Ripples in Saturn's rings reveal the planet's giant, slushy core

Saturn’s core is an unexpectedly immense mixture of ice, rock, and gas, surprising scientists who are trying to figure out how the planet formed and evolved to the enigmatic world we see today.

By observing waves in Saturn's rings, scientists were able to measure the size and shape of the planet's core—and it's much larger and more bizarre than anyone could have guessed.
Photograph by NASA, JPL

Hidden inside the solar system’s god of plenty is an unexpected bounty: Saturn’s mammoth core, spanning up to 60 percent of the planet’s width. The newly measured core, revealed through subtle waves in Saturn’s rings, appears to be ice, rock, and gas, blended into a soupy mass with blurry edges.

“It’s huge,” says Chris Mankovich of the California Institute of Technology, one of the authors of a new study describing Saturn’s core in the journal Nature Astronomy. “It’s definitely not something we expected to find.”

The characteristics of Saturn’s immense heart have scientists rethinking how the ringed planet may have formed, and how it generates its strangely uniform magnetic field. “It’s just more complex than we thought was going to be the case,” says Johns Hopkins University’s Sabine Stanley, who was not part of the new study.

To scrutinize Saturn’s innards, scientists turned to the planet’s rings, which act like a seismograph and record the gas giant’s internal sloshing and pulsing. By decoding subtle ripples in the rings, the team learned that Saturn’s nucleus is packed with 17 Earths worth of material, and that it isn’t a discrete, compact hunk of rock and iron as expected.

“It’s really hard to learn anything about the deepest parts of a planet, especially the giant planets,” Stanley says. “Any information we can get improves what we knew before.”

Now scientists have to figure out how giant planets such as Saturn can grow with such large, jumbled cores. And to complicate the matter, the newly revealed interior of Saturn makes it difficult to explain how the planet powers its enigmatic magnetic field. 

“A can of worms with Saturn is explaining the observations with Saturn’s magnetic field, which is a fairly bizarre magnetic field for many reasons,” Mankovich says. “This is a non-standard picture for the interior structure of the planet.”

A portal to the planet’s interior

Saturn is perhaps best known for its shimmering rings, which orbit the planet and appear solid from afar. In reality these rings are made of innumerable icy chunks—some as big as houses, others smaller than pebbles—that can be pushed, tugged, and sculpted by gravitational interactions with the planet and its moons. Some of those moons mow gaps into the rings, while others prune the edges and keep them neat.

The rings also record Saturn’s inner workings. Normally scientists use fluctuations in a planet’s gravitational field to take a look beneath its surface—but that technique cannot penetrate the deep interior of a gas giant like Saturn. The rings, however, do offer a window into the depths of the planet’s heart.

In the 1980s, planetary scientists surmised that movements within Saturn could create observable waves in the planet’s C ring, which is a wide but dim ring tucked near the planet. As its heart beats and its innards slosh, the planet pulses. Those oscillations interact with ring particles and sculpt what are known as spiral density waves, or ripples within the C ring.

These ideas “turned out to be 100 percent right,” Mankovich says. But it would take more than two decades and a multibillion-dollar space mission to confirm the predictions.

In 2013 scientists studying data from NASA’s Cassini spacecraft—which orbited Saturn from 2004 to 2017—read the seismic signatures in the rings and used them to peer inside the planet. That team coined the term “kronoseismology” to describe this new field of study, and they linked most of the observed ripples to movements within the planet. In 2019 scientists used kronoseismology to determine that Saturn rotates once every 10 hours and 33 minutes

“This is not a field for the impatient,” says Mark Marley, one of the early pioneers of kronoseismology and a reviewer of the new study. “It’s turned out that all these waves are really there, that there’s something like two dozen or so that are more or less where we predicted.”

But there’s also at least one mysterious wave that scientists didn't initially predict—and it’s the one Mankovich and Jim Fuller, also of Caltech, used to look straight into Saturn’s heart.

