We examine the meteorology of the giant planets using our automated cloud feature tracker. Through pattern recognition and correlation optimization, our software returns a dense, regular grid of wind vectors ideal for further analysis, in contrast with an irregular grid of relatively sparse vectors returned using manual (hand-eye) cloud tracking. We measure the winds in and around Jupiter's Great Red Spot (GRS) to reveal its distinctive "hollow" structure, its counter-rotating interior, and a newly-discovered cyclonic ring around its periphery. This cyclonic ring suggests the presence of a thermally indirect, downwelling secondary circulation at the periphery of the GRS. We also analyze a time-series of images of Jupiter's White Ovals. Over a decade, the system has evolved from three discrete, white anticyclones to one reddish vortex (Oval BA). Our measurements reveal non-uniform acceleration of the flow within Oval BA coincident with the coloration event, and areas of organized cyclonic circulation in apparent turbulent regions in the vicinity of the White Ovals and Oval BA. The proximity and apparent longevity of these cyclonic circulations implies a connection with the anticyclonic systems, perhaps through energy transfer and long-term maintenance of the systems. We have also directly measured the power spectrum of the turbulent kinetic energy present in Jupiter's atmosphere. Our measurements provide evidence consistent with an inverse cascade of energy from small to large scales that may fuel Jupiter's impressive jet streams and vortices. Finally, our analysis of near-infrared images of silhouetted clouds in Saturn's atmosphere demonstrates that the measured latitudinal zonal wind profile is largely similar to previous measurements using visible-wavelength images. This result, accompanied by a statistical analysis of the cloud features imaged in the near-infrared, implies that both visible and near-infrared images are observing a single cloud deck at different altitudes, though this implication does not necessarily extend to Saturn's jet streams. Furthermore, our measurements indicate that the equatorial jet stream at depth flows at relatively high speeds, suggesting that reports of significantly slower speeds within the equatorial jet are confined to the upper troposphere.

No description available.

ASPECT is a new convection code that uses more modern and advanced solver methods than geodynamics legacy codes. I use ASPECT to calculate 2-dimensional Cartesian as well as 2- and 3-dimensional spherical-shell convection cases. All cases use the Boussinesq approximation. The 2D cases come from Blankenbach et al. (1989), van Keken et al. (1997), and Davies et al. (in preparation). Results for 2D cases agree well with their respective benchmark papers. The time-evolutions of the root mean square velocity (Vrms) and Nusselt number agree, often to within 1%. The 3D cases come from Zhong et al. (2008). Modifications were made to the simple.cc and harmonic_perturbation.cc files in the ASPECT code in order to reproduce the initial conditions and temperature-dependence of the rheology used in the benchmark. Cases are compared using both CitcomS and ASPECT with different levels of grid spacing, as well as comparing uniform grid spacing and the ASPECT default grid spacing, which refines toward the center. Results for Vrms, average temperature, and Nusselt numbers at the top and bottom of the shell range from better than 1% agreement between CitcomS and ASPECT for cases with tetragonal planforms and 7000 Rayleigh number to as much as 44% difference for cases with cubic planforms and 10^5 Rayleigh number. For all benchmarks, the top Nusselt number from ASPECT is farthest from the reported benchmark values. The 3D planform and radially averaged quantity plots agree. I present these results, as well as recommendations and possible fixes for discrepancies in the results, specifically in the Nusselt numbers, Vrms, and average temperature. / Master of Science

A variety of fulgurites from diverse locations have been studied. Morphological features were measured and physical properties documented, and a classification scheme was developed. Three major types are introduced and described: Type I, Type II, and Type III, along with two minor types: Type IV and Melt Droplets. Fulgurites representative of each major taxonomic type were investigated using electron microprobe point analyses and x-ray mapping. A range of compositions were found, including nearly pure glass, detrital zircons with baddeleyite rims, Fe-metal with P-rich rims, and unusual Fe-Si metals. The fulgurite formation process is considered within the planetary context through a discussion of lightning detection and potential for formation on other terrestrial bodies. Finally, suggestions for future investigations are presented and discussed.

