Prior to 2009, the Moon was believed to be anhydrous. However, observations by three spacecraft revealed a hydrated surface by reporting a 3 µm absorption band attributed to hydroxyl and possibly molecular water. The Moon Mineralogy Mapper (M) spectrometer, onboard the Chandrayaan-1 spacecraft is mainly used to study the lunar 3 µm band but its spectral range ends at 3 µm. The limited wavelength range of Mhas allowed observed variations in the 3 µm band to be called into question due to uncertainties in thermal corrections.

To investigate the validity of variations in the lunar 3 µm band, we used the SpeX infrared spectrograph at the NASA InfraRed Telescope Facility at Maunakea Observatory in Hawaiʻi. With SpeX, we are able to obtain lunar data over a wavelength range of 1.67 to 4.2 µm at 1 – 2 km spatial resolution. The long wavelengths provide strong constraints on thermal emission corrections. We confirm that the 3 µm band varies with lunar time of day as well as with latitude and composition. Each observation reveals strong variations in abundances of hydroxyl and possibly molecular water. The data reveal a decrease in abundance with increasing lunar local time, an asymmetric trend about the equator that favors the southern latitudes, and higher concentrations in highland regions. The longer wavelengths provided by SpeX have allowed us to examine variations in the 3 µm band and provide definitive evidence that the variations are due to changes in hydration.