  The best constraints on the internal structures of giant planets have
historically come from measurements of their gravity fields. These gravity data
are inherently mostly sensitive to a planet's outer regions, providing only
loose constraints on the deep interiors of Jupiter and Saturn. This fundamental
limitation stymies efforts to measure the mass and compactness of these
planets' cores, crucial properties for understanding their formation pathways
and evolution. However, studies of Saturn's rings have revealed waves driven by
pulsation modes within Saturn, offering independent seismic probes of Saturn's
interior. The observations reveal gravity mode (g mode) pulsations which
indicate that a part of Saturn's interior is stably stratified by composition
gradients, and the g mode frequencies directly probe the buoyancy frequency
within the planet. Here, we compare structural models with gravity and seismic
measurements to show that the data can only be explained by a diffuse, stably
stratified core-envelope transition region in Saturn extending to approximately
60% of the planet's radius and containing approximately 17 Earth masses of ice
and rock. The gradual distribution of heavy elements constrains mixing
processes at work in Saturn, and it may reflect the planet's primordial
structure and accretion history.
