Zugl.: Köln, University, Diss., 2004.

Undergraduate honors paper--Mount Holyoke College, 2006. Dept. of Astronomy. / Includes bibliographical references (leaves 80-87).

Kiel, Universiẗat, Diss., 2000.

Heidelberg, University, Diss., 2001.

Thesis (Ph. D.)--University of Texas at Austin, 2006. / Vita. Includes bibliographical references.

According to both spectroscopic measurements and interior models, Jupiter, Saturn, Uranus and Neptune possess gaseous envelopes that are enriched in heavy elements compared to the Sun. Straightforward application of the dominant theories of gas giant formation - core accretion and gravitational instability - fail to provide the observed enrichment, suggesting that the surplus heavy elements were somehow dumped onto the planets after the envelopes were already in existence. Previous work has shown that if giant planets rapidly reached their cur rent configuration and radii, they do not accrete the remaining planetesimals efficiently enough to explain their observed heavy-element surplus. We ex plore the likely scenario that the effective accretion cross-sections of the giants were enhanced by the presence of the massive circumplanetary disks out of which their regular satellite systems formed. Perhaps surprisingly, we find that a simple model with protosatellite disks around Jupiter and Saturn can meet known constraints without tuning any parameters. Fur thermore, we show that the heavy-element budgets in Jupiter and Saturn can be matched slightly better if Saturn’s envelope (and disk) are formed roughly 0.1 — 10 Myr after that of Jupiter. We also show that giant planets forming in an initially-compact con figuration can acquire the observed enrichments if they are surrounded by similar protosatellite disks. Protosatellite disks efficiently increase the capture cross-section, and thus the metallicity, of the giant planets. Detailed models of planet formation must therefore account for the presence of such disks during the early stages of solar system formation. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

This thesis describes observations of the flne structure in Jovian decametric radiation made at Grahamstown during the 1967-68 apparition. It was found that pulses with duration less than 0.5 milliseconds were common during fine structure storms. The restrictions placed on the source for different theories of origin of the short pulses are discussed. The variation of the probability of occurrence from year to year is analysed on the assumption that the radiation is found in directions fixed with respect to the planet's magnetic field. It is concluded that there is a factor other than the declination of Earth and the Io effect which controls the probability of occurrence. A detailed analysis suggests a beam width of 3° in latitude at Jupiter but further work is necessary to check this.

Typescript (photocopy). / Digitized by Kansas Correctional Industries

The silicate "magma trigger" model of volcanism on Io has been evaluated numerically with finite element methods by considering the one-dimensional heat transfer between hot silicate magma and initially cold sulfur. It is found that for the probable range of initial magma temperatures and sulfur temperatures, the contact between silicate magma and a sulfur crust will be 700 (+OR-) 100 K, or approximately the vapor point of elemental sulfur. A silicate magma sill or laccolith on the order of 10 m thick will yield energetic vapor for a period of several weeks to several months depending on the vapor temperature and the amount of convective cooling of the silicate magma that occurs at the silicate-sulfur interface. This model may account for the origin of plumes and possible sulfur flows, as well as for their observed temperatures ((TURN) 600-700K) and lifetimes (several days to a few months). If the conducted heat flow is similar in high and low latitudes, then the low latitude occurrence of plumes may be explained as a result of lower temperatures at higher latitudes. Because the contact temperature of sulfur and silicate magma depends on the pre-existing sulfur temperature, a system in which sulfur vapor temperature is just reached at the equator would not generate sulfur vapor under lower initial sulfur temperatures existing at high latitudes. If the heat flow is higher in high latitudes, then the sulfur crust must be thinner than it is in low latitudes for the model to work as described above. Most of the heat flow from Io may be moved by convection from the interior to the surface, not by conduction. Heat flow may be modulated by the efficient transfer of silicate melts from 40 to 300 km depth, and emplaced as laccoliths at the sulfur-silicate crustal interfaces at a depth of 5-10 km. Sulfur flows, plumes, calderas and other areas of massive radiant heat dissipation continue the convective cycle to the surface. The temperature at the base of the sulfur crust may be less than the melting point of sulfur, and the silicate magma temperature can be as low as 1200 K. Low silicate magma temperatures will occur if the crust of Io is as differentiated as terrestrial rhyolites and trachytes. High alkalies in the Io plasma torus suggest the possibility that the Ionian crust is a highly differentiated silicate.

