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Dust and ice from Saturn’s rings are falling on nearby moons

Saturn’s rings were named alphabetically in the order they were discovered. The narrow F ring marks the outer boundary of the main ring system. Image & Caption Credit: NASA / JPL-Caltech

Particles of dust and ice from Saturn’s rings are falling onto five of the giant planet’s tiny moons, all of which are located close to and even within the ring system.

The phenomenon was detected by NASA’s Cassini orbiter, which conducted six close flybys of the rings between December of 2016 and April 2017 toward the end of its nearly 20-year-long mission. These flybys were part of Cassini‘s “Grand Finale,” during which the spacecraft plunged between Saturn and its ring system before being diving into the planet’s atmosphere in order to prevent any possible contamination of larger moons that could host microbial life with microbes from the spacecraft.

Saturn's moon Prometheus appears first in this animated sequence, on the inside of the F ring. Meanwhile Pandora orbits on the exterior of the ring. The impact of the two tiny worldlets can be clearly seen on the ting's frozen particles. Image Credit: NASA / JPL-Caltech

Saturn’s moon Prometheus appears first in this animated sequence, on the inside of the F ring. Meanwhile Pandora orbits on the exterior of the ring. The impact of the two tiny worldlets can be clearly seen on the ting’s frozen particles. Image Credit: NASA / JPL-Caltech

As Cassini conducted the closest ever observations of Saturn’s ring and moon system, six of its 12 science instruments caught material from the rings, including ice particles from Enceladus, a larger moon, falling and accreting onto small moons Daphnis, Pan, Atlas, Epimetheus, and Pandora.

One science instrument, the Visible and Infrared Mapping Spectrometer (VIMS), created a spectral map of Pan’s surface, which scientists used to infer the compositions of all five small moons.

“The daring, close flybys of these odd little moons let us peer into how they interact with Saturn’s rings. We’re seeing more evidence of how extremely active and dynamic the Saturn ring and moon system is,” said Bonnie Buratti of NASA’s Jet Propulsion Laboratory (JPL) In Pasadena, California.

Buratti led a team of 35 experts who conducted the study and published their results in the journal Science.

The five small moons, none of which is large enough to be spherical, were found to have porous surfaces, with extra material seemingly stuck on their equators. Researchers think these moons may have formed in stages, as ring materials fell onto dense cores that might have originally been part of a larger object destroyed during an impact.

“We found these moons are scooping up particles of ice and dust from the rings to form the little skirts around their equators. A denser body would be more ball-shaped because gravity would pull the material in,” Buratti explained.

Cassini Project Manager Linda Spilker suggested similar processes could be occurring throughout Saturn’s extensive ring system, with the larger ring particles gathering sufficient material over time to become tiny new moons themselves.

The Cassini mission concluded when the spacecraft plunged into Saturn's atmosphere in September of 2017. Image Credit: NASA / JPL-Caltech

The Cassini mission concluded when the spacecraft plunged into Saturn’s atmosphere in September of 2017. Image Credit: NASA / JPL-Caltech

“Detailed views of these tiny ring moons may tell us more about the behavior of of the ring particles themselves,” Spilker stated.

Of the five tiny moons, Daphnis and Pan, which are closest to Saturn, have a red color similar to that seen in the planet’s main rings. While their surface compositions have not yet been determined, scientists suspect they are composed of organic compounds and iron.

Further from Saturn, Atlas, Epimetheus, and Pandora are coated in a bright layer of ice that appears to originate from geysers being sprayed into space by Enceladus, a 310-mile (500-km) moon that is one of the solar system’s brightest objects due to its icy outer layer. Cryovolcanos observed near Enceladus’ south pole shoot out water vapor and volatiles that likely come from a subsurface ocean that could potentially harbor microbial life. Like Enceladus’ plumes, these three more distant tiny moons have a bluish color.

Saturn’s more distant E ring is also composed of materials ejected by Enceladus’ geysers.

An abstract of the paper published in Science notes the tiny moons’ surfaces are shaped by “two competing processes…contamination by a red material formed in Saturn’s main ring system and by accretion of bright icy particles of water vapor from volcanic plumes originating on the planet’s moon Enceladus.”

While Cassini‘s instruments gathered data on magnetic fields, dust, and plasma in the region between the rings and Saturn, scientists remain uncertain as to what may have initially started these tiny moons’ formation processes though they plan on inputting the new data into computer models in an attempt to answer this question.

Such answers could provide insight into worlds far beyond Saturn, including asteroids and other planets’ small moons. Ice giants Uranus and Neptune also have rings, and similar interactions could be occurring between their ring particles and closest moons, Buratti suggested.




Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.

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