Gradual Sequestration of Water at Lunar Polar Conditions due to Temperature Cycles

Migration of water molecules into the porous lunar soil can lead to sequestration of adsorbed water or ice at depth. Here, this process is modeled at several potential landing sites in the south polar region of the Moon. Desorption times are parameterized based on the Brunauer–Emmett–Teller (BET) isotherm model. Water sequestration is significant at depths where subsurface temperatures remain below the cold-trapping threshold, a condition satisfied at one of three sites considered, west of Haworth Crater. Model calculations predict a hydrated layer below a desiccated layer. The thickness of the desiccated layer is on the scale of the thermal skin depth, which for dust is typically centimeters. The underlying layer, decimeters thick, can be hydrated over a timescale of a billion years, reaching abundances on the order of 1 wt%. This sequestration process potentially simultaneously explains excess hydrogen concentrations outside of cold traps and the observed presence of a desiccated layer above a hydrogenous layer.