J. Geophys. Res. Planets 118, 1945–1954 (2013)

How do you catch a Martian cloud and pin it down? A full understanding of the CO2 and water cycle of Mars would be impossible without studying the formation of clouds. Given the difficulties in sampling one of the cirrus clouds in the Martian atmosphere, Daniel Cziczo and colleagues travelled to Karlsruhe, Germany, where a former nuclear reactor has been converted into an aerosol and cloud chamber. The three-storey-high facility is called the Aerosol Interactions and Dynamics in the Atmosphere (AIDA) Chamber.

The authors introduced Mojave Mars simulant dust (MMS) — mechanically ground olivine basalt — into a nitrogen atmosphere in the cold chamber, with water vapour frozen onto the walls. By varying the MMS pressure (the number of cloud 'seeds' to nucleate ice crystals) and the temperature, they grew ten clouds, using lasers to measure the number, density and composition. At the lowest temperature, 189 K, the relative humidity for forming a cloud is 190%, confirming previous experiments that used different techniques. Further studies that grow CO2 clouds at lower temperatures are needed to complete the picture.