Before plummeting to its fiery end in Saturn’s cloud tops in September 2017, NASA’s Cassini spacecraft discovered a raft of surprises in the space between the planet and its rings.

They include a radiation belt teeming with energetic particles. Researchers had known that Saturn had several such belts out beyond its majestic rings. But the newfound region, nestled close to the planet inside the rings, is cut off from the others — and provides a test bed for exploring some fundamental space physics ^{1 , 2} .

Cassini also found molecules, including methane ³ , and dust grains ⁴ falling from Saturn’s rings into its atmosphere, suggesting that the rings and the planet interact more closely than scientists had suspected. “There were many surprises,” says Elias Roussos, a planetary scientist at the Max Planck Institute for Solar System Research in Göttingen, Germany, who co-led the team that discovered the radiation belt.

Six papers in Science ^{1 , 3 , 4} and five in Geophysical Research Letters ² , all published on 4 October, lay out some of Cassini’s final discoveries. They date from the last 5 months of its 13-year orbit around Saturn, when the spacecraft dived repeatedly between the planet and its rings.

There Cassini found the inner radiation belt stretching from Saturn’s upper atmosphere into its innermost ring. The region is populated by protons whizzing along at nearly the speed of light. High-energy cosmic rays travelling through space slam into Saturn’s rings or atmosphere, generating a shower of further particles that ultimately decay into the protons that make up the belt.

Protons in the inner radiation belt lose their energy if they run into the planet's atmosphere or innermost ring. Protons in the outer radiation belts aren't at risk of such encounters, and so the outer radiation belts are denser.

The inside story

By comparing Saturn’s radiation belts with one another, and with radiation belts on other planets, such as Earth, researchers can better understand why the belts form where they do. Radiation is a big hazard for missions flying through interplanetary space; the Apollo astronauts had to zoom through Earth’s belts to avoid being bombarded with a deadly dose of radiation.

Saturn’s inner radiation belt is similar to one reported last year at Jupiter by NASA’s Juno mission — although less intense ⁵ . “Both planets seem to have these rather strange, small radiation belts quite close in,” says Fran Bagenal, a planetary scientist at the University of Colorado Boulder.

Studying these inner radiation belts could help to illuminate how charged particles and magnetic fields interact very close to gas-giant planets, she says. Until Cassini and Juno, no spacecraft had flown close enough for long enough to get the data. “Now we can really start to do comparative planetology,” Bagenal says.

At Saturn, Cassini also spotted other signs of how complex things get between the planet and the rings. The spacecraft measured methane, carbon monoxide and other compounds flowing inwards from the rings onto the planet in huge amounts — up to 45,000 kilograms every second ³ . Dust grains tens of nanometres across also shower from the rings into the atmosphere ⁴ .

“What all these observations show is that the region between the rings and the planet is a very strongly coupled system,” says Roussos. “Everything affects everything else.”