Mimas



Mimas is a moon of Saturn discovered in 1789 by William Herschel. It is named after Mimas, a son of Gaia in Greek mythology, and is also designated Saturn I.

Characteristics
With a diameter of 396 kilometers, it is the twenty-first-largest moon in the Sol System and is the smallest astronomical body in that system known to be rounded in shape because of self-gravitation. Mimas’s density is 1.1479±0.007 g/cm³ indicating that it is composed mostly of water ice with only a small amount of rock. Due to the tidal forces acting on it, Mimas is noticeably prolate ; its longest axis is about 10% longer than the shortest. Mimas’s mass is (3.7493±0.0031)×1019 kg,undefinedor 6.3×10−6 that of Earth’s. Mimas has a surface gravity of 0.064 m/s² or 0.00648 g.

Herschel Crater
Mimas's most distinctive feature is a giant impact crater named Herschel after the discoverer of Mimas. It is a huge crater in the leading hemisphere of Mimas, on the equator at 100° longitude. It is named after William Herschel.

 Herschel is the largest crater relative to its parent body of any equilibrium planetary moon in the Sol System. It is so large that astronomers have expressed surprise that Mimas was not shattered by the impact that caused it. It measures 139 kilometers across, almost 1/3 the diameter of Earth's moon; its walls are approximately 5 km high, parts of its floor are 10–12 km deep, and its central peak rises 6–8 km above the crater floor. If there were a crater of an equivalent scale on Earth it would be over 4,000 km in diameter and wider than Australia, with walls over 200 km high. The impact that formed Herschel must have nearly disrupted Mimas entirely; deep, elongated, steep-sided depressions can be seen on the opposite side of Mimas that may be stress fractures due to shock waves from the impact traveling through the moon and focusing there. The impact is also suspected of having something to do with the current temperature pattern on Mimas. Herschel has an estimated age of around 4.1 billion years.