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"A spacecraft orbiting Mars has detected carbon dioxide snow falling on the Red Planet, making Mars the only body in the solar system known to host this weird weather phenomenon. \nThe snow on Mars fell from clouds around the planet's south pole during the Martian winter spanning 2006 and 2007, with scientists discovering it only after sifting through observations by NASA's Mars Reconnaissance Orbiter (MRO). The Martian south pole hosts a frozen carbon dioxide \u2014 or \"dry ice\" \u2014 cap year-round, and the new discovery may help explain how it formed and persists, researchers said.\n\"These are the first definitive detections of carbon-dioxide snow clouds,\" lead author Paul Hayne, of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif., said in a statement. \"We firmly establish the clouds are composed of carbon dioxide \u2014 flakes of Martian air \u2014 and they are thick enough to result in snowfall accumulation at the surface.\"\nThe find means Mars hosts two different kinds of snowfall. In 2008, NASA's Phoenix lander observed water-ice snow \u2014 the stuff we're familiar with here on Earth \u2014 falling near the Red Planet's north pole. \n\nSEE ALSO: 7 Biggest Mysteries of Mars (SPACE.com) \nHayne and his team studied data gathered by MRO's Mars Climate Sounder instrument during the Red Planet's southern winter in 2006-2007. This instrument measures brightness in nine different wavelengths of visible and infrared light, allowing scientists to learn key characteristics of the particles and gases in the Martian atmosphere, such as their sizes and concentrations.\nThe research team examined measurements the Mars Climate Sounder made while looking at clouds \u2014 including one behemoth 300 miles wide \u2014 from directly overhead, and from off to the side. These combined observations clearly revealed dry-ice snow falling through the Red Planet's skies, researchers said.\n\"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,\" said co-author David Kass, also of JPL. \"Another comes from observations when the instrument is pointed toward the horizon, instead of down at the surface.\"\n\"The infrared spectra signature of the clouds viewed from this angle is clearly carbon-dioxide ice particles, and they extend to the surface,\" Kass added. \"By observing this way, the Mars Climate Sounder is able to distinguish the particles in the atmosphere from the dry ice on the surface.\"\nAstronomers still aren't entirely sure how the dry ice sustaining Mars' south polar cap \u2014 the only place where frozen carbon dioxide exists year-round on the planet's surface \u2014 is deposited. It could come from snowfall, or the stuff may freeze out of the air at ground level, researchers said.\n\"The finding of snowfall could mean that the type of deposition \u2014 snow or frost \u2014 is somehow linked to the year-to-year preservation of the residual cap,\" Hayne said.\nDry ice requires temperatures of about minus 193 degrees Fahrenheit to fall, reinforcing just how cold the Martian surface is.\nThe study will be published in an upcoming issue of the Journal of Geophysical Research. Hayne performed the research while a postdoc at Caltech in Pasadena.\nImage courtesy of NASA, Christine Daniloff/MIT News"^^<http://www.w3.org/2001/XMLSchema#string>
<http://mashable.com/2012/09/17/snow-on-mars/>