Quantitative compositional analysis of the lunar surface from reflectance spectroscopy: Iron, aluminum, and model for removing the optical effects of space weathering

Visible and near-infrared reflectance spectroscopy is widely recognized as an important tool for the compositional assessment of planetary surfaces and materials. However, in spite of numerous advances during the past two to three decades, geologic reflectance spectroscopy remains mostly a qualitative tool. Extensive, systematic analyses of lunar soils and remotely acquired data for the lunar surface were undertaken with the intent of creating the means to derive quantitative compositional information from lunar spectroscopic data. The spectral properties of lunar materials are altered as a result of exposure to space weathering processes (such as micrometeorite bombardment) at the lunar surface. Consequently, the spectral properties of lunar materials are a function both of composition and of exposure. The optical properties of two soils derived from identical lithologic sources will thus be markedly different if they have been exposed for different lengths of time. A ratio of the reflectance at a wavelength outside of the {dollar}1mu{lcub}rm m{rcub}{dollar} ferrous iron absorption band to the reflectance at a wavelength within the band can be used to determine the exposure state of highland soils. A mathematical model describing the evolution of lunar soil spectral properties due to exposure is presented. A procedure is developed whereby given a spectrum of a lunar material, the model can be used to alter the observed spectral properties to any desired exposure state. For example, the observed spectrum of a fresh crater can be altered to the exposure state of mature surrounding material for direct spectral comparison in the context of composition. Quantitative concentration data for the element iron can be derived for lunar highland materials from an accurate measurement of the {dollar}1mu{lcub}rm m{rcub}{dollar} absorption band depth. A combined analysis of Apollo X-ray spectrometer and Galileo Solid State Imaging system data reveals that the reflectance of a mature lunar soil can be used to estimate the aluminum content of the soil. Broad regions characterized by the mixing of mare and highland material (or of mafic primary basin ejecta and locally-derived highland material) are widespread on the lunar surface. Physical mixing by impact cratering can account for these zones.