NASA's Opportunity rover, older brother to the Curiosity rover that landed on Mars last month, has made a new discovery that geologists find both puzzling and exciting, the US space agency said Friday.

Opportunity, which has been on the Red Planet since 2004, has come across an outcrop of tiny spheres -- up to around an eighth of an inch (three millimeters) in diameter -- the likes of which scientists have never seen before.

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

"We never have seen such a dense accumulation of spherules in a rock outcrop on Mars," he added, in a statement.

At first glance, the researchers thought the objects resembled iron-rich spheres, nicknamed "blueberries," discovered near the Opportunity landing site.

The so-called Martian blueberries are formed when minerals separate from water and become hard masses inside sedimentary blocks -- they're part of the evidence Mars used to be wet.

But closer investigation revealed the new discoveries are "different in concentration. They are different in structure. They are different in composition. They are different in distribution," Squyres said.

"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."

Opportunity found the spheres at an outcrop called Kirkwood, in the Cape York segment of Endeavour Crater's western rim.

Although Opportunity's prime mission was completed more than eight years ago, the rover has continued on "bonus" missions since then.

Fellow Mars rover Spirit, also launched in the summer of 2003, continued working until March 2010.

The most recent Mars lander, Curiosity, is on a two-year quest to explore Mars geology. The $2.5 billion robot is currently in the Gale crater, trundling towards Mount Sharp.