Seasonal carbon dioxide exchange between the regolith and atmosphere of Mars: Experimental and theoretical studies

Abstract

Measurements of the rate of penetration of a CO 2 ‘pressure wave’ through basalt‐clay soils have been conducted under conditions of P CO 2 (∼6 mbar), Δ P CO 2 (∼2 mbar), soil density (ρ ∼ 1.3), and T (−40° to −70°C) which simulate the penetration of the cap‐induced seasonal CO 2 pressure wave through the topmost regolith on Mars. Results suggest that existing theoretical models for diffusion of a gas through a porous, highly adsorbing medium may be reliably used to assess the importance of Martian seasonal regolith‐atmosphere CO 2 exchange. Results also suggest that although Mars topsoils with a substantial component of fine‐grained weathering products could harbor, within a few meters of the surface, a substantial fraction of the atmospheric CO 2 inventory, seasonal cap‐atmosphere CO 2 exchange is still much more important than seasonal regolith‐atmosphere exchange. This is so because the high adsorptive capacity of such soils greatly slows CO 2 diffusion through them owing to the typically high ratio of mean residence time for CO 2 in the (static) adsorbed phase to that in the pore gas at T −40°C. At such temperatures, typical of the Martian regolith, the CO 2 diffusivity of such soils may be ∼1 × 10 −2 cm 2 s −1 —only somewhat higher than the thermal diffusivity and not high enough to allow seasonal isothermal regolith buffering of the cap‐induced CO 2 pressure wave to be effective. On the other hand, even the adsorption‐slowed diffusivities are still high enough to allow thermally driven regolith atmosphere exchange to be effective on a time scale of the obliquity cycle (∼10 5 yr). We also examined the maximum effect of thermally driven exchange between the topmost seasonally thermally affected regolith and the atmosphere. While of somewhat greater potential importance than the isothermal exchange, the thermally driven exchange would be recognizable only if the pressure wave from CO 2 thus exchanged at high latitudes did not propagate though the atmosphere at a faster rate than that at which the exchange itself occurred—an unreasonable assumption. Our results are insensitive to soil geometry and temperature and suggest that both isothermal and thermally driven regolith‐atmosphere seasonal CO 2 exchange effects are likely to be greatly dominated by cap‐atmosphere exchange. In principle, it is conceivable that a more profound understanding of the cap‐atmosphere‐regolith interaction could eventually allow us to use the externally imposed pressure waves as a natural experiment to set limits on global or regional soil properties. However, the regolith effects, while quite possibly dominant for H 2 O, are seemingly too weak and too complex for CO 2 to allow this to be accomplished.