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Ground truth gives scientists the confidence to interpret wide stretches of geography seen from afar. Powell climbed mile-high cliffs while scrutinizing their colorfully layered limestone, sandstone, marble, chert, volcanic rock, and gneiss. Once on top he could gaze across tens of miles of canyonland, sketch maps in his notebook, and knowingly label distant strata from their colors and continuities. Similarly, thanks to reports from the six-wheeled rovers exploring two small patches of Mars, researchers can make sense of the sometimes ambiguous images from satellites orbiting high above.

Take the sinuous channels visible on Mars's parched countenance. They so resemble huge, dry riverbeds that flowing water in some long-ago epoch seemed a very good bet. But proof was lacking. Some scientists argued that frigid surges of liquid carbon dioxide might have carved the channels. Others said Mars was never warm enough for more than the briefest pulses of water because it seems to lack the extensive limestone and other carbonate minerals that a warm, carbon-dioxide-rich atmosphere would have left behind. "I spent my career describing water on Mars," says Mike Carr, a geologist at the U.S. Geological Survey in Menlo Park, California. "But we just couldn't find the carbonate. I was starting to tell people my whole career maybe was ashes."

No longer. The rovers, Spirit and Opportunity, have found direct and convincing evidence that water sometimes sloshed across Mars, almost certainly during the planet's earliest epoch well over three billion years ago. Perhaps only puddles, streams, and flash floods came and went, drying repeatedly. But it's looking more and more likely that liquid water was once abundant, though short-lived. Even oceans, however ephemeral, remain a possibility, and on a planet once warmer and wet, life—past or present—is a tantalizing prospect.

The solar-powered robot explorers that have provided this ground truth rarely cover more than the length of a football field between sunrise and sunset. A flesh-and-blood geologist in a spacesuit (sent to Mars at a cost of untold billions) could do in a week what either machine has managed in more than a year. But the rovers are the best stand-ins yet for people on an alien planet. "From the start, we wanted a view with human visual resolution," says Jim Bell, a Cornell University astronomer and lead scientist on each rover's panoramic camera, or Pancam. The Pancam has stereo vision, like people, and its lenses ride at about human height: five feet up on a mast rising above a deck of solar panels.

For Larry Soderblom, a geophysicist and planetary specialist with the USGS in Flagstaff, Arizona, the view through those lenses triggers what he calls an enchantment. The detailed images of alien vistas, after one pores over them for days or weeks, commandeer the imagination. In one corner of the brain, says Soderblom, "you start to think and believe that you are actually there."

The scientists who send the rovers their daily marching orders often sound as if they're personally crunching across the red planet's surface, taking pictures, eyeballing sediments, and grinding holes.

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