Adsorption Kinetics of Water and Argon on Lunar Grains

Abstract

Adsorption residence times of water on lunar grains play an essential role for the migration and sequestration of water on the Moon, but estimates vary by over 10 orders of magnitude for the same temperature. Here, the physical chemistry literature for the interaction of water vapor and argon with silicates is reviewed, and available measurements of adsorption on lunar samples are used to quantify adsorption times. The wide range of adsorption energies, measured or postulated, is explained by activated surface sites with high desorption energies. Passivation, the opposite of activation, occurs quickly in laboratory settings and is expected on longer timescales at shallow depths on the Moon. A quantitative parameterization of desorption rates, intended for a wide range of adsorbate concentrations and temperatures, is constructed and used in model calculations. Grain surface diffusion plays a key role for desorption rates and also reduces photodestruction rates. For ice buried beneath lunar soil, multilayer adsorption is expected well before the depth of the ice is reached.