Matching Items (20)

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The effect of Rayleigh-Taylor instabilities on the thickness of undifferentiated crust on Kuiper Belt objects like Charon

Description

In this thesis I model the thermal and structural evolution of Kuiper Belt Objects (KBOs) and explore their ability to retain undifferentiated crusts of rock and ice over geologic timescales. Previous calculations by Desch et al. (2009) predicted that initially

In this thesis I model the thermal and structural evolution of Kuiper Belt Objects (KBOs) and explore their ability to retain undifferentiated crusts of rock and ice over geologic timescales. Previous calculations by Desch et al. (2009) predicted that initially homogenous KBOs comparable in size to Charon (R ~ 600 km) have surfaces too cold to permit the separation of rock and ice, and should always retain thick (~ 85 km) crusts, despite the partial differentiation of rock and ice inside the body. The retention of a thermally insulating, undifferentiated crust is favorable to the maintenance of subsurface liquid and potentially cryovolcanism on the KBO surface. A potential objection to these models is that the dense crust of rock and ice overlying an ice mantle represents a gravitationally unstable configuration that should overturn by Rayleigh-Taylor (RT) instabilities. I have calculated the growth rate of RT instabilities at the ice-crust interface, including the effect of rock on the viscosity. I have identified a critical ice viscosity for the instability to grow significantly over the age of the solar system. I have calculated the viscosity as a function of temperature for conditions relevant to marginal instability. I find that RT instabilities on a Charon-sized KBO require temperatures T > 143 K. Including this effect in thermal evolution models of KBOs, I find that the undifferentiated crust on KBOs is thinner than previously calculated, only ~ 50 km. While thinner, this crustal thickness is still significant, representing ~ 25% of the KBO mass, and helps to maintain subsurface liquid throughout most of the KBO's history.

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Date Created
2013

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The ancient rocky surfaces of Mars: analysis of spacecraft data and the development of laboratory instrumentation

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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

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.

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Date Created
2012

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Shock Effects and Mineral Assemblages in the Genomict Eucrite Northwest Africa 8677

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Shock effects in meteorites provide important insights into impacts on their parent bodies. Eucrites are among the Howardite-Eucrite-Diogenite (HED) class of achondrites that likely originate from the intact, differentiated asteroid Vesta. Brecciated eucrites provide a record of the impact processes

Shock effects in meteorites provide important insights into impacts on their parent bodies. Eucrites are among the Howardite-Eucrite-Diogenite (HED) class of achondrites that likely originate from the intact, differentiated asteroid Vesta. Brecciated eucrites provide a record of the impact processes that occurred after the crustal formation of the parent body. Radiometric dating of HEDs has shown that they were affected by resetting events at 3.4 – 4.1 and 4.48 Ga. Therefore, shock effects in HEDs are windows into ancient impacts on asteroids early in solar system history. Northwest Africa (NWA) 8677 is a genomict eucrite with lithologies that are texturally different, but compositionally similar. The clasts in the breccia include strongly shocked (S5) gabbroic fragments and weakly shocked (S3) basaltic clasts. Coesite, a high-pressure polymorph of quartz, is preserved in the core of a large (~250 μm) silica grain, indicating the gabbro was strongly shocked. A large thermal overprint from the surrounding melt resulted in the transformation of coesite to low-pressure silica phases of quartz and cristobalite on the rims of this grain. The shock melt, interstitial to the breccia fragments, exhibits well-developed quench textures and contains a low-pressure mineral assemblage of plagioclase and pyroxene, implying that crystallization occurred after pressure release. The heterogeneity in shock features between the gabbroic and basaltic lithologies suggests that NWA 8677 experienced a variable impact history, which included at least two impact events. An initial impact strongly shocked and brecciated the gabbro and ejected both onto the regolith of the parent body where a more weakly shocked basalt was incorporated. A second impact produced the interstitial melt between the breccia matrix. The temperature of this shock melt remained high after pressure release, resulting in crystallization of a low-pressure assemblage of pyroxene and feldspar, as well as the transformation of quartz + cristobalite rims on coesite

