This image taken by the Mars Exploration Rover Opportunity's panoramic camera shows the rover's rock abrasion tool before and after it ground into a rock at Meridiani Planum, Mars. The red dust coating on the instrument is thought to be a form of the mineral hematite. The image on the left was taken on the 29th martian day, or sol, of the rover's mission, and the image on the right on the 31st sol.