Memoirs of a Giant Planet

Saturn is ringing weakly. Exquisite data from the Cassini mission reveal the presence of f-mode oscillations as they excite density waves in Saturn's rings. These oscillations have displacement amplitudes of order 1 m on Saturn's surface. We propose that they result from large impacts in the past. Experiencing little dissipation inside Saturn on account of its weak luminosity, f-modes may live virtually forever, but the very ring waves that reveal their existence also remove energy from them, in 10⁴ to 10⁷ yr for the observed f-modes (spherical degree 2-10). We find that the largest impacts that arrive during these times excite the modes to their current levels, with the exception of the few lowest-degree modes. To explain the latter, either a fortuitously large impact in the recent past or a new source of stochastic excitation is needed. We extend this scenario to Jupiter, which has no substantial rings. With an exceedingly long memory of past bombardments, Jovian f-modes and p-modes can acquire much higher amplitudes, possibly explaining past reports of radial velocity detections, and are potentially detectable by the Juno spacecraft.