We haven't quite found Martians yet, but if the Opportunity rover's latest find is any indication, we could be inching closer. NASA scientists announced on Friday that the rover had found what appears to be a clay-rich rock -- a clue to past water on the planet. Of course, the water (and the life-promoting conditions it could have brought) would have been in abundance about three and a half billion years ago. It doesn't exactly tell us where to find contemporary aliens, but the forearm-length rock adds to the portrait of our sister planet. More importantly, it adds to scientists' knowledge of what it takes to make a planet habitable. Next stop, terraforming?