Evidence of Water Found on Mars
Confirmation of water's role came from a series of detailed measurements made over the past few days at El Capitan, a small section of the rock outcrop. Both microscopic images and spectral measurements, which can reveal specific chemicals and minerals, helped to convince scientists of water's historical role.
Squyres did, however, offer a caveat. While the outcrop was "definitively" altered by water percolating through it, he said, scientists are still not certain whether water played a role in its initial formation.
Small spherules embedded in the outcrop provided one early clue that water had transformed the rock. On Earth, similar spherules are known to precipitate out of water. (The science team has nicknamed the spherules "blueberries" because they are distributed throughout the rock matrix, the bulk of the rock, like blueberries in a muffin.) A second clue was sulfur detected on rock surfaces. The presence of sulfate minerals in a rock is often an indicator that rocks have been altered by water. Layering visible in the rock matrix suggested that water also might have been involved in the outcrop's initial formation.
These were tantalizing hints, but all of these phenomena could also be explained by volcanic processes in which water played no part. To an effort to settle the question, scientists directed the rover to grind into El Capitan at two different locations, and then used its microscopic imager and APX and Mössbauer spectrometers to explore what lay below the surface.
Microscopic images revealed that the rock's layers were not deformed in any way by the presence of the spherules. This, along with other visual evidence, said Squyres, led the science team to the conclusion that the spherules were concretions. "Concretions form when there's liquid water in a rock. [The water has stuff dissolved in it [that] begins to precipitate out. And as it does so, it grows around a nucleation point to make a small spherical object." If the spherules had not been concretions, the rock layers above and below them would have appeared deformed.
Microscopic images provided other visual evidence of water alteration, as well. Throughout the rock were small holes, or "vugs," about the size and shape of pennies. The distribution pattern of the vugs was a familiar sight to geologists. Similar formations are common on Earth. They occur when percolating water first deposits small crystals - of gypsum, for example - within pores in the rock, and some time later, after environmental conditions have changed, erodes or dissolves them away.
Sulfur: The Slam Dunk
The clincher, however, was mineralogical information uncovered by the two spectrometers on the rover's robotic arm. When the Alpha Particle X-ray Spectrometer (APXS) peered into the shallow holes ground by Opportunity's RAT (Rock Abrasion Tool), it detected "an enormous quantity of sulfur," said Squyres. "Too much to explain by any other mechanism than this rock being full of sulfate salts. That's a telltale sign of liquid water."
"We interpret this sulfur to be really the compound sulfate," said Ben Clark, a MER science team member. "And the salt that we think is probably most prevalent is magnesium sulfate." Magnesium sulfate can be found at your local drug store: it's what Epsom salt is made of. The mineral present on Mars is most likely kieserite, a dehydrated form of the compound.
Opportunity's Mössbauer spectrometer confirmed the presence of sulfates when it detected the mineral jarosite. Jarosite is an iron sulfate hydrate; it forms through the interaction of iron, sulfur and water. "Because it's a sulfate hydrate," Squyres said, "you've got to have water around to make it." Although it is rare on Earth, some scientists had predicted that it might be found on Mars.
The combination of these various lines of evidence, Squyres said, led the science team to conclude with confidence that water had altered the rocks that make up Opportunity Ledge (as the outcrop is known). But finding evidence of aqueous alteration doesn't necessarily imply that the martian surface was ever warmer and wetter than it is today, or that liquid water was ever present on the martian surface. Liquid water could have percolated through the rock, underground, even if Mars's surface was frozen and its atmosphere too thin for liquid water to be stable above ground.
Most significantly, the new results don't provide clear evidence that the rock matrix was originally formed by water. There is speculation among some members of the science team that the entire rock outcrop is a stack of evaporitic salts that were deposited as an ancient sea or lake dried up. In support of this argument, Clark points out that the total salt concentration in some portions of El Capitan, "may be as high as 40 percent. The only way you can form such large concentrations of salt on Earth normally is to dissolve it in water and have the water evaporate."
Chlorine and bromine detected in El Capitan also support the idea that the outcrop formed through evaporation. Two distinct geologic units are visible within El Capitan. The two units initially were distinguished by differences in their appearances. Recent data reveal that they differ in composition as well. Sulfur is present in its highest concentration in the upper unit, in a rock named Guadalupe. The highest concentration of bromine and chlorine, on the other hand, occurs in McKittrick, which is in the lower unit.
"This is what scientists call an 'evaporitic sequence,'" said Clark. "It happens whenever you have a salt-rich briny material" that precipitates out as water evaporates. "Each different type of salt will precipitate at a different time and a different level."
But there are also problems with this idea. One such problem, said Squyres, is that "it's difficult at this site to point to a well-defined basin that the water may have been confined in." But, he added, the topography of the region may have changed significantly since the time when the outcrop formed. So "I don't think the absence [of a visible basin] argues compellingly against these rocks having been laid down in liquid water. But I think all of us would agree that the jury's still out."
Reaching a verdict on this question is high on Opportunity's list of priorities. It the outcrop were, indeed, found to be an evaporitic column, the implication would be far-reaching. It would mean that liquid water once existed in a stable form on the planet's surface - that Mars at some point in the past was, without question, warmer and wetter than it is today.
John Grotzinger, another MER science team member, is hopeful that Opportunity will resolve this debate in the coming week, when the rover moves on to Last Chance, a rock near the right end of the outcrop. In Pancam images, different layers within Last Chance appear to lie at different angles. This is known as cross bedding.
Cross bedding typically occurs in sediments deposited in water, a result of the water washing back and forth across the lake or sea bed, but it can also occur in sediments tossed about by wind or volcanic gases. By examining Last Chance's cross beds in microscopic detail, Grotzinger says, scientists should be able to distinguish among these alternatives.