During the last days of its mission, NASA's Cassini spacecraft got an unprecedented look at how five of Saturn's tiny inner moons are influenced by their ringed home planet. The moons are covered with material from Saturn's iconic rings, as well as icy particles from its prominent, watery moon Enceladus.
"The daring, close flybys of these odd little moons let us peer into how they interact with Saturn's rings," says Bonnie Buratti of NASA's Jet Propulsion Laboratory in a press statement. Buratti and a team of 35 co-authors have released their findings in Science. "We're seeing more evidence of how extremely active and dynamic the Saturn ring and moon system is."
While humans have known about Saturn since ancient times, knowledge of these inner moons-known as Pan, Daphnis, Atlas, Pandora, and Epimetheus- is relatively modern. Pan, for example, was discovered in 1985, while Daphnis only dates back to 2005.
The Cassini spacecraft discovered several of those moons. And when the spacecraft's 19-year mission ended in 2017, NASA destroyed the beloved craft through the "Grand Finale," in which Cassini plummeted into Saturn's atmosphere via a carefully chosen route that would not disrupt any of the planet's moons. The route allowed for an unprecedented look at these inner moons, too, using Cassini's Visible and Infrared Mapping Spectrometer (VIMS).
"The optical properties of the moons’ surfaces are determined by two competing processes," according to the study's abstract. These include "contamination by a red material formed in Saturn’s main ring system, and by accretion of bright icy particles or water vapor from volcanic plumes originating on the planet’s moon Enceladus."
It seems that these moons are so affected by outside sources because they are highly porous, and the reason they're so porous, scientists believe, is because they formed in stages. Large remnants of Saturn's ring material long ago broke off and eventually formed these moons. Some of these moons, like Daphnis, act like a snowplow after a storm, clearing a path through the rings. Daphnis' orbit creates a space known as the Keeler Gap, a 26 mile (42 km) gap in one of Saturn's largest and brightest rings.
Their porousness contributes to their shape as well as their makeup. Instead of the common sphere, NASA describes the moons as "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," Buratti says. "A denser body would be more ball-shaped because gravity would pull the material in."
The process could be replicated throughout Saturn's rings, scientists say. "Perhaps this process is going on throughout the rings, and the largest ring particles are also accreting ring material around them. Detailed views of these tiny ring moons may tell us more about the behavior of the ring particles themselves," says Cassini Project Scientist Linda Spilker, also at JPL.
Cassini saw a reddish material on the moons closer to Saturn, Daphnis and Pan, that they believe to be a mixture of iron and organics. The further-out moons-Atlas, Prometheus and Pandora-feature icy blue that stems from material from larger moon Enceladus, which some believe has the potential for alien life.
The lunar origin stories might be replicated throughout the solar system, scientists say.
"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?" Buratti asked. "These are questions to be answered by future missions."
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