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What’s in a gas giant?
Not really. The interiors of Jupiter and Saturn are actually quite difficult to fathom. But Saturn’s unique and glorious ring system is proving to be a great tool for determining densities deep beneath its thick cloud layers, right down to the core.
This nucleus, according to a new analysis of the “oscillations” in Saturn’s innermost main ring, is probably not a dense ball of nickel and iron, as many believe, but a “fuzzy” region composed mostly of hydrogen and helium, with a gradual mixture of heavier elements, spanning 60 percent of the planet’s radius and containing about 17 land masses of ice and rock.
This observation, published in Nature astronomy, is similar to recent findings on the interior of Jupiter based on data from Juno, and it could change our assumptions about the original structure and history of Saturn’s formation.
“Fuzzy nuclei are like mud,” said planetary scientist and lead author of the study, Christopher Mankovich of Caltech.
“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 world’s oceans. Land where salinity increases as you reach deeper and deeper levels, thus creating a pattern. “
What might the core look like. (Caltech / R. Hurt, IPAC)
How can we learn this from the rings of Saturn? It all has to do with how the rumblings in Saturn’s belly influence the planet’s outer gravitational field.
“We used Saturn’s rings as a giant seismograph to measure oscillations inside the planet,” said Jim Fuller, co-author and astrophysicist 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.”
Acoustic waves and oscillations within cosmic bodies are a brilliant tool for probing their inner structure.
We do it here on Earth, where earthquakes send similar waves rippling across the planet; the way these waves bounce back in there can reveal different densities, allowing us to identify structures that we might never hope to see.
On the Sun and other stars, interior sound waves manifest themselves as fluctuations in brightness.
Saturn is no place for a seismometer, and it doesn’t experience fluctuations in brightness, but a few years ago scientists noticed signature patterns in Saturn’s C-ring, the innermost of its rings. main.
These, they concluded, were unlikely to be produced by Saturn’s moons, as such patterns are found in the outer rings; instead, they appear to be generated by oscillations deep inside the planet, which influence the gravitational field.
How Internal Oscillations Should Affect Saturn’s Rings. (Matthew Hedman / University of Idaho)
Thus emerges the field of kronoseismology: the study of the interior of Saturn by analyzing these waves in the C ring.
The Caltech team has now conducted a reanalysis of a previously characterized internal C-ring wave that was much lower in frequency than expected from the established inner model of Saturn. This frequency pattern, they discovered, places a strict new constraint on the inner makeup of Saturn.
“Our models impose strict constraints on the mass and size of Saturn’s heavy element nucleus, even though the diluted nature of this nucleus requires a more nuanced description than in traditional layered models,” they write in their article. .
Based on these constraints, they deduced that the mass of the core is about 55 times the mass of the Earth, containing 17 land masses of rock and ice.
The rest would be mostly hydrogen and helium; the whole is diffuse and gradually mixed, rather than a strictly delimited stratification, with a denser concentration of heavier elements at the very center.
This presents a challenge for models of planetary formation. The planets are believed to form from an ascending pebble accretion pattern, in which small pieces of rock are electrostatically bonded until the planetary “seed” is large enough to gravitationally attract more and more of matter – ultimately forming a planet.
For gas giants, like Jupiter and Saturn, it was believed that heavier materials sank towards the center, forming a solid core and allowing the lower density gas to rise to the outer regions.
Recent models suggest a more gradual distribution of the material; or it is possible that the convective mixing results in a more gradual distribution.
Even so, modeling the formation pathways of a fuzzy nucleus has proven difficult, and it is likely that more complex science-based poker games will be needed to fully understand how this can happen.
Maybe it’s putting the cart a little before the horse, though. The new study is based on a C ring wave. A little more kronoseismology would help validate the interpretation of a fuzzy nucleus of Saturn.
The research was published in Nature astronomy.
An earlier version of this article was published in May 2021.
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