The observed activity at the surfaces of the Galilean satellites mask subsurface internal dynamics. Tides generated from their primary, the massive planet Jupiter, deform these moons and can generate significant heat. This heat must then escape to space. The heat moving through a satellite has the potential to modify the interior structure, melting regions and causing internal interface boundary migration. We track how tidally generated heat will flow out of Io and Europa, and how this heat will modify the planetary structures. To do this we determine specific forms for the k Love numbers which depend on internal structure and temperature profiles. The Love numbers are used as a linear factor in tidal heating models. Because the Love numbers are very frequency dependent, it is important to consider every frequency term in a tide, as some may be much more significant than others. While some tidal heating models consider the orbital frequency only, we present the full tidal heating spectrum and isolate which terms are significant in which cases. In addition to finding the simple tidal heat forms for use with Io and Europa, we demonstrate with extreme examples, the tidal heating scenarios at Mercury and Venus. In these cases we find systems where the tidal heat delivery comes at frequencies related to sums and differences of mean motion and spin rate. Finally we use the Love numbers for Io and Europa we generated, in the tidal heating models as a term in thermal evolution models. We balance a tidal heat input into the bodies with heat convected upward into an elastic lid. The lid then conducts the once tidal heat into space. We probe the planetary structural and temperature space which leads to equilibrium conditions, where the tidal heat input is balanced by heat escape. For Io, we find no stable equilibrium scenarios for any structure we consider, and the body ultimately melts. This results because the dissipating volume is assumed to extend to the core. This generates much more heat than is able to conduct out through the lid and runaway melting occurs. Reducing the volume of the region, by adding a core or a subsurface fluid silicate layer, may reduce the volume enough for stable conditions to be found. In Europa, we do find stable equilibrium conditions at ice shells of roughly 50 km. For shells thicker than this, the tidal heating is too strong due to increased volume, and so the shell melts some in response until stable equilibrium is achieved. The Europa Clipper mission is slated to measure both shell thickness as well as the k Love number. With the relationships we draw between structural and orbital parameters, these measurements in the system can help constrain our models so that we can use Europa’s current state as a probe for its thermal history and an indicator of its thermal future.

This dissertation investigates the coupling between the large-scale atmospheric circulation and surface processes on Titan, with a particular focus on methane precipitation and its influence on surface geomorphology and hydrology. As the only body in the Solar System with an active hydrologic cycle other than Earth, Titan presents a valuable laboratory for studying principles of hydroclimate on terrestrial planets. Idealized general circulation models (GCMs) are used here to test hypotheses regarding Titan's surface-atmosphere connections. First, an Earth-like GCM simulated over a range of rotation rates is used to evaluate the effect of rotation rate on seasonal monsoon behavior. Slower rotation rates result in poleward migration of summer rain, indicating a large-scale atmospheric control on Titan's observed dichotomy of dry low latitudes and moist high latitudes. Second, a Titan GCM benchmarked against observations is used to analyze the magnitudes and frequencies of extreme methane rainstorms as simulated by the model. Regional patterns in these extreme events correlate well with observed geomorphic features, with the most extreme rainstorms occurring in mid-latitude regions associated with high alluvial fan concentrations. Finally, a planetary surface hydrology scheme is developed and incorporated into a Titan GCM to evaluate the roles of surface flow, subsurface flow, infiltration, and groundmethane evaporation in Titan's climate. The model reproduces Titan's observed surface liquid and cloud distributions, and reaches an equilibrium state with limited interhemispheric transport where atmospheric transport is approximately balanced by subsurface transport. The equilibrium state suggests that Titan's current hemispheric surface liquid asymmetry, favoring methane accumulation in the north, is stable in the modern climate.

The planets and moons in the solar system present a unique opportunity for enriching our understanding of Earth. The remarkable diversity of atmospheric compositions, dynamics, and weather patterns requires us to move beyond Earth-centric paradigms and search for more general theories that can explain the similarities and differences between them. In this work, Saturn's moon Titan is chosen as the point of comparison with Earth for its compositional similarity, active hydrological cycle, and distinct atmospheric dynamics. We investigate the governing physics of climate, clouds, and convection on both bodies with the goal of learning general truths that broadly apply to most, if not all, moist planetary atmospheres. We focus, in particular, on the effects of low to high moisture concentration in the atmosphere. First, it is found that Earth's climate is remarkably stable to significant changes in atmospheric moisture content. In Earth-like climate states, the vapor pressure path at the anvil cloud level is fixed due to spectroscopic properties of water vapor. The largest changes in Earth's climate occur at tipping points that involve transitions from multi-layered to single-layered convective clouds. Second, we demonstrate that the height of congestus cloud-top formation in the tropics is driven by a mid-tropospheric decline in the water vapor emissivity in clear-sky regions, which causes clouds to detrain preferentially between 5-6 km. This clear-sky theory of cloud formation is derived from basic assumptions of mass and energy balance and so should generalize well to other locations on Earth or to other planets and different atmospheric compositions. Third, we show that the transition from steady, quasi-equilibrium (QE) precipitation on Earth to non-steady, relaxed oscillator (RO) precipitation on Titan is predicted by the breakdown of a heat engine model of convection with increasing surface temperature and/or atmospheric moisture content. The breakdown of quasi-equilibrium dynamics occurs due to an imbalance between the work performed by the convective heat engine and the heat of condensation released by the convective motions themselves. The heat engine perspective offers a robust point of comparison between the atmospheres of Earth and Titan based on the first and second laws of thermodynamics, which are system invariant. The heat engine model is, in fact, agnostic of the working fluid and the condensing substance and, arguably, is the best framework to explain why dynamical similarities between present day Titan and a much warmer Earth exist.

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.