Deimos is probably an asteroid that was perturbed by Jupiter into an orbit that allowed it to be captured by Mars, though this hypothesis is still in some dispute.
Like most bodies of its size, Deimos is highly nonspherical with dimensions of 15 by 12 by 10 km.
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 two largest craters, Swift and Voltaire, measure about 3 kilometres across.
As seen from Deimos, Mars would be 1000 times larger and 400 times brighter than the full Moon as seen from Earth, taking up a full 1/11 of the width of a celestial hemisphere.