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2019-05

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Comparative evolution of the Shyok and Yarlung suture zones: implications for the collision Between India and Eurasia

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The collision between the Indian and Eurasian tectonic plates marked the onset of the rise of the Himalayan-Tibetan orogen, but also brought about profound changes to the Earth's oceans and climate. The exact sequence of events that occurred during this

The collision between the Indian and Eurasian tectonic plates marked the onset of the rise of the Himalayan-Tibetan orogen, but also brought about profound changes to the Earth's oceans and climate. The exact sequence of events that occurred during this collision is poorly understood, leading to a wide range of estimates of its age. The Indus and Yarlung sutures are generally considered to represent the final collision between India and Eurasia, and together form a mostly continuous belt that can be traced over 2000 km along strike. In the western portions of the orogen the Karakoram Fault introduces a key complexity to the study of timing of collision by offsetting the Indus and Yarlung sutures. Recent work has used the complexities introduced by the Karakoram Fault to suggest that the more northerly Shyok suture, not the Indus suture, represents the India-Eurasia collision zone. Estimates for timing of the India-Eurasia collision fall into one of three groups: 40-34 Ma, 55-50 Ma, and 66-60 Ma. Attempts to reconcile these models have thus far been unsuccessful. In order to provide additional data that might further clarify the timing and location of collision, studies have been performed along the Shyok suture in India and along the Yarlung suture in Tibet at Sangsang. A study along the Shyok suture argues that the suture formed between 92-85 Ma. This timing precludes an interpretation that the Shyok suture marks the location of the India-Eurasia collision. A second study demonstrates the utility of two new geochronometers, (U-Th)/Pb joaquinite and 40Ar/39Ar neptunite, that play an important role in unraveling the tectonic history of the Yarlung suture. A third study is an investigation of the structure and geochronology of the Sangsang ophiolite complex. Here, multiple (U-Th)/Pb and 40Ar/39Ar systems record magmatism and metamorphism spanning ca. 125-52 Ma. By tying these chronometers to tectonic process, a history is reconstructed of the southern margin of Tibet that includes Early Cretaceous to Late Cretaceous forearc rifting associated with mid ocean ridge subduction, Paleocene accretionary wedge uplift and erosion, and finally Eocene metasomatism and collision.

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Date Created
2016

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An accessible architecture for affordable access to space

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A design methodology for a new breed of launch vehicle capable of lofting small satellites to orbit is discussed. The growing need for such a rocket is great: the United States has no capabilities in place to quickly launch and

A design methodology for a new breed of launch vehicle capable of lofting small satellites to orbit is discussed. The growing need for such a rocket is great: the United States has no capabilities in place to quickly launch and reconstitute satellite constellations. A loss of just one satellite, natural or induced, could significantly degrade or entirely eliminate critical space-based assets which would need to be quickly replaced. Furthermore a rocket capable of meeting the requirements for operationally responsive space missions would be an ideal launch platform for small commercial satellites. The proposed architecture to alleviate this lack of an affordable dedicated small-satellite launch vehicle relies upon a combination of expendable medium-range military surplus solid rocket motor assets. The dissertation discusses in detail the current operational capabilities of these military boosters and provides an outline for necessary refurbishments required to successfully place a small payload in orbit. A custom 3DOF trajectory script is used to evaluate the performance of these designs. Concurrently, a parametric cost-mass-performance response surface methodology is employed as an optimization tool to minimize life cycle costs of the proposed vehicles. This optimization scheme is centered on reducing life cycle costs per payload mass delivered rather than raw performance increases. Lastly, a novel upper-stage engine configuration using Hydroxlammonium Nitrate (HAN) is introduced and experimentally static test fired to illustrate the inherent simplicity and high performance of this high density, nontoxic propellant. The motor was operated in both pulse and small duration tests using a newly developed proprietary mixture that is hypergolic with HAN upon contact. This new propellant is demonstrated as a favorable replacement for current space vehicles relying on the heritage use of hydrazine. The end result is a preliminary design of a vehicle built from demilitarized booster assets that complements, rather than replaces, traditional space launch vehicles. This dissertation proves that such capabilities exist and more importantly that the resulting architecture can serve as a viable platform for immediate and affordable access to low Earth orbit.

