NASA has shown what winter is like on Mars, where snow is cube-shaped, landscapes are frozen, and frost accompanies subzero temperatures. Precisely, the coldest temperatures are reached at the poles of the Red Planet, where they reach up to 123 degrees below zero.
Despite the cold, no region of Mars gets more than a few inches of snow, most of which falls on extremely flat areas, so there are no large snowdrifts. For winter to arrive on Mars, it takes several months to pass due to its elliptical orbit, which makes a year on this planet the equivalent of about two Earth years.
Still, the planet offers unique winter phenomena that scientists have been able to study, thanks to NASA’s robotic Mars explorers. Thus, Martian snow comes in two varieties: water ice and carbon dioxide, or dry ice.
“If you’re looking to ski, you’d have to go to a crater or cliff, where snow could accumulate on a sloping surface,” said Sylvain Piqueux, a scientist at NASA’s Southern California Laboratory.
Snow occurs only at the coldest extremes of Mars: at the poles, under cloud cover, and at night. Cameras on orbiting spacecraft cannot see through those clouds, and surface missions cannot survive in the extreme cold. As a result, no images of falling snow have ever been captured, but scientists know it happens thanks to some special scientific instruments.
Precisely, NASA’s Mars Reconnaissance Orbiter can peer through the cloud cover using its Mars Climate Sounder instrument, which detects light at wavelengths imperceptible to the human eye. That ability has allowed scientists to detect carbon dioxide snow falling to the ground.
In 2008, NASA sent the Phoenix lander about 1,000 miles from the north pole of Mars, where it used a laser instrument to detect frozen snow falling to the surface.
Because of how water molecules stick together when they freeze, snowflakes on Earth have six sides. In the case of carbon dioxide, the molecules of dry ice always stick together in a figure four when they freeze. “Because carbon dioxide ice has a symmetry of four, we know that dry ice snowflakes would be cube-shaped,” Piqueux said, noting that these snowflakes “would be smaller than the width of a human hair”.
Both water and carbon dioxide can form frost on Mars, and both types of frost occur much more widely across the planet than snow. Viking landers saw frozen water when they surveyed Mars in the 1970s, while NASA’s Odyssey orbiter observed frost formation.
At the end of winter, when all the accumulating ice begins to “thaw,” it takes on strange and beautiful shapes that have reminded scientists of “spiders, Dalmatian spots, fried eggs, and Swiss cheese.”
This “thawing” also causes geysers to erupt: The translucent ice allows sunlight to heat the gas below it, and that gas eventually erupts, sending fans of dust to the surface. In fact, scientists have begun studying these fans as a way to learn more about which way the Martian winds blow.