Revisiting the 3 µm feature on the lunar surface with an updated rough surface correction

Abstract:

Hydration of the lunar regolith has been debated for the past decade due to the detection of an absorption near 3 µm indicative of molecular vibrations of OH/H₂O bonds by several near-infrared spectrometers. The Mineralogy Mapper (M³) aboard Chandrayaan-1 currently provides the most complete spatial coverage of the 3 µm range. Previous studies have shown that absorption features longward of 2 µm can be complicated by thermal emission. We apply the first thermal correction which accounts for the roughness of the lunar surface that accounts for the sub-pixel distribution of surface temperatures to remove the thermal emission component. We contribute new insights into the distribution, or source of hydration across the lunar regolith.

We apply the Bandfield et al. (2018) roughness-based thermal correction to a wide range of terrains to better understand hydration on the Moon. Previous studies have shown conflicting evidence for the diurnal variation of the 3 µm feature, leading to debate about its source (OH or H₂O). We assess the 3 µm feature at a variety of local times to investigate the variability of this feature with solar incidence and over the lunar day. Lunar pyroclastic deposits have previously been shown to have a 3 µm enhancement, and we reproduce these studies with our updated data. We also test the correlation between the 3 µm feature and the local solar wind flux to assess whether the feature is primarily due to OH formed by H+ implantation into poorly crystalline regolith grains. To accomplish this, we study regions of interest on and near lunar swirls, surface features associated with magnetic anomalies which may be regions of reduced solar wind flux. We also test the latitudinal dependence of the 3 µm feature to assess how it varies with reduced solar wind flux at higher latitudes.

Initial results show that we do not observe diurnal variation in the 3 µm feature but may observe variations associated with the target properties of the lunar surface. We present analyses of the 3 µm features with time, latitude, and local terrain. This work provides new constraints on the source distribution and abundance of the enigmatic hydration observed on the lunar surface. We also constrain the variability in our model due to topography in our companion abstract Gonzales et al. (this conference).