Orbital Evidence for Clay and Acidic Sulfate Assemblages on Mars Based on Mineralogical Analogs from Rio Tinto, Spain

1Hannah H. Kaplan, 1Ralph E. Milliken, 2David Fernández-Remolar, 3Ricardo Amils, 1Kevin Robertson, 4Andrew H. Knoll
1Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI, 02912 USA
2British Geological Survey, Nicker Hill, Keyworth, NG12 5GG, UK
3Centro de Astrobiologia (INTA-CSIC), Ctra Ajalvir km 4, Torrejon de Ardoz, 28850, Spain
4Department of Organismic and Evolutionary Biology, Harvard University, Çambridge, MA, USA

Outcrops of hydrated minerals are widespread across the surface of Mars, with clay minerals and sulfates being commonly identified phases. Orbitally-based reflectance spectra are often used to classify these hydrated components in terms of a single mineralogy, although most surfaces likely contain multiple minerals that have the potential to record local geochemical conditions and processes. Reflectance spectra for previously identified deposits in Ius and Melas Chasma within the Valles Marineris, Mars, exhibit an enigmatic feature with two distinct absorptions between 2.2 – 2.3 µm. This spectral ‘doublet’ feature is proposed to result from a mixture of hydrated minerals, although the identity of the minerals has remained ambiguous. Here we demonstrate that similar spectral doublet features are observed in airborne, field, and laboratory reflectance spectra of rock and sediment samples from Rio Tinto, Spain. Combined visible-near infrared reflectance spectra and X-ray diffraction measurements of these samples reveals that the doublet feature arises from a mixture of Al-phyllosilicate (illite or muscovite) and jarosite. Analyses of orbital data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) shows that the martian spectral equivalents are also consistent with mixtures of Al-phyllosilicates and jarosite, where the Al-phyllosilicate may also include kaolinite and/or halloysite. A case study for a region within Ius Chasma demonstrates that the relative proportions of the Al-phyllosilicate(s) and jarosite vary within one stratigraphic unit as well as between stratigraphic units. The former observation suggests that the jarosite may be a diagenetic (authigenic) product and thus indicative of local pH and redox conditions, whereas the latter observation may be consistent with variations in sediment flux and/or fluid chemistry during sediment deposition.

Reference
Kaplan HH, Milliken RE, Fernández-Remolar D, Amils R, Robertson K, Knoll AH (2016)
Orbital Evidence for Clay and Acidic Sulfate Assemblages on Mars Based on Mineralogical Analogs from Rio Tinto, Spain. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2016.03.019]
Copyright Elsevier

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