Ring Seismology and Saturn's Winds, Composition Gradients, and Tides

Ring seismology offers information about Saturn's interior structure. So far this information comes in the form of precise frequency measurements for more than 30 prograde fundamental (f) modes, of which a handful at the lowest angular degree (l=2-5) are strongly mixed with gravity (g) modes that require at least one stably stratified region to exist in Saturn's deep interior. The remainder are more shallowly confined modes; their frequencies are more sensitive to the depth of the zonal winds and any stable stratification that may exist at or outward of the region where hydrogen transitions from its metallic to its molecular phase.

We present a minimally complicated interior model that aims to reproduce the full set of mode frequencies and Saturn's gravity coefficients simultaneously, accounting for the way differential rotation modifies Saturn's equilibrium shape and gravity field. Moving to a non-perturbative treatment of rotation in our mode calculations grants the precision necessary to conclude on the basis of the ring seismology alone that Saturn's zonal winds extend thousands of kilometers deep, buttressing a result already strongly supported by gravity science.

Small but significant frequency residuals in the best fitting model, especially for modes at lower angular degree, point toward a more complicated interior than the single stably stratified dilute core region and convective envelope structure we have considered so far. Multiple stable stratifications, such as would be expected from disjoint heavy element and helium gradients, complicate the mode spectrum by introducing multiple g-mode cavities. We discuss the promise of this multiple-cavity scenario for reconciling Saturn's low-degree mode spectrum.

We finally touch on the related topic of Saturn's dynamical tidal response, which could offer additional useful constraints on internal structure should one or more satellites prove to be locked in resonance with normal modes that dominate the (generally multifaceted) tidal response.