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A Review of New Testudine Specimens from the Oligocene and Miocene of the Great Plains of North America, Including Descriptions of Two New TaxabyEric M HoltDoctor of Philosophy in Integrative BiologyUniversity of California, BerkeleyProfessor Emeritus Anthony D Barnosky, ChairThe fossil record of Testudines in North America is well known for some parts of the Tertiary period, but for many stages representing millions of years our knowledge of Testudine diversity is both poor and outdated. The gaps in knowledge are pronounced for the Oligocene and Miocene in North America, despite an abundant fossil record. This study begins to address the gap in our understanding of fossil turtles by utilizing existing museum specimens from the Great Plains and adjacent Rocky Mountain basins, the majority of which have not been previously identified beyond the ordinal level. Updated identifications based on fossil shells are provided and the morphological criteria for recognizing many taxa are clarified, despite there being a bias in museum collections towards subadult and juvenile testudine specimens. A previously unknown regional variant of Gopherus edae is described, and initial steps are taken toward documenting the range of intraspecific morphological variation found within various taxa of Oligopherus, Gopherus, Stylemys, and Hesperotestudo. Chapter One lays the groundwork for future studies on tortoise diversity through time and space, and the relationship between their evolution, distribution, and climate change. In Chapter Two the largely complete shell of a new species of tortoise, Hesperotestudo nicholsorum n. sp., is described from Lemhi County, Idaho, USA. The tortoise genus Hesperotestudo Williams, 1950b, was widespread throughout North America from Canada to El Salvador from the earliest Miocene to the late Pleistocene but retains no extant members. The specimen was collected from Late Hemingfordian (~15-16.3 Ma) sediments in the Railroad Canyon Sequence of the Sixmile Creek Formation. Previous studies have established a late Eocene paleoelevation of 3.7 km for the area. Elevational decrease due to Basin and Range extension did not happen until the middle Miocene, indicating that the fossil locality was higher in the Hemingfordian than the roughly 2 km elevation it is today. This makes H. nicholsorum both the highest elevation specimen of Hesperotestudo currently described and one of the oldest members of its genus. The new taxon shares many morphological similarities with other Hesperotestudo but also exhibits characters more commonly found in Gopherus. In Chapter Three I describe the shell of another new tortoise, Hesperotestudo niobrarensis n. sp., from Cherry County, Nebraska, USA. The specimen was collected in 1933 from Clarendonian sediments in the Ash Hollow Formation of the Ogallala Group. An isolated left epiplastron from the Clarendonian Ash Hollow Formation near Merrit Reservoir in Cherry County, Nebraska is referred to the new species. This new taxon shares many morphological similarities with other Hesperotestudo but also exhibits a unique combination of characters not found in other members of the genus.

Giant planets are characterized by their predominant gaseous composition, with hydrogen and helium as the major constituents. The presence of trace gases in the atmosphere has two effects. Condensible trace gases form clouds, restricting visible and near-infrared observations mostly to the top-most cloud layer. This severely limits our understanding of the atmospheric composition and global circulation, both of which are hidden below the clouds. However, their interaction with radiation also provides a window through the clouds. We can use spectroscopy to retrieve the distribution below the clouds and to investigate the three-dimensional structure of the atmosphere through mid-infrared and radio observations.One of the key trace gases in the atmospheres of giant planets is ammonia (NH3). Ammonia constitutes a small fraction of the atmosphere (approximately 300 parts per million), but it serves as a valuable tracer for the background circulation by examining its distribution and temporal evolution. In the radio regime, the spectral absorption features of ammonia are highly pressure-broadened, allowing us to probe depths ranging from tens to thousands of bars. Through comparisons of radio observations with model atmospheres, where we can manipulate the distribution of trace gases and temperature profiles, we can constrain the atmospheric properties of Jupiter to hundreds of bars. These comparisons provide valuable insights into the processes shaping the atmosphere. Prior to the arrival of the Juno mission, radio observations had already indicated that Jupiter’s atmosphere is neither latitudinally nor vertically well-mixed below the cloud deck. Based on these earlier observations, it was believed that the water condensation layer around 6 bars acted as a barrier for atmospheric circulation, below which the atmosphere was considered fully convective and well-mixed. However, the presence of synchrotron radiation, the dominant source of radiation at lower frequencies, limited our understanding to altitudes above the water condensation layer. The arrival of the Juno mission in the Jovian system has enabled us to overcome this lim2 itation by descending below the synchrotron radiation belts (peaking at 1.5 jovian radii). Throughout my Ph.D. research, I have developed tools to independently calibrate and process instrument data from radio observatories, focusing primarily on NASA’s Juno Microwave Radiometer (MWR) and NRAO’s Very Large Array (VLA). The open-source pipelines I developed retrieve relevant observational quantities (brightness temperature and its dependence on the emission angle) along with their uncertainties. Subsequently, I developed and compared models with observations, which allowed me to study the distribution of ammonia on Jupiter. The enhancement in nitrogen (2.66+0.27 −0.17 solar) in Jupiter’s deep atmosphere was found to be lower than previously estimated, and consistent with the noble gases in the atmosphere as obtained by the Galileo probe.A more surprising discovery was the depth to which the atmosphere appears to be depleted (20-30 bar), much deeper than can be explained by dynamics or conventional microphysics. This finding highlights that Jupiter might have a stably-stratified troposphere. The distribution of ammonia revealed that zones and and belts on Jupiter can be traced below the cloud deck; in the tropical regions extending down to 20 bars, while the subtropics and mid-latitudes are confined to the upper few bars. Deeper in the atmosphere, around 30 bars, the signal reverses, showing enhanced belts compared to the depleted zones. This distribution could be explained by a stacked circulation cell model, where the middle and lower troposphere cells have opposite circulation directions.By examining the longitudinal structure of the atmosphere using data from both VLA and MWR, I was able to analyze the weather patterns on Jupiter. The majority of the variability in Jupiter’s atmosphere is confined to the upper few bars, affirming earlier theories and observations that weather is a shallow phenomenon on the gas giant. Even in the most dynamically active region (the North Equatorial Belt), the weather extends only to approximately the water condensation layer. The only three features I identified capable of affecting the lower troposphere were an anti-cyclone, an ammonia plume as part of the trapped equatorial Rossby wave, and the signature of precipitation, which appeared to deposit ammonia below the water condensation layer.The breakthrough in understanding came from studying the impact of a giant outbreak in the South Equatorial Belt. By comparing the MWR observations of the outbreak with orbit-averaged observations, I demonstrated that the storm affected the deep troposphere down to approximately 20 bars. These are the first observations showing that storms deplete the upper atmosphere in ammonia and deposit ammonia, while also cooling the lower troposphere at much greater depths than previously anticipated on Jupiter. The mechanism through which the atmosphere is depleted favors supercooled water, rapidly updrafted in storms, absorbing ammonia to form water-ammonia hail, which is subsequently deposited around 15-25 bars. This process shows how small- and large-scale processes interact to shape the atmosphere of Jupiter. This work has shown that the gas giant atmosphere are not as convective as previously thought, and are controlled by the interaction of both local microphysics and the background large-scale circulation.

The goal of this thesis is to characterize circumstellar disks and substellar objects, from moons to brown dwarfs, by improving and applying techniques from high-contrast imaging. To do so, I led four projects, two of which expand the range of targets accessible by high-contrast imaging. In Project 1, I develop an improved algorithm for high-contrast imaging data processing, enabling detection of fainter, smaller planets; this algorithm uses both spatial and temporal information on the speckle noise present in high-contrast images, and will be especially useful for next-generation photon-counting detectors such as Microwave Kinetic Inductance Detectors (MKIDs). In Project 2, I demonstrate a new technique to detect and monitor ejections of ice particles from subsurface oceans on outer solar system bodies, such as Jupiter’s icy moon Europa, using ground-based polarimetric imaging. Such plumes have not been confirmed on Europa, but are of significant interest for astrobiology and planning for the upcoming Europa Clipper mission.The remaining two projects use existing analysis techniques to investigate individual systems of interest that add to our understanding of planet formation. In Project 3, I characterize the atmosphere of a brown dwarf around HIP 93398, a recently discovered companion identified via the Hipparcos Gaia Catalog of Accelerations, using high-resolution spectra from SCExAO/CHARIS. With both a dynamical mass measurement and high-resolution spectroscopy, HIP 93398 B provides substantial constraints on models of brown dwarf evolution. In Project 4, I constrain the morphology and scattering properties of the debris disk around the bright star HD 155623, a unique “hybrid” debris disk that shows signs of youth despite its age, using polarimetric data from the Gemini Planet Imager. With the information from my investigation, the HD 156623 system can now serve as a benchmark for models aiming to explain how gas and dust interact during the later stages of planet formation. These investigations both further innovation in exoplanet imaging techniques and, respectively, provide insight into detections of extrasolar planets, the nature of oceans beyond Earth, the mechanisms of planet formation, and the atmospheres of giant planets and brown dwarfs.

Planets with radii between 1 and 4 Earth radii and orbital periods shorter than Mercury’s are the most common class of planet discovered to date. Detailed measurements of the masses and radii of these planets indicate that a ‘radius valley’ separates the smaller super-Earths, with densities consistent with an Earth-like composition, from the sub-Neptunes, larger planets with lower densities. A leading theory consistent with astronomical observations posits that these planets all formed with hydrogen atmospheres that greatly increase their observed radii. Over time, the super-Earths lost these atmospheres via a hydrodynamic wind, heated by the star’s bolometric luminosity and sustained by the heat released from the silicate-rich interior, while sub-Neptunes retained them. My dissertation work reveals that the nature of the connection between the interior silicates and atmospheric hydrogen has broad implications for the atmospheric composition and evolution of these planets. I find that the thermal coupling of the interior and atmosphere, which sustains atmospheric escape, can also lead to its end, as the interiors cool more efficiently as the atmosphere is stripped. I demonstrate that chemical equilibrium in the deep envelopes of sub-Neptunes, which implies high silicate vapor concentrations that decline with altitude, leads to structural changes in the atmosphere. Convection is inhibited by the strong molecular weight gradients, creating a highly super-adiabatic region that increases the atmospheric mass inferred around observed planets. I show that reactions between the silicates and hydrogen produce novel reduced silicon species and abundant endogenic water vapor in sub-Neptunes. Finally, I apply a hydrodynamic radiative transfer model to core-powered mass loss for the first time, demonstrating that the inclusion of multi-band opacities fundamentally alters predicted mass loss rates. Together, this work opens a new window into the atmospheres and interiors of the most abundant planets known in the Galaxy.

Evidence of wind-driven (aeolian) sediment transport has been observed on seven planetary bodies in our solar system to date. The movement of sediment across the surface of a planet can form depositional features, ranging from centimeter-scale ripples to vast and complex sand seas. Alternatively, this same transport process can cause erosion, carving complex structures out of bedrock over time. In some cases, aeolian features observed via remote sensing on other planets are not found on Earth, meaning our understanding of planetary processes is limited by the available analogs. The variety of atmospheric, sediment, and gravitational environments in which aeolian transport occurs indicates that though the conditions may vary, the fundamental physics involved is linked. Each of the research projects described here addresses an open question in a different area of planetary aeolian transport. The studies include two Mars-focused projects ((1) an analysis of dunes and ripples traversed by the Mars Science Laboratory Curiosity rover and (2) a survey of bedrock ridges in Gale crater), a field guide to the Ibex Dunes of Death Valley National Park, and a wind-tunnel experiment where analog sediments were used to simulate planetary aeolian transport.

