Curious Clouds Seen at Mars

With its thin atmosphere and scant moisture, Mars is often largely cloud-free. But new observations reveal clouds of dry ice thick enough to cast significant shadows on the red planet.
Dust storms are known to shroud vast swaths of Mars. Clouds have been photographed from the ground before, too.
The new research finds that carbon dioxide, the main component of martian air, freezes into clouds so dense they dim the sun by about 40 percent. Frozen carbon dioxide is commonly called dry ice here on Earth.