NASA scientist and astrobiologist Chris McKay said about a year ago that the center of astrobiological interest was probably not Mars, as many tend to assume, but more at the moons of the outer planets. Certainly the solid evidence of a liquid ocean on Europa has for many years raised astrobiological interest, with speculation that, where there is a stable quantity of water, there could likely be life. That possibility has led to making a Europa orbiter mission a first priority of planetary scientists, although we still await the formal report of the Planetary Decadal Survey of the National Research Council evaluating priorities.
Europan life is probably a long way from being discovered even if it exists. Ocean-based life would be buried below the moon’s ice layer, which could be quite thick, and there are no plans, sadly, for including a lander—let alone a drill—on the Europa mission.
But the possibility of life in the water has dramatically changed our view about the habitability of other worlds. This past year has seen a number of reports about the possibility of life on Titan, as well: in the atmosphere, on the surface, or in its hydrocarbon lakes. Some suggest there could be a methane-based bacteria and chemistry based on the liquid cycle of hydrocarbons rather than water. For those who find life without water too improbable there is Enceladus, another Saturnian moon of astrobiological interest, where salty water has been observed and measured by the Cassini spacecraft.
Even more bizarre there is now a suggestion, in a paper by presented at last month’s American Geophysical Union meeting, that Pluto may have enough internal heat from radioactive decay in rocks to create a liquid ocean under its frozen surface. Where there is water, there might be life. And while there is little suggestion of life on an asteroid, there are observations of amino acids and organics in meteorites, which lend credence to speculation about asteroids carrying the building blocks of life to the planets.
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