Martian moon Deimos, artwork. Deimos the smaller and outermost of the two Martian moons. An irregular shape, it measures 15 by 12 by 10 kilometres, and orbits some 20,000 kilometres above the Martian surface, taking 30.3 hours to complete an orbit. Deimos is named for the personification of terror in Greek mythology, one of the twin sons of Ares (named Mars by the Romans). The other moon is Phobos (see C018/0226).