DIRECT DETECTION OF MOLECULAR WATER ON THE ILLUMINATED MOON

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 30^th, 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.