[meteorite-list] Mars Reconnaissance Orbiter Observations Point to 'Dry Ice' Snowfall on Mars

[Ron Baalke]

Sept. 11, 2012

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: 12-315

NASA ORBITER OBSERVATIONS POINT TO 'DRY ICE' SNOWFALL ON MARS

PASADENA, Calif. -- NASA's Mars Reconnaissance Orbiter (MRO) data have 
given scientists the clearest evidence yet of carbon dioxide 
snowfalls on Mars. This reveals the only known example of carbon 
dioxide snow falling anywhere in our solar system. 

Frozen carbon dioxide, better known as "dry ice," requires 
temperatures of about minus 193 degrees Fahrenheit (minus 125 
Celsius), which is much colder than needed for freezing water. Carbon 
dioxide snow reminds scientists that although some parts of Mars may 
look quite Earth-like, the Red Planet is very different. The report 
is being published in the Journal of Geophysical Research. 

"These are the first definitive detections of carbon dioxide snow 
clouds," said the report's lead author Paul Hayne of NASA's Jet 
Propulsion Laboratory (JPL) in Pasadena, Calif. "We firmly establish 
the clouds are composed of carbon dioxide -- flakes of Martian air -- 
and they are thick enough to result in snowfall accumulation at the 
surface." 

The snow falls occurred from clouds around the Red Planet's south pole 
in winter. The presence of carbon dioxide ice in Mars' seasonal and 
residual southern polar caps has been known for decades. Also, NASA's 
Phoenix Lander mission in 2008 observed falling water-ice snow on 
northern Mars. 

Hayne and six co-authors analyzed data gained by looking at clouds 
straight overhead and sideways with the Mars Climate Sounder, one of 
six instruments on MRO. This instrument records brightness in nine 
wavebands of visible and infrared light as a way to examine particles 
and gases in the Martian atmosphere. 

The data provide information about temperatures, particle sizes and 
their concentrations. The new analysis is based on data from 
observations in the south polar region during southern Mars winter in 
2006-2007, identifying a tall carbon dioxide cloud about 300 miles 
(500 kilometers) in diameter persisting over the pole and smaller, 
shorter-lived, lower-altitude carbon dioxide ice clouds at latitudes 
from 70 to 80 degrees south. 

"One line of evidence for snow is that the carbon dioxide ice 
particles in the clouds are large enough to fall to the ground during 
the lifespan of the clouds," co-author David Kass of JPL said. 
"Another comes from observations when the instrument is pointed 
toward the horizon, instead of down at the surface. The infrared 
spectra signature of the clouds viewed from this angle is clearly 
carbon dioxide ice particles and they extend to the surface. By 
observing this way, the Mars Climate Sounder is able to distinguish 
the particles in the atmosphere from the dry ice on the surface." 

Mars' south polar residual ice cap is the only place on Mars where 
frozen carbon dioxide persists on the surface year-round. Just how 
the carbon dioxide from Mars' atmosphere gets deposited has been in 
question. It is unclear whether it occurs as snow or by freezing out 
at ground level as frost. These results show snowfall is especially 
vigorous on top of the residual cap. 

"The finding of snowfall could mean that the type of deposition -- 
snow or frost -- is somehow linked to the year-to-year preservation 
of the residual cap," Hayne said. 

JPL provided the Mars Climate Sounder instrument and manages the MRO 
Project for NASA's Science Mission Directorate in Washington. 

For more information about MRO, visit: 

http://www.nasa.gov/mro 
	
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