MIMAS

smallest and innermost of the eight large, regular satellites of Saturn. It has a diameter of 392 km (243 miles) and orbits the planet in a prograde, near-circular orbit at a mean distance of nearly 185,520 km (115,022 miles). The orbital period of 23 hours is half that of the Saturnian satellite Tethys, and therefore Mimas and Tethys are involved in an orbital resonance, always reaching conjunction on the same side of Saturn. The origin of this resonance is not fully understood. Mimas rotates synchronously with its orbital motion, always keeping the same hemisphere toward Saturn. The mean density of Mimas is only 1.2 times that of water, and its surface shows characteristics of water frost. For these reasons, Mimas is believed to be composed principally of ice. Its surface is bright and heavily marked with deep, bowl-shaped craters. The depth of the craters appears to be a consequence of the low surface gravity. In spite of Mimas' small size, there is some evidence of resurfacing, possibly resulting from a partial melting of the icy crust. The most noteworthy feature on Mimas is a 130-kilometre- (80-mile-) diameter crater named Herschel, which is near the centre of the leading hemisphere. The crater walls are 5 km (3 miles) high, its floor 10 km (6 miles) deep, and the central peak 6 km (4 miles) high. This is the largest impact structure, relative to the size of the body, so far discovered in the solar system.