Saturn: Moons: Mimas
Mimas (pronounced MY muss or MEE muss, adjective Mimantean) looks somewhat like a bull's eye if viewed from a certain angle. The feature that causes this is the huge 140-kilometer-wide (88-mile) Herschel Crater, which is one-third the diameter of Mimas. If the object striking Mimas had been larger or been moving faster, Mimas would probably have been "disrupted" into pieces that might have collapsed back into a new moon or might have scattered into another ring of Saturn. The walls of Herschel Crater are approximately 5 kilometers (3 miles) high, parts of the floor are approximately 10 kilometers (6 miles) deep, and the central peak towers are almost 6 kilometers (4 miles) above the floor of the crater. A comparable crater on Earth would be 4,000 kilometers (2,500 miles) in diameter.
Mimas is an inner moon of Saturn (the innermost of the major moons) that averages 396 kilometers (246 miles) in diameter. Shock waves from the Herschel impact may have caused the fractures, also called chasmata, on the opposite side of Mimas. This is not quite big enough to hold a round shape; the shape is somewhat ovoid with dimensions of 209 x 196 x 191 kilometers (130 x 122 x 119 miles, respectively). Mimas orbits at a range of 185,520 kilometers (115,280 miles) from Saturn in a time of 22 hours and 37 minutes. This orbit makes Mimas the closest major moon of Saturn. Mimas is tidally locked to Saturn with one side always facing in toward its parent. Mimas' close orbit means that Mimas probably receives several times the rate of collisions as the other moons of Saturn.
Mimas and another Saturn moon, Rhea, have been called "the most heavily cratered body in the Solar System." Mimas would probably have been more heavily cratered except, being closer to Saturn, Mimas was warmer (and consequently softer) for a longer time so early features have faded away. However, with so many impacts the youngest craters have tended to obliterate the older ones, and these moons are cratered about as much as they can get.
Most of the Mimas surface is saturated with impact craters ranging in size up to greater than 40 kilometers (25 miles) in diameter, although none are anywhere near the size of Herschel. However, the craters in the South Pole region of Mimas are generally 20 kilometers (12.4 miles) in diameter or less. This suggests that some melting or other resurfacing processes occurred there later than on the rest of the moon. (Interestingly, the South Pole area of Enceladus appears to be the source of that moon's geysers.)
Mimas' low density (1.17 times the density of liquid water) indicates that it is composed mostly of water ice with only a small amount of rock. It seems to be solidly frozen at a temperature of 209 degrees Celsius (-344 degrees Fahrenheit). This is puzzling because Mimas is closer to Saturn than Enceladus, and the Mimantean orbit is much more eccentric (out of round) than the Enceladean orbit. Thus, Mimas should have much more tidal heating than Enceladus. Yet, Enceladus has geysers of water, while Mimas has one of the most heavily cratered surfaces in the Solar System. This suggests a frozen Mimas surface that has persisted for a very long time. This paradox has led astronomers to use the "Mimas test" by which a theory to explain the partially thawed water of Enceladus must also explain the entirely frozen water of Mimas.
Mimas apparently cleared enough material to create the 4,800-kilometer (2,980-mile) wide gap (called the Cassini Division) between Saturn's two widest rings, the A and B rings. Observations from Cassini revealed that there is still some ring material in the Cassini Division, although it is sparse enough that the area appears dark from a distance.
Mimas is in resonance with two nearby moons, Dione and Enceladus. That is, these moons speed up slightly as they approach each other and slow down as they draw away, causing their orbits to vary slightly in a long series of complex changes, which help keep them locked in their positions.
Mimas strongly perturbs the tiny 3-kilometer (2-mile) diameter moon Methone, the 4-kilometer (3-mile) diameter moon Pallene, and the 2-kilometer (1-mile) diameter moon Anthe, all of which orbit between Mimas and the next major moon going out from Saturn, Enceladus. The vastly more massive Mimas causes the Methone orbit to vary by as much as 20 kilometers (12.4 miles). The perturbations are larger for tiny Anthe, and slightly smaller for Pallene.
Discovery
Ground-based astronomers could only see Mimas as little more than a dot until Voyagers I and II imaged it in 1980. The Cassini spacecraft has made several close approaches and provided detailed images of Mimas since Cassini achieved orbit around Saturn in 2004.
Origin of Name
William [Friederick Wilhelm] Herschel discovered Mimas in 1789. His son, John Herschel suggested that the moons of Saturn be associated with Greek mythical brothers and sisters of Kronus, known to the Romans as Saturn.
The name Mimas comes from the god (or Titan) Mimas in Greek mythology who was slain by one of the gods of Olympus in the war between the Olympians and the titans. Different accounts have Mimas dispatched by Hercules, by Ares (the god of war), or by Zeus himself using a thunderbolt. Legend has it that the island of Prochyte near Sicily rests on his body.
Astronomers also refer to Mimas as Saturn I based on its distance being the closest to Saturn. The International Astronomical Union now controls the official naming of astronomical bodies.
