Orbiting spacecraft previously detected the presence of clay-bearing deposits at a huge crater in Mars' southern hemisphere. Using that information as a guide, the six-wheel, solar-powered rover drove around the rim and encountered light-colored rocks never before seen in past explorations.
"This is the sweet spot," said mission chief scientist Steve Squyres of Cornell University. "This is the place where the orbital data tells us that the clays are present."
Squyres provided the update at a gathering of the American Geophysical Union in San Francisco.
Clays are important because they hold clues about the past Martian climate. They form in watery environments that are not too acidic and not too alkaline -- in other words, conditions that might have been more suitable for microbes.
Since landing in 2004, Opportunity has logged 22 miles, poking into four impact craters. The latest destination -- Endeavour Crater -- is the largest yet and contains the oldest deposits. Scientists believe the light-hued outcrops along Endeavour's western rim contain clays and will spend the next several months conducting experiments at the site before heading south to a region where a mother lode of clay minerals is thought to exist.
Opportunity previously uncovered geologic evidence of a more tropical past unlike today's frigid and radiation-scarred landscape. But Squyres said the minerals uncovered so far point to a more acidic environment. The clays, however, formed "in water you could drink" that had a neutral pH, he said.
Opportunity is not the only Mars rover chasing clays. NASA's newest spacecraft, the car-size Curiosity, is set to trek early next year to the slope of a mountain near the equator that's thought to have stacks of clay layers.
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