NASA explores a winter wonderland on Mars

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

When winter comes on Mars, the surface transforms into a truly otherworldly vacation scene. Snow, ice and frost are associated with sub-zero temperatures in the season. Some of the coldest ones occur at the planet’s poles, where temperatures drop to minus 190 degrees Fahrenheit (minus 123 degrees Celsius).

As cold as it is, don’t expect snowdrifts befitting the Rocky Mountains. No region of Mars gets more than a few feet of snow, most of it falling on very flat areas. And the red planet’s elliptical orbit means that winter will take several months: one year on Mars is about two years on Earth.

However, the planet presents 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 types: water and carbon dioxide ice, or dry ice. Because Martian air is so thin and temperatures so cold, frozen water ice solidifies, or gasifies, before it touches the ground. In fact, dry snow reaches the ground.

“Enough fall that you can snowshoe through it,” said Sylvain Beccio, 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, you’ll have to go to a crater or a slope, where snow can build up on an inclined surface.”

How do we know it is snowing

Snow only occurs at the coldest temperatures on Mars: at the poles, under cloud cover, and at night. Cameras on spacecraft in orbit can’t see through those clouds, and surface missions can’t survive the extreme cold. As a result, no snowfall photos were taken. But scientists know it happens, thanks to a few special science tools.

NASA’s Mars Reconnaissance Orbiter can zip through cloud cover using the Mars Climate Sounder instrument, which detects light in wavelengths imperceptible to the human eye. This ability allowed scientists to detect carbon dioxide snow falling on Earth. And in 2008, NASA sent the Phoenix probe 1,000 miles (about 1,600 kilometers) from the north pole of Mars, where it used a laser instrument to detect falling snow on the surface.

cube snowflakes

Because of how water molecules are bound together when they freeze, snowflakes on Earth have six sides. The same principle applies to all crystals: the way the atoms organize themselves determines the shape of the crystal. In the case of carbon dioxide, the molecules in dry ice are always linked into four shapes when frozen.

“Because carbon dioxide ice has a symmetry of four, we know the dry snowflakes will be cube-shaped,” Becchio said. “Thanks to the Martian climate safer, we can say that these snowflakes will be smaller than the width of a human hair.”

Jack Frost nibbles your rover

Both water and carbon dioxide can form frost on Mars, and both types of frost appear more widely across the planet than snow. The Viking lander saw water frost when they studied Mars in the 1970s, while NASA’s Odyssey rover observed frost forming and sublimating away in the morning sun.

Wonderful end of winter

Perhaps the most amazing discovery comes at the end of winter, when all the ice that has been created begins to “melt” and sublimate into the atmosphere. As it does so, this ice takes on strange, beautiful shapes that reminded scientists of spiders, Dalmatian spots, fried eggs, and Swiss cheese.

This “melting” also causes geysers to erupt: the transparent ice allows sunlight to heat the gas beneath, and that gas eventually explodes, sending fans of dust to the surface. Scientists have already begun studying these fans as a way to learn more about the way the Martian winds blow.