No, it's not a Martian doughnut or alien jumping-beans. As it turns out, the mystery rock may well have been an upturned stone shot out from under the rover Opportunity's wheel.
Examination of Pinnacle Island revealed high levels of elements such as manganese and sulfur, suggesting these water-soluble ingredients were concentrated in the rock by the action of water. "This may have happened just beneath the surface relatively recently," Arvidson said, "or it may have happened deeper below ground longer ago and then, by serendipity, erosion stripped away material above it and made it accessible to our wheels."
Now that the rover is finished inspecting this rock, the team plans to drive Opportunity south and uphill to investigate exposed rock layers on the slope.
Opportunity is approaching a boulder-studded ridge informally named the McClure-Beverlin Escarpment, in honor of engineers Jack Beverlin and Bill McClure. Beverlin and McClure were the first recipients of the NASA Medal of Exceptional Bravery for their actions on Feb. 14, 1969, to save NASA's second successful Mars mission, Mariner 6, when the launch vehicle began to crumple on the launch pad from loss of pressure.