The coldest season on Mars is when the snow forms cubes, the land is frozen, and there is frost.
When winter comes to Mars, the surface changes into a holiday scene that looks like it’s from another planet. The sub-zero temperatures of the season are accompanied by snow, ice, and frost. Some of these are as cold as minus 190 degrees Fahrenheit (minus 123 degrees Celsius), which happens at the planet’s poles.
Even though it’s cold, don’t expect snow drifts as big as the Rockies. Mars never gets more than a few feet of snow, and most of it falls on places that are very flat. Because Mars’s orbit is elliptical, winter takes a lot longer to come. One Mars year is about the same as two Earth years.
Still, the planet has unique winter events that NASA’s robotic Mars explorers have helped scientists study. Here are some things they’ve found out:
Two Kinds of Snow
There are two kinds of snow on Mars: water ice and carbon dioxide, also called “dry ice.” Because the air on Mars is so thin and it is so cold, water-ice snow turns into a gas before it even hits the ground. Dry-ice snow does, in fact, fall to the ground.
Sylvain Piqueux, a Mars scientist at NASA’s Jet Propulsion Laboratory in Southern California, said, “Enough snowfalls that you could snowshoe across it.” Piqueux studies a variety of winter events. “If you wanted to ski, you’d have to go to a crater or cliffside where snow could build up on a sloped surface.”
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How We Know It Snows
Snow only falls at the poles, at night, or when there are clouds over the surface. Cameras on spacecraft in orbit can’t see through these clouds, and missions to the surface can’t last in the extreme cold. Because of this, no pictures have ever been taken of snow falling. But scientists know it happens because they have a few special tools.
The Mars Climate Sounder on NASA’s Mars Reconnaissance Orbiter can see through clouds because it can pick up light at wavelengths that humans can’t see. Scientists have been able to find carbon dioxide snow falling to the ground because of this ability. In 2008, NASA sent the Phoenix lander within 1,000 miles (about 1,600 kilometers) of Mars’ north pole. There, it used a laser instrument to find water-ice snow falling to the surface.
Snowflakes with cubes
Snowflakes on Earth have six sides because of the way water molecules stick together when they freeze. All crystals follow the same rule: the shape of a crystal is determined by how its atoms are arranged. When dry ice freezes, molecules of carbon dioxide always stick together in groups of four.
“Dry-ice snowflakes would be cube-shaped because carbon dioxide ice has a symmetry of four,” Piqueux said. “Thanks to the Mars Climate Sounder, we know that these snowflakes would be smaller than the width of a human hair.
Jack Frost Biting Your Dog
Frost can be made from both water and carbon dioxide on Mars, and both types of frost are much more common than snow. When NASA’s Viking landers went to Mars in the 1970s, they saw water frost. NASA’s Odyssey orbiter has seen frost forming and melting in the morning Sun.
The Wonderful End of Winter
At the end of winter, when all the ice starts to “thaw” and “sublimate” into the air, this may be the most exciting thing to find out. As it moves, this ice forms strange and beautiful shapes that scientists have compared to spiders, Dalmatian spots, fried eggs, and Swiss cheese.
This “thawing” also causes geysers to erupt. This is because transparent ice lets sunlight heat gas underneath it, which eventually explodes, sending fans of dust to the surface. Scientists have started to look at these fans to find out more about where the winds on Mars are coming from.
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