Scientists studying Martian clouds have created them right here on Earth. Using a cloud chamber in Germany and rock from the Mojave Desert, their experiment shows that the Red Planet's ice clouds often need far more humidity to form than clouds on Earth.
The findings, published in the Journal of Geophysical Research: Planets, show that Martian clouds form under very different conditions than many scientists once thought – which may help researchers to better understand the planet’s water cycle.
Believe it or not, Mars does have clouds – they’re rather like the wispy cirrus clouds over Earth, which are filled with ice crystals and hover high in the atmosphere.
But unlike on Earth, where scientists can fly a plane through clouds to examine them with relative ease, studying clouds on Mars is something of a challenge.
Lead author Dan Cziczo, an atmospheric scientist at the Massachusetts Institute of Technology, realized there was a much easier way to study Martian clouds: make them on Earth.
The researchers went to the Aerosol Interaction and Dynamics in the Atmosphere facility in Karlsruhe, Germany – a former nuclear reactor that’s been turned into a chamber to study how Earth's clouds form.
“It’s sort of like a big, three-story-tall thermos,” Cziczo said. “You have to keep it cold because we’re doing ice-cloud formation. There’s an inner chamber where the cloud is formed, and an outer chamber that isolates it so it can stay cold.”
The system works by re-creating the interplay of pressure, temperature and humidity that causes clouds to form, Cziczo said.
“You fill the chamber with a little bit of water, and you put the particles in that you want to form the clouds on, and then you turn on these vacuum pumps and it drops the pressure in the chamber,” Cziczo said. “The pressure goes down, the temperature goes down, it gets cooler, the relative humidity goes up -- and at some point, that cloud forms.”
To run an experiment on Martian clouds, the researchers had to first find the right kind of rocky particles to seed them. Clouds can’t really form water droplets or ice unless they have a nucleus to grow around, and on Earth that can be anything from dust to man-made pollutants. On Mars, it’s mostly finely ground rock.
So the researchers used the most Mars-like rock they could -- Mojave Mars Simulant, an olivine basalt that looks very similar a Red Planet rock, donated by other researchers who found it in the desert.
They pulled the oxygen out of the chamber, because Mars doesn’t have much, filled it either with nitrogen or, for an even more Mars-like environment, carbon dioxide. Then they used the Mojave Mars Simulant to kick up a dust storm in the chamber and seed the clouds.
The cloud chamber was meant for warmer, Earth-like temperatures, but they lowered the thermostat as much as they could for a more Mars-like environment, testing a chilling temperature range of minus-73 degrees to minus-119 degrees Fahrenheit.
The scientists created 10 clouds that each formed in about 15 minutes.
Cirrus clouds generally form by the time humidity reaches about 100%, Cziczo said. But under the colder, Martian conditions, the clouds didn’t form until humidity reached a whopping 190%.
The study reveals the perils of making assumptions about Mars’ atmosphere based on Earth’s, the researcher said.
“So far, without any real data, models of the Martian atmosphere have sort of assumed that these clouds form in Earth-like conditions,” Cziczo said. “ That’s probably not valid … so we’re hoping that this helps move them in the right direction to understand the Martian atmosphere and the water budget of Mars.”
Next up? Perhaps a way to look at the carbon dioxide clouds filled with dry ice, not water.
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