There is a theory that a rogue planet came through our solar system and collided with earth to form the moon. The water could have come from that exchange of material and then subsequent comets."The collision that formed our moon would have been tremendous, enough to melt the surface of Earth," said co-author Geoff Bryden of NASA's Jet Propulsion Laboratory, Pasadena, Calif. "Debris from the collision most likely settled into a disk around Earth that eventually coalesced to make the moon.http://www.universetoday.com/2009/08/10/spitzer-fi ...