Seen from house: sand dunes on Mars coated by frost

It’s winter here on earth, for the denizens of the northern hemisphere. That means snow, rain, colder temperatures and all the other things we associate with “the Christmas season”. The same applies to Mars (aka “Earth’s twin”), which is also currently experiencing winter in its northern hemisphere. This means colder temperatures, particularly in the polar regions where it can drop as low as -123°C (-190°F), as well as ice, snow, frost and the expansion of the polar ice caps – which are composed of both water ice and frozen carbon dioxide ( “dry ice”).

While Mars doesn’t experience snowfall like Earth, the seasonal alternation leads to some very interesting phenomena. Thanks to the many robotic explorers NASA and other space agencies have sent to Mars over the past fifty years, scientists have been able to view these phenomena up close. These include the Viking orbiters and landers that probed the planet (with groundbreaking results) in the 1970s, to the Mars Reconnaissance Orbiter (MRO), the Mars Exploration Rovers (Spirit and Opportunity), and the Curiosity and Perseverance Rovers exploring the surface today.

Thanks to those dedicated orbiters, landers, and rovers, scientists have learned some key facts about snow on Mars: It comes in two varieties (water ice and dry ice), and it only ever snows in the coldest regions and times — at the poles, under cloud cover, and at night. Because Martian atmosphere is so thin and temperatures so extreme, water and carbon dioxide don’t freeze, but rather sublimate, turning from gas directly into ice (and back again). Additionally, dry ice snowflakes are cubic, meaning they have four sides instead of the familiar six-sided configuration we’re familiar with.

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As with water molecules, this is because the shape of a crystal depends on how atoms arrange themselves. With CO2, molecules always connect in groups of four. Also, snow on Mars never reaches the ground, but sublimates as it falls to the surface from the clouds. Because most orbiters cannot see through these clouds and rovers cannot withstand the extreme cold, images of falling snow have never been taken. But scientists do know that Mars is experiencing snowfalls, thanks to a handful of specialized instruments.

These include the Mars Climate Sounder (MCS) on board the MRO, which observes the Martian atmosphere in visible and infrared light to measure the temperature, humidity and dust content of the Martian atmosphere. This allows science teams to peer through cloud cover and spot CO2 snow falling to the ground. Sylvain Piqueux, a planetary science scientist at NASA’s Jet Propulsion Laboratory, explained the intricacies of Martian snow in a recent interview with NASA’s Mars News Report (a series dedicated to educating the public about exploration and exploration of the Red Planet). . As he explained:

“Enough waterfalls to cross with snowshoes. However, if you wanted to ski, you would have to go into a crater or cliff where snow could accumulate on a sloped surface. Because carbon dioxide ice has a symmetry of four, we know that dry ice snowflakes would be cube-shaped. Thanks to the Mars Climate Sounder, we can say that these snowflakes are smaller than the width of a human hair.”

Additionally, in 2008, NASA’s Phoenix mission landed within 1,000 miles (about 1,600 kilometers) of the north pole of Mars. As part of its scientific operations, the lander used a laser-based atmospheric sensor – part of a special meteorological station provided by the Canadian Space Agency (CSA) – to detect water ice snow falling on the surface. The Viking landers also discovered water frost at their landing sites, and NASA’s Odyssey orbiter observed frost forming and sublimating at sunrise many times during its mission.

On Mars, carbon dioxide frost and ice form over dunes in winter. When this sublimates in spring, darker “megadunes” (barchans) emerge. Credit: NASA/JPL-Caltech/UofA

As the CO2 ice sublimates towards the end of winter, the most iconic surface features on Mars emerge. These include the weird and beautiful shapes that scientists have dubbed “spiders,” “Dalmatian spots,” “fried eggs,” and “Swiss cheese.” The “spring thaw” also causes geysers to erupt as sunlight penetrates layers of translucent ice and heats pockets of gas beneath. This triggers eruptions that hurl dust at the surface and create a feature known as “spring fans,” which scientists are studying to learn more about which way the Martian winds are blowing.

As Piqueux explained, all of this data will be crucial when it comes to sending manned missions to Mars, which NASA hopes to do in the 2030s:

“[T]he Pheonix lander, the NASA mission that landed on Mars in 2008, observed beautiful frozen landscapes forming around it. The Pheonix lander was also able to scratch the surface and see this water ice just underground for the first time. This is the type of water ice that astronauts could potentially use in the future when we fly there.”

Many fascinating things accompany seasonal changes on Mars, and we are fortunate to be witnessing these things thanks to many generations of robotic missions. Soon, astronauts will experience Mars and its dynamic climate firsthand, and their research will fuel scientific breakthroughs and discoveries for generations to come!

Further reading: NASA

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