Martian rovers haven't found evidence of it. But the NASA interplanetary dune buggies have found evidence that water once existed on the Red Planet. If Spirit and Opportunity make it through the harsh Martian winter, they may discover more. Otherwise, NASA will settle for the spectacular images and research beamed back to Earth by the two mobile labs roaming Mars' surface.
After surviving landings in which both rovers bounced hard against the planet's surface, Spirit and Opportunity have been on the move. The rovers can drive only about 44 yards in one Martian day (slightly longer than a 24-hour day). But since practically every inch of Martian terrain the rovers roll over is an opportunity for a science project, NASA has been more than willing to take its time.
In the early spring, Opportunity discovered remnants of salt water-a discovery corroborated by Spirit in a different location. By April, both rovers completed their primary three-month missions. The extra eight months NASA counts as a bonus. Even at $820 million, the combined mission is a research marvel and a scientific bargain. The space agency plans to drive the rovers until the solar panels stop working.