DIRECT DETECTION OF MOLECULAR WATER ON THE ILLUMINATED MOON

Abstract

For a decade now the Moon has been known to host hydroxyl (OH) and possibly molecular water (H2O) on its surface [Pieters et al., 2009; Sunshine et al., 2009; Clark 2009]. Distinguishing between OH and H2O from spacecraft measurements at 3 microns however is difficult due to the similar spectral properties of OH and H2O. Currently the only direct measurement of H2O on the lunar surface is of ice deposits within permanently shadowed regions [Li et al., 2018]. On the illuminated Moon, H2O is inferred from LAMP UV observations assuming adsorbed H2O spectrally behaves like water ice, and that OH does not [Hendrix et al., 2019]. We have developed a new approach for unambiguous detection of the water molecule on the illuminated lunar surface. At 6 microns H2O exhibits the fundamental H-O-H bend that is strictly due to H2O without contribution by OH. This spectral region is also dominated by thermal emission so the mixture of reflectance and thermal emission is not an issue. 6 micron measurements are used in the FTIR community to determine the abundance of H2O present in thin sections [McIntosh et al. 2017]. A 6 micron feature has been observed on Mars [Bandfield et al., 2003] and on asteroids in Spitzer data [Marchis et al., 2012]. Currently there are no spacecraft capable of this measurement for the Moon and it cannot be conducted from groundbased telescopes because Earth’s atmosphere is opaque at 6 microns. The only current observatory capable of 6 micron observations of the Moon is the Stratospheric Observatory For Infrared Astronomy (SOFIA). SOFIA flies at 45,000 ft, above a majority of the Earth’s atmosphere allowing for 6 micron observations. Using SOFIA we conducted the first observations of the Moon at 6 microns on August 30th, 2018. Observations at high southern latitudes reveal the presence of a strong 6 micron emission feature due to H2O. The abundance of H2O present is ~940 ppm H2O based on 6 micron laboratory measurements of water bearing glasses with known H2O abundances. Comparison of the lunar 6 micron feature to the 6 micron features seen on asteroids and in meteorites show similar spectral shapes. These observations with SOFIA are the first direct and unambiguous detection of H2O on the illuminated lunar surface.