Cassini Ring Seismology as a Probe of Saturn's Interior I: Rigid Rotation

TL;DR

This study uses Saturn's ring waves and interior models to estimate its rotation period, finding it faster than previous measurements and suggesting minimal differential rotation in the outer envelope.

Contribution

It introduces a method to determine Saturn's internal rotation period using ring seismology and mode identification from Cassini data, providing new constraints on Saturn's internal rotation.

Findings

Estimated Saturn's interior rotation period as approximately 10h 33m 38s.

Identified fundamental modes responsible for observed ring waves.

Found no clear evidence of strong differential rotation in Saturn's outer envelope.

Abstract

Seismology of the gas giants holds the potential to resolve long-standing questions about their internal structure and rotation state. We construct a family of Saturn interior models constrained by the gravity field and compute their adiabatic mode eigenfrequencies and corresponding Lindblad and vertical resonances in Saturn's C ring, where more than twenty waves with pattern speeds faster than the ring mean motion have been detected and characterized using high-resolution Cassini Visual and Infrared Mapping Spectrometer (VIMS) stellar occultation data. We present identifications of the fundamental modes of Saturn that appear to be the origin of these observed ring waves, and use their observed pattern speeds and azimuthal wavenumbers to estimate the bulk rotation period of Saturn's interior to be $10{\rm h}\, 33{\rm m}\, 38{\rm s}^{+1{\rm m}\, 52{\rm s}}_{-1{\rm m}\, 19{\rm s}}$ (median and 5%/95% quantiles), significantly faster than Voyager and Cassini measurements of periods in Saturn's kilometric radiation, the traditional proxy for Saturn's bulk rotation period. The global fit does not exhibit any clear systematics indicating strong differential rotation in Saturn's outer envelope.