as a question about what the minimum requirements are for life to show up at some point, and whether some physical laws are more or less important. If we stick with carbon-based life then there's a ready laundry list, not least of which is the need for carbon in the first place. Carbon comes from the triple-alpha process in stellar nucleosynthesis. Intriguingly this set of nuclear reactions hinges critically on resonances between nuclear energy levels, allowing beryllium and helium nuclei to gracefully flop into forming a new carbon-12 nuclei. Without that coincidence then carbon would be little abundant in the universe, rather than the 4th most abundant element in the universe - after hydrogen, helium and oxygen. This resonance hinges in turn on the value of the fine-structure constant, describing the universal strength of electromagnetic interaction. Change this constant by a few percent and among the consequences you can kiss carbon goodbye.

Even if you feel you can do without carbon - preferring some alternative life chemistry - then a different fine structure constant also messes up things like covalent bonds in molecules, at least compared to the way they are around us. Before even getting to details like this if the universe had a different overall composition, or differing gravitational constant, or differing CP violation, then it could be radically different from the outset - perhaps never making any structures beyond individual atoms. In brief, there are a lot of things that seem to have to fall into place for something like us to come along.

Because we feel (rightly) incredibly uncomfortable with any notion we might be special to the universe (the universe clearly being special to us) then things get sticky, and answering question 3 hits a wall....unless we get to the first qualification I give above: the universe is one of many, whether contemporaneous (multiverses) or one of an endless succession (cyclical). An infinite number of universes, or quantum realities, are like the infinite number of monkeys with typewriters - somewhere amongst them will be the ones that can produce life like us. Much like the realization that carbon production requires something special - and indeed how Fred Hoyle came up with the physics for it - it may be that a version of the anthropic principle is actually telling us that there must be multiple universes, or else things are even more bizarre.

Astrobiology can't answer 3 yet, at least not in any satisfactory way. But finding out more about life in the universe, carbon-based or otherwise, could just possibly nudge us in the right direction.

The ten most important questions for astrobiology: Number 2

Question number 2 is close to home: Is there life elsewhere in our solar system?

For all the talk of exoplanets and habitable worlds out amongst the stars, our own humble solar system probably remains the most likely place for us to find the first convincing evidence for non-terrestrial organisms. Not only are there plausible environments for life to survive, they're close enough that we might hope to gather in-situ data. Mars is a great example, and for a mix of historical and practical reasons is still our main target for exobiological investigation. Evidence for past and present aqueous environments, as well as ongoing atmospheric chemical skulduggery - albeit contested and curiously correlated with some funky geophysical locations -  reinforce Mars as a critical target for astrobiology. The extraordinary high-def mapping of the martian surface by recent missions is key to helping us home in on places that might offer the best clues, and provide targets for upcoming missions like Curiosity, nee Mars Science Laboratory.

A sub-question that Mars raises, with its likely diverse climate history, is whether we're looking for fossils or living breathing organisms. In that context the real question at hand is perhaps better stated as whether there has ever been life elsewhere in the solar system. This opens up a whole slew of intriguing environments (while not excluding the possibility of extant life in any of them). The list includes: comet nuclei, big asteroids or dwarf planets, and moons like Europa, Ganymede, Titan, and Enceladus.

Cometary nuclei are more diverse than expected, the recent Hartley 2 flyby providing spectacular evidence of this. We already knew these bodies to be rich in organic chemistry and while finding signs of life within them may be pushing it a bit, they actually hit all the buttons for 'follow the water' and 'follow the chemistry' type strategies. As we move further out in the solar system then the typical volatile content (water and hydrocarbons) of bodies tends to increase - a characteristic left from  the proto-planetary disk temperature structure 4 billion years ago. The great moons of Jupiter and Saturn offer an extraordinary range of environments, from potentially vast subsurface oceans of liquid water (albeit likely full of noxious solubles) within Europa or Ganymede, to possible 'micro-environments' in Enceladus, to the novel and little probed cryo-chemistry of Titan's surface. A long way from the Sun these potential habitats can have a major energy input from phenomena like tidal heating, itself ultimately derived from nothing other than angular momentum.

Then there are nooks and crannies, some not so small, that may surprise us. The upper atmosphere of Venus can be positively temperate at the right altitude, and here on Earth it's increasingly apparent that microbial life among the clouds may be significant and substantial. Venus might not be a place to overlook after all.

