NASA has shown what winter looks like on Mars, where the ice is cube-shaped, the landscapes are frozen, and the temperatures are subzero. Precisely, the coldest temperatures are reached at the poles of the Red Planet, where they reach 123 degrees below zero.
Despite the cold, no part of Mars receives more than a few inches of snow, and most fall in very flat areas, so there are no large snowfalls. Winter on Mars requires several months to pass due to its elliptical orbit, which makes one year on the planet equal to about two Earth years.
However, the planet offers unique winter phenomena that scientists have been able to study, thanks to NASA’s robotic Mars explorers. Thus, Martian ice comes in two forms: water ice and carbon dioxide, or dry ice.
“If you want to ski, you have to go to a gully or a rock, where the snow accumulates on a sloped surface,” said Sylvain Picoux, a scientist at NASA’s Southern California Observatory.
Snow occurs only at the coldest extremes of Mars: at the poles, under cloud cover, and at night. Cameras on orbiting spacecraft can’t see through those clouds, and surface missions can’t survive the extreme cold. As a result, no images of avalanches have been captured yet, but scientists are thankful for some special scientific instruments.
Precisely, NASA’s Mars Reconnaissance Orbiter can look at cloud cover using its Mars Climatic Sounder, which detects light at wavelengths invisible to the human eye. That ability has allowed scientists to detect carbon dioxide snow falling to Earth.
In 2008, NASA sent the Phoenix lander about 1,000 miles from Mars’ north pole, where it used a laser instrument to detect frozen ice falling on 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 fours when it freezes. “Because carbon dioxide ice has fourfold symmetry, we know that dry ice is cubic,” Piqueux said, noting that these snowflakes are “smaller than the width of a human hair.”
Both water and carbon dioxide can form frost on Mars, and both types of frost occur more widely across the planet than snow. The Viking landers observed frozen water when they explored Mars in the 1970s, while NASA’s Odyssey orbiter observed ice forming.
At the end of winter, when all the accumulated ice begins to “melt”, it takes on strange and beautiful shapes that remind scientists of “spiders, Dalmatian spots, fried eggs and Swiss cheese”.
This “thawing” causes geysers to explode: translucent ice allows sunlight to heat the gas beneath it, and that gas eventually erupts, sending fans of dust to the surface. In fact, scientists have begun studying these fans to learn more about which way Martian winds blow.
