Neptune's lone large moon Triton is a unique object in the Solar System: its highly inclined, retrograde orbit, its composition and its status as Neptune's sole large moon betray a history and formation unlike that of the other large Solar System moons. Rather than having formed around Neptune, Triton is thought to have formed as one part of a binary pair of Kuiper belt-objects, whose disruption in an encounter with Neptune led to Triton being captured on a highly-eccentric orbit around the planet. The resulting process of tidally-driven circularisation to its present-day circular orbit will have released an amount of energy sufficient to melt Triton in its entirety several times over. Whether any such catastrophic consequences actually came to pass, however, and whether they may leave their mark into the present, depends strongly on when and where this energy is dissipated, factors for which previous authors have unfortunately found conflicting results. Such previous efforts attempting to model this process have relied on untested assumptions or a variety of simplified dynamical models, both of which were shown to lead to inconsistent predictions in previous work.

In an attempt to reconcile these past results, we therefore self-consistently couple a high-fidelity dynamical model developed in previous work to novel interior-evolution and deformation models of Triton. In doing so, we relax several assumptions applied in previous work, accounting for the non-synchronicity of Triton's rotation to its orbit, the frequency-dependence of Triton's deformation, higher-order terms in the Darwin-Kaula expansion, and the possibility of subsolidus convection in Triton's silicate interior. We then study the evolution of Triton with and without tides, and assess the sensitivity of Triton's evolution to variations in uncertain or unconstrained interior and initial parameters.

We find that not all assumptions applied in previous work are justified or even useful: premature truncation of the Darwin-Kaula expansion leads to a significant underestimation of tidal heating, whereas uncoupled dynamical-interior or fixed-interior models will significantly overestimate dissipation, and so none of these are useful in describing (even in a qualitative sense) the high-eccentricity regime of Triton's evolution. We find that Triton spends the majority of its tidal evolution stuck in higher-order spin-orbit resonances, being in an equilibrium but not synchronous rotational state (as was assumed in previous studies) until its eccentricity falls to ~0.2. However, this fact as well as the frequency-dependence of the tidal response play a limited role in Triton's evolution as we also find that Triton's tidal response does not vary by more than an order of magnitude over the range of frequencies excited at any given time, and so we find that the constant phase-lag model of MacDonald (1964), alone out of all simplified dynamical models, gives a qualitatively (though not quantitatively) correct description of its evolution.

In agreement with earlier results found using this constant phase-lag model, we find that Triton dissipates the vast majority of its orbital energy in its icy shell rather than in its silicate mantle: consequently, its shell recedes to thicknesses of 10 km or less over Gyr-timescales or longer, but circularisation leaves little if any mark in the mantle, nor in the shell after tidal dissipation ceases. Additionally, we find that these conclusions are not sensitive to choices in interior or initial parameters, though the timescale of circularisation varies between ~1-4 Gyr as a strong function of the reference viscosity of the icy shell. Moreover, Triton almost certainly reached the temperatures required to set on the development of an iron core, but not because of capture (as envisioned by early work on the moon): core formation is potentially promoted but never initiated by tidal dissipation.

JUICE will measure tidally-induced variations in Ganymede's gravitational field with unprecedented accuracy. Previous research focused on studying tides with a spherically-symmetric interior allowing to constrain the mean shell thickness and subsurface ocean. The presence of lateral variations in the interior alters the tidal response and produces additional signals. This research investigates whether JUICE measurements will be precise enough to constrain possible lateral variations in Ganymede. The structure of Ganymede's ice shell is crucial to the question of habitability, since it influences the process of tidal heating and the transport of heat and material in the interior. This research uses simulated range and Doppler observations during JUICE's orbital phase, to obtain the expected uncertainties in measuring the tides and compares them to the expected signals for a laterally heterogeneous Ganymede model. Results indicate that JUICE could detect zonal lateral variations of degree 1 and 2 if their amplitude is at least ~5%

