Mimas, the smallest and innermost of Saturn’s major moons, is dominated by the 139 km wide impact crater Herschel — so large relative to the moon that Mimas was nicknamed the “Death Star moon” after the original Star Wars film. The impact that formed Herschel very nearly shattered Mimas.

Its unusually low density suggests a bulk composition of mostly water ice with a small rocky core. A 2024 reanalysis of Cassini tracking data identified a libration pattern best explained by a young (5-15 million years old) subsurface liquid water ocean — a surprising result for so small and ancient-looking a body.