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.

30. Phobos

Phobos is 18 × 27 km in size and orbits 6000 km above the martian surface. The moon's major features are 10-km-diameter Stickney Crater and the grooves radiating away from the crater. Phobos would have been shattered if the object creating Stickney had been only slightly larger. The grooves are 100-200 m wide, about 20 m deep, and attest to the seismic disruption experienced by the satellite during the impact. The orbit of Phobos is slowly decaying and calculations indicate that the moon will strike the martian surface in about 50 million years. A new martian impact crater approximately 100 km in diameter will then result for future generations of planetary scientists to study in detail.

Viking mosaic P20776

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