Divining Water: The Plot Thickens?
Three of the instruments on the Mars rovers were designed to detect minerals that provide evidence of past water. As one scientist put the challenge before the missions began: "The best rock that we could find would be something like a carbonate or other sort of evaporite type of rock that on Earth we know, essentially only forms in the presence of standing water," according to Steve Ruff, a member of the Mini-TES team and a faculty research associate at the Mars Space Flight Facility at Arizona State University.
|"If we see one of these little spherical guys with a layer running through it," said Squyres, "that would favor the concretion idea." Concretions form when water flows through a rock, carrying tiny bits of dissolved sediment along with it.
Image Credit: NASA/JPL/Cornell
Since arriving at the Opportunity’s site–Meridiani Planum–scientists have found a host of detective challenges, ranging from many distinct mineral types to unusual rock shapes. In particular, the fortuitous bedrock found after landing in a crater has showered the region with tiny spherules and prompted various theories of how these spherical balls might have formed. Do the spherules point to an aqueous Martian past?
Visual inspection indicates that the region’s spherules are made of different material than the matrix rock. The layered rock is tan, while the spherules are gray. The bulk, or "matrix," of the rock, is composed of very thin layers. The spheres, or "spherules," embedded in the matrix, are about half a centimeter (about two-tenths of an inch) across.
These surfaces have been studied with the rover’s alpha particle X-ray spectrometer for identifying chemical elements and with a Mössbauer spectrometer for identifying iron-bearing minerals. "I think the key," Squyres said, "is going to be to use Pancam to find a place where there’s a lot of these spherules, go up to it, RAT it and then look at it carefully with the Mössbauer spectrometer.
|Empty nest view back to landing petal from the mobile Opportunity rover, which has ventured to the crater’s rim
One contending theory about the landform’s origin is that the spherules are "what geologists call concretions." Concretions form when water flows through a rock, carrying tiny bits of dissolved sediment along with it. The sediment "precipitates around a nucleation site," Squyres explained, "and it grows these little spherical granules within the rock."
It may be possible for mission scientists to determine which of these theories is correct using only visual clues. To this end, the science team took a series of microscopic images of different regions of the outcrop to better understand the physical relationship between the layers and the spherules.
If the spherules are concretions, there may be places where a layer cuts across both the matrix rock and a spherule. "So if we see one of these little spherical guys with a layer running through it," said Squyres, "that would favor the concretion idea."
But if the spherules are droplets of molten lava or rock melted by an impact, Squyres said, one might see places where hot, newly formed spherules had deformed the rock, "and then layers that are deposited subsequently could appear to be sort of draped over the top" of the spherules. Squyres ruled out the possibility that the rock could be a sandstone, because sandstone is composed of coarser grains. "This isn’t a coarse-grained rock," he said.
|Sandstone, if present, would have a coarser grain than what Meridiani’s outcrop offers on close inspection. Click image for larger view.
One particularly intriguing discovery found so far at Meridiani is the presence of sulfur on the surface of the bedrock. How the sulfur got there is still unknown. Scientists want to find out whether it is present merely within a surface coating, or deeper within the rock. "If we see it only at the surface and not below the surface," said Steve Squyres, principal investigator for the MER mission, "then it’s some kind of coating." That, he said, would "tell us something interesting about recent processes, but it doesn’t tell us about the formation of the outcrop itself."
If, on the other hand, Opportunity ground into the rock with its RAT and detected sulfur deeper within the rock, it would indicate that the sulfur was around long ago, when the bedrock formed. Scientists would then want to know which sulfate (sulfur-containing) minerals were present within the rock. There are many different types of sulfate minterals. Some form in volcanic environments; many others, such as gypsum, form in the presence of water.
According to Squyres, if the Mössbauer spectrometer detects "evidence for a sulfate that is the kind that forms only in the presence of liquid water, that would be an extraordinarily exciting finding. That would be probably the most interesting thing that we’d found yet" at Meridiani.
NASA officials announced that Mars scientists from the Jet Propulsion Laboratory in Pasadena, California, were flying to Washington for a "significant" announcement, but shied away from saying what it would be. The briefing is planned for 2 P.M. EST Tuesday. Scheduled to attend the briefing were lead rover scientist Steve Squyres, geologist John Grotzinger, chief space exploration scientist Benton Clark, project scientist Joy Crisp, and Jim Garvin, NASA’s lead scientist for Mars and the Moon.