Birth of a Moon 
At first glance, Earth's Moon and the largest moon of Mars don't seem to have much in common. Our moon is a sphere more than 2,000 miles in diameter. Phobos is shaped like a potato with a big bite out of one end, and is only 17 miles long.
But the two moons may have one important characteristic in common: both may owe their existence to "big whacks."
The leading theory says that Earth's moon was born after a body as big as Mars slammed into our planet. The impact spewed debris into orbit around Earth, which coalesced to form the Moon.
And a recent study by scientists in Europe says that Phobos may have formed in the same way.
Since the discovery of Phobos and Mars's other moon, Deimos, the leading theory has said that both were asteroids that were captured by Mars.
But by studying observations from two Mars-orbiting spacecraft, the European scientists concluded that the composition of Phobos is closer to that of Mars than the asteroids. They also found that Phobos is a pile of boulders with lots of empty space between them, suggesting that the moon formed when big chunks of rock clumped together.
If so, then Phobos most likely was born from the debris from a big collision -- a collision between an asteroid and either Mars itself or a bigger moon, which was pulverized.
The case isn't closed, though. Scientists need more evidence to know for sure if the moon was an interloper -- or the result of a giant collision.
Script by Damond Benningfield, Copyright 2011