Moons of Mars. Artwork of the Martian moons over Mars, seen from the porthole of a spaceship. Both moons are small (10-20 kilometres wide), irregular chunks of rock. Phobos (meaning fear, centre) orbits Mars in under 8 hours. Deimos (meaning terror, upper centre) is the outer moon, orbiting Mars in around 30 hours. The moons were discovered in 1877, by the American astronomer Asaph Hall. He was encouraged in the search by his wife (maiden name Stickney). The large Stickney crater (visible on Phobos) is named in her honour. Some scientists consider the moons to be captured asteroids. The Valles Marineris (lower left) and three volcanoes (upper left) are seen on the Martian surface.