Saturn’s main ring system is associated with a set of moons (Pan, Daphnis, Atlas, Pandora, and Epimetheus) that are either embedded within it, or interact with the rings to alter their shape and composition. The closest-ever flybys by NASA’s Cassini spacecraft revealed that the surfaces of these tiny moons are covered with material from the planet’s rings — and from icy particles blasting out of Saturn’s larger moon Enceladus.
During super-close flybys of Saturn’s rings, Cassini inspected the mini-moons Pan and Daphnis in the A ring; Atlas at the edge of the A ring; Pandora at the edge of the F ring; and Epimetheus, which is bathed in material that fans out from the moon Enceladus. Image credit: NASA-JPL / Caltech.
While Saturn has more than 60 confirmed moons, the planet’s main ring system is associated with a unique set of small moons that are either embedded within it or interact with the rings to alter their shape and composition.
Among questions about interactions between the ring system and these inner moons, whether the rings formed from the break-up of an inner moon, or, by contrast, whether the consolidation of existing ring material formed the moons remains unclear.
Between December 2016 and April 2017, Cassini performed six close flybys of Pan, Daphnis, Atlas, Pandora, and Epimetheus, collecting data on these moons’ morphology, structure, particle environment, and composition.
“The daring, close flybys of these odd little moons let us peer into how they interact with Saturn’s rings,” said Dr. Bonnie Buratti, a researcher at NASA’s Jet Propulsion Laboratory.
“We’re seeing more evidence of how extremely active and dynamic the Saturn ring and moon system is.”
The researchers found that dust and ice from the rings accretes onto the moons embedded within and near the rings.
They also found the moon surfaces to be highly porous, further confirming that they were formed in multiple stages as ring material settled onto denser cores that might be remnants of a larger object that broke apart. The porosity also helps explain their shape: rather than being spherical, they are blobby and ravioli-like, with material stuck around their equators.
“We found these moons are scooping up particles of ice and dust from the rings to form the little skirts around their equators,” Dr. Buratti said.
“A denser body would be more ball-shaped because gravity would pull the material in.”
“Perhaps this process is going on throughout the rings, and the largest ring particles are also accreting ring material around them,” added Cassini project scientist Dr. Linda Spilker, also from NASA’s Jet Propulsion Laboratory.
“Detailed views of these tiny ring moons may tell us more about the behavior of the ring particles themselves.”
Of the moons studied, the surfaces of those closest to Saturn — Daphnis and Pan — are the most altered by ring materials.
The surfaces of Atlas, Prometheus, and Pandora, farther out from Saturn, have ring material as well — but they’re also coated with the bright icy particles and water vapor from the plume spraying out of Enceladus.
“Do any of the moons of the ice giant planets Uranus and Neptune interact with their thinner rings to form features similar to those on Saturn’s ring moons?” Dr. Buratti asked.
“These are questions to be answered by future missions.”
The findings were published in the journal Science.
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B.J. Buratti et al. Close Cassini flybys of Saturn’s ring moons Pan, Daphnis, Atlas, Pandora, and Epimetheus. Science, published online March 28, 2019; doi: 10.1126/science.aat2349
