Images of the Martian sky taken by NASA's Opportunity rover show gauzy, high-altitude wisps similar to earthly cirrus clouds, likely consist of either carbon dioxide or water-based ice crystals.
Researchers at the Massachusetts Institute of Technology, hoping to learn more about how such clouds form, used a three-story-tall cloud chamber in Germany with temperature and relative humidity to match conditions on Mars.
Cloud formation in such Mars-like conditions required adjusting the chamber's relative humidity to 190 percent, they found, far greater than cloud formation requires on Earth.
"A lot of atmospheric models for Mars are very simple," MIT atmospheric chemistry Professor Dan Cziczo said. "They have to make gross assumptions about how clouds form: As soon as it hits 100 percent humidity, boom, you get a cloud to form. But we found you need more to kick-start the process."
It's unclear why Martian clouds need such highly humid conditions to take shape but more experiments could yield clues, he said.
"If we want to understand where water goes and how it's transported through the atmosphere on Mars, we have to understand cloud formation for that planet," Cziczo said. "Hopefully this will move us toward the right direction."