Volume: 02, Issue: 06 03/24/2004 
Interpretations of cross-lamination patterns presented as clues to this Martian rock's origin under flowing water are marked on images taken by the panoramic camera and microscopic imager on NASA's Opportunity. Image credit: NASA/JPL/Cornell/USGS
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This view of the lower portion of the martian rock called "Last Chance" shows a close-up of texture interpreted as cross-lamination evidence that sediments forming the rock were laid down in flowing water. Image credit: NASA/JPL/Cornell/ARC
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This image from the Mars Exploration Rover Opportunity is part of the first set of pictures that was returned to Earth after the rover exited "Eagle Crater." Image credit: NASA/JPL
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Other Articles in This Issue:
Fab Five Make Rare Appearance in Night Sky
Most Distant Object In Solar System Discovered
NASA Explains Dust Bowl Drought
Chat about Mars Rover
 

Opportunity Makes a Case for Standing Body of Water on Mars

NASA's Mars Exploration Rovers continue to uncover evidence that water once flowed freely on Mars. Opportunity recently discovered that some rocks on Mars probably formed as deposits at the bottom of a body of gently flowing saltwater.

"We think Opportunity is parked on what was once the shoreline of a salty sea on Mars," said Dr. Steve Squyres of Cornell University, Ithaca, N.Y., principal investigator for the science payload on Opportunity and its twin rover, Spirit.

Dr. James Garvin, lead scientist for Mars and lunar exploration at NASA Headquarters in Washington, said, "Many features on the surface of Mars that orbiting spacecraft have revealed to us in the past three decades look like signs of liquid water, but we have never before had this definitive class of evidence from the Martian rocks themselves. We planned the Mars Exploration Rover Project to look for evidence like this, and it is succeeding better than we had any right to hope."

Clues gathered so far do not tell how long or how long ago liquid water covered the area. To collect more evidence, the rover's controllers plan to send Opportunity out across a plain toward a thicker exposure of rocks in the wall of a crater.

NASA's Associate Administrator for Space Science, Dr. Ed Weiler, said, "This dramatic confirmation of standing water in Mars' history builds on a progression of discoveries about that most Earthlike of alien planets. This result gives us impetus to expand our ambitious program of exploring Mars to learn whether microbes have ever lived there and, ultimately, whether we can."

According to Dr. John Grotzinger, rover science-team member from the Massachusetts Institute of Technology in Cambridge, Mass., "Bedding patterns in some finely layered rocks indicate the sand-sized grains of sediment that eventually bonded together were shaped into ripples by water at least five centimeters (two inches) deep, possibly much deeper, and flowing at a speed of 10 to 50 centimeters (four to 20 inches) per second."

In telltale patterns, called crossbedding and festooning, some layers within a rock lie at angles to the main layers. Festooned layers have smile-shaped curves produced by shifting of the loose sediments' rippled shapes under a current of water.

"Ripples that formed in wind look different than ripples formed in water," Grotzinger said. "Some patterns seen in the outcrop that Opportunity has been examining might have resulted from wind, but others are reliable evidence of water flow."

The environment at the time the rocks were forming could have been a salt flat, or playa, sometimes covered by shallow water and sometimes dry, Grotzinger said. Such environments on Earth, either at the edge of oceans or in desert basins, can have currents of water that produce the type of ripples seen in the Mars rocks.

A second line of evidence, findings of chlorine and bromine in the rocks, also suggests this type of environment. Rover scientists presented some of that news three weeks ago as evidence the rocks had at least soaked in mineral-rich water, possibly underground water, after they formed. Increased assurance of the bromine findings strengthens the case rock-forming particles precipitated from surface water as salt concentrations climbed past saturation while water was evaporating.

"The particular type of rock Opportunity is finding, with evaporite sediments from standing water, offers excellent capability for preserving evidence of any biochemical or biological material that may have been in the water," Squyres said.

Earlier in the week, small mineral spheres analyzed by Opportunity hinted at a possible wet history on Mars. The spherules, fancifully called blueberries although they are only the size of BBs and more gray than blue, lie embedded in outcrop rocks and scattered over some areas of soil inside the small crater where Opportunity has been working.

Individual spherules are too small to analyze with the composition-reading tools on the rover. Those tools were used to examine a group of berries that had accumulated close together in a slight depression atop a rock called "Berry Bowl." The rover's Mössbauer spectrometer, which identifies iron-bearing minerals, found a big difference between the batch of spherules and a "berry-free" area of the underlying rock.

"This is the fingerprint of hematite, so we conclude that the major iron-bearing mineral in the berries is hematite," said Daniel Rodionov, a rover science team collaborator from the University of Mainz, Germany. On Earth, hematite with the crystalline grain size indicated in the spherules usually forms in a wet environment.

Scientists had previously deduced that the Martian spherules are concretions that grew inside water-soaked deposits. Evidence such as interlocking spherules and random distribution within rocks weighs against alternate possibilities for their origin. Discovering hematite in the rocks strengthens this conclusion. It also adds information that the water in the rocks when the spherules were forming carried iron, said Dr. Andrew Knoll, a science team member from Harvard University in Cambridge, Mass.

Halfway around Mars, Spirit has been exploring the rim of the crater nicknamed "Bonneville," which it reached last week. A new color panorama shows "a spectacular view of drift materials on the floor" and other features, said Dr. John Grant, science team member from the National Air and Space Museum in Washington. Controllers used Spirit's wheels to scuff away the crusted surface of a wind drift on the rim for comparison with drift material inside the crater.

A faint feature at the horizon of the new panorama is the wall of Gusev Crater, about 80 kilometers (50 miles) away, said deputy project scientist Dr. Albert Haldemann of NASA’s Jet Propulsion Laboratory, Pasadena, Calif. The wall rises about 2.5 kilometers (1.6 miles) above Spirit's current location roughly in the middle of Gusev Crater. It had not been seen in earlier Spirit images because of dust, but the air has been clearing and visibility improving, Haldemann said.

Controllers have decided not to send Spirit into Bonneville crater. "We didn't see anything compelling enough to take the risk to go down in there," said JPL's Dr. Mark Adler, mission manager. Instead, after a few more days exploring the rim, Spirit will head toward hills to the east informally named "Columbia Hills," which might have exposures of layers from below or above the region's current surface.

Engineers at JPL expect Opportunity and Spirit to operate several months longer than the initial rover's three-month prime missions on Mars. The main task for both rovers is to explore the areas around their landing sites for evidence in rocks and soils about whether those areas ever had watery environments suitable for sustaining life.

JPL, a division of the California Institute of Technology in Pasadena, manages the Mars Exploration Rover Project for NASA's Office of Space Science, Washington. For images and information about the project on the Internet, visit:
http://www.nasa.gov
http://marsrovers.jpl.nasa.gov
http://athena.cornell.edu

    
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