The presence of water in the Moon has a long and controversial history. It has been approached by laboratory measurements on returned lunar samples, by optical spectroscopy of the lunar surface and inferred from hydrogen analysis by neutron scattering. In addition, several experiments have been performed in the laboratory, under a wide variety of conditions, to simulate water production and stability in the regolith.
I will discuss a variety of phenomena that have been recognized incompletely or not at all in these studies and which are important for scientific analysis and to estimate water as a resource. Analyses of returned samples are compromised by contamination from terrestrial hydrogen and water, enhanced by extensive surface (or near surface) defects produced by exposure to solar radiation. The active damaged surface can incorporate hydrogen (or deuterium) from the gas phase and from liquids used in sample processing. Aqueous alterations by water are well known processes in geology and in chemical physics of spent nuclear fuel. Incorporation of hydrogen and water are governed by unknown isotope effects, which should be clarified in discussions of D/H ratios.
Returned samples have been exposed to atmospheric water. Even ultrapure nitrogen used in storage contains enough water to produce monolayer coverage in seconds or less. Further sample handling in the laboratory, such as the preparation of thin sections for microscopy are additional sources of water contamination. This topic needs to be studied thoroughly in anticipation of future sample return missions.
The implantation of solar wind protons into solids, such as lunar minerals and glasses, usually leads to trapping. The H can combine with other H and be desorbed or trapped at voids, or it can form bonds with surface atoms, which are often loosely identified as OH, SiH, or SiOH, a simplification to the complex polyatomic bonding in solids. Infrared spectroscopy is unable to identify these bonds except by correlation with what is known in terrestrial rocks that are relatively free of radiation damage. In addition, the distinction between adsorbed and incorporated water is often not made (estimates will be provided for maximum amount of adsorbed water on the Moon).
Remote identification of OH stretching vibrations by infrared spectroscopy can be compromised by the existence of other absorptions. Furthermore, water adsorption in optical elements in remote instruments can produce a background signal that can be affected by temperature and, hence by observation time.
I will also remark on the use of bulk values (such as ppm-weight) to quantify observations in cases where concentrations are hugely inhomogeneous such as in lunar fines. Related to this issue is the unknown extent of normal diffusion and radiation-enhanced diffusion of hydrogen in minerals at lunar temperatures and time scales.
Finally, the effects of solar ultraviolet light will be discussed if time permits.