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Astronomie - Cassini probe sniffs out strange radiation belt inside Saturn’s rings

5.10.2018

NASA spacecraft's swan song reveals unusual behaviour by charged particles close to the planet.

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 physics¹^,².

Cassini also found molecules, including methane³, and dust grains⁴falling 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 Science¹^,³^,⁴and five in Geophysical Research Letters², 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 intense⁵. “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 ringsonto the planet in huge amounts — up to 45,000 kilograms every second³. Dust grains tens of nanometres across also shower from the rings into the atmosphere⁴.

“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.”

Quelle: nature

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In its final days, Cassini bathed in 'ring rain'

Artist's rendering of Cassini flying below Saturn's rings during its "grand finale." (Credit: NASA/JPL-Caltech)

On its last orbits in 2017, the long-running Cassini spacecraft dove between Saturn’s rings and its upper atmosphere and bathed in a downpour of dust that astronomers call “ring rain.”

In research published today in Science, CU Boulder’s Hsiang-Wen (Sean) Hsu and his colleagues report that they successfully collected microscopic material streaming from the planet’s rings.

“Our measurements show what exactly these materials are, how they are distributed and how much dust is coming into Saturn,” said Hsu, lead author of the paper and a research associate at the Laboratory for Atmospheric and Space Physics (LASP).

The findings, which were made with Cassini’s Cosmic Dust Analyzer and Radio and Plasma Wave Science instruments, come a little more than a year after the spacecraft burned up in Saturn’s atmosphere. They stem from the mission’s “grand finale,” in which Cassini completed a series of risky maneuvers to zip under the planet’s rings at speeds of 75,000 miles per hour.

Diagram of Cassini's final orbits

In its 22 "grand finale" orbits (blue), Cassini zipped through the 1,200 mile-wide space between the Saturn's rings and its atmosphere. The spacecraft's penultimate series of orbits (yellow) grazed the planet's outermost rings. (Credit: NASA/JPL-Caltech)

Capturing dust under those conditions was an engineering and navigational coup, the researchers said—a snatch-and-run that the mission team had been planning since 2010.

“This is the first time that pieces from Saturn’s rings have been analyzed with a human-made instrument,” said Sascha Kempf, a co-author of the new study and a research associate at LASP and associate professor in the Department of Physics. “If you had asked us years ago if this was even possible, we would have told you ‘no way.’”

The research is one of a series of studies from Cassini’s last orbitsappearing today in Science. NASA’s Jet Propulsion Laboratory (JPL) managed the mission, which was a cooperative effort of NASA, the European Space Agency (ESA) and Italian Space Agency. Ralf Srama of the University of Stuttgart leads research using the spacecraft’s Cosmic Dust Analyzer, and William Kurth of the University of Iowa leads Radio and Plasma Wave Science.

Beautiful physics

Catching that ring rain—which astrophysicists had predicted based on studies of Saturn’s upper atmosphere—in action wasn’t easy: Getting too close to a planet’s rings risks shredding the spacecraft.

With Cassini running low on fuel in 2017, however, mission scientists decided to take the chance. Cassini made 22 passes around Saturn, threading between the planet’s closest ring and its upper atmosphere, a space less than 1,200 miles wide.

During eight of those final orbits, the Cosmic Dust Analyzer trapped more than 2,700 charged bits of dust. Based on the group’s calculations, that’s enough ring rain to send about one metric ton of material into Saturn’s atmosphere every second.

But those particles didn’t fall directly into the planet by gravity alone. Instead, the team suspects that they gyrate along Saturn’s magnetic field lines like a yo-yo before crashing into the atmosphere.

“It’s a beautiful display of physics at work,” said study co-author Mihály Horányi, a professor in physics at CU Boulder.

Dirty snowballs

The researchers were also able to study what that planetary dust was made of. Most of the particles were bits of water ice—the main component of Saturn’s rings. But the spacecraft also picked up a lot of tiny silicates, a class of molecules that make up many space rocks.

That finding is important, Hsu said, because it could help answer a nagging question about Saturn: how old are its rings? He explained that icy objects in space are a bit like bookshelves in your house.

“It is really difficult to maintain a pure ice surface in the solar system because you always have dirty material coming at you,” Hsu said. “One of the things we want to understand is how clean or dirty the rings are.”

If scientists can identify the exact types of silicates that coat Saturn’s rings, they may be able to tell whether those features are billions of years old or much younger. Hsu’s colleagues are currently working to make those identifications. Researchers at LASP are also building on what they learned from Cassini’s Cosmic Dust Analyzer to design similar dust-catching instruments for NASA’s Interstellar Mapping and Acceleration Probe(IMAP) and Europa Clipper missions.

As for Cassini, “I am sure there will be surprises yet to come,” said Horányi, who is also a co-investigator on the Cosmic Dust Analyzer. “We still have enormous amounts of data that we have to sort out and analyze.”

Other co-authors on the study include researchers at the University of Oulu; Heidelberg University; Free University of Berlin; University of Stuttgart; Potsdam University; JPL; University of Iowa; NASA Goddard Space Flight Center; Boston University; NASA Ames Research Center; University College London; University of London; and Baylor University.

Quelle: University of Colorado Boulder

Astronomie - Cassini probe sniffs out strange radiation belt inside Saturn’s rings

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