NASA's Opportunity rover has sent back a "puzzling" image from Mars.
Small spherical objects -- about 3 mm in diameter -- have been found, concentrated in the "Kirkwood" region of the red planet.
Researchers say the objects are different from Mars "blueberries" -- iron-laden beads that Opportunity has discovered in numerous locations.
"This is one of the most extraordinary pictures from the whole mission," said Opportunity's principal investigator, Steve Squyres, 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."
Researchers are calling the find a "geological puzzle."
While there are a few theories about what the mystery spheres might be, it may be some time before scientists come up with a definitive answer.