Noora Alsaeed has often thought about building a snowman on Mars.
Let’s go over that again. A snowman on Mars? That desertlike, desolate planet over there? The one covered in sand? What an unusual daydream.
But Alsaeed knows a few things that the rest of us don’t. She is a planetary scientist at the University of Colorado at Boulder whose work relies on data from a NASA spacecraft that orbits Mars. She studies the red planet’s polar regions and the peculiar molecules suspended in the atmosphere above them. She knows that on Mars, it snows.
Just like Earth, Mars has seasons, and during the winter—about twice as long as ours—icy crystals cascade from the clouds and accumulate on the frigid surface. This sounds unbelievable, given that Mars is notoriously dry. But Mars gets around that little technicality by substituting intricate, six-sided water snow for something else. The Martian atmosphere, many times thinner than Earth’s, is primarily composed of carbon dioxide. In the most bitter conditions, the carbon dioxide transforms from a gas into small, cube-shaped crystals of ice—specifically dry ice, the kind we earthlings use to set a spooky scene on Halloween. The ice is too heavy to remain in the Martian sky, so it flurries down, settling in shallow piles on the red planet.
Mars is the planet that, aside from Earth, has likely made the largest impression on the public imagination. We’re well acquainted with Mars as the planet with all the rovers, the place where Elon Musk wants people to make a second home, the obvious next destination now that humans have been to the moon. But under all that hype are subtler, downright fascinating details about the fourth planet from the sun, such as its mesmerizing soundscape and its richly textured rock formations, layered like mille-feuille. Carbon-dioxide snow is just one of Mars’s many curiosities.
Scientists began to suspect that Mars’s polar regions could become cold enough to turn carbon dioxide into snow as early as the 1800s, Paul Hayne, a planetary scientist at CU Boulder who studies Martian snowfall, told me. A NASA mission in the 1970s made observations that would later be interpreted as the first signs of carbon-dioxide snowfall. In 2008, a spacecraft that landed in Mars’s northern plains detected evidence of snow—the water-ice kind!—falling from the atmosphere. But there was no evidence that the water snow actually reached the ground; the air on Mars is so thin that the water sublimates into a gas before the crystals can touch the surface.
The Mars Reconnaissance Orbiter, which has been circling Mars for more than 15 years, has captured carbon-dioxide snow reaching the surface, though. (Scientists don’t have photographic or video evidence of carbon-dioxide snowfall, only detections made with laser technology and observations in wavelengths that are invisible to our eyes. “Since most of the snow on Mars falls in the darkness of polar night, we need to use wavelengths of radiation outside of the visible spectrum,” Hayne said.) The snow even accumulates, mostly near sloped areas such as cliff sides and crater edges, Sylvain Piqueux, a research scientist at NASA’s Jet Propulsion Laboratory who studies Mars, told me. He said that enough of it piles up to—hypothetically—snowshoe in.
That idea tickles the imagination. What might it be like to stand on the Martian surface in the middle of winter, the temperatures finally cold enough to loose some molecules from the sky? Snowfall occurs only during the cold Martian night, so if you brought some night-vision goggles, you’d see that you were enveloped in a bright haze. Carbon-dioxide snowflakes are tiny, smaller than the width of a strand of hair—much smaller than their six-sided, water-ice counterparts. “It wouldn’t look as magical as it does on Earth,” Alsaeed said.
But a Martian blizzard would be lovely in its own way. “It would be extraordinarily quiet,” Hayne said. You might even be able to catch the sound of little carbon-dioxide snow-cubes falling onto the ground. A gust of wind could kick up “an opaque column of glittering snow,” he said. “Glittering” and “snow”—two words that may reshape your mental picture of Mars.
So if astronauts could, in theory, snowshoe on the red planet, what else could they do? Skiing is likely out, Hayne said. “Part of what makes skiing possible on Earth is that a thin film of liquid water forms on the surfaces of the ice particles as your ski creates friction, lubricating your motion,” he said. On Mars, that friction would cause icy particles to turn into vapor and billow away, which “would probably make your skis a bit squirrelly.”
The experts don’t really know whether other classic winter activities could take place on Mars. “The idea of dealing with snow that’s made of CO2 is just so alien to me,” Alsaeed said. “It’s gonna be a completely different ball game.” Piqueux isn’t sure whether carbon-dioxide snow would clump enough to form a snowball, let alone a snowman; dry ice is not exactly a chemical enigma, but how the stuff behaves under Martian conditions is more mysterious, he said. At the very least, you might manage a snow angel. And as for opening your mouth wide to catch a cube-shaped snowflake? “You can’t stick your tongue out on Mars, ever!” Hayne said. (Sorry, I had to ask!)
There is much to learn. “Snow might be a universal process for [worlds] with an atmosphere,” Piqueux said. “Learning how it works might tell us quite a bit about planets—what shapes their surface, how they evolve, and what they look like.” Scientists theorize that Mars was more like Earth a few billion years ago—warm and balmy, with real lakes and seas. Perhaps it snowed more back then too, with chunky flakes of frozen water, and the influence of that ancient precipitation remains embedded at the planet’s poles.
Many decades ago, well before any space robots arrived on Mars, scientists imagined the red planet to be a bustling place, believing that the surface markings they saw through their telescopes were evidence of intelligent engineering. The astronomer Percival Lowell wrote at length about these markings, which he called canals, in The Atlantic in 1895, sparking in the public imagination the tantalizing promise of an inhabited Mars. That ended up not being the case: Any life that may have arisen on Mars is either long dead or hiding out of view, buried away from the glare of the sun. The dissimilarity to Earth was almost disappointing.
But still, there are familiar echoes, as Lowell himself recognized. “If astronomy teaches anything, it teaches that man is but a detail in the evolution of the universe, and that resemblant though diverse details are inevitably to be expected in the host of orbs around him,” he wrote. “He learns that though he will probably never find his double anywhere, he is destined to discover any number of cousins scattered through space.” Cousins like Martian snow—perhaps not enough to make a genuine snowman, but certainly enough to stir our imagination from millions of miles away.