Optical constants of synthetic potassium, sodium, and hydronium jarosite

Estimating mineral abundances of clay and gypsum mixtures using radiative transfer models applied to visible-near infrared reflectance spectra

“…The real indices of refraction can vary over the wavelength range (0.8 μm-2.5 μm) used in this study, but we adopt constant values of n = 1.51 for gypsum and n = 1.52 for montmorillonite, which are averages based on estimates from Roush (2005 ) and Roush et al (2007 ). This is in contrast to the wavelength-dependent Kramers-Kronig solutions for n used in other studies ( Roush, 2005;Roush et al, 2007;Sklute et al, 2015 ), which find that n may vary by up to 0.03 (e.g., Roush et al, 2007 ). However, for this study we make the assumptions that such differences are negligible and adopt a constant value over the wavelength range of interest.…”

“…We note that our implementation of the Hapke model is largely similar to that of Roush et al (2007) and Sklute et al (2015) , with the exception that we estimate the phase function by using fixed values for the Legendre polynomial coefficients ( b = -0.4 and c = 0.25, based on values for silicates from Mustard and Pieters (1989) and consistent with moderate forward scattering). As a test of the sensitivity to these values, we varied the b and c parameters by 0.3 and found that it changed the single scattering albedo values by only 1% (relative) and ultimately had a negligible effect on the modeling results.…”

“…In addition, we do not include the additional variable of the internal scattering coefficient, s , in the internal transmission factor because appropriate values for this variable are poorly constrained for different minerals and may be system-dependent (e.g., Roush et al, 2007;Sklute et al, 2015 ). Omission of the internal scattering coefficient typically results in a lower single scattering albedo, however the particles in our study may be considered optically thin (i.e.…”

“…RTMs are also advantageous in that they can provide estimates of the abundance and particle size of each component in a mixture. However, as noted in previous studies (e.g., Sklute et al, 2015 ), widespread application of these models to spectra of planetary surfaces has been somewhat limited, primarily due to the lack of accurate optical constants (real, n, and imaginary, k, components of the complex index of refraction) for appropriate minerals, which are required inputs to the models. This is particularly true for clay and sulfate minerals, and though optical constants at VIS-NIR wavelengths have been reported for montmorillonite ( Roush, 2005 ), gypsum ( Roush et al, 2007 ), bloedite, epsomite, hexahydrite ( Dalton and Pitman, 2012 ), and select Fe-sulfates ( Pitman et al, 2014;Sklute et al, 2015 ), they are not readily available for many other hydrated minerals that are of importance to Mars (e.g., nontronite, saponite, chlorite, bassanite, kieserite, etc.…”

End‐member identification and spectral mixture analysis of CRISM hyperspectral data: A case study on southwest Melas Chasma, Mars

“…The single‐scattering albedo is dependent on grain size and the optical constants (the real and imaginary indices of refraction n and k ), which define a mineral's optical properties. To make the spectral library of the single‐scattering albedos, mineral optical constants were first obtained using the Shkuratov radiative transfer model [ Shkuratov et al ., ; Martone and Glotch , ] or the Hapke radiative transfer model [ Sklute et al ., ]. Single‐scattering albedos were then calculated based on the Hapke model for a given grain size using the following equation: where S e is the reflectivity for the external incident light and is a function of both n and k : and S i is the reflectivity for the internal incident light and is expressed as …”