New Constraints on the Lunar Optical Space Weathering Rate

Space weathering processes form submicroscopic metallic iron particles that are optically active, darkening and reddening the lunar surface over time. The optical effects of these particles depend on their size; nanophase iron darkens and reddens, while microphase iron darkens without reddening. Using available Kaguya Multiband Imager parameter maps believed to estimate submicroscopic iron abundance, we investigate trends that may be associated with abundance of nanophase and microphase iron near dated lunar craters. We observe that nanophase iron is strongly correlated with crater age, while microphase iron exhibits a weaker correlation. We present models for the highlands nanophase and microphase iron accumulation rates from 100 ka to 1 Ga. Our observations suggest that highlands nanophase iron abundance is a direct result of space weathering exposure, while highlands microphase iron abundance is likely influenced by lunar source materials or stochastic impact-delivery mechanisms.