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Resonant stratification in Titan and other icy satellites with global oceans

Presentation #216.07 in the session Titan III: Surface and Interior (Oral Presentation)

Published onOct 23, 2023
Resonant stratification in Titan and other icy satellites with global oceans

Titan’s ice shell floats on top of a global ocean revealed in the large tidal Love number k2 = 0.616 ± 0.067 registered by Cassini. The Cassini observation is 3σ away from the predicted k2 if the ocean is pure water resting on top of a rigid ocean floor. Previous studies have hypothesized that the ocean is enriched in salts (>200 g/kg) to explain the 3σ signal in k2. Here we present an alternative to these previous interpretations and propose that, instead of highly enriched, Titan may host an ocean with an average salinity compatible with the typical values observed on Earth’s oceans and Enceladus’ plumes.

Our interpretation is based on the dynamic tidal response of a stratified ocean in resonance with 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 radial salinity profile where salt concentration increases with depth. We find multiple salinity profiles that lead to the k2 required by Cassini via resonant excitation of internal gravity waves or g-modes. In contrast with previous interpretations that neglect stratification, resonant stratification reduces the bulk salinity required by observations by an order of magnitude. Our interpretation requires a bulk salinity compatible with what is expected from water-rock interactions, contrary to the previously hypothesized heavy ocean. Consequently, no special formation event is required to enrich Titan’s ocean to a high salinity concentration as previously suggested.

The results and models reported here carry implications for the exploration of Ganymede’s interior. ESA’s JUICE mission is currently on cruise toward the Jovian system and will measure Ganymede’s k2 with a precision not seen before on an icy satellite. Our stratified ocean models predict the presence of g-modes in the ocean, which can significantly enhance k2 at certain given frequencies. The wealth of data expected from JUICE includes a frequency-dependent k2 that can be used to test our model of stratified oceans, advancing our general understanding of the conditions found in the oceans of icy satellites (e.g., circulation regime and heat flux from the core). An anomalous k2 near the low-frequency end of the tidal spectrum constitutes evidence for a stratified ocean in Ganymede.

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