The HiRISE camera aboard NASA’s Mars Reconnaissance Orbiter captured these images of frost-covered sand dunes just after the winter solstice. The frost here is a mixture of carbon dioxide (dry) ice and water ice and will disappear in a few months when spring arrives. Credit: NASA/JPL-Caltech/University of Arizona
Cube shaped snow, frozen landscapes and frost are part of the coldest season on the red planet.
When winter arrives on Mars, the surface transforms into an otherworldly holiday scene. Snow, ice and frost accompany the season’s sub-zero temperatures. Some of the coldest of these occur at the planet’s poles, where it reaches minus 190 degrees Fahrenheit (minus 123 degrees Celsius).
As cold as it is, don’t expect Rocky Mountain-worthy snowfall. No region on Mars receives more than a few feet of snow, most of which falls on extremely flat areas. And the elliptical orbit of the Red Planet means that winter takes many more months to arrive: a single Martian year is about two Earth years.
Snow falls and ice and ice also form on Mars. NASA spacecraft on the Red Planet and in orbit reveal the similarities and differences in how we experience winter on Earth. Mars scientist Sylvain Piqueux of JPL explains in this video. Credit: NASA/JPL-Caltech
Still, the planet offers unique winter phenomena that scientists have been able to study, thanks to NASA’s robotic Mars explorers. Here are some of the things they discovered:
Two types of snow
Martian snow comes in two varieties: water ice and carbon dioxide, or dry ice. Because the Martian air is so thin and the temperatures so cold, the water-ice snow sublimates, or turns into a gas, before it even hits the ground. Dry ice snow actually reaches the ground.
“There are enough falls that you can snowshoe,” said Sylvain Piqueux, a Mars scientist at NASA’s Jet Propulsion Laboratory in Southern California, whose research includes a variety of winter phenomena. “If you’re looking to ski, though, you’d have to go to a crater or a cliff, where snow could build up on a sloping surface.”
HiRISE captured these “megadunes”, also called barchans. During the winter, frosts of carbon dioxide and ice have formed on the dunes; when this begins to sublimate during the spring, the darker colored dune sand is revealed. Credit: NASA/JPL-Caltech/University of Arizona
How do we know it snows?
Snow only occurs at the coldest ends of Mars: at the poles, under cloud cover, and at night. Cameras on orbiting spacecraft cannot see through these clouds, and surface missions cannot survive the extreme cold. As a result, no images of the snowfall were ever captured. But scientists know it’s happening, thanks to a few special scientific instruments.
NASA’s Mars Reconnaissance Orbiter can peer through cloud cover using its Mars Climate Sounder instrument, which detects light at wavelengths imperceptible to the human eye. This ability has allowed scientists to detect carbon dioxide snow falling to the ground. And in 2008, NASA sent the Phoenix lander 1,000 miles (about 1,600 kilometers) from Mars’ north pole, where it used a laser instrument to detect water ice snow falling on the surface.
NASA scientists can measure the size and shape distribution of snow particles, layer by layer, in a storm. The Global Precipitation Measurement mission is an international satellite project that provides next-generation observations of rain and snow worldwide every three hours. Credit: NASA Goddard Space Flight Center/Ryan Fitzgibbons
Cubic snowflakes
Because of how water molecules bond when they freeze, snowflakes on Earth have six sides. The same principle applies to all crystals: the way the atoms are arranged determines the shape of a crystal. In the case of carbon dioxide, the molecules of dry ice are always joined in the form of four when frozen.
“Because carbon dioxide ice has fourfold symmetry, we know that dry ice snowflakes would be cube-shaped,” Piqueux said. “Thanks to the Mars Climate Sounder, we can say that these snowflakes would be smaller than the width of a human hair.”
The HiRISE camera captured this image of a crater rim in the dead of winter. The crater’s south-facing side, which receives less sunlight, has formed a bright, jagged frost, seen in blue in this color-enhanced image. Credit: NASA/JPL-Caltech/University of Arizona
Jack Frost pecking at your Rover
Water and carbon dioxide can form frost on Mars, and both types of frost are much more common across the planet than snow. Viking landers saw water frost when they studied Mars in the 1970s, while NASA’s Odyssey orbiter has observed frost forming and sublimating in the morning sun.
HiRISE captured this spring scene, when frozen water ice on the ground had split the ground into polygons. The translucent carbon dioxide ice allows sunlight to shine through and heat the gases escaping through the vents, releasing vents of darker material to the surface (shown as blue in this color-enhanced image). Credit: NASA/JPL-Caltech/University of Arizona
Wonderful end of winter
Perhaps the most fabulous discovery comes at the end of winter, when all the accumulated ice begins to “thaw” and sublimate into the atmosphere. In doing so, this ice takes on strange and beautiful shapes that have reminded scientists of spiders, Dalmatian spots, fried eggs and Swiss cheese.
This “thaw” also causes the geysers to erupt: the translucent ice allows sunlight to heat the gas below, and eventually that gas 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 are blowing.