From: NASANews@hq.nasa.gov
Newsgroups: gov.us.fed.nasa.announce
Subject: Water History. Rock Composition Among Latest Findings a Year after Mars Pathfinder
Date: 29 Jun 1998 14:32:09 -0400
Organization: National Aeronautics and Space Administration (NASA)
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Douglas Isbell
Headquarters, Washington, DC June 29, 1998
(Phone: 202/358-1547)
Diane Ainsworth
Jet Propulsion Laboratory, Pasadena, CA
(Phone: 818/354-5011)
RELEASE: 98-115
WATER HISTORY, ROCK COMPOSITION AMONG
LATEST FINDINGS A YEAR AFTER MARS PATHFINDER
A year after the landing of Mars Pathfinder, mission
scientists say that data from the spacecraft paint two strikingly
different pictures of the role of water on the red planet, and
yield surprising conclusions about the composition of rocks at the
landing site.
"Many of the things that we said last summer during the
excitement after the landing have held up well," said Dr. Matthew
Golombek, Pathfinder project scientist at NASA's Jet Propulsion
Laboratory (JPL), Pasadena, CA. "But we have now had more time to
study the data and are coming up with some new conclusions."
Similar to ongoing science results from NASA's Mars Global
Surveyor spacecraft currently in orbit around Mars, Pathfinder
data suggest that the planet may have been awash in water three
billion to 4.5 billion years ago. The immediate vicinity of the
Pathfinder landing site, however, appears to have been dry and
unchanged for the past two billion years.
Several clues from Pathfinder data point to a wet and warm
early history on Mars, according to Golombek. Magnetized dust
particles and the possible presence of rocks that are
conglomerates of smaller rocks, pebbles and soil suggest copious
water in the distant past. In addition, the bulk of the landing
site appears to have been deposited by large volumes of water, and
the hills on the horizon known as Twin Peaks appear to be
streamlined islands shaped by water.
But Pathfinder images also suggest that the landing site is
essentially unchanged since catastrophic flooding sent rocks
tumbling across the plain two billion years ago. "Since then this
locale has been dry and static," he said.
While the area appears to have been untouched by water for
eons, wind appears to have been steadily eroding rocks at the
landing site. Analysis of Pathfinder images shows that about one
to two inches (three to five centimeters) of material has been
stripped away from the surface by wind, Golombek noted.
"Overall, this site has experienced a net erosion in recent
times," said Golombek. "There are other places on Mars that are
net 'sinks,' or places where dust ends up being deposited.
Amazonis Planitia, for example, probably has about three to six
feet (one to two meters) of fine, powdery dust that you would sink
into if you stepped on it."
Chemical analysis of a number of rocks by the alpha proton X-
ray spectrometer (APXS) instrument on PathfinderÕs mobile
Sojourner rover, meanwhile, reveals an unexpected composition that
scientists are still trying to explain.
The current assessment of data from this instrument suggests
that all of the rocks studied by the rover resemble a type of
volcanic rock with a high silicon content known on Earth as
andesite, covered with a fine layer of dust. All of the rocks
appear to be chemically far different from meteorites discovered
on Earth that are believed to have come from Mars.
"The APXS tells us that all of these rocks are the same
thing with different amounts of dust on them," said Golombek.
"But images suggest that there are different types of rocks. We
don't yet know how to reconcile this."
When molten magma oozes up from a planet's mantle onto the
surface of the outer crust, it usually freezes into igneous rock
of a type that geologists call a basalt. This is typical on the
floors of Earth's oceans, as well as on the maria or "seas" of the
Moon and in many regions of Mercury and Venus. By contrast,
andesites typically form on Earth in tectonically active regions
when magma rises into pockets within the crust, where some of its
iron and magnesium-rich components are removed, leaving rock with
a higher silicon content. 'We don't believe that Mars has had
plate tectonics, so these andesites must have formed by a
different mechanism," Golombek said.
The rocks studied by Pathfinder most closely resemble
andesites found in Iceland and the Galapagos Islands, tectonic
spreading centers where plates are being pushed apart, said Dr.
Joy Crisp of JPL. Andesites from these areas have a different
chemical signature from andesites formed at subduction zones
(areas where one edge of a crustal plate descends below another),
mostly because wet ocean sediments carry more water down into the
mantle at the subduction zones. "On Mars, where the water content
is probably lower and there is no evidence of subduction, we would
expect a closer chemical similarity to Iceland andesites," said
Crisp.
The Martian rocks may have other origins, however. They
could be sedimentary and influenced by water processes; they could
be formed by melting processes resulting from a meteor impact; or,
a third alternative is that the rocks might be basaltic, but
covered by a silicon-rich weathering coating. "In any event, the
presence of andesites on Mars is a surprise, if it is borne out as
we study the data further," said Crisp. "Most rocks on Mars are
expected to be basalts lower in silicon. If these are in fact
andesites, they are probably not very abundant."
Scientists are looking forward to more data from the Thermal
Emission Spectrometer instrument on the Mars Global Surveyor to
reveal more about the chemical composition of the planet's
surface, especially once the orbiting spacecraft begins its prime
circular mapping mission in spring 1999.
In other recent Pathfinder science findings, Dr. Steven
Metzger of the University of Nevada found direct evidence of
gusting winds called "dust devils" in images from Pathfinder's
lander. Such dust devils had been seen in some Viking orbiter
images and inferred from measurements of atmospheric pressure and
winds by other instruments on the Pathfinder lander, but were not
spotted in actual surface images until Metzger's discovery.
JPL planetary scientist Dr. Diana Blaney has been using data
from Pathfinder, other spacecraft missions and ground-based
observations to study weathering on Mars. Her work suggests that
Mars is uniformly covered by a fine coating of dust formed by an
unusual process involving meteor impacts and volcanic gases that
add sulfur.
NASA's next Mars missions, the 1998 Mars Climate Orbiter and
Mars Polar Lander, are in testing now for launch in December and
January, respectively. Whereas Pathfinder's science focus was on
exploring rocks with its mobile robotic geologist, the Mars Polar
Lander will focus on a search for water under the planet's
surface, equipped with a robot arm that will dig into the soil at
the landing site near the planet's south pole.
Launched on December 4, 1996, Pathfinder reached Mars on July
4, 1997, directly entering the planet's atmosphere and bouncing on
inflated airbags as a technology demonstration of a new way to
deliver a lander and rover to Mars. The lander operated nearly
three times its design lifetime of 30 days, while the rover
operated 12 times its design lifetime of seven days.
During the mission, the spacecraft relayed 2.3 gigabits of
d'ta to Earth. This unexpectedly large volume of information
included 16,500 images from the lander's camera, 550 images from
the rover camera, 16 chemical analyses of rocks and soil, and 8.5
million measurements of atmospheric pressure, temperature and
wind.
Mars Pathfinder was designed, built and operated by JPL for
NASA's Office of Space Science, Washington, DC. JPL is a division
of the California Institute of Technology, Pasadena, CA.
-end-
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