Martian moon Phobos, artwork. Phobos is the larger and innermost of the two Martian moons. An irregular shape, it measures 19 by 21 by 27 kilometres, and orbits some 6000 kilometres above the Martian surface, taking 7.6 hours to complete an orbit. Stickney Crater (right) is 10 kilometres across. Phobos is named for the personification of fear in Greek mythology, one of the twin sons of Ares (named Mars by the Romans). The other moon is Deimos (see C018/0227).