Now in press at Astrobiology is a look at the possibilities of life on Enceladus that holds out hope for detecting biomarkers with data gathered during a Cassini flyby. That’s an exciting possibility, depending as it does not on an orbiter or lander mission from an indefinite future but on equipment we’ve currently got in Saturn space. And the Enceladus picture remains fascinating because of the possibility that some microbial systems on Earth that operate far beneath the surface may offer examples of how life could evolve on a cold and distant moon of Saturn.
We’ve already found a dozen icy particle jets coming out of Enceladus’ south polar regions, all pumping material into a plume that extends for thousands of kilometers. A 2005 Cassini flyby revealed, among other things, water vapor, methane and simple organic compounds, even as other Cassini instrumentation showed the moon’s south polar region to be anomalously warm. If there is liquid water under the south polar region, could life have evolved there? If so, the paper raises the possibility that methane may be a biomarker. For that matter, could life have come there from elsewhere? The paper argues both are possible:
The two categories of theories for the origin of life on Enceladus have different implications for how long habitable conditions must persist there for life to be probable. For the panspermia theories, life arrives at Enceladus intact ready to reproduce. In this case no origination time is required and life can utilize a pre-existing habitable environment instantly. For the organic soup or chemosynthetic theories for the origin of life within liquid water aquifers in Enceladus, the best we can conclude is that the origination time could be as low as 10 Myr or as along a 500 Myr. As discussed above, there are no direct geophysical estimates of the persistence of the jet activity on Enceladus. However, estimates for the timescale to freeze an ocean on Enceladus are ~30 Myr… which may be consistent with the timescale for the origin of life. Thus, there is no reason to conclude that any liquid water on Enceladus is too young to have been a site for the origin of life.
All of which is consistent with the possibility of living systems. Christopher McKay (NASA Ames) and colleagues run through three Earth ecosystems found deep below the surface that do not depend on oxygen or organic material produced by photosynthesis, relating these to plausible energy sources under Enceladan ice. An exciting possibility is that biological materials might be carried out into the plume streaming from Enceladus, available for future sample return missions. But Cassini itself, in its extended mission, may be able to use another flyby to determine whether the methane and other hydrocarbons found in the plume are consistent with biological origin by looking at the ratio of non-methane hydrocarbons to methane. The biological signature should be distinctive.
Fascinating stuff, as is the issue of whether any detected life would be biochemically related to life on Earth (via, perhaps, Solar System-wide panspermia) or completely different, evidence of a ‘second genesis.’ As the paper notes, both finds would be of huge interest, but the second would offer prospects both more exciting and more daunting in terms of detection, with life perhaps using a different set of basic molecules. The paper is McKay et al., “The possible origin and persistence of life on Enceladus and detection of biomarkers in the plume,” in press at Astrobiology.