Three weeks after reporting that the Opportunity rover’s landing site on Mars was once wet, scientists went even further on Tuesday, declaring that the now-barren rocks were formed at the bottom of an ancient body of saltwater. The findings, announced at a NASA news briefing in Washington, represent an important link in a chain of evidence hinting that the Red Planet was wet enough and warm enough for a long enough time to support the development of life. Moreover, if organisms ever did arise, their fossils should still exist within Martian rock, the scientists said.