Obliquity‐Driven CO ₂ Exchange Between Mars' Atmosphere, Regolith, and Polar Cap

Discerning the total mass of Mars’ obliquity‐timescale (∼10 5 ‐year) exchangeable CO 2 inventory has been elusive for decades due to the unknown adsorptive capacity of its regolith. Now, however, the stratigraphy of Mars’ recently discovered South Polar Massive CO 2 Ice Deposit (MCID) provides a record of orbit‐driven CO 2 exchange between its polar cap, atmosphere, and regolith with sufficient constraint to calculate the adsorptive capacity of its regolith and therefore the total mass of its exchangeable CO 2 inventory. We use a numerical climate model and Markov Chain Monte Carlo analysis to show that the observed MCID stratigraphic record is most consistent with a mobile CO 2 inventory of kg ( mbar, 68% confidence interval) that exchanges on obliquity timescales. We find that adsorptive CO 2 exchange with the regolith on obliquity timescales likely occurs in the depth range of >∼200 m to <∼1 km, with the deeper bound set by thermal processes and adsorptive surface availability. Our best‐fit model yields a peak mean annual surface pressure 40% lower than CO 2 exchange models that neglect an adsorbing regolith. We provide machine‐readable text files of our results to aid future study of Mars’ climate.