Thousands of kilometers of possible subduction sites have been identified on Venus near certain quasi-circular surface features called corona and near branches of rift zone trenches called chasmata. Subduction on Earth is driven by negative plate buoyancy with respect to the underlying mantle, however lithosphere on Venus may be significantly more buoyant than on Earth. On Earth, only 6-7 km of positively buoyant crust compete with the negative thermal buoyancy of the lithospheric mantle, but on Venus the enhanced positive buoyancy from 30 km of globally-averaged crust would significantly inhibit subduction initiation. Plume-lithosphere interactions have been proposed as a mechanism for subduction initiation, but the relatively high bending moments and elastic thicknesses near subduction sites indicate the lithosphere is thick and may not be easily penetrated by thermal upwellings. The following chapters present a series of 2D and 3D numerical models using the code, StagYY, to investigate the dynamics of regional-scale lithospheric recycling initiated at chasmata trenches on Venus with realistic crustal densities. Rather than subduction, a tectonic regime called ``peel-back delamination" (PBD) was observed in which the lithospheric mantle decouples and peels away from the overlying positively buoyant crust. Though net-negative lithospheric buoyancy is not a requirement for delamination, PBD is driven by the negative thermal buoyancy of the lithospheric mantle and resisted by the positive compositional buoyancy of the crust and the strength of the lithosphere. Aided by the pre-existing lithospheric weakness at chasmata, plume-rift interactions may accelerate timescales of delamination initiation. A thermal upwelling is shown to destabilize even thinner, more positively buoyant rift-adjacent lithosphere than in models without plume-rift interactions. Plume-induced PBD may require a minimum lithospheric thickness of approximately 150-200 km, and therefore may be most applicable to the inferred thick lithosphere near the Dali-Diana chasmata system. 3D delamination models show a radial retrograde migration of a trench and flexural bulge, compatible with the style of surface deformation observed at Artemis Corona in the Dali-Diana chasmata region of Venus.

Surveys dedicated to detecting exoplanets via the transit and radial velocity methods have revolutionized our understanding of planet formation and evolution by revealing the the prevalence of planets orbiting close to their stars. Transit surveys have been especially groundbreaking due to their abilities to discover large quantities of planets with small orbital separations, which can they be further characterized via transmission spectroscopy, emission spectroscopy, and thermal phase curve observations to reveal details about their atmospheres and surfaces. In recent years, the Transiting Exoplanet Survey Satellite (TESS) has provided the opportunity to expand these techniques into entirely new regimes, due to its ability to search for transiting planets around a greater variety of stars and around brighter stars that are more amendable to follow-up observations. This thesis focuses on the utilization of TESS data to search for and study the demographics of these planets.First, I present TRICERATOPS, a tool designed to statistically validate transiting exoplanets and identify likely astrophysical false positives in TESS data. To statistically validate a transiting exoplanet is to confirm its planetary nature by ruling out plausible false positive scenarios, such as those that arise when multiple stars are blended together in the data. This is a particularly pertinent problem for TESS, which is equipped with relatively low-resolution cameras that often cannot distinguish light originating from individual stars, especially in crowded fields. I discuss the design and efficacy of TRICERATOPS, demonstrating that it is an effective tool for identifying the most promising planet candidates detected by TESS and prioritizing follow-up observations with both ground-based and space-based telescopes.Next, I use TRICERATOPS and an array of ground-based follow-up observations to validate 13 hot and potentially terrestrial planets detected by TESS. These planets are unlike any rocky bodies in the Solar System; they orbit their stars at distances of only a few stellar radii and are so highly irradiated that many are expected to have molten surfaces. Their high temperatures also mean that they emit infrared light at levels detectable by JWST. Emission spectroscopy and thermal phase curve observations of these worlds can reveal the presence and composition of an atmosphere, measure Bond albedo, and calculate heat redistribution properties. Prior to TESS, very few of these types of planets were known around bright stars amenable to JWST observations. This sample therefore facilitates the investigation of hot Earth-size worlds at a population level.Finally, I conduct a search for planets smaller than Saturn orbiting A-type stars. A-type stars, which are roughly twice as massive and nearly twice as hot as Sun-like stars, have historically been avoided by transit and radial velocity surveys due to their large radii and rapid rotation rates, which hinder our ability to detect planets around them. As a consequence, early transit surveys like the Kepler mission acquired very little data of these stars, limiting our understanding of planet demographics to stars like the Sun and cooler. By observing all bright stars across nearly the entire sky, TESS has provided the best opportunity yet to search for small planets orbiting relatively hot stars. Through this search, I discover and validate a single planet: HD 56414 b, a Neptune-size planet orbiting one of the hottest planet-hosting stars known to date on a 29-day orbital period. The orbital period of this planet is long compared to the typical planet detected by TESS, suggesting that Neptune-size planets with smaller orbital separations may not exist around A-type stars. I display that atmospheric photoevaporation due to high levels of near-ultraviolet radiation offers one possible explanation for this phenomenon. To test this hypothesis more robustly, I calculate the occurrence rate of small planets with orbital periods under 10 days around A-type stars. I demonstrate, for the first time, that sub-Saturns and sub-Neptunes are rarer around A-type stars than they are around their cooler counterparts. I also find evidence that super-Earths are as common or less common around A-type stars than cooler stars. These results suggest that small planets are unable to form at, migrate to, or survive at the small orbital separations probed by TESS around these hot stars. Overall, these findings significantly advance our understanding of how planets form and evolve around stars hotter than the Sun, providing a more holistic picture of planetary populations throughout the galaxy.

The study of planetary ionospheres dates back thousands of years, to humanity's first attempts to explain the polar aurora. Ionospheric physics is a study of one pathway for energy to enter a planetary atmosphere: transfer from sunlight and solar wind plasma to planetary plasma, and from the planetary plasma to the neutral atmosphere. In the modern era of ionospheric physics, electrostatic analyzers with attached time-of-flight velocity analyzers are excellent tools for the study of planetary ionospheres because they can measure mass-resolved 3-dimensional ion velocity distribution functions over a wide range of energies and fluxes, with large fields-of-view and moderate angular resolution. One example of an electrostatic analyzer with attached time-of-flight section is the SupraThermal And Thermal Ion Composition (STATIC) instrument onboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. MAVEN has been collecting data since 2014 with the goal of illuminating how Martian climate and habitability have been affected by the escape of the atmosphere to space. To that end, STATIC was designed to measure the main ionospheric and escaping species, operating from deep in the collisional atmosphere out to the tenuous exosphere and magnetotail. STATIC samples ion velocity distribution functions every 4 seconds across a field-of-view covering 2$\pi$ steradians for ions with energies between 0.1 eV and 30 keV and masses between 1 and 60 amu. This work will treat STATIC as a case study in order to examine some of the challenges associated with using electrostatic analyzers in space, including the effects of background counts, spacecraft potential, and supersonic spacecraft motion on the measurement and analysis of distribution functions. Before the discussion of those details, we first provide an introduction to the geometry of electrostatic analyzers and how the measured count rates are related to physical quantities. This work also describes the results of several investigations in which MAVEN-STATIC data were used to examine the beginnings of cold ion outflow at Mars. Models of the Martian ionosphere show that the vast majority of ions are created at altitudes near 120 km, the location of the main ionospheric peak. The peak is located deep in the collisional atmosphere, where collisions are theorized to keep the ions in thermal equilibrium with the cold neutral atmosphere. The vast majority of the ionospheric ions are therefore gravitationally bound to the planet. Most ions that have the potential to escape the planet's gravity are created near a boundary called the exobase, where the mean free path between collisions becomes equal to the scale height and the ions no longer thermalize with the neutrals. The exobase can be considered the top of the collisional atmosphere and varies between 140 and 210 km altitude dependent on local time and season. In the present epoch, 10-20\% of atmospheric loss is attributable to the loss of ions from hundreds or thousands of kilometers above the main peak and the exobase region. The mechanisms by which ions gain enough energy to escape the planet's gravity between the exobase and the altitudes where they escape are not yet understood, but we have used STATIC data to begin analysis of ion distribution functions in the vicinity of the exobase. The analysis of ion velocity distribution functions measured near the exobase is experimentally challenging because the spacecraft travels supersonically with respect to the cold ions, affecting observations of both the energy and angular distributions. The details of how the instrumentation affects measurements of the distribution function must be understood in detail in order to extract accurate plasma parameters. In this work, we describe many of the procedures used to extract accurate plasma parameters from STATIC data. We use these corrections to begin to bridge the gap between studies of ion outflow conducted high above the exobase and studies of the cold, thermal ionosphere near the main peak. Specifically, we report the first measurements of Martian ionospheric ion temperatures since the Viking landers in 1975 and 1976, including the results of both a case study and a statistical study of over 10,000 MAVEN orbits. Unexpectedly, ion temperatures are significantly elevated over neutral gas temperatures many scale heights below the exobase, and none of the obvious mechanisms for ion heating explained the observed temperature difference. This surprising result was noted both in the case study and the study using the majority of the STATIC dataset, suggesting that a fundamental piece of physics is missing from current models of the Martian ionosphere.Finally, we also report the results of a statistical study with the goal of determining where signatures of ion energization are observed in STATIC data. By fitting the measured distribution functions with drifting Maxwell-Boltzmann distributions, we identified distributions in which suprathermal ions produced a significant portion of the measured energy flux. Most distributions are well-described by the Maxwell-Boltzmann distribution below the exobase region---in other words, the neutral atmosphere dominates low-altitude ion dynamics. Suprathermal ions are observed just above the exobase at all solar zenith angles. A comparison of results inside and outside Mars' strong crustal magnetic field regions showed that more thermal plasma is observed inside crustal fields on the dayside, while more suprathermal plasma is observed inside crustal fields on the nightside. These results suggest that crustal fields shield dayside plasma from energization by the solar wind while enhancing energization and outflow on the nightside. The techniques developed in this work provide tools for continued investigation into the physics of initial ion acceleration at Mars using STATIC data, while the results reported here provide context for case studies in which the processes responsible for ion acceleration can be analyzed. The study of initial ion acceleration at Mars is just one context in which electrostatic analyzers in combination with time-of-flight analyzers provide insight into the physics that cannot be matched by any other instrument.

Solar system giant planets are known for their opaque and colorful aerosols. Their weather layers harbor dynamic and fascinating cloud activities, convective plumes, and banded circulations. Observations also suggest remote giant planets, such as brown dwarfs, exhibit large-scale circulations. Weather layers also serve as the boundary condition of giant planets that control their evolution. It has been debated for several decades that moist convection is important to heat transport in the atmosphere and the formation of large-scale dynamics on giant planets. However, it is still unclear how moist convection operates the visible weather events, atmospheric circulations, and evolution of giant planets. In this thesis, I seek to combine state-of-the-art numerical simulations, analytical analysis, and telescope observations to shed light on the impact of moist convection on the atmospheric dynamics and cloud formation of giant planets and support ongoing and future space missions to enhance their future scientific return. Convection- and cloud-resolving simulations are computationally expensive due to the issue of atmospheric aspect ratio, in which the vertical resolution limits the numerical timestep and computational efficiency. Chapter~\ref{chap:1-VIC} presents a solution to this problem. We introduce a state-of-the-art vertically-implicit-correction scheme that can improve the computational efficiency by a factor of 10-to-1000 and correctly capture the small-scale turbulence and large-scale dynamics. The implementation of this scheme in the nonhydrostatic model, SNAP, greatly facilitates studies in later chapters, which require numerical simulations.The temperature structure of giant planet weather layers, which serve as the boundary of the planet and control the evolution of the planet, is poorly constrained. The molecular weight difference between the dominant hydrogen-helium mixture and the heavy cloud-forming species may inhibit thermal convection and alter the temperature structure of giant planets from adiabats to superadiabats. Chapter~\ref{chap:2-convective-inhibition} shows a new picture of convective inhibition with a significantly reduced inhibition threshold by consistently considering both downdrafts and updrafts in the weather layer. I argue that convective inhibition may have a broader impact on thermal convection in various planets with a reduced inhibition threshold than previously expected.The latent heat cycle, which is associated with the condensation of cloud-forming species and evaporation of their condensates, transports a significant amount of energy across the weather layer. Analytical and 2D numerical solutions in Chapter~\ref{chap:3-self-regulated-moist-convection} show that the cloud column density and mixing efficiency (i.e., eddy diffusivity) of cloud-forming species are significantly regulated by the latent heat flux, which is limited by internal heat fluxes of giant planets. The predicted cloud density, cloud opacity, and mixing efficiency with the self-regulated moist convection are several orders of magnitude smaller than traditional studies suggested. The simulated temperature structure and relative humidity profiles also support the conclusion from Chapter~\ref{chap:2-convective-inhibition} that convection inhibition and superadiabatic weather layer may occur with less abundant moisture.Chapter~\ref{chap:4-Jupiter-Superadiabat} carefully studies the heat transport and thermal structure at Jupiter's water weather layer in a localized 3D regime. Latent heat dominates heat transport in Jupiter's weather layer since thermal convection is turned off by the mass-loading effect. The mass-loading effect leads to a persistent and subsaturated stable layer that disjoints the weather layer into two convective zones. As a result, Jupiter's weather layer is superadiabatic. The simulated mixing efficiency and temperature structure support conclusions in Chapters~\ref{chap:2-convective-inhibition} and \ref{chap:3-self-regulated-moist-convection}. This research has important implications for interior and evolution studies. It also suggests that the ammonia depletion observed by Juno's microwave radiometer and ground-based telescopes may be caused by a warmer deep atmosphere. Brown dwarfs and direct-imaging planets exhibit rotational modulations in different wavelengths, leading to the observed rotational light curves. It is proposed that such variation is induced by patchy cloud patterns on these substellar objects. Chapter~\ref{chap:5-jupiter-light-curve} studies the rotational modulation of Jupiter from visible to mid-infrared and suggests that the wave-beating patterns such as 5 $\rm \mu$m hot spots and vortices like the great red spot have a significant contribution to emission light curves. These studies bridge theories and observations of moist convection and cloud activities on giant planets by analytical analysis and numerical simulations. Enriching implications from these studies can greatly benefit the understanding of giant planets' weather layers and support the ongoing Juno mission and the future flagship Uranus Orbiter and Probe.

