Martian volatiles: Their degassing history and geochemical fate

Abstract

Observations of Mars and cosmochemical considerations imply that the total inventory of degassed volatiles on Mars is 10 2 to 10 3 times that present in Mars' atmosphere and polar caps. The degassed volatiles have been physically and chemically incorporated into a layer of unconsolidated surface rubble (a “megaregolith”) up to 2km thick. Tentative lines of evidence suggest a high concentration (∼5g/cm 2) of 40 Ar in the atmosphere of Mars. If correct, this would be consistent with a degassing model for Mars in which the Martian “surface” volatile inventory is presumed identical to that of Earth but scaled to Mars' smaller mass and surface area. The implied inventory would be: ( 40Ar) = 4g/cm 2, (H 2O) = 1 × 10 5g/cm 2, (CO 2) = 7 × 10 3g/cm 2, (N 2) = 3 × 10 2g/cm 2, (Cl) = 2 × 10 3g/cm 2, and (S) = 2 × 10 2g/cm 2. Such a model is useful for testing, but differences in composition and planetary energy history may be anticipated between Mars and Earth on theoretical grounds. Also, the model demands huge regolith sinks for the volatiles listed. If the regolith were in physical equilibrium with the atmosphere, as much as 2 × 10 4g/cm 2 of H 2O could be stored in it as hard-frozen permafrost, or 5 × 10 4g/cm 2 if equilibrium with the atmosphere were inhibited. Spectral measurements of Martian regolith material and laboratory measurement of weathering kinetics on simulated regolith material suggest large amounts of hydrated iron oxides and clay minerals exist in the regolith; the amount of chemically bound H 2O could be from 1 × 10 4 to 4 × 10 4g/cm 2. In an Earth-analogous model, a 2 km mixed regolith must contain the following concentrations of other volatile-containing compounds by weight: carbonates = 1.5%, nitrates = 0·3%, chlorides = 0.6%, and sulfates = 0.1%. Such concentrations would be undetectable by current Earth-based spectral reflectance measurements, and (except the nitrates) formation of the “required” amounts of these compounds could result from interaction of adsorbed H 2O and ice with primary silicates expected on Mars. Most of the CO 2 could be physically adsorbed on the regolith. Thus, maximum amounts of H 2O and other volatiles which could be stored in the Mars regolith are marginally compatible with those required by an Earth-analogous model, although a lower atmospheric 40Ar concentration and regolith volatile inventory would be easier to reconcile with observational constraints. Differences in the ratios of H 2O and other volatiles to 40Ar between surface volatiles on the real Mars and on an Earth-analogous Mars could result from and reflect differences in bulk composition and time history of degassing between Mars and Earth. Models relating Viking-observable parameters, e.g., ( 40Ar) and ( 36Ar), to the time history and overall intensity of Mars degassing are given.