Investigation of the Spectral Properties of Martian Chloride-bearing Deposits as Observed by CRISM and THEMIS
Jensen, Heidi Bundgaard
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The initial identification of proposed chloride-bearing deposits on Mars by the Mars Odyssey Thermal Emission Imaging System (THEMIS) inspired a series of remote sensing studies investigating its geological context and morphology. This thesis focuses on spectral properties observed in remote sensing data for proposed chloride-bearing deposits. In Chapter 2, the issue of connecting the near infrared (NIR) spectral character of these deposits to chloride minerals is approached. In Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) near infrared (1.1 - 2.5 ??m) reflectance data for these deposits, a red slope (increase in reflectance with increasing wavelength) is observed in spectral ratios where the chloride-bearing region spectrum is divided by a spectrally neutral pyroxene-bearing region spectrum. This spectral character is inconsistent with the NIR spectral character of chloride minerals observed in the laboratory. Whereas the lack of features present in this wavelength region supports the hypothesis that these deposits are chloride-bearing, this spectral character is observed for unoxidized sulfide minerals and homopolar compounds (such as diamond) as well. NIR spectra of 1) simple mixtures of halite with labradorite and flood basalt and 2) halite crusts formed on labradorite and flood basalt were divided by spectra of pure labradorite or flood basalt, to see if the featureless red slope observed in CRISM ratio spectra could be emulated. These mixtures were formed with a variety of grain sizes in order to see if particle size has any effect on the ratio spectral shape. In order to rule out the possibility of unoxidized sulfides, acid washed pyrite was also mixed with labradorite and flood basalt of varying grain sizes, to see if the ratio spectra of these mixtures to pure labradorite/ flood basalt could also imitate the remote sensing spectral character. The results of this study allow unoxidized sulfide to be ruled out spectrally, and give some constraints on the composition and grain size of the chloride mixtures/ crusts that make up the Martian chloride-bearing deposits. Chlorides were originally discovered based on their unusual thermal infrared (TIR) spectral properties, characterized by featureless blue sloping (decrease in emissivity with increasing wavelength) spectra with a higher emissivity between 1267.62 and 850.846 cm-1 than the surrounding basaltic materials. Variations in the strength of the chloride unit spectral slope variations have not been previously investigated. Of particular interest is to identify potential factors resulting in chloride unit spectral slope variation that are not due to the mineralogy of the surface. The two factors investigated in this study are the characteristics of the TES training region used to manually correct for the atmosphere in the THEMIS data and the THEMIS image collection conditions. The result of this work indicates a weak correlation between chloride unit spectral slope and the minimum surface emissivity of the TES training region. Image acquisition conditions do not result in large differences in spectral character, but the quality of the atmospheric correction likely does influence the strength of the chloride unit spectral slope. Until chloride spectral variation that is not the product of surface mineralogy can be fully controlled, the spectral properties cannot be used to estimate characteristics such as the composition or chloride abundance.