The rub, and there is always one, is that getting to these places is tough. Then getting to the right spot, whether in a chasm on Mars or beneath ten kilometers of ice on Europa, is even tougher. Then there is the thorny issue of finding evidence of life if it's sparse. There are no magical CSI-like probes that conveniently produce the answers, and the problem of forward-contamination is ever-present. Nonetheless, question number 2 remains both very important and very much within our grasp, especially if we get past our space-exploration hangovers and recognize the critical role that our great wilderness in the sky can play in humanity's future.

The ten most important questions for astrobiology: Number 1

As our year approaches its end, with the planet slithering ever closer to where it was 12 months ago and towards its closest approach to the Sun on January 3rd, it seems like a good time to indulge in a highly biased and incomplete bit of rumination. There have been many tremendous discoveries and advances this past year, each inching us closer to tackling some of the core questions about life in the universe. Nonetheless, lots of big and important questions remain. Some approach the philosophical, some are quite narrow, but they're all interesting, and many overlap greatly. I thought I'd do a series of posts on the ones I particularly like, you may agree or disagree with the choices - a reasonable and necessary part of the process - but hopefully they will stimulate.

Number 1 is relatively non-challenging: Are there other planets like the Earth?

We're much, much closer to answering this than we were a year ago. As surveys for exoplanets increase in sensitivity, sample size, and sampling time it appears that small rocky worlds are essentially ubiquitous. With statistical estimates that at least 1 in 4 Sun like stars harbor Earth-sized planets within their habitable zones (extrapolating from shorter period objects) there is good reason to believe that Earth mass planets in comparable orbits are definitely out there. Kepler will help nail the rates to the wall. It will remain a tough question to tackle at a deeper level though. 'Like the Earth' is a bit ambiguous. Mass and orbit are one small piece. How many of these worlds have comparable chemistry, geophysics and climate? A good bet is that there are numerous cousin planets, recognizable but nonetheless a bit alien. That may of course be just fine. We're awfully biased about ourselves, often without recognizing the fact. One of my favorite mantras is that the modern Earth is not typical of our homeworld throughout its history. The suitability of the Earth today for life is a poor template to use.

So, if we treat 'like' as a broad qualification then the answer is almost certainly yes, and in another 12 months we'll have even better evidence supporting this.

Our leaky galaxy

A while back I wrote about the leakiness of our solar system - how significant amounts of material in the outer regions around the Sun could have really come from other stellar systems. Then I got on a bit of a jag about planets in other galaxies and the impending collision (well, one has to take a long-term view) with the Andromeda galaxy. Then lo and behold, like a fabulous new ice-cream flavor, someone turns up a new planet in the Milkyway that almost certainly came from another galaxy.

HIP 13044b is a giant world a little more massive than Jupiter orbiting a red giant star every 16 days, reported by Setiawan et al. in Science. Detected via the tell-tale velocity wobble of its host planet it might not garner much attention except for the fact that this system is part of the so-called Helmi star stream, some 2000 light years from us. The Helmi stream is the remnant of a dwarf galaxy that dove into the Milkyway sometime between 6 and 9 billion years ago, becoming shredded by galactic tidal forces.

Clearly the original host galaxy for this system was pretty puny, and one might argue that it was never truly distinct from the Milkyway - merely a part of its halo entourage, like some loose hair or slightly wayward limbs that would inevitably get subsumed. Nonetheless, it has some quite striking implications. The host star has an extremely low heavy element content - about a hundredth that of the Sun. This is the least element rich star yet found to host any type of planet. So the fact that it has at least one chunky world orbiting it is very interesting since lower heavy element abundance in stars correlates with a lower probability of giant planets. It's also an old and doddery star, well past its prime and has swollen in size as a result - possibly engulfing even shorter orbit planets, and possibly shedding some element-rich outer atmosphere.

Taken altogether it's another tick mark for planet formation being both a universal phenomenon and apparently a quite efficient one. As we learn more about the stellar populations in our galaxy we see just how messy they really are. Satellite galaxies can dive through the galactic plane, not just shedding their stars but also pulling out bona-fide Milkyway residents in their gravitational wakes. There may well be stars and planets that formed snug in our galactic disk that are now arcing over us, dragged tens of thousands of light years away from home. Remarkably, the opportunity to study planets with truly extragalactic origins may be all around.

The grains of Mars

We know that something is afoot on Mars. Apart from the surprising and intriguing measurements of atmospheric methane plumes there is clearly something funky about the chemistry of the Martian atmosphere that cleanses it of this organic gas in very short order. Various ideas have been put forward, including the possibility that there is some important effect of the dust that is often lofted skywards in the great Martian duststorms.

