DENVER, COLORADO -- A future astronaut traipsing across the landing sites of the Mars Exploration Rovers – Spirit and Opportunity – might be squishing into a welcome mat of microbes, according to one NASA scientist. ... Unfolding saga ... The observations... strongly suggested primitive life may have existed on Mars more than 3.6 billion years ago. ... Microbial mat ... The Mars rovers represent an incredibly successful mission, McKay said. Moreover, they seemingly have "uncovered" a major find. He points to pictures taken by the rovers that show areas of the martian surface disturbed by the retraction of landing airbags....