The evolution of a young ocean within Saturn’s moon, Mimas

Mimas is a small, icy moon (R=198.2 km) with a heavily-cratered surface, sparse tectonic features, and no evidence of past or present cryovolcanic activity. It has a high eccentricity (0.0196) and orbits close to Saturn (3.2 R_S), which ought to raise significant tides if Mimas’ interior is deformable. Taken together, Mimas’ geology and orbit suggest a frozen moon that has experienced negligible tidal heating over its surface age. However, analyses of Mimas’ librations and the precession of its orbit indicate that a global ocean under a 20-30 km thick ice shell provides the best fit to the observations. Studies that focused on Mimas’ geology to test the ocean hypothesis found that a thinning ice shell (and growing ocean) is compatible with constraints derived from tidal heating, the formation of Herschel crater, and the lack of extensive tectonic features. However, dissipation in an ocean-bearing Mimas ought to rapidly circularize its orbit, reducing tidal heating over time. Whether the ice shell can maintain a period of thinning even as the overall dissipation is decreasing is the key open question we address.

We modeled the co-evolution of Mimas’ eccentricity, tidal dissipation, and ice shell thickness assuming a past higher eccentricity and an initial ice shell thickness of 70 km (nearly the entire hydrosphere). We find that Mimas could evolve to its present-day eccentricity and inferred ice shell thickness if 1) the onset of melting occurred when Mimas’ eccentricity was 2.5-3 times the present-day value and 2) melting began on the order of 10 Myr ago. We further investigated the evolution of a growing ocean by tracking heat flow and stress within the ice shell for comparison with Mimas’ geology. Critically, we find that the initial ice shell thickness is far enough from thermal equilibrium that it can remain in a state of thinning, even as the eccentricity is dropping. We also find that Mimas’ low gravity and cold surface is sufficient to suppress viscous relaxation of impact craters subjected to enhanced heat flows caused by tidal heating. Finally, we find that Mimas’ orbital eccentricity will continue to decay, eventually leading the ocean to enter a state of freezing. As the ice shell thickens, radial fractures should crack the shell and enable surface eruptions of oceanic material, perhaps turning Mimas into a visibly active moon, like it’s neighbor, Enceladus.