Phobos is the larger and closer of the two moons of Mars. It is named after the Greek god Phobos (which means “fear”), a son of Ares (Mars). A small, irregularly shaped object, Phobos orbits about 3100 miles from the surface of Mars, closer to its primary than any other known planetary moon.
Phobos is one of the least-reflective bodies in the solar system. It is a Mohr-Coulomb body – in essence, a large pile of rubble composed of rock and ice.
Phobos is covered with a layer of fine-grained regolith at least 100 meters thick; it is believed to have been created by impacts from other bodies, but it is not known how the material stuck to an object with almost no gravity.
It is highly nonspherical, with dimensions of 27 × 22 × 18 km. Because of its shape alone, the gravity on its surface varies by about 210%; the tidal forces raised by Mars more than double this variation (to about 450%) because they compensate for a little more than half of Phobos’s gravity at its sub- and anti-Mars poles.
Phobos orbits Mars below the synchronous orbit radius, meaning that it moves around Mars faster than Mars itself rotates. Therefore it rises in the west, moves comparatively rapidly across the sky (in 4 h 15 min or less) and sets in the east, approximately twice each Martian day (every 11 h 6 min). Since it is close to the surface and in an equatorial orbit, it cannot be seen above the horizon from latitudes greater than 70.4°.
Phobos’s orbit is so low that its angular diameter, as seen by an observer on Mars, varies visibly with its position in the sky. Seen at the horizon, Phobos is about 0.14° wide; at zenith it is 0.20°, one-third as wide as the full Moon as seen from Earth. By comparison, the Sun has an apparent size of about 0.35° in the Martian sky.
Phobos’s phases, inasmuch as they can be observed from Mars, take 0.3191 days (Phobos’s synodic period) to run their course, a mere 13 seconds longer than Phobos’s sidereal period.