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Mars' Moon -- Deimos
Mars' Moon Deimos -- outermost of two small moons orbiting the planet Mars. Deimos orbits Mars at a distance of about 23,500 km (about 14,100 mi), completing an orbit once every 1.26 Earth days. The moon's orbit is almost circular and is only slightly tilted relative to the Martian equator.
Deimos is irregular in shape, measuring about 15 km (about 9 mi) along its longest side and about 11 km (about 6.6 mi) along its shortest side. It is the smallest known moon in the solar system.
If it was located farther from the earth among the moons of the outer solar system, it might not be known today. Deimos could easily fit inside a medium-sized crater on Earth's moon. The tiny moon is made of dark, carbon-rich rocks that are common in the outer asteroid belt, a group of asteroids that orbits between the orbits of Jupiter and Mars.
Deimos has a low density, however, which means it probably also contains water ice. Scientists theorize that Deimos and the larger Martian moon, Phobos, are asteroids captured by Mars billions of years ago. At that time Mars had a thick, dense atmosphere.
According to one theory, Deimos passed through the Martian atmosphere and was slowed enough that Mars's gravity could capture it. However, scientists are still uncertain about how Mars got its moons.
Deimos is peppered with craters dug out by collisions with meteoroids and small asteroids. The two largest craters, Swift and Voltaire, are each about 3 km (about 1.8 mi) across. Deimos's surface is generally smoother than that of Phobos, and is covered with a layer of loose material called regolith.
Deimos was discovered by American astronomer Asaph Hall when Mars came near Earth in 1877. The moon is named for a character in Greek mythology, who was the son of the war god Ares and the goddess of love and beauty, Aphrodite. In Roman mythology, Deimos was an attendant of the war god Mars. Features on Deimos are generally named for authors who wrote about the Martian moons.
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