(NASA)

Scientists have solved the mystery of the strange "jelly doughnut" rock on Mars.

NASA's Opportunity rover spotted an odd Martian rock that looked like a doughnut on Jan. 8. Four days earlier, however, the rock wasn't there at all. So how did the rock appear? Alien rock throwers? A nearby meteorite impact?

The truth is much less surprising. Scientists working with the intrepid robot have just confirmed that the rock (called Pinnacle Island) was simply kicked up by one of Opportunity's wheels as it made its way across the planet's surface. [Amazing Photos from NASA's Opportunity Mars Rover]

"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," Opportunity deputy principal investigator Ray Arvidson, of Washington University in St. Louis, said in a statement. "We drove over it. We can see the track. That's where Pinnacle Island came from."

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The rock stirred up enough controversy that a concerned citizen even filed a lawsuit against the space agency, claiming that NASA failed to properly investigate a possible fungus growing on the Red Planet.

Although researchers figured out where the rock came from, there are other weird aspects of the Pinnacle Island tale. Using Opportunity's tools, mission scientists have discovered that the rock has very high levels of sulfur and manganese. Both of those elements are water-soluble, suggesting that they were concentrated in the rock due to the "action of water," NASA officials said.

"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.

The rock is located in a spot on "Murray Ridge" along the wall of Endeavour Crater, where Opportunity is spending the Martian winter. Now that the rover is done examining Pinnacle Island, the Opportunity team is planning to drive the rover uphill to check out exposed rock layers on a different part of the Martian surface.

"We are now past the minimum solar-energy point of this Martian winter," Opportunity project manager John Callas, of NASA's Jet Propulsion Laboratory in Pasadena, Calif., said in a statement. "We now can expect to have more energy available each week. What's more, recent winds removed some dust from the rover's solar array. So we have higher performance from the array than the previous two winters."

Opportunity has been exploring Mars since 2004, landing on the Red Planet a few weeks after its twin, Spirit, touched down on the Martian surface. Both rovers were assigned 90-day missions, but Spirit gathered data until 2010 and Opportunity is still roving along.

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This image released Aug. 27, 2003 captured by NASA's Hubble Space Telescope shows a close-up of Mars when the telescope was 34,648,840 miles away. The picture, assembled from a series of exposures, was taken just 11 hours before the planet made its closest approach to Earth in 60,000 years.