The evolution of a planetary body often determines and is determined by its thermal properties. In my first project, I explore the consequences of heating upon pore closure, allowing me to estimate the heat flow through the Martian crust during the latest significant pore generation event—likely large basin-forming impacts. We apply a pore closure model developed for the Moon to Mars and take into account the geological processes that may alter the depth of a transition between porous and competent crust. If the 8–11 km deep discontinuity in seismic wave speed detected by the InSight lander marks the base of the uppermost porous layer of the Martian crust, then the heat flux at the time the porosity was created must exceed 60 mW m^−2, indicating a time prior to 4 Ga. Then, I explore how the global shape of an icy satellite allows us to infer its heat budget and interior—including the presence or absence of a subsurface global ocean. I apply this method in my second and third projects to Tethys and Mimas, respectively. We assume spatial variations in tidal heating are responsible for thickness or temperature variations in an isostatic ice shell, which manifests as surface topography. For Saturn’s moon Tethys, our best-fit models require Pratt isostasy and obliquity tides, with a normalized moment of inertia 0.340-0.345 and an average surface heat flux 1-2 mW m^−2. Then, we find that to account for its hydrostatic shape, Mimas’ normalized moment of inertia is 0.375, indicating a relatively undifferentiated world. Its remaining topography is consistent with a ∼30 km thick conductive ice shell in Airy isostasy atop a weakly convecting ∼30 km thick layer that itself mantles a ∼140 km radius ice-rock interior. For neither satellite do we find an ocean. However, the total power and pattern inferred to produce both satellites’ shapes from tidal heating indicate an ancient era of high obliquity. The common thread of all three projects is the flow of heat, and how our understanding of it can be revealed by or can reveal properties of the planetary bodies we study.

Icy satellites of the outer solar system have become the primary target for planetary exploration because of their relevance to understanding of solar-system evolution and to the origin of life. Despite this importance, it remains unclear how different combinations of tectonic deformation, climate conditions, and surficial and interior processes have shaped geologically diverse paths of satellite evolution, as evident from their widely different surface morphologies. Here I address this fundamental question by conducting geological mapping of Enceladus and Titan, the two end-member icy satellites of Saturn; Enceladus has tectonic activity expressed by erupting plumes along active faults while Titan has a thick atmosphere that exerts strong control on its surface processes and hence surface morphologies. My studies on Enceladus focus on two subjects: (1) the transport time scale for nanoparticles of silica from the ocean floor to the erupting plumes and (2) the role of the non-tidal stress in controlling the phase lag of time-varying plume fluxes that share the same periodicity with the diurnal tide. I assess the transport time scale of silica particles based on experimentally determined scaling relationships for convection systems under rotation and entrainment of particles in thermally-driven convecting fluids. The physics-based analytical relationships obtained from this approach allow the establishment of the size of the silica particles to the thermal regime of the core, which in turn provides the basis for estimating the transport time scale of the particle through the ocean, which I find to be on the order of months. To assess the role of the non-tidal stress in controlling the phase lag of plume eruption on Enceladus, I conducted detailed structural mapping along geyser-hosting faults zones (i.e., the informally named tiger stripes in the literature). My mapping shows that the geysers are preferentially located at local extensional structures along overall strike-slip faults. In order to have simultaneous strike-slip fault motion and local development of extensional structures along the strike-slip faults, coeval shear and tensile failure is required. Imposing this condition and assuming that the peak-eruption time is the result of the superposed tidal and non-tidal stresses reaching the maximum tensile-stress value, I am able to use a stress-decomposition model to determine the static non-tidal stress field along geyser-hosting faults. The required non-title stress field is best explained by lateral viscous flow induced by the gradient of gravitational potential stored in an unevenly thick ice shell.My research on Titan focuses on the geomorphological response in space and time to climate change and tectonic deformation. In this end, I established the spatial distribution and temporal relationships among morphologically distinctive terrains through mapping in the South Belet and Soi Crater regions. The major finding of the work is that dunes and lakes are the youngest geomorphologic units resulting from the youngest climate condition that are superposed on top of hummocky, labyrinth, pitted, and mountainous terrains. The presence of dune fields requires aeolian transport, the lake and labyrinth terrains surface and subsurface fluid-flow activities, and the pitted terrain removal of volatile materials. The oldest mountainous terrain is best explained by early tectonic deformation. The spatial distribution of dunes and lakes is consistent with the global mapping results that climate-sensitive terrains are distributed symmetrically with respect to the equator, reflecting the symmetry of the atmosphere circulation.

The Easter-Salas y Gómez seamount chain (ESC) is a natural laboratory for understanding the chemical heterogeneity in the Earth’s deep interior and the interactions between a mantle plume and spreading center as the Salas y Gómez plume (SyG) is near the East Pacific Rise (EPR), home of the fastest spreading ridge axis observed on Earth. The geochemical variability along the ESC chain was proposed to reflect increased entrainment of the depleted asthenospheric material towards the west as the plume flow is drawn towards the spreading center along lithospheric channels. Furthermore, the ESC is thought to be comprised of two sub-parallel chains, an enriched one that is in the north and a depleted chain in the south. The SyG plume was proposed to be striped with the enriched chain sampling the Pacific Large Low Shear Wave Velocity Province (LLSVP) and the depleted chain sampling the ambient deep Pacific mantle. Noble gases are uniquely useful tracers of mantle geochemical evolution and in identifying primitive material in the plume source. However, there is limited noble gas data along the ESC. Here we report on newhelium and neon isotopes from the ESC lavas and combine them with published data on Sr, Nd, Pb isotopic compositions and water abundances to better understand the origin of the geochemical variability along the ESC and the spatial geochemical variability in the ESC.We find the He isotopes co-vary with other lithophile isotopic compositions and withwater. The 4He/3He values range from a (primitive) value of 39,500 to values that are more radiogenic than MORBs and as high as 111,300. The 20Ne/22Ne values for five of the samples range from atmospheric values of 9.8 to values up to 12.61, which is more characteristic for a plume mantle source and suggests a solar volatile composition. For both the northern and southern chain the 21Ne/22Ne ratios are less radiogenic than MORBs requiring the presence of less degassed material than MORBs in both chains. Neon, therefore, maybe even more sensitive to the presence of primitive material in a plume than helium.We find that the previously observed transition towards more depleted composition in the west is due to incorporation of the more primitive FOZO component that carries low and less degassed 4He/3He ratios. FOZO is not as depleted as the source of mid ocean ridge basalts (MORBs) but is depleted compared to the recycled enriched components in the eastern part of the chain. Thus, the geochemical trend towards the west is due to the inherent heterogeneities present within the plume source. Furthermore, we see a greater proportion of the FOZO component in the southern chain. Previous work has shown that more primitive He isotopic ratios requires a reservoir that is less degassed and processed via partial melting compared to the MORB source and that the LLSVPs might be a suitable reservoir for such a source. If so, we find that the southern chainpreferentially samples material from the LLSVPs.The relationship between He and incompatible elements like water allows us to refine the juvenile water abundance of the FOZO component from the commonly used value of 750 ppm down to 150-230 ppm, indicating that FOZO is a lot drier than previously determined. While the 20Ne/22Ne value of up to 12.61 requires a solar source for neon, the ?D and He isotopic relationshipfor the juvenile water in the deep mantle appears to be derived from a chondritic source. Thus, the deep mantle indicates incorporation of volatiles from different sources during Earth’s early accretion.

The discovery of thousands of planets since 1995 has transformed how we perceive our place in this universe. One of the most profound findings since is the unexpected dearth of close-in planets of sizes 1.5 to 2.0 Earth radii, i.e., a radius valley. This valley divides the population of the most abundant class of planets yet known, those between the sizes of Earth and Neptune, into (1) super-Earths: planets smaller than 1.5 Earth radii with rocky, Earth-like bulk compositions, and (2) sub-Neptunes: planets larger than 2 Earth radii with hydrogen-rich atmospheres or interiors with substantial amounts of ices.The origin of the radius valley is typically attributed to atmospheric escape due to photoevaporation. Through this work, we have demonstrated that atmospheric mass-loss driven by the cooling luminosity of a planet and its host star's bolometric luminosity, i.e., the core-powered mass-loss mechanism, can also explain this observation, even in the absence of any other process. In a nutshell, our work shows that the typical observed exoplanet has an Earth-like interior composition and accreted a hydrogen atmosphere from the protoplanetary disk. However, over millions to billions of years, some planets lost their atmospheres because of core-powered mass-loss and transformed into super-Earths. In contrast, those that survived with their primordial atmospheres are today's sub-Neptunes.For this work, we used analytical theory and numerical simulations to model a planet's thermal evolution and atmospheric escape and explored the impact of this mechanism on planet demographics. We find that the core-powered mass-loss mechanism explains not just the bimodality in planet sizes but even the numerous trends observed in the planet demographics across various planetary and host star properties. For instance, we find that in the planet size-orbital period space, the radius valley slope is -0.11 across FGKM dwarfs, which is in excellent agreement with observations. In addition, my work gives several insights into the nature of these planets. As an example, we find that most close-in super-Earths and sub-Neptunes formed with hydrogen envelopes. This finding has major implications for the chemical evolution of their atmospheres. Finally, we also seek testable predictions of the core-powered mass-loss theory. For example, we predict that the slope of the radius valley decreases in magnitude in the planet size-stellar mass space as the stellar mass decreases from 1 to 0.1 Solar masses. In the same vein, with my collaborators, we find that a powerful diagnostic to distinguish between the signatures of core-powered mass-loss and photoevaporation is to explore the radius valley in the three-dimensional phase space of planet size-stellar mass insolation flux. Many of these tests are being employed today by observational studies. One of our significant findings, also corroborated by other contemporary theoretical and observational studies, is that most observed exoplanets have hydrogen atmospheres interacting with molten or super-critical interiors for millions to billions of years. In our Solar system, we see this for planets such as Jupiter and Neptune. Studies show that such interactions can have far-reaching implications for an atmosphere's composition, structure, and evolution. However, we hardly understand these interactions, and studying them in a laboratory is difficult. The last chapter presents a novel method to address this problem via computational experiments based on density functional theory molecular dynamics. Specifically, we examined how hydrogen and water interact under conditions similar to those found in planets such as Uranus and Neptune. We determined their phase diagram, which is in good agreement with laboratory experiments. Our findings indicate that planets like Neptune and Uranus have regions where hydrogen and water are thoroughly mixed, as well as regions of compositional gradients. We identify the pressure depths where these compositional changes likely happen in Uranus and Neptune and find that these locations are strongly correlated with the variations in observationally constrained density-pressure profiles of these planets. Furthermore, our results help elucidate the physical and chemical processes responsible for Neptune's higher internal heat flux compared to Uranus. We find that this difference in heat flux can be attributed to a higher degree of water-hydrogen demixing in Neptune's interior which would have led to the release of larger amounts of gravitational energy. Additionally, we identified regions where the magnetic fields of these planets are likely generated and discussed how their different magnitudes are also a possible by-product of hydrogen-water mixing properties. Our results highlight the importance of understanding the interaction between the fundamental building materials of planets if we want to develop a comprehensive understanding of how planets, in our Solar system and beyond, form and evolve.

