This started as a story about really cool clouds on Earth, but then it led to this: Does it snow on Mars?

Snow falling from clouds near Boulder, Colorado on Dec. 19, 2018 did not reach the ground — a classic example of virga. (Photo: ©Tom Yulsman) Mars is certainly cold. With temperatures that can plunge to more than negative 100 degrees Celsius, it's bloody frigid! But as cold as it might get, does it snow on Mars? This wasn't the first thing that came to mind when I photographed the scene above near Boulder, Colorado with my iPhone. But when I got home and started investigating the beautiful phenomenon I had documented, I eventually came around to that rather un-obvious question. How I came to it — as well as the answer — is a bit of an interesting journey, so I hope you'll keep reading. First, what about those clouds in the image above? If you live somewhere with wide open vistas, you've probably seen the phenomenon yourself: darkish streaks appearing to hang from a cloud deck. This is called "virga" — precipitation falling from the clouds but mostly not reaching the ground. I've seen virga many times before, but usually in summer. This is when temperatures are high and humidity near the surface can be relatively low, causing shafts of rain falling from clouds to evaporate before the drops reach the ground. But I shot the photo above on December 19th, two days before the winter solstice and well into meteorological winter. At the elevation of the cloud deck, temperatures were almost certainly well below freezing. So the virga probably consisted of little ice crystals. As they fell toward the ground, they simply sublimated — meaning they went from the frozen state to water vapor without first condensing into liquid. I decided to write a post about it here at ImaGeo, and as I did my research, my mind wandered far from Colorado. All the way to Mars, which I knew had its own clouds. I began wondering whether Martian clouds feature ice crystals streaming downward to form virga. Do ice crystals actually reach the ground as snow? And what might all of this look like from the surface? I haven't been the only person to think about these things. In their quest to confirm that frozen water exists on the surface of Mars, and also to learn the role it has played in the planet's geologic history, scientists have sought answers to those very same questions. Among them is Jim Whiteway of the Centre for Research in Earth and Space Science at York University in Toronto. He led a team of Canadian researchers that used observations from NASA's Phoenix spacecraft to address those questions. Scientists have known for quite some time that water clouds do indeed form in the Martian atmosphere. You can see it for yourself in the images that follow.