On Earth, snowfall happens when water in the air drops below the freezing point of water (32F) and falls to the ground in flakes. But on Mars, it snows dry ice: frozen carbon dioxide. When the CO2 in the Martian atmosphere falls below the freezing point of CO2 (-193F) flakes of frozen CO2 fall to the ground in what scientists believe is the only example of carbon dioxide snow in our solar system.
It's snowing dry ice on Mars
Mars may look just like Arizona, but the Red Planet's weather is even more extreme.
Every year in winter, Mars experiences a massive cloud that gathers over the south pole. This persistent cloud can be as much as 310 miles across. It is a huge seasonal blizzard that can dump inches - even feet - of fluffy white CO2 snow on the planet's surface.
Mars may look just like Arizona, but the Red Planet's weather is even more extreme. Of all the planets in our solar system, Mars has the most Earth-like set of seasons, due to the planet's tilt and the similarity to our proximity to the sun. Mars has observable seasons, polar ice caps, and Earth-like storms.
But that is where the similarity ends. The average temperature on Mars is, more or less, -67F, which is the sort of temperature Earth only sees in the depths of winter in the polar zones. Over the course of a year, the warmest day on Mars will top out at about 87F. The coldest day will bottom out at about -225F.
Most of the Martian atmosphere is carbon dioxide. This makes Mars' weather much more extreme and changeable, compared to the weather on Earth. We also have the benefit of our oceans, which help to moderate the climate. Not so on Mars, where weather patterns can sweep across the planet without anything to stop them.
Stranger still, in some places, at some times, the snow falls up. Mars experiences severe sudden warming at times, when the sun hits it on a clear day. When this happens in an area where CO2 ice has formed, the CO2 evaporates (sublimates) upwards. This creates upside down rivulets, as the CO2 trickles up a slope into the air. Dubbed "spider gullies," these were a mystery for years before NASA scientists were able to crack the case.
If Earth's climate engine is based on the process of water evaporating into the atmosphere, Mars' climate engine is based on the process of CO2 sublimating into its atmosphere. Scientists still don't fully understand all the ramifications of this intriguing difference. But they do point out that if you had a thick enough space suit, you could potentially go skiing in the CO2 snow one day.