Moonshine: Diurnally varying hydration through natural distillation on the Moon, detected by the Lunar Exploration Neutron Detector (LEND)

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 (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.

Water on the surface of the Moon as seen by the Moon Mineralogy Mapper: Distribution, abundance, and origins

A new thermal correction model and experimentally validated relationships between absorption strength and water content have been used to construct the first global quantitative maps of lunar surface water derived from the Moon Mineralogy Mapper near-infrared reflectance data. We find that OH abundance increases as a function of latitude, approaching values of ~500 to 750 parts per million (ppm). Water content also increases with the degree of space weathering, consistent with the preferential retention of water originating from solar wind implantation during agglutinate formation. Anomalously high water contents indicative of interior magmatic sources are observed in several locations, but there is no global correlation between surface composition and water content. Surface water abundance can vary by ~200 ppm over a lunar day, and the upper meter of regolith may contain a total of ~1.2 × 10¹⁴ g of water averaged over the globe. Formation and migration of water toward cold traps may thus be a continuous process on the Moon and other airless bodies.