Mars probe solves 'Doughnut' riddle

Researchers have determined that the now-infamous Martian rock resembling a jelly doughnut, dubbed Pinnacle Island, is a piece of a larger rock that was broken and moved by the wheel of NASA's Mars Exploration Rover Opportunity in early January.

The piece of rock about 1.5 inches wide (4 centimeters), the white-rimmed, red-centered rock caused a stir last month when it appeared in an image the rover took Jan. 8 at a location where it was not present four days earlier.

More recent images show the original piece of rock struck by the rover's wheel, slightly uphill from where Pinnacle Island came to rest.

"Once we moved Opportunity a short distance, after inspecting Pinnacle Island, we could see directly uphill an overturned rock that has the same unusual appearance," said Opportunity Deputy Principal Investigator Ray Arvidson of Washington University in St. Louis. "We drove over it. We can see the track. That's where Pinnacle Island came from.