Clouds of snow — frozen carbon dioxide crystals — at the Martian poles are made up of particles just 8–44 micrometres in diameter.
The Martian atmosphere consists mainly of carbon dioxide, which condenses at the poles during the winter. Renyu Hu and his colleagues at the Massachusetts Institute of Technology in Cambridge analysed temperature, pressure and topography data gathered over more than 9 years by the Mars Global Surveyor and Mars Reconnaissance Orbiter. The researchers found that snow accumulation is greater at the south pole than at the north and that the clouds spread halfway towards the equator before retreating in the spring.
J. Geophys. Res. http://dx.doi.org/10.1029/2012JE004087 (2012)