As we’ve discussed before, Mimas is a weird little moon. It’s a dead ringer for the Death Star, it’s close enough to Saturn to benefit from tidal heating but still appears to be frozen solid (which complicates our understanding of how tidal heating works, to the point where any new theory of the process now has to jump through a hoop called “the Mimas test"), and its pattern of surface heating inexplicably resembles Pac-Man rather than the pattern we expected to see.
Given all that, you might expect this is the part where the new stuff we learn about Mimas makes it gradually seem more normal and boring. And you’d be wrong. New Cassini data has revealed that Mimas is, as Space.com’s Kelly Dickerson put it, “weird inside”:
Mimas' rotation and its orbit around Saturn make the moon look like it's rocking and back forth and oscillating similar to the way a pendulum swings … [U]sing images of the moon captured by the Cassini spacecraft, Radwan Tajeddine, a research associate at Cornell University, discovered that the satellite's libration was much more exaggerated in one spot than predicted. He expects it must be caused by the moon's weird interior.
Could this mean Mimas has an underground, and potentially life-supporting, ocean? Probably not, but it’s a possibility. The explanation that seems to have the most currency is the idea that Mimas has an oval-shaped core, but then we have to ask the quite reasonable question of how it got an oval-shaped core, and what that oval’s composition is, and so on.
Whatever explanation we settle on to explain Mimas’ wobbliness, it’s a safe bet it will both solve and further complicate the “Mimas test," changing our understanding of how planetary gravity affects the composition of satellites. Which is completely fine, because it’s not as if we needed to already know what was going on. We just thought we did. And there, once again, was Mimas—reminding us that astronomy is still a very young science, and that there’s still an awful lot of theory we haven’t written yet.