Time-of-day-dependent behavior of surficial lunar hydroxyl/water: Observations and modeling

摘要

In this study we determine the depth of the absorption band around 3 mu m wavelength, which indicates the presence of OH/H2O in a thin surficial layer of the lunar regolith, for 18 lunar highland regions observed by the Moon Mineralogy Mapper (M-3) instrument at 4-8 different local times of day. For removing the thermal emission component, a physically motivated thermal equilibrium based method is used, which also takes into account the roughness of the regolith surface. We propose a continuity equation based model of the time-of-day-dependent column densities of surficial atomic hydrogen (H) and hydroxyl (OH). The considered source processes for H are implantation of solar wind protons and photolysis of OH, and for OH the reaction H + O -> OH. Sink processes are diffusive loss for H and OH, and photolysis for OH. Sputtering of H and OH is found to be negligible in comparison to the other sink processes. Additionally, we suggest a similar differential equation based model to describe the time-of-day-dependent behavior of micrometeoroid-delivered OH and H2O. The observed 3 mu m band depth values indicate that the surficial OH/H2O does not vanish even at local midday at low latitudes, while the model predicts nearly complete removal of surficial OH/H2O at midday. This apparent contradiction between model and observations is reconciled by adding to the model a region-specific "offset" OH component which is assumed to be stable against diffusive loss and photolysis and is therefore interpreted as a strongly bounded OH component. Meteoroid bombardment is found to be negligible in comparison with the solar wind source of OH. Fitting the model to the observed 3 mu m band depth values allows for estimating the H activation energy, the OH photolysis time, region-specific values of the offset OH component, and the proportionality factor between OH column density and 3 mu m band depth. The observed time-of-day-dependent behavior of the 3 mu m band depth at low and high latitud