METEOROLOGICAL VARIABILITY AND THE ANNUAL SURFACE PRESSURE CYCLE ON MARS

It is commonly admitted that the seasonal surface pressure cycle, obse rved on Mars by the two Viking landers, is due to condensation and sub limation of the atmospheric carbon dioxide in the polar caps. A three Martian year numerical simulation has been performed with a Martian Ge neral Circulation Model developed from the terrestrial model of the La boratoire de Meteorologie Dynamique. The conditions of the simulation were those of a typical clear-sky situation. The results, validated by comparison to Viking pressure measurements and to temperature fields retrieved from Mariner-9 measurements, show that the pressure cycle de pends on the location on the planet. They strongly suggest that, in ad dition to condensation and sublimation of the atmospheric carbon dioxi de, two other effects significantly contribute to the pressure cycle: an orographic effect resulting from the difference in mean height betw een the two hemispheres, and a dynamical effect resulting from the geo strophic balance between the mass and wind field. In high latitudes, t he pressure variation linked to the dynamical effect may have the same magnitude (about 25%) as the global mass variation due to the condens ation-sublimation cycle. A shorter dust storm simulation is also in go od agreement with observations, in particular as concerns the surface pressure variations and the low-level winds, independently estimated f rom observations of the bright streaks on the surface of the planet. T hese results show that the atmospheric mass budget cannot be correctly estimated from local measurements such as Viking measurements.