Modeling near-infrared reflectance spectra of clay and sulfate mixtures and implications for Mars

High-resolution mapping by visible and near-infrared orbital spectrometers has revealed a diversity of hydrated mineral deposits on the surface of Mars. Quantitative analysis of mineral abundances within these deposits has the potential to distinguish depositional and diagenetic processes. Such analysis can also provide important constraints on the nature of putative global and local-scale mineralogical transitions on Mars. However, the ability of models to extract quantitative mineral abundances from spectra of mixtures relevant to sedimentary rocks remains largely untested. This is particularly true for clay and sul-fate minerals, which often occur as fine-grained components of terrestrial sedimentary rocks and are known to occur in a number of sedimentary deposits on Mars. This study examines the spectral properties of a suite of mixtures containing the Mg-sulfate epsomite mixed with varying proportions of smec-titic clay (saponite, nontronite, and montmorrilonite). The goal of this work is to test the ability of checkerboard (linear) and intimate (non-linear) mixing models to obtain accurate estimates of mineral abundances under ideal and controlled laboratory conditions. The results of this work suggest that: (1) spectra of clay–sulfate mixtures can be reproduced by checkerboard and intimate mixing models to within 2% absolute reflectance or single scattering albedo, (2) clay and epsomite abundance can be mod-eled to within 5 wt.% when particle diameter is optimized, and (3) the lower threshold for modeling clay in spectra of clay–epsomite mixtures is approximately 10 wt.%, below which the models often fail to recognize the presence of clay. Visible-near infrared (VNIR) reflectance spectroscopy can be a powerful tool for identifying ancient records of environmental change on Mars because of its sensitivity to minerals that represent water–rock interaction. Such minerals include, but are not limited to, carbonates, sulfates, and clay minerals. High-resolution mapping by visible and near-infrared orbital spectrometers has revealed a diversity of hydrated minerals on the surface of Mars, suggesting a complex history of aqueous alteration and mineral precipitation (e. (Murchie et al., 2007) orbital spectrometers have detected clay mineral-bearing deposits in the ancient Noachian regions of Maw-whereas detections of mono-and polyhydrated sulfates occur predominantly in light-toned layered deposits in Hesperian-aged regions such as Terra Meridiani, Margaritifer Sinus, and in deposits within and on the plains surrounding Valles Marineris (Poulet et al., 2005; Gendrin et al., 2005; Bibring et al., 2006). It is generally observed that clay-and sulfate-bearing terrains are spatially and likely temporally distinct on the surface of Mars, a distinction purported to …