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Date Created
2011

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Using array seismology to study planetary interiors

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Stratification is a dominant feature of all planetary interiors. Fine-scale structure associated with layering, as well as heterogeneities hold important clues on a planet's compositional, thermal, and dynamical state, as well as its evolution. This research centers on using data

Stratification is a dominant feature of all planetary interiors. Fine-scale structure associated with layering, as well as heterogeneities hold important clues on a planet's compositional, thermal, and dynamical state, as well as its evolution. This research centers on using data from seismic arrays, networks of seismic sensors, and array processing methodologies to map the fine scale structure in the Earth's upper mantle and deep layering in the Moon - Earth and Moon are the only two planetary bodies with seismic available data for such analyses. Small-scale structure in the Earth's upper mantle can give rise to seismic wave scattering. I studied high frequency data from the Warramunga Array in Australia using array seismology. I developed and employed back-projection schemes to map the possible upper mantle scattering or reflection locations. Mapped scatterers show good correlation to strong lateral P-wave velocity gradients in tomography models and may be associated with the complex tectonic history beneath north of Australia. The minimum scale of scatterers relates to the seismic wavelength, which is roughly between 5 and 10 km in the upper mantle for the frequencies we study. The Apollo Passive Seismic Experiment (APSE) consisted of four 3-component seismometers deployed between 1969 and 1972 that continuously recorded lunar ground motion until late 1977. I studied the deep lunar interior with array methods applied to the legacy APSE dataset. The stack results suggest the presence of a solid inner and fluid outer core, overlain by a partially molten boundary layer, but their reflector impedance contrasts and reflector depths are not well constrained. With a rapidly increasing number of available modern broadband data, I developed a package, Discovery Using Ducttape Excessively (DUDE), to quickly generate plots for a comprehensive view of earthquake data. These plots facilitate discovery of unexpected phenomena. This dissertation identifies evidence for small-scale heterogeneities in Earth's upper mantle, and deeper lunar layering structure. Planetary interiors are complex with the heterogeneities on many scales, and discontinuities of variable character. This research demonstrates that seismic array methods are well-suited for interrogating heterogeneous phenomena, especially considering the recent rapid expansion of easily available dense network data.

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Date Created
2011

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High-resolution imaging of structure and dynamics of the lowermost mantle

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This research investigates Earth structure in the core-mantle boundary (CMB) region, where the solid rocky mantle meets the molten iron alloy core. At long wavelengths, the lower mantle is characterized by two nearly antipodal large low shear velocity provinces (LLSVPs),

This research investigates Earth structure in the core-mantle boundary (CMB) region, where the solid rocky mantle meets the molten iron alloy core. At long wavelengths, the lower mantle is characterized by two nearly antipodal large low shear velocity provinces (LLSVPs), one beneath the Pacific Ocean the other beneath Africa and the southern Atlantic Ocean. However, fine-scale LLSVP structure as well as its relationship with plate tectonics, mantle convection, hotspot volcanism, and Earth's outer core remains poorly understood. The recent dramatic increase in seismic data coverage due to the EarthScope experiment presents an unprecedented opportunity to utilize large concentrated datasets of seismic data to improve resolution of lowermost mantle structures. I developed an algorithm that identifies anomalously broadened seismic waveforms to locate sharp contrasts in shear velocity properties across the margins of the LLSVP beneath the Pacific. The result suggests that a nearly vertical mantle plume underlies Hawaii that originates from a peak of a chemically distinct reservoir at the base of the mantle, some 600-900 km above the CMB. Additionally, acute horizontal Vs variations across and within the northern margin of the LLSVP beneath the central Pacific Ocean are inferred from forward modeling of differential travel times between S (and Sdiff) and SKS, and also between ScS and S. I developed a new approach to expand the geographic detection of ultra-low velocity zones (ULVZs) with a new ScS stacking approach that simultaneously utilizes the pre- and post-cursor wavefield.. Strong lateral variations in ULVZ thicknesses and properties are found across the LLSVP margins, where ULVZs are thicker and stronger within the LLSVP than outside of it, consistent with convection model predictions. Differential travel times, amplitude ratios, and waveshapes of core waves SKKS and SKS are used to investigate CMB topography and outermost core velocity structure. 1D and 2D wavefield simulations suggest that the complicated geographic distribution of observed SKKS waveform anomalies might be a result of CMB topography and a higher velocity outermost core. These combined analyses depict a lowermost mantle that is rich in fine-scale structural complexity, which advances our understanding of its integral role in mantle circulation, mixing, and evolution.

