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

Abstract

Estimates have been made of the capacity of the Martian regolith to exchange adsorbed H20 and CO2 with the atmosphere-plus-cap system (APCS). These estimates are based upon measured isotherms for H:O and CO: adsorption on pulverized basalt at low temperatures and on theoretical considerations. A unit column (1 cm :) of regolith with a deep subsurface temperature of -77øC, considered average for the disk, will contain 40.4 g of adsorbed CO: and --• 1 g of adsorbed H•O per meter of depth. Under favorable circumstances the top 3 cm can exchange much more H•O 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 H•O and 5% of the CO2 surface inventories expected for a hypothetical Mars that has experienced degassing as intensive as that 9f earth. Long-term (• 105 yr) variations in surface insolation caused by variations in the obliquity of Mars could cause efficient desorption of adsorbed CO: and H:O from a 100-m regolith, and variations in solar output with periods >1 m.y. may well desorb H•O and CO2 efficiently from all the unconsolidated material onMars. Enough CO: 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 CO: 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.One approach to assessing the extent of Martian degassing is to assay all sinks into which volatiles supplied to the Martian surface might have eventually found their way. An analogous approach to the history of earth volatiles was employed by Rubey [1951] in his classical paper on the subject. He pointed out that the atmosphere, the polar caps, and even the oceans contain only a trivial fraction of certain released earth volatiles and that by far the most important fate of CO•. and some other volatiles that had been released to the surface was chemical recombination with igneous materials, resulting in incorporation into sediments. We will show that within the constraints currently supplied by observations of Mars it is possible and even likely that orders of magnitude more H•.O and CO•. are adsorbed on the Martian regolith than are present in the atmosphere-plus-caps system (APCS). Therefore any estimates of the degree of degassing of Mars based exclusively on the volatile inventory in the APCS are likely to be much too low for this reason alone, even if large amounts of H•.O and CO2 have not been removed by permafrost formation, chemical recombination, or exospheric escape. Further, we will attempt to show that exchange of adsorbed H•.O and CO•. between the Martian regolith and the APCS may play a major role in (1) seasonal and diurnal atmospheric phenomena, as discussed by Pollack et al. [1970] and Fanale and Cannon [ 1971 ], and (2) producing changes in atmospheric pressure...

H2O adsorption on smectites: Application to the diurnal variation of H2O in the Martian atmosphere

“…They incorporated much of the same physics as Flaser and Goody [1976] but had the advantage of using more accurate atmospheric H20 abundances based on the Viking observations. They assumed that H20 was a passive atmospheric tracer and employed soil adsorption isotherms using the data reported by Fanale and Cannon [1978] for H20 adsorption of basalt.…”

Stability and evolution of the climate system of Mars

“…On the other hand, volcanic degassing might have supplied CO 2 to the atmosphere (Baker et al, 1991). It is also important to consider the polar ice caps (e.g., Leighton and Murray, 1966) and the surface regolith (Fanale and Cannon, 1974) which might have been large CO 2 reservoirs to exchange CO 2 with the atmosphere.…”

Periodic climate change on Mars: Review of evidence and effects on distribution of volatiles

“…The depth for Mars is almost certainly shallower (10 km) because of higher gravity and higher heat flow (Carr, 1979). The unconsolidated rubble is probably considerably thinner than the brecciated zone but thicknesses of 1-1.5 km are plausible (Fanale and Cannon, 1974). The simple picture just outlined is locally complicated by deposits which postdate the early high-impact flux.…”

“…Diurnal temperature fluctuations affect only the top few centimeters; annual changes approximately the top meter. The exact values depend on the thermal conductivities and heat capacities of the materials involved (Fanale and Cannon, 1974). Long-term climatic changes can penetrate to much greater depths.…”