Saturn's rippling rings point to massive, soupy core hidden inside

Saturn's rings aren't just a beautiful adornment — scientists can use the feature to understand what's happening deep inside the planet.

By using the famous rings like a seismograph, scientists studied processes in the planet's interior and determined that its core must be "fuzzy." Instead of a solid sphere like Earth's, the core of Saturn appears to consist of a 'soup' of rocks, ice and metallic fluids that slosh around and affect the planet's gravity.

The new study used data from NASA's Cassini mission, which orbited Saturn and its moons for 13 years between 2004 and 2017. In 2013, data from the mission revealed for the first time that Saturn's innermost ring, the D-ring, ripples and swirls in ways that cannot be entirely explained by the gravitational influences of the planet's moons. The new study looked at these motions in Saturn's rings in greater detail to gain insight into the processes in its interior. 

"We used Saturn's rings like a giant seismograph to measure oscillations inside the planet," Jim Fuller, assistant professor of theoretical astrophysics at Caltech and one of the authors of the paper said in a statement. "This is the first time we've been able to seismically probe the structure of a gas giant planet, and the results were pretty surprising." 

Related: Cassini's greatest hits: The spacecraft's best images of Saturn

Not only does the planet's core seem sludgy, it also appears to extend across 60% of the planet's diameter, making it much larger than previously estimated. 

The analysis showed that Saturn's core might be about 55 times as massive as the entire planet Earth. Of the total mass of the core, 17 Earth masses are made of ice and rock, with the rest consisting of a hydrogen and helium-based fluid, the study suggests.

The lead author of the study, Christopher Mankovich, a postdoctoral scholar research associate in planetary science who works in Fuller's group, explained that the motions in the core cause Saturn's surface to constantly ripple. These surface waves create minuscule changes in the planet's gravity that subsequently affect the rings. 

"Saturn is always quaking, but it's subtle," Mankovich said in the statement. "The planet's surface moves about a meter [3 feet] 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."

According to the scientists, the nature of those ring ripples suggests that the core, despite its sloshing, is composed of stable layers of various densities. Heavier materials sit around the center of the planet and don't mix with the lighter materials closer to the surface. 

"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," Fuller said.

Mankovich compared the material in the core to sludge,  adding that the layered but liquid nature of the core is akin to the salinity of Earth's oceans, which increases with depth. 

"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," Mankovich said.

The findings might challenge some of the established models of the formation of gas giants, planets with no hard surface, which are composed mainly of hydrogen and helium, the study suggests. These models assume that the rocky cores of these planets formed first and then attracted large envelopes of gas. If the cores of the planets are, however, fuzzy as the study indicates, the planets might instead incorporate gas earlier in the process.

In fact, recent findings by NASA's Juno mission suggest that another of the solar system's gas giants, Jupiter, might also have a similarly fuzzy core. 

"Christopher [Mankovich] and Jim [Fuller] were able to show that one particular ring feature provided strong evidence that Saturn's core is extremely diffuse," said Matt Hedman, a planetary scientist at the University of Idaho, who was part of the team that first discovered that the motions in Saturn's rings can't be fully explained by the gravity of its moons.

"I am excited to think about what all the other ring features generated by Saturn might be able to tell us about that planet," added Hedman, who did not collaborate on the new paper.

The research is described in a paper published Monday (Aug. 16) in the journal Nature.

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