MODELING THE MARTIAN SEASONAL CO-2 CYCLE .2. INTERANNUAL VARIABILITY

A diurnal and seasonal thermal model that can successfully fit seasonal pressure variations observed at the Viking 1 landing site and observed retreat rates of the seasonal polar caps is used to investigate the sensitivity of Martian seasonal pressure curves to interannual variations in the heat balance of the north and south seasonal polar caps. Year-to-year differences between measured surface pressures at the Viking landing sites as a function of season are used as upper limits on the potential magnitudes of interannual variations in the mass of the Martian atmosphere. There are compared with model-calculated interannual pressure variations which we obtain by temporarily altering model input parameter values. The results show that uniform, year-long interannual variations in the albedos or emissivities of the north or south seasonal polar cap of greater than ∼2% absolute would be quite apparent in the Viking Lander data. A brief cessation of net frost condensation within the north seasonal polar cap due to elevated atmospheric temperatures during the 1977B global dust storm would have caused interannual pressure variations later in the year that are larger than those observed. Simulations are performed which show that the dust layers deposited onto the condensing north seasonal polar cap during global dust storms can substantially darken sublimating seasonal frost deposits when they become uncovered during the following spring, and yet have relatively minor effects on seasonal pressure variations. For the case of the 1977B global dust storm, the interannual repeatability of the Viking Lander pressure data could be compatible with a 10% decrease in the albedo of the north seasonal polar cap. This result leaves open the possibility that the asymmetric behavior of CO2 frost at the north and south residual polar caps could be due to differential dust contamination of accumulating north polar seasonal CO2 deposits during global dust storms. © 1992.