Kinetics of water adsorption on minerals and the breathing of the Martian regolith

Several observations of the total amount of water vapor in Mars atmosphere display diurnal variations. A possible explanation is an atmosphere/surface coupling that occurs through H2O exchange with the regolith, where adsorbed water molecules have been proposed as a consequent water reservoir. In order to test this hypothesis, experimental laboratory measurements of adsorption isotherms are needed together with adsorption kinetics measurements. Following our previous measurements of the adsorption isotherms of a series of Mars surface analog materials, we report here on kinetics measurements on the same samples at a temperature of 243 K (volcanic tuff, dunite, ferrihydrite, smectite, JSC-Mars1). We observed that even for thin samples (1 mm), diffusion through the sample might influence the adsorption process and significant caution is required to infer kinetics parameters of strongly adsorbing samples. The kinetics parameters k(d) and dk(a)/dP were extracted following the Langmuir theory. Results show that adsorption is fast but not instantaneous with regard to the diurnal time scale (k(d) = 10(-2)-10(-3) s(-1), dk(a)/dP = 10(-3)-10(-4) Pa-1 s(-1)). Large variations are found between the different samples, which suggest a possible geological control on the amount of exchangeable water between the regolith and the atmosphere. We estimate the impact of a noninstantaneous kinetics on the diurnal water vapor cycle by calculating the maximum amount of exchangeable water. We found that a significant amount of H2O can be trapped within the regolith, even in weakly adsorbing analog materials. The similarity in adsorption properties between the JSC-Mars1 and ferrihydrite samples suggests that the adsorption properties of the latter are controlled by the presence of iron oxyhydroxide. These materials have strong adsorption capacities, and their presence on the Martian surface might explain the observed spatial correlation between the average surface humidity and the abundance of surface dust.