Tidal dissipation in stably stratified and semi-convective regions of rotating giant planets: incorporating Coriolis forces

TL;DR

This paper investigates how stable stratification and semi-convective layers in rotating giant planets influence tidal dissipation, revealing that such layers can significantly enhance wave excitation and dissipation, aligning with observed planetary moon migration rates.

Contribution

The study introduces a detailed numerical model incorporating Coriolis forces to analyze tidal dissipation in stratified layers of giant planets, highlighting the role of stable layers in tidal evolution.

Findings

Stable stratified layers enhance inertial and gravito-inertial wave dissipation.

Efficient tidal dissipation can occur without resonance-locking.

Models predict tidal quality factors consistent with observed moon migrations.

Abstract

We study how stably stratified or semi-convective layers alter tidal dissipation rates associated with the generation of inertial, gravito-inertial, interfacial and surface gravity waves in rotating giant planets. We explore scenarios in which stable (non-convective) layers contribute to the high rates of tidal dissipation observed for Jupiter and Saturn in our solar system. Our model is an idealised spherical Boussinesq system incorporating Coriolis forces to study effects of stable stratification and semi-convective layers on tidal dissipation. Our detailed numerical calculations consider realistic tidal forcing and compute the resulting viscous and thermal dissipation rates. The presence of an extended stably stratified fluid core significantly enhances tidal wave excitation of both inertial waves (due to rotation) in the convective envelope and gravito-inertial waves in the dilute core. We show that a sufficiently strongly stratified fluid core enhances inertial wave dissipation in a convective envelope much like a solid core does. We demonstrate that efficient tidal dissipation rates (and associated tidal quality factors $Q'$) -- sufficient to explain the observed migration rates of Saturn's moons -- are predicted at the frequencies of the orbiting moons due to the excitation of inertial or gravito-inertial waves in our models with stable layers (without requiring resonance-locking). Stable layers could also be important for tidal evolution of hot and warm Jupiters, and hot Neptunes, providing efficient tidal circularisation rates. Future work should study more sophisticated planetary models that also account for magnetism and differential rotation, as well as the interaction of inertial waves with turbulent convection.