Mimas, one of Saturn‘s small moons with a big crater, is made of mostly water ice with a smattering of rock – so it is accurately described as a big dirty snowball.
The crater, named Herschel after the 1789 discoverer of Mimas, Sir William Herschel, spans about 130 kilometers and is pictured above.
Mimas‘ low mass produces a surface gravity just strong enough to create a spherical body but weak enough to allow such relatively large surface features.
The above image was taken during the 2005 August flyby of the robot spacecraft Cassini now in orbit around Saturn. A recent analysis of Mimas’s unusual wobble indicates that it might house a liquid water interior ocean.