Phobos is the larger of Mars' two moons; it is also closer to the planet than is Deimos. Phobos is pitted by many small impact craters. Both Phobos and Deimos resemble C-type, or carbonaceous chondrite, asteroids. Thy are very dark and have albedos of only a few percent. The asteroids close to Mars are not as dark as the moons, leading to the hypothesis that both Phobos and Deimos were once asteroids which resided much farther from the Sun. Jupiter could have perturbed their orbits and Mars subsequently could have captured them as satellites.