Impact craters dominate the landscapes of many planetary bodies. Among their most striking characteristics are their rays: radial streaks formed by high velocity ejecta launched to great distances. This dissertation investigates the influence of distal ejecta on planetary surfaces by studying two classes of features: secondary impact craters and lunar cold spots.Secondary impact craters form when rock fragments ejected from a primary crater re-impact the surface at high velocity. Individual primary craters have been shown to produce upwards of 10^6-10^9 secondary craters which form nearly instantaneously in geologic time. This has led many to question whether crater chronology models can be applied effectively. In chapter 2, we develop a model for the global accumulation of secondary craters with time for Mars, accounting for the spatial clustering of secondaries. We show that the number of km-scale secondaries produced on Mars may exceed primaries after only a few 100 Ma. However, most secondaries are clustered around their parent primaries, and regions far from large primaries have significantly fewer secondaries than the global average. The crossover diameter between primary and secondary crater production on a typical surface is estimated to exceed 1 km after ~1-2 Ga, though subsequent crater erasure has significantly influenced the number of secondaries visible today. In chapter 3, we produce updated global maps of nighttime temperature for the Moon using data from the Diviner Lunar Radiometer Experiment on the Lunar Reconnaissance Orbiter (LRO). We implement several improvements, including a correction for errors in instrument pointing, which result in a substantial increase in effective resolution. In addition, we develop a model which mostly removes the effect of topography on nighttime temperature by accounting for scattering and emission from the surrounding terrain. These improvements allow smaller and fainter thermal features to be identified than was previously possible.Lunar cold spots are extensive ray-like regions of reduced nighttime temperature surrounding young impact craters on the Moon. In chapter 4, we show that South Ray crater at the Apollo 16 landing site has a faint cold spot. Its temperature anomaly and ~2 Ma age are consistent with the fading rate of other large cold spots. Additionally, we show that the mean depth of astronaut footprints is greater at the Apollo 16 landing site than the other Apollo sites. This suggests that cold spots are caused by a decompaction of the upper regolith, consistent with estimates derived from thermal modeling. In chapter 5, we present the thermophysical properties of a global survey of cold spots and several new cold spots formed during the LRO mission lifetime. We show that the temperature anomaly of new cold spots scales with crater diameter, forming an upper envelope to the properties of pre-existing cold spots. This indicates a greater depth of regolith modification by larger cold spots. Using thermal modeling, we present bounds on the depth of regolith modification for new cold spots and estimate how this scales with crater size.

Many planetary bodies can generate and sustain large-scale magnetic fields. The kinetic energy of electrically conducting fluids inside the bodies converts into magnetic energy through so-called "dynamo" processes. Turbulent thermo-chemical convective flows in a planet’s electrically conducting fluid core often generate a planetary-scale, dipole-dominated magnetic field. The magnetic field generated by dynamo processes acts back on the convective flow via Lorentz forces, creating a complex turbulent magnetohydrodynamic (MHD) system. Investigating the planetary dynamos will elucidate the fundamental dynamics of planetary interiors, providing essential information on the formation and evolution of the host planet. Moreover, strong planetary-scale magnetic fields can shield the planets from high-energy cosmic radiation and charged particles from the stars, making dynamo study crucial for habitability and searching for life on candidate bodies in the solar system and other exoplanetary systems. In this thesis, I present a series of laboratory experiments to investigate the essential force balance and turbulence dynamics of planetary-core-style convection, in which magnetic field, rotation, and thermal buoyancy are applied to a liquid metal (gallium) fluid body on the RoMag device. These experimental investigations help bridge dynamics at planetary core boundaries and the interior of the fluid core, providing valuable insights into the fundamental physics of dynamo processes. They also provided an experimental pathway to connect small-scale dynamics that can be studied in the laboratory and large-scale dynamics within the planetary cores. This pathway is essential as current numerical simulations and experiments cannot capture large-scale dynamics directly. I investigate the MHD effects at the planetary boundaries using a simplified end-member system of core-style convection: non-rotating magnetoconvection (MC). I have characterized a self-sustaining thermoelectric effect in liquid metal turbulent MC with electrically conducting boundaries. The thermoelectric currents at the boundaries generate a large-scale precession of the turbulent convective flow. To explain this phenomenon, I have developed a solid-liquid analytical model that predicts precession frequencies agreeing with the lab data. This model also produces a set of new dimensionless parameters to describe under what conditions the thermoelectric effect could become prominent near the Earth's core-mantle boundary (CMB). Furthermore, I study liquid metal MC's heat transfer and behavior regimes from onset to highly supercritical. I have compared the effects of magnetic constraints in MC with the rotational constraints. With a better understanding of the MC system as a building block, I have carried out a set of rotating convection (RC) and rotating magnetoconvection (RMC) experiments to probe the internal flow dynamics of planetary cores. Our preliminary results hint that the turbulent liquid metal RC can form large-scale, barotropic vortices through an inverse cascade in kinetic energy. This stairway of energy cascade could potentially connect laboratory scale dynamics to planetary scale flows and provide insights into the dynamical origin of the observed large-scale magnetic structures.

Exoplanet science is now focusing on characterizing the physics and chemistry of exoplanet atmospheres, including those of terrestrial-class, potentially habitable planets. In this thesis, I use a combination of laboratory experiments and theoretical modeling to understand two themes related to these atmospheres: (1) their primordial outgassing compositions from an experimental cosmochemistry approach, and (2) the planetary context for observable biosignature gases using modeling tools. There is no first-principles understanding of how to connect a planet’s bulk composition to its initial atmospheric properties. Since terrestrial exoplanets likely form their atmospheres through outgassing, an important step towards establishing this connection is to assay meteorites, remnants of planetary building blocks, by heating and measuring their outgassed volatiles. In the first theme, I use multiple experimental techniques to determine meteorites’ outgassing compositions over a range of temperatures and pressures. I describe the results of heating carbonaceous chondrite samples and measuring their abundances of released volatiles as a function of temperature in a high-vacuum environment. I find that these meteorites outgas significant amounts of H2O, CO, CO2 and smaller quantities of H2 and H2S. I also discuss a complementary bulk element analysis to monitor outgassing of heavier elements (e.g., sulfur, iron, zinc). I compare these experimental results to thermochemical equilibrium models of chondrite outgassing and determine how these experiments can improve atmospheric models and inform the connection between bulk composition and early atmospheres.For the second theme, I perform a comprehensive analysis of the necessary planetary conditions for atmospheric methane to be a compelling biosignature gas. Methane is one of the only biosignatures that JWST can readily detect in terrestrial atmospheres. Therefore, it is essential to understand methane biosignatures to contextualize these imminent observations. Using a combination of multiphase thermodynamic and atmospheric chemistry models, I investigate abiotic sources of methane and determine the planetary conditions for which these sources could be enhanced on terrestrial planets so as to result in false positives. I determine that known abiotic processes cannot easily generate atmospheres rich in CH4 and CO2 with limited CO due to the strong redox disequilibrium between CH4 and CO2, providing the first tentative framework for assessing methane biosignatures.

This thesis consists of three loosely related projects exploring the physics of planetary bodies. The throughline in this research is that I explore how a planetary body's interior influences its exterior -- in particular how heat migrating outward drives evolution and leaves detectable traces of that evolution. Chapter One describes a novel form of volcanism -- volcanism on iron bodies, which we call ferrovolcanism. We predict that metallic bodies were able to host volcanism, making metal the third major type of crustal material capable of being volcanic, in addition to ice and silicate planets. We discuss the potential for its observation by the Psyche mission, its role in the evolution of metallic bodies, and its potential influence on the metallic meteorite record. Chapter Two lays out a way to significantly improve Europa Clipper's ability to measure Europa's global shape, without requiring any extra measurements. By using stellar occultations, measurements that Europa Clipper was already planning to collect, we can supplement radar altimetry to obtain more complete global coverage of Europa. We demonstrated the potential for this combined dataset to significantly improve global fits, which would allow Europa Clipper to better constrain the thickness, rheology, and history of Europa's ice shell. Chapter Three explores the relationship between rotation rate and tidal dissipation in the interior of Jupiter's moon Io. This is motivated by two separate lines of thinking: 1) Io's volcanoes appear to be offset in longitude from where tidal dissipation models predict they should form, and 2) if a satellite is sufficiently fluid - plausible for Io because it is so strongly heated - it is expected to rotate slightly faster than the synchronous rotation rate we see across solar system satellites. We find that because of the rigidity of its lithosphere, we do not expect Io to rotate nonsynchronously on geophysically relevant timescales.

This dissertation thesis is a combination of three projects on magnetism and gravity studies of the Moon and a magnetism study of Mars, each with a heavy focus on uncertainty estimation. The goal of each chapter is to elucidate some portion of the thermal history of the Moon and Mars. Analysis of crustal magnetic fields can explain past dynamo behavior, which is tied to the amount of heat within a planet and how long it retains that heat. Elastic thickness, determined from correlations between gravity and topography, indicates the heat flux at the time of load emplacement and we can use the elastic thickness of a region to determine its formation age. The results from this thesis place constraints on the ancient dynamo behavior of the Moon and Mars (magnetism studies) and on the formation ages of portions of the farside of the Moon (gravity studies).Chapter 1 focuses on lunar crustal magnetic anomalies. The Moon no longer has an active global magnetic field, but evidence of an ancient field can be found in portions of the crust, which have been magnetized in the presence of intense fields thought to be generated by an extant dynamo. Quantifying the magnetization directions of these anomalies elucidates the behavior of the paleo-magnetic field by determining the magnetic paleopole (i.e., the orientation of the dipolar axis). Previously, distinguishing between paleopole locations was impossible because of their large uncertainties. Without distinguishing between paleopole locations, determining the history of the lunar dynamo is impossible.I propose an alternative method of estimating uncertainty using a Monte Carlo method to add synthetic noise to the best-fit modeled fields, which allows us to determine how easily perturbed the magnetization direction is in the presence of noise (i.e., uncorrelated anomalies). The new method more accurately describes the uncertainty of the inversion method and allows for better discernment of paleopole locations. I determined that the dipolar axis of the lunar dynamo must have been misaligned with the spin-axis at some point in lunar history, or that there were significant multipolar components to the magnetic field.Chapter 2 focuses on gravity and topography studies of the Moon. I use admittance analysis to determine the lunar elastic thickness and how it varies across the Moon. Elastic thickness allows us to determine the heat flux at the time of load emplacement, which in turn elucidates the thermal history of the Moon. Regions of low elastic thickness indicate high heating at the time of loading, and we can infer these locations formed earlier in lunar history than areas with higher elastic thicknesses. However, as in Chapter 1, variations in elastic thickness are meaningless without a clear estimate of uncertainty to distinguish between values.In this chapter, I describe how to determine elastic thickness using the spectral domain and the Markov chain Monte Carlo (MCMC) technique. Careful consideration is given to where these techniques are valid, including an analysis of the uncertainties from the MCMC technique using synthetic testing. I find that there are several locations on the Moon with low elastic thickness, implying these regions formed very early (

