Saturn’s largest moon Titan (2575 km radius) is the second largest in the Solar system. Titan is the only known moon with a fully developed nitrogen-rich atmosphere with ionosphere extending to ~2000 km altitude, hosting complex organic chemistry. One of the main scientific interests of Titan’s atmosphere and ionosphere is the striking similarity to current theories of those of Earth ~3.5 billion years ago. The Cassini spacecraft has been in orbit around Saturn since 2004 and carries a wide range of instruments for investigating Titan’s ionosphere, among them the Langmuir probe, a “space weather station”, manufactured and operated by the Swedish Institute of Space Physics, Uppsala. This thesis reviews the first half of the PhD project on the production of pre-biotic molecules in the atmosphere of Titan and early Earth, focusing on the ion densities and dynamics in Titan’s ionosphere derived from the in-situ measurements by the Cassini Langmuir probe. One of the main results is the detection of significant, up to ~2300 cm-3, charge densities of heavy (up to ~13000 amu) negative ions in Titan’s ionosphere below 1400 km altitude. On the nightside of the ionosphere at altitudes below 1200 km, the heavy negative ion charge densities are comparable to the positive ion densities and are in fact the main negative charge carrier, making this region of the ionosphere exhibit properties of dusty plasma. The overall trend is the exponential increasing of the negative ion charge densities towards lower altitudes. Another important result is the detection of ion drifts that between 880-1100 km altitudes in Titan’s ionosphere translate to neutral winds of 0.5-5.5 km/s. Ion drifts define three regions by altitude, the top layer (above ~1600 km altitude) where the ions are frozen into the background magnetic field, the dynamo region (1100 – 1600 km altitudes) where the ions are drifting in partly opposing directions due to ion-neutral collisions in the presence of the magnetic and electric fields and the bottom layer (below 1100 km altitude) of the ionosphere, where the ions are coupled to neutrals by collisions.

Made available in DSpace on 2014-06-11T19:25:29Z (GMT). No. of bitstreams: 0 Previous issue date: 2007-07Bitstream added on 2014-06-13T20:33:09Z : No. of bitstreams: 1 rodrigues_tff_me_guara.pdf: 3882200 bytes, checksum: e4811cba55aa397290361e12d6eed278 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Universidade Estadual Paulista (UNESP) / Neste trabalho investigamos a estabilidade de sistemas de satélites coorbitais de Saturno. Estudamos os satélites Tétis e Dione que possuem coorbitais, mas também investigamos as regiões de Mimas e Encelado que ainda não possuem nenhum satélite coorbital conhecido, no intuito de investigar a possibilidade de existência e ainda delimitar as fronteiras das regiões estável e do instável. Para encontrarmos possíveis satélitescoorbitais a Mimas ou Encelado,ou ainda explicar a ausência dos mesmos, e ainda delimitar as regiões de estabilidade de Dione e Tétis utilizamos o método do expoente H, que é uma medida da difusão do sistema baseada em uma lei de potência. O expoente H foi calculado com difusão em semieixo maior, longitude e excentricidade. Além do mapeamento do expoente H, fizemos o gráfico da evolução do desvio padrão e a contagem de picos, para assim verificarmos se não há nenhum tipo de erro na contagem de picos e seu comportamento. Também fizemos o gráfico do valor do expoente H em função do semi-eixo maior para cada satélite, onde são encontrados resultados relevantes. Para Dione e Tétis constatamos que ambos possuem regiões de estabilidade e instabilidade. Para Encelado e Mimas concluímos que eles também possuem regiões de baixa difusão e que poderiam perfeitamente abrigar coorbitais, que possivelmente se perderam durante a migração de Encelado para sua posição atual. / In the present work we investigate the stability of Saturn’s coorbital satellite systems. We studied the satellites Thetis and Dione, which have coorbital satellites, but we also studied the regions of Mimas and Enceladus, which do not have any coorbital satellites known, in order to investigate the possibility of their existence and also to determine the borders of the stable and unstable regions. In order to find possible coorbitals of Mimas or Enceladus, or even explain the lock of them, and also to determine the stability regions of Dione and Thetis we used the method of the expoent H, which gives a measure of the system’s diffusion based on a power law. The H expoent was computed for diffusion in semi-major axis, longitude and excentricity. We generated maps of the H expoent and plots of the standard deviation evolution and of the pics, in order to identify any counting mistakes. We also produced plots the values of the H expoent as a function of the semi-major axis for each satellite, where we found some relevant results. In the cases of Dione and Thetis we found that both have regions of stability and unstability. In the case of Enceladus and Mimas we concluded that they also have low diffusion regions, that could harbour coorbital satellites. However, they were probably lost during the migration of Enceladus towards its present position.

