Phobos and Deimos, photographed here by the Mars Reconnaissance Orbiter, are tiny, irregularly-shaped moons that are probably strays from the main asteroid belt. Credit: NASA - See more at:

How Many Moons Does Mars Have?

Article Updated: 12 Aug , 2016

Many of the planets in our Solar System have a system of moons. But among the rocky planets that make up the inner Solar System, having moons is a privilege enjoyed only by two planets: Earth and Mars. And for these two planets, it is a rather limited privilege compared to gas giants like Jupiter and Saturn which each have dozens of moons.

Whereas Earth has only one satellite (aka. the Moon), Mars has two small moons: Phobos and Deimos. And whereas the vast majority of moons in our Solar System are large enough to become round spheres similar to our own Moon, Phobos and Deimos are asteroid-sized and misshapen in appearance.

Size, Mass and Orbit:

The larger moon is Phobos, whose name comes from the Greek word which means “fear” (i.e. phobia). Phobos measures just 22.7 km across and has an orbit that places it closer to Mars than Deimos. Compared to Earth’s own Moon — which orbits at a distance of 384,403 km away from our planet — Phobos orbits at an average distance of only 9,377 km above Mars.

This produces an orbit of short duration, revolving around the planet three times in a single day. For someone standing on the planet’s surface, Phobos could be seen crossing the sky in only 4 hours or so.

Phobos, the larger of Mars' two moons, with the Stickney crater seen on the right side. Credit: HiRISE, MRO, LPL (U. Arizona), NASA

Phobos, the larger of Mars’ two moons, with the Stickney crater seen on the right side. Credit: HiRISE, MRO, LPL (U. Arizona), NASA

Mars’ second moon is Deimos, which takes its name from the Greek word for panic. It is even smaller, measuring just 12.6 km across, and is also less irregular in shape. Its orbit places it much farther away from Mars, at a distance of 23,460 km, which means that Deimos takes 30.35 hours to complete an orbit around Mars.

When impacted, dust and debris will leave the surface of the moon because they do not have enough gravitational pull to retain the ejecta. However, the gravity from Mars will keep a ring of this debris around the planet in approximately the same region that the moon orbits. As the moon revolves, the debris is redeposited as a dusty layer on its surface.

Like Earth’s Moon, Phobos and Deimos always present the same face to their planet. Both are lumpy, heavily-cratered and covered in dust and loose rocks. They are among the darker objects in the solar system. The moons appear to be made of carbon-rich rock mixed with ice. Given their composition, size and shape, astronomers think that both of Mars’ moons were once asteroids that were captured in the distant past.

However, it appears that of these two satellites, Phobos won’t be orbiting the Red Planet for very much longer. Because it orbits Mars faster than the planet itself rotates, it is slowly spiraling inward. As a result, scientists estimate that in the next 10-50 million years or so, it will get so low that the Martian gravity will tear Phobos into a pile of rocks. And then a few million years later, those rocks will crash down on the surface of Mars in a spectacular string of impacts.

The Martian Moon of Deimos, as pictured by the Mars Reconnaissance Orbiter. Credit: HiRISE/MRO/LPL (U. Arizona)/NASA

The Martian Moon of Deimos, as pictured by the Mars Reconnaissance Orbiter. Credit: HiRISE/MRO/LPL (U. Arizona)/NASA

Composition and Surface Features:

Phobos and Deimos both appear to be composed of C-type rock, similar to blackish carbonaceous chondrite asteroids. This family of asteroids is extremely old, dating back to the formation of the Solar System. Hence, it is likely that they were acquired by Mars very early in its history.

Phobos is heavily cratered from eons worth of impacts from meteors with three large craters dominating the surface. The largest crater is Stickney (visible in the photo above). The Stickney crater is 10 km in diameter, which is almost half of the average diameter of Phobos itself. The crater is so large that scientists believe the impact came close to breaking the moon apart. Parallel grooves and striations leading away from the crater indicate that fractures were likely formed as a result of the impact.

