Resonant Stratification in Titan’s Global Ocean

Abstract

Titan’s ice shell floats on top of a global ocean, as revealed by the large tidal Love number k 2 = 0.616 ± 0.067 registered by Cassini. The Cassini observation exceeds the predicted k 2 by one order of magnitude in the absence of an ocean, and is 3σ away from the predicted k 2 if the ocean is pure water resting on top of a rigid ocean floor. Previous studies demonstrate that an ocean heavily enriched in salts (salinity S ≳ 200 g kg−1) can explain the 3σ signal in k 2. Here we revisit previous interpretations of Titan’s large k 2 using simple physical arguments and propose a new interpretation based on the dynamic tidal response of a stably stratified ocean in resonance with eccentricity tides raised by Saturn. Our models include inertial effects from a full consideration of the Coriolis force and the radial stratification of the ocean, typically neglected or approximated elsewhere. The stratification of the ocean emerges from a salinity profile where the salt concentration linearly increases with depth. We find multiple salinity profiles that lead to the k 2 required by Cassini. In contrast with previous interpretations that neglect stratification, resonant stratification reduces the bulk salinity required by observations by an order of magnitude, reaching a salinity for Titan’s ocean that is compatible with that of Earth’s oceans and close to Enceladus’ plumes. Consequently, no special process is required to enrich Titan’s ocean to a high salinity as previously suggested.

Tidal Forcing in Icy‐Satellite Oceans Drives Mean Circulation and Ice‐Shell Torques

“…It does not necessarily describe any specific dissipation process, though it has been used on Earth to parameterize the conversion from barotropic to baroclinic tides and the associated wave breaking (e.g., Jayne & St. Laurent, 2001). If icy satellite oceans are stratified, then this processes may be important (Abdulah et al, 2022;Idini & Nimmo, 2024;Rovira-Navarro et al, 2023). Prescribing drag in this linear way is common in tidal dissipation studies in icy satellite oceans (e.g., Matsuyama et al, 2018;Rovira-Navarro et al, 2020Tyler, 2011).…”