On Mars, it snows dry ice

Weather instruments aboard the Mars rover report sub-freezing temperatures even during the late summer. Water ice clouds have been observed north of Gale Crater where the Curiosity rover landed about a month ago.

This week, exometeorologists at the Massachusetts Institute of Technology and NASA's Jet Propulsion Laboratory (JPL) announced confirmation of a different kind of snow on Mars' south pole. The report confirmed snow clouds releasing a very fine snowfall of frozen carbon dioxide, better known as dry ice. This is based on observations made by the Mars Climate Sounder instrument aboard the Mars Reconnaissance Orbiter (MRO) during the 2006-2007 winter season on Mars.

This is the only known example of carbon dioxide (CO2) snowfall in our solar system.

"These are the first definitive detections of carbon-dioxide snow clouds," said the report's lead author, Paul Hayne of 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 relative brightness of particles and gasses in the Martian atmosphere across visible and infrared light are monitored from multiple angle. The frozen CO2 was observed in 300 mile wide clouds but is microscopic is size with the smallest flakes falling nearest to the south pole. These frozen particles are so small that clouds and resulting snowfall look more like fog than snowfall we are used to seeing here on Earth.

Scientists also found evidence of water based snow on Mars in 2008 when the Phoenix lander detected large ice-crystals in the atmosphere above its landing site near the planet's north pole.