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

Observations of the Martian planetary boundary layer lead to interpretations that are baffling and contradictory. In this paper we specifically address the question of whether or not water vapor finds a substantial diurnal reservoir in the Martian regolith. To address this issue, we have measured H 2 O adsorption kinetics on SWy‐1, a Na‐rich montmorillonite from Wyoming. The highest‐temperature (273 K) data equilibrate rapidly. Data gathered at realistic H 2 O partial pressures and temperatures appropriate to early morning show two phenomena that preclude a significant role for smectites in diurnally exchanging a large column abundance. First, the equilibration timescale is longer than a sol. Second, the equilibrium abundances are a small fraction of that predicted by earlier adsorption isotherms. The explanation for this phenomenon is that smectite clay actually increases its surface area as a function of adsorptive coverage. At Mars‐like conditions we show that the interlayer sites of smectites are likely to be unavailable.