Cube-shaped snow, icy landscapes, and frost are all part of the Red Planet’s coldest season.

when winter comes[{” attribute=””>Mars, the surface is transformed into a truly 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 gets as low as minus 190 degrees Fahrenheit (minus 123 degrees Celsius).

The HiRISE camera aboard NASA’s Mars Reconnaissance Orbiter captured these images of sand dunes covered by frost just after 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

Cold as it is, don’t expect snow drifts worthy of the Rocky Mountains. No region of Mars gets more than a few feet of snow, most of which falls over extremely flat areas. And the Red Planet’s elliptical orbit means it takes many more months for winter to come around: a single Mars year is around two Earth years.

Snowfall and ice and frost also form on Mars.[{” attribute=””>NASA’s spacecraft on and orbiting the Red Planet reveal the similarities to and differences from 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 a few of the things they’ve discovered:

Two Kinds of Snow

Martian snow comes in two varieties: water ice and carbon dioxide, or dry ice. Because Martian air is so thin and the temperatures so cold, water-ice snow sublimates, or becomes a gas, before it even touches the ground. Dry-ice snow actually does reach the ground.

“Enough falls that you could snowshoe across it,” 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 were looking for skiing, though, you’d have to go into a crater or cliffside, where snow could build up on a sloped surface.”

HiRISE captured these “megadunes,” also called barchans. Carbon dioxide frost and ice have formed over the dunes during the winter; as this starts to sublimate during spring, the darker-colored dune sand is revealed. Credit: NASA/JPL-Caltech/University of Arizona

How We Know It Snows

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 in the extreme cold. As a result, no images of falling snow have ever been captured. But scientists know it happens, thanks to a few special science instruments.

NASA’s Mars Reconnaissance Orbiter can peer through cloud cover using its Mars Climate Sounder instrument, which detects light in wavelengths imperceptible to the human eye. That ability has allowed scientists to detect carbon dioxide snow falling to the ground. And in 2008, NASA sent the Phoenix lander within 1,000 miles (about 1,600 kilometers) of Mars’ north pole, where it used a laser instrument to detect water-ice snow falling to the surface.

NASA scientists can measure the size and shape distribution of snow particles, layer by layer, during a storm. The Global Precipitation Measurement mission is an international satellite project that provides next-generation observations of rain and snow around the world every three hours. Credit: NASA Goddard Space Flight Center/Ryan Fitzgibbons

cubic snowflakes

Because of the way water molecules bond together when they freeze, snowflakes on Earth have six sides. The same principle applies to all crystals: the way the atoms arrange determines the shape of a crystal. In the case of carbon dioxide, the molecules in dry ice always bond in fours when frozen.

“Because carbon dioxide ice has a symmetry of four, we know that dry ice snowflakes would be cube-shaped,” Piqueux said. “Thanks to the Mars Climate Sounder, we can tell that these snowflakes would be smaller than the width of a human hair.”

The HiRISE camera captured this image of the rim of a crater in the dead of winter. The south-facing slope of the crater, which receives less sunlight, has formed an uneven, shiny frost, visible in blue in this enhanced color image. Credit: NASA/JPL-Caltech/University of Arizona

Jack Frost bites your Rover

Water and carbon dioxide can each form frost on Mars, and both types of frost occur much more widely on the planet than snow. Viking landers saw frozen water when they studied Mars in the 1970s, while NASA’s Odyssey orbiter saw observed the formation and sublimation of frost in the morning sun.

HiRISE captured this springtime scene, when frozen water ice in 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 fans of darker material on the surface (shown in blue in this enhanced color image). Credit: NASA/JPL-Caltech/University of Arizona

The wonderful end of winter

Perhaps the most fabulous discovery comes at the end of winter, when all the ice that has accumulated begins to “thaw” and sublimate into the atmosphere. In doing so, this ice takes on strange and beautiful shapes that have reminded scientists spiders, Dalmatian spots, fried eggsand swiss cheese.

This “thaw” also causes geysers to erupt: the translucent ice allows sunlight to heat the gas below, and this gas eventually erupts, sending dust lovers on the surface. Scientists have actually started studying these ventilators in order to learn more about in which direction do the martian winds blow.