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False-color image of gully channels in a crater in the southern highlands of Mars, taken by the High Resolution Imaging Science Experiment camera on the Mars Reconnaissance Orbiter.
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NASA's Mars Reconnaissance Orbiter (MRO) is
examining several features on Mars that address the role of water at
different times in Martian history.
Features examined with the orbiter's advanced instruments include
material deposited in two gullies within the past eight years, polar
ice layers formed in the recent geologic past, and signs of water
released by large impacts when Mars was older.
Last year, discovery of the fresh gully deposits from before-and-after
images taken since 1999 by another orbiter, Mars Global Surveyor,
raised hopes that modern flows of liquid water had been detected on
Mars. Observations by the newer orbiter, which reached Mars last
year, suggest these deposits might instead have resulted from
landslides of loose, dry materials. Researchers report this and other
findings from the MRO in five papers in Friday's issue of the journal
Science.
"The key question raised by these two deposits is whether water is
coming to the surface of Mars today." said Alfred McEwen of the
University of Arizona, Tucson, lead scientist for the spacecraft's
High Resolution Imaging Science Experiment camera and co-author of
three of the papers. "Our evidence suggests the new deposits did not
necessarily involve water."
One of the fresh deposits is a stripe of relatively bright material
several hundred yards long that was not present in 1999 but appeared
by 2004. The orbiter's Compact Reconnaissance
Imaging Spectrometer for Mars reveals the deposit is not frost, ice
or a mineral left behind by evaporation of salty water. Also, the researchers inspected
the slopes above this and five other locations that have bright and
apparently young deposits. The slopes are steep enough for sand or
loose, dry dust to flow down the gullies. Bright material seen uphill
could be the source.
Other gullies, however, offer strong evidence of liquid water flowing
on Mars within the last few million years, although perhaps at a
different phase of repeating climate cycles. Mars, like Earth, has
periodic changes in climate due to the cycles related to the planets'
tilts and orbits. Some eras during the cycles are warmer than others.
These gullies are on slopes too shallow for dry flows, and images
from MRO's high-resolution camera show clear indicators of liquid
flows, such as braided channels and terraces within the gullies.
Another new finding from that camera may help undermine arguments that
very ancient Mars had a wet climate on a sustained basis. Landscapes
with branched channels and fan-like deposits typical of liquid flows
were found around several impact craters. Images show close
association between some of those flow features and ponded deposits
interpreted as material melted by the impact of a meteoroid into
ice-rich crust. This new evidence supports a hypothesis that ancient
water flows on the surface were episodic, linked to impact events and
subsurface heating, and not necessarily the result of precipitation
in a sustained warmer climate. Crater-digging impacts were larger and
more numerous during the early Martian era when large drainage
networks and other signs of surface water were carved on many parts
of the planet.
The MRO has examined ice-rich layered deposits near the Martian poles
with the ground-penetrating Shallow Subsurface Radar instrument, and
other experiments. The radar detected layering patterns near the
south pole that suggest climatic periods of accumulating deposits
have alternated with periods of erosion, report Roberto Seu of the
University of Rome and co-authors. Maria Zuber of the Massachusetts
Institute of Technology, and collaborators used effects of Mars'
gravity on the orbiter to check whether layered deposits at the south
pole are high-density material, such as rock, or lower-density such
as ice. Their observations add to other evidence that the layers are
mostly water. Kenneth Herkenhoff of the U.S. Geological Survey,
Flagstaff, and others used the high-resolution camera to trace a
series of distinctive layers near the north pole.
An accompanying paper by Windy Jaeger of the U.S. Geological Survey,
Flagstaff, and co-authors uses images from the high-resolution camera
to show lava flows completely draping a young Martian channel network
called Athabasca Valles This creates ponded lava over an expanse that
other researchers had interpreted in 2005 as a frozen sea.
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This video provides zoom and pan moves to emphasize the context and features of a Martian valley named Athabasca Valles, perhaps the youngest outflow channel on Mars. [Video]
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Richard Zurek, project scientist for MRO at NASA's Jet Propulsion
Laboratory (JPL), Pasadena, Calif., said, "These latest increases in
observational capabilities, individually and in combination, reveal a
more complex Mars, a planet with a rich history that we are still
learning to read."
JPL manages the MRO mission for NASAs Science Mission Directorate,
Washington. Lockheed Martin Space Systems, Denver, is the prime
contractor and built the spacecraft. The University of Arizona
operates the High Resolution Imaging Science Experiment camera, built
by Ball Aerospace & Technology Corp., Boulder, Colo. The Johns
Hopkins University Applied Physics Laboratory, Laurel, Md., operates
the Compact Reconnaissance Imaging Spectrometer for Mars. The Shallow
Subsurface Radar was provided by the Italian Space Agency; its
operations are led by the University of Rome, and its data analyzed
by a joint Italian-U.S. science team.
Images from the new reports are available online at:
http://www.nasa.gov/mission_pages/MRO/multimedia/20070920.html
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