Lunar Ice: Adsorbed Water on Subsurface Polar Dust

Differential scanning calorimetry indicates that adsorbed water and goethite, a product of hydrated ilmenite, are thermally stable over geologic time in the lunar polar regions. Adsorbed water can undergo burial as a result of several mechanisms, thereby achieving protection from sputtering or Lyman α radiation losses. Adsorbed, subsurface water layers on lunar dust, and any hydrated minerals present, could account for a majority of the hydrogen at the north lunar pole as well as account for a portion of that found at the south pole, particularly in small (<10 km) craters. Lunar ice, if it forms by condensation of water vapor in polar cold traps, will initially be in the form of amorphous solid water, and its rate of crystallization will depend on trap temperature and the composition of the surfaces upon which it has condensed. Between 95 and 110 K, diurnal temperature fluctuations cause surface ice deposits to migrate through the lunar regolith. Via such migration, stable and immobile layers of adsorbed water will be formed. In this temperature range, which can be expected at the margins of large craters and in smaller craters, any water resource would be a mixture of relatively unstable bulk ice and stable adsorbed water on subsurface dust and fines.