Less than 400 km in diameter, crater-covered Mimas is the smallest and innermost of Saturn's major moons. Its most distinguishing feature is a giant impact crater -- named Herschel after the moon's discoverer -- which stretches a third of the way across the face of the moon, making it look like the Death Star from "Star Wars." Herschel is 130 km across, with outer walls about 5 km high and a central peak 6 km high. The impact that blasted this crater out of Mimas probably came close to breaking the moon apart.
At a mean distance of less than 200,000 km from the massive planet, Mimas takes only about 23 hours to complete an orbit. It keeps the same face toward Saturn as it flies around the planet, just as our Moon does with Earth. Its low density suggests that it consists almost entirely of water ice, which is the only substance ever detected on Mimas.
That Mimas appears to be frozen solid is puzzling because Mimas is closer to Saturn and has a much more eccentric (elongated) orbit than Enceladus, which should mean that Mimas has more tidal heating than Enceladus. Yet Enceladus displays geysers of water, which implies internal heat, while Mimas has one of the most heavily cratered surfaces in the solar system, which suggests a frozen surface that has persisted for enough time to preserve all those craters. This paradox has prompted the "Mimas Test" by which any theory that claims to explain the partially thawed water of Enceladus must also explain the entirely frozen water of Mimas.
Mimas orbits Saturn exactly twice as often as the more distant moon, Tethys, a phenomenon known as "orbital resonance." Similar orbital resonances between Mimas and parts of Saturn's rings are thought to be responsible for the Huygens gap, which marks the boundary between the B Ring and the Cassini Division, and for several density waves within the A Ring. In addition, Mimas' slight inclination (1.574 degrees with respect to the ring plane) gives rise to several vertical bending waves within the A Ring.
Mimas is also in resonance with Dione and Enceladus, and perturbs the orbits of Methone, Pallene and Anthe.
Mimas was discovered on 17 September 1789 by English astronomer William Herschel, using his 40-foot reflector telescope.
How Mimas Got its Name:
The mythological Mimas was a giant who was killed by Mars in the war between the Titans and the gods of Olympus. Even after his death, Mimas' legs -- which were serpents -- hissed vengeance and sought to attack his killer.
Mimas was named by John Herschel, the son of discoverer William Herschel, who explained his choice of names for the first seven of Saturn's moons to be discovered by writing, "As Saturn devoured his children, his family could not be assembled round him, so that the choice lay among his brothers and sisters, the Titans and Titanesses."