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?
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.
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.
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.
Phobos (pron.: /ˈfoʊbəs/ FOH-bəs; Greek: Φόβος; systematic designation: Mars I) is the larger and closer of the two natural satellites of Mars. With a mean radius of 11.1 km (6.9 mi), Phobos is 7.24 times as massive as the second moon Deimos. It is named after the Greek god Phobos (which means "fear"), a son of Ares (Mars) and Aphrodite (Venus) . Both moons were discovered in 1877.
A small, irregularly shaped object, Phobos orbits about 9,400 km (5,800 mi) from the center of Mars, or about 6,000 km (3,700 mi) from the Martian surface, closer to its primary than any other known planetary moon. Phobos is one of the least reflective bodies in the Solar System, and features a large impact crater, Stickney crater. It orbits so close to the planet that it moves around Mars faster than Mars rotates. As a result, from the surface of Mars it appears to rise in the west, move across the sky in 4 h 15 min or less, and set in the east twice each Martian day. Due to its short orbital period and tidal interactions, Phobos's orbital radius is decreasing and it will eventually break up into a planetary ring.