[meteorite-list] Trough Deposits on Mars Point to Complex Hydrologic Past

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 15 Dec 2009 14:58:03 -0800 (PST)
Message-ID: <200912152258.nBFMw35m009912_at_zagami.jpl.nasa.gov>

http://www.psi.edu/press/

Trough Deposits on Mars Point to Complex Hydrologic Past
Ed Stiles
Public Information Office
Planetary Science Institute
520-248-7119
psinews at psi.edu

Dec. 15, 2009 - Catherine Weitz, a senior scientist at the
Tucson-based Planetary Science Institute, has reported new evidence for
multiple, water-related geologic processes on Mars.

She and her colleagues studied light-toned deposits (LTDs) within
troughs of the Noctis Labyrinthus region in western Valles Marineris
using data gathered by three Mars Reconnaissance Orbiter (MRO)
instruments: the High Resolution Imaging Science Experiment (HiRISE)
camera, the Context Camera (CTX) and the Compact Reconnaissance Imaging
Spectrometer for Mars (CRISM).

Weitz presented the research results today during a morning session of
the American Geophysical Union Conference in San Francisco, Calif.

"We analyzed ten troughs containing well-exposed LTDs, and we found a
lot of variability that we didn't expect to see," she said. "We found
that each of the troughs with LTDs has a unique mineralogy, and,
therefore, the processes occurring in each trough were very localized."

Weitz and her team identified various types of clays, hydrated silicas,
and sulfates in these small basins, which are typically 30 to 100
kilometers across. One LTD included dozens of beds of varying thickness,
brightness, color and erosional structure, suggesting that significant
amounts of water once existed there. In addition, sulfates were mixed
with clays within the deposits, indicating that ph levels may have
fluctuated between acidic and alkaline conditions.

Another LTD is buried several meters beneath wind-deposited material and
is only exposed in the trough's upper walls, indicating it is older than
the trough. In still another area, clays are buried beneath younger
plains along the trough floor, while in the same trough, but a few
kilometers away, there are exposures of hydrated silica and calcium
sulfate.

The wide variability in deposits and mineralogy in these and the other
basins suggests a complex hydrologic history, including multiple events
in some troughs, Weitz said.

"Clearly, these areas were affected by water," she added. "In some cases
there had to be multiple events. But we don't know how much water was
involved or whether it was always a flowing liquid."

"It might have been groundwater coming from Tharsis, the large volcanic
complex to the west," she said. "There could have been active volcanism
that produced water by melting snow, ice, or underground, hydrothermal
processes. These little basins could then have filled or partially
filled with some of that water. Another possibility is that material was
already in several of the troughs, perhaps as volcanic ash or lava
flows, and some kind of hydrothermal activity may have altered these
pre-existing deposits."

Weitz and her colleagues identified LTDs using the wide-range CTX camera
that covers about a 30 km swath at a spatial resolution of 6 meters per
pixel. Then they zeroed in on the areas of interest using HiRISE visible
light images that cover about a 5-km-wide area with a resolution of
about 26 centimeters per pixel.

Finally, the corresponding CRISM data -- in the visible and near
infrared regions -- revealed the hydrated minerals within each LTD.

"It's great to have these complementary data sets," Weitz said.
"Together, the synthesis of these three datasets provides valuable
morphologic and mineralogic information needed to interpret the geologic
setting and origin of the light-toned deposits within the troughs of
Noctis."

The LTDs both pre-date and post-date trough formation and occurred
between the Noachian (4.6 to 3.5 million years ago) and Amazonian (1.8
billion years ago to the present) eras. The sediments in the LTDs could
have formed at the bottom of lakes or pools, but materials also could
have been deposited by the wind or fallen as volcanic ash and then been
altered by water.

Noctis Labyrinthus is a region of Mars that's located between the
volcanic Tharsis upland and Valles Marineris, a huge system of canyons.

Those working with Weitz on the research include Janice Bishop, of SETI
Institute/NASA Ames; Leah Roach, of Frontier Technology; Ralph Milliken,
of the Jet Propulsion Laboratory/Caltech; and J. Alexis Rodriguez, of
the Planetary Science Institute.
Received on Tue 15 Dec 2009 05:58:03 PM PST