NASA’s Opportunity rover
Small spherical objects fill the field in this mosaic combining four images from the Microscopic Imager on NASA's Mars Exploration Rover Opportunity. Image credit: NASA/JPL-Caltech/Cornell Univ./ USGS/Modesto Junior CollegeReuters

NASA's Opportunity rover, which has been on Mars since 2004, has sent an image of the Martian surface that is puzzling geologists.

Opportunity has come across an outcrop of tiny spheres which measure as much as one-eighth of an inch (3 millimeters) in diameter.

Initially, the researchers thought the spherical objects resembled iron-rich spheres nicknamed "blueberries" that the rover had found at its landing site in early 2004 and at many other locations to date.

"This is one of the most extraordinary pictures from the whole mission," Opportunity's principal investigator, Steve Squyres of Cornell University in Ithaca, N.Y., said.  

Opportunity is investigating an outcrop called Kirkwood in the Cape York segment of the western rim of Endeavour Crater. The analysis is still in the first round, but it indicates that these spheres do not have the high iron content of the Martian blueberries. 

"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 blueberries found in another place are concretions formed by the action of mineral-laden water inside rocks, which is an evidence of a wet environment on early Mars. Concretions are formed by the precipitation of mineral cement within the spaces between sediment grains.

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

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

In 2003, NASA launched the Mars rovers Spirit and Opportunity and both completed their three-month prime missions in April 2004.

Although, Opportunity's mission was completed in 2004, it has continued on an extended bonus mission. While the other Mars rover, Spirit, finished communicating with Earth in March 2010.