The interiors of Jupiter and Saturn are currently very difficult to explore. Saturn’s magnificent and extensive ring structure, on the other hand, is proving to be an excellent way to determine densities deep below the dense cloud layers all the way to the center. According to a new study of the “wobbles” in Saturn’s innermost main ring, the core is probably not a dense ball of nickel and iron as previously thought, but a “fuzzy” region composed mainly of hydrogen and helium, with gradual mixing of heavier elements, extending to about 60% of the planet’s radius and comprising about 17 Earth masses of ice and rock.
This discovery, recently published on the preprint server arXiv and currently under peer review, is similar to recent findings about Jupiter’s interior based on Juno data, and it could change our assumptions about Saturn’s early structure and formation history. Based on Saturn’s rings, how can we tell? It’s about the way Saturn’s internal rumblings affect the planet’s external gravitational field.
Saturn’s enigmatic wobblers revealing clues to its out of focus interior
Acoustic waves and vibrations inside celestial bodies are an excellent probe of their internal structure. We do it on Earth, where quakes send identical waves across the planet; the way these waves move reveals densities and helps us detect objects we may never see. Internal acoustic waves manifest as light variations on the Sun and other stars.
Saturn is not a suitable place for a seismometer, and its brightness does not vary, but scientists discovered signature patterns in Saturn’s C ring, the innermost of the planet’s main rings, a few years ago.
They concluded that these patterns are probably not generated by Saturn’s moons, as is the case with the outer rings; rather, they appear to be generated by oscillations deep inside the planet that affect the gravitational field.
Thus was born the field of cronoseismology: the study of Saturn’s interior by analyzing these C-ring waves.
Now, astrophysicists at Caltech, Christopher Mankovich and Jim Fuller, have conducted a new study of a previously identified inner C-ring wave whose frequency was significantly lower than the developed Saturn interior model predicted. They discovered that this frequency pattern imposes a new stringent limit on the composition of Saturn’s interior.
“Our models place tight constraints on the mass and size of Saturn’s heavy-element core, even though the rarefied nature of this core requires a more nuanced description than in traditional layered models,” they write in their paper.
From these constraints, they deduced that the density of the core is about 55 times that of Earth, including 17 Earth masses of rock and ice. The rest would be mostly hydrogen and helium; the overall structure would be diffuse and gradually mixed, rather than strictly stratified, with a denser concentration of heavier elements in the center.
This poses a problem for models of planet formation. Planets are thought to form through a bottom-up model of pebble accretion, in which small pieces of rock are electrostatically bound together before the planetary “seed” becomes large enough to gravitationally absorb additional material and eventually form a planet.
In the case of gas giants such as Jupiter and Saturn, it was previously believed that heavier material sinks to the center, creating a solid core, and lower density gas rises to the outer regions.
More recent models suggest a more gradual distribution of material; instead, convective mixing may lead to a more gradual distribution.
Nevertheless, modeling the formation pathways for a fuzzy core has proven difficult, and it is likely that more complex scientific shenanigans will be necessary to fully understand how this occurs.
However, that could put the cart before the horse. The new research focuses on observing a single C-ring wave. Additional crown seismology will help validate the understanding of a hazy Saturn core.