They pumped out all of the oxygen out from the cloud chamber at the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) facility in Karlsruhe. The chamber is 22,000 gallons (84.5 meters3) in volume and has adjustable temperature, pressure and humidity. Scientists usually use it to study high-altitude cirrus and polar stratospheric clouds that appear on Earth.
The research team pumped into AIDA either inert nitrogen or carbon dioxide, the most common gases in Mars' atmosphere.
They blew in bits of dust that are of a similar size and material as Martian dust. Just as on Earth, Martian clouds form around grains of dust, which act as the clouds' nuclei.
They turned down the temperature in AIDA, gradually lowering it from about -81 degrees Fahrenheit, the lowest temperature at which clouds are known to form on Earth, to -120 degrees Fahrenheit, which would count as a warm day on Mars. It's also just about as cold as AIDA can get. The researchers kept the pressure low, as in Mars' atmosphere, and tried different humidities, to see which combinations of conditions would form clouds.
They crisscrossed the chamber with lasers to measure what was happening inside. Any cloud formation should interfere with the passage of light through the chamber.