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On its last orbits in 2017, the long-running Cassini spacecraft dove between Saturn鈥檚 rings and its upper atmosphere and bathed in a downpour of dust that astronomers call 鈥渞ing rain.鈥�

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

鈥淥ur measurements show what exactly these materials are, how they are distributed and how much dust is coming into Saturn,鈥� said Hsu, lead author of the paper and a research associate at the (LASP).

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

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

鈥淭his is the first time that pieces from Saturn鈥檚 rings have been analyzed with a human-made instrument,鈥� said Sascha Kempf, a co-author of the new study and a research associate at LASP and associate professor in the . 鈥淚f you had asked us years ago if this was even possible, we would have told you 鈥榥o way.鈥欌��

The research is one of a appearing today in Science. NASA鈥檚 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鈥檚 Cosmic Dust Analyzer, and William Kurth of the University of Iowa leads Radio and Plasma Wave Science.

Beautiful physics

Catching that ring rain鈥攚hich astrophysicists had predicted based on studies of Saturn鈥檚 upper atmosphere鈥攊n action wasn鈥檛 easy: Getting too close to a planet鈥檚 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鈥檚 closest ring and its upper atmosphere, a space less than 1,200 miles wide.听

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

But those particles didn鈥檛 fall directly into the planet by gravity alone. Instead, the team suspects that they gyrate along Saturn鈥檚 magnetic field lines like a yo-yo before crashing into the atmosphere.听

鈥淚t鈥檚 a beautiful display of physics at work,鈥� said study co-author Mih谩ly Hor谩nyi, 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鈥攖he main component of Saturn鈥檚 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.

鈥淚t is really difficult to maintain a pure ice surface in the solar system because you always have dirty material coming at you,鈥� Hsu said. 鈥淥ne of the things we want to understand is how clean or dirty the rings are.鈥�

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

As for Cassini, 鈥淚 am sure there will be surprises yet to come,鈥� said Hor谩nyi, who is also a co-investigator on the Cosmic Dust Analyzer. 鈥淲e still have enormous amounts of data that we have to sort out and analyze.鈥�

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.