Today's Moon is Phobos, one of the two small moons of Mars.
Phobos is the first moon we've featured with an irregular shape. Since it is a very small moon (with a mean radius of only 11.1 km), there is insufficient gravity to pull it into a sphere. This also means that the escape velocity is very low: only about 40 km/h (compare with Earth's 40,269 km/h escape velocity). Since its composition and density have much in common with C-Type asteroids, many astronomers believe that Phobos and Deimos are captured from the main belt between the orbits of Mars and Jupiter. But this theory is not entirely proven, as there is so far no definitive explanation for the circularizing of their orbits.
Take a look at this incredible full size version of the picture above, courtesy of the Mars Reconnaissance Orbiter.
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