Scientists have concluded the part of Mars NASA’s Opportunity rover is exploring was soaking wet in the past.
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, Ithaca, N.Y., principal
investigator for the science instruments on Opportunity and its
twin, Spirit. “We’ve been able to read the tell-tale clues the
water left behind, giving us confidence in that conclusion,” he
said.
Dr. James Garvin, lead scientist for Mars and lunar exploration
at NASA Headquarters, 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 first views Opportunity sent of its landing site in Mars’
Meridiani Planum region five weeks ago delighted researchers at
NASA’s Jet Propulsion Laboratory (JPL), Pasadena, Calif.,
because of the good fortune to have the spacecraft arrive next
to an exposed slice of bedrock on the inner slope of a small
crater.
The robotic field geologist has spent most of the past three
weeks surveying the whole outcrop, and 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, Denver. “Elements that can form chloride
or even bromide salts have also been detected.”
At the same location, the rover’s Moessbauer spectrometer,
which identifies iron-bearing minerals, detected a hydrated
iron sulfate mineral called jarosite. Germany provided both
these instruments. 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.
The water evidence from the rocks’ physical appearance comes in
at least three categories, said Dr. John Grotzinger,
sedimentary geologist from the Massachusetts Institute of
Technology, Cambridge: indentations called “vugs,” spherules
and crossbedding.
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.
The images obtained to date are not adequate for a definitive
answer. So scientists plan to maneuver Opportunity closer to
the features for a better look. “We have tantalizing clues, and
we’re planning to evaluate this possibility in the near
future,” Grotzinger said.
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 information about NASA and the Mars mission on the
Internet, visit:
Images and additional information about the project are also
available on the Internet at:
http://marsrovers.jpl.nasa.gov
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