Implications of surface roughness in models of water desorption on the Moon

The observed presence of water molecules in the dayside lunar regolith was an unexpecteddiscovery and remains poorly understood. Standard thermophysical models predict temperaturesthat are too high for adsorbed water to be stable. We propose that this problem can becaused by the assumption of locally flat surfaces that is common in such models. Here weapply a model that explicitly considers surface roughness, and accounts for solar illumination,shadows cast by topography, self–heating, thermal reradiation, and heat conduction. We couplethe thermophysical model to a model of first–order desorption of lunar surface water anddemonstrate that surface roughness substantially increases the capability of the Moon to retainwater on its sunlit hemisphere at any latitude, and within 45 of the poles, at any time of thelunar day. Hence, we show that lunar surface roughness has a strong influence on lunar wateradsorption and desorption. Therefore, it is of critical importance to take account of surfaceroughness to get an accurate picture of the amount of water on the Moon’s surface and in itsexosphere.