CO2 SNOWFALL ON MARS - SIMULATION WITH A GENERAL-CIRCULATION MODEL

Although CO2 snowfall has never been directly observed on Mars, it has been suggested that such precipitation may explain the puzzling infra red measurements obtained by Mariner 9 and Viking during the polar nig ht in each hemisphere. The radiative effect of the snow would strongly alter the radiative balance of the condensing polar caps and thus the CO2 cycle and the global climate. We have simulated this phenomenon w ith a general circulation model (GCM). For that purpose, a new paramet erization of CO2 condensation in the atmosphere and on the ground has been developed, paging particular attention to mass and energy conserv ation and allowing for the possible sublimation of sedimenting COL ice particles. Atmospheric condensation may result from radiative cooling on the one hand (especially when the atmosphere is dust laden) and fr om 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 CO2 snowfall 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 CO2 ice particles, snow metamorphism on the ground). Without tuning the model parameters, we have been able t o accurately reproduce the general behavior of the features observed b y Viking in the thermal infrared. These modeling results support the C O2 snowfall scenario suggested from the observations. Overall, this ne w 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 CO2 condensation-sublimation cycle rea listically. In particular, the seasonal variations of the surface pres sure recorded by the Viking Landers can now be reproduced without arti ficially decreasing the condensation rate as was done in previous stud ies. (C) 1998 Academic Press.