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Saturn’s insides are sloshing around

A new paper suggests Saturn’s core is more like a fluid than a solid, and makes up more of the planet’s interior than we thought.

August 16, 2021
Saturn colored rings
Saturn's rings as viewed by the Cassini probe.NASA

With its massive rings stretching out 175,000 miles in diameter, Saturn is a one-of-a-kind planet in the solar system. Turns out its insides are pretty unique as well. A new study published in Nature Astronomy on Monday suggests the sixth planet from the sun has a “fuzzy” core that jiggles around. 

It’s quite a surprising find. “The conventional picture for Saturn or Jupiter’s interior structure is that of a compact core of rocky or icy material, surrounded by a lower-density envelope of hydrogen and helium,” says Christopher Mankovich, a planetary scientist at Caltech and coauthor of the new study, along with his colleague Jim Fuller

What Mankovich and Fuller glimpsed “is essentially a blurred-out version of that conventional structure.” Instead of seeing a tidy boundary dividing the heavier rocks and ice from the lighter elements, they found that the core is oscillating so that there is no single, clear separation.

This diffuse core extends out to about 60% of Saturn’s radius—a huge leap from the 10 to 20% of a planet’s radius that a traditional core would occupy. 

One of the wildest aspects of the study is that the findings did not come from measuring the core directly—something we’ve never been able to do. Instead, Mankovich and Fuller turned to seismographic data on Saturn’s rings first collected by NASA’s Cassini mission, which explored the Saturnian system from 2004 to 2017.

“Saturn essentially rings like a bell at all times,” says Mankovich. As the core wobbles, it creates gravitational perturbations that affect the surrounding rings, creating subtle “waves” that can be measured. When the planet’s core was oscillating, Cassini was able to study Saturn’s C ring (the second block of rings from the planet) and measure the small yet consistent gravitational “ringing” caused by the core. 

Mankovich and Fuller looked at the data and created a model for Saturn’s structure that would explain these seismographic waves—and the result is a fuzzy interior. “This study is the only direct evidence for a diffuse core structure in a fluid planet to date,” says Mankovich.

Mankovich and Fuller think the reason the structure works is that the rocks and ice near Saturn’s center are soluble in hydrogen, allowing the core to behave like a fluid rather than a solid. Their model suggests that Saturn’s diffuse core contains rocks and ice adding up to more than 17 times the mass of the entire Earth—so that’s a lot of material wobbling around. 

A diffuse core could have a few big implications for how Saturn works. The most significant is that it would stabilize part of the interior against convective heat, which otherwise would roil Saturn’s insides with turbulence. In fact, this stabilizing influence gives rise to the internal gravity waves that influence Saturn’s rings. Moreover, the diffuse core would explain why Saturn’s surface temperatures are higher than what traditional convective models would suggest. 

Still, Mankovich acknowledges that the model is limited in some important ways. It can’t explain what scientists have observed about Saturn’s magnetic field, which is bizarre in a lot of ways (for example, it exhibits a nearly perfect symmetry on its axis, which is quite unusual). He and Fuller hope that future investigations can constrain the interior more narrowly and clue scientists in to how the planet’s core might affect its magnetic field.

They also hope that NASA’s Juno mission might reveal a similar diffuse core within Jupiter. That would go a long way to affirming suspicions that when giant planets form, the process naturally creates gradients of material as opposed to clean and solid cores. Some research using gravity data collected by Juno seems to support this idea as well

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Illustration by Rose Wong

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