Mars’s smaller and more distant moon is Deimos. It is one of Mars’s two known natural satellites, alongside the larger, closer moon Phobos.
Deimos is an irregular body only about 15 kilometres across at its longest dimension. Its surface appears dark and reddish, and it is widely thought to be asteroid-like in composition. The moon’s name comes from Deimos, a figure representing dread or terror in Greek mythology and a companion of Ares, the Greek equivalent of Mars.
Deimos orbits Mars at an average distance of roughly 23,460 kilometres from the planet’s centre. It takes about 30.3 hours to complete one orbit, so it moves more slowly across the Martian sky than Phobos. Like Phobos, Deimos is tidally locked, always showing nearly the same face to Mars.
The two moons may be captured asteroids, but their exact origin remains debated. Their low density and appearance have also inspired models involving debris from an ancient impact.