SUMMARY

Mars' moons are among the smallest in the solar system. Phobos is a bit larger than Deimos, and orbits only 3,700 miles (6,000 kilometers) above the Martian surface. No known moon orbits closer to its planet. It whips around Mars three times a day, while the more distant Deimos takes 30 hours for each orbit. Phobos is gradually spiraling inward, drawing about six feet (1.8 meters) closer to the planet each century. Within 50 million years, it will either crash into Mars or break up and form a ring around the planet.

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To someone standing on the Mars-facing side of Phobos, Mars would take up a large part of the sky. And people may one day do just that. 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.

​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 and may be captured asteroids.

​Phobos has only 1/1,000th as much gravitational pull as Earth. A 150-pound (68 kilogram) person would weigh two ounces (68 grams) there. Yet NASA's Mars Global Surveyor has shown evidence of landslides, and of boulders and dust that fell back down to the surface after being blasted off the moon by meteorites.

​Phobos and Deimos. Phobos (fear) and Deimos (panic) were named after the horses that pulled the chariot of the Greek war god Ares, the counterpart to the Roman war god Mars. Both Phobos and Deimos were discovered in 1877 by American astronomer Asaph Hall. The moons appear to have surface materials similar to many asteroids in the outer asteroid belt, which leads most scientists to believe that Phobos and Deimos are captured asteroids

Two Moons Passing in the Night

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Two Moons and the Pleiades from Mars

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1st Animation of Two Moons Passing in the Night

This animation shows Phobos as a large white dot against a black background, representing Phobos, moving from the lower center toward the upward right corner. Above and to the left of it is Deimos, a smaller white dot, that also moves upward and to the right but at a much slower pace. In fact, Phobos starts out below Deimos, the smaller dot, and quickly moves past it in the sequence.

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2nd Animation of Two Moons Passing in the Night

This animation shows a large white dot against a black background, representing Phobos, moving from the lower center toward the upward right corner. Above and to the left of it is a smaller white dot, representing Deimos, that also moves upward and to the right but at a much slower pace.

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1st Image of Two Moons and the Pleiades from Mars

This image shows on the upper right side, is a small white circle against a black background representing Phobos, with a smaller black circle representing Deimos a little above and to the right of it.

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2nd Image of Two Moons and the Pleiades from Mars

This image shows a large white circle representing an enhanced image of the light from Phobos with an oblong rock-like object representing Phobos itself inserted in the middle. A little above and to the right of that is a smaller white circle representing Deimos. In the lower left corner are several white pinpoints labeled 'The Pleiades.'

PHOBOS

Phobos is the larger of the two heavily-cratered Martian moons and is dominated by three large craters. The largest of Phobos' craters, Stickney, was named after the wife of Asaph Hall, the astronomer who discovered the moons of Mars. Stickney crater is 10 km in diameter, which is almost half of the average diameter of Phobos! The crater is so large relative to the size of Phobos that the satellite probably came close to breaking up. Radiating away from Stickney are sets of parallel grooves or striations. These fractures undoubtably formed as a result of the impact that produced Stickney.

Another interesting feature about Phobos is the duration of its orbit. Phobos revolves around Mars at an astounding rate. In fact, it revolves around Mars 3 times during one Martian day! As a result, Phobos appears to rise in the west, and set in the east!

The image on the right is the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter took two images of the larger of Mars' two moons, Phobos, within 10 minutes of each other on March 23, 2008.

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Curiosity Observes Phobos Eclipse
Crater Chain on Phobos
Two Moons Passing in the Martian Night
Phobos is the larger of the two heavily-cratered Martian moons and is dominated by three large craters.
The Stickney crater, the largest crater on Phobos. Stickney is about10 km in diameter and was formed by a giant impact.
The image of Hall crater on Phobos. The crater is 6 km in diameter. Just to the lower right of the crater is Phobos' south pole.

DEIMOS

Deimos is the smaller of the two Martian moons and is less irregular in shape. The largest crater on Deimos is approximately 2.3 km in diameter, 1/5 the size of the largest crater on Phobos. Although both moons are heavily cratered, Deimos has a smoother appearance caused by the partial filling of some of its craters. When impacted, dust and debris will leave the surface of the moon because it doesn't 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.

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Curiosity Observes Deimos 'Eclipse'
Close-up image of Mars' satellite Deimos from Viking 2 showing craters and associated streaks. The streaks run from upper right to lower left, and may have been formed by the base surge "wind" of an impacting meteoroid.
Viking 2 view of the Martian satellite Deimos taken from 500 km distance. Deimos is about 8 km across at its widest part. Note the smoother appearance of the craters, caused by partial burial in dusty regolith.
This Viking 2 image shows the surface of Deimos from a distance of 30 km. Features as small as 3 meters across can be seen. Note many of the craters are covered over by a layer of dust estimated to be about 50 meters thick. Large blocks, 10 to 30 meters across, are also visible.
Deimos is the smaller of the two Martian moons and is less irregular in shape.
Two Moons Passing in the Martian Night