Deimos is the outer and smaller of
Mars's two moons. Discovered in 1877 along with its inner counterpart
Phobos by
Asaph Hall, it was likewise named after a son of
Ares and is Greek for "panic." Similar to Phobos in many other ways as well, it resembles the
C-type asteroids found in the
asteroid belt in both its composition and its irregular, cratered shape. It appears to be covered with a thick coat of
regolith produced by asteroid impacts.
At 23,459 km from the planet's center, Deimos' completes an orbit every 1.26244 (Earth) days. Due to its irregular shape, its "diameter" varies from 15 to 11 km, with a mean of 12.6 km. Its mass is about 1.8e15 kg with a density of 1.7 gm/cm3.
Deimos' surface is actually less rough than that of Phobos, despite its lesser mass (and corresponding gravity) that should increase the chance of debris from asteroid impacts flying off into space. Its most prominent features are craters named Swift and Voltaire, each after the well-known writer of the same name. Swift (the writer) originally predicted twin Martian satellites in Gulliver's Travels, a century and a half before Hall's discovery. Gritchka tells me Voltaire mentioned Mars' moons in Micromégas.