1789

Mimas

William Herschel (1738–1822), Caroline Herschel (1750–1848), John Herschel (1792–1871)

Shortly after discovering Enceladus, William Herschel struck Saturnian gold again, spotting the seventh known moon of the ringed planet in September of 1789. At the time, this newest one was the closest to Saturn itself, with an orbital period of less than a day. William’s son, John Herschel, would later name this moon Mimas, after a giant in Greek Titan mythology.

Herschel’s ability to detect faint moons close to bright planets was made possible by his innovative telescope designs, which could collect unprecedented numbers of photons from distant sources. For example, his discoveries of both Mimas and Enceladus were made using a 40-foot (12-meter) reflecting telescope with a primary mirror 4 feet (1.2 meters) in diameter—the largest telescope in the world at the time.

Telescopic spectroscopy of Mimas showed that it has a water ice–dominated surface, like all the other Saturnian moons except for Titan. Little else was known about Mimas until the Voyager and Cassini spacecraft encounters obtained close-up images and other data. The images revealed Mimas to be a small world, only about 250 miles (400 kilometers) in diameter (less than one-eighth the size of our Moon), with a density of just under 1.2 grams per cubic centimeter, indicating a primarily icy interior. Astronomers believe Mimas is close to the smallest possible astronomical body that can form into a spherical shape from its own self-gravity. Actually, Mimas is noticeably oblate (about 10 percent wider at the equator than at the poles) because of the strong gravitational pull of nearby Saturn.

Herschel’s famous 40-foot (12-meter) telescope, used to discover both Mimas and Enceladus.

Mimas is heavily cratered, but not uniformly so. That and the presence of canyons and fractures indicate that some areas might have been resurfaced in the distant past by tectonic forces or cryovolcanism (ice volcanoes). The most distinctive feature on Mimas is a single enormous crater, almost a third of the moon’s diameter across, that gives Mimas its distinctive “Death Star” look.

NASA Cassini Saturn orbiter photo of Mimas taken in October 2010. The large (80-mile [130-kilometer] wide) impact crater that dominates this satellite’s hemisphere is named Herschel, after the moon’s discoverer.