Tiny wobbles in the incredible rings of Saturn have allowed astronomers an indirect look into the gas giant's interior, and it appears that its core differs from expectations. A new study suggests it is larger and "fuzzier" than previously thought.
As reported in Nature Astronomy, researchers used data from the Cassini mission – which studied Saturn for 13 years before plummeting to its death – to look at patterns in its rings. Some of them are created by the many moons orbiting the planet (some within the rings themselves), but others are believed to be due to the gravitational effects of the planet’s sloshing interior.
Instead of having a hard sphere of rocks and metals, as some theories had suggested, the core appears to be a sloppy mixture of ice, rocks, and metallic fluids. This is usually referred to as a fuzzy, or diffuse core, without clearly defined boundaries. The findings suggest that the core extends to about 60 percent of the planet's radius, much larger than previously thought. This puts it at around 55 times as massive as Earth and containing enough ice and rocks to make 17 Earths.
"We used Saturn's rings like a giant seismograph to measure oscillations inside the planet," co-author Assistant Professor Jim Fuller from Caltech said in a statement. "This is the first time we've been able to seismically probe the structure of a gas giant planet, and the results were pretty surprising."
"The fuzzy cores are like a sludge," added lead author Dr Christopher Mankovich, also from CalTech. "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. 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."
The rings of Saturn are a collection of fine dust and ice organized into large bands with gaps within them. They are susceptible to many perturbations and the idea that they could be used to probe the interior of the planet has been around for the last three decades. Like earthquakes on our planet, oscillations in Saturn's interior can make the planet jiggle around ever so slightly. Those motions, in turn, cause ripples in Saturn's rings.
"Saturn is always quaking, but it's subtle," explained Mankovich. "The planet's surface moves about a meter every one to two hours like a slowly rippling lake. Like a seismograph, the rings pick up the gravity disturbances, and the ring particles start to wiggle around."
It was only with the Cassini mission and its detailed observations of the Saturnian system that such an idea could be tested. Mankovich and Fuller's analysis has highlighted multiple spiral patterns within Saturn’s C-ring driven by the planet's gravitational field. They believe that no other influence, such as the gravitational pull of Saturn's moons, is causing these fluctuations.
They posit that the gravitational ripples seen show that while Saturn's interior sloshes around, it is still made of stable layers with heavier materials having sunk to the middle and very little mixing with lighter material above.
"In order for the planet's gravitational field to be oscillating with these particular frequencies, the interior must be stable, and that's only possible if the fraction of ice and rock gradually increases as you go in toward the planet's center," said Fuller.
Jupiter is believed to have a similarly fuzzy core based on recent evidence from NASA’s Juno mission. Cassini was a collaborative mission between NASA, the European Space Agency, and the Italian Space Agency.