Volume: 02, Issue: 05 03/10/2004 
This image, taken by Opportunity’s panoramic camera, shows a close up of the rock dubbed "El Capitan," located in the rock outcrop at Meridiani Planum, Mars.  Photo courtesy NASA/JPL/Cornell.
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This false-color image taken by Spirit’s panoramic camera shows the rock dubbed "Humphrey" and the hole drilled into the rock by the rover.  Photo courtesy NASA/JPL.
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This image, taken by Opportunity’s microscopic imager, shows a spherule on the rock outcrop at Meridiani Planum, Mars dubbed "El Capitan." Photo courtesy NASA/JPL/US Geological Survey.
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Other Articles in This Issue:
A New Look and a New Name
Hubble Unveils Earliest Galaxies
Space Station Research Yields New Information About Bone Loss
Cassini Captures Stunning View of Saturn
Chat Live with NASA Astronaut Daniel Brandenstein

Spirit and Opportunity Find Signs of Water

Both of NASA’s Mars Exploration Rovers have found evidence that water once existed on the surface of Mars. Spirit found hints of past water activity at Gusev Crater just days after Opportunity led scientists to conclude that the part of Meridiani Planum it is exploring was soaking wet in the past.

Spirit discovered indications of a water history in a rock at Gusev Crater in a dark volcanic rock dubbed "Humphrey." According to Dr. Ray Arvidson of Washington University in St. Louis, the rock, which is about 60 centimeters (2 feet) tall, shows bright material in interior crevices and cracks that looks like minerals crystallized out of water.

"If we found this rock on Earth, we would say it is a volcanic rock that had a little fluid moving through it," said Arvidson, the deputy principal investigator for the rovers' science instruments. If this interpretation is correct, the fluid—water with minerals dissolved in it—may have been carried in the original magma that formed the rock or may have interacted with the rock later, he said.

The clues appear in an interior exposure of "Humphrey" where Spirit's rock abrasion tool scraped away the rock's surface to a depth of 2 millimeters (.08 inch). To gain more confidence that the bright material seen in cracks and pores is not dust that has intruded from the surface over the millenia, scientists intend to have Spirit grind more deeply into another dark rock, not yet selected. The bright material is not debris from the grinding process, said Stephen Gorevan of Honeybee Robotics, New York, lead scientist for the abrasion tool.

The amount of water suggested by the possible crystals in "Humphrey" is far less than what is indicated by the minerals and structures that Opportunity has revealed in rocks on the other side of the planet at Meridiani Planum. Rover scientists announced the Opportunity findings earlier this week.

Evidence the rover found in a rock outcrop led scientists to the conclusion. Clues from the rocks' composition, such as the presence of sulfates, and the rocks' physical appearance, such as niches where crystals grew, helped make the case for a watery history.

"Liquid water once flowed through these rocks. It changed their texture, and it changed their chemistry," said Dr. Steve Squyres of Cornell University in Ithaca, N.Y., principal investigator for the science instruments on Opportunity and Spirit. "We've been able to read the tell-tale clues the water left behind, giving us confidence in that conclusion."

Dr. James Garvin, lead scientist for Mars and lunar exploration at NASA Headquarters in Washington, said, "NASA launched the Mars Exploration Rover mission specifically to check whether at least one part of Mars ever had a persistently wet environment that could possibly have been hospitable to life. Today we have strong evidence for an exciting answer: Yes."

Opportunity has more work ahead. It will try to determine whether, besides being exposed to water after they formed, the rocks may have originally been laid down by minerals precipitating out of solution at the bottom of a salty lake or sea.

The robotic field geologist has spent most of the past three weeks surveying the whole outcrop, then turning back for close-up inspection of selected portions. The rover found a very high concentration of sulfur in the outcrop with its alpha particle X-ray spectrometer, which identifies chemical elements in a sample.

"The chemical form of this sulfur appears to be in magnesium, iron or other sulfate salts," said Dr. Benton Clark of Lockheed Martin Space Systems in Denver. "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 a hydrated iron sulfate mineral called jarosite. Opportunity's miniature thermal emission spectrometer has also provided evidence for sulfates.

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.

Pictures from the rover's panoramic camera and microscopic imager reveal the target rock, dubbed "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.

Round particles the size of BBs 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.

A new color view, combining several frames from Opportunity's panoramic camera, adds information about the rover's likely destination after finishing work in and around the small crater where it landed. From partway up the inner slope of that 22-meter-diameter (72-foot-diameter) crater, the rover has an improved view of a crater nicknamed "Endurance," about 10 times as big and about 700 meters (2,300 feet) to the east.

The same new view across the flat plain of Meridiani also shows Opportunity’s jettisoned heat shield, a trail of marks left by the airbag bounces and a solitary dark rock about 40 centimeters (16 inches) across.

Both rovers carry magnets supplied by Denmark for experiments to analyze Martian dust. Dust covers much of Mars' surface and hangs in the atmosphere, occasionally rising into giant dust storms. One of the magnets is designed to exclude any magnetic dust particles from landing in the center of a target area. During Spirit's time on Mars, dust has accumulated on other parts of the target while the center has remained "probably the cleanest area anywhere on the surface of the rover," said Dr. Morten Madsen, science team member from the Center for Planetary Science, Copenhagen, Denmark.

"Most, if not all, of the dust particles in the Martian atmosphere are magnetic," Madsen said.

Another of the magnets is within reach of the rover's robotic arm. Examination of dust on the target by instruments on the end of the arm will soon yield further information about the composition of the dust, he said.

Opportunity completed its 40th sol (Martian day) at Meridiani at 9:32 a.m. PST on March 5. Spirit completed its 60th sol at Gusev late on March 4.

"Between the two rovers, we've had a terrific 100 days on Mars,” said Opportunity Missioni Manager Matt Wallace of JPL. “This last week has been particularly exciting."

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, D.C. Images and additional information about the project are available from JPL at http://marsrovers.jpl.nasa.gov and from Cornell University at http://athena.cornell.edu.

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