The goal of this thesis is to investigate the geologic origins of two different lunar magnetic anomalies: Reiner Gamma on the lunar nearside, and the Gerasimovich-area anomalies on the farside. Chapters 2 and 3 are concerned with Reiner Gamma, while Chapter 4 is concerned with the Gerasimovich-area anomalies. Understanding the geologic origins of these lunar magnetic anomalies is key to progressing our understanding of the Moon’s magnetic history, and this work carries out these investigations using data from lunar orbiters.In Chapter 2, I present evidence that the magnetic anomaly Reiner Gamma overlies a relative-negative Bouguer gravity anomaly. This gravity anomaly is likely a buried impact crater, and I determined its age, and thus the age of the Reiner Gamma magnetic source bodies, to be between ∼3.3 Ga and ∼3.9 Ga, which are the approximate temporal bounds of mare volcanism. This range of ages coincides with the putative high-field era of the lunar dynamo (∼3.56–3.9 Ga), thus, the high magnetization of Reiner Gamma could be due to deposition during a time of a high-magnitude ambient field.In Chapter 3, I present observations that a portion of Reiner Gamma appears to have been demagnetized by the emplacement of a dome in the nearby Marius Hills volcanic complex. I created three different models to determine if the observed magnetic anomaly was diminished by thermal demagnetization. First, I created a flexure model, which approximated the dome as a buried laccolith and determined the burial depth of the laccolith based on its surface expression. Second, I created a thermal model, which determined the time-temperature history of areas around the laccolith. Third, I created a dipole model that simulated thermal demagnetization via decreasing the moments of some of the source dipoles. These three models, taken together, suggest that thermal demagnetization did occur at Reiner Gamma, and we use this result to estimate source body burial depths of

In this thesis, I investigate the nature of two small body populations; the irregular satellite populations of the giant planets and the properties of fragmented nuclei of comets. In both cases the objective is to understand evolutionary processes acting on primitive solar system objects. An optical color survey of 43 irregular satellites enabled color comparisons with other small body populations that may reflect upon the origin of the irregular satellites. Ultrared matter (color index B-R ≥ 1.6), while abundant in the excited Kuiper belt and Centaur populations, is depleted from the irregular satellites. Also, the color distributions of the irregular satellites at each giant planet are statistically similar to each other, consistent with a common source region and/or evolutionary mechanism. Separately, the current observed supply of comets allows for estimates on the masses of their outer solar system source regions, however, comet fragmentation may occur more often than previously thought, which will lead to shorter estimates of comet lifetimes than predicted. As a case study, I analyzed archival Hubble Space telescope images of comet 73P/Schwassmann-Wachmann 3 (73P). The measured rotation period of the nucleus is much longer than the critical period for rotational instability for any reasonable nucleus density and shape, even in the absence of tensile strength. The data also show hundreds of fragments within 73P-B and 73P-G on which photometry was used to measure the brightness distribution of the fragments. I also measure the motion of these fragments and find the relative speeds of the fragments within 73P-B are a few m/s, implying an impulsive breakup about 7 days prior to the observations. Both the irregular satellites and comets are small bodies comprised of primitive material. The origin and evolution of the small bodies describe the early formation and evolution of the solar system itself.

The principal theme of this thesis is to see the planetary processes underlying observable variations. Various planetary processes in atmosphere, surface and interior exert long-term or short-timescale influence on the superficial properties that can be easily observed. This thesis combines observations with theoretical modelling to mine out the essential information of the Venus atmosphere and Enceladus’s ice shell and promote the understanding of variations and evolution of our Solar System. The Venus atmosphere is essential for understanding why Earth and Venus have evolved so differently even though they are similar in mass and radius. However, the complicated coupling among atmospheric dynamics, chemistry and clouds on Venus is still not well investigated. Using chemical-transport models (CTMs), I aimed to disentangle the effects from various atmospheric processes and guide observations of future Venus missions (DAVINCI+, VERITAS and EnVision).Recent ground observations from TEXES/IRTF have for the first time revealed the co-evolution of SO2 and H2O at the cloud top of Venus. The two species exhibit a temporal anti-correlation. I used a one-dimensional CTM to investigate the mechanism of this anti-correlation. I found that the anti-correlation can originate from the sulfur photochemistry in the middle atmosphere, while the variations can be caused by the lower-atmosphere perturbations. Eddy diffusion alone cannot explain the observations. This study emphasizes the urgent need of detecting the cloud layer and the deep atmosphere of Venus.The instrumentation TEXES/IRTF also found a two-peak feature in the local-time distribution of SO2 at the cloud top, consistent with SPICAV/VEx observations. I developed a two-dimensional CTM and connected it to a Venus GCM to investigate this feature. My work revealed that the two peaks can be explained by the combination of the semi-diurnal tides and the retrograde superrotating zonal (RSZ) flow. SOIR/VEx also observed a statistical difference between terminators for CO in the upper atmosphere. From my simulations, this difference can be explained by the transition from the RSZ flow to the sub-solar to anti-solar (SS-AS) circulation. My work also discussed mechanisms underlying the local-time distributions of other species and implied a complex coupling of photochemistry and dynamics in the Venus mesosphere.The Cassini flyby observed that Enceladus currently experiences a high surface heat flow. This leads to the question whether its ice shell is in steady state or its sub-surface ocean is freezing with time. To support the steady state of the ice shell, amounts of endogenic heat are required, which are currently thought coming from tidal dissipation. However, the exact process that produces sufficient tidal dissipation to match the observations remains elusive. I used a libration model to investigate the heating effect of the diurnal forced libration. I found that although the forced libration enhances the tidal dissipation in the ice shell, the total heating in the shell is still insufficient to match the observed surface heat loss. If Enceladus is in steady state, there should exist a large heat source beneath the shell, either in the ocean or in the core. If in steady state, Enceladus is likely to be in a stable thermal equilibrium, which resists small perturbations on the ice shell. This implies that thermal runaway or episodic heating is unlikely to originate from the librations of the ice shell.

This thesis studies the formation of planets and the destruction of stars that explode as supernovae. To understand planet formation and recreate the diversity of exoplanets that we see in our galaxy, we need a better understanding of protoplanetary disks. We develop a time-evolving model for the solid particles in these disks, tracking maximum particle size and surface density in their outer regions. This time dependent particle modeling shows us that disks pass through several regimes as more particles drift inwards, lowering maximum particle sizes and surface densities. By combining this model with our model for pebble accretion, we are able to estimate growth rates for injected protoplanetary cores. Applying our models to a sample of seven disks, we find that planetesimals should grow rapidly, particularly early in the disk's lifetime before it has drained too much. To reproduce observed planetary masses, we find that protoplanetary cores must reach planetesimal sizes before the ages of typically observed disks.Turning our attention to smaller particles, we develop a new model for pebble accretion to explore the growth of lower mass protoplanetary cores. We apply full gas effects to the dynamics of small cores, finding that their new velocities play a crucial role in understanding their growth. Additionally, we model full probability distributions for the relative velocities of interacting particles, rather than simply studying the mean velocity. As we extend the model down to cm scales, we find that gas interactions play the dominant role in setting relative velocities for inter-particle collisions. At these small scales especially, particles in the low velocity tail of the velocity distribution can be accreted particularly rapidly, enhancing growth. In examining these rare growth interactions, our model suggests a path for solid body growth across the meter-scale barrier, up to planetesimal masses.Finally, I also present my work statistically modeling the ages of Type Iax supernova progenitor stars. In this study, we use Hubble Space Telescope photometry of the stellar regions around Type Iax supernovae explosion sites to estimate ages for these regions. This is performed by statistically rigorous fitting of theoretical stellar models to our multi-band photometry. We are ultimately able to generate probability distributions for the ages of each supernova we consider, generating strong constraints on the formation channel for these events.

Ultra-low frequency (ULF) waves have been commonly detected in inner and outer Solar System planetary magnetospheres. ULF waves with periods that are of the order of the Alfv�n wave transit time in a planetary magnetosphere are often associated with resonant field lines that can be excited by either internal or external triggers. In this dissertation, we present a comparative study of standing Alfv�n waves in realistic magnetic field and plasma density models for the Kronian and Terrestrial magnetospheres.At Saturn, we carried out the first calculation of standing wave resonances in a realistic model of the Kronian magnetosphere. The resulting eigenperiods of the 4th harmonic vary little with radial distance from 5 to 20RS, matching the quasi-periodic 60-minute (QP60) waves that have been reported in observations at a wide range of local times and radial distances. We have used 13 years of the Cassini magnetometer data and identified quasi-periodic fluctuations with periodicities of around 30 minutes (QP30), 60 minutes (QP60), and 120 minutes (QP120) that reoccur at the period of planetary period oscillations (PPO) of roughly 10.7 h. We suggest that these correspond to even-mode harmonics of Saturn’s magnetic field lines that are excited by the periodic vertical flapping of Saturn’s magnetotail.At Earth, we evaluated the field line resonances for several plasma density models and investigated the effects of geomagnetic activity on the field line eigenperiods up to L = 10 at all magnetic local times. We find that the dipole-field and the time-of-flight approximations used to estimate the fundamental eigenperiods of standing waves lead to significantly different eigenperiods, especially during active times. Additionally, the eigenperiods are shown to be more sensitive to the magnetic field configuration and equatorial plasma densities than the distribution of the mass density along the field lines.Saturn’s and Earth’s magnetospheres have large differences in scale, rotational speed, and plasma distribution. Despite these differences in the parameter regimes, field line resonances are important at both magnetospheres. The QP30, QP60, and QP120 waves at Saturn reoccur consistently at a PPO period, with the highest transverse to parallel magnetic perturbation power in the post-dusk sector, suggesting mainly an internal driver such as the vertical flapping of the magnetotail. At Earth, the field line resonances that are associated with Pc4 to Pc5 waves do not demonstrate similar periodicity in reoccurrence as at Saturn and are likely to be driven externally by processes such as the solar wind dynamic pressure variations.

The three planets of the Inner Solar System with significant atmospheres, Venus, Earth, and Mars, can be described as representing three “climate archetypes” of terrestrial planets: Venus is hot, dry, and rotates slowly; Mars is cold and dry, with fast rotation similar to Earth; Earth is the “middle ground”, warm enough to sustain liquid water on its surface but not so warm it evaporates away. These archetypes can be placed as endpoints on a spectrum of climates, where adjusting one or more planetary parameters can move a climate from one archetype to another, e.g. drying the surface can move an Earth-like planet towards the Venus and Mars archetypes. In addition to the three inner planets, there is one additional body in the Solar System that has a thick atmosphere and solid surface: Titan, a moon of Saturn. Titan presents a unique opportunity in observable planetary climates because it has a volatile liquid, or condensable, on its surface in the form of methane. This methane is able to evaporate to form clouds (Turtle et al., 2018) and likely rain (Turtle et al., 2011), but is mostly restricted to large polar lakes (Lunine and Lorenz, 2009) with the rest of the surface a vast desert (Mitchell and Lora, 2016). This means Titan’s climate archetype is between the ocean-dominated Earth and the fully-dry Venus/Mars.In this dissertation, we seek to further investigate the “in-betweens” of these climate archetypes, focusing on the transition between an Earth-like planet and a Titan-like one. To accomplish this, we recreate a Titan-like climate using an Earth-like global climate model (GCM) by varying a small set of planetary parameters. We first limit the available water by placing a continental land strip centered on the equator and varying its width. This mimics Titan’s dry tropics and wet poles, and could be similar to past continental arrangements in Earth’s history. Second, we take three of these land strip widths and vary the rotation period, starting with Earth’s rotation and moving towards Titan’s (16 Earth days). Third, for the same three land strip widths and using Earth’s rotation, we vary the volatility of the condensable via a constant multiplied to the saturation vapor pressure. Titan’s condensable, methane, is more volatile under Titan’s surface conditions than water is on Earth, resulting in high specific humidities. By artificially increasing the saturation vapor pressure, we can approximate this effect without changing the properties of the condensable.We find that simply replicating Titan’s parameters in our simulations does not fully reproduce Titan-like conditions. In addition, we find that it is possible to reproduce key Titan-like features by varying only the width of the equatorial land strips. This may indicate that there are many possible “in-between” states an Earth-like planet can have that span the gap between the Earth and Titan climate archetypes. It also suggests Titan’s current climate is primarily dependent on its surface liquid distribution, meaning an Earth-like planet with similar topography is likely to display the same features.

