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Web Extra Wednesday, December 15, 2004

Correction: Opportunity did not observe a new mineral, as reported below, but Spirit observed a new rock type.

Evidence for water on Mars flows

This week, scientists further confirmed the presence of water on Mars, almost a year after the Mars Exploration Rovers landed on the fourth rocky planet from the sun. Spirit, which is now in the Columbia Hills and 336 martian days into its travels on Mars, finally found evidence of water, several months after its twin Opportunity did. And Opportunity continues to see further signs of water on Mars, from the ground to the sky.

This panorama image of Burns Cliff on Mars was taken by the rover Opportunity, while it was perched below the tens of meters of rhythmic layers in the cliff wall. The composite image makes the gently curving cliff face appear to bulge towards the rover. Courtesy of NASA/JPL.

After landing in Gusev Crater last January (halfway around the planet from twin rover Opportunity's landing site later that month), Spirit exited its terrain of volcanic deposits and bare soils around its 150th day on Mars. Seven martian days later, it entered the hills that it is now traversing, where it has seen sedimentary rocks with fine, almost horizontal layering that contains sometimes-cemented granules that vary in size and angularity.

"We think these are deposits from a fluid," said Raymond Arvidson of Washington University in St. Louis, Mo., at a press conference on Monday at the annual fall meeting of the American Geophysical Union in San Francisco. The team is now debating whether the granules deposited are from volcanic ash or impact deposits from a later event, with chemical and mineralogical signatures that are very different from the volcanic deposits that Spirit first saw. Whether or not "it's an impact ash transported by fluid, we just can't tell yet," Arvidson said.

In its transition to a different landscape, Spirit also has stumbled across a surprising mineral that further confirms the presence of water in some form: goethite, a mineral that is common on Earth. "It doesn't necessarily need to have liquid water" to form, said Goestar Klingelhoefer of Johannes Gutenberg University, Mainz, Germany, who is the lead scientist for the Moessbauer Spectrometers onboard both rovers. Klinghoefer said that the goethite [pronounced GUR-tite] could have formed in the presence of water vapor or maybe ice. "That's one of the things we need to look more closely at in the data."

Until last Sunday (Earth time), Spirit's twin Opportunity continued to examine the Endurance Crater, and in particular Burns Cliff, a thick sequence of sulfate-rich rocks. Opportunity managed to climb halfway up the cliff, and is mapping the lithologic layers by comparing what it has found with exposures on other faces in the crater. The result should be a 3-D map of at least one layer.

And, said principal investigator Steve Squyres of Cornell University in Ithaca, N.Y., Opportunity found a distinct unconformity between what looks like aeolian dunes topped by finer layers probably deposited in liquid water. "Don't think deep ocean; think 'a playa,'" he said, with intermittently wet and dry conditions. Squyres also noted that Opportunity observed a new mineral in the past few days, something that the team has not yet had time to identify, but which he will show pictures of in his talk later this week.*

With the new observations that the team is reporting this week, Squyres said, "what we thought we were seeing is now confirmed." In the Dec. 3 issue of Science, the Mars Exploration Rovers team first reported on the martian atmosphere and rocks that Opportunity examined (Spirit had its first installment of papers in the Aug. 6 Science). One of the clues that Opportunity found for the presence of water in Mars' past includes tiny, spherical hematite concretions that peppered one of the first sedimentary formations that the rover examined. Researchers came to the conclusion that the "blueberries," as they were nicknamed, could have formed in a playa environment, something that alternated between a shallow body of water that saturated the rock layers below and a windblown, dry sandy dessert. Another tidbit that reinforced that theory was the presence of jarosite, a mineral that needs water to form, the details of which were also reported in the suite of Science papers.

Opportunity has also been looking at the sky for signs of water. After watching a persistent cloud belt that appears in winter, when Mars is farthest from the sun, scientists have finally gotten the ground's and bird's eye view of the clouds. They look like cirrus clouds on Earth, as reported by Michael Wolff, a research scientist at the Space Science Institute, in Brookfield, Wisconsin, and a member of the Mars rover team, at the press conference. The droplet formations, however, are different (generally much smaller) than those in Earth clouds, as the amount of water vapor present is much less, he said. Opportunity is experiencing Mars' southern winter, and in the northern summer, water is coming off the polar ice cap. "The water and temperature is just right in combination, [which] has implications to Mars' climate we don't yet understand," he said.

In the meantime, although the researchers hesitated to say so, the rovers seem to be trucking along fine. Arvidson documented a few of the problems the robot explorers are having, from slight solar power deficits to a grumpy wheel on Spirit's right front. But the scientists were cautiously optimistic that the rovers would at least celebrate their first anniversaries in the field. "Yesterday we drove out of Endurance Crater," Squyres said. Opportunity is "about to go off to the south … and [to] new explorations."

Naomi Lubick


Mars coverage in Geotimes
NASA home page for the Mars Exploration Rovers

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