The "jelly doughnut" rock that seemed to appear out of nowhere on Mars last month did not fall out of an extraterrestrial pastry box.
The rock had been mysterious to scientists because Mars rover Opportunity photographed it in a spot where the rock had not been present just four days earlier. Steve Squyres, lead scientist of the Mars Exploration Rover mission, described it as a white rock with a dark red low spot in the middle. The rock, more than 1.5 inches wide, was named Pinnacle Island.
So where did it come from, then?
Researchers now say Pinnacle Island is a piece of a larger rock, which Opportunity broke and moved with its wheel in early January. Further images from the rover reveal the original rock that the rover's wheel must have struck.
"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 Ray Arvidson of Washington University in St. Louis, deputy principal directory of Opportunity, in a statement. "We drove over it. We can see the track. That's where Pinnacle Island came from."
No, that's not as exciting as if the rock had crawled into view on its own or been dropped there by aliens. But now that this puzzle has been solved, the rover team plans to drive Opportunity elsewhere to look at exposed rock layers on slope south of its current location.
The rock has high levels of manganese and sulfur, which may have been concentrated in the rock because of water, researchers say.
"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."
Opportunity weighs 384 pounds and measures about 5 feet in both length and height. It recently celebrated 10 years since its January 25, 2004, landing. Opportunity's twin, Spirit, landed on January 4, 2004.
Spirit stopped communicating in 2010, but Opportunity continues to reveal details about the Red Planet's former habitable conditions.
Opportunity is the older, less technologically advanced of the two NASA rovers currently exploring Mars.
The car-sized Curiosity rover, representing a $2.5 billion mission, is now on its way to Mount Sharp, a sedimentary formation that will allow the rover to explore Mars' history by driving up the peak's slope and exploring rock chemical composition layer by layer.
NASA is planning to launch another Curiosity-sized rover in 2020, which could collect samples that later missions might return to Earth.