No description available.

The role of physical collisions in shaping Saturn's F ring is explored using a mixture of dynamical theory, image analysis and computer simulations. The F ring is highly dynamic, being perturbed by the nearby moons, Prometheus and Pandora, and by a population of small bodies, whose presence is inferred by their influence on the ring, charged particle data and, occasionally, direct detection. Small-scale features, termed `mini-jets', are catalogued from images taken by the Imaging Science Subsystem of the Cassini spacecraft. More than 1000 are recorded, implying a population of 100 objects on nearby orbits, colliding with the ring at velocities of a few ms 1. Many are seen to collide several times, forming repeated structures, and must have enough physical strength, or self-gravity, to survive multiple passages through the core. Larger features, called `jets', share a similar morphology. They are likely caused by a more distant population which collide at higher velocities ( 10 ms 1) and are roughly an order of magnitude less common. Differential orbital motion causes jets to shear out over time, giving the ring its multi-stranded appearance. Jets have different orbital properties to mini-jets, probably because they result from multiple, overlapping collisions. Simulations using an N-body code show that the shape of collisional features depends heavily on the coefficient of restitution, particularly the tangential component. When both components are < 1 large objects merely sweep up small particles. Features like jets and mini-jets require large particles in both the target and impactor, as is the case for two similarly-sized aggregates colliding. A single population of aggregates is proposed, ranging from large, unconsolidated clumps, embedded in the core, through mini-jet-forming objects to the more distant, jet-forming colliders. Prometheus may be ultimately responsible for all of these features as its gravity can trigger clump formation as well as perturb particles.

Saturn's magnetosphere is unique in the solar system. The rotation-driven convection consists of alternating channels of cool plasma from an interior source moving outward and hot plasma from outside moving inward, making Saturn’s inner magnetosphere a dynamical region. This thesis describes work on developing numerical models to simulate the plasma convection pattern in Saturn's inner magnetosphere. Chapter 2 introduces the numerical Rice Convection Model (RCM), a multi-fluid model that was originally developed for Earth’s magnetosphere. We adapt it for Saturn’s conditions in this thesis. In Chapter 3, we show results of initial RCM simulation runs, in which only cool plasma from the interior source is considered. We also include the Coriolis force and the pickup effect. Because the cool plasma is much denser than the hot plasma and always dominant in determining the convection pattern, it is important and necessary to investigate it first. Chapter 4 compares several cool plasma source models and determines the one that produces the best simulation results when compared to Cassini spacecraft observations. In Chapter 5, we add the finite temperature and associated plasma pressure of the cool plasma. The effect of ionospheric Pedersen conductance is also investigated. Finally in Chapter 6, we add hot plasma at the outer boundary, and simulate the V-shape signatures of the injection-dispersion events, which are considered the most definitive evidence of rotation-driven convection in Saturn's inner magnetosphere. Our simulations conform to the observed fact that wider, slower outflow channels of cooler, denser plasma alternate with narrower, faster inflow channels of hotter, more tenuous plasma. Comparisons between simulated and observed results show great consistency.

