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

Observations of the Martian planetary boundary layer lead to interpretation s that are baffling and contradictory. In this paper we specifically addres s the question of whether or not water vapor finds a substantial diurnal re servoir in the Martian regolith. To address this issue, we have measured H2 O adsorption kinetics on SWy-1, a Na-rich montmorillonite from Wyoming. The highest-temperature (273 K) data equilibrate rapidly. Data gathered at rea listic H2O partial pressures and temperatures appropriate to early morning show two phenomena that preclude a significant role for smectites in diurna lly exchanging a large column abundance. First, the equilibration timescale is longer than a sol. Second, the equilibrium abundances are a small fract ion of that predicted by earlier adsorption isotherms. The explanation for this phenomenon is that smectite clay actually increases its surface area a s a function of adsorptive coverage. At Mars-like conditions we show that t he interlayer sites of smectites are likely to be unavailable.