NASA's stalwart Opportunity rover has captured an image of the Martian surface that is puzzling scientists, TG Daily reports.

Spherical objects concentrated at an outcrop Opportunity reached last week differ in several ways from iron-rich spherules nicknamed "blueberries" the rover found at its landing site in early 2004 and numerous other locations to date.

The long-lived rover is currently investigating an outcrop known as Kirkwood in the Cape York segment of the western rim of Endeavour Crater. The spheres measure as much as one-eighth of an inch (3 millimeters) in diameter. Although the analysis is still preliminary, the resulting data indicates the spheres do not have the high iron content of Martian blueberries.

"This is one of the most extraordinary pictures from the whole mission," explained Opportunity's principal investigator, Steve Squyres of Cornell University in Ithaca, N.Y. "Kirkwood is chock full of a dense accumulation of these small spherical objects. Of course, we immediately thought of the blueberries, but this is something different. We never have seen such a dense accumulation of spherules in a rock outcrop on Mars."

The Martian blueberries found elsewhere by Opportunity are concretions formed by action of mineral-laden water inside rocks, evidence of a wet environment on early Mars. Concretions result when minerals precipitate out of water to become hard masses inside sedimentary rocks. Many of the Kirkwood spheres are broken and eroded by the wind. Indeed, where wind has partially etched them away, a concentric structure is clearly evident.

Opportunity used the microscopic imager on its arm to look closely at Kirkwood. Researchers checked the spheres' composition by using an instrument called the Alpha Particle X-Ray Spectrometer on Opportunity's arm.

"They seem to be crunchy on the outside, and softer in the middle. They are different in concentration. They are different in structure. They are different in composition. They are different in distribution,” Squyres said.



"So, we have a wonderful geological puzzle in front of us. We have multiple working hypotheses, and we have no favorite hypothesis at this time. It's going to take a while to work this out, so the thing to do now is keep an open mind and let the rocks do the talking."