The Mars Rover Opportunity is the first of the two twin robots to discover strong evidence that water once flowed on mars. "That does not mean life was there. We don't know that, but this was a habitable place on Mars at one point in time," said Cornell University astronomer Steve Squyres who works as the principal investigator for NASA's Spirit and Opportunity rover missions. But as Squyres explains, "This was the kind of place that would have been suitable for life."