Martian Mayhem 
The largest moon of Mars might have had a violent birth. A large asteroid might have slammed into the planet, blasting billions of tons of material into space. Some of this material settled into orbit, then coalesced to form Phobos, a chunk of rock that’s only about 15 miles long.
In decades past, many astronomers thought that Phobos and its sister moon, Deimos, were born in the asteroid belt. They were later expelled from the belt, and eventually made their way closer to the Sun, where Mars captured them. It’s a complicated process, requiring extra bodies or a cloud of debris around Mars, but it seemed the most likely scenario.
More recently, though, a second idea has gained favor: that like our own Moon, one or both of the Martian moons were born as the result of a “big whack.”
Two recent Mars orbiters have analyzed the surface of Phobos, and found that its composition is similar to the surface of Mars. One of the orbiters also found a mineral that requires a good source of heat -- like a big whack. And it found that Phobos is most likely a big pile of boulders with lots of empty space between them -- suggesting that it formed when many small chunks of rock clumped together.
The scientists who studied the mission results say the debate isn’t over yet. But future missions -- including a Russian probe that’s scheduled to land on Phobos -- should tell us for sure whether the little moon was adopted -- or born of Mars.
Script by Damond Benningfield, Copyright 2010