I studied the atmospheric jet streams of Jupiter and Saturn through numerical simulations. Jupiter and Saturn have approximately 30 and 15 jet streams, respectively, alternatively blowing eastward and westward at the cloud level. My studies are motivated by recent space probe observations of the giant planets, which are revealing vertical structures and time dependent behaviors of the atmospheric jets. Such new findings are important keys to understanding how the jets are driven and maintained. My first project tested the hypothesis that a large convective storm on Saturn observed in 1990 decelerated the equatorial jet. The equatorial jet's speed is reported to be ∼275 ms⁻¹ today, half of the speed measured by the Voyagers in 1980-81. It has been hypothesized that the large storm is responsible for causing the observed slowdown. Our result shows that the storm's effect is insufficient to cause a slowdown of the observed magnitude. The second project investigated the formation of Jovian jet streams, namely, whether Jupiter-like atmospheric jets emerge from self-organization of small initial vortices. Thunderstorms are observed on Jupiter and have been proposed to be the sources of small-scale vorticity. Our result shows that self-organization of initial small vortices leads to east-west jets under various Jupiter-like conditions. Third, I tested the stability of shallow atmospheric jets under Jovian conditions. Deep atmospheric jets have been shown to be stable on Jupiter; however, the possibility that those jets are shallow, with the point of zero-motion at perhaps ∼100-bar level, is not well explored and deserves a thorough examination.

No description available.

Thesis (MS)--Montana State University--Bozeman, 2009. / Typescript. Chairperson, Graduate Committee: David R. Lageson. Includes bibliographical references (leaves 93-100).

Resumo: Neste trabalho investigamos a estabilidade de sistemas de satélites coorbitais de Saturno. Estudamos os satélites Tétis e Dione que possuem coorbitais, mas também investigamos as regiões de Mimas e Encelado que ainda não possuem nenhum satélite coorbital conhecido, no intuito de investigar a possibilidade de existência e ainda delimitar as fronteiras das regiões estável e do instável. Para encontrarmos possíveis satélitescoorbitais a Mimas ou Encelado,ou ainda explicar a ausência dos mesmos, e ainda delimitar as regiões de estabilidade de Dione e Tétis utilizamos o método do expoente H, que é uma medida da difusão do sistema baseada em uma lei de potência. O expoente H foi calculado com difusão em semieixo maior, longitude e excentricidade. Além do mapeamento do expoente H, fizemos o gráfico da evolução do desvio padrão e a contagem de picos, para assim verificarmos se não há nenhum tipo de erro na contagem de picos e seu comportamento. Também fizemos o gráfico do valor do expoente H em função do semi-eixo maior para cada satélite, onde são encontrados resultados relevantes. Para Dione e Tétis constatamos que ambos possuem regiões de estabilidade e instabilidade. Para Encelado e Mimas concluímos que eles também possuem regiões de baixa difusão e que poderiam perfeitamente abrigar coorbitais, que possivelmente se perderam durante a migração de Encelado para sua posição atual. / Abstract: In the present work we investigate the stability of Saturn's coorbital satellite systems. We studied the satellites Thetis and Dione, which have coorbital satellites, but we also studied the regions of Mimas and Enceladus, which do not have any coorbital satellites known, in order to investigate the possibility of their existence and also to determine the borders of the stable and unstable regions. In order to find possible coorbitals of Mimas or Enceladus, or even explain the lock of them, and also to determine the stability regions of Dione and Thetis we used the method of the expoent H, which gives a measure of the system's diffusion based on a power law. The H expoent was computed for diffusion in semi-major axis, longitude and excentricity. We generated maps of the H expoent and plots of the standard deviation evolution and of the pics, in order to identify any counting mistakes. We also produced plots the values of the H expoent as a function of the semi-major axis for each satellite, where we found some relevant results. In the cases of Dione and Thetis we found that both have regions of stability and unstability. In the case of Enceladus and Mimas we concluded that they also have low diffusion regions, that could harbour coorbital satellites. However, they were probably lost during the migration of Enceladus towards its present position. / Orientador: Othon Cabo Winter / Coorientador: Décio Cardoso Mourão / Banca: Silvia Maria Giuliatti Winter / Banca: Ricardo Reis Cordeiro / Mestre

No description available.