Waves in Saturn’s rings reveal the nature of its core

Image credit: NASA

Oscillations inside Saturn cause the gas giant to move slightly. Those movements, in turn, cause ripples in Saturn’s rings. In a new study accepted in the journal Nature Astronomy, two Caltech astronomers have analyzed those rippling rings to reveal new information about Saturn’s core. For their study, they used data captured by NASA’s Cassini mission, a spacecraft that orbited the ringed giant for 13 years before it plunged into the planet’s atmosphere and disintegrated in 2017.

The findings suggest that the planet’s core is not a ball of hard rock, as some previous theories had proposed, but rather a fuzzy soup of ice, rock, and metallic fluids, or what scientists call a “fuzzy” nucleus. The analysis also reveals that the core extends over 60 percent of the planet’s diameter, making it substantially larger than previously estimated.

“We use the rings of Saturn as a giant seismograph to measure oscillations within the planet,” says co-author Jim Fuller, an assistant professor of theoretical astrophysics at Caltech, in a statement. “This is the first time that we have been able to seismically probe the structure of a gas giant planet, and the results were quite surprising.”

“Detailed analysis of Saturn’s undulating rings is a very elegant form of seismology to infer the characteristics of Saturn’s core,” Jennifer Jackson, a professor of Mineral Physics at Caltech’s Seismological Laboratory, said in a statement. study, but uses different types of seismic observations to understand the composition of the Earth’s core and to potentially detect seismic events on Venus in the future.

The study’s lead author is Christopher Mankovich, a postdoctoral research associate in planetary sciences who works in Fuller’s group. The findings offer the best evidence yet for Saturn’s diffuse core and align with recent evidence from NASA’s Juno mission, which indicates that the gas giant Jupiter may also have a similarly diluted core.

“Fuzzy nuclei are like mud,” Mankovich explains. “The planet’s hydrogen and helium gradually mix with more and more ice and rocks as it moves towards the center of the planet. It’s a bit like parts of Earth’s oceans where salinity increases as you get to levels. deeper and deeper, creating a stable configuration. “

The idea that Saturn’s oscillations could generate ripples in its rings, and that the rings could therefore be used as a seismograph to study the interior of Saturn, first emerged in studies in the early 1990s by Mark Marley and Carolyn Porco, who later became the Cassini imaging team leader. The first observation of the phenomenon was made by Matt Hedman and P.D. Nicholson in 2013, who analyzed data taken by Cassini. The astronomers found that Saturn’s C ring contained multiple spiral patterns driven by fluctuations in Saturn’s gravitational field and that these patterns were distinct from other ripples in the rings caused by gravitational interactions with the planet’s moons.

Now, Mankovich and Fuller have analyzed the wave pattern in the rings to build new models of the splashing interior of Saturn. “Saturn is always shaking, but it is subtle,” says Mankovich. “The surface of the planet moves about a meter every one to two hours like a slowly rippling lake. Like a seismograph, the rings pick up the disturbances of gravity and the particles in the ring begin to move,” he says.

The researchers say the observed gravitational waves indicate that Saturn’s deep interior, as it churns as a whole, is made up of stable layers that formed after heavier materials sank into the middle of the planet and stopped mixing with. lighter materials on top of them.

“For the gravitational field of the planet to oscillate at these particular frequencies, the interior must be stable, and that is only possible if the fraction of ice and rock gradually increases as it moves towards the center of the planet,” says Fuller.

Their results also indicate that the nucleus of Saturn is 55 times more massive than the entire Earth, with 17 land masses being ice and rock and the rest being a fluid of hydrogen and helium.

Hedman, who is not part of the current study, says: “Christopher and Jim were able to show that one particular feature of the ring provided strong evidence that Saturn’s core is extremely diffuse. I’m excited to think of all the other features of the ring. generated by Saturn could tell us about that planet. “