Proof that water once was abundant on Mars provided by rover Opportunity, Cornell's Steven Squyres reports

By Blaine P. Friedlander Jr.

A close up of the rock dubbed "El Capitan" shows fine, parallel lamination in the upper area of the rock, which also contains scattered sphere-shaped objects ranging from 1 to 2 millimeters (.04 to .08 inches) in size. There are also more abundant, scattered vugs, or small cavities, about 1 centimeter (0.4 inches) long. NASDA/JPL/CornellCopyright © Cornell University Click on the image for a high-resolution version (1024 x 1024 pixels, 1196K)

WASHINGTON, D.C. (March 2)-- Water was once abundant on Mars, scientists from NASA, the Jet Propulsion Laboratory and Cornell University reported today at a press conference at NASA headquarters.

Over the past three weeks the rover Opportunity has unveiled daily clues to the existence of water. The final proof, said the Mars mission's science team leader Steven Squyres, Cornell University professor of astronomy, was found at a rocky outcropping, dubbed El Capitan, in the crater at Meridiani Planum where Opportunity landed Jan. 25.

In the outcropping, mission scientists found a hydrated iron sulfate mineral called jarosite, an uncommon mineral on Earth, which forms in dilute sulfuric acid in ground water. Jarosite was first discovered on Earth in 1852 in ravines in the mountainous coast of southeastern Spain.

"We have concluded that the rocks here were once soaked in liquid water. It changed their texture, and it changed their chemistry," said Squyres. "We've been able to read the tell-tale clues the water left behind, giving us confidence in that conclusion," he said.

"We believe that this place on Mars for some period in time was habitable," Squyres said. "It was a ground-water environment, the kind of place that would have been suitable for life. That doesn't mean life was there. We don't know that. But this was a habitable place on Mars at one point in time."

Using all the tools aboard Opportunity, including five scientific instruments, Squyres said the clues appeared one by one.

After surveying the whole El Capitan outcrop, the rover -- a robotic field geologist -- turned back for close-up inspection of selected portions. Using its alpha particle X-ray spectrometer, which identifies chemical elements in a sample, the rover found a very high concentration of sulfur in the outcrop.

"The chemical form of this sulfur appears to be in magnesium, iron or other sulfate salts," said Benton Clark of Lockheed Martin Space Systems, Denver, at the NASA briefing. "Elements that can form chloride or even bromide salts have also been detected."

At the same location, the rover's Mössbauer spectrometer, which identifies iron-bearing minerals, detected jarosite. Opportunity's miniature thermal emission spectrometer has also provided evidence for sulfates.

This image, taken by the microscopic imager on the Mars Exploration Rover Opportunity, illustrates the shapes of the vugs, or small cavities, located on "El Capitan." Several vugs have disk-like shapes with wide midpoints and tapered ends. This feature is consistent with sulfate minerals that crystallize within a rock matrix, either pushing the matrix grains aside or replacing them. These crystals are then either dissolved in water or eroded by wind activity to produce vugs. NASA/JPL/US Geological Survey Click on the image for a high-resolution version (1537 x 1149 pixels, 1011K)

On Earth, rocks with as much salt as this Mars rock either have formed in water or, after formation, have been highly altered by long exposures to water. Jarosite may point to the rock's wet history having been in an acidic lake or an acidic hot springs environment.

The water evidence from the rocks' physical appearance comes in at least three categories, said John Grotzinger, a sedimentary geologist from the Massachusetts Institute of Technology: indentations called "vugs," spherules and crossbedding.

A sample of ther mineral jarosite from a site in Greece. Photo Copyright © Steve Rust Click on the image for a high-resolution version (578 x 458 pixels, 372K)

Pictures from Opportunity's panoramic camera and microscopic imager reveal El Capitan is thoroughly pocked with indentations about a centimeter (0.4 inch) long and one-fourth or less that wide, with apparently random orientations. This distinctive texture is familiar to geologists as the sites where crystals of salt minerals form within rocks that sit in briny water. When the crystals later disappear, either by erosion or by dissolving in less-salty water, the voids left behind are called vugs, and in this case they conform to the geometry of possible former evaporite minerals.

Cornell's Steven Squyres shows a jarosite sample from Earth to Rep. Sherwood Boehlert, chair of the House Science Committee, and explains why scientists think the discovery of jarosite on Mars means that planet once had abundant water. Squyres was summoned to the congressman's office after the March 2 press briefing at NASA's Washington, D.C. headquarters. Robert Barker/Cornell University PhotographyCopyright © Cornell University Click on the image for a high-resolution version (0 x 0 pixels, 1921K)

Round particles the size of BBs, dubbed "blueberries," are embedded in the outcrop. From shape alone, these spherules might be formed from volcanic eruptions, from lofting of molten droplets by a meteor impact, or from accumulation of minerals coming out of solution inside a porous, water-soaked rock. Opportunity's observations that the spherules are not concentrated at particular layers in the outcrop weigh against a volcanic or impact origin, but do not completely rule out those origins.

Layers in the rock that lie at an angle to the main layers, a pattern called crossbedding, can result from the action of wind or water. Preliminary views by Opportunity hint the crossbedding bears hallmarks of water action, such as the small scale of the crossbedding and possible concave patterns formed by sinuous crestlines of underwater ridges.

The rock at El Capitan has an 8 to 9 percent jarosite content, but the mineral is not seen on the rock's crust. "This gives us a compelling case that this was once a liquid water environment," Squyres said.

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