08:30 〜 08:45
[S19-1-01] A Window into Giant Planet Structure using Saturn's Natural Seismograph
Saturn's nonradial oscillations perturb the orbits of ring particles. Saturn's C ring is fortuitous in that it spans several resonances with Saturn's fundamental acoustic modes, and its moderate optical depth allows the characterization of wave features using stellar occultations. The growing set of C-ring waves with pattern frequencies and azimuthal order m measured by Cassini provide significant constraints on Saturn's internal structure, with the potential to resolve long-standing questions about the planet's distribution of helium and heavier elements, its means of internal energy transport, and its rotation state.
We construct Saturn evolutionary models and calculate their mode eigenfrequencies at the solar age, mapping the planet mode frequencies to resonant locations in the rings to compare with observed density and bending waves. A new feature of the models is the immiscibility of neutral helium in the liquid metallic hydrogen mantle, which leads to helium rain and the establishment of a deep stabilizing composition gradient. Similar to the model of Fuller (2014), this stratified region provides a deep cavity wherein modes close to Saturn's fundamental frequency can propagate as internal gravity waves, yielding modes that have an overall mixed character, as seen in pulsating red giant stars. The result is a more densely packed spectrum of modes that can excite resonances in the C ring, helping to explain several of the observed waves that a conventional homogeneous Saturn model--supporting only acoustic modes--cannot.
I'll assess the hypothesis that helium rain is the origin of Saturn's deep stable stratification, as well as the role of the possibly eroded core-mantle boundary. I'll touch on the related question of convective versus double-diffusive heat transport through the stably stratified region. Finally, I'll give an update on an ongoing campaign using Doppler imaging to search for Jovian oscillations from the 3m Shane telescope at Lick Observatory.
We construct Saturn evolutionary models and calculate their mode eigenfrequencies at the solar age, mapping the planet mode frequencies to resonant locations in the rings to compare with observed density and bending waves. A new feature of the models is the immiscibility of neutral helium in the liquid metallic hydrogen mantle, which leads to helium rain and the establishment of a deep stabilizing composition gradient. Similar to the model of Fuller (2014), this stratified region provides a deep cavity wherein modes close to Saturn's fundamental frequency can propagate as internal gravity waves, yielding modes that have an overall mixed character, as seen in pulsating red giant stars. The result is a more densely packed spectrum of modes that can excite resonances in the C ring, helping to explain several of the observed waves that a conventional homogeneous Saturn model--supporting only acoustic modes--cannot.
I'll assess the hypothesis that helium rain is the origin of Saturn's deep stable stratification, as well as the role of the possibly eroded core-mantle boundary. I'll touch on the related question of convective versus double-diffusive heat transport through the stably stratified region. Finally, I'll give an update on an ongoing campaign using Doppler imaging to search for Jovian oscillations from the 3m Shane telescope at Lick Observatory.