Time-of-day-dependent Global Distribution of Lunar Surficial Water/hydroxyl

A new set of time-of-day-dependent global maps of the lunar near-infrared water/hydroxyl (HO/OH) absorption band strength near 2.8 to 3.0 μm constructed on the basis of Moon Mineralogy Mapper (M) data is presented. The analyzed absorption band near 2.8 to 3.0 μm indicates the presence of surficial HO/OH. To remove the thermal emission component from the M reflectance spectra, a reliable and physically realistic mapping method has been developed. Our maps show that lunar highlands at high latitudes show a stronger HO/OH absorption band in the lunar morning and evening than at midday. The amplitude of these time-of-day-dependent variations decreases with decreasing latitude of the highland regions, where below about 30°, absorption strength becomes nearly constant during the lunar day at a similar level as in the high-latitude highlands at midday. The lunar maria exhibit weaker HO/OH absorption than the highlands at all, but showing a smaller difference from highlands absorption levels in the morning and evening than at midday. The level around midday is generally higher for low-Ti than for high-Ti mare surfaces, where it reaches near-zero values. Our observations contrast with previous studies that indicate a significant concentration of surficial HO/OH at high latitudes only. Furthermore, although our results generally support the commonly accepted mechanism of HO/OH formation by adsorption of solar wind protons, they suggest the presence of a more strongly bounded surficial HO/OH component in the lunar highlands and parts of the mare regions, which is not removed by processes such as diffusion/thermal evaporation and photolysis in the course of the lunar day.