An instrument from NASA (the Mars Climate Sounder) captured CO2 (dry ice) snowflakes falling from Mars' sky, enough to form large piles of snow. Scientists have known that frozen CO2 existed on Mars. Afterall, the temperature on Mars at the pole (where this image was capture), is -139 degrees Farenheit. It's the first time CO2 snow has ever been documented. The snow is very tiny, about the size of a cell. You'd need a microscope to see a flake! The snow accumulates as it falls; scientists predict it would look like a dense fog.
FYI: Dry ice is the solid form of carbon dioxide (CO2). CO2 is the air we exhale with every breath. It was also used by Michael Jackson in Thriller. Using dry ice is a cheap way to make fog since it sublimates (turns immediately from a solid to a gas--it's a cool kind of phase change 'cause it skips a step in the middle). It sublimates at -109 degrees Farenheit (way colder than room temperature, which is about 65-70 degrees Farenheit). CO2 also explains why Coke explodes when you drop a Mentos into it. Coke is packed with dissolved CO2 at the factory. The Mentos has a ton of tiny pits, covering its surface. This gives the CO2 more surface area cling to, allowing the gas to easily escape and, thus, erupt, as a gas. Check out Mythbusters, Mentos and Coke episode.