LROC WAC ULTRAVIOLET CHARACTERIZATION OF LUNAR SPACE WEATHERING. B. W.

Introduction: The effects of space weathering as manifested in changes in ultraviolet (UV) reflectance have been observed in laboratory spectra of Apollo samples and samples subjected to simulated space weathering, as well as in comparisons of laboratory spectra of meteorites and telescopic spectra of asteroids [1]. In contrast to visible and near-infrared wavelengths, where increased space weathering causes an increase in spectral slope (“reddening”), at wavelengths shorter than ~400 nm the spectral slope decreases with increased space weathering. This phenomenon can be explained by the nanophase iron coatings deposited on grains due to exposure to the space weathering environment, where the addition of relatively spectrally neutral metallic iron diminishes the strong dropoff toward UV wavelengths that occurs in silicates. The decrease in UV slope with increased maturity has been hypothesized to occur at lower levels of exposure to the space weathering environment [1] than at longer wavelengths. Lunar Reconnaissance Orbiter Camera (LROC) Wide Angle Camera (WAC) [2] observations at 321, 360, 415, 566, 604, 643 and 689 nm enable resolved studies of the changes in ultraviolet reflectance that occur due to maturation of the lunar surface. Image data: This study utilizes a preliminary seven-band, 400-m/pixel global WAC mosaic that was photometrically normalized using a simplified Hapke function [3]. Photometric parameters were derived independently for each 5° by 5° surface bin using three months of WAC coverage [4]. Clementine-based maps of optical maturity (OMAT) as determined from 750 and 950 nm reflectance [5] were used for comparison. UV and maturity: To explore the effects of maturity in the UV, we examined ratios of 321/415 nm reflectance. In this index, variations due to maturity are relatively subdued, and many large ray systems are not easily distinguished from background mature terrain (Fig. 1). Closer to crater rims however, many fresh craters have a region (~1 crater diameter) of low 321/415 ratio. To examine the changes of UV slope with