(Title borrowed from Kurt Vonnegut)
In the novel and film 2010, when the Monolith builders force Jupiter into nuclear ignition they also program poor put-upon HAL to broadcast, non-stop, "All these worlds are yours except Europa. Attempt no landings there."
Arthur C. Clarke was deemed uncannily prescient when he wrote this, because many astrobiologists believe that life may exist under Europa's thick ice crust; the moon harbors an underground water ocean and has geothermal energy courtesy of its huge planet. But recent news from the Cassini-Huygens mission could prove the prophet wrong. Before we encounter life on Europa, we may find it on Titan.
The Cassini data essentially show complex surface chemistry, as the Voyager data did for Mars. They also show mysterious absences of items expected to be abundant, given Titan's specifics - acetylene and hydrogen in particular. Such results always carry the cautionary sentence that "non-biogenic processes yet unknown" could cause these anomalies. But organisms feeding on the missing chemicals are definitely on the list of these processes, something that several astrobiologists (Chris McKay, Derek Schulze-Makuch, David Grinspoon) suggested five years ago by speculating that acetylene would be tasty to Titanian life.
Titan, unlike bone-dry Mars, has enormous lakes - although they contain liquid hydrocarbons, rather than water. The lightest in that family (methane and ethane) are poor solvents because they're non-polar, unlike water and ammonia. Nevertheless, they do act as solvents for the rich organic soup churned by Titan's thick atmosphere of ammonia and methane (Carl Sagan's "tholins," from the Hellenic word for murky). And although chemical reactions will be slow in Titan's ambient temperature of -190 Celsius (room temperature is 24 Celsius), all bets are off once enzymes are involved.
If we can conclude definitively that there is life on Titan, we will have walked one step further to the right of the Drake equation. We share material with Mars by meteorite exchange, so any life that existed or still exists there may have shared its beginnings with us. There can be no such ambiguity for Titan, given its distance and conditions. Whatever we find there, from bacteria to placidly grazing hydrogen balloons, it will be the product of an independent genesis. And it will be very different from us, finally making it possible to rigorously determine which aspects of life are parochial and which are universal.
This brings us full circle to HAL's warning. If life exists elsewhere in the solar system, it will be both a boon and a burden. Such a discovery will give a major boost to astrobiology, which will finally have a legitimate topic to explore beyond the armchair vaporings of famous physicists - and to crewed space exploration beyond the depressing and trivial prospect of sending more people in fungus-infested tincans into low terrestrial orbit.
At the same time, as I wrote elsewhere, we may destroy alien life even if we are careful. Such an outcome will deprive us of precious, irrecoverable knowledge that will help us make sense of our universe and our own planet, even if the new life consists entirely of bacteria (to say nothing of the moral equivalent of genocide if it's more advanced than that). It may be that none of these worlds are ours, except for us exploring them and becoming their stewards.
Note: This article also appeared on the author's blog, with two nifty images and the references that discuss the Cassini-Huygens data.