Since different parts of the Moon display varying magnetic field strengths, our goal was to determine whether these differences are due to specific geological characteristics. We found that older materials tend to be more magnetic than younger materials. Additional statistical studies found that the mare regions of the Moon are less magnetic than the plains and terra regions. We did not find a simple relationship between lunar magnetism and crustal thickness, and this is inconsistent with the hypothesis that thicker crust is more magnetic since there is additional material. Thus, it is not just a matter of the amount of magnetic material that determines the magnetic field strength of the lunar crust. Our results demonstrate that magnetism and crustal thickness have a complex relationship, with multiple distinct groups corresponding to various regions of the Moon. The lunar maria formed a particularly distinct group, consisting of low magnetism and thin crust, while the lunar highlands consist of thick crust but typical magnetic field values. The ejecta thickness and magnetic field distributions for specific craters showed that larger impact basins have a thicker and more widespread ejecta blanket than smaller craters. We did not find a consistent pattern of magnetic field enhancements near specific craters, but evidence for these strong magnetic signatures was present for Mare Crisium and Mare Nectaris. These results may support the hypothesis that ejecta materials are carriers of magnetism, and this may be the reason for their tendency to have higher magnetic field strengths.

No description available.

Cassini spacecraft images of Enceladus' surface have revealed diverse terrains---some heavily cratered, others almost devoid of craters, and even some with ridges and fractures. We have documented crater morphologies in regions for which high-resolution data are available (140 to 360 W and 90 S to 60 N). The south polar region shows a dearth of craters, in sharp contrast to the heavily cratered northern latitudes. Tectonized regions such as Sarandib and Diyar Planitiae also have low crater densities. Viscously relaxed craters are found in the apparently young regions of the anti-Saturnian and trailing hemispheres, as well as in the older, upper northern latitudes. By modeling the viscoelastic relaxation of craters on Enceladus using TEKTON, a finite-element code, we predict large geographical variation in heat flow and a complicated thermal history on Enceladus. Our results are consistent with the planitiae being older examples of the South Polar Terrain, supporting a satellite-reorientation hypothesis.

Meteorites provide information about the early history of our solar system and the formation and evolution of planetesimals. One of the few direct observations of internal geophysical processes within planetary bodies is the presence or absence of a dynamo-driven magnetic field. These observations provide essential constraints on the degree of differentiation, core solidification timescales and the driving forces for convection. This thesis focusses on the paleomagnetic information recorded by iron and stony-iron meteorites, providing us with a unique view-point for the generation and variability of core dynamo activity. Iron and stony-iron meteorites are primarily comprised of FeNi metal. The Widmanstätten pattern; an intergrowth of taenite and kamacite lamellae. Between these lamellae, a range of microstructures develop, dictated by the ‘M-shaped’ Ni diffusion profile. Among these microstructures is the cloudy zone, a region of tetrataenite islands in a Fe-rich matrix, formed by spinodal decomposition. The tetrataenite islands are extremely reliable paleomagnetic recorders. The direction of magnetisation and composition of FeNi microstructures was imaged using synchrotron X-rays. Magnetic contrast is generated using X-ray magnetic circular dichroism. The dimensions of tetrataenite islands within the cloudy zone directly correlate with cooling rates. Cooling rates vary from ~0.5–10,000°C/Myr and correspond to island sizes of ~500–10nm, respectively. The slowest cooled group of iron meteorites reveal multidomain magnetic behaviour within the cloudy zone, whereas in faster-cooled meteorites islands are vortex state. This demonstrates that cooling rate influences the magnetic properties of the cloudy zone. The subtly different cooling rates between different pallasite meteorites means that each meteorite provides a ‘snapshot’ of the parent body magnetic field at a different point during its thermal evolution. Paleointensity results provide the first observations of a quiescent period in dynamo activity preceding core solidification. This also helps to constrain the paleomagnetic signals associated with core nucleation, which, in turn, constrains the mechanism of solidification. Paleomagnetic studies of meteoritic metal were complemented with measurements of magnetic inclusions in olivines. Alternating-field and thermal demagnetisation experiments were carried out using both 2G SQUID and WSGI small-bore SQUID magnetometers. Results suggest that pallasite silicates are unreliable paleomagnetic recorders, and a planetary-strength paleointensity cannot be recovered. Paleomagnetic fidelity was also investigated for a dusty olivine grain from the Semarkona chondrite. Lorentz microscopy, transmission X-ray microscopy, nanotomography and micromagnetic simulations were used to rigorously test the behaviour of Fe-nanoparticles. The final study in this thesis focusses on the IAB iron meteorites. These meteorites have an unusual and complex history. Paleomagnetic results are accompanied by a detailed microstructural study using X-PEEM and electron backscatter diffraction to constrain the formation of two microstructures: pearlitic and spheroidised plessite. Paleomagnetic results suggest the IAB parent body did not have an active core dynamo. Meteorites represent the oldest material in our solar system, and their complex histories and susceptibility to alteration make them some of the most challenging samples to extract reliable paleointensity estimates from. Advanced electron microscopy and synchrotron techniques are now making it possible to extract reliable paleomagnetic information, with profound implications for the formation and evolution of the solar system.

