NASA’s Cassini spacecraft, which conducted closest-ever flybys of Saturn, beamed back detailed images of tiny moons which resemble raviolis nestled in the planet’s rings.
The images reveal that the surfaces of these unusual moons are covered with material from Saturn’s rings and icy particles blasting out of the planet’s larger moon Enceladus.
The research may help explain how competing processes shape these mini-moons, NASA said in a statement.
“The daring, close flybys of these odd little moons let us peer into how they interact with Saturn’s rings,” said Bonnie Buratti of NASA’s Jet Propulsion Laboratory in the US.
“We’re seeing more evidence of how extremely active and dynamic the Saturn ring and moon system is,” said Buratti, who led the team.
The research, published in the journal Science, from data gathered by six of Cassini’s instruments before its mission ended in 2017, is a clear confirmation that dust and ice from the rings accretes onto the moons embedded within and near the rings.
Scientists 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,” Buratti said.
“A denser body would be more ball-shaped because gravity would pull the material in,” he said.
“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 behaviour of the ring particles themselves,” said Cassini Project Scientist Linda Spilker, also at JPL.
Of the satellites studied, the surfaces of those closest to Saturn — Daphnis and Pan — are the most altered by ring materials.
The surfaces of the moons Atlas, Prometheus and Pandora, farther out from Saturn, have ring material as well — but they are also coated with the bright icy particles and water vapour from the plume spraying out of Enceladus.
The key puzzle piece was a data set from Cassini’s Visible and Infrared Mapping Spectrometer (VIMS), which collected light visible to the human eye and also infrared light of longer wavelengths.
The six uber-close flybys of the ring moons, performed between December 2016 and April 2017, engaged all of Cassini’s optical remote sensing instruments that study the electromagnetic spectrum.
They worked alongside the instruments that examined the dust, plasma and magnetic fields and how those elements interact with the moons.
Questions remain, including what triggered the moons to form. Scientists will use the new data to model scenarios and could apply the insights to small moons around other planets and possibly even to asteroids.