COMPOSITION AND EXPOSURE AGE OF THE APOLLO-16 CAYLEY AND DESCARTES REGIONS FROM CLEMENTINE DATA - NORMALIZING THE OPTICAL EFFECTS OF SPACE WEATHERING

In order to quantitatively assess the composition of the lunar surface from reflectance spectroscopy, it is necessary to be able to differen tiate between the optical effects due to composition and those due to exposure to space weathering processes. Laboratory analyses of returne d lunar soils are used to show that limited quantitative compositional information and the exposure age of the lunar surface can be determin ed using straightforward calculation from remotely acquired reflectanc e data sampled with a small number of spectral channels. Exposure age, approximated by I-s/FeO (where I-s is the characteristic ferromagneti c signal from single-domain iron metal), is estimated using a ratio of the reflectance at a wavelength outside of the 1-mu m ferrous iron ab sorption band to the reflectance at a wavelength within the band. The iron plus titanium content of a soil can then be calculated from its I -s/FeO and reflectance. These combined techniques are applied to Cleme ntine UVVIS multispectral data of the lunar highlands in the vicinity of the Apollo 16 landing site. The I-s/FeO and FeO+TiO2 contents are c alculated for this region. Materials associated with recently formed i mpact craters and recent and/or on n; downslope movement are easily de tected as nonmature. The FeO+TiO2 content of the Descartes Mountains i s found to be 1-2 wt% less than the Cayley plains west of the Apollo 1 6 landing site. Localized areas of compositional heterogeneity associa ted with some small impact events are also revealed. A model for norma lizing the optical contribution due to exposure to space weathering pr ocesses is applied to the Clementine UVVIS data in order to bring the data to an equivalent mature exposure state. This approach allows resi dual spectral differences between the observed materials to be confide ntly interpreted as being due to compositional differences alone rathe r than to differences both of composition and of exposure. The exposur e-normalized data reveal that this region of the highlands is characte rized by limited large-scale compositional heterogeneity that is detec table by a small number of spectral bandpasse. The Descartes Mountains are identified as being more anorthodsitic than the Cayley plains uni ts. The Descartes Mountains are identified as 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.