The poles of Mars, much like Earth's polar regions, are covered by kilometer-thick sheets of ice that interact with the Martian atmosphere and can record climatic changes in their stratigraphy. These polar caps are composed of several icy sections that interact with the Martian environment over different timescales. This dissertation describes my investigation of two of these units: The South Polar Residual Cap (SPRC), and the North Polar Layered Deposits (NPLD). The overarching theme of my work is to explore the connections between these caps and the current (SPRC) and past (NPLD) climate of Mars using a wide variety of data from spacecraft missions, and applying numerical models of surface properties and processes to interpret the observations. The SPRC is a ~10 meter thick slab of bright carbon dioxide ice that is covered by pits and scarps formed by differential sublimation. It is unclear whether this cap is in a state of net accumulation or net ablation. During the summer of Mars Year 28 (2006/2007), The High Resolution Imaging Science Experiment (HiRISE) observed an apparent increase in brightness near the edges of these pits that had not been seen before, and was not seen in the few years following. I analyzed hundreds of images from HiRISE and the Mars Orbiter Camera (MOC), as well as data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) to search for compositional and/or grain-size changes in the ice that could explain these "halos". I coupled my observations with numerical modeling of the spectral reflectance of the ice, to explore the effects of different ice grain sizes and minuscule dust inclusions on the observed brightness. I concluded that the features were caused by the occurrence of a global dust storm, after which the depositing dust actually acted to darken the SPRC. The halos were thus areas that had been kept relatively "clean" of the depositing dust, thanks to winds driven by enhanced sublimation from the pit walls. The fact that the halos did not show up in subsequent years means that they had not been exhumed, and that flat areas of the SPRC are in a state of net accumulation. It is likely that events like these result in new flat surfaces formed by snowfall driven by the depositing dust, which could explain the persistence of the cap throughout history. The polar layered deposits (PLD) are kilometer-thick stratified dome structures composed of dusty water ice that make up the bulk of the polar caps. The layers that make up the PLD are thought to record climatic variations over timescales of millions of years, in a similar way to Earth's ice caps. These caps are dissected by deep troughs that allow us to observe outcrops of their internal layers and map the stratigraphy. In the past, researchers have done this using layer brightness. However, remotely observed brightness has been shown to be affected by many external factors and may not represent an intrinsic property of the layers. Using Digital Terrain Models (DTM) made from HiRISE stereo images of NPLD outcrops, I mapped the change with depth of each layer's topographic protrusion from the scarp slope, defining the stratigraphy with a property related to the layers' resistance to erosion. I mapped the protrusion stratigraphy of 16 sites throughout the NPLD, and correlated the stratigraphic profiles from a subset of these sites, with Context Camera (CTX) images and signal-matching algorithms. This correlation combined topographic information with brightness information, resulting in an improvement of the current state of stratigraphic mapping of the NPLD, providing further evidence that layer sequences are continuous across the NPLD, and setting lower limits on relative accumulation rates for large sections of the cap.In order to search for a connection between the Martian paleoclimate and the NPLD stratigraphic record, I identified overlapping periodicities in the stratigraphic structure and compared them to periodicities in the climatic history, represented by the change in insolation with time at the North Pole over the last 5 Myr. I found that the ratio of stratigraphic wavelengths is systematically lower than the ratio between dominant modes of oscillation of the north polar insolation. However, a similar wavelet analysis of synthetic stratigraphic profiles created with a simple climate-driven model of accumulation revealed that a detectable non-linear relationship exists between the variation of insolation on the North Polar region of Mars and the stratigraphic record preserved in the NPLD. The dissertation is organized into four principal chapters and one final chapter with concluding remarks and future directions. Chapter 1 gives an introduction to Mars' polar regions and to the history of research in astronomically forced climate change through cyclostratigraphy, along with a short summary of the scope and main questions of this study. Chapter 2 details my study of the SPRC halos. Chapter 3 deals with the stratigraphic mapping of the NPLD through high-resolution topography, and Chapter 4 presents the results of my search for an astronomical forcing signal in the NPLD stratigraphy. Chapter 2 was published in the journal Icarus, in a special issue on the dynamic geologic processes of Mars and the science learned from continuous monitoring of these processes through remote sensing. Chapter 3 has been accepted for publication in the Journal of Geophysical Research. A modified version of Chapter 4 will be submitted to Nature Geoscience.

