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.