QUASI-PERIODIC ATMOSPHERE-REGOLITH-CAP CO2 REDISTRIBUTION IN THE MARTIAN PAST

Our earlier Mars regolith-atmosphere-cap CO2 distribution model (Fanal e et al., 1982, Icarus 50, 381-407) has been improved, revised, and ex tended back over Mars' mid to late history. The present model takes in to 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 fo r the full range of obliquities, latitudes, and epochs, and a CO2 adso rption relation is used, together with a conservation of mass constrai nt, to calculate atmospheric pressures and exchangeable CO2 mass as fu nctions of obliquity and epoch for the regolith, atmosphere, and polar caps for two assumed thicknesses of a basalt regolith. It is found th at the heat conduction term in the surface boundary condition has an i mportant effect in reducing the range of atmospheric pressures over th e obliquity cycle at all epochs. Its main effect is to maintain the at mospheric CO2 pressure near 1 mb at very low obliquity (10 degrees) as opposed to similar to 0.01 mb without this term (Fanale ef 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 pr esent due to the increasing solar constant. At intermediate to high ob liquities at which there was no perennial CO2 polar cap, atmospheric p ressures decreased toward the present to a point due to the decreasing CO2 inventory and then increased to the present due to the increasing solar constant. In past times up to the point considered in this stud y, CO2 pressures have been as Iowas similar to 0.6 mbar at the lowest obliquity and only slightly higher than at present at the highest obli quity. Atmospheric CO2 pressures < similar to 0.1 mbar are still sugge sted by the model, but only in cases where near zero obliquity occurs episodically or chaotically as predicted by recent models of the obliq uity and eccentricity variation. At low to intermediate obliquities th e polar caps contained mast of the exchangeable CO2, At intermediate t o high obliquities the regolith contained most of the CO2, at least in more recent epochs, while the polar caps may have been the dominant r eservoir during earlier epochs. In physical equilibrium the atmosphere contained a very small portion of the exchangeable CO2 for most cases . (C) 1994 Academic Press, Inc.