Saturn's intricate ring system is one of the most spectacular sights in the solar system, and has understandably captivated astronomers for centuries. But while the origin of the main ring system—which spans about 300,000 kilometers—is still a mystery, planetary scientists Ryuki Hyodo and Keiji Ohtsuki of Kobe University may have figured out how Saturn's bizarre F ring came into existence.
Located 3,400 kilometers beyond the edge of the main rings, this tiny circle of debris is only about 100 kilometers wide, and is gravitationally sculpted by the two "shepherd moons" called Prometheus and Pandora, which appear to herd the ring into place like planetary sentries.
In a paper published this morning in Nature Geoscience, Hyodo and Ohtsuki propose that the F ring is the fallout of a collision between two "moonlets"—a term for very small moons—at the outer edge of Saturn's main ring system.
The team demonstrated that the force of this impact could eject material into a new ring. Moonlets made primarily from ice would not survive these collisions, but so-called "rubble-pile satellites"—moonlets with dense silicate cores—would remain relatively intact.
In the case of the F ring, two rubble-pile satellites would have smashed together and ejected each other from the main ring system, along with a bunch of loose debris. The debris now forms the F ring itself, their research suggests, while the satellites evolved into Prometheus and Pandora.
"We cannot estimate directly from our model when such a collision formed the F ring," Ohtsuki told me. He added, however, that these kinds of collisions are in line with current models of late-stage satellite formation in Saturn's rings. "Therefore, at least we can say that such a collision took place a long time after the birth of the main ring," he said.
Hyodo and Ohtsuki's research isn't applicable just to Saturn. It may also explain the formation of similar structures in our solar system, such as Uranus's epsilon ring.
"Our model of collision between rubble-pile moons is definitely applicable to the origin of the narrow epsilon ring and its shepherd moons around Uranus," Hyodo told me. "This is because Saturn and Uranus have multiple satellite systems that have common characteristics."
The new research suggests that narrow rings fringed with shepherd moons might be a fairly common configuration in large ring systems, both in our solar system and beyond it. Unfortunately, it is impossible to detect such small rings around exoplanets at this time, but Hyodo is hopeful that the next generation of telescopes will make it easier to study larger ring structures in other solar systems.
"It would be very challenging to directly detect a narrow structure [like Saturn's F ring] even by using current telescopes," he said. "However, it could be possible to detect wide rings such as Saturn's main rings in the near future."
In the meantime, Hyodo, Ohtsuki, and other planetary scientists will continue trying to crack the mystery behind Saturn's main ring system. "The origin of Saturn's main rings is still debated," Hyodo said, adding that "the origin of the main rings is very important, because it could be the direct results of planet formation."
"In other words, it could be directly related to the evolution of all planets in the solar system," he said. "So working on the origin of Saturn's main rings would be key to understanding the origin and evolution of the solar system."