Saturn’s largest moon, Titan, provides a new perspective on planetary climate. It is larger than Mercury, has a 16-day rotation period, 29.5-year annual cycle, and a ~1.5-bar nitrogen atmosphere. Titan has a fully developed atmosphere, analogous to that of Earth, and methane plays a similar role to water in the hydrological cycle on our planet, generating clouds, storms and precipitation. Titan’s clouds have been under investigation since their detection in 1995 with ground-based telescopes and were observed in detail during the Cassini-Huygens mission to the Saturn system. “Cassini” orbited Saturn and its moons from 2004 to 2017, giving unparalleled views that have led to countless discoveries and clouds are one of many fascinating Titan phenomena revealed by it. To this day, cloud formation mechanisms, dynamics and duration of the associated storms are still not fully understood and are the subject of ongoing study. The central goal of this work is to provide a general physical interpretation of observed storms and their relation to atmospheric dynamics of the moon. Two previous studies are the foundation for this research: Mitchell et al. (2011), who developed a process for interpreting Titan’s cloud morphologies and precipitation through a combined analysis of observations and general circulation model (GCM) simulations; and Turtle et al. (2011), who reported the first evidence of seasonal changes on the moon obtained from Cassini cloud data.This dissertation presents a survey of Titan’s mid-latitude clouds, as seen from space by the Cassini Imaging Science Subsystem (ISS) instrument, compares a subset of the observed clouds to the methane storms produced in a climate model of Titan, and infers the underlying storm dynamics by connecting the two. ISS is a multi-wavelength – ultraviolet to near-infrared – camera specifically designed to take high resolution images from the top of the atmosphere to the surface of Titan piercing through the thick haze located in the stratospheric layer of the moon, which typically blocks the view of tropospheric clouds underneath it.This study starts with an analysis of the physics of clouds applied to Titan’s conditions and a microphysical cloud scheme to show how the abundant haze particles in the atmosphere are likely the seeds for methane droplets that catalyze cloud formation. Next, the ISS image archive is searched for cloud phenomena and various types of storms are surveyed. This is followed by Image processing, that require the conversion of raw images into maps with global locations of the clouds and the production of enhanced views of cloud features against the surface background to reveal their morphology. We then search for storms with temporally resolved observations and use their spatio-temporal distributions to identify the atmospheric dynamics behind them, including Rossby and gravity waves. Although many clouds/storms are identified, only two of them provide clear spatial and temporal information that allow this type of analysis.The manuscript then pivots to analysis of methane storms in model simulations of Titan’s climate using the Titan Atmosphere Model (TAM; Lora et al. 2015) with full surface hydrology (Faulk et al., 2019). The spatio-temporal features of observed clouds combined with the simulated storms at the same season as the observations suggest that just as waves organize storms on Earth, they do so on Titan as well. The results of the study strongly indicate that Titan’s cloud formation and propagation are associated with Rossby and equatorial Kelvin waves, and perhaps combinations thereof, and that the clouds/storms can persist for weeks and perhaps much longer as they propagate around the moon’s globe – a phenomenon referred to in this work as “persistence”.These findings offer a glance into the complex phenomenology, dynamics and persistence of Titan’s clouds. The methodology developed in the course of this work for comparing the spatio-temporal distribution of observed clouds to analog storms in TAM is novel, while also being consistent with previous studies focused either on the spatial distribution or seasonal evolution of observed clouds. Future missions to Titan, including the funded Dragonfly mission, will facilitate further model-data comparisons, for instance the long-term persistence of the Kelvin wave in TAM. This methodology, documented in detail in a “cookbook”, provides a set of useful tools and guidance for future explorations of the clouds and storms of Titan.

An understanding of planetary histories and characteristics requires an empirical connection between planet formation and evolved planets---a long-sought goal of astrophysics and planetary science. This connection is now increasingly possible due to simultaneous revolutions in the observations of protoplanetary disks and exoplanet atmospheres. A crucial step towards relating these observations of different evolutionary stages is to characterize the fundamental properties of both disks and atmospheres. The work presented in this dissertation uses microphysics---i.e., the physics that governs the evolution of small particles--- to constrain the fundamental properties of both disks and atmospheres. This dissertation provides evidence that protoplanetary disks are more than an order of magnitude more massive than previously appreciated, that the detailed properties of clouds shape observations of exoplanet atmospheres, and that the physics of modeling clouds gives a new understanding of the solid content and composition in protoplanetary disks. Clouds on extrasolar worlds are abundant and interfere with observations; however, little is known about their properties. Herein, cloud properties are predicted from first principles and are used to investigate and explain the novel observational properties of hot Jupiters---massive planets close to their host stars. This work describes the use of clouds in tracing fundamental planetary properties and develops a method for probing non-uniform cloud properties using near-future observations. The total mass available in protoplanetary disks is a critical initial condition for understanding planet formation, however, the surface densities of protoplanetary disks are largely unconstrained due to uncertainties in the dust-to-gas ratio and carbon monoxide (CO) abundance. In this dissertation, a new set of models (dust-line models) are developed that reconcile theory with observations of protoplanetary disks and create a new set of initial conditions for planet formation models. These models use recent, resolved, multiwavelength observations of disks in the millimeter to constrain the aerodynamic properties of dust grains and infer the total disk mass without an assumed dust surface density or tracer-to-total mass ratio. This work provides a picture of protoplanetary disks where they are significantly more massive than was previously appreciated. These qualitative changes to models of protoplanetary disks thus have significant implications for theories of planet formation; particularly for the important processes where the amount of gas determines the evolution of the solids. The techniques used in modeling clouds in exoplanet atmospheres are then combined with the dust-line models of protoplanetary disks to show that the observed depletion of CO gas in well-studied disks is consistent with freeze-out processes in a moderately diffusive environment. This new model of ice formation and evolution in disks is able to use existing observations to constrain three crucial parameters that control planetary formation, namely: the solid and gaseous CO inventory at the disk midplane where planets form, the bulk disk diffusivities and mixing characteristics, and the disk mass--through resolving inconsistencies in estimates of total mass using different tracers.

This thesis is constructed around two distinct topics. The first is the formation history of the Earth and the Moon. The hafnium/tungsten (Hf/W) isotopic system can act as a chronometer for planets forming in the early solar system. To study possible planetary formation scenarios, I model the isotopic evolution of planetary embryos as they form rocky planets during collisions in N-body simulations. In Chapter 2, I show that the fast accretion timescales of the Grand Tack scenario require highly efficient re-equilibration of W to produce an Earth with observed mantle W isotope anomaly (excess of radiogenic tungsten compared to non-radiogenic). Such a high level of re-equilibration is not supported by fluid dynamic experiments, and this result suggests the Grand Tack scenario builds the Earth too quickly. The Earth and Moon share a very similar isotopic fingerprint: many chemical isotopes found in lunar rocks are nearly identical to ones found on Earth. It is particularly interesting that they also share a near-identical W isotope anomaly, because simply starting from similar material is not sufficient. This system evolves depending on the collision history of bodies, so the Earth and Moon sharing W isotopic values require an explanation. The canonical model of the Moon formation holds that it is mostly made up of material from Theia, the impactor into Earth. In Chapter 3, I apply the isotopic evolution model to the canonical Earth-Moon impact formation scenario. Using 242 N-body simulation results, I demonstrate the likelihood of forming an Earth and Moon with near-identical W isotope anomaly is less than 5%. This suggests that an alternate explanation for forming the Moon with Earth material may be necessary to explain the similarity in W isotope anomaly.The second topic is understanding the temperature, zonal wind, and general circulation that occurs in the middle atmosphere of Jupiter. The Voyager and Cassini spacecraft, along with many ground-based telescopic observations, have provided zonally averaged distributions of temperature, gaseous species, and haze particles in Jupiter’s upper troposphere and stratosphere. Measurements of wind speed are derived from cloud movement near the 0.5 – 1 bar pressure level, but we have no measurements of circulation in the stratosphere. Historical models of this region have used 2D linearized equations of motion and simple radiative calculations. In Chapter 4, I present a state-of-the-art 2D dynamical model of Jupiter’s middle atmosphere with realistic radiative transfer. A dynamical model with a simple frictional drag force, representing eddy forces that damp the mean zonal wind, is able to reproduce some of the small latitudinal temperature variations seen on Jupiter. However, none of the models tested were able to produce strong >5 K variations observed in the low-to-mid latitudes between 1 – 500 mbar. This suggests that localized wave forcing plays a dominant role in shaping the temperature distribution in the middle atmosphere of Jupiter. I also show that polar temperatures are strongly dependant on the chosen optical properties of stratospheric haze, and further work constraining haze opacity is needed to accurately model heating and cooling at the poles.

In this work, exoplanet-research is combined with the study of the solar system in order to assess differences and similarities between rocky bodies in the Milky Way. To evaluate rocky bodies outside of the solar system, I utilize optical spectroscopy to study polluted white dwarf stars, dense stars that show accretion of planetary material. By observing polluted white dwarfs, we can measure elemental abundances from the rocky and icy bodies that previously orbited the star. Specifically, I conduct observations using the KAST Spectrograph on the Shane 3-meter telescope at Lick Observatory and the High-Resolution Echelle Spectrometer (HIRES) on the Keck I Telescope, as well as evaluate compiled literature data. Generally, the elemental compositions of extrasolar planetesimals closely resemble those of rocky bodies in the solar system. In this work, a more detailed comparison with solar system meteorites and planets shows that oxidation of planetesimals prior to planet formation is common among extrasolar rocks. Overall, the processes that lead to the geochemistry and much of the geophysics of Earth is normal compared to the current sample of extrasolar planetesimals. Additionally, the origin of excesses in spallogenic nuclides in polluted white dwarfs is investigated. The MeV proton fluence required to form the high Be/O ratio in the accreted parent bodies of two polluted white dwarfs (GALEX J2339-0424 and GD 378) is consistent with irradiation of ice in the rings of a giant planet within its radiation belt, followed by accretion of the ices to form a moon that is later accreted by the WD.

Polished thin sections of ten eucrites, meteorites originating from the asteroid Vesta, were examined using polarized light microscopy, photomicrograph mapping, and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) to investigate their compositions, with particular emphasis on elements known to be volatile at planetary formation conditions. The samples were assessed for pristinity and grouped into impact melts, granular, and subophitic textures. LA-ICP-MS analysis was performed on selected samples for individual pyroxene and plagioclase grains – the main silicate phases - as well as fusion crust, to measure major and trace element abundances, with an emphasis on moderately volatile elements (MVE’s) such as Cs, Zn, K, Rb, and Pb. Incompatible trace element abundances are in generally good agreement between samples, and both plagioclase and pyroxene grains show depletions in the MVE’s. Modal reconstructions were done using plagioclase and pyroxene and compared to bulk rock data. They fairly accurately recreate the bulk rock, besides the most incompatible elements, which must exist in minor accessory minerals known to occur in eucrites (e.g., apatite, zircon, baddeleyite). Fusion crust measurements, along with ALHA 81001 (a fine-grained impact melt,) are almost identical to the bulk rock data, confirming their usefulness as a proxy for bulk rock composition. Volatile abundances were assessed using Rb/Ba, Rb/Sr, and Zn/Fe ratios, which compare an MVE to a refractory element. These show eucrites having similar levels of volatile depletion to the Moon. Finally, normalized elemental abundances were plotted versus the element’s volatility, in which volatile elements fall along a downward trend, with the slope indicating the level of volatiles present. From this perspective, eucrite volatile abundances fall in between those of the Earth and the Moon.

