Quasi-periodic Atmosphere-Regolith-Cap CO 2 Redistribution in the Martian Past

Abstract

Our earlier Mars regolith-atmosphere-cap CO 2 distribution model (Fanale et al., 1982, Icarus 50, 381-407) has been improved, revised, and extended back over Mars' mid to late history. The present model takes into account four new factors: (1) a more realistic long-term obliquity cycle, (2) thermal conduction as it affects the surface energy balance, (3) the changing solar constant, and (4) atmospheric erosion 3.5 byr ago to the present. Solar insolation and temperatures are computed for the full range of obliquities, latitudes, and epochs, and a CO 2 adsorption relation is used, together with a conservation of mass constraint, to calculate atmospheric pressures and exchangeable CO 2 mass as functions of obliquity and epoch for the regolith, atmosphere, and polar caps for two assumed thicknesses of a basalt regolith. It is found that the heat conduction term in the surface boundary condition has an important effect in reducing the range of atmospheric pressures over the obliquity cycle at all epochs. Its main effect is to maintain the atmospheric CO 2 pressure near 1 mb at very low obliquity (10°) as opposed to ∼0.01 mb without this term (Fanale et al., 1982, Icarus 50, 381-407). At low to medium obliquities when a perennial CO 2 polar cap was present, atmospheric pressures increased toward the present due to the increasing solar constant. At intermediate to high obliquities at which there was no perennial CO 2 polar cap, atmospheric pressures decreased toward the present to a point due to the decreasing CO 2 inventory and then increased to the present due to the increasing solar constant. In past times up to the point considered in this study, CO 2 pressures have been as low as ∼0.6 mbar at the lowest obliquity and only slightly higher than at present at the highest obliquity. Atmospheric CO 2 pressures <∼ 0.1 mbar are still suggested by the model, but only in cases where near zero obliquity occurs episodically or chaotically as predicted by recent models of the obliquity and eccentricity variation. At low to intermediate obliquities the polar caps contained most of the exchangeable CO 2. At intermediate to high obliquities the regolith contained most of the CO 2, at least in more recent epochs, while the polar caps may have been the dominant reservoir during earlier epochs. In physical equilibrium the atmosphere contained a very small portion of the exchangeable CO 2 for most cases.