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Date Created
2012

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Element use and acquisition strategies in biological soil crusts

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Biological soil crusts (BSCs) are critical components of arid and semiarid environments and provide the primary sources of bioavailable macronutrients and increase micronutrient availability to their surrounding ecosystems. BSCs are composed of a variety of microorganisms that perform a wide

Biological soil crusts (BSCs) are critical components of arid and semiarid environments and provide the primary sources of bioavailable macronutrients and increase micronutrient availability to their surrounding ecosystems. BSCs are composed of a variety of microorganisms that perform a wide range of physiological processes requiring a multitude of bioessential micronutrients, such as iron, copper, and molybdenum. This work investigated the effects of BSC activity on soil solution concentrations of bioessential elements and examined the microbial production of organic chelators, called siderophores. I found that aluminum, vanadium, copper, zinc, and molybdenum were solubilized in the action of crusts, while nickel, zinc, arsenic, and zirconium were immobilized by crust activity. Potassium and manganese displayed behavior consistent with biological removal and mobilization, whereas phosphorus and iron solubility were dominated by abiotic processes. The addition of bioavailable nitrogen altered the effects of BSCs on soil element mobilization. In addition, I found that the biogeochemical activites of BSCs were limited by molybdenum, a fact that likely contributes to co-limitation by nitrogen. I confirmed the presence of siderophore producing microbes in BSCs. Siderophores are low-molecular weight organic compounds that are released by bacteria to increase element solubility and facilitate element uptake; siderophore production is likely the mechanism by which BSCs affect the patterns I observed in soil solution element concentrations. Siderophore producers were distributed across a range of bacterial groups and ecological niches within crusts, suggesting that siderophore production influences the availability of a variety of elements for use in many physiological processes. Four putative siderophore compounds were identified using electrospray ionization mass spectrometry; further attempts to characterize the compounds confirmed two true siderophores. Taken together, the results of my work provide information about micronutrient cycling within crusts that can be applied to BSC conservation and management. Fertilization with certain elements, particularly molybdenum, may prove to be a useful technique to promote BSC growth and development which would help prevent arid land degradation. Furthermore, understanding the effects of BSCs on soil element mobility could be used to develop useful biomarkers for the study of the existence and distribution of crust-like communities on ancient Earth, and perhaps other places, like Mars.

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Date Created
2012

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Glacial processes on Earth and Mars: new perspectives from remote sensing and laboratory analyses

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Chemical and physical interactions of flowing ice and rock have inexorably shaped planetary surfaces. Weathering in glacial environments is a significant link in biogeochemical cycles – carbon and strontium – on Earth, and may have once played an important role

Chemical and physical interactions of flowing ice and rock have inexorably shaped planetary surfaces. Weathering in glacial environments is a significant link in biogeochemical cycles – carbon and strontium – on Earth, and may have once played an important role in altering Mars’ surface. Despite growing recognition of the importance of low-temperature chemical weathering, these processes are still not well understood. Debris-coated glaciers are also present on Mars, emphasizing the need to study ice-related processes in the evolution of planetary surfaces. During Earth’s history, subglacial environments are thought to have sheltered communities of microorganisms from extreme climate variations. On Amazonian Mars, glaciers such as lobate debris aprons (LDA) could have hosted chemolithotrophic communities, making Mars’ present glaciers candidates for life preservation. This study characterizes glacial processes on both Earth and Mars.

