WASHINGTON (Reuters) - NASA (news - web sites)'s Mars rover Opportunity is sending data back to Earth from an ancient martian seashore, scientists reported on Tuesday. "We think Opportunity is now parked on what was once the shoreline of a salty sea on Mars," said Steve Squyres, principal investigator for the science payload on Opportunity and its twin Mars exploration Rover, Spirit. On March 2, astronomers announced that the Red Planet was "drenched with water" at some point. But the rovers' analysis of Mars rocks has now produced the first concrete evidence that liquid water might actually have flowed on...