This image from the Mars Reconnaissance Orbiter shows Mars' small moon, Deimos. Deimos is the smaller of the two moons of Mars. It has a much smoother surface than its twin, Phobos, and orbits nearly twice as far from Mars. The smooth surface of this moon is caused by a layer of regolith, which is fine layer of loose material. This material may have been formed from the fine debris from the many small impact craters. Deimos is only 7.5 miles (12 kilometers) in diameter, and is believed to be an asteroid that was captured by the gravity of Mars.