Image: Trench
A photo taken by the Opportunity rover's camera system shows the 4-inch-deep trench that the rover excavated by spinning one of its wheels.

The Opportunity rover spun one of its six wheels to dig into Mars for the first time, taking just minutes to excavate a 4-inch-deep (10-centimeter-deep) trench that should open a window into the planet's past, scientists on the NASA mission said Tuesday.

"Yesterday, we dug a nice big hole on Mars," rover planner Jeffrey Biesiadecki told a news conference at NASA's Jet Propulsion Laboratory.

Opportunity sat parked on the gentle slope of a small crater while digging the hole, which took six minutes of wheel spinning to complete. The rover spun its cleated, front right wheel in one direction and then in the other, repeating the two-part process six times in all, to scoop out the 20-inch-long (50-centimeter-long) trench, Biesiadecki said.

Clotted chunks
The operation exposed material buried beneath the surface, including clotted chunks that could be seen protruding from the wall of the trench, said deputy main scientist Ray Arvidson, of Washington University.

"It's not like sand in an hourglass," Cornell University scientist Rob Sullivan said of the clodlike material.

Black-and-white images taken during the digging showed its progress, including the small piles of excavated material that the rover gently tamped down to ensure they didn't slide back into the hole.

The rover measured the mineralogy, texture and elemental composition of the trench site before digging and then began a second set of observations once it completed the task. Full results from the before-and-after measurements were expected Thursday.

Scientists chose the dig site because earlier observations revealed it is rich in an iron-bearing mineral called hematite, which typically forms in water.

Measurements should reveal if the mineral is limited to the surface or is present throughout the soil, Arvidson said. The scientific observations could also turn up traces of other minerals, including goethite, that might point to past water activity at the site.

Opportunity and its twin, Spirit, are searching for geologic evidence that Mars was once a wetter place hospitable to life.

Another distance record set
Spirit, on the other side of Mars, continued its 1,150-foot (350-meter) trek to an impact crater. As of Tuesday, Spirit had traveled 356 feet (108.5 meters), overtaking the 337 feet (102.7 meters) covered by the far smaller Sojourner rover during 1997's Pathfinder mission, project manager Richard Cook said.

Image: Spirit's tracks
NASA / JPL / Cornell
The Spirit rover's panoramic camera looks back at the probe's tracks in the Martian surface.
Cook replaced Pete Theisinger as head of the $820 million double mission, NASA announced Tuesday. Theisinger now heads the Mars Science Laboratory mission. NASA plans to launch the nuclear-powered rover, about the size of a sport utility vehicle, to Mars in October 2009.

Spirit could reach its destination in as little as 12 more days of driving, depending on how many stops it makes on the way to examine rocks flung out from the impact that created the crater, Arvidson said.

"The idea is to drive and look, drive and look," Arvidson said.

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