The two Mars Rovers, Opportunity and Spirit, working on opposite sides of the planet, have already achieved their shared goal: to find evidence of liquid water on the barren world. Although they didn’t find actual glistening pools of Martian water, they did discover strong evidence that Mars was once a world drenched in water, with rivers and streams flowing into larger basins and perhaps even an ocean or two. Important questions remain unanswered: Where did the water go? How long was it on the planet? And, because water is one of the key elements needed for life as we know it, was there — is there — life on that hunk of rock next door?