NASA this week released new images, taken by Opportunity's Pancam and microscopic imager, that show small spheres embedded in the layered rock. Steve Squyres, principle investigator for the MER mission, said the spheres looked "like blueberries in a muffin."
The rock, which has been dubbed "Stone Mountain," is at the far right edge of Opportunity Ledge, the bedrock outcrop that lies exposed within the crater where the rover landed. 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.
|The blue filter on Pancam applied to the feature called 'Snout' on the far right side of an arc that is thought to be bedrock and stretches half the circumference of the crater. Click image for larger view.
The spherules "may be different in composition" than the layered rock, Squyres said. The layered rock is tan, while the spherules are gray. And the spherules, many of which are scattered on the ground in front of Stone Mountain, do not appear to weather as readily as the matrix, leading scientists to suspect they are made up of harder material.
The science team has come up with a smorgasbord of theories about how the matrix and the spherules embedded within it may have formed.
The best clue to the composition of the layered rock is its weathering pattern. It is weathering "the way you'd expect an extremely fine-grained sediment, or a very fine-grained ash, to weather," Squyres said. This has led scientists to conclude that it is composed either of volcanic ash or of wind-blown dust that has been compacted into a sedimentary rock. In neither case would water have played any role in forming the layered rock.
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.
|Sphere embedded in matrix layer. Click image for larger view.
Three hypotheses remain on the table about the origin of the spherules, said Squyres, but "one of them's fading fast." Losing favor is the notion that they are lapilli, small globs of airborne volcanic ash that stick together, forming spheres that then fall to Earth. Although the size and shape of the spherules in Stone Mountain are consistent with lapilli, Squyres said, lapilli "tend to be made of the same stuff as the matrix in which they're embedded." Visual inspection, however, indicates that the spherules are made of different material than the matrix rock.
Another possibility, one that is still in vogue, is that the spherules formed from molten rock that was sprayed or splashed into the air, where it froze into little droplets. An impact with enough energy to melt rock or lava splashed around during a volcanic eruption "can throw these little spherical grains up in the air and then you'll get these little glass beads that'll fall down on the surface," Squyres said.
The third contender 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 plans to take 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.
|The Instrument Deployment Device is a four-instrument robotic arm, which swivels like a turret to apply different rock diagnostics for visual, chemical or mechanical analyses. Click image for larger view.
Credit:NASA/JPL/ Cornell/ D. Maas
If visual evidence doesn't give the science team enough information to pick one theory over another, additional measurements taken by the rover's instruments, particularly by the Mössbauer spectrometer, should help them to make a final determination.
As for hematite, the iron-bearing mineral that initially motivated scientists to send a rover to Meridiani Planum, its story may lie elsewhere. The science team is all but certain that there is no hematite in the layered matrix rock that comprises the bulk of the bedrock outcrop. But whether or not there is hematite in the spherules remains an open question.
"I think the key to answering that," 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. There is no question, though, that the highest concentration of hematite is actually above the outcrop."