This view from the front hazard-avoidance camera on NASA's Mars Exploration Rover Opportunity shows the rover's arm's shadow falling near a bright mineral vein informally named "Homestake." The vein is about the width of a thumb and about 18 inches (45 centimeters) long. Opportunity examined it in November 2011 and found it to be rich in calcium and sulfur, possibly the calcium-sulfate mineral gypsum. Credit: NASA/JPL-Caltech