Now a new work here on Earth suggests that both our planet and Mars may share something in dusty atmospheric chemistry. Shaheen et al. report in the Proceedings of the National Academy of Sciences that they have painstakingly identified a set of chemical reactions that occur between ozone, water, and carbon dioxide on an incredibly thin layer on the surface of atmospheric dust. The smoking gun is an enhancement of the oxygen 17 isotope in carbonate material on the dust grains compared to other natural carbonate sources. Isotopic fractionation points towards a different chemical history, involving the formation of hydrogen peroxide on the dust grain surfaces.

Remarkably, a similar isotopic excess was known to lurk in one of the famed Martian meteorites from the Allan Hills finds in Antarctica. While this doesn't necessarily prove a common chemical mechanism it is very suggestive that the type of grain surface chemistry that occurs in the Earth's atmosphere may happen on Mars too. Dust-grain surfaces offer a fast way for chemistry to occur by capturing reactants into a solid/liquid layer. As Shaheen et al. point out, the unusual carbonate deposits in this meteorite that have provoked so much intrigue as potential micro-fossils could possibly just be aerosol produced carbonates - we just didn't know about this mechanism for their formation.

Dust grain chemistry is not just confined to planetary atmospheres. Interstellar and interplanetary dust (admittedly far more microscopic than most terrestrial or Martian dust) plays a critical role in astrochemistry - accelerating reactions that might otherwise take an eternity, and engaging in all manner of cycles for molecular chemistry in the relatively warm and wet environment of protoplanetary disks. Given that it's perhaps not so surprising that it should play an important role in the soupy airways of rocky planets. Interesting questions remain - does this chemistry offer clues to Mars' ability to scrub itself clean of methane, and can it provide another window into paleo-climate both on Mars and the Earth?

From Andromeda with Love

The galactic theme in the context of planets and life is an interesting one. Take our own particular circumstances. As unappealingly non-Copernican as it is there is no doubt that the Milkyway galaxy today is 'special'. This should not be confused with any notion that special galaxy=special humans, since it's really totally unclear that the astrophysical specialness of the galaxy has significant bearing on the likelihood of us sitting here picking our teeth. Nonetheless, the scientific method being what it is we need to pay attention to any and all observations with as little bias as possible - so asking the question of what a 'special' galaxy might mean for life is ok, just don't get too carried away.

First of all the Milkyway galaxy is big. As spiral galaxies go it's in the upper echelons of diameter and mass. In the relatively nearby universe it and our nearest big galaxy Andromeda are the sumo's in the room. This immediately makes it somewhat unusual, the great majority of galaxies in the observable universe are smaller. The relationship to Andromeda is also very particular. In effect the Milkyway and Andromeda are a binary pair, our mutual distortion of spacetime is resulting in us barreling together at about 80 miles a second. In about 3 billion years these two galaxies will begin a ponderous collision lasting for perhaps 100 million years or so. It will be a soft type of collision - individual stars are so tiny compared to the distances between them that they themselves are unlikely to collide, but the great masses of gas and dust in the two galaxies will smack together - triggering the formation of new stars and planetary systems.

Some dynamical models suggest that our solar system could be flung further away from the center of the merging galaxies, others indicate it could end up thrown towards the newly forming stellar core of a future Goliath galaxy. Does any of this matter for life? For us the answer may be moot. In about only 1 billion years the Sun will have grown luminous enough that the temperate climate we enjoy on the Earth may be long gone. In 3 billion years it may be luminous enough that Mars, if not utterly dried out and devoid of atmosphere by then, could sustain 'habitable' temperatures. Depending on where the vagaries of gravitational dynamics take the solar system as Andromeda comes lumbering through we might end up surrounded by the pop and crackle of supernova as the collision-induced formation of new massive stars gets underway. All in all it doesn't look too good. But for other places, solar systems that we see forming today, it could be a very different story.

Imagine a terrestrial world just beginning to form right now. By the time Andromeda is ploughing through our galaxy in 3-4 billion years, merging and settling, there could be a species sitting there writing a blog about what a 'special' time and place they exist in. They might marvel at how fortuitous it was for their sun to be thrown further out from the new galactic center - away from all those nasty supernova. They might stare in awe at the myriad new planetary systems being formed as the gas from these cousin galaxies collides, a golden era for new worlds. They would doubtless write papers on how galactic mergers might be signposts to highly habitable regions of the universe - unlike all those boring isolated galaxies out there. It sounds like they're getting carried away, but really they're not doing anything we don't already. Getting perspective is one of the most difficult things to do, especially from our tiny mote of existence.

