Hydrated silica on Mars: Global comparison and in-depth analysis at Antoniadi Crater
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Hydrated silica is found in a variety of Martian deposits within suites of minerals that indicate aqueous alteration, mostly because hydrated silica forms easily in different environments. Because of its relative ubiquity on Mars, it makes a good tracer mineral to compare otherwise dissimilar deposits and relate their relative degrees of aqueous alteration. The first portion of this dissertation uses near-infrared and thermal-infrared spectroscopy to determine the relative degree of crystallinity of hydrated silica and the bulk SiO2 abundance of hydrated-silica-bearing surfaces. This analysis reveals that Martian hydrated silicas exhibit a range of crystalline structures, from noncrystalline (opal-A or hydrated glass) to crystalline (quartz), implying a range in the maturity of these silica deposits. However, bulk SiO2 abundances show less diversity, with most Martian silica deposits having relatively low SiO2 abundances, similar to a basaltic andesitic composition that exists over much of the surface and that suggests little interaction with water. The second portion of this dissertation focuses on one location that contains the only detected quartz on the planet. High-resolution satellite imagery and thermal and near-infrared spectroscopy is used to construct a geologic history of these deposits and their local context. The quartz-bearing deposits are consistently found co-located with hydrated silica, and this spatial coherence suggests that the quartz formed as a diagenetic product of amorphous silica, rather than as a primary igneous mineral. Diagenetic quartz is a mature alteration product of hydrated amorphous silica, and indicates more persistent water and/or higher temperatures at this site. There is also spectral evidence for smectites beneath the silica-bearing rocks, in the lowermost exposed Noachian-aged breccia. A similar stratigraphic sequence -- smectite-bearing breccias beneath deposits containing minerals indicating a greater degree of alteration -- has also been found at nearby exposures at Nili Fossae and Toro Crater, suggesting a widespread sequence of alteration. The two sections of this dissertation provide a global and in-depth view of Martian hydrated silica deposits, thereby broadening and refining our search for past water on Mars.