Mars is accompanied by two small moons, Phobos and Deimos, discovered in 1877 by the American astronomer Asaph Hall. The irregular shapes of both satellites as well as the dark surface material spectrally similar to that on some asteroids suggest that Phobos and Deimos may be captured asteroids.

29. Deimos

The 10 × 16 km moon Deimos orbits Mars at a distance of 20,000 km above the planet's surface. The surface of Deimos appears very smooth because of a thick layer of light, fragmental material that partly fills most craters. Closer views of this moon, however, reveal house-sized blocks, probably ejected during formation of the impact craters.

Viking Orbiter frame 428B22

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