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.