Seismic waves in Saturn’s rings reveal the strange ‘fuzzy core’ interior of the planet within.

NASA’s Cassini spacecraft continues to uncover amazing facts about the ringed planet Saturn. A recent study in the August edition of Nature: Astronomy highlighted an intriguing method to indirectly probe the interior of the planet.

Typically, missions to the gas giant planets Jupiter and Saturn rely on direct measurements of gravity fields during flybys and orbits to characterize planetary interiors. This method, however, has a chief drawback, as it doesn’t describe whether the planet has a small, solid rocky or large liquid core.

For the recent study, astronomers took a different approach. Specifically, the Cassini mission looked at stellar occultations of distant stars passing behind the rings of Saturn. These observations were carried out in the final days of the mission known as the risky Grand Finale ring plane-crossing phase, done before the final destructive disposal of the spacecraft in the atmosphere of Saturn on September 15th, 2017.

The final orbits for Cassini during the Grand Finale phase for the mission, during which astronomers took advantage of radio occultations of the spacecraft as seen from Earth, and visual occultations of stars from behind the ring particles. Credit: NASA/JPL-Caltech

During the occultations, Cassini observed very low frequency spiral waves traversing the inner C ring. The ring system itself is complex: thought to be part of a shredded moon, the major moons of Saturn pull and braid the outer ring system, while tiny shepherd moons are seen carving out paths in the rings, like the groves in a record. But it’s the planet itself that tugs on the inner sections such as the C-ring.

Astronomers soon realized that it was the surface of Saturn itself that was quivering to the tune of a meter every few hours, exerting tidal forces rippling through the ring system.

A Fuzzy Core

And what’s more, this oscillation says something about what’s going on inside Saturn itself. The best fit model suggests that rather than having a rocky distinct core, Saturn has a ‘fuzzy’ indistinct core extending about 60% of the way to the planet’s surface. Computer models suggest that the core is 55 times as massive as the Earth (Saturn iteself is 95 times as massive as Earth), with rock and ice only making up 17 Earth masses worth.

A new model for the interior structure of Saturn. Credit: Caltech/R. Hurt (IPAC).

To stabilize a sloshing liquid core, astronomers realized that it had to be gradually mixed with heavier material, like layers in a cake. The Earth’s oceans produce a similar stabilized scenario, as salinity increases with depth.

“The fuzzy cores are like a sludge,” says Christopher Mankovich, a Caltech research associate in planetary science in a recent press release. “The hydrogen and helium gas in the planet gradually mix with more and more ice and rock as you move toward the planet’s center.”

Standard gravitational measurements made by Cassini also back up the ‘fuzzy/sloshy core’ model for Saturn. Many of us remember the mantra we were all taught as school kids, that Saturn has a low enough density to float in water… if you could somehow construct a pool for it that would not collapse into a planet in its own right.

A simulation of a stellar occultation, versus Saturn’s rings. Credit: NASA/JPL/University of Colorado.

Here another strange and (fun) fact that Cassini sought to solve before its destructive demise in 2017: we don’t actually know the exact rotational period of Saturn to as high a degree of precision as you might think. Like Jupiter and the Sun itself, Saturn is mainly a ball of spinning gas rather than a single solid