Mimas: Small Moon with a Big Crater
Whatever hit Mimas nearly destroyed it. What remains is one of the largest impact craters on one of Saturn 's smallest round moons. Analysis indicates that a slightly larger impact would have destroyed Mimas entirely. The huge crater, named Herschel after the 1789 discoverer of Mimas, Sir William Herschel , spans about 130 kilometers and is featured here . 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. Mimas is made of mostly water ice with a smattering of rock - so it is accurately described as a big dirty snowball. The featured image was taken during the closest-ever flyby of the robot spacecraft Cassini past Mimas in 2010 while in orbit around Saturn . Interactive: Take a trek across Mimas January 14: Zoom APOD Lecture hosted by the Amateur Astronomers of Association of New York