Near-infrared spectroscopy of boulders with dust or patina coatings on the Moon: A two-layer radiative transfer model

Abstract

Previous remote sensing studies focus on lunar surface regolith, which contains abundant mixtures of rock fragments and dust, making it hard to track the petrologic origin. Igneous boulders exposed on lunar surface, however, carry pristine mineralogy and chemistry since its formation, therefore are direct evidence of lunar thermal evolution events. High spatial-resolution remote sensing images and rover explorations of the Moon allow us to study the spectroscopy of igneous boulders. We modeled the optical scattering properties of rocks using the Legendre and Double Henyey–Greenstein phase functions, porosity parameter and grain size, and provided a modified radiative transfer model for rocks rather than powdered minerals. Considering that space weathering could generate a layer of dust or patina on the surface of boulders, we introduced a two-layer radiative transfer modeling algorithm to solve the spectroscopy of the substrate rock for dust- or patina-coated boulder. The modeled substrate rock spectra show less reddening, larger reflectance, and stronger absorption band depth compared to dust- or patina-coated rock, consistent with the measurements of Apollo rock samples. We applied this two-layer model on the dust-coated boulder detected by Yutu-2 rover and derived the spectrum of the substrate rock. Using Kaguya Multiband Imager data, we calculated the substrate rock spectra for an anorthosite boulder, and our result shows good consistency with laboratory measured anorthosite rock spectrum. This work is a beginning of understanding lunar boulder spectroscopy for a more precise interpretation of lunar thermal history.