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Phobos - Click for a bigger image

Technical Data:-

Diameter 22.2 km(Phobos) 12.6 km(Deimos)
Average Distance from Mars 9,378 km(Phobos) 23,459 km(Deimos)
Max surface Temperature -4C(Phobos)
Min surface Temperature -112C(Phobos)
Length of Day 1.026 days(Phobos) 1.026 days(Deimos)
Length of Year 0.3191(Phobos) 1.26244(Deimos)
Eccentricity of Orbit 0.015(Phobos) 0.0005(Deimos)

2 gm/cm3(Phobos) 1.7 gm/cm3(Deimos)

Atmosphere n/a

More pictures of Deimos and Phobos Here

    Phobos, Mars' closest satellite, is named after the Greek word meaning 'fear', whilst Deimos means 'terror', both following on from the Mars/War theme. Orbiting at a meagre 6000km from the surface of Mars, Phobos is the closest moon to its parent planet in the Solar System (at least that we know of), whilst Deimos is the smallest known planetary moon in the Solar System.

Phobos and Deimos are both believed to be captured asteroids, due to their irregular shape and small size (Phobos having a 22.2km wide diameter, whilst Deimos is only 12.6km wide!). Both moons are heavily cratered and are believed to be made up of rock and ice. Like our moon they have a coating of dust on their surfaces. Phobos' orbit is such that it rises in the west and sets in the east, often more than once in one day.

However, I'm afraid there doesn't really appear to be much of a future left for Phobos as an independant moon of Mars. This is because in little over 50 million years it is destined to crash into Mars (with a thud) due to the planet's tidal forces lowering its orbit (presently at about 1.8cm/year). Nevertheless, it (and Deimos) have been seen by many spacecraft since it was discovered by Asaph Hall in 1877, such as the Mariner 9, Viking 1, Phobos and Phobos 2, the last of which even detected an outgassing coming from Phobos.