Strength of the H2O near-infrared absorption bands in hydrated minerals: Effects of measurement geometry

Abstract : Quantitative effects of incidence and emergence angles variations on the 1.9- and 3-μm H2O bands are experimentally investigated for two hydrated minerals: smectite and altered volcanic tuff. Near-infrared bidirectional reflectance spectra are measured with various incidence and emergence angles leading to phase angles varying between 10° and 140°. We observe a decrease of both band strengths when phase angle increases. The relationship between band strength and phase angle depends on both the absorption intensity, i.e., strong or weak band, and the particle size of the material. We compare experimental results with radiative transfer modeling with optical constants of a smectite. Numerical models can reproduce the changes in band strength with incidence and emergence angles if the value of the single particle scattering anisotropy varies over the absorption band. We attribute this wavelength-dependent scattering effect to enhanced surface scattering within the absorption bands compared to volume scattering outside the bands. Relative variations of band strength induced by measurement geometry variations can reach tens of percent at high phase angle. Therefore these effects have to be taken into account when mapping spectral parameters on a planetary surface or comparing spectral parameters between two surfaces if measurement geometries are subject to large variations.