Planetary News: Space Missions (2011)

Click to enlarge > Homestake Opportunity took this picture of Homestake, the striking vein running through bedrock, in early November with her stereo panoramic camera (Pancam). Stuart Atkinson, of UMSF.com, the MER poet dude and a frequent image contributor to the MER Update, processed it here in rich Martian color. "This is different than anything we've ever seen with either rover," said MER PI Steve Squyres last month. Today at the American Geophysical Union fall meeting, he announced that Homestake is harboring gypsum, evidence of an ancient Water environment right there. Credit: NASA / JPL-Caltech / Cornell / S. Atkinson

Intriguingly, the rover hadn’t seen anything like these veins anywhere around the 33 kilometers (20 miles) of crater-pocked plains that it explored for 90 months before it reached Endeavour, nor in the higher ground of the rim.

About the width of a human thumb, 1 to 2 centimeters (0.4 to 0.8 inch), 40 to 50 centimeters (16 to 20 inches) long, Homestake protrudes slightly higher than the bedrock on either side of it. Last month, Opportunity used its microscopic imager (MI) and alpha particle X-ray spectrometer (APXS), as well as multiple filters of the panoramic camera (Pancam) on its mast to examine the vein.

The APXS identified calcium and sulfur, in a ratio pointing to relatively pure calcium sulfate, according to Ray Arvidson, deputy principal investigator, of Washington University St. Louis. "As far as we can tell that vein is almost pure calcium and sulfur," he told the MER Update. "Since Mini-TES [miniature thermal emission spectrometer] is not working, we really couldn't pin down the mineral phase based on the infrared. So the next best thing was to look at the vein with Pancam. It shows a dip at 1 micrometer, which is probably related to the presence of molecular water in the mineral, so it's calcium, and sulfur, it's water bearing. The best bet is it's gypsum, which is calcium sulfate with 2 waters in the unit cell," he explained.

Click to enlarge > Homestake up close and in 3D Holy gypsum. Get your blue-red glasses and check out this close up of Homestake, rendered here in 3D by UMSF's Stuart Atkinson. For more of Atkinson's writings and renderings, visit his Road to Endeavour blog at: http://roadtoendeavour.wordpress.com/2011/12/ Credit: NASA / JPL-Caltech / Cornell / S. Atkinson

Both Spirit and Opportunity have found signs of past water however, so what makes this, as Squyres put it, Opportunity's "bullet-proof" evidence for past liquid water on Mars?

"This is the first time that we found anything like this that's really in situ, in place," Squyres explainedduring an interview with the MER Update. "All of the sulfates that we have found, all the gypsum that's been found elsewhere is stuff that was transported from where it was originally deposited. The other good gypsum deposits on the planet are the dunes around the north pole, and that's stuff that's been blown around by the wind and who the hell knows where that stuff came from. Everything we've seen at Meridiani has been blown around by the wind, moved around by water. Nothing is where it came from. But this is stuff that is filling fracture in bedrock in place. This is where it was deposited."

One hypothesis is that the Homestake deposit, whether gypsum or another calcium sulfate, formed from water dissolving calcium out of volcanic rocks. The calcium combined with sulfur that was either leached from the rocks or introduced as volcanic gas, and it was deposited as calcium sulfate into an underground fracture that later became exposed at the surface.

Click to enlarge > Opportunity's roving grounds This graphic combines a perspective view of the Botany Bay and Cape York areas of the rim of Endeavour Crater on Mars, and an inset with mapping-spectrometer data. Major features are labeled. In the perspective view, the landscape's vertical dimension is exaggerated five-fold compared with horizontal dimensions. Opportunity examined targets in the Cape York area during the second half of 2011. The perspective view was generated by producing an elevation map from a stereo pair of images from the High Resolution Imaging Science Experiment (HiRISE) camera on the Mars Reconnaissance Orbiter, then draping one of the HiRISE images over the elevation model. Credit: NASA/JPL-Caltech/UA/JHUAPL

But why here? Why is Opportunity seeing veins of gypsum here in the Bench around Cape York? Neither rover has ever before seen such a pure form of the stuf in a vein.

