Excitation and Damping of Oscillation Modes in Gaseous Planets

Abstract

The excitation and damping mechanisms for oscillation modes of gas giant planets are undetermined. We show that differential rotation may greatly enhance convective viscosity in giant planets, resulting in damping times of $t_{\rm damp} \sim 10^5-10^6 \, {\rm years}$ for f modes and low-order p modes. Radiative diffusion damps p modes on time scales of $t_{\rm damp} \sim 10^3-10^7 \, {\rm years}$. While the lethargic convective motions cannot effectively excite f mode or p modes, storms driven by condensation of water and/or silicates may play a role. High-order p modes are most effectively excited by cometary/asteroid impacts. Applying these calculations to solar system planets, water storms, rock storms, and impacts may all contribute to exciting the observed f modes amplitudes of Saturn via ring seismology. Similar f mode amplitudes with fractional gravitational perturbations of $\delta \Phi/\Phi \sim 10^{-10}-10^{-9}$ are expected for Jupiter and Uranus, apart from their lowest $\ell$ f modes which could have larger gravitational perturbations of $\delta \Phi/\Phi \sim 10^{-7}$. Rock storms may contribute to mode driving in Jupiter, while water storms are more important for Uranus. The highest-amplitude p modes are predicted to have periods of $\sim$10-30 minutes, with surface velocities of $\sim$10 cm/s for Jupiter and Saturn, and $\sim$1 cm/s for Uranus. These oscillation modes may be detectable with radial velocity measurements, ring seismology, or spacecraft Doppler tracking. However, both the damping and excitation physics are uncertain by orders of magnitude, so more careful examination of the relevant physics is required for robust estimates.