Saturn Ring Seismology: Evidence for Stable Stratification in the Deep Interior of Saturn

TL;DR

This study uses Saturn's ring density waves to infer the planet's internal structure, providing evidence for stable stratification deep within Saturn, and matching observed mode frequencies with theoretical models.

Contribution

It introduces models that incorporate stable stratification to explain observed mode frequencies and fine splittings in Saturn's interior, advancing understanding of planetary internal structures.

Findings

Fundamental mode frequencies match observations within 1%.

Presence of gravity modes indicates stable stratification deep inside Saturn.

Models struggle to reproduce fine mode splitting, implying additional effects like differential rotation.

Abstract

Seismology allows for direct observational constraints on the interior structures of stars and planets. Recent observations of Saturn's ring system have revealed the presence of density waves within the rings excited by oscillation modes within Saturn, allowing for precise measurements of a limited set of the planet's mode frequencies. We construct interior structure models of Saturn, compute the corresponding mode frequencies, and compare them with the observed mode frequencies. The fundamental mode frequencies of our models match the observed frequencies (of the largest amplitude waves) to an accuracy of $\sim 1 \%$, confirming that these waves are indeed excited by Saturn's f-modes. The presence of the lower amplitude waves (finely split in frequency from the f-modes) can only be reproduced in models containing gravity modes that propagate in a stably stratified region of the planet. The stable stratification must exist deep within the planet near the large density gradients between the core and envelope. Our models cannot easily reproduce the observed fine splitting of the $m=-3$ modes, suggesting that additional effects (e.g., significant latitudinal differential rotation) may be important.