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New discoveries have emerged about five tiny moons nestled in and near the rings of Saturn. The closest flyovers ever made by NASA's Cassini spacecraft reveal that the surface of these unusual moons is covered with materials from the planet's rings – and icy particles from Saturn's largest moon, Enceladus. The work paints a picture of the competing processes that shape these mini-moons.
"The audacious and close flights of these strange little moons allow us to see how they interact with the rings of Saturn," said Bonnie Buratti of NASA's Jet Propulsion Laboratory in Pasadena, California. Mr. Buratti led a team of 35 co-authors who published their work in the journal Science on March 28th. "We are seeing more and more evidence of the extreme activity and dynamism of the Saturn ring and moon system."
The new research, based on data collected by six Cassini instruments before the end of its mission in 2017, clearly confirms that the dust and ice of the rings accumulate on embedded moons in and out of near the rings.
Scientists have also found that lunar surfaces are very porous, confirming that they have formed in several stages, with the ring material depositing on denser nuclei that may be the remains of a larger object. who broke up. Porosity also helps to explain their shape: Rather than being spherical, they look like blobs and ravioli, with material glued around their equators.
"We found that these moons were collecting ice and dust particles in the rings to form small skirts around their equators," said Buratti. "A denser body would be more ball-shaped because gravity would attract material to the interior."
"Perhaps this process continues throughout the rings and that the larger ring particles also accumulate annular material around them." Detailed views of these tiny ring moons can tell us more about the behavior of the particles of the ring themselves, "said Linda, project scientist Cassini Spilker, also at JPL.
Among the satellites studied, the surfaces closest to Saturn – Daphnis and Pan – are the most altered by annular materials. The surfaces of the Atlas moons, Prometheus and Pandora, further away from Saturn, also have a ring material – but they are also covered with glossy icy particles and water vapor from the plume that is spreading over Enceladus. (A broad outer ring of Saturn, called an E ring, is formed by the icy material that escapes from the Enceladus Plume.)
The centerpiece of the puzzle was a set of data from the Cassini Visible and Infrared Mapping Spectrometer (VIMS), which collected light visible to the human eye as well as infrared light of longer wavelengths. It was the first time that Cassini was close enough to create a spectral map of the innermost moon surface. By analyzing the spectra, VIMS was able to learn about the composition of the materials of the five moons.
VIMS has seen that the rings closest to Saturn appeared the most red, similar to the color of the main rings. Scientists do not yet know the exact composition of the material that appears in red, but they think it's probably a mixture of organic matter and iron.
On the other hand, the moons outside the main rings look bluer, like the light of Enceladus's icy feathers.
The six ultra-close flyovers of the circular moons, carried out between December 2016 and April 2017, mobilized all the Cassini optical remote sensing instruments that study the electromagnetic spectrum. They worked alongside instruments that examined dust, plasma and magnetic fields and how these elements interact with moons.
Questions remain, including what triggered the formation of the moons. Scientists will use new data to model scenarios and apply this knowledge to small moons surrounding other planets and possibly to asteroids.
"Do the moons of the giant ice planets, Uranus and Neptune, interact with their thinner rings to form features similar to those of the ringed moons of Saturn?" Buratti asked. "These are questions that future missions will have to answer."
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