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 larger and innermost 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 more massive than Deimos, Mars's outer moon. Phobos is named after the Greek god Phobos, a son of Ares (Mars) and Aphrodite (Venus) which was 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 than any other known planetary moon. It is so close that it orbits Mars 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 15 min or less, and set in the east, twice each Martian day. Phobos is one of the least reflective bodies in the Solar System, and features a large impact crater, Stickney. The temperatures range from about −4 °C (25 °F) on the sunlit side to −112 °C (−170 °F) on the shadowed side.

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 2 meters closer to Mars every one hundred years, and it is predicted that in 30 to 50 million years it will collide with the planet or break up into a planetary ring.

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