"This tells a slam-dunk story that water flowed through underground fractures in the rock," said Steve Squyres of Cornell University. This colour view of a mineral vein called 'Homestake' comes from Opportunity's panoramic camera. Opportunity examined it in November 2011 and found it to be rich in calcium and sulphur, possibly the calcium-sulphate mineral gypsum.
Credit: NASA
MARYLAND: Bright veins of a mineral, apparently gypsum, have been found near water along the rim of Endeavour Crater on Mars, say NASA scientists.
The vein examined most closely by NASA's Mars Rover Opportunity is about the width of a human thumb (1 to 2 cm) and 40 to 50 cm long. Observations by the rover reveal this vein and others like it within an apron surrounding the rim of Endeavour Crater. Nothing like it was seen in the 33 km of crater-pocked plains that Opportunity explored for 90 months before it reached Endeavour, nor in the higher ground of the crater's rim.
"This tells a slam-dunk story that water flowed through underground fractures in the rock," said Steve Squyres of Cornell University in New York, principal investigator for Opportunity, who presented the findings earlier this week at the American Geophysical Union's conference in San Francisco. "This stuff is a fairly pure chemical deposit that formed in place right where we see it. It's the kind of thing that makes geologists jump out of their chairs."
Mars water mysteries
Last month, researchers used the Microscopic Imager and Alpha Particle X-ray Spectrometer on the rover's arm and multiple filters of the Panoramic Camera on the rover's mast to examine the vein, which is informally named 'Homestake'. The spectrometer identified plentiful calcium and sulphur, in a ratio pointing to relatively pure calcium sulphate.
Calcium sulphate can exist in many forms varying by how much water is bound into the minerals' crystalline structure. The multi-filter data from the camera suggest gypsum, a hydrated calcium sulphate. On Earth, gypsum is used for making drywall and plaster of Paris.
Observations from orbit had detected gypsum on Mars previously. A dune field of windblown gypsum on far northern Mars resembles the glistening gypsum dunes in White Sands National Monument in New Mexico. The origin of that windblown gypsum is, however, uncertain.
"It is a mystery where gypsum sand on northern Mars comes from," said Opportunity science-team member Benton Clark of the Space Science Institute in Boulder, Colorado. "At Homestake, however, 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."
Ancient, wet environment
The Homestake deposit, whether gypsum or another form of calcium sulphate, likely formed from water dissolving calcium out of volcanic rocks. The calcium combined with sulphur that was either leached from the rocks or introduced as volcanic gas, and it was deposited as calcium sulphate into an underground fracture that later became exposed at the surface.
The discovery of gypsum fits the emerging picture of an ancient wet environment with the presence of Mars water. Throughout Opportunity's long traverse across Mars' Meridiani plain, the rover has driven over bedrock composed of magnesium, iron and calcium sulphate minerals that also indicate the presence of water billions of years ago.
The highly concentrated calcium sulphate at Homestake could have been produced in conditions more neutral than the harshly acidic conditions indicated by the other sulphate 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," Clark said.
Exploration continues
Opportunity has been exploring Mars for nearly eight years, far exceeding than the rover's original three-month mission, which began in 2004. Gypsum veins are just the latest example of an important discovery about wet environments on ancient Mars that may have been favourable for supporting microbial life. Opportunity's equally productive twin, Spirit, stopped communicating in 2010.
Opportunity continues exploring for Mars water, currently heading to a sun-facing slope on the northern end of the Endeavour rim fragment called 'Cape York' to keep its solar panels at a favorable angle during the mission's fifth Martian winter.
