Saturn is surrounded by a natural seismograph: Its rings record the faint signatures of vibrations inside the planet.
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Saturn is surrounded by a natural seismograph: Its rings record the faint signatures of vibrations inside the planet.

In Saturn’s Rings, a Portal to the Planet’s Interior

In addition to being spectacularly beautiful, Saturn’s rings act as a cosmic seismograph: They keep a record of the shifts and vibrations happening deep within the giant world. These internal movements pull ever so gently on the ring particles and alter their orbits around Saturn, creating visible gaps and waves in the rings close to the planet.

By reading that pattern of gaps and waves, scientists can reconstruct what’s happening in Saturn’s gassy interior – and, as reported at the American Geophysical Union’s meeting last month, they’re finding some things they didn’t expect.

“What’s great about Saturn, of course, is that it has rings. And this allows us to detect oscillations of the planet itself, which otherwise is almost impossible to do,” said Caltech postdoc Jim Fuller, who presented the work. “This is the first seismology, to my knowledge, that’s provided any useful constraints on a planet other than the Earth.”

Reading the Rings

First described as a theoretical possibility three decades ago by Caltech’s Dave Stevenson, what’s now called kronoseismology enjoyed a brief emergence in the early 1990s. Scientists studying Voyager images had singled out particular features in Saturn’s C ring – one notable gap and a handful of spiral density waves — that couldn’t be explained by the gravitational tugs and nudges of its many moons.

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Seismologically useful spiral density waves are found in Saturn’s C ring. Click to see full-size version of this ring map (highly recommended). (NASA/JPL/Space Science Institute)

Back then, astronomer Mark Marley and planetary scientist Carolyn Porco, extending Stevenson’s idea, predicted that those features are produced by the planet itself, and that the enigmatic, embedded ripples could be used to probe Saturn’s interior structure.

“It’s like the rings are giving you a window into the interior of the planet, much like earthquakes and the frequencies of the waves they generate give you a window into the structure of the Earth,” says Porco, who’s now the imaging team leader for NASA’s Cassini mission. “We’re using the position and character of the gaps and waves to figure out something about the interior structure.”

But it would be almost two decades before Cassini arrived at the ringed world and could capture the waves with sufficient precision for scientists to accurately read them (“It is fun to see it actually working out,” says Stevenson, who saw Fuller’s presentation at AGU). Now, when a distant background star seems to pass behind the rings as Cassini looks through them, the flickering of the star’s light can reveal telltale patterns of alternating dense and sparse regions — the planet’s seismic signature written in waves and gaps.

So far, University of Idaho planetary scientist Matt Hedman and Philip Nicholson of Cornell University have studied more than a dozen different density waves and coined the term “kronoseismology” to describe the field.

“The particular features we’re looking at cluster in the inner part of the ring system,” Hedman says. “They show up well. It’s also where these sorts of features are expected to be common.”

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Patterns in the rings can be made by Saturn’s moons. In this image, Daphnis is sculpting material near the Keeler Gap (left) and Pan is producing waves near the Encke Gap (right). (NASA/JPL/Space Science Institute)

Saturnian Symphony

Giant planets aren’t put together like Earth, which has a rigid crust, squishier mantle and dense, multilayered core. While giant planets do have a very large chunk of ice and rock at their center, their cores are swaddled in expansive, turbulent layers of gas. Deep within the planet, tremendous pressures compress the gas and cause it to behave more like a liquid, or even a metal (scientists suspect both Saturn and Jupiter have cores surrounded by a layer of liquid metallic hydrogen, for example).

Planets, like musical instruments, oscillate at particular frequencies. When that happens in a planet like Saturn, its odd, gassy innards slosh around. Sloshing produces pressure waves and slight perturbations in the planet’s gravitational field — gentle nudges that rearrange ring particles and leave seismic inscriptions in particular locations.

“It’s a really small change, but the rings are very sensitive to really small changes,” Fuller says. “So we can see the effect.”

Just as with a musical instrument, the way a planet is put together decides which tones it can play. At Saturn, scientists are working backward. They’re using the sheet music hiding in the rings to determine which instruments are playing, and how they’re built.

“We try to measure the notes Saturn plays, and then use that to figure out what’s inside the planet,” Fuller says.

Sometimes that means requisitioning the blueprints for an unexpected instrument. When Hedman disentangled the density waves he’d observed in 10 years of Cassini data, he found a few bizarre notes that didn’t match predictions.

In other words, something deep inside the planet was acting peculiarly.

So, Fuller got to work simulating different planet anatomies, calculating the notes those varying innards would play, and then looking for a match with the waves Hedman saw.

It’s a complicated calculus, but Fuller’s results suggest the presence of a calm, stable layer of gas near Saturn’s rocky core – an arrangement that doesn’t fit with classical theories describing planet innards and instead aligns more closely with what’s going on inside stars like the sun.

“The fluid is basically not moving at all,” Fuller says. “It’s a very non-turbulent region.”

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An early view of Saturn, seen through the eyes of Cassini. (NASA/JPL/Space Science Institute)

What Lies Beneath

Normally, scientists expect a giant planet’s gassy layers to be slowly churning all the way to the core, not just part of the way there. So what, then, is going on? It turns out, there are a few ways to grow a stable layer even if you’re a planet. One of these includes helium behaving strangely, either by forming a liquid ocean near the planet’s core, or raining down in the deep atmosphere, or slowly transitioning into a liquid metallic form of itself.

Another, more alarming sounding possibility, is that Saturn’s core is dissolving into the metallic hydrogen immediately around it. “That sounds pretty exotic at first, and it kind of is, but I think most people are familiar with the idea that solids can dissolve in liquids,” Fuller says. “The same thing might happen deep down inside planets.”

Stevenson notes that three decades ago, he would have thought such a configuration “unlikely,” but that it now seems highly reasonable, given the last five years of work understanding core solubility.

More observations and simulations should help Fuller pinpoint how far beneath Saturn’s yellowish-brown surface that stable layer lies. That’ll help him figure out exactly what’s going on inside our beloved ringed world, and perhaps even offer some clues about the innards of Saturn’s giant kin.

Maybe similar signatures are hiding in the rings of Jupiter, Uranus and Neptune as well.

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What else is hiding in Saturn’s rings? (NASA/JPL/Space Science Institute)