If you haven’t seen them yet, check out the photos of clouds on Mars taken by Curiosity. Clouds are a rare occurrence on Mars as the atmosphere is very thin. Most clouds form near the equator at the coldest time of the Martian year, but NASA noticed clouds forming two years ago earlier than expected, so they decided to be ready for them this time around. The clouds are mostly wispy clouds full of ice crystals, similar to Earth’s cirrus clouds. Unlike cirrus clouds though, most Martian clouds are low lying, no more than 37 miles in the air. These new clouds are higher in the atmosphere, where it is colder, meaning these ice crystals may actually be dry ice, or frozen carbon dioxide.