[meteorite-list] Marks on Martian Dunes May Be Tracks of Dry-Ice Sleds

[Ron Baalke]

June 11, 2013

Dwayne Brown 
Headquarters, Washington      
202-358-1726 
dwayne.c.brown at nasa.gov 

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

RELEASE: 13-180

MARKS ON MARTIAN DUNES MAY REVEAL TRACKS OF DRY ICE SLEDS

PASADENA, Calif. -- NASA research indicates hunks of frozen carbon 
dioxide -- dry ice -- may glide down some Martian sand dunes on 
cushions of gas similar to miniature hovercraft, plowing furrows as 
they go. 

Researchers deduced this process could explain one enigmatic class of 
gullies seen on Martian sand dunes by examining images from NASA's 
Mars Reconnaissance Orbiter (MRO) and performing experiments on sand 
dunes in Utah and California. 

"I have always dreamed of going to Mars," said Serina Diniega, a 
planetary scientist at NASA's Jet Propulsion Laboratory (JPL) in 
Pasadena, Calif., and lead author of a report published online by the 
journal Icarus. "Now I dream of snowboarding down a Martian sand dune 
on a block of dry ice." 

The hillside grooves on Mars, called linear gullies, show relatively 
constant width -- up to a few yards or meters across -- with raised 
banks or levees along the sides. Unlike gullies caused by waterflows 
on Earth and possibly on Mars, they do not have aprons of debris at 
the downhill end of the gully. Instead, many have pits at the 
downhill end. 

"In debris flows, you have water carrying sediment downhill, and the 
material eroded from the top is carried to the bottom and deposited 
as a fan-shaped apron," said Diniega. "In the linear gullies, you're 
not transporting material. You're carving out a groove, pushing 
material to the sides." 

Images from MRO's High Resolution Imaging Science Experiment (HiRISE) 
camera show sand dunes with linear gullies covered by carbon dioxide 
frost during the Martian winter. The location of the linear gullies 
is on dunes that spend the Martian winter covered by carbon dioxide 
frost. The grooves are formed during early spring, researchers 
determined by comparing before-and-after images from different 
seasons. Some images have even caught bright objects in the gullies. 

Scientists theorize the bright objects are pieces of dry ice that have 
broken away from points higher on the slope. According to the new 
hypothesis, the pits could result from the blocks of dry ice 
completely sublimating away into carbon-dioxide gas after they have 
stopped traveling. 

"Linear gullies don't look like gullies on Earth or other gullies on 
Mars, and this process wouldn't happen on Earth," said Diniega. "You 
don't get blocks of dry ice on Earth unless you go buy them." 

That is exactly what report co-author Candice Hansen, of the Planetary 
Science Institute in Tucson, Ariz., did. Hansen has studied other 
effects of seasonal carbon-dioxide ice on Mars, such as spider-shaped 
features that result from explosive release of carbon-dioxide gas 
trapped beneath a sheet of dry ice as the underside of the sheet 
thaws in spring. She suspected a role for dry ice in forming linear 
gullies, so she bought some slabs of dry ice at a supermarket and 
slid them down sand dunes. 

That day and in several later experiments, gaseous carbon dioxide from 
the thawing ice maintained a lubricating layer under the slab and 
also pushed sand aside into small levees as the slabs glided down 
even low-angle slopes. 

The outdoor tests did not simulate Martian temperature and pressure, 
but calculations indicate the dry ice would act similarly in early 
Martian spring where the linear gullies form. Although water ice, 
too, can sublimate directly to gas under some Martian conditions, it 
would stay frozen at the temperatures at which these gullies form, 
the researchers calculate. 

"MRO is showing that Mars is a very active planet," Hansen said. "Some 
of the processes we see on Mars are like processes on Earth, but this 
one is in the category of uniquely Martian." 

Hansen also noted the process could be unique to the linear gullies 
described on Martian sand dunes. 

"There are a variety of different types of features on Mars that 
sometimes get lumped together as 'gullies,' but they are formed by 
different processes," she said. "Just because this dry-ice hypothesis 
looks like a good explanation for one type doesn't mean it applies to 
others." 
The University of Arizona Lunar and Planetary Laboratory operates the 
HiRISE camera, which was built by Ball Aerospace & Technologies Corp. 
of Boulder, Colo. JPL manages MRO for NASA's Science Mission 
Directorate in Washington. Lockheed Martin Space Systems of Denver 
built the orbiter. 

To see images of the linear gullies and obtain more information about 
MRO, visit: 

http://www.nasa.gov/mro 

For more about HiRISE, visit: 

http://hirise.lpl.arizona.edu 

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