Ice and Fire

It's pretty much a golden age for exobiologists. Once the preserve of cranks, science fiction writers and supermarket tabloids, the search for evidence of life on other planets is may still be based on speculation and extrapolation rather than actual hard evidence, but it's now a sober and respectable area of legimate scientific research, covered soberly and respectfully in popular science magazines and broadsheet newspapers.

Latest headlines concern findings by the Cassini spacecraft: it seems that not only is hydrogen in Titan's atmosphere flowing down to the moon's surface and disappearing, but there's a distinct lack of acetylene at the surface, too; acetylene should be formed from methane and ethane and snow out onto the surface, but so far it hasn't been detected. Most likely, the disappearance of hydrogen and low levels of acetylene are due to some kind of catalytic chemical process, but exobiologists have pointed out that it could be the signature of some form of biological activity. The surface of Titan is far too cold to support any form of life that uses water as a solvent, but there's abundant liquid methane and ethane, which rains out of the atmosphere, carves rivers into the surface, and forms lakes and seas. Acetylene would be the best energy source for life based on liquid methane, which would consume hydrogen much as we consume oxygen. Right now, on Titan, some frigid, curious slime mould could be edging its way with infinite patience across a pebbly beach towards the Huygens lander, wondering if it's evidence for life above the eternal clouds.

Jupiter's moon Io is one the least likely venues for life in the Solar System. Although it's only a little larger than Earth's moon, it exhibits extremes of geological activity. Tidal effects stretch and flex its crust, and the resulting friction generates enough heat to power volcanoes that spew molten sulphur at temperatures of over 1600 degrees Centigrade. Areas between the volcanoes are covered with sulphur dioxide snow chilled to -130 degrees C, almost all water has long ago been driven off, and besides all that, it's drenched in Jupiter's radiation fields. But astrobiologist Dirk Schulze-Makuch has pointed out that Io would have possessed plenty of water when it first formed, and if life arose before the water was blasted away by Jupiter's radiation, it could have retreated underground, into rocks or even lava tubes, which would provide protection from radiation and conserve and moisture and nutrients (and would be ideal homes for any intrepid human colonists).

It's unlikely, but not impossible. Extremophile bacteria, the poster children of exobiology, exploit all kinds of forbidding niches on Earth, from hydrothermal vents to Antarctic dry valleys, and lithotrophic extremophiles extend the biosphere deep into the Earth's crust, obtaining energy from all kinds of organic and inorganic sources. The oddest example is Desulfurodis audaxviator, discovered in 2008 deep in a South African gold mine. It survives without light or oxygen, using energy generated by the radioactive decay of uranium and other elements in the surrounding rock to drive its thrifty metabolism. When you contemplate this microscopic, one-species, completely self-sufficient ecosystem, life on Io or Titan doesn't seem so unlikely after all.