Mars: The regolith-atmosphere-cap system and climate change

Abstract

A new model for predicting the behavior of the Martian regolith-atmosphere-cap CO 2 regime and describing its role in climate change is derived. The model describes the time-temperature histories of 90 regolith “chunks” on a latitude-depth grid, accounting for CO 2 exchange by means of laboratory-derived expressions relating temperature, CO 2 pressure ( P co 2 ), and adsorbed CO 2 mass. CO 2 polar cap formation and sublimation is accounted for, subject to the constraint of constant total CO 2 in the atmosphere-regolith-cap system. The influence of differences in regolith adsorption laws for basalt and clay and the influence of variations in regolith depth with latitude, regolith thermal diffusivity, and total exchangeable CO 2 inventory on predicted variations in atmospheric pressure and cap mass are examined. The following results are indicated: (1) The atmosphere acts as a low-capacity conduit, through which flows between 10 and 100 times the current atmospheric mass of CO 2, between two reservoirs: a regolith “ocean” of adsorbed CO 2 and a polar cap “cryosphere.” (2) Exchange between these reservoirs is driven by variations of obliquity (θ), with the polar cap the dominant CO 2 sink at low θ and the regolith dominating at high θ. Atmospheric pressure ranges between ∼0.1 (low θ) and ∼15 mbar (high θ). Pressures of up to 25 mbar could result from pre-Tharsis θ variations. Pressure up to ∼40 mbarare achieved only in models which also call for a huge quasi-permanent present CO 2 cap, which is not apparent. (3) At high θ, the atmospheric pressure is buffered by the regolith and satisfies adsorption equilibrium for the 90 “chunks,” as well as conservation of CO 2 mass. At low θ a potetially large CO 2 cap appears and the atmospheric pressure is buffered by the cap, not the regolith. (4) The periodic reservoir flushing can account for the observation of simultaneous nonenrichment of 18O and enrichment of 13N. (5) Mars climate history may be divided into three parts: (a) post-Tharsis formation history as described above, (b) pre-Tharsis formation with qualitatively similar processes but larger oscillations between reservoirs and larger atmospheric pressure and cap mass variation, and (c) the very early period of channel formation. (6) The nature of the response of the three-part regolith-atmosphere-polar cap Martian climate system to surface insolation variations may be analogous to that of Earth's ocean-atmosphere-cryosphere system.