Meteorite RBT04262 is one of only two Martian meteorites made of composite lithologies. Lithology 1 is composed of coarse-grained olivines enclosed in poikilitic pigeonites, resembling lherzolitic shergottites. Lithology 2 is finer-grained and composed of olivine phenocrysts set within a groundmass of augite, olivine, plagioclase (shocked to maskelynite) and accessory phases such as Ca-phosphates, representing an olivine-phyric shergottite. Lithology 1 may be an early-formed cumulate while Lithology 2 may represent a cooled liquid laden with accumulated olivine crystals. Geochemical and textural observations suggest that all components of RBT04262 formed as intrusions. As the majority of Martian meteorites also have a cumulate origin, it is speculated here that crust formation on Mars may be dominated by magmatic underplating or intrusion rather than by subaerial flows. An intrusive-origin for Martian crust has important implications as intrusions can reset the radiometric ages or magnetic signatures of the Martian crust without erasing the cratering history.

<p> The results from this study show that during the middle Amazonian period (2 Ga), Arabia Terra did exhibit glacial and fluvial activity as predicted by the Global Circulation Model. Arabia Terra is the largest expanse of ancient terrain north of the Martian equator. Arabia Terra extends across the Martian prime meridian from approximately 30&deg;E (330&deg;W) westward to 300&deg;E in longitude and from the equator to 50&deg;N. Glacial and fluvial activity during the Amazonian era in the north-central region (20&deg;-40&deg;N, 0&deg;-30&deg;E) of Arabia Terra was of high interest because of the Global Circulation Model (GCM) predictions of lower latitude precipitation during periods of high obliquity (> 35&deg;) [Forget et al., 2006]. The Ejecta morphologies, interior morphologies, and preservational states of 3732 craters with diameters ranging from 1.0-161.7 km were analyzed during the study. The interior morphologies which displayed glacial and fluvial activity were analyzed in detail for this project but aeolian, tectonic, and complex features were also studied. Distribution maps of all the interior morphologies and size-frequency distribution plots were created to determine the location and ages of these activities. Younger craters which did not display glacial and fluvial morphologies were used to create a lower bound for the size-frequency distribution plots for the region. Older craters were used to create upper bounds to show older periods of glacial and fluvial activity. Glacial and fluvial morphologies were mainly concentrated north of 30&deg; but there were some variations in concentration for the glacial morphologies.</p>

<p> This dissertation examines the influence which a geophysical process (giant impacts) has on a geochemical marker (composition) during terrestrial planet formation. Simultaneously studying all planets maximizes the available constraints and permits examination of controls on the overall composition of the Earth. I also examine the Galilean satellite system to determine the universality of the terrestrial conclusions. </p><p> The late stages of planetary accretion involve stochastic, large collisions. Impact-related erosion and fragmentation can have profound consequences for the rate and style of accretion and the bulk chemistries of terrestrial planets. However, the previous predominate assumption in computer models of accretion was that all collisions resulted in perfect merging despite the likelihood of these collisions producing a range of outcomes (e.g., hit-and-run, removal of material from target, or production of several post-collision bodies). In this work, I investigate the effects of late-stage accretion with multiple collision types and the consequences on the bulk (mantle/core) and isotopic (Hf&ndash;W) composition. </p><p> My model is composed of two parts: (1) N-body accretion code tracks orbital and collisional evolution of the bodies and (2) geochemical post-processing evolves composition in light of impact-related mixing, partial equilibration and radioactive decay. For terrestrial planets, Part (1) is Chambers (2013, Icarus) and incorporates multiple collisional outcomes. For Galilean satellites, Part (1) is Ogihara &amp; Ida (2012, Icarus) and assumes perfect merging for all collisions thus the model is not self-consistent (it likely overestimates compositional changes). </p><p> For the terrestrial planets, the results are consistent with observed mantle/core ratios and tungsten isotopic anomalies. A moderate (approx. 0.4) core equilibration factor is preferred due to protracted accretion time. It is important to include multi-modal collisions when modeling planet formation if composition, timescales, or spatial distribution of mass are being investigated. </p><p> I could not reproduce the observed ice fraction gradient of the Galilean satellites, even with an initial compositional gradient and vaporization of water ice. Some other physical process(es) are needed, perhaps tidally-driven volatile loss at Io and Europa. Extensive inward radial migration smooths out initial compositional gradients.</p>

