37th DPS Meeting, 4-9 September 2005
Session 67 Laboratory Research
Oral, Friday, September 9, 2005, 2:00-3:30pm, Law LG19

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[67.03] Optical constants of Gypsum in the regions of weak absorptions: Comparisons of independent estimates.

T.L. Roush (NASA Ames Research Center), F. Esposito, L. Colangeli (INAF-Osservatorio Astronomico di Capodimonte), G.R. Rossman (California Institute of Technology)

Recent and ongoing remote observations indicate that sulfates are present in significant abundances at various locations on Mars. NASA's Mars Reconnaissance Orbiter Compact Reconnaissance Imaging Spectrometer for Mars (MRO-CRISM) will return hyperspectral data at high spatial resolution (~20 m/pixel). Thus, the optical properties of sulfates, in general, are of interest to quantitative interpretation of existing and future remotely sensed data. Here we focus upon gypsum due to: 1) its applicability to Mars; 2) its availability in sample sizes suitable for study allowing various spectral measurements; 3) existence of several independent infrared optical constants applicable to evaluate our approach; and 4) a general lack of data regarding the optical constants for gypsum at wavelengths where observations are being obtained and planned (0.3-5 um). Diffuse reflectance spectra of multiple grain size fractions and two independent scattering calculation approaches were used to determine the imaginary index of refraction (k) from the measured reflectance of gypsum. The k's resulting from the two scattering calculations compare favorably, and in some regions are similar to k-values of gypsum reported in the literature. We find that in the regions of maximum k-values, the scattering theory results provide a poor determination compared to more traditional techniques. However, we find that the scattering results are more sensitive when determining k for weak spectral features that are absent when using more traditional techniques. In the 0.4-2.5 um region, we compare the results obtained from the diffuse reflectance to results obtained from direct transmission measurements of a gypsum crystal. We find a very good comparison to the k-values derived from transmission, except where gypsum becomes transparent. In the transparent region discrepancies may arise from: 1) difference in samples; 2) increased effective path length in the particulate samples; 3) light scattering during transmission measurement; and 4) some other experimental effects.

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