Phobos is the largest of the two moons of mars, it is also the innermost. The diameter of, the heavily cratered, Phobos is approximately 16 by 13 by 11 miles. It is also the closest Moon to any planet in the Solar System, with a distance of 3728.2 miles from the surface of Mars.
This moon was discovered on August 17, 1877 by Asaph Hall, and is named after, Fear, the Greek mythological son of Ares, the god of war. Ares is the Greek counterpart of the Roman god of war, Mars.
The most major feature on Phobos is the large crater named Stickney, after the maiden name of Hall's wife. This large crater may have been created when the moon initially crashed into Mars, while the crash left a large crater the moon stayed in orbit with the planet.
Phobos may be composed of carbon-rich rock- like asteroids, it is extremely dark, and very similar to carbonaceous chondrite meteorites. It is speculated that since the density is very low that it is most likely composed of a mixture of rock and ice.
In 1988 the Soviet Union launched the spacecraft Phobos 2 to do a scientific exploration of Phobos. When it arrived it detected a outgoning gas from the moon, but the spacecraft died before further analysis could be completed. It is speculated that the gas, was melting ice water vapor being released, as the moon was heated by the Sun.
Unfortunately, Phobos is on a collision course with Mars, it orbit below synchronous altitude tidal forces are lowering its orbit at approximately 1.8 meters every 100 years. It is speculated that it will crash into the planet in 50 million years or break up upon contact and create a ring around the planet.
There has been discussion, between scientists, of using Phobos as a space station, for use during the future explorations of Mars.