12 May 2020

Sounding Saturn’s depths with its seismic icy rings

Posted by larryohanlon

By Larry O’Hanlon

The secrets of Saturn’s veiled interior are leaking out by way of the planet’s spectacular rings, according to a line of research that has taken four decades to come to fruition. In the last few years, what was first considered a sort of wacky hypothesis – that scientists can use Saturn’s rings to learn about  its structure — has turned into a singular window into Saturn’s surprisingly fluid and leviathan depths.

“People thought it was crazy back in the 1980s,” said Christopher Mankovich, a planetary scientist at Caltech and author of a new commentary in AGU Advances explaining the story behind the remarkable science. “Today we’re using the rings to listen to Saturn’s structure.”

A simplified and exaggerated model of how the bell-like seismic ringing of Saturn is transmitted gravitationally to the planet’s icy rings. Credit: Christopher Mankovich.

It all started with a hypothesis in a 1982 Eos article entitled, ‘Are Saturn’s Rings a Seismograph for Planetary Inertial Oscillations?,’ by Dave Stevenson. He proposed the billions of small ice crystals which form Saturn’s rings ought to be measurably affected by seismic vibrations within the giant planet. Theoretically, effects on the rings could be used as a sort of seismograph to learn about structures inside of Saturn, using the same science that allows seismologists to use the ringing of seismic waves on Earth to explore the structure of our own planet. 

The seismic waves bouncing around inside of Saturn are not moving through space to reach the rings – because seismic waves don’t move through outer space. But Saturn’s gravity has a tight grip on the icy particles in the rings, and the bell-like ringing mass of the planet could stir the rings by way of oscillations in Saturn’s gravitational field, according to the hypothesis.

If this hypothesis was correct, it would be a boon to planetary scientists who had been unable to penetrate the nebulous depths of Saturn well enough to even confirm the planet’s daily rotation rate, much less any details about its internal structure. The trouble is, it’s a hard hypothesis to test without watching the rings very closely, which isn’t possible from Earth. 

“Voyager hinted at it,” said Mankovich referring to the flybys of Voyager 1 and Voyager 2 in 1980 and 1981. They imaged unexpected and enticing complexity in the rings that were later traced to the periodic gravitational tugs from Saturn’s moons.

In the years that followed, planetary scientists fleshed out Stevenson’s original idea in a series of papers in 1990, 1991 and 1993. But it wasn’t until the Cassini spacecraft reached Saturn in 2004 and started gathering detailed observations that the seismic rings hypothesis could be tested.

Cassini was able to detect and measure the oscillations in the rings by peering through Saturn’s rings from various angles to see bright stars in the background. By measuring how the starlight varied over time, the oscillations caused by Saturn’s gravitational field could be observed  spiraling outward in the rings.

Even more amazing, the scientists found that ice particles in different locations in the rings resonate with different Saturnian seismic frequencies, similar to how plucking an E string on a harp can cause other E strings at different octaves to vibrate. When the ice particles at a certain distance from Saturn  resonate, they can launch waves that propagate outwards through the rings. This breakthrough was published in 2013 by researchers Matthew Hedman and Phillip Nicholson.

“Saturn’s rings thus, incredibly, form a natural frequency-domain seismograph for the planet’s normal mode oscillations,” Mankovich writes in the new commentary.

What the ring seismology has recently suggested, as described in a 2014 paper by Jim Fuller, is that planetary scientists had it wrong about Saturn’s interior. Instead of being all angry, turbulent depths with  huge convection currents, Saturn appears to have some areas with a very calm, stably-layered hydrogen-rich envelope that smoothly transitions down to an ice and rock core.

It’s not a detailed picture yet, but it’s a start and an entirely new and unexpected way to use Cassini data to study one of the most inaccessible places in the solar system.

“So it’s going to be a major way to study the interior of the planet,” Mankovich said.

Larry O’Hanlon is a freelance geoscience writer and editor in New Mexico. He manages the AGU Blogosphere.