"In fact, on Mars, gypsum us a pretty rare mineral," Arvidson said. "The big outcrop or exposure of gypsum is in the north polar dunes. We don't really see gypsum exposed in any way we can confirm with some degree of validity anywhere in the layered sulfates as seen from CRISM or OMEGA." The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) is onboard NASA's Mars Reconnaissance Orbiter (MRO), and OMEGA is the spectral imaging instrument onboard ESA's Mars Express mission.

Why here in the Bench near at Cape York?

"I think it's because the Bench is sitting right on top of this ancient Noachian outcrop that is Endeavour's rim," Arvidson offered. "Everywhere else we've been, the outcrop is meters deep. So the gypsum may be there, but not at a place accessible to Opportunity. And it turns out when you have water percolating up through basaltic rock and that water encounters sulfate sandstones, gypsum is the most insoluble phase. It will drop out as a precipitate right away; whereas, the magnesium sulfates and ferric sulfates that we've seen elsewhere, from CRISM, OMEGA and Opportunity, will just keep going in solution. So I think it's basically a boundary layer effect, where we're looking at the sedimentary unit that is sitting right on the Noachian outcrop. And it's another piece of the puzzle of the chemistry of rising groundwater."

The scientists agree that there is much work to be done. "This is an emerging story," Squyres said. "We’re still working it."

"It will be important to see if there are deposits like this in other areas of Mars," pointed out MER science team member Benton Clark, of the Space Science Institute in Boulder, CO.

Throughout Opportunity's long traverse across the Meridiani plain, the rover has driven over bedrock composed of magnesium, iron and calcium sulfate minerals that also indicate a wet environment billions of years ago. Those past environments generally appeared to boast highly acidic water. The highly concentrated calcium sulfate at Homestake, however, could have been produced in conditions more neutral than the harshly acidic conditions indicated by the other sulfate deposits observed by Opportunity. "It could have formed in a different type of water environment," Clark said, "one more hospitable for a larger variety of living organisms."

Click to enlarge > Gypsum from Earth This is an image of a chunk of gypsum found on Earth. Credit: Kendal Museum / S.Atkinson

Gypsum belongs to a group of minerals called the sulfates, and is the most common of the approximately 150 sulfate minerals. Specifically, it is a soft sulfate mineral composed of calcium sulfate dihydrate. Its chemical formula is CaSO4·2H2O.

On Earth, gypsum is found in both mineral and rock form. As a mineral, it can form beautiful, sometimes extremely large, crystals. As a rock, gypsum is a sedimentary rock, usually found in thick beds or layers.

Gypsum rocks form on Earth in lagoons where ocean waters high in calcium and sulfate content slowly evaporate and are regularly replenished with new sources of water, leaving accumulations of sedimentary gypsum. Since it is deposited in this kind of environment, gypsum is often associated with rock salt and sulfur deposits.

Part of the composition of alabaster, gypsum was used as a decorative stone in Ancient Egypt. These days, gypsum isprocessed and used as prefabricated wallboard or as industrial or building plaster, used in cement manufacture, and agriculture among other things.

With the buzz in recent weeks suggesting that Homestake might be harboring carbonates or the phylliosilicates that Opportunity is hoping to find before the Mars Science Laboratory rover Curiosity lands next August, the discovery of gypsum, at first blush, may seem much less important or exciting. But, according to the MER scientists, it's not.

"This tells a slam-dunk story that water flowed through underground fractures in the rock," Squyres said. "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."

It was Opportunity's time. Spirit made the scientists jump out of their chairs with something it found in 2005. After years of exhaustive, tedious research, MER science team members announced in June 2010 that Spirit had found magnesium iron carbonate in a rock called Comanche while hiking down Husband Hill, a discovery that points to near neutral water, and a potential habitat friendly to the emetgences of life. It remains one of the top discoveries in the mission.

Click to enlarge > Triple crunch After checking out the vein dubbed Homestake in November 2011, Opportunity was instructed to perform a "triple crunch," as Deputy PI Ray Arvidson calls it. "We backed over the vein, drove forward, then turned in place and left. We crunched it," he said. It turned out to be filled with "sparkly bits" we now know to be pieces of gypsum. Stuart Atkinson processed it here in "retina burning" false color. What a mess, but what a mix of stuff. Credit: NASA / JPL-Caltech / Cornell / S. Atkinson