Spectral Indexing of Chemical Weathering in the Mid-infrared: New Means to Evaluate Weathering on Mars

Introduction: Spectroscopic observations offer the best means to evaluate chemical weathering on Mars at regional and global scales. Near-infrared (NIR) spectral data from OMEGA and CRISM have revealed hydrated minerals that indicate aqueous alteration, including different clay minerals [1-3], sulfates [1,4], silica [5], and carbonates [6], which are regionally localized across the Martian highlands. Mid-infrared (MIR) spectra from TES have indicated globally disseminated carbonate as a constituent of Martian dust [7]. TES spectra have shown the widespread occurrence of high-silica materials [8] thought to be related to aqueous alteration at high latitudes [9-12]. In addition , TES data have indicated that silicate alteration products may occur at low-but-significant levels at low latitudes [12], where variable olivine detection might be a sign of weathered regolith [13]. NIR data from OMEGA and CRISM and MIR data from TES and THEMIS are complementary. The NIR and MIR datasets seem, thus far, to afford generally consistent interpretations, though neither dataset is fully capable of observing what is seen in the other, owing to differences in what is measured between NIR and MIR wavelengths and how those spectral regions are affected by the physical and mineralogical conditions on the ground. It is necessary to employ both spectral regions to achieve a robust picture of Mars' alteration. Studying weathering in the MIR: The MIR offers particular challenges to investigating chemical weathering , but with these challenges come great possibilities as well. MIR fundamentally measures lattice vibrations and can be used to detect important silicate minerals not seen in NIR, most notably feldspars. Differences in remotely sensed igneous mineral assemblages may result from aqueous weathering, and MIR data are uniquely suited to investigate weathering by seeking out mineralogical variations in primary silicate phases. Additionally, MIR data are sensitive to silicate alteration minerals that can go undetected in NIR, for instance in rock weathering rinds and silica rock coatings [14,15]. Consequently, MIR data provide a unique and robust means of investigating aqueous alteration. Among the challenges, MIR observations of rocks and sand are adversely affected by dust deposition, temperature variations within a scene, and the cold temperatures often encountered on Mars. Yet the most significant challenge to chemical weathering studies in MIR is related to its strength—the vast majority of minerals that one attempts to measure are silicates.