Post by Angel One on Feb 12, 2004 23:17:20 GMT
Source:
www.space.com/missionlaunches/opportunity_update_040209.html
Spheres of Influence: Water among Hypotheses for Mars Rock Formation
By Robert Roy Britt
Senior Science Writer
posted: 02:35 pm ET
09 February 2004
Scientists are narrowing the number of explanations for what created tiny spheres of material embedded in a rock outcropping discovered at the landing site of the Mars rover Opportunity. One idea still in the running is that the tiny beads formed in the presence of water.
The researchers cannot rule out a volcanic origin, but a thorough examination of the broad outcropping over the next few days could produce a definitive answer.
The tiny spherical objects are embedded in layers of a single rock initially nicknamed Snout. It has now been renamed Stone Mountain, though it is actually a small stone -- part of an outcrop of rocks that is only a few inches tall overall.
"This is wild looking stuff," said Steven Squyres, principal investigator for the Mars Exploration Rover (MER) project from Cornell University, referring to microscopic images revealing the rocks layers and embedded spherical grains.
At a press conference today, Squyres said the layers of the rock are made of fine material, probably either dust or sediment, and not sandstone. Each layer is just a few millimeters thick.
The tiny spherules, as they are called, are embedded in the layers "like blueberries in a muffin," he said.
The spherules are clearly made of a different material than the rock's primary layers, or matrix, Squyres said. The matrix is tan or buff in color, he said, and the spherules are very gray.
"That's a hint that they may be different in composition," he said.
Millions of years of sandblasting in the harsh Martian environment have exposed many of the spherules. Some have dropped out of the rock and others are hanging on like a child's loose tooth. This makes them ripe for observation.
The spherules are tougher material than the rock matrix, Squyres said. Some are "strung like beads" along a crack in the rock. But his team does not yet know what they're made of or how they formed.
There are three leading hypotheses:
The spherules might have formed when ash from a volcanic eruption was suspended in the air, stuck together, and fell from the sky. That idea is rapidly falling from favor, Squyres said, because it would tend to produce spherules made of the same material as the rock's main matrix.
They might have formed when molten rock -- either from a volcano or a meteor impact -- froze in mid-air into glass beads.
Most interesting, they might be "concretions," which form when a fluid, possibly water, carrying dissolved minerals flows through a rock and "precipitates" a grain that typically grows into a sphere.
Squyres said all three ideas should be testable with Opportunity's suite of instruments.
If a spherule was born as a layer formed, for example, "you might see the layering preserved within the sphere," he said, favoring the concretion method. If they fell from above, one or more might have created indentations in layers of the rock.
Importantly, Squyres said the spherules do not appear to have formed by the erosive action of running water, when larger pieces of material can be worn down into smaller, spherical beads. Nor were they likely created by similarly erosive ocean waves. Concretion, however, would require liquid, perhaps water. It's not yet clear what form that liquid might have taken -- if it ever existed -- in the grand scheme of Meridiani Planum, where Opportunity landed.
Scientists speculated before the mission that the region might have been covered by a giant lake or ocean of water. That remains a possibility given the current state of the investigation.
The rover will be on a "shoot and scoot" mission for the next few days. It will take close-up pictures of the broad rock outcropping, scoot to the next site along the front of the wall, and shoot more pictures. Scientists will look for spots where the matrices of various rocks are well exposed and, hopefully, spherules will be abundant.
Once a thorough survey is complete, Opportunity would then be sent back to a couple locations to drill into rocks with its rock abrasion tool (RAT).
Squyres dangled another interesting finding Opportunity had sent back today: The rock outcropping "has got a lot of sulfur in it," he said. "That's a clue, but we don't know what it means yet."
Meanwhile, one of the minerals du jour at the Opportunity landing site, hematite, has been pushed to the back burner in terms of exploration. Hematite usually forms in the presence of standing water, and Opportunity's landing site was chosen in part because previous orbiter observations revealed an apparent abundance of hematite.
"When we look hard at the outcrop from a distance with many tests, we don't see high concentrations of hematite," Squyres said, adding that the rock matrix -- its primary material -- is probably not hematite. The spherules might contain hematite, however, and more tests should find out.
He said the highest concentrations of hematite appear to be above the rock outcropping and beyond, where Opportunity has yet to investigate up-close. "We don't know what's up there," he said. But it's possible the limited amounts of hematite found in the shallow depression where the rover landed might have rolled down from above, or it could have come from the rock outcropping.
After a thorough exploration of the shallow crater, Opportunity is expected to wheel up to the surrounding plain.