“They show really convincingly that you can only explain this extra wave, along with all the other ones, if Saturn has this fuzzy, gradual core,” Marley says. “This particular ring wave is very sensitive to deep in the planet.”

The colossal heart of Saturn

Using that ring ripple, Mankovich and Fuller discovered that Saturn is mostly heart, with a core that occupies the majority of the planet. Contrary to expectations, the planet’s core is a diffuse, soupy mix of hydrogen, helium, ice, and rock, rather than a solid pit of stony iron. If you were to slice Saturn in half, you wouldn’t see discrete layers like the ones inside onions, candy gobstoppers, or planet Earth. Instead the core has a fuzzy boundary, and the deeper you dive, the denser the material becomes.

At the extreme temperatures and pressures in Saturn’s core, gases behave more like metallic fluids than puffs of air, and the whole assemblage is a blend of exotic material that’s difficult to replicate in labs on Earth. Mankovich says that when he and Fuller saw how strange their picture of Saturn was, they tried to find another explanation for the seismic signature in the rings.

“We were trying to shut it down as hard as we could,” he says, but the picture of Saturn’s core in the new study “does seem to be what the data required.”

Although unexpected, the model of Saturn’s core fits nicely with the wealth of gravity data that scientists have collected from Saturn. It also echoes findings from NASA’s Juno spacecraft, which suggest Jupiter’s core might be a similarly diffuse mixture of ingredients.

Jupiter, however, doesn’t have a thick ring system to record its inner jostles. “We need to blow up one of Jupiter’s little moons,” Marley jokes, to create a ring that will record the pulses of Jupiter’s heart. 

Many mysteries to solve

The classical origin story for a large gassy world starts with small chunks of material that clump together, growing more and more massive until the protoplanet’s gravity draws in all the nearby gas. But it’s not clear whether that scenario can craft a core like the one Mankovich and Fuller observed.

It’s possible that Saturn’s heart evolved and changed during its 4.5 billion years of existence, perhaps slowly dissolving into liquid metallic hydrogen or being modified by other unknown processes. “We just don’t know yet,” Mankovich says.

Another surprise is that Mankovich and Fuller deduced that the core is not convective, meaning it doesn’t move heat around as expected—an observation that might explain why Saturn is surprisingly bright in the infrared. “Jupiter is about as bright as you would expect Jupiter to be after 4.5 billion years, but Saturn is too bright,” Marley says. “Because this [core] doesn’t want to convect, it slows down the cooling and is brighter than it should be.”

But a non-convective core presents a substantial challenge for understanding the planet’s magnetic field. Normally planetary magnetic fields are powered by a dynamo—a rotating, convective layer of electrically conductive fluid deep within a planet’s core. But that’s not possible inside Saturn, where 60 percent of the planet isn’t convective, according to the new study. Scientists are now wondering whether a thin layer of liquid metallic hydrogen might be churning within the core, or perhaps just outside it.

But even those ideas struggle to account for Saturn’s oddly symmetric magnetic field, which is unlike the tilted, irregular ones of Earth and Jupiter. One possibility is that a layer of helium rain might be smoothing the magnetic field lines before they reach the planet’s surface, but the researchers don’t have a good explanation for how the giant core could affect this process.

“It’s really hard to do that with magnetic fields, and you can’t really do that with a dynamo, and then we have a giant planet doing it,” says Stanley, who is working with Mankovich and Fuller to untangle the magnetic puzzle. “That’s why science is fun.”

Answering these Saturnian questions will require poring over the wealth of information gathered by the Cassini spacecraft, performing detailed simulations of planetary interiors with supercomputers, and crafting experiments with Earth-based telescopes. In the future, scientists could also use these methods to scrutinize the rings of other planets such as Uranus and Neptune, uncovering any secrets that might be written into their icy particles.

“There’s a number of mysteries about those rings,” Marley says, “so we’re looking to see if the same kind of thing happened there.”

Editor's Note: This story has been updated to reflect the fact that a number of planetary scientists developed theories in the 1980s that led to this new study.

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