Terrestrial Planets, Page 18: [Previous Page] [Next Page] [Class Home Page] Textbook pages 181 - 201

Mars' Moons: Phobos & Deimos

Mars has two small moons: 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.

Phobos
Phobos orbits Mars below the synchronous orbit radius. Thus it rises in the west, moves very rapidly across the sky and sets in the east, usually twice a day. It is so close to the surface that it cannot be seen above the horizon from all points on the surface of Mars.

Phobos is doomed: because its orbit is below synchronous altitude tidal forces are lowering its orbit (current rate: about 1.8 meters per century). In about 50 million years it will either crash onto the surface of Mars or (more likely) break up into a ring. (This is the opposite effect to that operating to raise the orbit of the Moon.)

Deimos
Deimos is the smaller and outermost of Mars' two moons. It is the smallest known moon in the solar system. The largest crater on Deimos is approximately 12.6 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.



Small Moons!
Phobos is 11 km, and Deimos just 6km in radius. That's not much bigger than a major city! Note the relative size of the two moons placed to scale on a regional map of Phoenix, AZ.