Origin & evolution of Mimas (Neveu & Rhoden, Icarus, 2017)

The origin and evolution of a differentiated Mimas

Marc Neveu, Alyssa R. Rhoden

Icarus, 296, 183-196 (Link)

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The second paper describes possible paths for the evolution of Saturn’s “death star” moon Mimas. Mimas is Enceladus’ sibling moon, but unlike Enceladus (where tides from Saturn heat up the interior to create an ocean that is being erupted into space) Mimas looks pretty inactive. This is weird, because Mimas is actually closer to Saturn and its orbit is more elliptical than that of Enceladus, so tides on Mimas should be 30 times stronger. Another oddity is the way Mimas sways along its orbit, which suggests that it has a rocky core and an icy shell (it’s not just a homogeneous mix of rock and ice), but for rock and ice to separate, ice had to melt at some point. The thing is that when ice warms up to melt, it becomes all squishy and that should really heat Mimas up (just like driving with under-inflated, squishy tires heats them up, which wears them out) and make its orbit more circular, which is not what we’re seeing.

To find out how Mimas could separate rock from ice/water without showing any sign of activity and without getting its orbit more circular, we’ve built a model that accounts for all of these effects. We find two possible solutions:
  1. Mimas formed very early and hot, its ice completely melted, letting the rock sink through and making its orbit a circle. Then, it froze quickly and had 4.5 billion years to get beaten up by impacts and for its orbit to become more and more elliptical due the (tiny) tides that Mimas raised on Saturn;
  2. Mimas was always cold and inactive, but formed from the rings of Saturn. Mimas’ rocky core formed first, because rock has more cohesion and so is less easily torn apart by tides, then the ice was added on around it just like when building a snowman. The ice didn’t get torn apart, because at that point Mimas had begun moving away from Saturn by raising tiny tides on it (just like our Moon moves away from us because of the tides it raises twice a day). Further away, the tearing forces were not so strong. This way we also get a two-layer Mimas that is inactive and on an elliptical orbit.
We tried to use the same model for Enceladus, but it didn’t work so well. We’ve found a better way, and we’re writing the paper right now, stay tuned…