Large Water Inventory in Highly Adsorptive Regolith Simulated With a Mars Global Climate Model

Abstract

This study explores the subsurface water distribution near the surface throughout Mars, using a Mars Global Climate Model (MGCM). We coupled a regolith model that solves the equations of water transport, adsorption, and condensation in the regolith with our MGCM to investigate the effects of inhomogeneous regolith properties on the subsurface water distribution. This regolith model includes a regolith property model that calculates inhomogeneous regolith properties (grain size, porosity, and adsorption coefficient) based on the results of laboratory experiments. The simulations were performed with five different regolith situations over 500 Martian years, aiming for a quasi-steady state of adsorbed water concentration within the top meters of the regolith. Our results revealed a high adsorbed water concentration near the equator (2.0–4.0 kg m⁻² for depths within 2 m, with a high adsorption coefficient of ∼200 kg m⁻³), which was caused by the highly adsorbed regolith, which has a large specific surface area. This indicates the nonnegligible effects of the spatial inhomogeneity of the adsorption coefficient on the adsorbed water distribution. Highly adsorptive regolith also promotes pore ice accumulation in shallower layers at high latitudes because in the winter the freezing of adsorbed water is faster than the water transport in the regolith. In the determination of long-term water retention and runoff on Mars, highly adsorptive regolith acts to retain subsurface water, decelerating water transport within the regolith because the water vapor concentration at equilibrium decreases with increasing adsorption coefficient. This study highlights the significant effects of regolith adsorbability on the subsurface water distribution.