The Gusev crater, named after 19th-century Russian astronomer Matvey Gusev, has long been suspected of harboring—in the distant past—a lake of liquid water and ice, making it a useful source of evidence for exobiologists investigating the possibility of ancient Martian life. When NASA's Spirit rover was sent to Mars ten years ago, project leaders had it traverse a four-mile stretch of prime Martian real estate along the crater. You can follow Spirit's journey in this haunting computer-generated video:
Sadly, Spirit didn't find any immediate evidence of water and ice deposits in the Gusev crater. There was some ambiguous evidence, sure—and nothing to exclude the possibility that the crater once contained water—but we didn't find the clear, unmistakable evidence that most of us had hoped, and some of us had expected, to find. The case went cold.
But one of the wonderful things about the scientific endeavor is that you don't always need new evidence to prove something—sometimes, analysis of old evidence is enough. And in this case, a team of geologists has reopened the case—and found that, although we'd missed it at the time, the Spirit rover did uncover evidence of a lake in the Gusev crater after all. It's one more piece of evidence that points to a Martian landscape teeming with water, ice, and—quite possibly—life.