abstract: Impact cratering has played a crucial role in the surface development of the inner planets. Constraining the timing of this bombardment history is important in understanding the origins of life and our planet's evolution. Plate tectonics, active volcanism, and vegetation hinder the preservation and identification of existing impact craters on Earth. Providing age constraints on these elusive structures will provide a deeper understanding of our planet's development. To do this, (U-Th)/He thermochronology and in situ 40Ar/39Ar laser microprobe geochronology are used to provide ages for the Haughton and Mistastin Lake impact structures, both located in northern Canada. While terrestrial impact structures provide accessible laboratories for deciphering Earth's impact history, the ultimate goal for understanding the history of the reachable inner Solar System is to acquire robust, quantitative age constraints for the large lunar impact basins. The oldest of these is the South Pole-Aitken basin (SPA), located on the lunar farside. While it is known that this basin is stratigraphically the oldest on the Moon, its absolute age has yet to be determined. Several reports released in the last decade have highlighted sampling and dating SPA as a top priority for inner Solar System exploration. This is no easy task as the SPA structure has been modified by four billion subsequent years of impact events. Informed by studies at Mistastin - which has target lithologies analogous to those at SPA - sampling strategies are discussed that are designed to optimize the probability of a high science return with regard to robust geochronology of the SPA basin. Planetary surface missions, like one designed to explore and sample SPA, require the integration of engineering constraints with scientific goals and traverse planning. The inclusion of in situ geochemical technology, such as the handheld X-ray fluorescence spectrometer (hXRF), into these missions will provide human crews with the ability to gain a clearer contextual picture of the landing site and aid with sample high-grading. The introduction of hXRF technology could be of crucial importance in identifying SPA-derived melts. In addition to enhancing planetary field geology, hXRF deployment could also have real implications for enriching terrestrial field geology. / Dissertation/Thesis / Ph.D. Geological Sciences 2014

The debate on whether water exists on Mars has been resolved by recent data from the Mars Phoenix Polar Lander. The lander found water ice just below the surface in the high northern latitudes of Mars. The questions to be answered now are: how much water was present in the past, how much water is currently present, what was the state the water in the past, and what is the current state of water on Mars. The morphology and spatial relationships are examined between three different landforms (pitted cones, giant polygons, and putative shorelines) considered to be the result of water-related processes. At two locations, Utopia Planitia and Cydonia Mensae, these three features exhibit the same topographic relationship. Non-water-related features adjacent to or nearby the landforms, such as the Dichotomy Boundary, multi-ringed basins, and locations of recent methane release, are examined for possible relationships to the formation of these 3 landforms. My results support previous work that indicates a large water body existed in the northern lowlands of Mars at some time in the past. In addition large amounts of sediment must have been shed from the highlands to the lowlands during this period to support the mud volcanism and giant polygon formation. Evidence also exists that mud volcanism was a common phenomenon during, and possibly after, the existence of the water body.

<p> Chondritic meteorites are undifferentiated fragments of asteroids that contain the oldest solids formed in our Solar System. Their primitive, solar-like chemical compositions indicate that they experienced very little processing following accretion to their parent bodies. As such, they retain the best records of chemical and physical processes active in the protoplanetary disk during planet formation. Chondritic meteorites are depleted relative to the sun in volatile elements such as S and O. In addition to being important components of organic material, these elements exert a strong influence on the behavior of other more refractory species and the composition of planets. Understanding their distribution is therefore of key interest to the scientific community. While the bulk abundance of volatile elements in solid phases present in meteorites is below solar values, some meteorites record volatile-rich gas phases. The Rumuruti (R) chondrites record environments rich in both S and O, making them ideal probes for volatile enhancement in the early Solar System. </p><p> Disentangling the effects of parent-body processing on pre-accretionary signatures requires unequilibrated meteorite samples. These samples are rare in the R chondrites. Here, I report analyses of unequilibrated clasts in two thin sections from the same meteorite, PRE 95404 (R3.2 to R4). Data include high resolution element maps, EMP chemical analyses from silicate, sulfide, phosphate, and spinel phases, SIMS oxygen isotope ratios of chondrules, and electron diffraction patterns from Cu-bearing phases. Oxygen isotope ratios and chondrule fO2 levels are consistent with type II chondrules in LL chondrites. Chondrule-sized, rounded sulfide nodules are ubiquitous in both thin sections. There are multiple instances of sulfide-silicate relationships that are petrologically similar to compound chondrules, suggesting that sulfide nodules and silicate chondrules formed as coexisting melts. This hypothesis is supported by the presence of phosphate inclusions and Cu-rich lamellae in both sulfide nodules and sulfide assemblages within silicate chondrules. Thermodynamic analyses indicate that sulfide melts reached temperatures up to 1138 &deg;C and fS₂ of 2 x 10^-3 atm. These conditions require total pressures on the order of 1 atm, and a dust- or ice-rich environment. Comparison with current models suggest that either the environmental parameters used to model chondrule formation prior to planetesimal formation should be adjusted to meet this pressure constraint, or R chondrite chondrules may have formed through planetesimal bow shocks or impacts. The pre-accretionary environment recorded by unequilibrated R chondrites was therefore highly sulfidizing, and had fO₂ higher than solar composition, but lower than the equilibrated R chondrites. </p><p> Chalcopyrite is rare in meteorites, but forms terrestrially in hydrothermal sulfide deposits. It was previously reported in the R chondrites. I studied thin sections from PRE 95411 (R3 or R4), PCA 91002 (R3.8 to R5), and NWA 7514 (R6) using Cu X-ray maps and EMP chemical analyses of sulfide phases. I found chalcopyrite in all three samples. TEM electron diffraction data from a representative assemblage in PRE 95411 are consistent with this mineral identification. TEM images and X-ray maps reveal the presence of an oxide vein. A cubanite-like phase was identified in PCA 91002. Electron diffraction patterns are consistent with isocubanite. Cu-rich lamellae in the unequilibrated clasts of PRE 95404 are the presumed precursor materials for chalcopyrite and isocubanite. Diffraction patterns from these precursor phases index to bornite. I hypothesize that bornite formed during melt crystallization prior to accretion. Hydrothermal alteration on the parent body by an Fe-rich aqueous phase between 200 and 300 &deg;C resulted in the formation of isocubanite and chalcopyrite. In most instances, isocubanite may have transformed to chalcopyrite and pyrrhotite at temperatures below 210 &deg;C. This environment was both oxidizing and sulfidizing, suggesting that the R chondrites record an extended history of volatile-rich interaction. These results indicate that hydrothermal alteration of sulfides on the R chondrite parent body was pervasive and occurred even in low petrologic types. This high temperature aqueous activity is distinct from both the low temperature aqueous alteration of the carbonaceous chondrites and the high temperature, anhydrous alteration of the ordinary chondrites. </p>

