The Moons of Mars:
Mars has two relatively small natural moons, Phobos and Deimos, which orbit close to the planet. Asteroid capture is a long-favored theory, but their origin remains uncertain. Most probably, they were two asteroids captured during the Late Heavy Bombardment of the Solar System 3 or 4 billion years ago.
Enhanced-color HiRISE image of Phobos, showing a series of mostly parallel grooves and crater chains, with its crater Stickney at right.
From the surface of Mars, the motions of Phobos and Deimos appear very different from that of our own moon. Phobos rises in the west, sets in the east, and rises again in just 11 hours.
Enhanced-color HiRISE image of Deimos (not to scale), showing its smooth blanket of regolith.
Deimos, being only just outside synchronous orbit – where the orbital period would match the planet's period of rotation – rises as expected in the east but very slowly. Despite the 30 hour, orbit of Deimos, 2.7 days elapse between its rise and set for an equatorial observer, as it slowly falls behind the rotation of Mars. No telling what tidal effects these two moons had on the oceans of Mars when they existed.
Video Clip on Martian Moons
Because the orbit of Phobos is below synchronous altitude, the tidal forces from the planet Mars are gradually lowering its orbit. In about 50 million years, it could either crash into Mars's surface or break up into a ring structure around the planet.
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