New research reveals that Saturn, like Jupiter, has a “fuzzy” core that extends 60% of the way to its surface. This finding joins others in changing how we think about the formation of giants in our solar system and beyond.
Saturn quivers like heavy Jello, its gaseous surface heaving by a meter every couple of hours. These subtle shakes pull on the giant’s rings, whipping up spirals that reveal the planet’s interior structure.
Now, a study of the rings’ spirals has revealed that Saturn, like Jupiter, has a “fuzzy” core that extends 60% of the way to its surface. The finding, which was published August 16th in Nature Astronomy, completely changes our understanding not just of Saturn but of all giant planets.
Astronomers have long debated what the guts of Saturn look like: Is the core solid or liquid? Is it all mixed up or is it layered? And how does the planet generate a magnetic field whose axis is almost exactly centered on the planet’s spin axis — an unusual thing for planets whose magnetic fields come from churning in the core.
To investigate these ideas, astronomers have used observations of the planet’s rings from NASA’s Cassini spacecraft, which watched the planet for 13 years before diving in for a grand finale in 2017. While the moons tug on the outer rings, it’s the planet itself that pulls on the inner rings, shaping the ice particles into spirals.
Cassini did not image the ring spirals directly; instead, astronomers used the spacecraft to look through the rings at a background star. As the star passed behind a given ring, the spacecraft’s instruments measured how much the star dimmed to determine how densely packed the ring material was. Using multiple such stellar occultations, astronomers were able to measure spirals in the C ring.
However, astronomers at the time couldn’t make sense of all of what they saw. In particular, one of the waves had a very low frequency that they couldn’t explain. Now, Christopher Mankovich and Jim Fuller (both at Caltech) make the case that this frequency indicates a quiver that penetrated deep into the planet.
Such a low-frequency quiver indicates that there’s no hard boundary between the core and the envelope around it. Instead, the rocks and ices in the planet’s core are “smeared out,” dissolved into the fluid helium and hydrogen under intense pressure, Mankovich explains.
“From the demands of ring seismology on one hand and expectations from materials physics on the other, a solid core in Saturn is looking very unlikely,” Mankovich adds.
Combining this seismic data with Cassini’s measurements of local gravitational fields and with computer models of Saturn’s interior, the researchers conclude that the core of the planet is 55 times Earth’s mass, with rock and ice making up 17 Earths’ worth (hydrogen and helium make up the rest).
To maintain stability in the face of sloshing, all this material must be layered, with the heaviest layers at the bottom.
“The hydrogen and helium gas in the planet gradually mix with more and more ice and rock as you move toward the planet's center,” Mankovich says. “It's a bit like parts of Earth's oceans, where the saltiness increases as you get to deeper and deeper levels, creating a stable configuration.”
In addition to increasing rock-and-ice “salt,” there’s also a gradual change in the mix of hydrogen and helium. When Mankovich and Fuller folded in Cassini’s measurements of gravity, they could see that helium must also be more concentrated toward Saturn’s center, consistent with previous ideas of “helium rain,” in which blobs of helium condense out of the hydrogen and settle down toward the core.
“This is a very interesting result that indeed changes the way we think about Saturn, and giant planets in general,” says planetary scientist Ravit Helled (University of Zurich), who was not involved in the study.
For example, one planet-formation scenario has hydrogen and helium gas glomming onto a rocky core to make planets like Saturn. It could be that the rocky core then disintegrated under the pressure, diffusing outward into the current fuzzy core — or it could be there was never a rocky core to begin with.
Saturn’s layer-cake interior also affects its magnetic field. In most planets, the churning electric fluids that create the global magnetic fields are in the core. But if Saturn’s core is layered, it can’t also be churning. The planet’s magnetic field would instead have to come from the outer gaseous envelope. “This should be investigated further,” Helled says.
The results reach further than the solar system. When we examine giant planets around other stars, we assume we understand our own system’s giants. Helled thinks this new understanding of Saturn’s interior will affect how we characterize exoplanets as well.