In its final days, Cassini zipped under Saturn's rings, bathed in 'ring rain'


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.

“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. “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 orbits appearing 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 Analyze, and William Kurth of the University of Iowa leads Radio and Plasma Wave Science. 

Cassini made 22 “grand finale” orbits in 2017. During eight out of those passes, 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.

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 icy objects in space tend to accumulate dust the longer they sit around—just like your bookshelves. By studying the exact types of silicates that coat Saturn’s rings, researchers 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, proving that despite Cassini’s demise, the spacecraft still isn’t done wowing scientists. 

“I am sure there will be surprises yet to come,” said Mihály Horányi, a professor in CU Boulder’s Department of Physics and a co-author of the study. “We still have enormous amounts of data that we have to sort out and analyze.”