MarsLab Investigation of the Spectral Signature of Gypsum Bearing Rocks of Differing Composition and Formation Environment

Introduction: Infrared spectroscopy has long been used to identify the presence of minerals in laboratory, satellite, and most recently field-based measurements. However, high fidelity compositional identification has largely been limited to laboratory based studies where the numerous mineralogical and textural factors that affect spectral signature can be identified and constrained. In this study, we used field based infrared spectroscopy to compare naturally formed gypsum bearing rocks of differing composition and formation environment to enhance our knowledge of the role of several mineralogical and textural factors in clouding high fidelity compositional identification. Infrared spectroscopy is a primary method used to explore the mineralogy of Mars remotely. The goals of this project, and ongoing MarsLab efforts, are: (1) define the types of materials that infrared airborne (satellite analog) and ground-based (rover analog) spectrometers identify and miss, and explain why; and (2) define implications for the exploration of Mars [1,2]. Formation Environments: We selected locations at Bristol Lake, a dry lake in the Mojave desert near Amboy, CA, and Alunite, an alunite rich outcrop ~15 miles SE of Las Vegas, NV. The Bristol Lake location typifies gypsum precipitation from saturated surface water. The gypsum bearing rocks form a compositional “bathtub ring” around the central area of halite bearing rocks that formed as different materials saturated and precipitated as the lake dried and shrank [3,4]. The gypsum deposits sampled at this location are mixed with silt and clay (Figs. 1,3A). The Alunite location is an example of gypsum precipitation from saturated hydrothermal fluid. Gypsum is found in veins cutting through the host rock. This unit is exposed in a small railroad cut (Figs. 2,3B). There is a curiously high concentration of alunite present surrounding the outcrop, however, this was not seen spectroscopically or visually in the rail-cut itself. Heavens et al. further investigates discrepancies between infrared spectroscopic identifications and energy-dispersive X-ray spectroscopy (EDXS) mineralogy at this site [2]. Data: The 2003 rover MiniTES (~6-25 μm) measures thermal infrared, hyperspectral images similar to the MarsLab instruments RamVan and Tonka (~7.5-12.5 μm, 181 bands) [6]. Figure 1: Bristol Lake Site. Gypsum bearing rock exposed as slightly brighter regions, darker regions are silty. Three aluminum targets used for downwelling radiance correction are approximately 1 x 1 meter [5].