Deimos is composed of rock rich in carbonaceous material, much like C-type asteroids and carbonaceous chondrite meteorites. It is cratered, but the surface is noticeably smoother than that of Phobos, caused by the partial filling of craters with regolith. The regolith is highly porous and has a radar estimated density of only 1.1 g/cm³. The two largest craters, Swift and Voltaire, each measure about 3 kilometres across.
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