Mission controllers at Jet Propulsion Laboratory directed the two rovers - Spirit at Gusev Crater and Opportunity at Meridiani Planum, both on near the equator but half a world away from each other - to perform two important but very different drives after recent weeks of probing more sites for Martian geological history.

Opportunity finished examining sedimentary evidence of ancient water at a crater called Erebus, and now is rapidly crossing flat ground toward a much larger crater called Victoria.

Despite the recent and permanent failure of its right-front wheel, Spirit has been able to study signs of an ancient at a bright, low plateau called Home Plate. At first, controllers tried to move Spirit up a north-facing slope so its solar panels could gather maximum sunlight during the approaching winter. When that attempt failed, they quickly designed an alternate route.

"For Spirit, the priority has been to reach a safe winter haven," said Steve Squyres of Cornell University, the principal investigator for the Mars Exploration Rover project.

To date, the rovers have operated more than eight times longer than their original 90 Martian day, or sol, mission. Each has driven more than 6.8 kilometers (4.2 miles) about 11 times farther than planned.

Together, the two golf-cart-sized rovers have returned more than 150,000 images, and both have found sites on Mars that once had been wet and possibly harbored a habitable environment.

Opportunity has spent most of the past four months at Erebus, a highly eroded impact crater about 300 meters (1,000 feet) in diameter, where the rover found extensive exposures of thin, rippled layering interpreted as a fingerprint of flowing water.

"What we see at Erebus is a thicker interval of wetted sediment than we've seen anywhere else," said mission team member John Grotzinger of the California Institute of Technology. "The same outcrops also have cracks that may have formed from wetting and drying."

In mid-March, Opportunity began a 2 kilometer (1.6-mile) trek from Erebus to Victoria, a crater about 800 meters (half a mile) across, featuring a thick sequence of exposed sedimentary rocks. In the past three weeks, Opportunity already has driven more than one-fourth of that distance.

At Home Plate, Spirit found coarse layering overlain by finer patterns that suggest the accumulation of material falling to the ground after a volcanic or impact explosion. In one place, the layers are deformed where a golfball-size rock appears to have fallen on them while they were soft.

Geologists call this type of formation a bomb sag, and it presents strong evidence for some kind of explosive origin, Squyres said. "We would like to have had time to study Home Plate longer, but we needed to head for a north-facing slope before winter got too bad."

Spirit sits slightly in the Martian southern hemisphere, where the Sun has been crossing lower in the northern sky each day. Because the rovers rely on solar power, the amount of sunlight available will keep dropping until the shortest days of the Mars winter, in four months.

To keep producing enough electricity to run overnight heaters that protect vital electronics, Spirit's solar panels must be tilted toward the winter sun by driving onto north-facing slopes.

On March 13, however, the right-front wheel's drive motor gave out. Spirit subsequently has driven about 80 meters (262 feet) using five wheels and dragging the sixth, but an initial route toward a large hill proved impassable due to soft ground.

Last week, the team chose a smaller nearby ridge dubbed Low Ridge Haven as the winter destination. Spirit reached the ridge Sunday and has gained a favorable 11-degree northern tilt.

"We have to use care choosing the type of terrain we drive over," said JPL rover planner Ashitey Trebi-Ollennu, adding that the mission team has been practicing a maneuver to gain additional tilt by perching the left-front wheel on a basketball-size rock.

The plan now is for Spirit to spend the next eight months or so at Low Ridge Haven. That stint should provide time for the rover to conduct many long-duration studies that mission scientists have desired since early on.

Ray Arvidson, of Washington University in St. Louis, who is deputy principal investigator for the rovers, said these studies will include detailed mapping of rocks and soils; in-depth determination of rock and soil composition; monitoring of clouds and other atmospheric changes; watching for subtle surface changes due to winds, and learning properties of the shallow subsurface by tracking surface-temperature changes over a span of months.

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