Determining the eruption temperature of Io's dominant silicate lavas would constrain Io's present interior state and composition. We have examined how eruption temperature can be estimated at lava tube skylights through synthesis of thermal emission from the incandescent lava flowing within the lava tube. Lava tube skylights should be present along Io's long-lived lava flow fields, and are attractive targets because of their temporal stability and the narrow range of near -eruption temperatures revealed through them. We conclude that these skylights are suitable and desirable targets (perhaps the very best targets) for the purposes of constraining eruption temperature, with a 0.9:0.7-mu m radiant flux ratio <= 63 being diagnostic of ultramafic lava temperatures. Because the target skylights may be small - perhaps only a few m or 10 s of m across - such observations will require a future Io-dedicated mission that will obtain high spatial resolution (<100 mipixel), unsaturated observations of Io's surface at multiple wavelengths in the visible and near-infrared, ideally at night. In contrast to observations of lava fountains or roiling lava lakes, where accurate determination of surface temperature distribution requires simultaneous or near simultaneous (<0.1 s) observations at different wavelengths, skylight thermal emission data are superior for the purposes of temperature derivation, as emission is stable on much longer time scales (minutes, or longer), so long as viewing geometry does not greatly change during that time. (C) 2016 Elsevier Inc. All rights reserved.

The use of electrodynamic tethers for propulsion and power generation is attractive for missions to the outer planets, which are traditionally handicapped by large propellant requirements, large times of flight, and a scarcity of power available. The proposed electrodynamic tether propulsion scheme is shown to be capable at reducing or eliminating these mission constraints. In this work, the orbital dynamics of a spacecraft using electrodynamic tether propulsion during the mission phases of capture, apojove pump-down and perijove pump-up in the Jovian system are investigated. The main result is the mapped design space involving mission duration, tether length and minimum perijove radius. Phase-free flyby sequences and bang-bang control laws are also included, which provide performance upper bounds for a given mission architecture. It is found to be advantageous to utilize in-bound only flybys of the Galilean moons during capture, and few, if any, out-bound only flybys during apojove pump-down. The electrodynamic tether system is also shown to be capable of lowering the spacecraft’s orbit to a Europa-Ganymede Hohmann orbit with a total flight time after entering Jupiter’s sphere of influence of just under two years. The benefits of leveraging solar third body perturbations, ballistic flyby tours, and adding a secondary propulsion system are also considered. / text

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Researchers analyzing the upper clouds of Jupiter and Saturn are unable to theoretically reproduce the data returned by Pioneers 10 and 11 and Voyagers 1 and 2 with an approach based on Mie theory. Ammonia crystals are believed to be an important constituent of Jupiter's upper clouds, but both their shape and scattering properties were unknown at the start of this work. Ammonia crystals and water crystals were grown in a cold chamber at temperatures 20°C below their freezing points (0°C and -77.7°C, respectively). The H₂O crystals formed had shapes in agreement with published growth habit diagrams. The NH₃ crystals formed were usually irregular in shape, but regular four-sided pyramids were commonly observed. This four-sided pyramidal shape is in agreement with ammonia's primitive cubic crystal structure. Ammonia crystals could not be formed at temperatures above -95°C due to nucleation problems. A scattering measuring instrument was constructed with fifteen separate lens-detector combinations aimed at a common point in the center of the cold chamber. A laser beam (6328Å wavelength) traversed the chamber center, illuminating any crystal aerosal clouds present. A computer was used to rapidly sample the outputs of the fifteen detectors and to drive a photoelectric modulator to change the slow speed polarization properties of the laser beam. The measurements resulted in a determination of the single scattering phase function and degree of linear polarization for the crystal species present. Water crystals were found to have scattering properties similar to that reported by previous researchers. The H₂O crystal scattering possesses a smaller backscatter peak and smaller polarization features than is common for water spheres of similar size. A negative polarization of 5% occurred in the forward scattering hemisphere and a positive polarization of 10% in the rear. Ammonia particles were observed to have a backscattering peak four times higher than for water crystals. The NH₃ particle light scattering produced very little polarization of the scattered light. A small (∼ 4%) negative polarization occurred in the forward scattering hemisphere. Work is continuing here to make scattering measurements using blue light illumination nearly simultaneous with the red HeNe laser wavelength illumination.

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Paterae (volcano-tectonic depressions) are among the most prominent topographic features on Io. They are unique, yet in some aspects they resemble calderas known and studied on Earth, Mars, and Venus. They have steep walls, flat floors, and arcuate margins, typical of terrestrial and Martian basalt shield calderas. However, they are much larger (2 km - 202 km diameter, mean 42 km 3 km) and typically lack obvious shields. They are often angular in shape or are found adjacent to mountains, suggesting tectonic influences on their formation. A preferential clustering of paterae at the equatorial sub- and anti-jovian regions is likely a surface expression of tidal massaging and convection in the asthenosphere. Paterae adjacent to mountains have a mean diameter 14 km 9 km larger than that for all paterae, which may indicate paterae grow larger in the fractured crust near mountains. Nightside and eclipse observations of Pele Patera by the Cassini and Galileo spacecraft reveal that much of Pele’s visible thermal emission comes from lava fountains within a topographically confined lava body, most likely a lava lake. Multiple filter images provided color temperatures of 1500 80 K from Cassini ISS data, and 1420 100 K from Galileo SSI data. Hotspots found within paterae (79% of all hotspots) exhibit a wide range of thermal behaviors in global eclipse images. Some hotspots are similar to Pele, consistently bright and confined; others, such as Loki, brighten or dim between observations and move to different locations within their patera. A model for patera formation begins with heating and convection within a high-temperature, low-viscosity asthenosphere. Magma rises through the cold, dense lithosphere either as diapirs [for thermal softening of the lithosphere and sufficiently large diapirs (20 km - 40 km diameter, >5 km thickness)] or through dikes. Magma reaches zones of neutral buoyancy and forms magma chambers that feed eruptions. Collapse over high-level chambers results in patera formation, filling of the patera with lava to create a lava lake, or lateral spreading of the magma chamber and subsequent enlargement of the patera by consuming crustal materials.

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.

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Inaug.-diss.--Bonn. / Vita.