17/09/2012 05:50 BST | Updated 16/11/2012 05:12 GMT

Mars Rover Opportunity Discovers Mysterious Ancient Spheres On Mars (PICTURES)

The Mars rover Opportunity has discovered mysterious spheres on the surface of the Red Planet, Nasa has said.

The images of the spherical objects were described as a "mystery" and are said to be "puzzling researchers".

Opportunity had found similar objects in 2004, which were rich in iron and nicknamed "blueberries".

But the new objects do not have the same iron content, and measure about an eight of an inch in diameter.

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

"They seem to be crunchy on the outside, and softer in the middle," he added.

The "blueberries" found elsewhere on Mars were formed by mineral-laden water interacting with rocks, indicating the dry, dusty planet was once rich with water.

But the new spheres are different in structure and composition to the older "blueberries", and researchers are still scratching their heads about what they could be.

Some have partially eroded away, revealing their structure underneath.

"We have a wonderful geological puzzle in front of us," Squyres said.

"We have multiple working hypotheses, and we have no favourite 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."

The Opportunity rover is still functioning on the surface of the planet, even as the Curiosity rover takes the headlines after landing in July for a two-year mission.

The older rover is still investigating an outcrop ('Kirkwood') in the Cape York area of the Endeavour Crater, and is still turning up interesting science.