The magnetic fields of planets and other bodies are created and sustained due to the turbulent motions of an internal fluid layer, a process known as dynamo action. Forward models are required to characterize the dynamics of rotating convective turbulence driving dynamo action due to the inability to obtain direct measurements of the internal fluid layers of planetary bodies. The characteristic flow velocities and length scales of dynamo systems remain poorly constrained due to the difficulty of modeling realistic planetary core conditions. Thus, the goal of this dissertation is to explore these key properties of core-style convection. To do so, I have conducted novel experiments aimed to better quantify the features of quasi- geostrophic turbulence using the UCLA large-scale rotating convection device, ‘NoMag’.I have completed a systematic study to simultaneously measure the heat transfer and bulk velocities of different rotating convective regimes at some of the most extreme laboratory conditions possible to date. The study of heat transfer is employed in most forward models of core-style convection. In laboratory experiments in particular, due to the relative difficulty of collecting velocity measurements, those of heat transfer alone are assessed, the dynamics of which are assumed to describe the the bulk velocity dynamics of the system. On the contrary, I utilize laser doppler velocimetry to obtain measurements of bulk velocities concurrently with the collection of temperature measurements for the characterization of system heat transfer. I find that heat transfer behavior is consistent with the results of past studies and is largely controlled by boundary layer dynamics. I further find that velocity behaviors do not directly coincide with heat transfer behaviors in the parameter space studied. Instead, I show that a dynamical flow regime of quasi-geostrophic turbulence relevant to core flows is robustly reached, suggesting that it is possible to access realistic bulk dynamics in models that remain far from planetary core conditions.Using the results of this study, I estimate the characteristic length scales of the flows of each experiment. These estimates from my data are compared with length scale estimates of numerous numerical models of planetary core convection. I conclude from this meta-analysis of forwards models that all evidence to date suggests that the theorized characteristic length scales of planetary dynamo systems co-scale with one another and are thus non-separable.In two other studies that comprise the remainder of this dissertation, I further examine the applicability of laboratory models towards planetary settings. An experimental study on the influence of centrifugal buoyancy on rotating convection in water and in liquid metal was completed, where results agree with the recent numerical work of Horn and Aurnou (2018). It is found that the transition from Coriolis to centrifugally dominated convection depends on the strength of the centrifugal buoyancy relative to the gravitational buoyancy and the geometry of the cylinder in which experiments are conducted. These results are useful to ensure that the regime of rotating convection explored in a given experiment is relevant to planetary core flows, i.e. not centrifugally dominated. Separately, I conducted a series of spin up experiments with well-established theory to calibrate the NoMag apparatus and its measuring components. Further, the results from spin up experiments conducted with rough boundaries might have geophysical implications for the possible viscous coupling at Earth’s core mantle boundary, as well as turbulent mixing in the global ocean.The results of the studies presented in this dissertation clarify the relevance of long theorized and poorly tested dynamic length and velocity scalings of planetary core flows. Flows that are quasi-geostrophically turbulent are robustly observed in the laboratory data collected in this dissertation. The need for next generation models of planetary core flows is motivated by the results of the work herein. In particular, studies in which the characteristic length scales of core-style flows are directly quantified will undoubtedly enhance ourunderstanding of the multi-scale turbulent physics driving planetary dynamo systems.

This thesis is composed of two distinct themes. The first concerns the chemical structure of Venus's atmosphere. Venus's atmosphere can be roughly separated into lower and middle regions separated by a thick cloud deck. When modeling the chemistry of Venus' atmosphere past researchers have focused on these regions separately. In doing so they have made conflicting assumptions about the cloud region that connects them. In Chapter 2 I present the first detailed chemical model of Venus' atmosphere that includes both the lower and middle atmosphere. This model is used to characterize the chemical recycling pathways of observed trace species. In this study I find that there also exists a yet unidentified sink of sulfur-dioxide in the Venusian clouds.The second theme of this thesis concerns understanding the interior structure and history of Kuiper Belt objects. In July 2015, NASA's New Horizons spacecraft made its close flyby of Pluto. This opened a new era for understanding not only of Pluto, but also the nature of Kuiper Belt objects generally. To this end I developed a model of how the bulk density of Kuiper Belt objects would evolve through time due to changes in porosity and the melting and refreezing of a subsurface ocean. In Chapter 3 I apply this model to Pluto and its largest moon Charon. I found that the density contrast between Pluto and Charon is large enough that it can only be reasonably explained by a difference in bulk composition (eg. rock to ice ratio). In Chapter 4 I apply this model to bulk density measurements of Kuiper Belt objects (KBOs) generally. It has previously been observed that small KBOs have a much lower bulk density than their larger counterparts. I have found that this can naturally be explained by smaller KBOs being more porous. This difference in porosity is due to the longer cooling timescale of large KBOs causing them to warm more from the heat of radioactive decay. This in turn causes the ice to viscously relax away porosity. Small KBOs in contrast can efficiently conduct out this heat while staying cold and rigid. Because this depends on the abundance and heat production of radioactive elements, I have used this density information to place a constraint on when these objects formed.In Chapter 5 I revisit Pluto examining what the observed tectonics imply about the history of Pluto's subsurface ocean. Namely I consider whether these observations are more consistent with Pluto having that ocean shortly after formation or developing it over a longer timescale through radioactive decay of long lived radioisotopes. If Pluto did form its ocean slowly, this would have caused global compression for which we find no tectonic evidence. If, however, Pluto started with an ocean we predict two stages of extension which is far more consistent with the observed geology.

This thesis comprises two separate but interesting projects that attempt to constrain the internal history of planetary bodies. The first set attempts to interpret the Moon's internal thermal history from the relaxation state of lunar impact basins. As the Moon cools, impact structures degrade at a slower and slower rate. This can be observed in maps of lunar topography and crustal thickness. This analysis, however, was greatly enhanced by the GRAIL spacecraft mission to the Moon. In Chapter 2,I present the first relaxation analysis of the most up-to-date complete lunar impact basin catalog. With the addition of ~6 new impact basins and the re-qualification of other basins, a basin relaxation transition is clearly observed in the lunar impact record. This relaxation transition signal can be used to constrain and link lunar solidification and cooling models with impact chronology models. In that study, I find that if the lunar surface experienced a lull in basin-class impacts it must have solidified and cooled rapidly following its formation.The second project involves two studies that try to understand the thermal history of Pluto and its moon Charon. In July 2015, the field of Kuiper Belt Objects was greatly widened with the arrival of the New Horizons spacecraft at the Pluto-Charon system. One of the major discoveries of that mission was the prevalence of extensional tectonic features on both worlds, a likely signal of a frozen-out (or possibly still freezing) subsurface ocean. In Chapter 3, I characterize the large extensional tectonics features in the encounter hemisphere on Pluto. Then by comparing the features to topographic flexure models, I was able to constrain the maximum surface heat flux experienced by Pluto. This showed that Pluto's internal evolution matches thermal models that primarily use a radiogenic heat source.Although Chapter 3 put maximum heat flux constrains on the thermal history of Pluto, the constraints can be improved upon and expanded to include analysis of Charon's surface. In Chapter 4, I create and use limb profile topography of Pluto andCharon to understand the differences in the morphological and interior history of the two worlds. This is achieved by calculating the topographic variance spectra from limb profiles, which typically results in a single-power law spectrum. While this typical case holds for Pluto, it does not for Charon which displays a characteristic wavelength. My analysis further constrains an upper limit for Pluto's maximum surface heat flux, but it also sets a range for the absolute maximum heat flux for Charon that cannot be solely explained by radiogenic heating. This implies that an extra heat source, probably tidal heating, was necessary.

The low obliquity of Mercury and the Moon causes topographic depressions located near their poles to cast persistent shadows, which may cold-trap volatiles for geologic time periods. Despite their similar thermal environments, telescopic and remote sensing observations have previously detected thick, pure water ice deposits near the poles of Mercury but not the Moon - where ice was found to be superficial or mixed with the regolith. This work attempts to resolve the apparent difference between the two planetary bodies employing physical models and spacecraft observations. We study how topographic roughness affects the temperature distribution and the ensuing prevalence of cold-traps, and constrain the amount, age and origin of polar ice deposits on Mercury and the Moon. Our results suggest that the difference between the amount of cold-trapped volatiles on these planetary bodies may not be as significant as previously thought, and that the presence of heavier carbonaceous volatiles on Mercury may explain the higher purity of its ice deposits relative to the Moon.

The ancient crust of Mars contains a remarkable record of the planet's early history. With 45% of the martian crust dated to the Noachian (>3.7 Ga) and another 30% of the crust dated to the Hesperian (>3 Ga), Mars provides a unique opportunity to examine processes that can influence the early evolution of terrestrial planets. Observations of Mars' surface motivate hypotheses as to what processes controlled the planet's early evolution and how the surface developed into its present form. Key features to explain for early Mars are the formation of the crustal dichotomy, the massive Tharsis volcanic province, and the prevalence of features indicating widespread early water. Hypotheses for the formation of these features can be tested using geodynamic models. For the origin of the crustal dichotomy, both endogenic (mantle convection) and exogenic (giant impact) mechanisms have been proposed. We examine if a hybrid model in which a giant impact can influence the global pattern of mantle convection. We find that a superplume can form on Mars following an early giant impact, and examine the subsequent superplume dynamics and migration in relation to the formation of Tharsis and remanent crustal magnetism. For a purely giant impact origin of the crustal dichotomy, we find that the crustal structure of the dichotomy is difficult to reproduce with high-resolution impact models. We also examine evidence for early oceans on Mars, finding that deviations in proposed paleoshoreline elevations from present-day equipotential surfaces can be explained by deformation due to the emplacement of Tharsis and other surface loads. Our results suggest that Mars paleoshorelines follow paleo-equipotentials, supporting the ocean hypothesis. Overall, our results provide insight into the early processes that may have occurred on Mars, and their effect on the subsequent evolution of the planet.

We have created a comprehensive set of laboratory bidirectional measurements of lunar soil samples from Apollo missions 11, 12, 15, 16 and 17. The lunar soil samples successfully fit photometric models from the laboratory data to the solar spectrum averaged hemispheric reflectance as a function of the incidence angle. The Apollo 11 (vial 10084) and Apollo 16 (vials 61141 and 68810) soil samples are representative end member samples from the Moon’s dark lunar maria and bright lunar highlands. We used our solar spectrum averaged albedos in a thermal model and compared our model-calculated normal bolometric infrared emission curves with those measured by the LRO Diviner Lunar Radiometer Experiment. We found excellent agreement at all of the Mare Apollo sites (Apollo 11, 12, 15 and 17) with Diviner at their landing sites. However, the sample from the Apollo 16 site, had an albedo measured in the laboratory that was 33% brighter than those required to fit the Diviner infrared data. We attribute this difference to increased compaction and decreased maturity of the laboratory sample relative to the natural lunar surface and to local variability in surface albedos at the Apollo 16 field area that are below the spatial resolution of Diviner. We address the idea of compaction as a possibility for the discrepancy of the Apollo 16 soil with additional measurements. We show that increasing the density and decreasing the porosity of the Apollo 11 and 16 samples increases the albedo of the samples. The albedo of the Apollo 16 highlands soil sample is nearly twice as sensitive to the effects of compaction as the mare Apollo 11 soil sample.