A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of Doctor of Philosophy. October 2017. / Since the arc assembly and continental collision of the East African Orogen some 640 million years ago, Madagascar has gone through several geodynamic and tectonic episodes that have formed and subsequently modified its lithosphere. This thesis aims to investigate the structure of the crust, the uppermost mantle, and the mantle transition zone beneath Madagascar to gain insights into the relationship between present-day lithosphere structure and tectonic evolution, and to evaluate candidate models for the origin of the Cenozoic intraplate volcanism. To address these issues, local, regional, and teleseismic events recorded by several temporary seismic networks; the MAdagascar-COmoros-MOzambique (MACOMO), the SEismological signatures in the Lithosphere/Asthenosphere system of SOuthern MAdagascar (SELASOMA), and the Réunion Hotspot and Upper Mantle – Réunions Unterer Mantel (RHUM-RUM) were used to complement the seismic events recorded by the permanent seismic stations in Madagascar. The different methods used and the primary results of this study are explained in each section of this thesis. In the first part of this thesis, crustal and uppermost mantle structure beneath Madagascar was studied by analyzing receiver functions using an H-κ stacking technique and a joint inversion with Rayleigh-wave phase-velocity measurements. Results reflect the eastward and northward progressive development of the western sedimentary basins of Madagascar. The thickness of the Malagasy crust ranges between 18 km and 46 km. The thinnest crust (18-36 km thick) is located beneath the western basins and it is due to the Mesozoic rifting of Madagascar from eastern Africa. The slight thinning of the crust (31-36 km thick) along the east coast may have been caused by crustal uplift and erosion when Madagascar moved over the Marion hotspot and India broke away from it. The parameters describing the crustal structure of Archean and Proterozoic terranes, including thickness, Poisson’s ratio, average shear-wave velocity, thickness of mafic lower crust, show little evidence of secular variation. Slow shear-wave velocity of the uppermost mantle (4.2-4.3 km/s) are observed beneath the northern tip, central part and southwestern region of the island, which encompass major Cenozoic volcanic provinces in Madagascar. The second part of the thesis describes a seismic tomography study that determines the lateral variation of Pn-wave velocity and anisotropy within the uppermost mantle beneath Madagascar. Results show an average uppermost mantle Pn-velocity of 8.1 km/s. However, zones of relatively low-Pn-velocity (~7.9 km/s) are found beneath the Cenozoic volcanic provinces in the northern, central, and southwestern region of the island. These low-Pn-velocity zones are attributed to thermal anomalies that are associated with upwelling of hot mantle materials that gave rise to the Cenozoic volcanism. The direction of Pn anisotropy shows a dominant NW-SE direction of fast-polarization in the northern region and around the Ranostara shear zone, in the south-central Madagascar. The anisotropy in the uppermost mantle beneath these regions aligns with the existing geological framework, e.g. volcanic complex and shear zones, and can be attributed to a fossil anisotropy. The Pn anisotropy in the southwestern region, around the Morondava basin, is E-W to NE-SW-oriented. It can be attributed either to the mantle flow from plate motion, the African superplume, or the Mesozoic rifting from Africa. Results from this study do not show any substantial evidence of the formation of a diffuse boundary of the Lwandle plate, cutting through the central region of Madagascar. Station static delays reflect the significant variation in the Moho depth beneath the island. In the third part of the thesis, the thickness of the mantle transition zone beneath Madagascar, which is sensitive to the surrounding temperature variation, has been estimated by stacking receiver functions. Single-station and common-conversionpoint stacking procedures show no detectable thinning of the mantle transition zone and thus no evidence for a thermal anomaly in the mantle under Madagascar that extends as deep as the mantle transition zone. Therefore, this study supports an upper mantle origin for the Cenozoic volcanism. However, the resolution of the study is not sufficient to rule out the presence of a narrow thermal anomaly as might arise from a plume tail. Overall, the findings in this research are broadly consistent with the crustal and upper mantle structure of Madagascar determined by previous studies, but provides significantly greater detail with regard to the crustal and uppermost mantle structure as more seismic stations were used. / LG2018