The first unambiguous spectroscopic observation of CO2 ice clouds on Mars came from Montmessin et al. (2007), but observations have been reported for a decade now (Clancy et al. 2003). The CO2 ice clouds are a rare phenomenon in the Solar System, since 95% of the martian atmosphere consist of CO2 gas, and thus on Mars we are dealing with the condensation of the main component of the atmosphere. The condensation is moreover occuring in a rarefied atmosphere (large Knudsen numbers) that can have dramatic consequences on the crystal growth through the limiting effect of the heat transfer. CO2 ice cloud modeling has turned out to be challenging : recent efforts (e.g. Colaprete et al. 2008) fail in explaining typical small sizes (80 nm-130 nm) observed in equatorial mesospheric ice clouds (Montmessin et al. 2006). Recent modelling studies suggest that the effect of the thermal tide in cooling the mesosphere is a prerequisite for the cloud formation (Gonzalez-Galindo et al. 2011), but other perturbations are required to attain the CO2 condensation temperatures. A recent study has shown a strong correlation between mesospheric ice cloud observations and the filtering of gravity waves through the atmosphere (Spiga et al. 2012). Those waves could create cold pockets in wich T ≤ Tcond, and thus provoke a supersaturated environment in which the clouds can form. However, the nature of the key microphysical processes in the formation of CO2 ice clouds remains unclear, especially in the mesosphere. We have adapted a microphysical model previously developed for water ice clouds formation on Mars (Montmessin et al. 2004) for modeling these CO2 ice clouds.
