NASA released this image taken by the Curiosity on January 6th, 2012.
The pale circle is the result of the rover's DRT aka Dust Removal Tool. The DRT is a wire bristle brush attached to the turret on the end of Curiosity's arm. The motorized duster removed the debris from a 1.85 inches by 2.44 inches piece of rock.
Got a confidential tip? Submit it here.