The Galaxy is Not Enough

Megalomania can sometimes be good. Well, with a lot of emphasis on the 'sometimes'. Here we are as a species, scooping up detections of planets around other stars at a ferocious rate. Our galaxy is however a big place, and we're going to be sifting through its two hundred billion or so stars for a very, very long time. Eventually though, presuming that we remain aware of the deeper meanings of the word civilization, we might find ourselves not content to be just parochially curious. We talk about life in the universe but the Milkyway is merely one of a couple hundred billion galaxies in the observable cosmos. Can we seek out planets in other galaxies?

Even now the answer is a surprising 'almost'. A paper by Ingrosso et al. about a year ago in the Monthly Notices of the Royal Astronomical Society, discusses in detail the possibility of seeing evidence for planetary mass objects in gravitational microlensing events occurring in the Andromeda galaxy (M31) - a mere 2.5 million light years away across our backyard. As with lensing detection of planets in our own galaxy the mechanism is that a foreground star with planets (in this case within Andromeda) acts as a gravitational lens on the light from a background star (also in Andromeda), dramatically magnifying it as the two systems drift into close alignment from our point-of-view. At the distance of Andromeda the best we can do is see 'pixel-lensing', which is a variation in the number of photons coming from individual pixels in our digital cameras, each of which contains the light from many indistinguishable stars - too close together on the sky to be separated. During magnification the tiny lensing effect of planets can also be greatly enhanced, adding characteristic spikes and wiggles to the way the light varies during the alignment.

By carefully modeling what one might expect to see, Ingrosso et al. demonstrate that there is a pretty good chance that in a few percent of Andromeda lensing events you should find the signs of Jupiter-scale planets if they are there. They even go so far as to offer support for the detection of a 6 Jupiter mass planet in a rather funky lensing event seen in Andromeda by An et al. with the gripping name of PA-99-N2. As with all micro-lensing studies there are huge challenges owing to the amount of data needed to spot events and to fully characterize them as they play out over days to weeks. The pay-off is an incredible statistical cleanliness and sensitivity.

Probing for planets in Andromeda may therefore be fruitful. What of further afield? There's the rub. The same technique might be applied to more distant galaxies, but it will require bigger telescopes, capable of fitting more pixels across the faces of these stellar swarms. It could be worth it though. Suppose we were able to evaluate the planetary populations (albeit statistically) in a representative chunk of intergalactic space. That would be an incredibly interesting set of numbers. It would tell us what galaxy types - elliptical to spiral - are most fertile grounds for other worlds, and take us closer to answering the question of the true capacity of this universe for life.

Necropanspermia

It's a fitting title for a few days past Halloween. Sugar withdrawal and the odor of gently scorching pumpkin fresh in our minds. This term - necropanspermia - is a new one to me, and comes from a thorough and carefully put together paper that popped up this morning from Paul Wesson. Although it is ostensibly a review of the ideas of panspermia (and a nice balanced one at that), it also gets to some rather interesting points.

The transfer of planetary - potentially life-carrying - material between stellar systems seems only likely for tiny particles. Dust grains of a few to a few tens of microns across are the ultimate solar sailors. The radiation pressure of photons from one star accelerates these minute pieces across interstellar space. Routes that lead into new solar systems result in the gentle deceleration of the dust as it plunges into the photon cloud of a star, and the possibility exists of being swept into the atmosphere of any handy planets - just as happens here on Earth. The question is what, if any, viable organisms can hold up over millions of years of interstellar transport - subject to cosmic rays and high-energy photons - to re-plant on a new world?

Wesson's discussion takes a rather interesting direction. He argues both that viral material may be prevalent, and that this type of fragmented DNA/RNA may be quite sufficient to help 'seed' life in new environments. One motivation for suggesting this is the conclusion that most transferred organisms are dead-on-arrival, there just doesn't seem to be a way that intact DNA and/or cells are going to survive their trans-galactic journeys. Another motivation is simple physics - a microscopic dust grain might barely have space for one poorly protected bacterium, but it could easily harbor a hundred much smaller viruses.

The hypothesis that there was an ancient 'viral world' here on the Earth - complete with unique viral genes that help replication - has been around for a while. While even viruses might not arrive intact after interstellar transit, this might be far less of a problem, since they play fast and loose with their molecules in the first place. It's an interesting twist to the long and checkered history of the idea of panspermia - perhaps we should be rushing cosmic dust particles from high in our atmosphere off to the labs for some quick looks for viral DNA?