The optical properties of the finest fraction of lunar soil: implications for space weathering

The fine fraction of lunar soils (<45 m) dominates the optical properties of the bulk soil. Definite trends can be seen in optical properties of size separates with decreasing particle size: diminished spectral contrast and a steeper continuum slope. These trends are related to space weathering processes and their affects on different size fractions. The finest fraction (defined here as the <10 m fraction) appears to be enriched in weathering products relative to the larger size fractions, as would be expected for surface correlated processes. This <10 m fraction tends to exhibit very little spectral contrast, often with no distinguishable ferrous iron absorption bands. Additionally, the finest fractions of highland soils are observed to have very different spectral properties than the equivalent fraction of mare soils when compared with larger size fractions. The spectra of the finest fraction of feldspathic soils flatten at longer wavelengths, whereas those of the finest fraction of basaltic soils continue to increase in a steep, almost linear fashion. This compositional distinction is due to differences in the total amount of nanophase iron that accumulates in space weathering products. Such ground-truth information derived from the <10 m fraction of lunar soils provides valuable insight into optical properties to be expected in other space weathering environments such as the asteroids and Mercury.