Saturn’s ‘Fuzzy Core’ seen in Ring Ripples

Seismic waves in the rings of Saturn reveal the strange “fuzzy core” inside the planet.

NASA’s Cassini spacecraft continues to reveal amazing facts about the ringed planet Saturn. A recent study in the August issue of Nature: Astronomy highlighted a fascinating way to indirectly study the interior of the planet.

Typically, missions to the gas giant planets Jupiter and Saturn rely on direct measurements of gravitational fields during flybys and orbits to characterize the planet’s interior. There is one major drawback to this method, however, as it does not describe whether the planet has a small, solid rock core or a large liquid core.

For the current study, astronomers took a different approach. Specifically, the Cassini mission examined stellar eclipses of distant stars that pass behind the rings of Saturn. These observations were made in the final days of the mission, known as the risky phase of crossing the ring in the Grand Finale, before the spacecraft was finally destroyed in Saturn’s atmosphere on September 15, 2017.

The final orbits for Cassini during the Grand Finale phase of the mission, when astronomers took advantage of the spacecraft’s radio eclipses from Earth and visual occultations of stars behind the ring particles. Photo 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: the large moons of Saturn are believed to be part of a tattered moon, pulling and weaving the outer ring system, while tiny shepherd moons like the groves in a record carve paths into the rings. But it is the planet itself that is tugging at the inner sections like the C-ring.

Astronomers soon realized that it was the surface of Saturn itself that trembled at meter pitch every few hours, exerting tidal forces that rushed through the ring system.

A fuzzy core

In addition, this vibration says something about what is going on in Saturn itself. The best-fit model suggests that Saturn does not have a rocky, clear core, but rather a “fuzzy” indistinct core that extends about 60% of the way to the planet’s surface. Computer models suggest that the core is 55 times as massive as Earth (Saturn itself is 95 times as massive as Earth), with rock and ice making up only 17 times the mass of the earth.

A new model for the internal structure of Saturn. Photo credit: Caltech / R. Injured (IPAC).

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

“The fuzzy nuclei are like a mud,” says Christopher Mankovich, a Caltech researcher in planetary research, in a recent press release. “The hydrogen and helium gases on the planet gradually mix with more and more ice and rocks as you near the center of the planet.”

Standard gravitation measurements carried out by Cassini also underpin the “fuzzy / sloshy core” model for Saturn. Many of us remember the mantra we were all taught as school children that Saturn is low density to swim in water … if you could somehow build a pool for it that wouldn’t collapse into a planet of its own.

A simulation of a star eclipse compared to the rings of Saturn. Photo credit: NASA / JPL / University of Colorado.

Here’s another weird and (funny) fact that Cassini was trying to resolve before its devastating demise in 2017: We don’t know the exact period of rotation of Saturn as well as you might think. Like Jupiter and the Sun itself, Saturn is primarily a sphere of spinning gas and not a single solid object: Confusing is the fact that visual observations and radio measurements from spacecraft do not exactly match. The current radiometric value for the rotation period of Saturn is 10 hours and 39 minutes.

Studies of Saturn’s magnetic field structure could also support or refute the fuzzy core hypothesis; in the fuzzy core model, for example, a stable layered core would not also rotate and create a huge planet-wide magnetic field; instead, Saturn’s magnetic field would be generated in the outer gas envelope.

This finding also has implications for the review of our way of thinking about the complex interiors of gas giant planets, both in our solar system and on exoplanets beyond. NASA’s Juno mission is also investigating the interior of Jupiter in detail.

Unfortunately, it will still be a while before we return to Saturn: The next mission in the pipeline will not be until 2027, when the nuclear-powered Dragonfly helicopter will fly to Saturn’s large moon Titan.

You can see Saturn for yourself and ponder all these amazing facts – the ringed world is fresh from the opposition on August 2nd and at sunset in late August, together with Jupiter, rules the evening sky deep in the east.

The view from late August at dusk looking east. Credit: Stellarium.

What other secrets await humanity in this most photogenic of all worlds?

-Lead Image: A simulated image of Saturn created with Cassini images colored to show different optical depths compared to ring particle sizes. NASA / JPL-Caltech

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