Giant Waves Solve Saturn Ring Mystery

By Lisa Grossman
November 1, 2010 |
2:57 pm |
Categories: Space

Saturn’s largest ring appears to behave like a mini spiral galaxy. NASA’s Cassini spacecraft caught enormous waves sloshing back and forth across Saturn’s B ring, similar to waves believed to give galaxies their spiral shapes.

“This is a major result,” said Cassini imaging team leader Carolyn Porco of the Space Science Institute. “Saturn’s rings are tiny tiny tiny compared to a galaxy, but we see the same physics.”

The new observations also show two warped regions, including a tall arc of spiky peaks that rise almost two miles above the ring plane. These perturbations may have been sculpted by small moons that migrated across the ring disk, a process believed to be important in shaping planetary systems.

Saturn’s most massive ring, the B ring, has baffled astronomers since the Voyager spacecraft flew by in 1980 and 1981. Those observations showed the B ring was sculpted into a flattened football shape with a sharp outer edge by the moon Mimas. But even in the Voyager images, it was clear the B ring was too complex and chaotic to be shaped by Mimas alone.

Now, in a new analysis published in the Astronomical Journal, thousands of Cassini images gathered over the course of four years have revealed three separate wave patterns that are not driven by any moons, but spring up spontaneously by drawing energy from the small, random motions of ring particles. The waves, which can be hundreds of miles long, keep themselves going by reflecting off the ring’s edges.

“Think of it like waves in a pool,” Porco said. If two kids are hopping up and down at either end of a pool, she says, the waves they send sloshing across the water will pass through each other and reflect off the edge of the pool.

In Saturn’s rings, the waves are more like compressions in a Slinky than water waves, but the physics is similar. “These waves just go back and forth, and keep reflecting until they finally grow large enough so that we can actually see them,” Porco said.

“Normally viscosity, or resistance to flow, damps waves — the way sound waves traveling through the air would die out,” said planetary ring expert Peter Goldreich of Caltech and the Institute for Advanced Study in Princeton, who was not involved in the new study, in a press release. “But the new findings show that, in the densest parts of Saturn’s rings, viscosity actually amplifies waves, explaining mysterious grooves first seen in images taken by the Voyager spacecraft.”

Cassini has also observed similar waves on smaller scales, with wavelengths around 300 feet. Computer models of galaxies and protoplanetary disks around other stars have shown similar randomly generated waves with proportionally larger wavelengths. But because those waves would take hundreds of millions of years to complete one slosh, astronomers can’t observe them directly.

“This is the first time we’ve seen these things in nature,” Porco said. “It underscores the deep, physical connection between what we’re studying at Saturn’s rings, and disk systems across the universe at a very large range of spatial scales.”

Cassini has also snapped pictures of sharp, stalagmite-like peaks at the edge of the B ring that made themselves known by throwing long spiky shadows (below).

The new study suggests this region of the rings contains small moons that compress the ring material like a soda can and force it upward. This idea is supported by the presence of at least one moonlet, caught during Saturn’s summer equinox when it cast a shadow across the rings.

These moonlets may have migrated across Saturn’s rings, and become trapped in a gravitational resonance with the larger moon Mimas. This process of migration and trapping is exactly how scientists believe the solar system achieved its current architecture.

In this way, Saturn serves as a nearby laboratory to study celestial structures on all scales, from planets to solar systems to galaxies.

“There are basically two shapes in the universe, there’s disks and there’s spheres,” Porco said. “Saturn’s rings allow us to understand one of the two main structures in the universe: a celestial disk system.”

“This is not just a slight addition, it’s something significantly new,” Goldreich told Wired.com. Goldreich and colleagues predicted the presence of these waves in 1985, but the Cassini observations provide the first proof.

“A lot of times, you don’t expect to be around to see whether you made a prediction that worked,” Goldreich said. “I was quite pleased to see it.”

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