Saturn's Rings as a Seismograph to Probe Saturn's Internal Structure

Abstract

As it has already done for Earth, the Sun, and the stars, seismology has the potential to radically change the way the interiors of giant planets are studied. In a sequence of events foreseen by only a few, observations of Saturn's rings by the Cassini spacecraft have rapidly broken ground on giant planet seismology. Gravity directly couples the planet's normal mode oscillations to the orbits of ring particles, generating spiral waves whose frequencies encode Saturn's internal structure and rotation. These modes have revealed a stably stratified region near Saturn's center and provided a new constraint on Saturn's rotation.Just like measuring earthquakes around the world can tell scientists about Earth's deep structure, vibrations of gas giant planets can tell us about their deep structure. But these vibrations are very hard to detect. At Saturn, help has come in the form of Saturn's icy rings, where gravity causes the orbits of ring material to pick up the planet's steady vibrations. This makes waves in the rings that are now being used as a powerful tool to study the inner workings of Saturn itself. Surprisingly, these waves have shown that the fluid motions in the deepest parts of the planet are relatively tame, compared to the forceful churning motions that were generally expected. They have also provided a measurement of the length of a Saturn day, a tough quantity to determine. Cassini characterized more than 20 waves in Saturn's rings caused by Saturn's oscillations, opening the door to giant planet seismology The frequency spectrum has revealed that Saturn's deep interior is stably stratified and yields a seismological rotation rate for Saturn The existing data can quantify the location and strength of Saturn's deep stable stratification, as well as Saturn's differential rotation