NASA made another discovery about Mars recently, and it’s not from that little vehicle rolling around the rocks and craters at the moment.

The Mars Reconnaissance Orbiter (MRO) has found a 500-km wide cloud pouring snow down on the planet’s southern cap. Parts of the cap are covered with snow anywhere between a few feet to more than a mile deep.

But aside from the fact that it’s on an alien planet, there’s something else fundamentally different about this snow. It’s made from carbon dioxide molecules, not water. Because the polar ice cap on Mars reaches -125 degrees Celcius, the CO₂ is able to freeze into small particles within the cloud, then fall to Mars’ surface.

To put the temperature of the snow into perspective, to touch it would give you instant frostbite. In fact, you’ve probably already seen the somewhat volatile quality of frozen CO₂ when exposed to our atmosphere during kids’ Halloween parties or grade school science experiments – it’s dry ice.

Scientists have known for a while that the southern cap of Mars is covered in dry ice, but they’ve never seen it actually snow until now. It’s also a relatively rare phenomenon; Mars is the only planet we know of in our solar system that has carbon dioxide snow storms.

It also might be a bit of an anomaly to the planet itself. On the opposite pole, NASA’s Phoenix lander found water-based snow in 2008.

So what does this mean? Nothing, yet. But it is a new chapter in the increasingly interesting saga of life (or non-life) on Mars.

Photo: Mars’ southern polar cap taken by NASA’s Mars Orbiter Camera