Time-of-day-dependent global distribution of lunar surficial water/hydroxyl.

Abstract

A new set of time-of-day-dependent global maps of the lunar near-infrared water/hydroxyl (H₂O/OH) absorption band strength near 2.8 to 3.0 μm constructed on the basis of Moon Mineralogy Mapper (M³) data is presented. The analyzed absorption band near 2.8 to 3.0 μm indicates the presence of surficial H₂O/OH. To remove the thermal emission component from the M³ reflectance spectra, a reliable and physically realistic mapping method has been developed. Our maps show that lunar highlands at high latitudes show a stronger H₂O/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 H₂O/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 H₂O/OH at high latitudes only. Furthermore, although our results generally support the commonly accepted mechanism of H₂O/OH formation by adsorption of solar wind protons, they suggest the presence of a more strongly bounded surficial H₂O/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.