Deimos

Deimos is the smaller and outer of the two Martian moons. In Greek mythology, Deimos is one of the sons of Ares (Mars) and Aphrodite (Venus); "deimos" is Greek for "panic". Deimos, like Phobos, has often been likened to an enormous potato because of its shape.

Description
Deimos is very similar to Phobos in appearence, but unlike Phobos it is much larger in diameter than it is in width, and has a highly irregular "hump" along its northern hemisphere. Whereas Phobos has a scarred landscape, the surface of Deimos is theorized to be extremely smooth due to the fact that it reflects sunlight to an unusual extent for a body its size. Deimos and Phobos are composed of carbon-rich rock and ice, much like C-type asteroids.

Origins
Phobos and Deimos are probably asteroids perturbed by Jupiter into orbits that allowed them to be captured by Mars. There is, however no completely satisfactory theory as to how Deimos and Phobos came to be in orbit around Mars. The theory that these moonlets are in fact asteroids is backed up by tha fact that Deimos orbits Mars at an unusual distance for a moon of a planet Mars' size.