Backward Modeling of Reflectance Spectra of Single Mineral Assemblages

Abstract:

Hapke’s radiative transfer theory provides a complete framework for modeling reflectance spectra of minerals and intimate mixtures of minerals, as those forming asteroidal regoliths. Deconvolving reflectance spectra of intimate mineral mixtures to determine grain size and abundance of each component mineral is crucial to study the mineralogy of asteroids. According to Hapke’s theory, reflectance can be calculated provided measurement geometry, phase function, grain size, internal scattering coefficient and the wavelength-dependent optical constants (real and imaginary parts of refraction index) are known. In practice, however, this information is barely known and/or cannot be independently obtained with sufficient accuracy. Backward modeling of mineral radiative transfer properties from reflectance spectra of single mineral assemblages can be a solution to this problem. In this work, we performed simulated annealing fits to visible/near-infrared reflectance spectra of single-mineral assemblages of varying nominal grain sizes to retrieve optical constants, phase function, internal scattering coefficient and grain size. Our results show that considering a grain size distribution has a major impact in improving fit quality. Other factors affecting fit results will also be identified and discussed.