The Bay Area is in the middle of a frigid
cold snap, driven by a blast of icy Arctic air.
But winter temperatures on Mars can drop over 200 degrees lower. Amid these bitterly cold conditions, the Red Planet develops ice, frost and — at its coldest extremes — several feet of snow.
This isn’t water-based snow, like what’s piled up in California’s
Sierra Nevada. Because the Red Planet is so dry and so cold, scientists don’t think that such snowflakes would make it down to the surface.
But another kind of snow, made up of carbon dioxide, does.
“Enough falls that you could snowshoe across it,” said Sylvain Piqueux, a Mars scientist at NASA’s Jet Propulsion Laboratory.
There are a few reasons why this dry-ice snow falls from Martian skies. For one, the Red Planet has a distinct winter season — not the case for all planets. The planet spins around a tilted axis and, like Earth, the northern and southern hemispheres experience pronounced seasons over the course of one lap around the sun.
This lap is about twice as long as one Earth year because Mars is farther away from the sun than we are. That means that not only does Mars receive less warmth from the sun, it has winters that are about twice as long.
“Things are really, really cold,” Piqueux said.
During frigid Martian winters, patches of carbon dioxide frost (blue, in this enhanced-color image) can form on the surface, like in this Southern Hemisphere crater.
NASA / JPL-Caltech / University of ArizonaThese frigid temperatures cause carbon dioxide and water frost to form on the surface of the Red Planet, especially at the poles, where the chilliest conditions occur. That’s where scientists think that up to several feet of dry-ice snow falls during Martian winters.
Rather than looking like Earth’s six-sided snowflakes, Martian snowflakes would have an entirely alien appearance.
“They’re probably looking like little cubes,” Piqueux explained.
This difference is due to how carbon dioxide molecules pack themselves when frozen. The snowflakes would also be tiny — smaller than the width of a human hair.
Scientists know so much about frost and snow on Mars because of detailed measurements by satellites circling the planet, which use special instruments to peer down onto the surface.
The Martian surface develops a thin layer of water frost during the winter, as captured by Viking 2 on May 18, 1979.
NASA / JPLSpacecraft, like the Viking lander, have also made it to the ground and provided a close-up view of frost on the Martian surface.
“Those were incredibly beautiful images,” Piqueux said.
New Year’s on Mars kicked off Dec. 26, 2022. Since a Martian year is 687 Earth days, the date shifts around our current Gregorian calendar. This also marks the start of spring, when ice that built up during the winter starts to thaw. But rather than turning to liquid, dry-ice frost and snow go straight to forming a gas — a process known as sublimation.
When the sun shines through translucent ice, layers closer to the surface can disappear first, causing pockets of gas to form. When enough pressure builds up, this carbon dioxide tears free.
“You start creating geysers,” Piqueux said.
When carbon dioxide gas escapes through ice on the Martian surface, it can release fans of darker material (blue, in this enhanced-color image). Here, water ice also split the ground into polygons.
NASA / JPL-Caltech / University of Arizona
As Martian ice thaws, the surface can also take on unique fracture patterns, like weirdly shaped polygons and
spiders.
“You have nothing like it on Earth,” Piqueux said.
Jack Lee (he/him) is a San Francisco Chronicle staff writer. Email: [email protected]