Scientists are stumped as to how a rock mysteriously appeared in images taken two weeks apart by NASA's Mars rover Opportunity.

The rover, which landed in an area known as Meridiani Planum a decade ago, is exploring the rim of a crater for signs of past water.

Another rover, Curiosity, touched down on the opposite side of the planet in 2012 for a more ambitious mission to look for past habitable environments.

For the moment, however, scientists are pondering a more immediate question. On January 8, while preparing to use its robotic arm for science investigation, Opportunity sent back a picture of its work area.

Oddly, it showed a bright white rock, about the size of a doughnut, where only barren bedrock had appeared in a picture taken two weeks earlier. Scientists suspect the rock was flipped over by one of the rover's wheels.

It also may have been deposited after a meteorite landed nearby.

Either way, the rock, dubbed "Pinnacle Island" is providing an unexpected science bonus.

"Much of the rock is bright-toned, nearly white," NASA said in a statement on Tuesday. "A portion is deep red in color. Pinnacle Island may have been flipped upside-down when a wheel dislodged it, providing an unusual circumstance for examining the underside of a Martian rock."