Impact events are a significant surface-modifying process on bodies lacking frequent resurfacing by atmospheric or igneous processes. To explore the effects of impacts on surface materials, we measured water and trace element compositions of tektites from Vietnam. Fourier transform infrared spectroscopy was used for water measurements and laser ablation inductively coupled plasma mass spectrometry was used for trace element measurements. Consistent with previous investigations of tektites, we found that the samples are depleted in volatile metals (e.g., Zn, Pb) as well as in water compared to the average continental crust, though water contents are high for rocks melted at atmospheric pressure. While Zn and Pb concentrations are linearly correlated with each other, there is no correlation between H 2 O and Zn or Pb contents. Combined with water contents of other tektites in the Australasian strewnfield, our results demonstrate that the source impact occurred at a wet site near the Indochina peninsula.

No description available.

<p>Impact craters are common geomorphological features on Mars. The density of craters is different among various regions. Higher crater density means older terrain. Craters can be divided into two types by the interior morphology: simple and complex. The cavity of Simple craters is bowl-shape, and complex craters display various interior features, such as central peaks. The depth/diameter ratio (d/D) of simple craters is larger than that of complex craters. The transition diameter from simple to complex morphologies ranges between 5 and 10 km, and is commonly cited to be about 7 km in the equatorial regions and 6 km near the poles, but the exact value also could vary with terrain type. In this research, seven regions, Amazonis Planitia, Arabia Terra, Chryse Planitia, Hesperia Planum, Isidis Planitia, Solis/Syria/Sinai Planum, and Terra Sirenum, were selected to investigate the onset diameter of complex craters and the relationship of crater diameter and depth in these regions on Mars in order to understand how the geology affects crater d/D. The analysis revealed that the slopes of the d/D relations are different, and these are linked to the surface material in different regions. The onset diameters in young volcanic regions with stronger material are slightly higher than older volcanic regions, and much higher than that of volatile regions. The research proves the different geological units can affect the morphology and morphometry of craters. </p>

<p> A survey of observations and analyses of the Mars year (MY) 25 planet-encircling dust storm (PDS) on Mars is presented. The environmental causes and dynamical mechanisms responsible for PDS initiation, expansion, decay, and interannual frequency are not fully understood. PDS seasonal occurrence suggests the presence of climatic and environmental components, yet interannual variability suggests that initiation and expansion mechanisms are not solely seasonal in character. The objectives of this research were to better understand the dynamical processes and circulation components responsible for MY 25 PDS initiation and evolution and to analyze why a PDS developed in MY 25 and not in MY 24 or 26.</p><p> Negative anomalies in temperature data with &sim;3-sol periodicity indicate the presence of baroclinic eddies. After comparing these eddies with dust storms observed in satellite imagery, the author hypothesized that six eastward-traveling transient baroclinic eddies triggered the MY 25 precursor storms due to the enhanced dust lifting associated with their low-level wind and stress fields. They were followed by a seventh eddy that contributed to dust storm expansion. All seven eddy cold anomalies were less than &ndash;4.5 K. It is possible that the sustained series of high-amplitude eddies in MY 25 were a factor in PDS onset and expansion.</p>