Are Thalassa and Despina in Resonance Lock with Neptune's Oscillations?

TL;DR

This paper investigates the resonance dynamics of Neptune's moons Naiad, Thalassa, and Despina, proposing a resonant-lock mechanism with Neptune's internal oscillations to explain their current orbital configurations.

Contribution

It introduces the idea that Neptune's innermost moons may be in resonance lock with Neptune's internal oscillation modes, providing a new explanation for their long-term stability.

Findings

Naiad-Thalassa resonance is unstable on Myr timescales due to perturbations from Despina.

Resonant-lock tides with Neptune's internal modes can stabilize the moons' resonances over longer periods.

Low-order g-modes at Neptune may drive the resonant-lock evolution of the moons.

Abstract

The two innermost moons of Neptune, Naiad and Thalassa, are currently in a 73:69 mean-motion resonance. This resonance relies on the large inclination of Naiad, and we estimate that Naiad requires multiple Gyr to reach its $4.7^{\circ}$ inclination through this resonance. However, we find through direct numerical simulations that the current Naiad-Thalassa resonance is unstable on Myr timescales due to perturbations from the neighboring moon Despina. As this instability is a product of convergent tidal evolution predicted by equilibrium tidal theory, we propose that the innermost moons of Neptune may migrate through resonant-lock tides. If both Despina and Thalassa are locked to two resonant oscillations modes within Neptune, the frequencies of which evolve approximately in parallel, Naiad-Thalassa resonance can be stable for much longer. We find that Lindblad resonances with low-order $l=m=1$, $n=1$ g-modes at Neptune may be suitable candidates for driving the resonant-lock evolution of Thalassa and Despina, and possibly even Galatea.