Saturn's main ring system is associated with a set of moons (Pan, Daphnis, Atlas, Pandora and Epimetheus), which are either embedded in it or interact with the rings to change their shape and composition. The closest aerodromes of NASA's Cassini spacecraft showed that the surfaces of these small moons are covered with material from the planet's rings – and from icy particles that burst out of Saturn's larger moon Enceladus.
While Saturn has more than 60 confirmed moons, the planet's primary ring system is associated with a unique set of small moons either embedded in it or interacting with the rings to change their shape and composition.
Among the issues of interactions between the ring system and these inner moons, whether the rings formed from breaking up an inner moon or, in contrast, whether the consolidation of existing ring material formed the moons remains unclear.
Between December 2016 and April 2017, Cassini performed six dense flies of Pan, Daphnis, Atlas, Pandora and Epimetheus, collecting data on the morphology, structure, particle environment and composition of these lunar moons.
"The bold, dense flybys of these odd little moons make us see how they interact with Saturn's rings," says Dr. Bonnie Buratti, a researcher at NASA's Jet Propulsion Laboratory.
"We see more evidence of how extremely active and dynamic is Saturn ring and moon system."
The researchers found that dust and ice from the rings are accreted on the moons embedded in and near the rings. They also found the lunar surfaces to be highly porous, further confirming that they were formed in multiple phases as ring material settled on denser cores, which could be remnants of a larger object that broke apart. The porosity also helps explain their shape: Instead of being spherical, they are blobby and ravioli-like, with material fixed around their equators.
"We found that these moons pick up particles of ice and dust from the rings to form small skirts around their equator," said Dr. Buratti. "A closer body would be more spherical because gravity would pull the material in." "Perhaps this process is going through the rings and Biggest ring particles also accretes ring material around them," added Cassini project researcher. Linda Spilker, also from NASA's Jet Propulsion Laboratory.
"Detailed overviews of these small ring months can tell us more about the construction of the ring particles themselves."
Of the moons studied, the surfaces of those closest to Saturn – Daphnis and Pan – are the most altered of ring materials .
Atlas, Prometheus and Pandora surfaces are further out from Saturn also ring material – but they are also coated with the bright icy particles and water vapor from the plume that ejects from Enceladus.
"Do any of the moons on the ice giant planets Uranus and Neptune with their thinner ring shape features like those on Saturn's ring moons?" Buratti asked.
"These are questions to be answered by future missions."
The results were published in the journal Science . BJ Buratti et al. . Close to Cassini flybys of Saturn's ring moons Pan, Daphnis, Atlas, Pandora and Epimetheus. Science published online March 28, 2019; doi: 10.1126 / science.aat2349