PHOTOS: Weirdest Mars Craters
"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.
NEWS: Weirdest Mars Craters
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.