A diffuse core in Saturn revealed by ring seismology

TL;DR

This paper uses ring seismology to reveal that Saturn has a diffuse, stably stratified core extending to about 60% of its radius, providing new insights into its internal structure and formation history.

Contribution

It demonstrates that seismic data from Saturn's rings can constrain the planet's deep interior, revealing a diffuse core-envelope transition not detectable by gravity measurements alone.

Findings

Saturn's core is diffuse and extends to 60% of its radius.

Seismic data indicates a stably stratified interior with composition gradients.

The core contains approximately 17 Earth masses of ice and rock.

Abstract

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.