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

Specks of dust from Saturn’s rings appear to float much farther above and below the planet than scientists thought possible, suggesting the rings are more like a giant dusty doughnut.

The main structure of Saturn’s rings is extremely thin, extending outwards for tens of thousands of kilometres but only vertically for around 10 metres, which creates the planet’s striking appearance when viewed from Earth. There is some variation in this shape, however, such as the puffier outer E ring fed by Saturn’s moon Enceladus, which spurts out ice from its underwater ocean.

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

Cassini’s spectrometer, the Cosmic Dust Analyzer, found hundreds of tiny rocky particles near the top of Cassini’s trajectory that had a similar chemical make-up to grains found in the main ring, which are low in iron. “It’s a really distinct spectral type we never see anywhere else in the Saturnian system,” says Postberg.

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

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To get so high, more than 100,000 kilometres from the main ring, Postberg and his team calculated that particles would need velocities of more than 25 kilometres per second to escape Saturn’s gravity and magnetic forces.

It is unclear what process might give them those speeds, says Postberg. The most straightforward explanation is that tiny meteorites smash into the rings and send particles flying, but this wouldn’t produce fast enough shrapnel.

However, micrometeorites colliding with Saturn’s rings could generate temperatures hot enough to vaporise rock, according to a recent study which suggested that Saturn’s rings are far older than previously thought. Postberg and his colleagues suggest this vaporised rock can shoot out of the rings at far higher velocities than shrapnel and later condense at distances far from the planet.

To find dust so far from the main ring is surprising, says Frank Spahn at the University of Potsdam, Germany, who wasn’t part of the study. This is because the particles in Saturn’s main ring are small, making them collide infrequently, and sticky, so collisions tend to be more like snowballs hitting each other than billiard balls, he says.

Micrometeorite collisions happen all over the solar system, so the same thing could also be happening on other ringed planets, such as Uranus. “If you have high velocity impacts onto icy rings, then this process could be universal. You would expect similar dust halos above and below other rings,” says Postberg.

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