Deimos. Viking I orbiter image of the Martian moon Deimos. This is the smaller and outermost of Mars' two moons, the other being Phobos. It is an irregular chunk of rock rich in carbonaceous material that orbits Mars in just over 30 hours. Some astronomers think that Deimos and Phobos were asteroids that were captured by Mars, although this is not proven. Viking 1 entered Mars orbit on 19 June 1976 and stopped operations on 17 August 1980.