Molecular water detected on the sunlit Moon by SOFIA

Abstract

Widespread hydration was detected on the lunar surface through observations of a characteristic absorption feature at 3 µm by three independent spacecraft 1-3. Whether the hydration is molecular water (H 2 O) or other hydroxyl (OH) compounds is unknown and there are no established methods to distinguish the two using the 3 µm band 4. However, a fundamental vibration of molecular water produces a spectral signature at 6 µm that is not shared by other hydroxyl compounds 5. Here, we present observations of the Moon at 6 µm using the NASA/DLR Stratospheric Observatory for Infrared Astronomy (SOFIA). Observations reveal a 6 µm emission feature at high lunar latitudes due to the presence of molecular water on the lunar surface. On the basis of the strength of the 6 µm band, we estimate abundances of about 100 to 400 µg g −1 H 2 O. We find that the distribution of water over the small latitude range is a result of local geology and is probably not a global phenomenon. Lastly, we suggest that a majority of the water we detect must be stored within glasses or in voids between grains sheltered from the harsh lunar environment, allowing the water to remain on the lunar surface. Using SOFIA and the Faint Object infraRed CAmera for the SOFIA Telescope (FORCAST) instrument, we conducted observations of the lunar surface at 6 µm on 31 August 2018 in a search for molecular water. FORCAST is well suited to look for 6 µm lunar water due to its wavelength coverage from 5 to 8 µm, spectral resolution of R = 200 and high signal-to-noise ratios. The FORCAST entrance slit that defines the portion of the Moon observed is 2.4 × 191 arcsec sampled with 248 pixels. At the lunar centre of disk the slit has a spatial extent of 4.8 × 1.5 km 2 (the spatial resolution near the limb is lower due to foreshortening). During the observations, the Moon was at a phase angle of 57.5°. We observed a region at high southern latitudes near Clavius crater and a low-latitude portion of Mare Serenitatis (Extended Data Fig. 1). Details regarding observations, site selection and data reduction can be found in the Methods. Data from SOFIA reveal a strong 6 µm emission band at Clavius crater and the surrounding terrain (Fig. 1) relative to the control location near the lunar equator, which shows low hydration in some analyses (see Methods). All spectra from the Clavius region exhibit this 6 µm emission feature. The majority of these emission peaks (98%) exceed 2σ significance relative to the background noise, and about 20% exceed 4σ significance (Extended Data Fig. 2). To determine whether the spectral properties of the lunar 6 µm band are consistent with spectra of particulate water-bearing materials, we examined other planetary materials that show a 6 µm

Hydroxylation of Apollo 17 Soil Sample 78421 by Solar Wind Protons

“…present at high lunar latitudes (Honniball et al, 2020), suggesting that the lunar feature near 3 µm, in that region as well as possibly others, may be a combination of implanted OH and some form of water.…”

Dynamics of Subsurface Migration of Water on the Moon

“…During 4-12% of the days at the investigated sites a cumulative water mass of >1 µg per m 2 is released from the surface. This gives rise to the hypothesis that the release of temporarily trapped water could be one potential contributor to recent observations of surface and near-surface water in sunlit areas (Honniball et al, 2020;Pieters et al, 2009). A second mechanism that could lead to such observations is the release of shallow water accumulations via impacts.…”

“…pumping at mid-latitudes and attempt to test a hypothesis for water observations in illuminated nonpolar areas (e.g., Honniball et al, 2020). All six sites have distinct thermal characteristics, representing different boundary conditions for thermal pumping.…”

Molecular Dynamics Simulations of Dielectric Breakdown of Lunar Regolith: Implications for Water Ice Formation on Lunar Surface

“…Honniball et al. (2020) recently reported that the lunar regolith holds individual water molecules that could be the result of micrometeoroid impact inducing water formation from hydroxyl. It is interesting to note that the detection in Honniball et al.…”

“…Hence, dielectric breakdown induced by SEP may be a potentially important mechanism for surface water formation in PSRs as well as, to a lesser extent, globally on the Moon. Honniball et al (2020) recently reported that the lunar regolith holds individual water molecules that could be the result of micrometeoroid impact inducing water formation from hydroxyl. It is interesting to note that the detection in Honniball et al (2020) could also be consistent with this study, which shows water molecules attaching to regolith grains after a breakdown event.…”