AIRLESS SOLAR SYSTEM BODIES: IMPROVEMENTS TO RADIATIVE TRANSFER MODELING OF REFLECTANCE SPECTRA

In order to improve modeling of planetary reflectance spectra, we have undertaken a project to make new measurements of the optical constants of iron over the entire wavelength range of interest to planetary reflectance spectrometry. Reflectance spectrometry across the ultraviolet, visible and near-infrared spectrum has long been a powerful tool for remotely obtaining information on the surface composition of airless Solar System bodies. The optical constants (real and imaginary parts of the index of refraction) are required as inputs to theoretical radiative transfer models (such as the model by Hapke and by Shkuratov) that permit extraction of quantitative compositional information from reflectance spectra. Metallic iron is a key component in the lunar regolith because it can be present in both coarse-grained and nanophase forms; this is also likely to be the case on asteroids and there is evidence that nanophase iron is present in Mercury's regolith. Nanophase iron, produced by reduction of ferrous iron during space weathering, has a strong effect on the optical properties of the regolith. The optical constants we determine, using modern variable-angle spectroscopic ellipsometers, will be used in conjunction with an improved treatment of the effects of metal in our Hapke theoretical model. In addition, we are seeking to develop better handling of lunar agglutinates, which are complex aggregates of mineral, glass and lithic fragments bonded together by impact melt glass. Together, these improvements will allow more accurate compositional and resource mapping with new datasets collected by spacecraft instruments such as the Chandrayaan-1 Moon Mineralogy Mapper, Kaguya Spectral Profiler, MESSENGER Mercury Atmosphere and Surface Composition Spectrometer, and to be returned by the Dawn Visible and InfraRed mapping spectrometer.