Neither Phobos nor Deimos have atmospheres. They are too small, with no volatiles to create an atmosphere, and their gravity too low to retain one.

Like Phobos, Deimos has characteristics that make it similar to a C-type asteroid. It is not a spherical moon.

Deimos has a much smoother surface than Phobos, although it does have some craters. The two largest are named Swift and Voltaire, after 19th century writers.

Demos has a nearly circular orbit around Mars. It may also have been a captured asteroid, but its orbit would have needed a long time to become circular.

An observer looking at Deimos from the surface of Mars would see a small body which at “full moon” would be about as bright as Venus looks from Earth.

One theory suggests that both Phobos and Deimos were part of a swarm of smaller objects around early Mars. They may have come to the Red Planet as part of a collision between planetesimals.

Deimos has not been formally visited by a spacecraft. However, most of the missions to Mars have also studied this little moon as they imaged and mapped Mars.

Missions have been proposed to study both moons of Mars. One is called Phobos and Deimos and Mars Environment (PADME). Another is called OSIRIS-REx 2, which is a followup to OSIRIS-REx, a regolith and surface mission currently in development at NASA.