Composition and exposure age of the Apollo 16 Cayley and Descartes regions from Clementine data: Normalizing the optical effects of space weathering

Abstract

In order to quantitatively assess the composition of the lunar surface from reflectance spectroscopy, it is necessary to be able to differentiate between the optical effects due to composition and those due to exposure to space weathering processes. Laboratory analyses of returned lunar soils are used to show that limited quantitative compositional information and the exposure age of the lunar surface can be determined using straightforward calculations from remotely acquired reflectance data sampled with a small number of spectral channels. Exposure age, approximated by Is/FeO (where Is is the characteristic ferromagnetic signal from single‐domain iron metal), is estimated using a ratio of the reflectance at a wavelength outside of the 1‐μm ferrous iron absorption band to the reflectance at a wavelength within the band. The iron plus titanium content of a soil can then be calculated from its Is/FeO and reflectance. These combined techniques are applied to Clementine UVVIS multispectral data of the lunar highlands in the vicinity of the Apollo 16 landing site. The Is/FeO and FeO + TiO2 contents are calculated for this region. Materials associated with recently formed impact craters and recent and/or ongoing downslope movement are easily detected as nonmature. The FeO + TiO2 content of the Descartes Mountains is found to be 1–2 wt% less than the Cayley plains west of the Apollo 16 landing site. Localized areas of compositional heterogeneity associated with some small impact events are also revealed. A model for normalizing the optical contribution due to exposure to space weathering processes is applied to the Clementine UVVIS data in order to bring the data to an equivalent mature exposure state. This approach allows residual spectral differences between the observed materials to be confidently interpreted as being due to compositional differences alone rather than to differences both of composition and of exposure. The exposure‐normalized data reveal that this region of the highlands is characterized by limited large‐scale compositional heterogeneity that is detectable by a small number of spectral bandpasses. The Descartes Mountains are identified as being more anorthositic than the Cayley plains units. Most small impacts in the region excavated material compositionally similar to material surrounding them, with the notable exceptions of the impacts that created South Ray Crater and an unnamed crater on the floor of Abulfeda.