Systematic designation: Mars I.
Phobos is the larger and innermost of Mars' two moons, and is named after a human mythical figure. Phobos orbits closer to a major planet than any other moon in the Sol system, less than 6000 km above the surface of Mars, and is also one of the smallest known moons in the solar system. The adjectival form of the name is Phobian, and in fairly common usage. Phobos houses several domed colony cities and a fast train transportation connecting them.
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 a day (every 11 h 6 min). It is so close to the surface (in a low-inclination equatorial orbit) that it cannot be seen above the horizon from latitudes greater than 70.4°.
This low orbit means that Phobos will eventually be destroyed: tidal forces are lowering its orbit, currently at the rate of about 1.8 metres per century, and in about 50 million standard years it will either impact the surface of Mars or (more likely) break up into a planetary ring. Given Phobos' irregular shape (intermediate between a prolate and oblate spheroid) and modeling it as a pile of rubble, it has been calculated that Phobos is stable with respect to tidal forces, but it is estimated that Phobos will pass the limit for its description when its orbital radius drops to about 8400 km, and will probably break up soon afterwards.
Because of its ellipsoidal shape alone, the gravity on Phobos' surface varied 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' gravity at its sub- and anti-Mars poles. However, current technology has managed to stabilize this variation.
Phobos was a dark body that is composed of C-type surface materials. It is similar to the C-type (blackish carbonaceous chondrite) asteroids that exist in the outer asteroid belt of this system. Phobos's density is too low to be pure rock, however, and it is probably composed of a mixture of rock and ice. Phobos was covered with a layer of fine dust about a metre thick, similar to the regolith on Luna.
Phobos is highly nonspherical, with dimensions of 27 × 21.6 × 18.8 km. It is heavily cratered, and the most prominent surface feature is the large crater named Stickney, after the maiden name of a certain ancient human astronomer's wife. Like Mimas's crater Herschel on a smaller scale, the impact that created Stickney must have almost shattered Phobos. The grooves and streaks on the surface were probably also caused by the Stickney impact. The grooves are typically less than 30 m deep, 100 – 200 m wide, and up to 20 km in length.
Phobos' density is 1.85 grams per cubic centimeter, revealing that the moon is not solid, but probably filled with giant caverns. This kind of body is basically a clump of rocks held together by gravity. Phobos has a layer of regolith at least 100 meters thick, perhaps formed by impacts. It is not known how this material stuck to an object with so little gravity.
Furthermore, the low density of Phobos means it has a large reservoir of ice within it, despite the surface regolith having no evidence of hydration.
Phobos and Deimos both have much in common with carbonaceous (C-type) asteroids, with very similar spectra, albedo and density to those seen in C-type asteroids. This has led to speculation that both moons could have been captured into Martian orbit from the main asteroid belt. However, both moons have very circular orbits which lie almost exactly in Mars' equatorial plane, while captured moons would be expected to have eccentric orbits in random inclinations. Some evidence suggests that Mars was once surrounded by many Phobos- and Deimos-sized bodies, perhaps ejected into orbit around it by a collision with a large planetesimal.
Ball games are not very popular on Phobos, as the gravity of the moon is so small balls often reach escape velocity and drift into space.