NASA's Mars Exploration Rover Opportunity inspected this mineral vein, called "Homestake," in November 2011 at the northern end of the "Cape York" section of Endeavour Crater's western rim. The vein is about the width of a thumb and about 18 inches (45 centimeters) long, extending beyond the portion shown here.
This view, showing subtle linear texture on the bright vein, combines close-up detail recorded by Opportunity's microscopic imager (MI) and enhanced color information from Opportunity's panoramic camera (Pancam). The area covered in this view spans about 2 inches (5 centimeters) across. The MI exposures used in this view were taken while the vein was fully shadowed by the rover during the mission's 2,766th Martian day, or sol (Nov. 4, 2011).
A Pancam view encompassing more of the Homestake vein is at http://photojournal.jpl.nasa.gov/catalog/PIA15033 . Researchers using the alpha particle X-ray spectrometer (APXS) on Opportunity determined that this vein is rich in calcium and sulfur, possibly the calcium-sulfate mineral gypsum.
Image Credit: NASA/JPL-Caltech/USGS/Cornell Univ./Arizona State Univ.