Improved space weathering maps of the lunar surface through radiative transfer modeling of Kaguya multiband imager data

Abstract

Space weathering is a continuous process operating on the surface of the Moon producing nanophase and microphase iron on the rims of mineral grains and within agglutinates. The spectral effects due to these nanophase and microphase iron particles in the lunar regolith are well characterized in the visible and near-infrared wavelengths. In this work, we used the Hapke radiative transfer technique to model multispectral data (415–1550 nm) from the Kaguya Multiband Imager to map the nanophase and microphase iron abundances across the lunar surface. In order to produce these maps, we developed a reflectance model for silicate host particles as a function of FeO content using magnetic and Lunar Soil Characterization Consortium (LSCC) data. We tested the accuracy of our radiative transfer technique and found that the model can predict the nanophase and microphase iron abundances within 0.1 wt% Fe and 0.6 wt% Fe, respectively. By using a FeO and ilmenite map and our relationship between host particle reflectance and FeO abundance, we produced a nanophase, microphase, and submicroscopic iron abundance map. We found the approximate saturation limit for nanophase and microphase iron with respect to FeO. In general, the nanophase and microphase iron saturation limit increases with FeO content, except in soils that have FeO content of 6–15 wt%, the nanophase iron saturation is nearly constant. We also observed that the microphase iron abundance saturates before the nanophase iron abundance. These maps could be used to determine relative age and maturity, but with limitations. Our analyses of these maps showed that (1) swirls contain low abundances of nanophase iron, but no anomalous microphase iron abundances, (2) nanophase and microphase iron abundances are lower at higher latitudes, which suggests lower solar wind and micrometeoroid impact flux at these latitudes, (3) decreased nanophase iron abundances in the nearside suggest a lower solar wind flux due to Earth's magnetotail, and (4) in contrast to Mercury, the microphase to nanophase iron abundance ratio is lower on the Moon due to lower surface temperatures and less intense space weathering.