Nat. Astron. https://doi.org/c4c3 (2019)

Saturn’s inner moons, Mimas, Enceladus, Tethys, Dione and Rhea, have different interior compositions of rock and ice. Surprisingly, these are not correlated with their mass or distance from Saturn itself. While an ocean is present on Enceladus, Mimas shows no sign of geological activity despite having a closer and more eccentric orbit that would be expected to give rise to much stronger tides. What is known about the moons’ geology is not in line with their orbits.

Credit: NASA/JPL-Caltech/Space Science Institute

Numerical simulations performed by Marc Neveu and Alyssa Rhoden may now provide an explanation. The simulations follow the evolution of the moons’ orbits from their formation until today, over a time span of 4.5 billion years, and account for the development of their thermal, geophysical and orbital properties. Assuming that Mimas was formed less than 1 billion years ago from Saturn’s rings, and the outer moons are much older, the different geologies are reconciled with the observed orbits.