Browsing by Author "Silver, Eli A."
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Item Characterization of phyllosilicates observed in the central Mawrth Vallis region, Mars, their potential formational processes, and implications for past climate(2009) McKeown, Nancy K.; Bishop, Janice L.; Noe Dobrea, Eldar Z.; Ehlmann, Bethany L.; Parente, Mario; Mustard, John F.; Murchie, Scott L.; Swayze, Gregg A.; Bibring, Jean-Pierre; Silver, Eli A.Mawrth Vallis contains one of the largest exposures of phyllosilicates on Mars. Nontronite, montmorillonite, kaolinite, and hydrated silica have been identified throughout the region using data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). In addition, saponite has been identified in one observation within a crater. These individual minerals are identified and distinguished by features at 1.38-1.42, approximately 1.91, and 2.17-2.41 mu m. There are two main phyllosilicate units in the Mawrth Vallis region. The lowermost unit is nontronite bearing, unconformably overlain by an Al-phyllosilicate unit containing montmorillonite plus hydrated silica, with a thin layer of kaolinite plus hydrated silica at the top of the unit. These two units are draped by a spectrally unremarkable capping unit. Smectites generally form in neutral to alkaline environments, while kaolinite and hydrated silica typically form in slightly acidic conditions; thus, the observed phyllosilicates may reflect a change in aqueous chemistry. Spectra retrieved near the boundary between the nontronite and Al-phyllosilicate units exhibit a strong positive slope from 1 to 2 mu m, likely from a ferrous component within the rock. This ferrous component indicates either rapid deposition in an oxidizing environment or reducing conditions. Formation of each of the phyllosilicate minerals identified requires liquid water, thus indicating a regional wet period in the Noachian when these units formed. The two main phyllosilicate units may be extensive layers of altered volcanic ash. Other potential formational processes include sediment deposition into a marine or lacustrine basin or pedogenesis.Item Interpretation of reflectance spectra of clay mineral-silica mixtures: implications for Martian clay mineralogy at Mawrth Vallis(2011) McKeown, Nancy K.; Bishop, Janice L.; Cuadros, Javier; Hillier, Stephen; Amador, Elena; Makarewicz, Heather D.; Parente, Mario; Silver, Eli A.The Al-clay-rich rock units at Mawrth Vallis, Mars, have been identified as mixtures of multiple components based on their spectral reflectance properties and the known spectral character of pure clay minerals. In particular, the spectral characteristics associated with the approximately 2.2 mu m feature in Martian reflectance spectra indicate that mixtures of AlOH- and SiOH-bearing minerals are present. The present study investigated the spectral reflectance properties of the following binary mixtures to aid in the interpretation of remotely acquired reflectance spectra of rocks at Mawrth Vallis: kaolinite-opal-A, kaolinite-montmorillonite, montmorillonite-obsidian, montmorillonite-hydrated silica (opal), and glass-illite-smectite (where glass was hydrothermally altered to mixed-layer illite-smectite). The best spectral matches with Martian data from the present study's laboratory experiments are mixtures of montmorillonite and obsidian having approximately 50% montmorillonite or mixtures of kaolinite and montmorillonite with approximately 30% kaolinite. For both of these mixtures the maximum inflection point on the long wavelength side of the 2.21 mu m absorption feature is shifted to longer wavelengths, and in the case of the kaolinite-montmorillonite mixtures the 2.17 mu m absorption found in kaolinite is of similar relative magnitude to that feature as observed in CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) data. The reflectance spectra of clay mixed with opal and of hydrothermally altered glass-illite-smectite did not represent the Martian spectra observed in this region as well. A spectral comparison of linear vs. intimate mixtures of kaolinite and montmorillonite indicated that for these sieved samples the intimate mixtures are very similar to the linear mixtures with the exception of the altered glass-illite-smectite samples. However, the 2.17 mu m kaolinite absorption is stronger in the intimate mixtures than in the equivalent linear mixture. Modified Gaussian modeling of absorption features observed in reflectance spectra of the kaolinite-montmorillonite mixtures indicated a strong correlation between percent kaolinite in the mixture and the ratio of the area of the 2.16 mu m band found in kaolinite to the area of the 2.20 mu m band found in montmorillonite.Item Variability of rock texture and morphology correlated with the clay-bearing units at Mawrth Vallis, Mars(2013) McKeown, Nancy K.; Bishop, Janice L.; Silver, Eli A.The clay units at Mawrth Vallis have been well-characterized in hyperspectral data; however, a similar study of high spatial resolution High Resolution Imaging Science Experiment (HiRISE) data has not been previously conducted. Here the textures of the clay units are described and related to mineralogy across the central Mawrth Vallis region. The nontronite-bearing rocks appear tan in HiRISE COLOR data and are polygonally fractured with polygons 2–5m across. In some cases, the fractures appear wider and/or have darker fill or the rocks are a darker brown. The montmorillonite-bearing rocks appear blue with regular polygons 0.5–1.5m across; sometimes, there are larger polygons surrounded by regular polygons, a square fracture pattern, or the color appears yellow or mottled blue-yellow. Kaolinite-rich rocks are the brightest outcrops and are nonpolygonally fractured. Regions with spectra consistent with hydrated silica or the ferrous mineral component do not have unique textures. Hydrated silica-bearing rocks appear yellow or mottled with a regular polygonal texture or yellow with hummocky appearance with no polygons. It is also possible that dust/sand on the surface alters the montmorillonite spectrum to appear like that of hydrated silica. The ferrous component may be expressed as mottled coloring or as a bright fracture fill. The nontronite- and montmorillonite-bearing units have remarkably consistent textures in this region, allowing them to be uniquely identified in the Mawrth Vallis region in nonhyperspectral data sets such as CTX and HiRISE. The morphology of the polygons in these two units suggests that their formation is likely dominated by desiccation and controlled by composition.