Astrophysicists use mysterious wobble in Saturn's rings to study Saturn's core

What's in a gas giant? The interiors of Jupiter and Saturn are actually very difficult to probe. But Saturn's unique ring system has proven to be an excellent tool for calculating the density beneath its thick clouds, all the way to the core.

According to a new analysis of the "wobble" of Saturn's innermost main ring, the core is likely not a dense sphere of nickel and iron as currently thought, but rather a "fuzzy" region of mostly hydrogen and helium, The heavy elements are gradually mixed, extending to 60% of the planet's radius and containing about 17 Earth masses of ice and rock.

The discovery, published on the preprint server arXiv pending peer review, is similar to a recent discovery of Jupiter's interior based on data from the Juno probe, and it could change our assumptions about Saturn's early structure and formation history.

How do we learn this from Saturn's rings? It all has to do with the way the rumble of Saturn's core affects the planet's outer gravitational field.

Sound waves and oscillations inside cosmic objects are excellent tools for probing their internal structure. We do this on Earth, where earthquakes send similar ripples; how those waves bounce there can reveal different densities, allowing us to identify structures we never expected to see. On the Sun and other stars, internal sound waves appear as fluctuations in brightness.

Saturn is an inhospitable place to probe with seismometers, nor does it experience brightness fluctuations, but a few years ago scientists noticed a characteristic pattern in Saturn's C ring (the innermost part of Saturn's main rings).

They concluded that these are unlikely to be produced by Saturn's moons, as such patterns are in the outer rings; instead, they appear to be produced by oscillations deep within the planet's interior that affect the gravitational field.

Thus, the field of Kronots seismology was born: studying the interior of Saturn by analyzing these waves in the C ring.

Now, Caltech astrophysicists Christopher Mankovich and Jim Fuller have conducted a new analysis of a previously characterized inner ring wave that is much lower in frequency than established models of Saturn's interior expectations. They found that this frequency pattern places severe new constraints on the makeup of Saturn's interior.

"Our model imposes strict constraints on the mass and size of Saturn's heavy-element core, even though the dilutive nature of this core requires a more detailed description than traditional hierarchical models," they wrote in the paper.

Based on these constraints, they deduced that Saturn's core, which is about 55 times the mass of Earth, contains 17 Earth masses of rock and ice, and the rest is mostly hydrogen and helium; the entire Saturn's structure is diffuse rather than rigid Layered, heavier elements are more abundant at the center of Saturn.

This presents a challenge to models of planet formation. Planets are thought to form from a bottom-up model of pebble accretion, in which small rocks are electrostatically bound together until the planetary "seed" is large enough to gravitationally attract more and more matter— - eventually forming a planet.

For gas giants like Jupiter and Saturn, the heavier material is thought to sink toward the center, forming a solid core that allows less dense gas to rise to the outer regions.

Recent models suggest a slower distribution of material; alternatively, convective mixing may result in a more gradual distribution.

Even so, modeling the formation paths of fuzzy theoretical kernels has proven to be a challenge, and more complex science is likely to be required to fully understand how it happens.

However, this may be putting the cart before the horse. The new study, which is based on a C-ring wave, and a little deeper into Kronots seismology, could help validate the explanation for Saturn's fuzzy theoretical core.