PC is polar cap, A is atmosphere, HLR and LLR are the high and low latitude regoliths, respectively. A 400 m thick nontronite model is assumed, which probably provides an upper limit on the amount of CO2 that the regolith can realisticallly hold. Atmospheric pressure as function of obliquity according to the model of Fanale et al. At low obliquities, the atmosphere is buffered by permanent polar caps. Pressures at high obliquities are controlled by equilibrium with the regolith.
Arrow refer to various estimates for obliquity limits.