Chemical Weathering Trends from Tir Spectral Models: Implications for Deriving Weathering Trends from Martian Spectral Data

Introduction: Chemical weathering and petrologic trends of the martian surface have been estimated from thermal infrared (TIR) spectral models from the Thermal Emission Spectrometer (TES) and Mini-TES on the Spirit and Opportunity rovers [1,2]. Interpretations of chemical data from APXS on the MER rovers and TIR data from TES and the Thermal Emission Imaging System (THEMIS) suggest planet-wide acidic chemical weathering with low water-to-rock ratios, causing the dissolution of olivine and the precipitation of Fe-oxides, Fe-sulfates, and amorphous silica [1,3]. TIR analysis of terrestrial chemically weathered basalts show that chemical weathering causes surface coatings that result in non-linear spectral mixing of rock and coating components [4-6]. If spectral modeling of weathered rocks produces inaccurate mineral models, bulk chemistry derived from those spectral models will also be inaccurate. The effects of chemical weathering on TIR spectral model-derived chemical trends are not well known. The chemical weathering trends in weathered basalts from Baynton, Australia have been well characterized [7,8]. We have collected weathered basalts from the Baynton region, and, here, we compare our spectral model-derived chemical weathering trends to measured chemical weathering trends. The goal of this study is to understand how chemical weathering affects TIR spectral model-derived chemistry and if accurate chemical weathering trends can be derived from TIR spectral models. Methods : Weathered basalt samples from Baynton, Australia were cut perpendicular to the weathering rind. TIR spectra of the weathered surfaces and the interiors were measured at the Mars Space Flight Facility at ASU, using a Nicolet Nexus spectrometer configured to measured emitted energy [9]. We modeled the TIR spectra using linear deconvolution [10] with two slightly different spectral libraries containing end-members common in weathered basalt: Library 1 is commonly used to model TES spectra [11]. Library 2 has more clay mineral end-members than Library 1 , including kaolinite, halloysite, nontronite, and chlorite. Mineral abundances derived from these models were used to calculate chemical compositions according to the procedure in [12]. The model-derived chemical compositions were compared those measured by Nesbitt and Wilson (1992) [8] to assess the accuracy of