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NORTHERN HEMISPHERE, MARS
We’re dreaming of a white Christmas… on Mars. These sand dunes in the planet’s northern hemisphere have been dusted with an unusual kind of snow, formed not from water but from carbon dioxide (CO ). Better known as dry ice, it appears during the Martian winter when temperatures drop and atmospheric CO freezes, forming ice on the planet’s surface or falling from the clouds as snow.
As the Sun reappears in spring, the pristine covering begins to crack, releasing gaseous carbon dioxide that carries dark sand up from the ground below. The result is these beautiful patterns, captured last May by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA’s Mars Reconnaissance Orbiter.
“Frozen CO is common on Mars,” explains Dr Candice Hansen, senior scientist at the Planetary Science Institute in Tucson, Arizona. “A seasonal polar cap made of dry ice forms each year at the north and south poles, and we even get patchy deposits close to the equator.”
We won’t be getting any here on Earth, though. CO requires temperatures of -78.5°C to freeze, so keep your fingers crossed for normal snow instead.