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 a metre 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 10^4 to 10^7 yrs 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 detectable by the Juno spacecraft.