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.

The surface of Mars in front of the rover on December 26, 2013 (left) and on January 8, 2014 (right).

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.