More Kronoseismology with Saturn's rings

TL;DR

This paper advances Kronoseismology by analyzing additional waves in Saturn's rings, identifying resonances with planetary normal modes, and observing temporal changes in resonant radii, revealing insights into Saturn's internal structure.

Contribution

It applies and extends previous tools to identify new wave resonances with Saturn's internal modes and reports a drifting resonant radius indicating changing planetary oscillation frequencies.

Findings

Identification of a wave generated by an m=10 normal mode.

Detection of five waves with pattern speeds near Saturn's rotation rate.

Observation of a drifting resonant radius over thirty years.

Abstract

In a previous paper (Hedman and Nicholson 2013), we developed tools that allowed us to confirm that several of the waves in Saturn's rings were likely generated by resonances with fundamental sectoral normal modes inside Saturn itself. Here we use these same tools to examine eight additional waves that are probably generated by structures inside the planet. One of these waves appears to be generated by a resonance with a fundamental sectoral normal mode with azimuthal harmonic number m=10. If this attribution is correct, then the m=10 mode must have a larger amplitude than the modes with m=5-9, since the latter do not appear to generate strong waves. We also identify five waves with pattern speeds between 807 degrees/day and 834 degrees/day. Since these pattern speeds are close to the planet's rotation rate, they probably are due to persistent gravitational anomalies within the planet. These waves are all found in regions of enhanced optical depth known as plateaux, but surprisingly the surface mass densities they yield are comparable to the surface mass densities of the background C ring. Finally, one wave appears to be a one-armed spiral pattern whose rotation rate suggests it is generated by a resonance with a structure inside Saturn, but the nature of this perturbing structure remains unclear. Strangely, the resonant radius for this wave seems to be drifting inwards at an average rate of 0.8 km/year over the last thirty years, implying that the relevant planetary oscillation frequency has been steadily increasing.