Life on Enceladus?
A very interesting hypothesis paper in the latest issue of Astrobiology asks the question: could life exist on Enceladus? More to the point, could life exist under Enceladus… because this little moon (its radius is only about 250 km) of Saturn is covered by water ice but appears to have a subsurface reservoir of liquid water. Images taken by the Cassini probe clearly show about a dozen jets of particulate ice shooting from the southern pole of the moon. Samples of this geysering ice were analyzed by onboard instruments on Cassini as it passed through one such plume, and the results indicate the presence of water, nitrogen, carbon monoxide (maybe) and some simple organic molecules.
In the new Astrobiology paper, Christopher McKay (at NASA Ames) and coworkers examine whether life under Enceladus is feasible, given what we know about the tiny world specifically and the chemical and energetic requirements for life in general. McKay and company conclude that there are several paths by which any hypothetical microbes on Enceladus could make a living.
One way life could make a go on Enceladus is by producing methane. Methanogens on Earth do this, by using hydrogen produced by chemical weathering of rocks, combined with carbon dioxide, to generate methane and biochemical energy. Such a metabolism might be feasible under the Enceladan ice. Another path might be for deep microbes living in a subsurface water ocean to glean energy from radioactive decay, through radiolytic oxidation of chemicals there. Some microbes on Earth make a living from this weird approach, as I’ve reported previously here. That doesn’t mean it’s actually happening on Enceladus, but a big part of science is trying to figure out what’s possible and what’s precluded. This new paper makes the case that life under Enceladus is physically possible, and offers some ways life there might be detected in future missions, even without landing and drilling.
Personally, I remain skeptical of life getting an independent start under Enceladus. The energy might be there, but liquid water under the surface might be a more recent phenomenon. The conditions enabling life there might have only been intermittent through the last 4.6 billion years. There may not be enough radioactive elements in the interior to support continuous heat generation for this long. And so on. But those are ifs that don’t yet have answers… and there is only one way of really getting them: go back.