Simultaneous adsorption of CO 2 and H 2 O 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 CO 2 in isolation. Under some conditions, H 2 O can poison adsorbate surfaces, such that CO 2 uptake is greatly reduced. We have made the first measurements of the simultaneous adsorption of CO 2 and H 2 O under conditions appropriate to the Martian regolith and have found that at H 2 O monolayer coverage above about 0.5, CO 2 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, H 2 O does not substantially displace CO 2 , 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 H 2 O in the pores, preventing high monolayer coverages of H 2 O from displacing CO 2 . The peak atmospheric pressure at high obliquity does increase as the total inventory of exchangeable CO 2 increases.