A mysterious wobble in Saturn’s rings reveals traces of its “fuzzy” interior

What’s in the gas giant?

No really. The interiors of Jupiter and Saturn are actually quite difficult to explore. However, Saturn’s uniquely famous and extensive ring system is proving to be an excellent tool for detecting densities deep beneath its thick layers of clouds, all the way to the core.

According to a new analysis of “fluctuations” in Saturn’s innermost main ring, this nucleus is probably not a dense sphere of nickel and iron, as it is currently thought, but a “fuzzy” region of predominantly hydrogen and helium, with gradual mixing of heavier elements, reaching up to 60 percent of the planet’s radius and containing about 17 Earth’s masses of ice and rocks.

This finding, published on the arXiv prepress server and waiting peer review, is similar recent knowledge about the interior of Jupiter based on Juno data, and this could change our assumptions about the initial structure and history of Saturn’s formation.

How can we learn this from Saturn’s rings? It all has to do with how the rumble in Saturn’s abdomen affects the planet’s external gravitational field.

Acoustic waves and oscillations inside cosmic bodies are a great tool for studying their internal structure. We do this here on Earth, where tremors send out similar waves rippling across the planet; how these waves are reflected there reveal different densities, which allows us to identify structures that we could never hope to see. In the Sun and other stars, internal acoustic waves manifest as fluctuations in brightness.

Saturn is not a place for a seismometer and is not subject to brightness fluctuations, but A few years agoScientists have noticed signature patterns in Saturn’s C ring, the innermost of its main rings.

They concluded that they were unlikely to be formed by Saturn’s moons because such patterns are in the outer rings; instead, they appear to be generated by oscillations deep inside the planet that affect the gravitational field.

chronoseismology2015 diagram showing how internal oscillations should affect Saturn’s rings. (Matthew Hedman / University of Idaho)

Thus arose the field of chronoseismology: the study of the interior of Saturn by the analysis of these waves in the C-ring.

Now astrophysicists Christopher Mankovich and Jim Fuller of Caltech have performed a new analysis of the previously characterized inner circular wave C, the frequency of which was much lower than expected from the established Saturn Interior model. They found that this frequency pattern created a new severe constraint on Saturn’s internal structure.

“Our models impose strict limits on the weight and size of the core of Saturn’s heavy elements, although the dilute nature of this core requires a finer description than in traditional layered models.” they wrote in their work.

Based on these limitations, they concluded that the mass of the core is approximately 55 times the mass of the Earth and contains stone and ice worth 17 masses of the Earth. The rest would be predominantly hydrogen and helium; the whole thing is scattered and gradually mixed, rather than strictly defined stratification, with a denser concentration of heavier elements at the very center.

This is a challenge for planetary models. Planets are thought to form from a bottom-up model of pebble accretion, in which small pieces of rock are electrostatically bonded together until the planetary “seed” is large enough to gravitate more and more material – eventually forming a planet.

For giant giants such as Jupiter and Saturn, the heavier material was thought to sink toward the center, forming a solid core and allowing the lower density gas to rise to the outer regions.

Recent models suggest more gradual distribution of material; or it is possible convection mixing leads to a more gradual distribution.

Nevertheless, modeling the shaping path for the fuzzy core has proven to be challengingand it is likely that more complex scientific jiggers will be needed to fully understand how this can happen.

However, this can put the cart in front of the horse a bit. The new study is based on a single circular wave C. A little more cronoseismology would help verify the interpretation of the fuzzy Saturn nucleus.

Research is available at arXiv.

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