NASA Observations Point to 'Dry Ice' Snowfall on Mars
NASA's Mars Reconnaissance Orbiter data have given scientists the clearest evidence yet of carbon-dioxide snowfalls on Mars.
PASADENA,  Calif. -- NASA's Mars Reconnaissance Orbiter 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 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  snowfalls 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 the Mars  Reconnaissance Orbiter. 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 analysis was conducted while Hayne was a post-doctoral  fellow at the California Institute of Technology in Pasadena.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 the Red Planet 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,  a division of the California Institute of Technology in Pasadena, provided the  Mars Climate Sounder instrument and manages the Mars Reconnaissance Orbiter Project  for NASA's Science Mission Directorate in Washington.For  more information about the Mars Reconnaissance Orbiter, visit:http://www.nasa.gov/mroandhttp://mars.jpl.nasa.gov/mro.