Sept. 12, 2012 — Catching snowflakes on your tongue might be a fun event during a winter flurry here on Earth, but you probably wouldn't want to try it on Mars — the flakes there are made of carbon dioxide "dry ice" and at temperatures of -125ºC (-193ºF) the fun level would be dramatically decreased!
Scientists have long known that Mars has frozen carbon dioxide on its surface, both as surface and subsurface frost and in enormous ice caps covering its north and south poles. But it wasn't exactly understood how the dry ice got there... did it condense from the air at ground level, building up as layers of frost? Or did it accumulate from the top down, precipitating from clouds like water ice snow does on Earth?
The latest results from research conducted with NASA's Mars Reconnaissance Orbiter shows clear evidence of the latter.
During Mars' southern winter in 2006-2007, researchers from JPL observed large clouds above the polar ice cap at oblique angles using MRO's Mars Climate Sounder instrument. The clouds were found to be composed of carbon dioxide with evidence of particles extending all the way down to the surface of the ice… a dry ice blizzard.
"These are the first definitive detections of carbon-dioxide snow clouds," said Paul Hayne of NASA's Jet Propulsion Laboratory in Pasadena, the report's lead author. "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."
Mars Climate Sounder looks at the horizon of Mars from orbit to observe the atmosphere in slices, with measurements every 5 kilometers (3 miles) down in each slice through the atmosphere detecting temperature, pressure, water vapor, and dust. These profiles are combined into three-dimensional global weather maps.
Further evidence that actual snow was occurring comes from the scale of the particles, which were detected at sizes large enough to fall.
These findings show that snowfall is likely a contributor to the persistence of the residual ice caps on Mars. It's also the first time that solid evidence for carbon dioxide snow has been identified anywhere in the solar system.