Near-infrared reflectance spectroscopy of bulk analog materials for planetary crust

In the present study, the differences in near-infrared (NIR) reflectance spectra of powders and bulk surfaces of basalt and pyroxene samples were examined. Furthermore, the effect of a variation in the roughness of the bulk surfaces on the spectra was investigated. The study revealed significant differences n the spectra of powders and bulk surfaces, as observed previously. The most important differences concern the NIR continuum slope and the band depths of the pyroxene absorption bands I (located close to 1 μm) and II (located close to 2 μm). Compared to powders, the spectra of bulk surfaces are characterized by lower band depths and a preference for a blue-sloped continuum. The effects on bulk surface spectra produced by a variation of the roughness are similar to the well-known effects that arise from a variation of the grain size in powder spectra. Detailed chemical and physical analyses of the mineral and rock samples have enabled a better understanding of the spectral properties than in past studies. A simple model of lateral light passage through bulk surfaces is proposed to explain this analogy between roughness and grain size. The results of this study have implications for the spectroscopic laboratory investigation of analog materials as well as for the interpretation of reflectance spectra acquired in remote sensing situations.