Phobos

Mars I

Phobos Facts

• Phobos is the larger and innermost of Mars's two moons:
  • distance from (the center of) Mars: 9378 km
  • diameter: 22.2 km (27 x 21.6 x 18.8)
  • mass: 1.08e16 kg
  Phobos is closer to its primary than any other moon in the Solar System, less than 6000 km above the surface of Mars; it is also one of the smallest moons in the solar system.

• Pronounced "FOH bus"

• In Greek mythology, Phobos is one of the sons of Ares (Mars) and Aphrodite (Venus). "phobos" is Greek for "fear" (the root of "phobia").

• Discovered 1877 August 12 by Hall, photographed by Viking 1 in 1977.

• Phobos orbits Mars below the synchronous orbit radius. 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 pulling it toward Mars. In less than 100 million years it will either break up into a ring or crash onto the surface of Mars.

• Phobos and Deimos may be composed of carbon-rich rock like C-type asteroids. But their densities are so low that cannot be pure rock. They are more likely composed of a mixture of rock and ice. Both are heavily cratered.

• The most prominent feature on Phobos is the large crater named Stickney (the maiden name of A. Hall's wife). Like Mimas's crater Herschel (on a smaller scale) the impact that created Stickney must have almost shattered Phobos. The grooves and streaks on the surface were probably also caused by the Stickney impact.

• The Soviet spacecraft Phobos 2 detected a faint but steady outgassing from Phobos. Unfortunately, Phobos 2 died before it could determine the nature of the material; water is the best bet.

• Phobos and Deimos are widely believed to be captured asteroids. There is some speculation that they originated in the outer solar system rather than in the main asteroid belt.

• Phobos and Deimos may someday be useful as "space stations" from which to study Mars or as intermediate stops to and from the Martian surface; especially if the presence of ice is confirmed.

Pictures

1. (above) Phobos's crater Stickney 21k jpg; 143k gif
2. another view of Stickney 121k gif; 26k jpg

3. the other side 35k gif

4. Map of Phobos 163k gif
5. Phobos, Deimos, Gaspra and Ida compared 96k jpg
6. Close up 17k gif
7. Close up 11k gif
8. Close up 13k gif

9. Close up 17k gif

10. Close up 18k gif

More about Phobos

• from LANL

Open Issues

• It seems likely that Phobos and Deimos formed elsewhere and subsequently captured by Mars. But how did the capture occur? Was it made possible by a thicker atmosphere long ago?