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Mars' Moon -- Phobos
Mars' Moon Phobos -- in astronomy, innermost moon, or natural satellite, of Mars.
Phobos orbits Mars at a distance of only 9,378 km (5,627 mi), closer to its planet than any other moon in the solar system. In fact, it is so close that the force of Mars's gravity is stronger than the force keeping the moon in its orbit, so the radius of Phobos's orbit is decreasing at the rate of about 1.8 m (about 6 ft) per century.
In 40 million years, Phobos will either break apart into a ring around Mars or crash into the Martian surface. The little moon travels around Mars three times each Earth day in a circular orbit and, as seen from the surface of Mars, crosses the Sun's disk about 1,300 times a year.
Phobos's shape is irregular�the moon's widest diameter is 27 km (16 mi) and its smallest diameter is 19 km (11 mi). Though it is larger than Deimos, the other Martian moon, Phobos could easily fit inside a medium-sized crater on the earth's moon.
Phobos is made of dark, carbon-rich rocks common in the outer asteroid belt, a band of thousands of objects that orbit the sun between the orbits of Mars and Jupiter.
The little moon has a lower density than it would have if it were solid rock, so it probably also contains water ice. Phobos was probably once an asteroid, but no one knows how it came to orbit Mars.
Phobos's most distinctive feature is a 10-km (6-mi) wide crater called Stickney. The asteroid collision that blasted out the crater nearly shattered the little moon. Cracks cover Phobos's surface and near Stickney the cracks are up to 700 m (2,100 ft) wide and 90 m (295 ft) deep.
Phobos is too small to have an atmosphere, though the Soviet Mars probe Phobos 1 detected a tiny amount of water vapor near the moon in 1989. This vapor probably comes from water ice inside Phobos.
Phobos was discovered by American astronomer Asaph Hall in 1877. The moon is named for a character in Greek mythology who was the son of Ares and Aphrodite. The Romans considered him an attendant of the war god Mars. Features on Phobos are generally named for astronomers who searched for or studied the two Martian moons; however, the crater Stickney bears the maiden name of Asaph Hall's wife, who convinced him to continue looking for Martian moons after he became discouraged and decided to give up.
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