The Mists of Mars The Planetary Society

Late last month, visitors to Grand Canyon National Park in Arizona witnessed a rare and spectacular sight. A temperature inversion (where a layer of cold air is trapped beneath warm air) led to a canyon filled to the brim with clouds. On the very same day, a robotic spacecraft at the planet Mars captured a similar scene. This one was a much more common event, but one that still makes for incredible imagery. Valles Marineris is a network of canyons that in many ways looks similar to the Grand Canyon–except that at more than 4,000 kilometers in length, if it were on Earth it would stretch across most of the United States.
This canyon, too, sometimes fills with clouds, made of tiny particles of water ice, though it’s not caused by an inversion. Despite the Red Planet’s well-earned reputation as a dry desert, there are hints of water on its surface and in its atmosphere. The Mars Color Imager (MARCI) on board the Mars Reconnaissance Orbiter makes daily observations of the entire planet. On November 29 and 30, MARCI returned pictures of wispy clouds clinging to the summits of Olympus Mons and the other towering volcanoes. It also showed Valles Marineris, a long horizontal scar probably formed in part by the tectonic effects of all those volcanoes. As happens seasonally, the canyon was clearly filled with clouds.