NASA's Mars rover Opportunity discovered a mineral vein telling of a warmer, wetter and potentially more conducive to life past for the seemingly cold, dry and desolate Red Planet. The mineral vein, found in the crater Endeavor, is composed of a calcium sulfate called gypsum. Unlike some other forms of calcium sulfates, gypsum retains water in high temperatures after precipitating, in other words its presence indicates the existence of water.

The find is the single most compelling piece of evidence for water activity found on Mars according to Opportunity's principal investigator Prof. Steve Squyres, astronomy.

Squyres' team named the find "Homestake" after the largest goldmine in the U.S. "The implication is not that Homestake is literal gold," Squyres said, "but scientific gold."

Homestake is just one of many such newly discovered Martian gypsum veins. "The size of the Homestake vein itself is actually quite small: the width of your thumb, or only about half a meter long," said Squyres. "But there are quite a lot of them; this is not a singular find.

There is a geologic unit which surrounds the rim of the Endeavour crater, and there are lots and lots of these little veins. Homestake is just the first one that we have investigated."

Squyres' team identified the mineral using several of the rover's on-board instruments. The rover's Microscopic Imager, a cross between a microscope and a camera, produced close-up pictures of the mineral vein.

The Alpha Particle X-ray Spectrometer helped measure the ratio of the elements in this case, calcium and sulfur, present in the sample, and the multi-filter Panoramic Camera revealed that the calcium sulfate was hydrated, as opposed to being in its forms which contain less or no water.

"The fact that Mars once had liquid water is not a new discovery," Squyres said. Opportunity and its defunct sister-rover, Spirit, had found evidence of running water on early Mars in the form of an iron sulfate deposit called jarosite.

Like gypsum, jarosite only forms in the presence of liquid water. However, as Squyres said, it forms only under very acidic conditions.

"What that mineral [jarosite] tells you is that, at the time that that mineral formed, when people talk about water on Mars they're really talking about sulfuric acid," Squyres said. Gypsum, on the other hand, forms from water with a pH suitable for life.

Though gypsum has been spotted on Mars before, in sand dunes by its North Pole, those deposits were blown about and transported by the wind. "Where that previous gypsum came from is anybody's guess," Squyres said.

"This newly discovered gypsum vein formed right here: it's in the local bedrock, this is where it happened." Observing gypsum in the exact location where it formed is critical to understanding the existence of water on Mars.

Despite the importance of this discovery, Opportunity offers little clues to answering how long ago water was present on the surface of Mars since its current equipment is not capable of determining the vein's age. "Radiometric age dating requires a complicated laboratory," Sqyures said, "not the kind of instrumentation you can actually send to Mars on a rover these days."

What we're doing now is we're adding great depth and detail to our understanding of water on Mars: what its chemistry was, when it was present, how long it was present, how much there was, what form it took, how it got to where it was."

Having exceeded its design lifetime of 90 days by over twentyfold, Opportunity is now buckling down for its fourth harsh Martian winter. "Our chances of getting through this winter are very good," Squyres said.

Come spring, the rover will travel south along the rim of Endeavour crater where Homestake was found and continue its hunt for evidence of water. It will search for clay minerals already detected from orbit that require water for their formation.

The next generation Mars rover Curiosity is scheduled to land at Gale crater in August later this year.