Mars: CO₂ adsorption and capillary condensation on clays—significance for volatile storage and atmospheric history

Abstract

We report results on the adsorbate-adsorbent system CO2-nontronite at 230øK, 196øK, and 158øK, covering the range of subsurface regolith temperature on Mars. These results, together with Viking observations have allowed construction of a three-part regolith-atmosphere-cap model describing storage and exchange of atmosphere-exchangeable CO2. Cold nontronite, and 'expanding' clays in general, are far better adsorbers for CO2 than cold pulverized basalt and exhibit far more complex adsorptive properties as well. Both observations result from a limitation on interlayer H20 content of clays at low temperature (even when the humidity is high). This makes available energetically favorable adsorption sites for CO2 and creates a micropore structure allowing capillary condensation of CO2 at Martian near-polar temperatures. The layered terrain, and possibly the adjacent debris mantle, contains ---2% or more by mass of atmosphere-exchangeable CO2 (---12 cc STP/g) and the total regolith inventory of available adsorbed CO2 is estimated to be •6 x 1020 g or 4 X 102 g/cm 2 as a global average. Thus the 'ocean' of adsorbed CO2 in the regolith, not the atmosphere or surface caps, is the main repository of atmosphere-exchangeable CO2. Such a large near-polar subsurface reservoir of CQ2 at relatively constant temperature can, either directly or via a small quasi-permanent cap, buffer the atmospheric CO2 pressure on a long-term basis while allowing annual caps to form in response to the wide seasonal variations in surface temperature. For example, isothermal removal (or addition) of atmospheric CO2 causes the large regolith buffer to contribute (or adsorb) atmospheric CO2 in response. Thermal changes also cause important buffering responses. The best understood of these is the obliquity variation, which can produce a ---15 ø increase or decrease in near-polar temperatures relative to present values. An increase would produce rapid dissipation of any quasi-permanent CO2 cap and then the regolith would buffer pressures which could rise to ---20 mb. A decrease would give rise to a situation in which the cap would buffer the pressure as it falls to a minimum of <1 mb. As the pressure falls, the regolith probably transfers much larger amounts of CO2 to the growing cap than the cap would acquire from the atmosphere. The predicted variation in atmospheric pressure (greater than a factor of 10 independent of postulates concerning the existence of a massive surface CO2 reservoir) together with the predicted periodic variations in cap size may have helped generate the layered terrain and other surface features. Also the predicted periodic 'flushing' of the atmosphere resulting from regolith-cap CO2 transfer can explain the near absence of 180 enrichment in the atmosphere observed by the Viking probe mass spectrometer, while other aspects of the model are consistent with and help to explain certain of the Viking lander gas chromatograph-mass spectrometer (GCMS) and gas exchange experiment (GEX) gas evolution observations. tion of circumstantial...

Geochemistry of Carbonates on Mars: Implications for Climate History and Nature of Aqueous Environments

“…A previously unidentified deposit of solid CO 2 was discovered by the Mars Reconnaissance Orbiter, in the form of bodies of CO 2 ice embedded within the martian south polar layered deposits (Phillips et al 2011) with an amount equivalent to a global pure CO 2 atmospheric pressure of 4-5 mbar. More substantial amounts of CO 2 could be adsorbed in the regolith, up to ≈140 mbar (Fanale and Cannon 1979). CO 2 may also be present in the martian cryosphere in the form of CO 2 clathrate hydrates (Mousis et al, this issue), although in unknown amounts.…”

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

“…Conversely, at minimum obliquity, polar temperatures would be 15 ° lower than at present and equatorial temperatures slightly warmer (Ward, 1974). Fanale and Cannon (1979) showed that for their nontronite model, these temperature fluctuations result in an atmospheric pressure that varies between 0.5 and 22 mb. A regolith of fine-grained basalt would result in smaller variations.…”

“…It is reasonable, therefore, to conclude that these eroded debris blankets at mid to high latitudes are similar to the materials analyzed at the Viking sites; that is, they consist largely of weathered debris and may contain a significant fraction of montmorillonitic clays (Toulmin et al, 1977). Fanale and Cannon (1979) showed that montmorillonite clays have a substantial adsorptive capacity for CO2 and that the capacity depends sensitively on temperature. For example, under a 6.1-mb CO2 atmosphere, nontronite, an Fe-rich montmorillonite, can adsorb 11 cm a (STP) COz/g at 158°Kbut only 3.5 cm 3 at 196°K.…”

“…The effects on adsorption should, however, be greatest close to the surface where the temperature fluctuations are largest and the near-surface materials have the smallest grain size and the largest fraction of weathered products. Fanale and Cannon (1979) modeled the effects of long-term climate changes on the adsorption and desorption of COR on the basis of a mean regolith thickness of 150 m, using the term regolith simply to mean finegrained debris. They suggested that the regolith could be thought of in terms of three zones: a polar zone with a mean annual temperature of 158°K, an intermediate latitude zone at 196 ° K, and an equatorial zone at 233°K.…”

Mariner 9 observations of the south polar cap of Mars: Evidence for residual CO2 frost

“…Currently, the available observations point to the conclusion that the total mass of permanent, solid CO2 at the south residual cap is not sufficient to control the mass of the Martian atmosphere over climatic time scales. Fanale and Cannon [1979] have estimated that if the cap was uniformly covered with a 400-m-thick layer of CO2 frost, it could contain at most the equivalent of I present Martian atmospheric mass of solid CO2. If a residual water ice cap was exposed in 1969 [Jakosky and Barker, 1984] We now present the IRIS and imaging data sets and describe our data reduction and analysis procedures.…”