Martian clouds mostly hover at an altitude of no more than 60 kilometers and are composed of water ice. However, the team of the rover Curiosity, which has been working on the Red Planet for almost nine years, made a discovery: the clouds that appear there earlier than usual are at a higher height and consist, apparently, of frozen carbon dioxide.
Cloudy days rarely occur on the Red Planet: clouds are usually low and are found near the equator in the coldest time of the year, when Mars is farthest from the star in its elongated orbit. However, in 2019, a team from NASA's Jet Propulsion Laboratory working with the Curiosity rover noticed clouds forming over Mount Sharp in Gale Crater earlier than expected.
Two years later, scientists managed not only to catch this phenomenon again, but also to get pictures of it. Thin clouds began to form at the end of January, and at an altitude of over 60 kilometers. They were filled with ice crystals that scattered light from the Sun, as a result of which the clouds seemed to shimmer. Since they were higher than usual, experts assumed that the clouds were not composed of water ice, but frozen carbon dioxide (dry ice).
“If you see a shimmering cloud in pastel shades, it’s because all the particles are almost the same size,” said Mark Lemmon of the Boulder Institute for Space Sciences. "This usually happens right after the clouds have formed and expanded at the same rate."
It will take some time and new color images from the rover's camera to tell exactly which of Curiosity's recent images captured clouds of water ice and which of dry ice. They will show the crystalline twinkling of the clouds and determine their height: when the Sun is above the level of the clouds, dry ice crystals shine with the brightest light, and when the luminary drops below, they lose some of their brilliance.
Scientists recently stated that thanks to special icy clouds that create a greenhouse effect, the Red Planet maintained a warm and arid climate in ancient times. They used a global climate model to test the cloud greenhouse's ability to warm the planet and maintain an average annual temperature of 265 Kelvin - sufficient for low-latitude lakes.