Best place to find water on Mars may be out of rover's range

PASADENA, Calif. -- NASA's Spirit rover traveled 300 million miles to search for evidence that frozen, dry Mars was once a wetter planet capable of supporting life. But scientists now say their best chance for finding that evidence may be out of the robot's range.

Spirit landed in Gusev Crater, a 95-mile-wide depression thought by some to have contained a lake in the ancient past, but it has turned out to be far from a pristine dry lake bed.

The broad depression appears to have been blanketed by volcanic debris and scoured by the wind, with the deposits of lake sediments that scientists had hoped to find either buried or erased by 4 billion years of vigorous geologic activity.

"Goodness knows what you might have stripped away," said Mars scientist Maria Zuber, of the Massachusetts Institute of Technology.

That means Spirit might have to roam farther than NASA expected to find the evidence scientists seek. The most promising place appears to be in a group of hills that NASA is not even sure the rover can reach.

"It might take a bit of searching to find," said Dave Des Marais, of NASA's Ames Research Center.

Scientists say the hills, which rise hundreds of feet above the Martian plain, represent a far bigger slice of the planet's history than the relatively shallow crater Spirit is visiting first.

But the hills are farther away than Spirit was designed to travel, even if the solar-powered rover outlasts its three-month lifetime. Still, mission engineers continue to calculate how much ground Spirit would have to cover to reach their slopes.

Scientists are also holding out hope that the rocks in Spirit's more immediate surroundings could be the smoking gun. Examples would include any carbonate rocks, such as limestone, that form in water.

"Rocks are like little time capsules. They remember what formed them and the time that formed them," Des Marais said.

Short of that, the closest evidence could be as close as 825 feet away, inside a crater that the air bag-swaddled Spirit nearly landed in when it bounced down on the planet two weeks ago.

As Spirit zigzags toward that depression, rolling dozens of yards a day, the robot geologist should come across older rocks. Once at the crater, scientists hope Spirit can reach the rim and peer down. The rover has nine cameras, including a microscopic imager, and a rock drill to examine the terrain.

Scientists hope to find the crater walls striped with horizontal layers of sedimentary rocks, which would suggest that Gusev once sloshed with water.

A second martian spacecraft, Opportunity, is on track to land Jan. 24 halfway around the planet from Spirit. Its landing site abounds in a mineral called gray hematite, which is associated with liquid water.

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