Beneath its colourful bands of cloud, Saturn is an enigma. We probably know more about the gas giant’s many moons than we do about the planet itself. And intriguing clues to its interior have only deepened the mystery.
Gravitational fingerprints in the planet’s rings point to planet-wide tsunamis racing around the equator, and hint at surprising structures inside – giant whirlpools thousands of kilometres deep, a buried sphere of light or perhaps something even stranger.
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“Unless the laws of physics have been repealed on Saturn, none of it makes sense”
In 1980, the first Voyager mission found spiral waves ruffling Saturn’s rings, a little like the arms of a spiral galaxy. Most, caused by the gravity of Saturn’s moons, radiate outwards. But a few move inwards.
Researchers suspected that these are the result of vibrations inside the planet. According to conventional wisdom, Saturn is a uniform fluid ball, a smooth mixture of hydrogen and helium. In theory, this substance can vibrate, forming waves that race around the equator. The undulating gravity of the peaks and troughs on one of these planetary waves could be enough to tease out a spiral wave in the rings above.
But based on the limited data from Voyager, no one could be sure. So Phillip Nicholson and his colleagues at Cornell University started picking through observations gathered by the Cassini space probe since 2004. They traced out several spirals in the inner rings which back up the basic idea: waves are indeed racing around the planet. Then things got weird.
If the planet is a simple fluid ball, the theory goes, the speed of each wave should be fixed by its number of peaks. A three-peaked wave travels more slowly than a two-peaked wave, and so on. The researchers expected to see one example of each type of spiral, each whizzing around at a unique speed. Instead, Nicholson’s team found , as well as two separate two-armers. “This is a puzzle,” says Nicholson.
The simplest explanation would be a large solid core, vibrating in its own way, interfering with the simple fluid waves above. While that’s in line with conventional ideas of planet formation, it would take some fine-tuning to generate these particular waves.
Alternatively, there may be a layer in the planet where the hydrogen-helium mixture behaves differently. At some point the molecules of hydrogen and helium should break up into separate atoms, which would make the mixture relatively transparent, creating a luminous sphere. This could also make it vibrate differently. If so, the spirals could be telling us something about what happens to matter under these pressures, a regime still beyond our computer simulations.
Strangest of all, a few spiral waves are moving around at almost exactly the same speed as Saturn’s rotation. One explanation is the presence of permanent hills and valleys on the planet. But if Saturn is fluid, this would be like finding fixed hills on the sea.
Unless the laws of physics have been repealed on Saturn, fluid hills aren’t an option. Nicholson’s colleague Maryame El Moutamid, who has found , has another tentative suggestion: massive vortices deep inside the planet, less dense than the surrounding fluid and so exerting less gravity. This would create dents in Saturn’s gravitational field to explain those perplexing spirals. She is now building a simulation of Saturn’s interior to find out if this works.
Next year, Cassini’s mission ends with a dive into Saturn. Sadly, it will burn up long before it reaches any of these obscure wonders, but on its way the spacecraft will make half a dozen close orbits of the planet that may tell us more about Saturn’s gravity. With luck, that might illuminate what is really stirring inside the giant.
Read more: The 6 greatest mysteries of the solar system
(Image: NASA/JPL/Space Science Institute)
This article appeared in print under the headline “What’s inside Saturn?”
