The Chesapeake Bay impact structure is one of the largest and most well preserved impact structures on Earth. It has a unique morphology composed of an inner crater penetrating crystalline basement surrounded by a wider crater in the overlying sediments. In 2004, the U.S. Geological Survey conducted a seismic survey with the goals of constraining crater structure and in support of the drilling of a borehole into the deepest part of the crater. Travel-time and waveform inversion were applied to the data to produce a high-resolution velocity model of the crater. Low-fold reflection processing was also applied. Northeast of the crystalline crater, undeformed, eastward-sloping crystalline basement is ~1.5 km deep. The edge of the inner crater is at ~ 15 km radius and slopes gradually down to a depth of 1.5 - 1.8 km. A central peak of 4-5 km radius rises to a depth of ~0.8 km. Basement velocity in the crystalline crater is much lower than undeformed basement, which suggests ~10% fracturing of the crater floor, and up to 20% fracturing of the central uplift. A basement uplift and lateral change of velocity, interpreted as the edge of the transient crater, occurs at a radius of ~ 11 km. Assuming a 22 km diameter transient crater, scaling laws predict a ~30 km diameter crater and central peak diameter of 8-10 km. This indicates that post-impact collapse processes that created the ~ 30 km diameter crystalline crater were unaffected by the much weaker rheology of the overlying sediments. / Master of Science

abstract: Early spacecraft missions to Mars, including the Marnier and Viking orbiters and landers revealed a morphologically and compositionally diverse landscape that reshaped widely held views of Mars. More recent spacecraft including Mars Global Surveyor, Mars Odyssey, Mars Express, Mars Reconnaissance Orbiter, and the Mars Exploration Rovers have further refined, enhanced, and diversified our understanding of Mars. In this dissertation, I take a multiple-path approach to planetary and Mars science including data analysis and instrument development. First, I present several tools necessary to effectively use new, complex datasets by highlighting unique and innovative data processing techniques that allow for the regional to global scale comparison of multiple datasets. Second, I present three studies that characterize several processes on early Mars, where I identify a regional, compositionally distinct, in situ, stratigraphically significant layer in Ganges and Eos Chasmata that formed early in martian history. This layer represents a unique period in martian history where primitive mantle materials were emplaced over large sections of the martian surface. While I originally characterized this layer as an effusive lava flow, based on the newly identified regional or global extent of this layer, I find the only likely scenario for its emplacement is the ejecta deposit of the Borealis Basin forming impact event. I also re-examine high thermal inertia, flat-floored craters identified in Viking data and conclude they are typically more mafic than the surrounding plains and were likely infilled by primitive volcanic materials during, or shortly after the Late Heavy Bombardment. Furthermore, the only plausible source for these magmas is directly related to the impact process, where mantle decompression melting occurs as result of the removal of overlying material by the impactor. Finally, I developed a new laboratory microscopic emission and reflectance spectrometer designed to help improve the interpretation of current remote sensing or in situ data from planetary bodies. I present the design, implementation, calibration, system performance, and preliminary results of this instrument. This instrument is a strong candidate for the next generation in situ rover instruments designed to definitively assess sample mineralogy and petrology while preserving geologic context. / Dissertation/Thesis / Ph.D. Geological Sciences 2012

From the early stages of planetary accretion and differentiation to the geomorphology of planetary surfaces and the evolution of life, fluids play an integral role in shaping planetary bodies. Fluid properties and processes were investigated under a range of planetary conditions through (1) experimental simulations of impact events and petrographic analysis of terrestrial impactites to determine the effects of shock metamorphism on fluid inclusion properties; and (2) numerical thermodynamic equilibrium modeling of aqueous alteration processes on Mars. Results of impact experiments and analyses of fluid inclusions in rocks from the Ries Crater and Meteor Crater indicate that fluid inclusions reequilibrate systematically with increasing shock pressure: stretching and decrepitating under low shock pressure conditions and collapsing at moderate shock pressures. Above the Hugenoit Elastic Limit, fluid inclusion vesicles are destroyed due to plastic deformation and phase transitions within the host mineral. This suggests that impact processing may result in the destruction of fluid inclusions, leading to shock devolatilization of target rocks. In addition, the absence of fluid inclusions in planetary materials does not preclude the presence of fluids on the meteorite's parent body. Thermodynamic modeling of aqueous alteration of basalt under Mars-relevant conditions provides constraints on the conditions under which secondary sulfates are likely to have formed. The results of this study indicate that jarosite is likely to form as a result of water-limited chemical weathering of basalts. Magnesium sulfates are only predicted to form as a result of evaporation. This suggests that in order to form the alteration assemblages recently observed by the Mars Exploration Rover Opportunity at Meridiani Planum, water must have been removed from the system after a geologically short period of time, before fluids came into equilibrium with mafic surface materials and became alkaline. / Ph. D.

Les cratères d'impact hypervéloces sont les structures morphologiques les plus abondantes à la surface des corps planétaires telluriques du système solaire, sauf sur Terre où ils sont effacés par les processus de surface. La structure interne des cratères d'impact de type complexe ne peut être étudiée en détail que sur Terre par des études géophysiques et géologiques de terrain. De telles approches - combinées à de la modélisation - peuvent révéler comment le processus de cratérisation, la composition des roches cibles, l'érosion et d'autres processus post-impact peuvent conduire aux anomalies géophysiques observées, qui peuvent également être détectées par des données satellitaires sur d’autres planètes. La cartographie du champ magnétique, les mesures gravimétriques, les sondages électromagnétiques (EM34), les analyses paléomagnétiques, le magnétisme des roches et les techniques pétrographiques sont utilisées. Pour la première fois, nous révélons que la structure de Tunnunik récemment découverte présente des anomalies de gravité négative et de champ magnétique positif typiques, ce qui nous aide à reconsidérer l'étendue de la fracturation dans les roches cibles. La structure d’Haughton, moins érodée que Tunnunik, montre des signes d'une aimantation augmentée au centre de son soulèvement central, ce qui est causé par l’altération hydrothermale induite par l’impact. Le paléomagnétisme aide à contraindre les âges différents des deux impacts des lacs à l’eau claire au Québec. Ce travail a des implications importantes pour notre compréhension du processus de cratérisation dans le système solaire, notamment en ce qui concerne l'étude des surfaces planétaires. / Hypervelocity impact craters are the most abundant morphologic features on rocky planetary bodies of the solar system, except on Earth where they are erased by surface processes. The internal structure of complex impact craters can only be studied on Earth by using ground-truth geophysical and geological studies. Such approaches - combined with modeling - can reveal how impact cratering, target geological composition, erosion and other post-impact processes can lead to the observed geophysical anomalies, which could also be detected by remote geophysical data on other planetary surfaces. Magnetic field mapping, gravimetry measurements, electromagnetic soundings (EM34), paleomagnetic analyses, rock magnetism and petrography techniques are used. For the first time, we reveal that the recently-discovered Tunnunik impact structure has typical negative gravity and positive magnetic field anomalies, which help us to reconsider the brecciation extent in the target rocks. The Haughton crater, which is less eroded than Tunnunik, shows evidence for an enhanced-magnetization in the core of the central uplift, caused by impact-generated hydrothermal alteration. Paleomagnetism helps to constrain the different ages of the East and west clearwater lake impacts. This work has important implications for our understanding of impact-cratering in the solar system, especially concerning the study of planetary surfaces.