Water vapor adsorption on Mars

Abstract

Near-surface air moisture observations from Phoenix and Curiosity display diurnal adsorption, which appears insensitive to mineralogy of regolith. To study this, adsorptive column model simulations are made for midsummer at Phoenix, validated against recalibrated TECP water vapor pressures p. Two standard-form (= obeying the van't Hoff equilibrium law) adsorption isotherms with quite different specific surface areas As do produce nearly identical and good matches with the observed p, whereas a widely used non-standard isotherm fails. When made standard, it also produces good results. Adsorbed amounts differ in the three good simulations but their surface fluxes and surface vapor concentrations are nevertheless nearly identical. Properties of regolith except enthalpy are shown to disappear in the model's adsorption term, explaining insensitivity to As. The van't Hoff plot of observed ln p vs. model's 1/Tg during adsorption and desorption suggests enthalpy of about 22 kJ/mol. Enthalpies of the three standard-form isotherms are close to this, explaining their excellent match with each other and with observations. Hence the low-pressure water vapor adsorption to martian regolith appears essentially nonspecific and is associated with low enthalpy, as is typical for physisorption in general.