Space Weathering and Lunar OH/H2O-Insights from Ab Initio Mie Modeling of Submicroscopic Iron

Lunar OH/H2O has been confirmed and mapped by analyzing the 3 μm absorption band in spectra acquired by the Moon Mineralogy Mapper (M3) instrument. Space weathering leads to accumulation of submicroscopic iron particles in the uppermost layer of the regolith which gradually changes the spectral signature of airless planetary bodies and thus may affect the detection of lunar OH/H2O. The contribution of this paper is twofold. (1) Our new technique combines Hapke reflectance modeling and ab initio Mie scattering calculations to model the scattering behavior of submicroscopic iron which governs the optical effects due to space weathering. (2) Thermally corrected M3 spectra of mature and immature sample points in mare and highland regions are used to assess the performance of the simulation framework and are analyzed to understand maturity-related changes of the OH/H2O band depth. We find that the simulation method can convincingly reproduce the spectral changes of maturing lunar soil. It becomes clear that there is only a minor effect on the 3 μm absorption feature. This finding makes the analysis of the lunar OH/H2O mapping largely invariant with respect to space weathering. In general, the absorption features around 1 and 2 μm are more strongly obstructed than the feature around 3 μm. Further, we discuss agglutination as the main cause for slight deviations found around the 2 μm band and layering/clustering as a likely reason to explain predicted iron particle sizes that are larger than observed.