The two satellites of Mars, Phobos and Deimos, are very small, irregularly shaped objects. Little was known of these bodies until the Mariner 9, Viking, and the Mars Global Surveyor missions provided detailed photographs of them. Roughly speaking, they are triaxial ellipsoids about 10 to 20 km across. Their surfaces are heavily cratered, but not identical in appearance. In particular, Phobos has a system of long linear depressions or grooves each of which is about 100 to 200 m wide and 10 to 20 m deep. There are no grooves on Deimos. The grooves on Phobos are probably related to fractures caused by a large impact, perhaps the one responsible for the Stickney crater. There are no craters on Deimos as large as Stickney; this may explain the absence of grooves on Deimos. The low mean density of Phobos, 2000 kg m−3, and its reflectance spectrum suggest that it is made of a dark gray material similar to Types I or II carbonaceous chondrite meteorites.
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