NASA's Opportunity mission, laboring halfway around Mars from the headline-stealing Curiosity rover, is giving geologists their first up-close glimpse of Martian clays, leftovers from an ancient watery environment on the red planet.

Opportunity captured this view of its own late afternoon shadow at Endeavour Crater on Aug. 23, 2012. Credit: NASA/JPL-Caltech
 
Following a signature detected remotely from a satellite orbiting Mars, Opportunity traversed an expansive plain to Endeavour Crater, where there is evidence of clay-bearing minerals. The material could have only formed when Mars was warmer and wetter, according to scientists.

"Clays are tremendsouly important on Mars," said Steve Squyres, principal investigator for the Opportunity rover. "Clays form in a watery environment. That's important, but even more important is clay will only form in kind of a neutral pH - water that's not acid."

NASA's Curiosity rover, a larger, more capable robot than Opportunity, was dispatched to Gale Crater to pursue a signature of clays detected there. But scientists do not expect Curiosity to reach the 'sweet spot' for clays in Gale Crater until some time next year.

Although it is not equipped with the same advanced instrumentation as Curiosity, the Opportunity rover is giving researchers their first long-sought ground truth on clay minerals.

Since landing in January 2004, Opportunity has explored an array of geologic features, rocks and soil rife with signs of water in an earlier period on Mars. But scientists say clues found by Opportunity so far point to acidic conditions hostile to life.

Clay minerals pinpointed on the edge of Endeavour Crater by NASA's Mars Reconnaissance Orbiter are much older than the surrounding unit of sandstones.

"The thing that's different here is that these clay minerals point towards a neutral chemistry - water you could drink," Squyres said last week at the American Geophysical Union's fall meeting in San Francisco. "It's particularly interesting to me that it correlates with the oldest stuff that we've seen on Mars. It seems that these more suitable water chemistries, the evidence for those, are concentrated in the oldest materials on Mars. This is our first glimpse ever at conditions on ancient Mars that clearly show us a chemistry that would have been suitable for life at the Opportunity site."

After the science team decided on Endeavour as Opportunity's next destination, the rover drove 13 miles over three years to reach the crater.

Controllers have driven Opportunity more than 22 miles, stopping at several craters, examining rock outcrops and collecting millions of measurements to characterize its landing site at Meridiani Planum, a dusty plain pockmarked with impact sites.

But Endeavour Crater, spanning 14 miles across, is the largest crater visited by Opportunity.

"Endeavour is fundamentally different from every other crater we've looked at with Opportunity," Squyres said. "For one thing, it's big - 22 kilometers in diameter is much larger than anything we've looked at before. But more importantly, it's old."

Scientists estimate the crater was formed 3 billion years ago, scouring a deep basin and exposing older material around its edges.

A team led by Ray Arvidson, Opportunity's deputy principal investigator, refined data from MRO to find the exact locations where the clay minerals were deposited along the crater's edges, according to Squyres, a professor at Cornell University.

Opportunity is now at one of the clay sites - a feature scientists call Matijevic Hill, named for the late Jacob Matijevic, who led the engineering team for Opportunity and its twin rover Spirit for several years.

This false-color image of a light-toned clay-bearing rock named Whitewater Lake was taken by Opportunity's panoramic camera on Sept. 6, 2012. Whitewater Lake is the large flat rock in the top half of the image. From left to right, it is about 30 inches (0.8 meter) across. Credit: NASA/JPL-Caltech/ Cornell University/Arizona State University
 
The rover drove in a circular pattern around the hill in October and November, surveying rocks and identifying features for follow-up observations. It found an array of flat-lying, light-toned material interspersed with outcrops of tougher, darker rock.

Squyres, who has led the rover science team for more than a decade, called the Matijevic Hill site "one of the most delightful geologic puzzles that we have ever found with this rover on Mars."

Scientists say the flat-lying material, which looks to be composed of fine grains, is remarkably soft and is made up of elements common on Mars.

"This has got to be the clay-bearing stuff," Squyres said of the light-toned rock type, which has been dubbed Whitewater Lake.

But scientists were in for a surprise when Opportunity's microscopic camera looked at a fin-like protrusion rising above the Whitewater Lake material.

The images revealed a cluster of spherical objects embedded in the rock, and Squyres said scientists are puzzled about their origin. Earlier in its mission, Opportunity discovered similar iron-rich deposits nicknamed blueberries, which were made of a mineral known as hematite.

But the tiny spheres, each about the size of a BB, at Matijevic Hill are not made of hematite.

"I've been calling them 'newberries' because they're something new, and I don't know what they are," Squyres said.

Opportunity's microscopic imager discovered 'newberries' in a rock formation at Matijevic Hill. Credit: NASA/JPL-Caltech/Cornell University/USGS/Modesto Junior College
 
They could be concretions of a different mineral, residue from an ancient faraway asteroid or comet impact, a byproduct of volcanic activity, or something else, scientists said.

The rover will spend at least the first few months of 2013 analyzing the clays and 'newberries' at Matijevic Hill. According to Squyres, the science team wants to learn the origin and constrain the age of the soft light-colored clay-bearing rock prevalent at the site and whether it was deposited by water, wind or a violent process such as an asteroid strike.

"Clearly, this is a place where water was present," Squyres said. "The clay mineral signal clearly says that. Clearly, this is very ancient. The processes are something we're still working out, so rather than guessing, we're going to do the geology."

The examination will require many observations using Opportunity's X-ray spectrometer, an instrument designed to discern the elemental composition of rocks and soil.

"You can't look at the newberries and the clays independently," Squyres said. "One is embedded in the other. Their stories are woven together."

Diana Blaney, Opportunity deputy project scientist at the Jet Propulsion Laboratory, said the rover's microscopic imager, rock grinding tool, and cameras are all in good shape after nearly 9 years on Mars.

Opportunity and Spirit were designed for 90-day missions after landing. Spirit last communicated with Earth in March 2010.

"Against all odds, this rover is still operating after 9 years, and we have fortuitously just run up against what I think is one of the most interesting, most challenging, most subtle geologic problems that we've encountered the entire mission," Squyres said.