Although the Mars rover Curiosity may have been getting all the attention of late, there is another rover on the planet, Opportunity, that is still viable and conducting science. In fact, this rover, which has been active on the planet since 2004, has made an interesting discovery: an outcropping of mysterious round balls.

The NASA release dubs them “puzzling little Martian spheres” that were captured in an image by Opportunity a couple weeks ago. They were found in an outcrop called Kirkwood through Opportunity’s Microscopic Imager. The field of view of the objects is only about six centimeters across. The “spherules” are only about three millimeters in diameter.

NASAs Rover Opportunity Spots New Spherules That Differ in Composition to Earlier Blueberries

(Photo: NASA/JPL-Caltech/Cornell Univ./ USGS/Modesto Junior College)

This is not the first time Opportunity has come across spherules such as this either. When it first landed, it spotted “blueberries” similar in shape that NASA says held a high iron content and supported Mars being wet earlier in its history.

NASAs Rover Opportunity Spots New Spherules That Differ in Composition to Earlier Blueberries

Spherules spotted by Opportunity in 2004 called "blueberries." These exhibited a high level of iron content. (Photo: NASA/JPL)

Unlike those spheres though, these ones more recently found do not exhibit the same iron-rich composition, according to NASA. Not only that but the structure appears to be different as well. As some parts of the spherules have eroded, a concentric structure has been observed that differs from the earlier spherules.

“This is one of the most extraordinary pictures from the whole mission,” Opportunity’s principal investigator, Steve Squyres of Cornell University  said in a statement. “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.”

With all the differences in this spherule, compared to the blueberry versions found a little more than eight years ago, Squyres said the researchers have a “wonderful geological puzzle in front of us.”

The team has many hypotheses and Squyres says none of them are coming out as a favorite yet, so he envisions much more work has yet to be done in observing the outcrop before more on the origins of the rocks are known.

“[...] the thing to do now is keep an open mind and let the rocks do the talking,” Squyres said.

Next up for Opportunity on Mars is what’s described as a pale outcrop, also in the Cape York region of the Endeavor Crater.

You may remember there is a third rover still on the red planet: Spirit. Arriving at the same time as Opportunity, a rover that has gotten a clean bill of health from NASA, Spirit stopped communicating with scientists in 2010.