Astronomers using data from NASA’s Cassini spacecraft on ripples in Saturn’s rings have revealed new information about the gas giant’s mysterious core. Their results, published in the journal Nature Astronomy, suggest that Saturn’s core is not a hard ball of rock, as some previous theories had proposed, but a diffuse soup of ice, rock, and metallic fluids — a so-called ‘fuzzy’ core. They also reveal that the core extends across 60% of the planet’s diameter, which makes it substantially larger than previously estimated.

An illustration of Saturn and its ‘fuzzy’ core. Image credit: R. Hurt, IPAC / Caltech.

The idea that Saturn’s oscillations could make waves in its rings and that the rings could thus be used as a seismograph to study the planet’s interior first came about in studies in the early 1990s by Mark Marley and Carolyn Porco, who later became the leader of the Cassini imaging team.

The first observation of the phenomenon was made by Matt Hedman and P.D. Nicholson in 2013, who analyzed data taken by Cassini.

They found that Saturn’s C-ring contained multiple spiral patterns driven by fluctuations in the gas giant’s gravitational field and that these patterns were distinct from other waves in the rings caused by gravitational interactions with the Saturnian moons.

In the new study, Caltech astronomers Christopher Mankovich and Jim Fuller analyzed the pattern of waves in the rings to build new models of Saturn’s sloshing interior.

“Saturn is always quaking, but it’s subtle,” Dr. Mankovich said.

“The planet’s surface moves about a meter every one to two hours like a slowly rippling lake.”

“Like a seismograph, the rings pick up the gravity disturbances, and the ring particles start to wiggle around.”

The observed gravitational ripples indicate that the deep interior of Saturn, while sloshing around as a whole, is composed of stable layers that formed after heavier materials sunk to the middle of the planet and stopped mixing with lighter materials above them.

“The fuzzy cores are like a sludge,” Dr. Mankovich said.

“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.”

“It’s a bit like parts of Earth’s oceans where the saltiness increases as you get to deeper and deeper levels, creating a stable configuration.”

“In order for the planet’s gravitational field to be oscillating with these particular frequencies, the interior must be stable, and that’s only possible if the fraction of ice and rock gradually increases as you go in toward the planet’s center,” Dr. Fuller said.

The results indicate that the core of Saturn is 55 times as massive as the entire Earth, with 17 Earth-masses of that being ice and rock and the rest a fluid of hydrogen and helium.

They also pose challenges to current models of gas giant formation, which hold that rocky cores form first and then attract large envelopes of gas.

If the cores of the planets are indeed fuzzy as the study indicates, the planets might instead incorporate gas earlier in the process.

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