It’s no secret that Saturn’s moon Enceladus contains an ocean of liquid water, sprayed into the sky by geysers on the surface. This is what makes Enceladus such an exciting candidate for extraterrestrial life. But if you look at a little closer to Saturn, you’ll find a frozen little moon that bears an uncanny resemblance to the Death Star—and it’s Mimas’ probable lack of an underwater ocean, despite its closer proximity to Saturn, that has challenged many astronomers’ theories of how underground oceans are formed and sustained.
As the Cassini page on Mimas explains:
[Mimas] seems to be solidly frozen at a temperature of -209 degrees Celsius (-344 degrees Fahrenheit). This is puzzling because Mimas is closer to Saturn than Enceladus, and the Mimantean orbit is much more eccentric (out of round) than the Enceladean orbit. Thus, Mimas should have much more tidal heating than Enceladus … This paradox has led astronomers to use the "Mimas test" by which a theory to explain the partially thawed water of Enceladus must also explain the entirely frozen water of Mimas.
The Mimas test hurdle becomes easier to meet if we can find some evidence of water pockets beneath Mimas’ surface, but so far there’s no evidence of such. One 2012 heat map of Mimas gave some observers hope that there was water under the surface, but NASA scientists tend to believe that this merely reflects differences in surface temperature brought on by electron collision or some other general factor. Compare to the shape of the 2005 heat map of Enceladus, which practically telegraphs an underwater ocean at the south pole, and it’s hard to explain how such a wide distribution of surface heat could be indicative of anything warm underground.
That said, Mimas is 246 miles in diameter and close enough to Saturn to benefit from internal tidal heating; the possibility of liquid underground water, while remote, can’t completely be discounted. And if Mimas itself is an unlikely candidate for extraterrestrial life, the differences between it and other, more habitable moons can help us understand the astrophysical forces that might shape life on other worlds.