Asking whether there’s life on Saturn’s moon Tethys is a little bit of a trick question, because if living microbes are being expelled by the geysers of Enceladus, it’s very likely that Tethys—which passes through these geysers—picks up some of them. But what happens next? Can they actually thrive, and evolve, on the surface of Tethys?
It’s unlikely, but not impossible. As we’ve previously discussed, life on a planet or moon without an atmosphere almost certainly depends on an underground ecosystem. For every form of life we can predict, that means an underground ocean. We can be reasonably confident that Tethys, which is made up mostly of water ice, had just such an ocean at one point; it’s difficult to imagine how it would have one now unless Tethys had geological activity to keep the water in liquid form, as the surface temperature averages out around -305°F/-187°F.
One 2008 study found that there is some evidence of geological activity on Tethys in the form of ammonia hydrate:
The detection of ammonia hydrate on the surface of any body in the Solar System is of particular interest because its presence enables geologic activity. The melting temperature of ammonia hydrate [is] considerably lower than that of pure water ice. On a saturnian satellite, photolysis would deplete [ammonia ice] …Therefore, the presence of ammonia hydrate on the surface of an icy satellite implies recent emplacement, possibly by cryovolcanic activity.
That said, the evidence for geological activity is otherwise fairly weak. Photographs from the Cassini probe seem to point to a flat surface that doesn’t change its shape often—the 280-mile-wide Odysseus crater has sat largely undisturbed on Tethys’ surface for a billion years—and obviously that does not tend to correspond with geological activity. We won’t know whether there’s an underground ocean until we’ve actually investigated the moon in more detail, but right now the evidence for it—and, subsequently, the evidence for life on Tethys—seems to be fairly weak.
But Tethys could tell us whether or not there’s life on Enceladus; in the process of collecting matter from Enceladus’ geysers, Tethys could function well as a museum of frozen extraterrestrial microbes. If there’s a feasible way to have an unmanned probe scrape Tethys’ surface for samples, it could pay off.