This close-up view of a mineral vein called 'Homestake' comes from the microscopic imager on NASA's Mars Exploration Rover Opportunity; the vein is found to be rich in calcium and sulfur, possibly the calcium-sulfate mineral gypsum.

This close-up view of a mineral vein called "Homestake" comes from the microscopic imager 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.

This view blends three exposures taken by the microscopic imager during the 2,765th and 2,766th Martian days, or sols, of Opportunity's career on Mars (Nov. 3 and 4, 2011).

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