Phobos (moon) Phobos (moon) Phobos ( , , ; systematic designation: ) is the innermost and larger of the two natural satellites of Mars, the other being Deimos. Both moons were discovered in 1877 by American astronomer Asaph Hall. Phobos is a small, irregularly shaped object with a mean radius of and is seven times as massive as the outer moon, Deimos. Phobos is named after the Greek god Phobos, a son of Ares (Mars) and Aphrodite (Venus) and the personification of horror (cf. phobia). Phobos orbits from the Martian surface, closer to its primary body than any other known planetary moon.
Solar System moons. The centaurs and many short-period comets also orbit in this region. Due to their greater distance from the Sun, the solid objects in the outer Solar System contain a higher proportion of volatiles, such as water, ammonia, and methane than those of the inner Solar System because the lower temperatures allow these compounds to remain solid. The four outer planets, or giant planets (sometimes called Jovian planets), collectively make up 99% of the mass known to orbit the Sun. Jupiter and Saturn are together more than 400 times the mass of Earth and consist overwhelmingly of hydrogen and helium.