Variations in surface adsorbed H2O on lunar soils and relevant minerals

Abstract

Spectral variations due to the removal of surface adsorbed H2O at 3 and 6 μm in reflectance spectra on lunar soils and relevant minerals (olivine, pyroxene, and plagioclase) have been assessed. This study characterizes variations in hydration features as a function of lunar relevant surface temperatures, to further understand current (i.e., M3, HRI-IR, VIMS) and future (i.e., Lunar Trailblazer) observations of diurnal changes in surface hydration. Additionally, we explore the utility of using the 6 μm H2O feature to discern the speciation of surface hydration at 3 μm. We perform controlled temperature measurements (25–200 °C) in a Linkam THMS600 Environmental Stage fixed to a Bruker LUMOS Microscope Fourier Transform IR (μFTIR) spectrometer. We observe clear and systematic changes in the strength of the 3 μm H2O/OH feature associated with the thermal removal of adsorbed H2O, in addition to changes in the overall shape and band position of the feature in both the terrestrial and lunar samples. The strength of the 3 μm feature for the compositionally distinct and relatively brighter Apollo highland soil (62231) is stronger and more symmetric than the 3 μm feature observed for the darker mare soil (10084). While several silicate related absorption features are identified near 6 μm, neither a distinguishable hydration feature nor any changes in reflectance that could be attributed to the presence or a change in the amount of surface adsorbed H2O were observed at 6 μm.