How Saturn's Shepherd Moons Herd Its Rings

Saturn and its main rings seen by Cassini spacecraft. Saturn's F ring is located just outside the main rings. NASA/JPL/Space Science Institute.

Just on the edge of Saturn's main set of rings is a delicate, thread-like band: the F ring. This ring is made up of carefully herded dust and ice particles, tended by the team effort of two moons. These shepherd-moons keep a watchful eye on the ring from either side.

How this combination of ring and moons came to be is a mystery. In the past, moon-like clumps have appeared and vanished in the F ring so how these two moons have endured is intriguing. Two scientists have created a simulation to see how different ring structures form from different moon collisions. And they think that it might have revealed how this feature came to be. You can see their results published in Nature Geoscience.

If you look up into the sky with a telescope, it is possible to see Saturn's spectacular dusty rings. However, in ancient times this feature was even more massive than what we see today, and more compact. The restless particles in the ring bumped, thumped and knocked together and slowly but surely inched away from Saturn. As the rings got wider and further away from their host, the planet's gravity had less influence over them. 

The F ring seems to have been formed in the Goldilocks zone of moon collisions. Saturn's gravitational power is "not too destructive nor too moderate," Ryuki Hyodo, from Kobe University and one of the paper's authors, explained to IFLScience. This means that early collisions between the moons were only partially destructive, so a ring could form but the shepherd satellites could also circle either side of it.

It was these moons that kept the outer ring in check over time. The moons are named after Prometheus, a Titan in Greek mythology who gifted man with fire stolen from Mount Olympus, and Pandora, the woman created from earth by the gods as a punishment for Prometheus' theft. They are the celestial pair that maintain the ring of matter separating them in mint condition. 

The scientists noted that, in their simulation, if these moons are composed entirely of ice particles then they would crumble in a collision, with the resulting debris contributing to the ring. However, if the moons have a secret core made of silicate then they're equipped to survive a collision. The Cassini spacecraft, which orbits Saturn, has seen evidence of moons forming with dense centers like the ones described. Atlas and Pan, for example, show signs of being dense, ice-covered rocks.

"At the final stage of such satellite system formation processes, two small satellites that are formed just outside the ring's outer edge collide with each other – the mantle part of the satellites were broken into pieces, but thanks to the dense cores that these satellites have inside, they were not disintegrated completely," Keiji Ohtsuki, professor at Kobe University and one of the paper's authors, summarized to IFLScience.

Hyodo went on to explain how the moons created a stable structure. "Our simulations also showed that after the first impact, their [separation] becomes large enough to avoid second impact," and as a result their trajectories are steady enough to influence the ring in the long term.

This simulation doesn't just apply to Saturn: it also has applications to another ring found in the Solar System: The epsilon ring found encircling Uranus, which also has shepherd moons. 

"Thus, those ring dynamics and formation of shepherd moons are not unique to Saturn but can be ubiquitous around giant planets in the universe," concluded Hyodo.

Image in text (top): Saturn's F ring and its moons Prometheus (inner) and Pandora (outer). Saturn is to the right. NASA/JPL/Space Science Institute. 

Image in text (bottom): Prometheus (far right) and Pandora (left) encircle Saturn's F ring. NASA/JPL/Space Science Institute. 

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