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Abstract

Polar Mesospheric Clouds (PMCs) are H2O ice clouds occurring at high latitudes in the summer mesopause of Earth. They have a counterpart in the mesosphere of Mars consisting of CO2 ice particles. Both types of clouds most likely form via heterogeneous nucleation on nanometer-sized meteoric smoke particles. However, the onset conditions for ice particle formation are only poorly known. Therefore, I investigated the microphysical formation process of these clouds in the laboratory. Experiments on adsorption, nucleation and growth processes of H2O and CO2 molecules on meteoric smoke analogue particles were performed using the MICE-TRAPS setup. For Earth, it was found that amorphous solid water is the primary phase which forms under the extreme temperatures of the summer mesopause. The vapor pressure of this ice phase was measured and is 2 to 4 times higher than previously assumed. Nevertheless, ice formation is activated at low supersaturation, which is described by a newly developed activation model. In contrast, CO2 ice formation on Mars initiates at high supersaturation, which is described using classical nucleation theory with the parameters determined in this work. The results presented in this dissertation significantly enhance the understanding of mesospheric ice cloud formation on Earth and Mars.