Time-of-day-dependent behavior of surficial lunar hydroxyl/water: Observations and modeling

Detection of an excessively strong 3-μm absorption near the lunar highland crater Dufay

“…In this study we assumed a surface with a root-mean-square slope angle of 9 • . It has been shown (Wöhler et al 2017b;Grumpe et al 2019) that the 3-µm band depth remains qualitatively similar when a value of 20 • as proposed by Bandfield et al (2018) is used instead, which is due to the weak influence of the surface roughness on the thermal emission at low incidence angles Grumpe et al 2019), as is the case here. A detailed analysis of the range of surface roughness values to be used for thermal-emission removal was conducted by Grumpe et al (2019).…”

“…The global M 3 OH/H 2 O maps by Wöhler et al (2017b) indicate that in the equatorial lunar highlands the 3-µm band depth differs by less than about 10% between morning and midday or between midday and afternoon. In contrast, recent modeling by Grumpe et al (2019) has shown that near the equator in the lunar highlands the time-of-day-dependent OH component always decreases to zero at midday due to diffusive loss and photolysis for all examined settings of physical parameters. This finding is confirmed by the model of Farrell et al (2017), which predicts H column densities on the surface at low latitudes to be about 10-100 times lower at midday than in the morning and afternoon.…”

“…Our analysis is therefore based on the M 3 level-1B spectral radiance images available on the Planetary Data System (PDS). We applied our previously described method for thermal-emission removal (Wöhler et al 2017a,b;Grumpe et al 2019), which is based on the Hapke model (Hapke 1984(Hapke , 2002 and on the thermal equilibrium approach of Shkuratov et al (2011), to model simultaneously the temperature, spectral emissivity, and bidirectional spectral reflectance of the surface. An initial estimate of the surface temperature is obtained by fitting the superposition of a reference reflectance spectrum linearly extrapolated to the 3-µm range and a black-body emission spectrum to the observed M 3 reflectance spectrum.…”

“…The 3-µm band depth was computed similar to previous works (Wöhler et al 2017b;Grumpe et al 2019) but with adjusted wavelength ranges. First, we fitted a linear continuum c(λ) to the reflectance spectrum R(λ) across the wavelength range 2.258-2.497 µm and extrapolated it up to 2.936 µm (Fig.…”

“…The analysis of NIR reflectance spectra of the lunar surface furthermore led to the identification of timeof-day-dependent variations of the 3-µm band depth (Sunshine et al 2009;Li & Milliken 2017;Wöhler et al 2017a,b). A commonly accepted explanation of the existence of lunar surficial OH/H 2 O is the reaction between adsorbed protons from the solar wind with oxygen atoms contained in the surface material (e.g., Starukhina 2001;Farrell et al 2015Farrell et al , 2017Grumpe et al 2019).…”