MODELING THE MARTIAN SEASONAL CO-2 CYCLE .1. FITTING THE VIKING LANDER PRESSURE CURVES

A diurnal and seasonal thermal model is used to simulate the seasonal exchange of carbon dioxide between the atmosphere and polar caps of Mars. Surface CO2 frost condensation and sublimation rates are determined by the net effects of radiation, latent heat, and heat conduction in subsurface soil layers. We show that this model can successfully reproduce the measured seasonal pressure variations at the Viking Lander 1 site using constant values for frost albedo and emissivity in each hemisphere. An exact treatment of heat conduction is found to have important effects, as our best-fit CO2 frost albedos and emissivities are not unique, but depend on the value of soil thermal inertia assumed in each hemisphere. We find that Martian seasonal pressure variations are primarily due to frost condensation and sublimation in the 55° to 70° latitude regions in both hemispheres. The observed retreat of the north and south seasonal polar caps can also be matched closely by this model, but no combination of best-fit frost albedos and emissivities was consistent with the stability of permanent CO2 deposits at either pole. The fact that this relatively simple model can provide such a good fit to the Viking Lander 1 pressure curve makes it a useful platform for studying the Martian CO2 cycle over seasonal, interannual, and climatic timescales. © 1992.