New finds have emerged around five small moons located in and near Saturn's rings. The closest aerodromes of NASA's Cassini spacecraft reveal that the surfaces of these unusual moons are covered with material from the planet's rings – and from icy particles that burst out of Saturn's larger moon Enceladus. The work paints a picture of the competing processes that form these mini-moons.
"The bold, dense flybys of these weird little moons enable us to see how they interact with Saturn's rings," said Bonnie Buratti of NASA's Jet Propulsion Laboratory in Pasadena, California. Buratti led a team of 35 co-authors who published their work in the journal Science on March 28. "We see more evidence of how extremely active and dynamic Saturn ring and moon system is."
The new research, from data collected by six of Cassini's instruments before his mission ended in 201
Researchers 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. Porosity also helps to explain their shape: Instead of being spherical, they are blossom and ravioli, with material fixed around their equator.
"We found that these moons pick up particles of ice and dust from the rings to form small skirts around their equators," Buratti said. "A closer body would be more spherical because gravity would pull the material in."
"Perhaps this process is going through the rings, and the largest ring particles also accretes ring material around them. Small ringmen can tell us more about the behavior of the ring particles themselves," says Cassini project researcher Linda Spilker, also at JPL.
Of the satellites studied, the surfaces of those closest to Saturn – Daphnis and Pan – are the most altered of ring materials. The Moon's surfaces Atlas, Prometheus and Pandora, further out from Saturn, also have ringing material – but they are also coated with the bright icy particles and water vapor from the plume spraying out of Enceladus. (A wide outer ring of Saturn, known as the E-ring, is formed by the ice-cold material moving out from Enceladus plume.)
The key puzzle was a dataset from Cassini's Visible and Infrared Mapping Spectrometer (VIMS ), which collected light visible to the human eye and also longer-wavelength infrared light. It was the first time Cassini was close enough to create a spectral map of the surface of the inner moon panel. By analyzing the spectra, VIMS was able to learn about the composition of materials on all five moon.
VIMS saw that the ring assemblies closest to Saturn seem the redest, corresponding to the color of the main rings. Scientists do not yet know the exact composition of the material that looks like red, but they think it is probably a mixture of organic and iron.
On the other hand, the moons just outside the main rings look more blue, just like the light from Enceladus' ice cold.
The 6 uber-close flybys of the ring months, performed between December 2016 and April 2017, involved all of Cassini's optical remote sensing devices studying the electromagnetic spectrum. They worked with the instruments that examined the dust, plasma and magnetic fields, and how these elements interact with the moons.
Questions remain, including what triggered the moons to form. Scientists will use the new data to model scenarios and be able to apply insights to small moons around other planets and possibly even to asteroids.
"Do any of the moons on the ice giant planets Uranus and Neptune with their thinner rings to form features similar to those of Saturn's ring months?" Buratti asked. "These are questions to be answered by future missions."