Posted by jburk to Planetology at March 7, 2004 11:33 AM

The Mars rover Opportunity's examination of Martian rocks last week provided the first convincing evidence that our neighbour world was once "awash" in water, as one NASA scientist described it. But where did the water come from? And why does Mars have no liquid water now, while Earth apparently has been covered with the stuff for 4 billion years?