Mars rovers prepare for a dramatic last act

By the end of this week, though, Spirit should arrive at its promised land: the westernmost spur of the hills. "We really don't know what we're going to find," says Dr. Rice. "Mars has a story to tell and our job is to unravel that story."

From space, it looks fairly convincing that the story of Gusev Crater involves water. Signs of erosion and flooding suggest that large amounts of liquid water once reshaped the landscape.

That would make the Columbia Hills islands in a Martian ocean that spread to the bend of each horizon - and their flanks might show evidence. When Spirit arrives, it will look for an outcrop or a rock that fell from the highlands, and begin its most eagerly anticipated work of the mission. Later, the rover might scoot up the side of the hills, yielding a panorama, but scientists don't expect to summit the peaks.

That Spirit is even still going is something of a marvel. Spirit has now traveled six times its expected distance, "and we've got a fair amount of rover left," says Dr. Squyres. The performance has been so robust, in fact, that the scientists are considering shutting the rovers down at the height of the Martian winter (September) and then starting them back up in the spring when they can draw more solar power from longer days.

For its part, Opportunity has a broken heater that will eventually ruin one of its noncritical sensors. But the biggest problem might not be power or heaters, but basic traction. At the moment, it stands at the edge of a 60-foot-deep crater called Endurance. Ahead lies a trove of science - and the possibility of getting stuck.

Scientists will proceed slowly down a slope that would challenge an intermediate skier, testing Opportunity's traction. But there's the chance that Opportunity will never emerge.

"The worst thing that could happen is that we spend the rest of the mission in the candy store," says Squyres, adding that the science at Endurance "is much more of a sure thing" than in the Columbia Hills.

That is because, scientists have already filled in part of Meridiani's story with the research completed at the small landing crater. It's just a question of going deeper into the geological record. From the bedrock in the landing crater, which is less than 10 feet deep, scientists already know that there was water at some time in the past. Now, the deeper and older bedrock at Endurance will tell them what was there before.

It is a profound question. If the rocks show no water, they might offer clues about Martian climate change. If scientists find further evidence of water, though, it suggests that water stayed for a long time, raising the likelihood of life. Says Bruce Betts of the Planetary Society: "The duration is a very significant question for organic life."

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