The pale rock in the upper center of this image, about the size of a human forearm, includes a target called "Esperance," which was inspected by NASA's Mars Exploration Rover Opportunity. Data from the rover's alpha particle X-ray spectrometer (APXS) indicate that Esperance's composition is higher in aluminum and silica, and lower in calcium and iron, than other rocks Opportunity has examined in more than nine years on Mars. Preliminary interpretation points to clay mineral content due to intensive alteration by water.
This image is a composite of three exposures taken by Opportunity's panoramic camera (Pancam) during the 3,262nd Martian day, or sol, of the rover's work on Mars (March 28, 2013). The component images were taken through three different Pancam filters -- centered on wavelengths of 753 nanometers (near-infrared), 535 nanometers (green) and 432 nanometers (violet). The view is presented in false color to make some differences between materials easier to see.
Image credit: NASA/JPL-Caltech/Cornell/Arizona State University
Note: For more information, see Mars Rover Opportunity Examines Clay Clues in Rock.