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

The fine fraction of lunar soils (<45 <mu>m) dominates the optical properti es of the bulk soil. Definite trends can be seen in optical properties of s ize separates with decreasing particle size: diminished spectral contrast a nd a steeper continuum slope. These trends are related to space weathering processes and their affects on different size fractions. The finest fractio n (defined here as the <10 <mu>m fraction) appears to be enriched in weathe ring products relative to the larger size fractions, as would be expected f or surface correlated processes. This <10 <mu>m fraction tends to exhibit v ery little spectral contrast, often with no distinguishable ferrous iron ab sorption bands. Additionally, the finest fractions of highland soils are ob served to have very different spectral properties than the equivalent fract ion of mare soils when compared with larger size fractions. The spectra of the finest fraction of feldspathic soils flatten at longer wavelengths, whe reas those of the finest fraction of basaltic soils continue to increase in a steep, almost linear fashion. This compositional distinction is due to d ifferences in the total amount of nanophase iron that accumulates in space weathering products. Such ground-truth information derived from the <10 <mu >m fraction of lunar soils provides valuable insight into optical propertie s to be expected in other space weathering environments such as the asteroi ds and Mercury.