Originally published by NASA on December 22, 2022. Edits by EarthSky.
Winter wonderland on Mars
When winter comes to one hemisphere or the other of Mars, the surface may be transformed into a truly otherworldly scene. Snow, ice, and frost often accompany the season’s sub-zero temperatures. Some of the coldest of these occur at the planet’s poles, where it gets as low as minus 190 degrees Fahrenheit (minus 123 degrees Celsius).
Cold as Mars is, don’t expect snow drifts worthy of Earth’s Rocky Mountains. Because Mars’ atmosphere is so thin, no Martian region gets more than a few feet of snow, most of which falls over extremely flat areas. And the 687-day orbit of Mars means it takes many more earthly months for winter to come around. A single Mars year is around two Earth years. So seasons last twice as long on Mars as they do on Earth.
Still, the planet offers unique winter phenomena that scientists have been able to study, with the aid of NASA’s robotic Mars explorers. Below are a few of the things they’ve discovered.
Two kinds of snow
Martian snow comes in two varieties: water ice and carbon dioxide, or dry ice. Because Martian air is so thin and the temperatures are so cold, water-ice snow sublimates, or becomes a gas, before it even touches the ground. But dry-ice snow does reach the ground.
Sylvain Piqueux is a Mars scientist at NASA’s Jet Propulsion Laboratory in Southern California whose research includes a variety of winter phenomena. He said:
Enough falls that you could snowshoe across it. If you were looking for skiing, though, you’d have to go into a crater or cliffside, where snow could build up on a sloped surface.
How we know it snows
Snow occurs only at the coldest extremes of Mars: at the poles, under cloud cover, and at night. Cameras on orbiting spacecraft can’t see through those clouds, and surface missions can’t survive in the extreme cold. As a result, no images of snow falling on Mars have ever been captured. But scientists know it happens, thanks to a few special science instruments.
NASA’s Mars Reconnaissance Orbiter can peer through cloud cover using its Mars Climate Sounder instrument. That’s because it can detect light in wavelengths imperceptible to the human eye. That ability has allowed scientists to detect carbon dioxide snow falling to the ground.
And, in 2008, NASA sent the Phoenix lander within 1,000 miles (1,600 km) of Mars’ north pole, where it used a laser instrument to detect water-ice snow falling to the surface.
Cubic snowflakes
Because of how water molecules bond together when they freeze, snowflakes on Earth have six sides. The same principle applies to all water crystals. The way in which atoms arrange themselves determines a crystal’s shape.
In the case of carbon dioxide, molecules in dry ice always bond in forms of four when frozen. According to Piqueux:
Because carbon dioxide ice has a symmetry of four, we know dry-ice snowflakes would be cube-shaped. Thanks to the Mars Climate Sounder, we can tell these snowflakes would be smaller than the width of a human hair.
Jack Frost nipping at your rover
Water and carbon dioxide can each form frost on Mars. And both types of frost appear far more widely across the planet than snow does. The Viking landers saw water frost when they studied Mars in the 1970s. And NASA’s Odyssey orbiter – which arrived at Mars in late 2001 – observed frost forming and sublimating away in the morning sun.
When the winter wonderland on Mars ends
Perhaps the most fabulous discovery comes at the end of winter, when all the ice that built up begins to thaw and sublimate into the atmosphere. As it does so, this ice takes on bizarre and beautiful shapes that have reminded scientists of spiders, Dalmatian spots, fried eggs, and Swiss cheese.
This thawing also causes geysers to erupt. When translucent ice allows sunlight to heat up gas underneath it, and that gas eventually bursts out, sending fans of dust onto the surface. Scientists have begun to study these fans as a way to learn more about which way Martian winds are blowing.
Bottom line: NASA’s HiRISE camera captured fascinating images of the winter wonderland on Mars. When Mars thaws in the spring, the ice takes on new and bizarre shapes even resulting in erupting geysers.
Read more: Hear a dust devil on Mars for the first time
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