This thesis presents a methodology to characterize the interior structure of Ganymede, Jupiter’s largest moon, through a joint Bayesian inversion framework that integrates gravity, magnetic induction, tidal, and libration observations. The motivation arises from the growing scientific interest in icy moons, which not only provide insights into Solar System evolution but also represent promising candidates for habitability due to their subsurface oceans. Ganymede stands out as the largest moon in the Solar System and the only one known to possess its own intrinsic magnetic field. It is also believed to host a subsurface ocean beneath its icy crust, making it an interesting target for scientific exploration. However, much of its internal structure remains uncertain. The upcoming ESA’s Juice mission will carry out detailed observations of the moon through a series of flybys followed by an extended orbital tour, delivering high-precision data that will help constrain Ganymede's structure.
This work addresses the challenge of constraining Ganymede's interior structure using multiple datasets - gravity, magnetic induction, tidal, and libration observations - each sensitive to different interior parameters and affected by parameter degeneracies. We first perform a global sensitivity analysis to identify how each observable relates to specific interior properties. Our model assumes a five-layer spherical structure comprising a metallic core, silicate mantle, high-pressure ice, liquid salty ocean, and outer ice shell. The sensitivity analysis shows that magnetic induction is most sensitive to ocean thickness and composition, tidal displacement to ice shell thickness and rigidity, and libration amplitude to shell rigidity. Degeneracies also emerge, such as between shell thickness, ocean density, and shear modulus, highlighting the need for a joint inversion approach.
The Bayesian inversion is carried out progressively. Starting with Ganymede’s moment of inertia, we constrain the core and mantle but leave the hydrosphere largely unconstrained. Including magnetic induction data substantially improves the estimation of ice shell and ocean thicknesses, while the real part of the tidal Love number k₂ refines constraints on the densities of the ice shell and ocean and enables inference of ocean composition. These observations also provide information on the rigidity of high-pressure ice and the compressibility of interior layers. Finally, the combination of moment of inertia, magnetic induction amplitude, and both the real and imaginary parts of k₂ offers constraints on the viscosities of the ice layers, related to the tidal dissipation within Ganymede.
Beyond advancing our understanding of Ganymede, this study demonstrates the effectiveness of joint Bayesian inference for the characterization of planetary interior.This framework contributes to the scientific preparation for the ESA's Juice mission, identifies the most critical measurements to break degeneracies in interior model parameters, and can be extended to other icy moons, such as Europa and Enceladus, where spacecraft data will provide similar observational constraints.

Saturn’s moon Enceladus harbours a global subsurface ocean beneath its icy crust. Tidal dissipation within the moon’s core generates a substantial amount of heat which leads to ocean convection. Observations of the moon indicate ocean thickness variations of up to 20 km from equator to pole and heterogeneous heat generation within the core likely results in latitude-dependent temperature gradients. The effects of meridional thickness variations and heterogeneous temperature gradients on rotating thermal convection have not been simulated in previous studies. Here we simulate a non-uniform spherical shell employing a degree-2 zonal thickness profile. Using direct numerical simulations, we analysed various properties associated with heat transfer behaviour for flows in a uniform and non- uniform spherical shell domain driven by thermal convection with the Rayleigh number in the range 1.6×10⁵ ≤ Ra ≤ 5.0×10⁶ and constant Ekman number of Ek = 3×10^-4 and Prandtl number of Pr = 1. Our results demonstrate that different regimes of convection exist, which depend on the relative influence of rotation. With increasing thermal forcing, convection moves from being restricted to equatorial regions to filling the whole fluid domain. Global scaling behaviour for both domains was found to be consistent with literature, although weaker polar convection in non-uniform shells caused a decrease in heat transfer efficiency and thus a diminished heat transfer scaling behaviour. The diminishing transport of heat at the poles in the non-uniform shell deviates from the predicted heat flux profile at Enceladus, suggesting that stronger thermal heterogeneities are required to enhance polar heat transfer.