Some of Mars' Missing Carbon Dioxide May be Buried

Some of Mars' Missing Carbon Dioxide May be Buried
Jet Propulsion Laboratory
March 08, 2011

HOUSTON -- Rocks on Mars dug from far underground by crater-blasting
impacts are providing glimpses of one possible way Mars' atmosphere has
become much less dense than it used to be.

At several places where cratering has exposed material from depths of
about 5 kilometers (3 miles) or more beneath the surface, observations
by a mineral-mapping instrument on NASA's Mars Reconnaissance Orbiter
indicate carbonate minerals.

These are not the first detections of carbonates on Mars. However,
compared to earlier findings, they bear closer resemblance to what some
scientists have theorized for decades about the whereabouts of Mars'
"missing" carbon. If deeply buried carbonate layers are found to be
widespread, they would help answer questions about the disappearance of
most of ancient Mars' atmosphere, which is deduced to have been thick
and mostly carbon dioxide. The carbon that goes into formation of
carbonate minerals can come from atmospheric carbon dioxide.

"We're looking at a pretty lucky location in terms of exposing something
that was deep beneath the surface," said planetary scientist James Wray
of Cornell University, Ithaca, N.Y., who reported the latest carbonate
findings today at the Lunar and Planetary Science Conference near
Houston. Huygens crater, a basin 467 kilometers (290 miles) in diameter
in the southern highlands of Mars, had already hoisted material from far
underground, and then the rim of Huygens, containing the lifted
material, was drilled into by a smaller, unnamed cratering event.

Observations in the high-resolution mode of the Compact Reconnaissance
Imaging Spectrometer for Mars (CRISM) instrument on the Mars
Reconnaissance Orbiter show spectral characteristics of calcium or iron
carbonate at this site. Detections of clay minerals in lower-resolution
mapping mode by CRISM had prompted closer examination with the
spectrometer, and the carbonates are found near the clay minerals. Both
types of minerals typically form in wet environments.

The occurrence of this type of carbonate in association with the largest
impact features suggests that it was buried by a few kilometers (or
miles) of younger rocks, possibly including volcanic flows and
fragmented material ejected from other, nearby impacts.

These findings reinforce a report by other researchers five months ago
identifying the same types of carbonate and clay minerals from CRISM
observation of a site about 1,000 kilometers (600 miles) away. At that
site, a meteor impact has exposed rocks from deep underground, inside
Leighton crater. In their report of that discovery, Joseph Michalski of
the Planetary Science Institute, Tucson, Ariz., and Paul Niles of NASA
Johnson Space Center, Houston, proposed that the carbonates at Leighton
"might be only a small part of a much more extensive ancient sedimentary
record that has been buried by volcanic resurfacing and impact ejecta."

Carbonates found in rocks elsewhere on Mars, from orbit and by NASA's
Spirit rover, are rich in magnesium. Those could form from reaction of
volcanic deposits with moisture, Wray said. "The broader compositional
range we're seeing that includes iron-rich and calcium-rich carbonates
couldn't form as easily from just a little bit of water reacting with
igneous rocks. Calcium carbonate is what you typically find on Earth's
ocean and lake floors."

He said the carbonates at Huygens and Leighton "fit what would be
expected from atmospheric carbon dioxide interacting with ancient bodies
of water on Mars." Key additional evidence would be to find similar
deposits in other regions of Mars. A hunting guide for that search is
the CRISM low-resolution mapping, which has covered about three-fourths
of the planet and revealed clay-mineral deposits at thousands of locations.

"A dramatic change in atmospheric density remains one of the most
intriguing possibilities about early Mars," said Mars Reconnaissance
Orbiter Project Scientist Richard Zurek, of NASA's Jet Propulsion
Laboratory, Pasadena, Calif. "Increasing evidence for liquid water on
the surface of ancient Mars for extended periods continues to suggest
that the atmosphere used to be much thicker."

Carbon dioxide makes up nearly all of today's Martian air and likely was
most of a thicker early atmosphere, too. In today's thin, cold
atmosphere, liquid water quickly freezes or boils away.

What became of that carbon dioxide? NASA will launch the Mars Atmosphere
and Volatile Evolution Mission (MAVEN) in 2013 to investigate processes
that could have stripped the gas from the top of the atmosphere into
interplanetary space. Meanwhile, CRISM and other instruments now in
orbit continue to look for evidence that some of the carbon dioxide in
that ancient atmosphere was removed, in the presence of liquid water, by
formation of carbonate minerals now buried far beneath the present surface.

The Johns Hopkins University Applied Physics Laboratory, Laurel, Md.,
provided and operates CRISM, one of six instruments on the Mars
Reconnaissance Orbiter. JPL, a division of the California Institute of
Technology in Pasadena, manages the Mars Reconnaissance Orbiter project
and the Mars Exploration Program for the NASA Science Mission
Directorate, Washington. For more about CRISM, see . For more about the Mars Reconnaissance
Orbiter, visit .

Guy Webster 818-354-6278
Jet Propulsion Laboratory, Pasadena, Calif.

Lauren Gold 607-255-9736
Cornell University, Ithaca, N.Y.

Geoffrey Brown 240-228-5618
Johns Hopkins University Applied Physics Laboratory, Laurel, Md.



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