Characterizing phyllosilicate distribution, abundance, and origin on Mars

We present a method for mapping exposures of Fe/Mg-rich phyllosilicates on the Martian surface using THEMIS daytime multispectral thermal infrared images. The primary utility of this technique comes from the fact that THEMIS coverage is nearglobal at 100 m/pixel, so it offers the opportunity to extend and improve upon phyllosilicate maps created using CRISM and/or OMEGA near-infrared data alone. The spectral index formulation used for this mapping consists of a sum of spectral band ratios, weighted in different proportions. An initial formulation of the index was determined using a suite of synthetic spectra containing varying proportions of a Martian basalt spectrum and a laboratory phyllosilicate spectrum. A scene-specific formulation was then optimized by exploiting the correlation between pixels the initial formulation flags in each THEMIS scene as containing phyllosilicates and those identified in overlapping CRISM data. Once the optimal formulation of the index is determined, it is applied to the entire THEMIS scene, extending the map of phyllosilicates beyond the area of overlapping CRISM coverage. The method is illustrated using 12 THEMIS scenes from the Mawrth Vallis and Nili Fossae regions. The new THEMIS-derived maps of Fe/Mgrich phyllosilicates in these two regions have approximately double the coverage provided by CRISM data. The cross-wavelength and cross-instrument approach we use to derive phyllosilicate indices for THEMIS data could potentially be used to define 18 spectral indices for mapping other compositions of interest on Mars that possess spectral features in more than one spectral region. 1.0 INTRODUCTION In the past three decades, missions to Mars have revealed surface compositions through in situ and remotely sensed observations. Because liquid water is unstable in the current Martian surface temperature and pressure regime, the spectral identification of hydrated minerals has been a key piece of evidence for the presence of liquid water on the surface of Mars early in its history. The implications of such minerals for past climate conditions on Mars are not well constrained: was liquid water stable at the surface on geologic timescales, or was it ephemeral? Was persistent water interaction a widespread phenomenon, or was the extent of this activity limited spatially? The nature, abundance, and distribution of hydrated minerals, such as clay minerals, preserve evidence for the conditions that existed in the ancient Martian environment. The identification and mapping of these minerals allows us to better understand the geologic context of their formation (e.g., in long-standing surface water, or hydrothermal alteration in the subsurface) and reveals whether formation processes were localized or global. Furthermore, terrestrial clay minerals are known to have long crustal residence times, low permeability upon burial, and binding properties that can trap organic molecules to charged surfaces, important for the preservation of biosignatures [e.g., Farmer and Des Marais, 1999], so understanding their distribution on Mars can guide future exploration of sites where potential biosignatures may be preserved. Generally, clay minerals (a hydrous aluminum-bearing subset of phyllosilicates, generally <2 ! m) result from