Two years ago as the Cassini probe made its daring final plunge into the atmosphere of Saturn, it flew past for a closer look at a few of the gas giant's inner moons. Now a NASA team has analyzed the data and uncovered some intriguing new details about these tiny worlds, including how they're busily scooping up material from Saturn's rings and growing into weird shapes.
Between December 2016 and April 2017, Cassini used six instruments to examine the shape, structure and composition of five of Saturn's innermost moons, Pan Daphnis, Atlas, Pandora and Epimetheus. These moons all orbit within or very near to the planet's iconic rings, so it's no surprise that they interact with the dusty disks.
"The daring, close flybys of these odd little moons let us peer into how they interact with Saturn's rings," says Bonnie Buratti, lead author of the study. "We're seeing more evidence of how extremely active and dynamic the Saturn ring and moon system is."
These Saturnian moons are famous for their weird shapes, resembling flying saucers or pieces of ravioli due to their lumpy bodies and bulging middles. The new data revealed a possible explanation for how that comes to be. The team found that the moons have porous surfaces and possibly denser cores, suggesting that they're growing bigger as they gather up particles from the rings.
"We found these moons are scooping up particles of ice and dust from the rings to form the little skirts around their equators," says Buratti. "A denser body would be more ball-shaped because gravity would pull the material in."
Cassini's close pass of Pan, the innermost moon, revealed more clues about what these objects are made of. The spacecraft used a Visible and Infrared Mapping Spectrometer (VIMS) to make a spectral map of the moon's surface, in both visible and infrared light.
That showed that the moons closest to Saturn – Pan and Daphnis – are reddish in color, which is similar to the main rings. While the team doesn't yet know what minerals make it that color, they suspect a mix of organic compounds and iron. Meanwhile the moons outside the main rings, including Atlas, Prometheus and Pandora, look more blue, thanks to layers of icy particles and water vapor sprayed over them by Enceladus' icy plumes.
The team still isn't really sure how these moons form, but one possibility is that a larger, denser object broke apart in the distant past and the fragments swept through Saturn's rings, collecting the dust and ice and gradually building up. The team plans to model different scenarios to find the most likely answers.
The research was published in the journal Science.
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