Martian clouds form in much more humid conditions

Clouds captured above a vast plain of sand by the Opportunity rover near Victoria Crater on Mars on October 2006. Credit: NASA

At first glance, Mars' clouds might easily be mistaken for those on Earth: Images of the Martian sky, taken by NASA's Opportunity rover, depict gauzy, high-altitude wisps, similar to our cirrus clouds.

Given what scientists know about the Red Planet's atmosphere, these clouds likely consist of either carbon dioxide or water-based ice crystals.

But it's difficult to know the precise conditions that give rise to such clouds without sampling directly from a Martian cloud.

Researchers at MIT have now done the next-best thing: They've recreated Mars-like conditions within a three-story-tall cloud chamber in Germany, adjusting the chamber's temperature and relative humidity to match conditions on Mars—essentially forming Martian clouds on Earth.

While the researchers were able to create clouds at the frigid temperatures typically found on Mars, they discovered that cloud formation in such conditions required adjusting the chamber's relative humidity to 190 percent—far greater than cloud formation requires on Earth.

The finding should help improve conventional models of the Martian atmosphere, many of which assume that Martian clouds require humidity levels similar to those found on Earth.

Dan Cziczo

"A lot of atmospheric models for Mars are very simple," says Dan Cziczo, the Victor P. Starr Associate Professor of Atmospheric Chemistry at MIT.

"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."

Cziczo says the group's experimental results will help to improve Martian climate models, as well as scientists' understanding of how the planet transports water through the atmosphere.

He and his colleagues have reported their findings in Journal of Geophysical Research: Planets.

Seeding Martian clouds
The team conducted most of the study's experiments during the summer of 2012 in Karlsruhe, Germany, at the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) facility—a former nuclear reactor that has since been converted into the world's largest cloud chamber.

The facility was originally designed to study atmospheric conditions on Earth. But Cziczo realized that with a little fine-tuning, the chamber could be adapted to simulate conditions on Mars.

To do this, the team first pumped all the oxygen out of the chamber, and instead filled it with inert nitrogen or carbon dioxide—the most common components of the Martian atmosphere.

They then created a dust storm, pumping in fine particles similar in size and composition to the mineral dust found on Mars.

Much like on Earth, these particles act as cloud seeds around which water vapor can adhere to form cloud particles.