This before-and-after pair of images of the same patch of ground in front of the rover, taken 13 days apart, documents the arrival of a bright rock on to the scene. The one on the left is from 3,528th Martian day, or sol, of the rover Opportunity's work on Mars (December 26, 2013). The one on the right, with the newly arrived rock, is from Sol 3540 (January 8, 2014).
Photo courtesy: NASA/JPL-Caltech