Uranus and Neptune are representatives of the `ice giants', one of the most common classes of exoplanets \citep{Fressin2013}. Thousands of exoplanets have been discovered thanks to the \textit{Kepler} mission, and soon the James Webb Space Telescope will characterize their atmospheres in unprecedented detail. Such work will rely on the observations, techniques, and analysis used to study the Solar System's gas giants. However, in many ways our own ice giants remain poorly understood. In this dissertation, I use multi-wavelength observations of Neptune to better constrain the bulk properties and dynamic patterns within the planet's upper atmosphere. At visible and near-infrared wavelengths, sunlight is reflected off the cloud tops and hazes populating the upper atmospheres of the giant planets. Bright cloud features can be tracked to extract velocities. By doing this over many latitudes, a global velocity field called the `zonal wind profile' can be made. Here, I present zonal wind profiles for Jupiter and Neptune. These are constructed from \textit{Hubble Space Telescope} WFC3 global maps of Jupiter taken between $2009-2016$ and Keck NIRC2 images of Neptune taken in the H-band ($1.4-1.8 \mu$m) and Kp-band ($2.0-2.4 \mu$m) in 2013 and 2014. I show that Jupiter's zonal wind profile is stable throughout the observed period, apart from variations on the order of 10 m/s at the $24^{\circ}$N Northern Temperature Belt (NTB). These variations arise during periodic plume outbreaks at the NTB and are coupled to a decrease in the albedo. These findings suggest that material, normally unseen, is dredged upward due to these plumes. If plumes are a signature of deeper activity, the decrease in velocity we see at the NTB during outbreaks may be evidence of vertical wind shear.I also find evidence of vertical wind shear at Neptune's equator, with the H-band zonal wind profile offset eastward by 100 m/s at the equator relative to the Kp-band profile. I apply a new thermal wind equation applicable at the equator to reconcile this observed vertical wind shear with Neptune's horizontal thermal and composition profiles. In order to match \textit{Voyager}/IRIS derived temperatures \citep{Fletcher2014}, the equator must be enriched in methane compared to the mid-latitudes at pressures greater than 1 bar. I discuss the implications of this finding with regards to global dynamics and compare and contrast to the other giant planets.Radio wavelengths probe below the visible cloud deck. I analyze maps of Neptune taken with the Atacama Large Millimeter/Submillimeter Array (ALMA) and extended Very Large Array (VLA) to constrain Neptune's deep opacity sources. The opacity source at radio wavelengths is dominated by H$_2$S and NH$_3$ as well as the the collision-induced absorption of H$_2$ with H$_2$, He, and CH$_4$. Clear brightness temperature variations are present across Neptune's disk caused by variations in these trace gases. These observations are the first to achieve the sensitivity, resolution, and wavelength coverage required to simultaneously extract the abundance profiles of H$_2$S, CH$_4$, and NH$_3$. I retrieve disk-average properties assuming both wet and dry adiabats. The disk-averaged data are consistent with profiles where trace gases are enriched by $30\times$ their protosolar value, apart from NH$_3$ which is $1\times$ its protosolar value. In both the ALMA and VLA maps, I identify seven distinct latitudinal bands with discrete transitions in the brightness temperature. I use the radiative transfer code Radio-BEAR to generate model spectra of Neptune's brightness temperature as a function of temperature and composition. I find best-fitting parameters to the H$_2$S, NH$_3$, and CH$_4$ abundance profiles in each of the seven identified latitude bands using $\chi^2$-statistics and MCMC retrievals. Of note, the equator is more complicated than expected. Trace gases are enriched in the $2-12^{\circ}$N band compared to neighboring latitudes. Here, the best-fit deep CH$_4$ abundance is $45\times$ the protosolar value (or 2.2$\%$ mixing ratio). H$_2$S is $30\times$ solar (or $7\times10^{-4}$ mixing ratio) and supersaturated at the H$_2$S-ice cloud formation. I relate these findings to my near-infrared work and present a new schematic of Neptune's global circulation structure.

When NASA's Dawn mission arrived at Ceres on March 6, 2015 it made history by becoming the first spacecraft to enter orbit around a second extraterrestrial object after leaving the asteroid Vesta in September 2012. Dawn thoroughly investigated the surface and deep interior of the dwarf planet Ceres, the largest object in the main asteroid belt, through a series of successively lower mapping orbits until its end of mission on November 1, 2018. Prior to Dawn’s arrival Ceres was known to be the largest C-type asteroid, and was suspected of being rich in water ice and other hydrated materials. As a putative remnant of the earliest phases of rocky planet formation, Ceres was thought to contain clues as to how planetesimals accreted and how volatiles arranged themselves throughout the inner solar system during the tumultuous era of planet formation. The Dawn mission’s goals were to further elucidate the structure and composition of the early solar system, which would lead to an increased understanding of the conditions present during terrestrial planet formation, and to determine the chemical, geological, and structural nature of the largest surviving planetary embryos: Vesta and Ceres. At Ceres, this was accomplished by meticulously characterizing the surface geology, surface and near-surface geochemistry, and interior structure via a combination of photo geology; visible, infrared, and nuclear spectroscopy; and gravimetry. This dissertation contributes to the objectives of the Dawn mission by aiding in the global geologic mapping effort of Ceres, identifying and classifying geological features indicative of its compositional and physical properties, and then applying geophysical techniques to these features in order to estimate these properties, particularly the water ice content of the near-surface. Understanding the quantity and distribution of water ice in the upper layer of Ceres is paramount for understanding both its geochemical evolution and the nature of the early solar system. The investigations presented in this dissertation reveal that Ceres is ubiquitously covered in geologic features suggestive of significant quantities of near-surface ground ice, namely large constructional mountains and hills, and a broad spectrum of lobate flow and mass wasting deposits. The observed mass wasting features exhibits physical characteristics and runout efficiencies similar to many ground ice mediated flows on Earth, Mars, and Iapetus such as long run-out landslides and frozen debris flows. Additionally, many craters on Ceres were observed to emanate fluidized appearing ejecta similar to examples found on Mars, Ganymede, and other icy worlds. Analyzing the mobilities of these ejecta using a kinematic-dynamic sliding ejecta emplacement model indicated that the cerean crust is significantly weaker than competent silicate rock at impacting conditions, and that the frictional properties of its surface are consistent with a rock-ice mixture. Finally, a unique fractured terrain named Nar Sulcus was identified on Ceres’ southern hemisphere that displayed topography suggestive of elastic flexure. By applying a flexural-cantilever model to the observed topography, the flexural rigidity of the cerean crust was estimated to be similar to those of outer solar system moons such as Europa, Ganymede, and Enceladus, which are several orders of magnitude less rigid than the terrestrial planets. From the aforementioned observations and investigations, the ground ice content of the cerean crust is estimated to be ~30-70 vol %, although there is likely significant regional heterogeneity in its distribution. This result is significant as it independently supports the interpretation that Ceres is a water rich dwarf planet, and that large quantities of ice can be sequestered within massive C-type asteroids over geologically long time periods. This is particularly exciting as carbonaceous chondrites are the most spectrally similar meteorites to C-type asteroids, and their water is the most isotopically similar to the Earth’s oceans. In light of the results presented in this dissertation, and by a myriad of other authors, it increasingly appears that a significant portion of the Earth’s water was likely delivered by Ceres-like asteroids.

Although sub-surface oceans have been detected on Europa and Enceladus, two of the best studied icy satellites in the outer solar system, the birth and evolution of these oceans and their impacts on the tectonic history of the overlying ice shells remain poorly understood. One of the fundamental questions regarding Europa and Enceladus is how the apparently young surfaces, evident from a lack of impact craters, are resurfaced and how this is related to the evolution of the underlying oceans. In this dissertation, I address this question by examining the cross-cutting relationships and spatial association of key morphological features on the two satellites. The results are expressed as sets of geomorphological maps that quantify the spatial and temporal relationships of the geologic features, which can then be related to the physical states of the ice shells.For the Europa studies, I constructed a global geologic map at 1:15M scale and the first detailed structural-geology map using the highest-resolution (~14 m/pixel) Galileo SSI images with a corresponding regional geological map for context. The main mapping results of my work include: (1) establishment of a global three-stage evolutionary model of the features on Europa’s surface, and (2) recognition of a transition from distributed to discrete deformation potentially resulting from ice-shell cooling and thickening. In order to further understand how the observed morphological relationships have developed, I used a gravity-scaled physical analogue model to analyze the formation of the ridged plains, the oldest surface features visible on Europa. I performed a series of two-layer physical analogue experiments, with a brittle layer above a ductile layer. The model permits a systematic test of two competing hypotheses for the formation of the ridged plains: horizontal ice-shell compression versus horizontal ice-shell extension. With these experiments, I showed that the similarity between the experimentally generated landscape and the observed ridged plains morphology strongly favors the compression mechanism.For the Enceladus study, I conducted a systematic investigation of high-resolution Cassini ISS images focusing on establishing regional structural relationships and the kinematics of major shear zones across the southern part of the Leading Hemisphere Terrain (LHT). The study area displays a unique morphological feature informally known as the “cratered islands” and referred to in this study as the relic cratered blocks. The blocks are characterized by isolated heavily cratered domains surrounded by younger ridged terrain that is essentially free of craters. The main finding of this work is that the relic cratered blocks have experienced vertical-axis rotation during distributed ductile strike-slip shear deformation. In order to understand how the ductile shear deformation evolved, I developed an analytical thermo-mechanical model that relates the shear strain rate of ductile deformation constrained by surface observations to the heat flow during the formation of the deformed features. The model results indicate that the ice shell of Enceladus likely experienced several periods of intense but local heating (> 0.3 W/m2), causing ice-shell thinning, ductile flow, and local resurfacing.By evaluating the formation mechanisms of first-order surface morphological features on Europa and Enceladus, I show that the predicted past ice-shell thicknesses on Europa and Enceladus are considerably thinner than their current values. This in turn implies that the underlying oceans may have experienced episodic development caused by the evolution of the overlying ice shell. As a result, this may have an effect on the potential habitability of the underlying oceans if the thermal and chemical environment is not stable over geologic time.

In order to have a better understanding of the early history of the solar system, it is scientifically important to study comets and active asteroids, which are believed to be leftovers from the formation epoch. This work presents our studies of two of the least understood families of cometary objects -- active asteroids and near-Sun comets, in terms of their non-gravitational effects and physical properties. We also present a summary of our two short-term surveys specifically for Kreutz-group comets, and an examination of a widely adopted cometary nucleus-extraction technique.We first systematically investigate the non-gravitational effects of the known active asteroids. Two of the members (313P/Gibbs \& 324P/La Sagra) exhibit statistically significant non-gravitational effects, while for the remaining members we are only able to place upper limits. The result is broadly consistent with the fact that the mass loss of active asteroids is generally less than that of typical comets. Thus, previous dynamical studies of active asteroids without consideration of the non-gravitational effects are still valid, and the majority were likely formed in situ in the main belt rather than from other cometary sources.For near-Sun comets, we conducted ground-based surveys from the Canada-France-Hawaii telescope (CFHT) and the VLT Survey Telescope (VST) in 2012 and 2014, respectively, without detection of any dwarf Kreutz-group comets. Our non-detection of two bright members which were later discovered by the {\it Solar and Heliospheric Observatory} ({\it SOHO}) but within our CFHT search region suggests that dwarf Kreutz comets brighten much more rapidly than previously thought, or they undergo outburst at larger heliocentric distances. We also present an unprecedentedly detailed study of near-Sun comet C/2015 D1 ({\it SOHO}), which was the first sunskirting comet observed from the ground over the past half century. This comet disintegrated around perihelion due to excessive thermal stress within its nucleus, or to rotational instability. The enormous mass loss ($\dot{M}_\mathrm{N} \sim 10^5$ kg s$^{-1}$) caused a strong non-gravitational effect. Together with photometric measurements, the nucleus mass and radius are inferred ($M_\mathrm{N} \sim 10^8$-10$^9$ kg, $R_\mathrm{N} \sim 50$-150 m), and we probe the emission of dust grains and dust-size distribution based on the morphology, as well as its composition using {\it SOHO} multiband observations.Finally, we examine the nucleus-extraction technique, an important tool for revealing cometary nucleus sizes, based on which statistics of the nucleus-size distribution are established. By testing the method on our synthetic comet images, we identify an obvious systematic bias stemming from neglect of the distortion of the coma brightness profile after convolution with point-spread functions (PSF). Thus, we conclude that published nucleus-size determinations using this technique are likely invalid. Our main suggestion is to better apply the technique on high spatial resolution images of weakly active comets when the nucleus signal occupies $\gtrsim$10\% of the total around the central region.