Late winter has arrived in Mars’s Northern Hemisphere, and NASA recently released images of the season captured by its Mars Reconnaissance Orbiter. The frosty scenes are a foreign-yet-familiar display of beauty.
Like Earth, the Red Planet experiences snow and frost and is home to water ice. So, in a way, its winters look like ours. But that’s about where the resemblance stops.
In a Martian winter, the planet’s average temperature—already a frozen minus 80 degrees Fahrenheit—plunges to 190 below. In this bone-chilling weather, the Red Planet also hosts a second kind of ice made from carbon dioxide, known as dry ice.
Unlike our water frost at home, Mars’s carbon dioxide frost doesn’t melt. Instead, when temperatures warm, it converts directly from a solid to a gas in a shift called sublimation. In the process, fantastic formations are created on the ground, ranging from spider-like intertwining lines to scattered polka dots. Scientists have named these formations after some familiar items and patterns, from Dalmatian spots, to fried eggs, to Swiss cheese.
Carbon dioxide ice makes for another Martian winter oddity: cube-shaped snowflakes. Dry ice molecules bond in fours when frozen, so these four-sided flakes look different from the six-sided ones on Earth. They’re also very tiny. “These snowflakes would be smaller than the width of a human hair,” Sylvain Piqueux, a Mars scientist at NASA’s Jet Propulsion Laboratory, says in a statement.
While Mars hosts both water frost and carbon dioxide frost, the cube-shaped flakes made from dry ice are the only kind of snow that blankets the ground. But no place on the planet gets more than a few feet of snow, and most of it falls on flat land.
“Enough falls that you could snowshoe across it,” Piqueux says in the statement. “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.”
Mars’s water-based snow doesn’t accumulate. The frigid temperatures and thin atmosphere make falling water snowflakes turn into gas before they reach the ground.
Though NASA knows that Mars gets snow, the agency has never photographed the flakes as they fall. Orbiters can’t see through the planet’s dense clouds to capture such images, and robots on the ground can’t survive in the planet’s coldest extremes. Yet those are the places where Mars’s snow falls—in the frigid areas shaded by clouds, at the poles and at night.
Short of photo evidence, NASA has used other methods for finding and studying Mars’s falling snow. In 2008, the agency’s Phoenix lander sent a laser up into the atmosphere and measured returning signals from clouds and snowflakes. “Basically, you light up the sky, and you can see when snow falls,” Piqueux says in a video. Plus, the Mars Reconnaissance Orbiter, which studies the weather in infrared and visible light, has detected dry ice snow falling.
Martian frost is beautiful, as NASA’s images prove, but to future human astronauts on the Red Planet, it might also have a practical use. Beyond hydrating extraterrestrial travelers, water frost could be used for agriculture or as a propellant for spacecraft.
“Access to water is a key consideration for space exploration,” Richard Davis, assistant director for science and exploration in NASA’s Science Mission Directorate, said in a 2020 statement. “Technological advancements that enable humans to ‘live off the land’ on distant worlds and use resources such as water, will unlock significant opportunities to explore our universe first-hand.”