Mimas

Mimas is the smallest and innermost of Saturn's round moons, and the most-cratered body its size in the solar system. Its iconic feature is the 130-km Herschel crater — one-third the moon's diameter — which gives Mimas a striking resemblance to the fictional Death Star. The impact that created Herschel was so violent it nearly shattered Mimas; fracture lines on the opposite hemisphere likely propagated through the moon as shock waves. Recent analysis of Cassini data suggests Mimas hides a global subsurface ocean beneath its battered icy crust — making it a possible "stealth ocean world."

Mimas is essentially a giant frozen snowball — 98% water ice with a small rocky core. Its low density and uniform brightness suggest the icy mantle is geologically inactive at the surface. The surprising 2024 finding of a subsurface ocean came not from imaging but from analysing Mimas's wobble in its orbit, which only matches a model with a liquid layer ~30 km below the surface.

Voyager 1 captured the first close-up images of Mimas in 1980, revealing the iconic Herschel crater. Cassini's 13-year mission performed multiple Mimas flybys, mapping the surface in detail and gathering the orbital data later used to infer the subsurface ocean.