Exchange of adsorbed H2O and CO2between the regolith and atmosphere of Mars caused by changes in surface insolation

Estimates have been made of the capacity of the Martian regolith to exchange adsorbed H2O and CO2 with the atmosphere-plus-cap system (APCS). These estimates are based upon measured isotherms for H2O and CO2 adsorption on pulverized basalt at low temperatures and on theoretical considerations. A unit column (1 cm2) of regolith with a deep subsurface temperature of −77°C, considered average for the disk, will contain ∼0.4 g of adsorbed CO2 and ∼ 1 g of adsorbed H2O per meter of depth. Under favorable circumstances the top 3 cm can exchange much more H2O with the lower atmosphere each day than is necessary to produce the diurnal brightening. The process appears to be seasonally reversible. The total regolith may contain, in the adsorbed phase alone, as much as 1% of the H2O and 5% of the CO2 surface inventories expected for a hypothetical Mars that has experienced degassing as intensive as that of earth. Long-term (∼105 yr) variations in surface insolation caused by variations in the obliquity of Mars could cause efficient desorption of adsorbed CO2 and H2O from a 100-m regolith, and variations in solar output with periods >1 m.y. may well desorb H2O and CO2 efficiently from all the unconsolidated material on Mars. Enough CO2 could be desorbed to initiate major climatic change even if there is not enough in the APCS. The paradox presented by the fact that the APCS contains barely enough CO2 molecules to form caps is mitigated by the observation that the APCS is itself in transient equilibrium with a much larger reservoir of adsorbed CO2.