Moons

Phobos and Deimos

Two small, lumpy moons named after the horses of a war god. One of them is on a very slow collision course.

A grey, potato-shaped moon covered in craters, with one very large crater — Phobos, photographed by Mars Reconnaissance Orbiter in 2008.Photo: NASA / JPL-Caltech / University of Arizona, Public domain, Commons

A small, smooth, lumpy grey moon — Deimos, photographed by Mars Reconnaissance Orbiter (colours enhanced).Photo: NASA/JPL-caltech/University of Arizona, Public domain, Commons

Two little moons

Mars has two small moons, Phobos and Deimos. NASA Science: Mars moons · checked 25 Oct 2026 Both moons are potato-shaped, because they have too little mass for gravity to pull them into balls. They may be captured asteroids. NASA Science: Mars facts

Asaph Hall discovered both moons in 1877. NASA Science: Phobos They are named after the horses that pulled the chariot of Ares, the Greek god of war. NASA Science: Mars facts

Phobos

Phobos, the larger moon, goes round Mars three times a day. NASA Science: Mars moons Phobos is only 27 by 22 by 18 km. NASA Science: Phobos

Phobos’s biggest crater, Stickney, is named after Chloe Angeline Stickney Hall, a mathematician and the wife of Asaph Hall. NASA Science: Phobos

Falling, very slowly

Phobos is creeping closer to Mars by 1.8 m every hundred years. In about 50 million years it will crash into Mars or break up into a ring. NASA Science: Mars moons

Deimos

Deimos, the smaller moon, is just 15 by 12 by 11 km and goes round Mars every 30 hours. NASA Science: Deimos Deimos looks smoother than Phobos because loose dirt fills many of its craters. NASA Science: Mars facts

And rings?

Mars has no rings. NASA Science: Mars facts

Not yet, anyway: see what might happen to Phobos, above.

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