Ripples in Saturn’s Rings Reveal the “Fuzzy” Nature of the Gas Giant’s Core

Saturn makes waves in its own rings

In the same way that earthquakes make our planet rumble, the oscillations within Saturn make the gas giant tremble very lightly. These movements, in turn, cause ripples in Saturn’s rings.

In a new study accepted in the journal Nature astronomy, two Caltech astronomers analyzed these undulating rings to reveal new information about Saturn’s core. For their study, they used older data captured by Nasa‘s Cassini, a spacecraft that orbited the Ringed Giant for 13 years before plunging into the planet’s atmosphere and disintegrating in 2017.

The results suggest that the planet’s core is not a hard rock ball, as some previous theories have proposed, but a diffuse soup of ice, rock, and metallic fluids – or what scientists call a core. “vague”. The analysis also reveals that the nucleus spans 60% of the diameter of the planet, making it considerably larger than expected.

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

“Detailed analysis of Saturn’s undulating rings is a very elegant form of seismology for inferring the characteristics of Saturn’s nucleus,” says Jennifer Jackson, William E. Leonhard professor of mineral physics at the Caltech Seismological Laboratory, who has no participated in the 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 lead author of the study is Christopher Mankovich, a postdoctoral associate researcher in planetary science who works in Fuller’s group.

The results offer the best evidence to date for Saturn’s fuzzy core and match recent evidence from NASA’s Juno mission, which indicates that the gas giant Jupiter can also have a diluted core in the same way.

“Fuzzy cores are like mud,” explains Mankovich. “The planet’s hydrogen and helium gas gradually mix with more and more ice and rock as you move toward the center of the planet. It’s a bit like parts of the Earth’s oceans where salinity increases as you reach deeper and deeper levels, creating a stable pattern. “

The idea that Saturn’s oscillations could make waves in its rings and that the rings could thus be used as a seismograph to study the interior of Saturn first appeared in studies in the early 1990s by Mark Marley ( BS ’84) and Carolyn Porco (PhD ’83), who later became the leader of the Cassini imaging team. The first observation of the phenomenon was made by Matt Hedman and PD Nicholson (PhD ’79) in 2013, who analyzed the data collected by Cassini. Astronomers discovered that Saturn’s C-ring contained several spiral patterns driven by fluctuations in Saturn’s gravitational field and that these patterns were distinct from other ring waves 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 Saturn’s tossed interior.

“Saturn is still shaking, but it’s 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 disturbances in gravity and the particles in the ring start to wiggle, ”he says.

The researchers say the observed gravitational ripples indicate that Saturn’s deep interior, while swaying as a whole, is made up of stable layers that formed after heavier materials sank in the middle of the planet and stopped settling. mix with lighter materials on top of them.

“For the planet’s gravitational field to oscillate with these particular frequencies, the interior must be stable, and this is only possible if the fraction of ice and rock gradually increases as you get closer to the center of the planet. “Fuller explains.

Their results also indicate that Saturn’s core is 55 times as massive as the entire Earth, with 17 land masses of ice and rock and the rest a fluid of hydrogen and helium.

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

Additionally, the results pose challenges to current models of gas giant formation, which argue that rock cores form first and then attract large envelopes of gas. If the planets’ cores are indeed fuzzy as the study indicates, the planets might instead incorporate gas earlier in the process.

Reference: “A diffuse nucleus of Saturn revealed by ring seismology” by Christopher R. Mankovich and Jim Fuller, August 16, 2021, Nature astronomy.
DOI: 10.1038 / s41550-021-01448-3

the Nature astronomy The study, titled “A Diffuse Core in Saturn Revealed by Ring Seismology,” was funded by the Rose Hills Foundation and the Sloan Foundation.