The lunar far‐UV albedo: Indicator of hydration and weathering

We present an analysis of Lunar Reconnaissance Orbiter (LRO) Lyman Alpha Mapping Project (LAMP) measurements of the dayside lunar surface at far‐ultraviolet wavelengths. We use the strong 165 nm H 2 O absorption edge to look for diurnal variations in hydration. We find that diurnal variations in spectral slope are indeed present; they are superimposed on latitudinal and spatial variations related to composition and weathering. We use two different spectral regions (164–173 nm and 175–190 nm) to separate out these effects. Highlands and mare regions have distinct reflectance spectra, with mare regions being spectrally bluer than highlands regions, a consequence of the greater abundance of opaque minerals in mare regions. Bright ray terrains and areas known to be young such as Giordano Bruno crater, are found to be relatively spectrally flat or red in the far‐UV; this is consistent with a lack of space weathering, which tends to make the far‐UV spectrum bluer due to the spectral behavior of nanophase iron. Large‐scale latitudinal variations in FUV slope are distinct and are likely due to a gradient in space weathering. The diurnal variation in hydration is consistent with a solar wind origin and with loss of H 2 O at temperatures above ∼320 K. Far‐UV spectroscopy is thus shown to represent a viable method for mapping aqueous alteration, even on the dayside of the Moon, and potentially elsewhere in the solar system.