[meteorite-list] Study Links Fresh Mars Gullies to Carbon Dioxide

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 29 Oct 2010 12:38:12 -0700 (PDT)
Message-ID: <201010291938.o9TJcCCU006009_at_zagami.jpl.nasa.gov>

http://www.jpl.nasa.gov/news/news.cfm?release=2010-359

Study Links Fresh Mars Gullies to Carbon Dioxide
Jet Propulsion Laboratory
October 29, 2010

PASADENA, Calif. -- A growing bounty of images from NASA's Mars
Reconnaissance Orbiter reveals that the timing of new activity in one
type of the enigmatic gullies on Mars implicates carbon-dioxide frost,
rather than water, as the agent causing fresh flows of sand.

Researchers have tracked changes in gullies on faces of sand dunes in
seven locations on southern Mars. The periods when changes occurred, as
determined by comparisons of before-and-after images, overlapped in all
cases with the known winter build-up of carbon-dioxide frost on the
dunes. Before-and-after pairs that covered periods only in spring,
summer and autumn showed no new activity in those seasons.

"Gullies that look like this on Earth are caused by flowing water, but
Mars is a different planet with its own mysteries," said Serina Diniega,
lead author of a report on these findings in the November issue of the
journal Geology. She analyzed these gullies while a graduate student at
the University of Arizona, Tucson, and recently joined NASA's Jet
Propulsion Laboratory, Pasadena. "The timing we see points to carbon
dioxide, and if the mechanism is linked to carbon-dioxide frost at these
dune gullies, the same could be true for other gullies on Mars."

Scientists have suggested various explanations for modern gullies on
Mars since fresh-looking gullies were discovered in images from NASA's
Mars Global Surveyor in 2000. Some of the proposed mechanisms involve
water, some carbon dioxide, and some neither.

Some fresh gullies are on sand dunes, commonly starting at a crest.
Others are on rockier slopes, such as the inner walls of craters,
sometimes starting partway down the slope.

Diniega and co-authors at the University of Arizona and Johns Hopkins
University Applied Physics Laboratory, Laurel, Md., focused their study
on dune gullies that are shaped like rockier slope gullies, with an
alcove at the top, a channel or multiple channels in the middle, and an
apron at the bottom. The 18 dune gullies in which the researchers
observed new activity range in size from about 50 meters or yards long
to more than 3 kilometers (2 miles) long.

"The alcove is a cutout at the top," Diniega said. "Material being
removed from there ends up in a fan-shaped apron below."

Because new flows in these gullies apparently occur in winter, rather
than at a time when any frozen water might be most likely to melt, the
new report calls for studies of how carbon dioxide, rather than water,
could be involved in the flows. Some carbon dioxide from the Martian
atmosphere freezes on the ground during winter and sublimates back to
gaseous form as spring approaches. The dunes studied are poleward of 40
degrees south latitude.

"One possibility is that a pile of carbon-dioxide frost accumulating on
a dune gets thick enough to avalanche down and drag other material with
it," Diniega said. Other suggested mechanisms are that gas from
sublimating frost could lubricate a flow of dry sand or erupt in puffs
energetic enough to trigger slides.

At an increasing number of sites, before-and-after images have
documented changes in Martian gullies. The new report uses images from
the Mars Orbiter Camera on Mars Global Surveyor, which operated from
1997 to 2006, and from the High Resolution Science Imaging Experiment
(HiRISE) camera and Context Camera on Mars Reconnaissance Orbiter, which
has been examining Mars since 2006.

"The Mars Reconnaissance Orbiter is enabling valuable studies of
seasonal changes in surface features on Mars," said Sue Smrekar of
NASA's Jet Propulsion Laboratory, Pasadena, Calif., deputy project
scientist for this orbiter. "One key to doing that has been the
capability to point from side to side, so that priority targets can be
checked more frequently than just when the spacecraft flies directly
overhead. Another is the lengthening span of years covered by first Mars
Global Surveyor and now this mission."

JPL, a division of the California Institute of Technology in Pasadena,
manages the Mars Reconnaissance Orbiter for NASA's Science Mission
Directorate in Washington. Lockheed Martin Space Systems, Denver, built
the orbiter. The University of Arizona Lunar and Planetary Laboratory
operates the HiRISE camera, which was built by Ball Aerospace &
Technologies Corp., Boulder, Colo. Malin Space Science Systems, San
Diego, built and operates the Context Camera and formerly did the same
for the Mars Orbiter Camera. For more about the Mars Reconnaissance
Orbiter, visit http://www.nasa.gov/mro. For more about HiRISE, visit
http://hirise.lpl.arizona.edu.

Guy Webster 818-354-6278
Jet Propulsion Laboratory, Pasadena, Calif.
guy.webster at jpl.nasa.gov

Daniel Stolte 520-626-4402
University of Arizona, Tucson
stolte at email.arizona.edu

2010-359
Received on Fri 29 Oct 2010 03:38:12 PM PDT