This false-color view of a mineral vein called "Homestake" comes from the panoramic camera (Pancam) on NASA's Mars Exploration Rover Opportunity. 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.
"Homestake" is near the edge of the "Cape York" segment of the western rim of Endeavour Crater.
Exposures combined into this view were taken through Pancam filters admitting light with wavelengths centered at 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.
The exposures were taken during the 2,769th Martian day, or sol, of Opportunity's career on Mars (November 7, 2011).
Photo credit: NASA/JPL-Caltech/Cornell/ASU
Note: For other pictures of Homestake Vein, see PIA15033: 'Homestake' Vein in Color, PIA15035: Close-up View of 'Homestake' Vein, PIA15036: Western Edge of 'Cape York,' with Bright Vein and http://photojournal.jpl.nasa.gov/catalog/PIA15037. Also, see the NASA Science News article, "Slam Dunk" Sign of Ancient Water on Mars.
Update: (3 May 2012) A further discussion of Opportunity at Endeavour Crater and the previously wet climate there (as indicated by the Homestake Vein, shown above) can be found at Paydirt at 8-Year-Old Mars Rover's 'New Landing Site'