Le sujet de cette thèse est la phase d'accrétion emballée du gaz lors de la formation des planètes géantes, au moyen de simulations hydrodynamiques. Une planète de la masse de Jupiter est simulée au sein d'un disque circumstellaire autour d'une étoile de masse solaire. Grâce aux grilles emboitées du code JUPITER, le voisinage de la planète est résolu suffisamment pour étudier le disque circumplanétaire. Des simulations 3D localement isothermes révèlent que l'accrétion est un processus fondamentalement tridimensionnel, avec 90% du gaz accrété verticalement à travers le sillon ouvert par la planète, via une circulation méridienne entre les disques circumstellaire et circumplanétaire. Le taux d'accrétion est mesuré à partir de simulations sans viscosité, en accord avec les conditions qui règnent dans l'environnement planétaire. On trouve que Jupiter doublerait sa masse en un demi million d'années durant cette phase emballée, ce qui est similaire au temps de dispersion du disque, et pourrait donc expliquer la rareté des exoplanètes très massives (plus de 3 masses de Jupiter). En ajoutant les effets thermiques au code Jupiter, nous avons réalisé des simulations radiatives, avec des températures plus réalistes. Celles-ci montrent que la température de la planète influence fortement les propriétés de la matière circum-planétaire : même une planète assez massive pour ouvrir un sillon ne peut former qu'une enveloppe planétaire supportée par la pression si sa température est élevée (~13000 K), comme une planète de faible masse. Au contraire, dans les simulations où la température au voisinage de la planète est bornée à 1000-2000 K, un disque circum-planétaire se forme. / This thesis is focusing on the runaway gas accretion phase of giant planet formation with hydrodynamic simulations. A Jupiter-mass planet is simulated embedded in a circumstellar disk around a Solar-mass star. Thanks to the JUPITER-code nested meshing technique, the planet vicinity is resolved with high resolution allowing to study the circumplanetary disk formed around the giant planet. Isothermal, 3-dimensional simulations revealed that the accretion is truly 3D process, with 90% of the gas accreted from the vertical direction through the planetary gap. This vertical influx is part of a meridional circulation between the circumstellar and circumplanetary disks. The accretion rate to planet was determined from inviscid simulation, in order to account for the presumably low viscosity environment in the forming planet’s vicinity. In this inviscid limit, the mass doubling time in the runaway phase can be as long as half a million years, competing with the gas dispersal timescale, hence providing a possible solution for the missing population of massive (>3 Jupiter-mass) giant planets. Incorporating the thermal effects into the JUPITER-code, radiative simulations with more realistic temperature information were carried out as well. These simulations revealed that the planetary temperature greatly determines the properties of the circumplanetary material. Even a gap-opening giant planet could only form a circumplanetary, pressure-supported envelope, if the planet temperature is high (~13,000 Kelvin), similarly to low-mass planets. In contrary, in the simulations were the central temperatures were capped at 1000-2000 Kelvins, circumplanetary disks were formed.