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Extreme Worlds of the Outer Solar System: Dynamic Processes on Uranus & Io


A central goal of planetary science is the creation of a framework within which the properties of each solar system body can be understood as the product of initial conditions acted on by fundamental physical processes. The solar system’s extreme worlds - those objects that lie at the far ends of the spectrum in terms of planetary environment - bring to light our misconceptions and present us with opportunities to expand and generalize this framework. Unraveling the processes at work in diverse planetary environments contextualizes our understanding of Earth, and provides a basis for interpreting specific signatures from planets beyond our own solar system. Uranus and Io, with their unusual planetary environments, present two examples of such worlds in the outer solar system.

Uranus, one of the outer solar system’s ice giants, produces an anomalously low heat flow and orbits the sun on its side. Its relative lack of bright storm features and its bizarre multi-decadal seasons provide insight into the relative effects of internal heat flow and time- varying solar insolation on atmospheric dynamics, while its narrow rings composed of dark, macroscopic particles encode the history of bombardment and satellite disruption within the system.

Jupiter’s moon Io hosts the most extreme volcanic activity anywhere in the solar system. Its tidally-powered geological activity provides a window into this satellite’s interior, permitting rare and valuable investigations into the exchange of heat and materials between interiors and surfaces. In particular, Io provides a laboratory for studying the process of tidal heating, which shapes planets and satellites in our solar system and beyond. A comparison between Earth and Io contextualizes the volcanism at work on our home planet, revealing the effects of planetary size, atmospheric density, and plate tectonics on the style and mechanisms of geological activity.

This dissertation investigates the processes at work on these solar system outliers through studies of Uranus’ atmosphere and rings and of Io’s thermal activity. I show that Uranus’ rings are spectrally flat in the near-infrared, setting them apart from all other ring systems in the solar system. I investigate the vertical profile of species in Uranus’ atmosphere, and demonstrate evidence for seasonal trends in the upper atmosphere on decadal timescales.

Based on a large high-cadence dataset of Io’s volcanism obtained with adaptive optics over 100 nights, I show that the thermal timelines of Io’s volcanoes indicate at least two distinct classes of eruption. The asymmetric spatial distribution of Io’s volcanic heat flow suggests additional mechanisms at work modulating the effects of tidal heating. I present the detection of one of the most powerful eruptions ever seen on Io, which I use to derive a eruption temperature of >1300 K, consistent with a highly mafic magma composition. Geophysical modeling of the thermal timeline of Loki Patera, a distinctive volcanic feature on Io, indicates low lava thermal conductivities also consistent with a highly-mafic silicate composition. Ultra-high-resolution thermal mapping of this patera reveals a multi-phase volcanic resurfacing process that hints at the plumbing system underlying this massive volcanic feature.

The results presented here are founded on near-infrared observations of unprecedented resolution in the spatial, spectral, and temporal domains. The interpretation of the data utilizes rigorous statistical techniques to draw meaningful conclusions. In addition to the scientific impact of the findings, this work therefore also pioneers specific ground-based tele- scope capabilities and analysis tools, and demonstrates their utility to solar system science. Chapter 2 presents the first high-resolution spectra of Uranus’ rings. Chapter 3 introduces Markov Chain Monte Carlo simulations into ice giant atmospheric radiative transfer model- ing, permitting a rigorous analysis of parameter uncertainties and correlations. Chapters 4-7 present results from the first multi-year, high-cadence ground-based observing campaign to study Io’s volcanism with sufficient spatial resolution to directly resolve individual volcanoes. The thermal timelines of these volcanoes provide unprecedented insight into the variability and distribution of Io’s volcanism over a wide range of timescales. Chapter 7 uses geometric arguments to deduce topography of a volcanic feature on Io based on observations at a range of viewing angles. Finally, Chapter 8 presents the first ground-based observations to map a thermal feature on Io at a spatial resolution of ∼10 km on Io’s surface, derived from the first mutual satellite occultation event to be observed with adaptive optics on a dual-telescope interferometric system. These techniques can all be expanded and applied to these and other targets in future near-infrared studies.

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