Much like Phobos, it’s surface is pockmarked and cratered from numerous impact. The largest crater on Deimos is approximately 2.3 km in diameter (1/5 the size of the Stickney crater). Although both moons are heavily cratered, Deimos has a smoother appearance caused by the partial filling of some of its craters.


Compared to our Moon, Phobos and Deimos are rough and asteroid-like in appearance, and also much smaller. In addition, their composition (as already noted) is similar to that of C-type asteroids that are common to the Asteroid Belt. Hence, the prevailing theory as to their origin is that they were once asteroids that were kicked out of the Main Belt by Jupiter’s gravity, and were then acquired by Mars.


History of Observation:

Phobos and Deimos were originally discovered by American astronomer Asaph Hall in August of 1877. Ninety-four years after the moons’ discovery, NASA’s Mariner 9 spacecraft got a much better look at the two moons from its orbit around Mars. Upon viewing the large crater on Phobos, NASA decided to name it after Hall’s wife – Stickney. Subsequent observations conducted by the HiRISE experiment, the Mars Global Surveyor, and the Mars Reconnaissance Orbiter have added to our overall understanding of these two satellites.

Someday, manned missions may be going to Phobos and Deimos. Scientists have discussed the possibility of using one of the Martian moons as a base from which astronauts could observe the Red Planet and launch robots to its surface, while shielded by miles of rock from cosmic rays and solar radiation for nearly two-thirds of every orbit.

Here’s an article about how Phobos is going to crash into Mars in the future. And here are some great images of both Phobos and Deimos.

Here’s NASA’s fact sheet on Mars, including information about the moons, and additional info from Starry Skies.

Finally, if you’d like to learn more about Mars in general, we have done several podcast episodes about the Red Planet at Astronomy Cast. Episode 52: Mars, and Episode 91: The Search for Water on Mars.


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4 Responses

  1. CJSF says:

    I thought there was considerable rethinking about Phobos’ grooves being related to Stickney, and in fact lend credence to the current idea that Phobos will likely be torn apart in a relatively short time (geologically speaking).


  2. FarAwayLongAgo says:

    One could mention Mars’ trojan asteroids, about half a dozen observed.

    I suppose there are many thousands near Mars asteroids, which maybe could be helpful for Mars exploration. Some surely come close to both Earth and Mars and are natural Earth-Mars cyclers with resources we could extract without having to launch them from Earth.

    I think some of the sources used for this article are outdated. More research has been done in recent years, so astronomers are less certain now 🙂 Their orbits are as if they formed together with Mars, but it is funny that Phobos only has about 30 million years left if it formed 4500 million years ago. One possibility is that they formed like the Moon, out of debris from a relatively recent impact on Mars.

    Humans to Mars’ moons is seriously being considered for the favorable conjunctions in 2031 and 2033. Much easier and cheaper than to land on Mars’ surface. Should fit politically as a compromise in the endless Mars, Moon and asteroids target choice debate.

  3. Pasander says:

    Just a week or so ago there was an article saying the grooves might be due to Phobos being almost breaking up already. It is orbiting Mars inside Roche limit for fluid bodies (edit: for Phobos’ mass, I assume).

    That gave me an idea and I asked Wolfram Alpha what is the gravitational binding energy of Phobos. Turns out it’s about 100 MT of TNT (maybe less inside Roche limit?), so with a Tsar Bomba using third bomb stage uranium tamper we could blow it to pieces and have a ringed Mars!

    • FarAwayLongAgo says:

      Hush hush, DON’T TELL ELON MUSK, the Mars polar nuker!

      The mass of Phobos is less than 1/1000 of the Saturnian rings, but Mars is only 1/20 the radius of Saturn and Phobos is about 10 times closer than the rings to Saturn. Once disrupted, Phobos’ debris will form rings within just a few weeks! Maybe Elon retired to the desolate deserts on Mars, wants a spectacular view for his entertainment, and dear Phobos will be forever gone.

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