Mars watchers got excited last month when NASA's Opportunity Rover snapped a surprising set of pictures: a jelly doughnut-shaped rock had seemingly appeared where none previously existed. However, it appears the rock, dubbed "Pinnacle Island" by researchers, has a surprisingly simple explanation. The Opportunity Rover drove over it, kicking the white-rimmed, red-centered rock into its later position.

"Once we moved Opportunity a short distance away... we could see directly uphill an overturned rock that has the same unusual appearance," said mission investigator Ray Arvidson of Washington University. "We drove over it. We can see the track. That's where Pinnacle Island came from."

It's not the first time an intriguing object spotted on the Martian surface has turned out to be something fairly ordinary. What originally appeared to be a metal "doorknob" or "flower" on Mars last February was later determined to be a ventifact, a rock formation carved by the wind.

In December, observers noticed that the rocky Martian surface is beginning to do cosmetic damage to the Mars Curiosity Rover's wheels. And in other rover news, China's lunar rover Jade Rabbit has come back from the dead.