One of a pair of irregular, small moons orbiting Mars, Phobos was discovered by the American astronomer Asaph Hall in 1877 along with its fellow satellite, Deimos.

Both moons are grey in colour, cratered and generally similar in appearance to asteroids that orbit between Mars and Jupiter. One theory is that both moons were once asteroids that were captured by Mars's gravitational force, but this has not been confirmed.

Photo: Phobos taken by the Mars Global Surveyor probe (NASA/JPL/Malin Space Science Systems)

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About Phobos

Are Mars's moons really captured asteroids?

About Phobos

Phobos (systematic designation: Mars I) is the innermost and larger of the two natural satellites of Mars, the other being Deimos. Both moons were discovered in 1877 by American astronomer Asaph Hall.

Phobos is a small, irregularly shaped object with a mean radius of 11 km (7 mi), and is seven times larger than the outer moon, Deimos. Phobos is named after the Greek god Phobos, a son of Ares (Mars) and Aphrodite (Venus), and is the personification of horror. The name "Phobos" is pronounced /ˈfbəs/ FOH-bəs or /ˈfbɒs/ FOH-bos, or like the Greek Φόβος.

Phobos orbits 6,000 km (3,700 mi) from the Martian surface, closer to its primary body than any other known planetary moon. It is indeed so close that it orbits Mars much faster than Mars rotates, and completes an orbit in just 7 hours and 39 minutes. As a result, from the surface of Mars it appears to rise in the west, move across the sky in 4 hours and 15 minutes or less, and set in the east, twice each Martian day.

Phobos is one of the least reflective bodies in the Solar System, with an albedo of just 0.071. Surface temperatures range from about −4 °C (25 °F) on the sunlit side to −112 °C (−170 °F) on the shadowed side. The defining surface feature is the large impact crater, Stickney, which takes up a substantial proportion of the moon's surface.

Images and models indicate that Phobos may be a rubble pile held together by a thin crust, and that it is being torn apart by tidal interactions. Phobos gets closer to Mars by about 2 meters every one hundred years, and it is predicted that within 30 to 50 million years it will either collide with the planet, or break up into a planetary ring.

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