The properties of five small moons located close to Saturn's rings have been illuminated, thanks to data from the Cassini spacecraft's final orbits. These small inner moons have unusual morphologies, and their optical properties are determined by two competing processes - an understanding of which helps explain any difference in color between the moons and their adjacent ring, the authors say. While Saturn has more than 60 confirmed moons, Saturn'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. Whether any volatiles other than water ice exist on these moons has also been a question. Between December 2016 and April 2017, the Cassini spacecraft performed six close flybys of the moons Pan, Daphnis, Atlas, Pandora, and Epimetheus, collecting collect data on these moons' morphology, structure, particle environment, and composition. The analysis involved several instruments onboard Cassini. From these flybys and the data captured, Bonnie Buratti and colleagues report that no volatiles other than water ice exist on the ring moons. The moons' geology was shaped by a complex history, including groove formation caused by tidal stresses. Finally, depending on the position of the moon with respect to the rings, with Pan being reddest, and Epimetheus being bluest, the optical properties of the moons are determined by two competing process, the authors say: contamination by a red chromophore from the main rings and ice particles or water vapor from the E-ring. The authors say the low densities of the small moons as measured here support a multi-stage moon formation scenario involving accretion of ring material.
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Journal
Science