NASA last week announced a startling discovery on the Red Planet using its Mars Recoinnassance Orbiter (MRO), a satellite that's been circling Mars since 2006: It snows dry ice around the Martian south pole in the winter.
NASA pointed out in a news release that this is the only known example of such a phenomena anywhere in the solar system. The discovery was made by NASA Jet Propulsion Laboratory scientist Paul Hayne when he was a post-doctoral fellow at Caltech, using the MRO's Mars Climate Sounder instrument, which measures and records visible and infrared light in the Martian atmosphere. Analyzing data obtained from the sounder over the winter 2006-2007 period, Hayne and his colleagues detected a 300-mile-diameter cloud made of carbon dioxide over the south pole, with measurements showing particles falling all the way down to the surface.
Scientists still aren't sure what process allows the carbon dioxide crystals to fall to the surface, or whether they simply form as frost on the ground.
Here's a NASA MRO infrared image showing the concentration of dry ice snow deposits on the surface, with the largest grains in darker colors.