So far the only way to get any information about the interior of rocky planets was from the moment of inertia which is related to a planets mass distribution. This only allowed to estimate if a planet has a mantle and what the estimated thicknesses of the core and mantle are. Of course, other (remote sensing) methods like potential (gravity and magnetic) fields can also give more information about a planets interior. However, the only solid way to determine the layer interface and their depth and thicknesses is seismic imaging, a popular technique used to image Earth’s subsurface and interior.
Cassini – An exhilarating story of planetary explorationStandard
For more information: Cassini at Saturn
More on the Grand Finale (2nd dive: May 2, 2017; End of mission: Sept 15, 2017)