Chemical weathering at Robertson Glacier, a small alpine glacier in the Canadian Rocky Mountains, is examined with a multidisciplinary approach. The relative proportions of differing dissolution reactions at various stages in the glacial system are empirically determined using aqueous geochemistry. Synthesis of laboratory and orbital thermal infrared spectroscopy allows identification of dissolution rinds on hand samples and characterization of carbonate dissolution signals at orbital scales, while chemical and morphological evidence for thin, discontinuous weathering rinds at microscales are evident from electron microscopy. Subglacial dissolution rates are found to outpace those of the proglacial till plain; biologically-mediated pyrite oxidation drives the bulk of this acidic weathering.

Second, the area-elevation relationship, or hypsometry, of LDA in the midlatitudes of Mars is characterized. These glaciers are believed to have formed ~500 Ma during a climate excursion. Hypsometric measurements of these debris-covered glaciers enable insight into past flow regimes and drive predictions about past climate scenarios. The LDA in this study fall into three major groups, strongly dependent on basal elevation, implying regional and climatic controls on ice formation and flow.

I show that biologically-mediated mineral reactions drive high subglacial dissolution rates, such that variations within the valley can be detected with remote sensing techniques. In future work, these insights can be applied to examining Mars’ glacial regions for signs of chemical alteration and biosignatures.

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Date Created
2015

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The petrogenesis of angrites and martian meteorites inferred from isotope and trace element systematics

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The present understanding of the formation and evolution of the earliest bodies in the Solar System is based in large part on geochemical and isotopic evidences contained within meteorites. The differentiated meteorites (meteorites originating from bodies that have experienced partial

The present understanding of the formation and evolution of the earliest bodies in the Solar System is based in large part on geochemical and isotopic evidences contained within meteorites. The differentiated meteorites (meteorites originating from bodies that have experienced partial to complete melting) are particularly useful for deciphering magmatic processes occurring in the early Solar System. A rare group of differentiated meteorites, the angrites, are uniquely suited for such work. The angrites have ancient crystallization ages, lack secondary processing, and have been minimally affected by shock metamorphism, thus allowing them to retain their initial geochemical and isotopic characteristics at the time of formation. The scarcity of angrite samples made it difficult to conduct comprehensive investigations into the formation history of this unique meteorite group. However, a dramatic increase in the number of angrites recovered in recent years presents the opportunity to expand our understanding of their petrogenesis, as well as further refine our knowledge of the initial isotopic abundances in the early Solar System as recorded by their isotopic systematics. Using a combination of geochemical tools (radiogenic isotope chronometers and trace element chemistry), I have investigated the petrogenetic history of a group of four angrites that sample a range of formation conditions (cooling histories) and crystallization ages. Through isotope ratio measurements, I have examined a comprehensive set of long- and short-lived radiogenic isotope systems (26Al-26Mg, 87Rb-87Sr, 146Sm-142Nd, 147Sm-143Nd, and 176Lu-176Hf) within these four angrites. The results of these measurements provide information regarding crystallization ages, as well as revised estimates for the initial isotopic abundances of several key elements in the early Solar System. The determination of trace element concentrations in individual mineral phases, as well as bulk rock samples, provides important constraints on magmatic processes occurring on the angrite parent body. The measured trace element abundances are used to estimate the composition of the parent melts of individual angrites, examine crystallization conditions, and investigate possible geochemical affinities between various angrites. The new geochemical and isotopic measurements presented here significantly expand our understanding of the geochemical conditions found on the angrite parent body and the environment in which these meteorites formed.

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Date Created
2012