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)
Are Mars's moons really captured asteroids?
An American astronomer discovers Mars' moons.
The 19th and early 20th century American astronomer Asaph Hall discovered Mars' small moons, Phobos and Deimos. Observations of the moons' orbits allowed others to calculate the Red Planet's gravity, which is just under half that of Earth's.
Patrick Moore describes the asteroid belt and its discovery.
Sir Patrick Moore explains how the asteroid belt between Mars and Jupiter was discovered. He also talks about Bode's Law, an 18th century empirical rule that was once thought to predict planets' orbits. It is now discredited by astronomers, including Sir Patrick.
Before launch in 1988, Phillip Clark explains the ill-fated Russian Phobos missions.
The Soviet Phobos 1 and 2 probes were launched in 1988 and were meant to study Mars's moon Phobos in detail. However, Phobos 1 failed due to a software glitch and Phobos 2 malfunctioned shortly before mission controllers attempted to send two landers from the orbiter onto the moon's surface. In this clip Soviet space programme analyst Phillip Clark explains the probes' missions.
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 // FOH-bəs or // 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.