NASA ponders mysterious rock on Mars that suddenly appears

A mystery rock on Mars that suddenly appeared in front of NASA's Opportunity rover may look like a tasty donut, but it is like nothing ever seen on the Martian surface before.

This before-and-after pair of images of the same patch of ground in front of NASA's Mars Rover Opportunity 13 days apart documents the arrival of a strange, bright rock at the scene. The rock, called "Pinnacle Island," is seen in the right image on Jan. 8, 2014. The image at left was taken on Dec. 26, 2013.

"It looks like a jelly donut," said Steve Squyres, the rover's lead scientist at Cornell University in Ithaca, N.Y., during a recent NASA event marking Opportunity's 10th year on Mars. "And it appeared, it just plain appeared, at that spot and we haven't driven over that spot." [Amazing Photos from NASA's Opportunity Mars Rover]