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While the Martian clouds may look like the kind seen in Earth's skies, they include frozen carbon dioxide, or dry ice.
Red-and-green-tinted clouds drift through the Martian sky in a new set of images captured by NASA's Curiosity rover using its Mastcam - its main set of "eyes." Taken over 16 minutes on Jan. 17 (the 4,426th Martian day, or sol, of Curiosity's mission), the images show the latest observations of what are called noctilucent (Latin for "night shining"), or twilight clouds, tinged with color by scattering light from the setting Sun.
Sometimes these clouds even create a rainbow of colors, producing iridescent, or "mother-of-pearl" clouds. Too faint to be seen in daylight, they're only visible when the clouds are especially high and evening has fallen.
Martian clouds are made of either water ice or, at higher altitudes and lower temperatures, carbon dioxide ice. (Mars' atmosphere is more than 95% carbon dioxide.) The latter are the only kind of clouds observed at Mars producing iridescence, and they can be seen near the top of the new images at an altitude of around 37 to 50 miles (60 to 80 kilometers). They're also visible as white plumes falling through the atmosphere, traveling as low as 31 miles (50 kilometers) above the surface before evaporating because of rising temperatures. Appearing briefly at the bottom of the images are water-ice clouds traveling in the opposite direction roughly 31 miles (50 kilometers) above the rover.
Dawn of Twilight Clouds
Twilight clouds were first seen on Mars by NASA's Pathfinder mission in 1997; Curiosity didn't spot them until 2019, when it acquired its first-ever images of iridescence in the clouds. This is the fourth Mars year the rover has observed the phenomenon, which occurs during early fall in the southern hemisphere.
Mark Lemmon, an atmospheric scientist with the Space Science Institute in Boulder, Colorado, led a paper summarizing Curiosity's first two seasons of twilight cloud observations, which published late last year in Geophysical Research Letters. "I'll always remember the first time I saw those iridescent clouds and was sure at first it was some color artifact," he said. "Now it's become so predictable that we can plan our shots in advance; the clouds show up at exactly the same time of year."
