Phobos is the larger of Mars' two moons. It is an oblong shaped moon measuring 27 X 21 X 19 km. It orbits Mars with a period of 7.3 hours, less than a Martian day. This makes the moon appear to rise in the west and set in the east. Phobos is heavily cratered with interesting parallel grooves about 150 m long and 25m deep. The grooves seem to radiate from the largest crater to an oddly shaped area on the other end of the moon. Because of this it is presumed that the grooves may have formed with the impact of the largest crater.

Deimos is also oblong shaped, 15 X 12 X 11 km and orbits Mars in 30.3 hours.

Deimos appears to have little surface detail because it is covered with a thick layer of dust. The dust fills craters and covers some surface detail. The reason Phobos may not have as much dust and debris on its surface is because of its close orbit to Mars where the planet's gravity would tend to pull debris off the moon. Both moons have low escape velocities, Phobos is only 12 m/sec and Deimos is even lower.

Both moons are probably about 2 billion years old and appear to have a composition very similar to carbonaceous chondrites (meteorites rich in water and organic content.) Since carbonaceous chondrites form in the asteroid belt, it is considered highly probable that both Phobos and Deimos are captured asteroids.

Both moons have a density of about 2 g/cu. cm, lower than Mars 4 g/cubic cm. This, along with their appearance makes astronomers think that Phobos and Deimos are more similar to the asteroids than Mars.

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Kathy Miles, Author, and Chuck Peters, Systems Administrator
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