Telescopic Observations of Lunar Hydration: Variations and Abundance

Abstract

Prior to 2009, the surface of 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 (M3) spectrometer on board the Chandrayaan-1 spacecraft has been used to study the lunar 3 μm band, but its spectral range ends at 3 μm. The limited wavelength range of M3 has allowed observed variations in the strength of 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 Hawai‘i. 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 separating thermal emission from solar reflectance. We confirm that the 3 μm band varies with lunar time of day as well as with latitude and composition. Pole-to-pole observation chords reveal strong variations in abundances of hydroxyl and possibly molecular water. The data reveal a decrease in abundance as lunar noon is approached, 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.