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NASA’s Cassini spacecraft captured one of its last images of Saturn in September 2017 from a distance of 870,000 miles.

Flyby of Saturn’s C ring prompts plateau puzzlement

As NASA’s Cassini spacecraft threaded its way through Saturn’s rings to acquire the last drops of data before its fatal plunge into the planet nearly two years ago, it collected spectral information about the enchanting C ring and its bright plateaus.

Instead of uncovering definitive scientific answers, the spectral images from the Cassini flyby triggered more questions, according to new research published June 13 in Science.

“The plateaus are quite prominent,” said Phil Nicholson, Cornell professor of astronomy and a member of Cassini’s Visual and Infrared Mapping Spectrometer team. “These plateaus are four times brighter and denser than the rest of the C ring, but when we examined the ring’s infrared spectrum the plateaus were almost indistinguishable from the rest of the C ring.”

Saturn’s C ring is the planet’s innermost main ring. From an Earth-based, telescopic perspective, it is the faintest ring, but it seems to sport a variety of textures. The bright bands are known as plateaus because they are flat-topped – like mesas in the Southwestern states, for example. Nicholson expected to see a spectral difference between the plateaus and the remainder of the C ring but did not. “That was a surprise,” he said. “In other parts of the rings – such as the A and B rings – we do see spectral variations that track ring density and brightness.”

The plateaus have long been a puzzle, and scientists have theorized they may be due to differences in particle size within the rings, as their mass per square meter is similar to that of the rest of the C ring. Nicholson said there must be some underlying factor causing the C ring plateaus and the rest of that ring to look different.

“So, when we examined the ring with the spectral camera, we expected something to look different. Now, we have no hints as to what is going on,” said Nicholson, who was funded by NASA for this research. “It’s driving us a bit batty.”

The paper, “Close-Range Remote Sensing of Saturn’s Rings During Cassini’s Ring-Grazing Orbits and Grand Finale,” is authored by 25 astronomers, including Matt Tiscareno of the nonprofit SETI Institute and a former senior researcher in the laboratory of co-author Joe Burns, Cornell professor emeritus of astronomy and engineering; and Matt Hedman, University of Idaho assistant professor and a former senior researcher with Burns and Nicholson.

Data were gathered during ring-grazing orbits from December 2016 to April 2017, and during the mission’s grand finale, when Cassini flew above Saturn’s clouds. As the spacecraft ran out of fuel, the mission team deliberately plunged it into the planet’s atmosphere in September 2017.

The paper provides new detail on how features such as tiny moons sculpt the rings. It also explains how the rings are a window into the astrophysical disk processes that shape our solar system, according to NASA’s Jet Propulsion Lab.

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