Simultaneous adsorption of CO2 and H2O under Mars-like conditions and application to the evolution of the Martian climate

The Martian regolith is the most substantial volatile reservoir on the planet; estimates of its adsorbed inventory have been based on simple measurements of the adsorption of either water or CO₂ in isolation. Under some conditions, H₂O can poison adsorbate surfaces, such that CO₂ uptake is greatly reduced. We have made the first measurements of the simultaneous adsorption of CO₂ and H₂O under conditions appropriate to the Martian regolith and have found that at H₂O monolayer coverage above about 0.5, CO₂ begins to be displaced into the gas phase. We have developed an empirical expression that describes our co-adsorption data and have applied it to standard models of the Martian regolith. We find that currently, H₂O does not substantially displace CO₂, implying that the adsorbate inventories previously derived may be accurate, not more than 3-4 kPa (30-40 mbar). No substantial increase in atmospheric pressure is predicted at higher obliquities because high-latitude ground ice buffers the partial pressure of H₂O in the pores, preventing high monolayer coverages of H₂O from displacing CO₂. The peak atmospheric pressure at high obliquity does increase as the total inventory of exchangeable CO₂ increases.