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Sand Dunes on Mars Covered in Frost and Oddly Shaped

It's winter on Earth, for those living in the Northern Hemisphere. This means snow, rain, colder temperatures and all the other things we associate with the “festive season.” The same goes for Mars (aka. "Earth's Twin"), which is currently also experiencing winter in the Northern Hemisphere. This means colder temperatures, especially around the polar regions which can get as low as -123 °C (-190 °F), as well as ice, snow, frost, and expanding polar ice caps – made up of water ice and frozen carbon dioxide. ("dry ice").

Although Mars doesn't experience snowfall like Earth, the changing seasons produce some very interesting phenomena. Thanks to the many robotic explorers that NASA and other space agencies have sent to Mars in the last fifty years, scientists have been able to get a close look at this phenomenon. This includes the Viking orbiters and landers who studied the planet in the 1970s (with groundbreaking results) to the Mars Reconnaissance Orbiter (MRO), Mars Exploration Rovers ( Spirit and Opportunity ), and the Curiosity and Perseverance rovers exploring the surface today.

Thanks to these dedicated orbiters, landers and rovers, scientists have learned several important facts about snow on Mars: Snow comes in two types (water ice and dry ice), and it only ever falls in the coldest regions and times – at the poles, under cloud cover, and at night. Because the Martian atmosphere is so thin and the temperatures are so extreme, water and carbon dioxide don't freeze but sublime, changing from gas directly to ice (and back again). Additionally, dry ice snowflakes are cubic in shape, meaning they have four sides instead of the six-sided configuration we are familiar with.

Like a water molecule, this is because the shape of the crystal depends on how the atoms arrange themselves. In the case of CO2, the molecules always bond in groups of four. Additionally, snow never reaches the ground on Mars but sublimes as it falls from the clouds to the surface. Because most orbiters cannot see through these clouds, and rovers cannot withstand the extreme cold, no pictures of falling snow have ever been taken. But scientists know that Mars experiences snowfall, thanks to some special instruments.

These include the Mars Climate Sounder (MCS) aboard the MRO, which scans the Martian atmosphere in visible and infrared light to measure temperature, humidity and dust content in the Martian atmosphere. This allows the science team to peer through the cloud cover and detect CO 2 snow that has fallen to the ground. Sylvain Piqueux, a planetary research Scientist at NASA's Jet Propulsion Laboratory, explained the ins and outs of Martian snow in a recent interview with NASA's Mars News Reports (a series dedicated to educating the public about the exploration and study of the Red Planet). As he explains:

“Fall enough that you can snowboard across it. However, if you want to ski, you'll need to head to the crater or cliff side, where snow can accumulate on the sloping surface. Because carbon dioxide ice has quadrilateral symmetry, we know a dry ice snowflake will be cube-shaped. Thanks to the Mars Climate Sounder, we were able to tell that these snowflakes would be smaller than the width of a human hair."

Additionally, NASA's Phoenix mission landed within 1,000 miles (about 1,600 kilometers) of the Martian north pole in 2008. As part of its science operations, the lander uses laser-based atmospheric sensors – part of a dedicated meteorological station provided by Canada. Space Agency (CSA) – to detect snow water ice falling to the surface. The Viking lander also detected water frost at their landing site, and the NASA Odyssey orbiter observed the formation and sublimation of frost at sunrise many times during its mission.

When CO 2 ice sublimates towards the end of winter, Mars' most iconic surface results emerge. These include strange and wonderful shapes that scientists have nicknamed “spiders,” “Dalmatian spots,” “fried eggs,” and “Swiss cheese.” The "spring thaw" also causes geysers to erupt as sunlight passes through the translucent permafrost, heating pockets of gas beneath. This triggers an eruption that sends dust to the surface, creating a feature known as a "Spring Fan" that scientists study to learn more about which way Mars' winds blow.

As Piqueux explains, all of this data will be critical when it comes time to send manned missions to Mars, which NASA hopes to undertake in the 2030s:

“The Pheonix lander, a NASA mission that arrived on Mars in 2008, surveyed the beautiful icy landscape that was forming around it. The Pheonix lander was also able to scratch the surface and, for the first time, saw this icy water just beneath the ground. It's a type of water ice that astronauts could potentially use in the future when we go there."

Many exciting things accompany the changing seasons on Mars, and we are fortunate to witness these thanks to many generations of robotic missions. Soon, astronauts will witness Mars and its dynamic climate first-hand, and their research will fuel scientific breakthroughs and discoveries for generations to come!