CO₂Snowfall on Mars: Simulation with a General Circulation Model

Although CO₂snowfall has never been directly observed on Mars, it has been suggested that such precipitation may explain the puzzling infrared measurements obtained by Mariner 9 and Viking during the polar night in each hemisphere. The radiative effect of the snow would strongly alter the radiative balance of the condensing polar caps and thus the CO₂cycle and the global climate. We have simulated this phenomenon with a general circulation model (GCM). For that purpose, a new parameterization of CO₂condensation in the atmosphere and on the ground has been developed, paying particular attention to mass and energy conservation and allowing for the possible sublimation of sedimenting CO₂ice particles. Atmospheric condensation may result from radiative cooling on the one hand (especially when the atmosphere is dust laden) and from adiabatic cooling in upward motions on the other hand. This latter process can be very efficient locally. On this basis, we have modeled the effect of the CO₂snowfall on the infrared emission by decreasing the local emissivities when atmospheric condensation was predicted by the model. This parameterization is based on physical considerations (radiative transfer through the CO₂ice particles, snow metamorphism on the ground). Without tuning the model parameters, we have been able to accurately reproduce the general behavior of the features observed by Viking in the thermal infrared. These modeling results support the CO₂snowfall scenario suggested from the observations. Overall, this new parameterization, used in combination with the digital terrain model topography and with allowance for a varying atmospheric dust content, allows the GCM to simulate the CO₂condensation-sublimation cycle realistically. In particular, the seasonal variations of the surface pressure recorded by the Viking Landers can now be reproduced without artificially decreasing the condensation rate as was done in previous studies.