Mystery of Mars 'doughnut rock' solved

Scientists have found that the mysterious rock resembling a jelly doughnut discovered on the red planet is a piece of a larger rock broken and moved by the wheel of NASA's Mars Rover Opportunity in early January.

About 4 cm wide, the red-centered rock, dubbed Pinnacle Island, caused a stir last month when it appeared in an image the rover took on January 8 at a location where it was not present four days earlier.

Recent images have shown the original piece of rock struck by the rover's wheel, slightly uphill from where Pinnacle Island came to rest, scientists said.

"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," Arvidson said.

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 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," Arvidson said.

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.