Phobos is the largest and innermost of Mars' two moons. 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. Phobos was discovered on August 12 1877 by Asaph Hall, and was photographed by Mariner 9 in 1971, Viking 1 in 1977, and Phobos 2 in 1988.
Phobos orbits Mars below the synchronous orbit radius, meaning that it moves around Mars faster than Mars itself rotates. 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. This orbit means that Phobos will soon be destroyed: tidal forces are lowering its orbit (currently at the rate of about 1.8 meters per century), and in about 50 million years it will either impact the surface of Mars or (more likely) break up into a planetary ring.
Phobos is a dark body that appears to be composed of C-type surface materials. It is similar to the C-type (blackish carbonaceous chondrite) asteroids that exist in the outer asteroid belt. Phobos' density is too low to be pure rock, however. It is probably composed of a mixture of rock and ice. 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, but it is most likely water. Recent images from Mars Global Surveyor indicates that Phobos is covered with a layer of fine dust about a meter thick, similar to the regolith on the Earth's Moon.
Phobos is heavily cratered. The most prominent feature on Phobos is the large crater named Stickney, the maiden name of Asaph Hall's wife. Like Mimas' 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.
Phobos is widely believed to be a captured asteroid. There is some speculation that they originated in the outer solar system rather than in the main asteroid belt. It is not known how the capture could have taken place.
- Orbital radius: 9378 km (note: measured from the center of Mars)
- Diameter: 22.2 km (27 x 21.6 x 18.8)
- Mass: 1.08×1016kg
- Orbital period: 0.31910 days
- Density: 2.0 g/cm3
- Orbital inclination: 1.0°