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Saturn, lit from behind. The F ring is the outermost, visible in white. NASA/JPL-Caltech/SSI
Saturn’s rings are not joined together in a single glorious rainbow of rock. The striations you see in photos of the planet are caused by gaps, which create five discrete rings. The thinnest and outermost of these is the F ring. It’s also the most active, having been observed to undergo changes in a matter of hours.
Now researchers from Kobe University, Japan, have revealed that the F Ring was likely created by a collision between two small satellites, which occurred during the final stage of the planet’s satellite formation.
According to doctoral student Hyodo Ryuki and professor Ohtsuki Keiji, the rings of Saturn used to contain many more particles. These particles accrete to form larger satellites near the outer edges of the rings in their final stage, resulting in multiple larger pieces of rock. Observations by NASA’s Cassini indicate that these small satellites have very dense cores.
The F Ring, Hyodo and Ohtsuki discovered through a computer simulation, likely came into being because of a collision between two of these densely cored satellites, which partially shattered on impact. The resulting debris formed into the ring, while the dense cores of the satellites formed the moons Prometheus and Pandora on the inside and outside of the ring, respectively.

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Prometheus carves a pattern into the F Ring. NASA/JPL/Space Science Institute
We already know that much of the F Ring’s change has to do with the irregular, potato-shaped Prometheus, which is what is known as a shepherd moon. As it orbits Saturn on the inside of the F Ring, it skims along the edge and sometimes carves a knotted channel into it as its gravity collects material from the ring.
“Through this study, we were able to show that the current rings of Saturn reflect the formation and evolution processes of the planet’s satellite system,” Hyodo said.

The information, the researchers hope, will help understand how satellites form around other planets, such as the satellites and rings of Uranus, which are similar to those of Saturn.
“As plans are underway in and outside of Japan to explore the satellite system of Jupiter and the satellites of Mars, we will continue to unravel the origin of satellite systems, which is key to understanding the formation process of planetary systems,” Ohtsuki said.
The research is available online in the journal Nature Geoscience.

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