Saturn’s rings form a giant dusty doughnut encircling the planet

Dust grains from Saturn’s rings appear to float much farther above and below the planet than scientists thought possible, suggesting that the rings are more like a giant dusty donut.

The main structure of Saturn’s rings is extremely thin, extending outward for tens of thousands of kilometers, but only vertically for about 10 meters, giving the planet a striking appearance when viewed from Earth. There are some variations in this shape, however, such as the more inflated outer E ring powered by Saturn’s moon Enceladus, which projects ice from its underwater ocean.

Now, Frank Postberg of the Free University of Berlin and colleagues analyzed data from NASA’s Cassini spacecraft during 20 orbits in 2017, the mission’s final year, when it took extremely steep paths through the rings, starting at distances of up to three times Saturn’s radius above the planet and sweeping down to the same distances below.

Cassini’s spectrometer, the Cosmic Dust Analyzer, found hundreds of tiny rock particles near the top of Cassini’s trajectory that had a chemical composition similar to grains found in the main ring, which are poor in iron. “This is a really distinct spectral type that we don’t see anywhere else in the Saturnian system,” says Postberg.

“There’s a lot more stuff close to the ring plane, but it’s still surprising that we see these ring particles so high up, both above and below the ring plane,” he says.

To reach such a height, more than 100,000 kilometers from the main ring, Postberg and his team calculated that the particles would need a speed greater than 25 kilometers per second to escape Saturn’s gravity and magnetic forces.

It’s unclear exactly what process could give them these speeds, Postberg says. The simplest explanation is that tiny meteorites crash into the rings and send particles flying, but that wouldn’t produce shrapnel fast enough.

However, micrometeorites colliding with Saturn’s rings could generate temperatures high enough to vaporize the rock, according to a recent study suggesting that Saturn’s rings are much older than previously thought. Postberg and his colleagues suggest that this vaporized rock can shoot out of the rings at speeds much higher than shrapnel and later condense at distances far from the planet.

Finding dust so far from the main ring is surprising, says Frank Spahn of the University of Potsdam, Germany, who was not involved in the study. That’s because the particles in Saturn’s main ring are small, making them infrequent and sticky, so collisions tend to look more like snowballs colliding than billiard balls, he says.

Micrometeorite collisions occur throughout the solar system. The same thing could therefore also happen on other ringed planets, such as Uranus. “If you have high-speed impacts on ice rings, then this process could be universal. You would expect similar dust halos above and below the other rings,” says Postberg.

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