NASA's Opportunity rover has found veins of a mineral that appears to be gypsum, and which could only have been deposited by flowing water.
Opportunity's focused most closely on a vein that's about the width of a human thumb, 16 to 20 inches long, and protrudes slightly higher than the bedrock on either side of it. The team's dubbed it 'Homestake'.
Together with other, similar veins, it was discovered within an apron surrounding a segment of the rim of Endeavour Crater. Nothing like it's been seen before, either in the 20 miles of ground that Opportunity covered before it reached Endeavour, nor in the higher ground of the rim.
"This tells a slam-dunk story that water flowed through underground fractures in the rock," says Steve Squyres of Cornell University, principal investigator for Opportunity.
"This stuff is a fairly pure chemical deposit that formed in place right where we see it. That can't be said for other gypsum seen on Mars or for other water-related minerals Opportunity has found. It's not uncommon on Earth, but on Mars, it's the kind of thing that makes geologists jump out of their chairs."
Last month, researchers used the Microscopic Imager and Alpha Particle X-ray Spectrometer on the rover's arm, together with a number of filters of the Panoramic Camera on the rover's mast, to examine the vein. The spectrometer identified plentiful calcium and sulfur, in a ratio pointing to relatively pure calcium sulfate.
Calcium sulfate can exist in many forms, depending on how much water is bound into the minerals' crystalline structure. The data from the camera suggests that the vein is gypsum, a hydrated calcium sulfate often used for making drywall and plaster of Paris.
It's not the first time gypsum has been opbserved on Mars. Orbital observations have detected a dune field of windblown gypsum on far northern Mars, resembling the glistening gypsum dunes in White Sands National Monument in New Mexico.
"It is a mystery where the gypsum sand on northern Mars comes from," says Opportunity team member Benton Clark of the Space Science Institute. "At Homestake, we see the mineral right where it formed. It will be important to see if there are deposits like this in other areas of Mars."
The Homestake deposit, whether gypsum or another form of calcium sulfate, probably formed from water dissolving calcium out of volcanic rocks. The minerals combined with sulfur that either leached from the rocks or was introduced as volcanic gas, creating calcium sulfate in an underground fracture that later became exposed at the surface.
NASA says the Homestake vein could have been produced in conditions more neutral than the harshly acidic environment indicated by the other sulfate deposits observed by Opportunity.
"It could have formed in a different type of water environment, one more hospitable for a larger variety of living organisms," says Clark.
The newly-discovered veins appear in a zone where the sulfate-rich sedimentary bedrock of the plains meets older, volcanic bedrock exposed at the rim of Endeavour - and the location may offer a clue about their origin.
"We want to understand why these veins are in the apron but not out on the plains," says the mission's deputy principal investigator, Ray Arvidson, of Washington University.
"The answer may be that rising groundwater coming from the ancient crust moved through material adjacent to Cape York and deposited gypsum, because this material would be relatively insoluble compared with either magnesium or iron sulfates."