To celebrate the winter solstice on Earth and the start of winter in the northern hemisphere, NASA has released some photos captured on Mars by its Mars Reconnaissance Orbiter space probe’s HiRISE camera showing what some Martian areas look like when it’s winter on the red planet. Seeing snowfall is still beyond the possibilities of the available instruments but frozen or at least frost-covered landscapes show some of the marvels of Mars.
Temperatures on Mars can drop to extremely low levels even when compared to the polar areas or those hit by particularly severe snowstorms on Earth. At the Martian poles, they can drop as low as -123° Celsius with some consequences that may be familiar but conditions are very different from Earth’s, so the landscapes remain alien in many ways. There are two types of snow since carbon dioxide can freeze just like water.
Despite the cold, conditions on Mars don’t lead to snowfall capable of completely covering the territory unless it’s very flat. Snowfall occurs particularly at the poles, under clouds, and at night. These conditions prevent orbiting instruments from seeing through the clouds, so there are no images of snowfall. These phenomena can only be detected through other instruments such as the ones that can detect carbon dioxide falling to the ground or with lasers that can detect water snow.
Carbon dioxide snow is very different from water snow because the two molecules form different bonds when they freeze. The so-called dry ice contains molecules that generate cubic crystals so, even if we have never seen it snow, we know what carbon dioxide snowflakes look like.
On Mars, frost is more common than snow. Both water and carbon dioxide can form frost over large areas and in this case, there are photos such as the ones just released by NASA that show it in various winter landscapes. Water frost has been sighted on Mars since the 1970s by NASA’s Viking landers and subsequently studied by the various space probes that observed the territory from orbit. This made it possible to study how frost is formed and sublimates, turning into vapor when the Sun rises. This process can generate enough pressure to cause avalanches if there’s sand under the frost.
At the end of winter, even on Mars, temperatures rise enough to thaw most of the ice that formed in the previous months. This process may be accompanied by geysers produced by the sublimation of underground ice which is heated by light passing through the uppermost layer when it’s transparent.
Winter landscapes photographed on Mars can be evocative and at the same time provide insights into climate and weather. Observations that have been going on for decades are being used to reconstruct the atmospheric phenomena taking place on the red planet even when it’s a little whiter than normal.
