These Two Moons Smashed Together to Produce Saturn’s Unusual F Ring

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A new theory proposes that Saturn’s outermost ring formed in the wake of an ancient collision between two icy satellites, and that similar collisions may account for comparable ringed structures around other planets.

Saturn’s discrete F Ring is a narrow 62-mile (100-km) band located some 2,110 miles (3,400 km) away from the outer edge of the main ring system. It’s the most active ring in the Solar System, exhibiting structural changes on an hourly basis.


The F Ring is also unique in that it’s accompanied on either side by two small shepherd satellites, Prometheus and Pandora. According to models produced by Keiji Ohtsuki and Ryuki Hyodo of Kobe University in Japan, the F Ring, unlike the interior bands, formed after the collision of two small satellites. The details of their work can now be found in Nature Geoscience.

Prometheus and Pandora are referred to as shepherd satellites because their gravitational forces and respective positions within the F Ring maintain its structure. Calculations show that these two moons hit and partially destroyed each other, leaving a significant amount of debris that went on to form the F Ring. (Image credit: Hyodo et al., 2015/Nature Geoscience)


Fascinatingly, these moons struck each other in Saturn’s sweet spot—a boundary that separates Saturn’s rings from its moon system. ABC Science explains:

Saturn’s main ring system is located inside the planet’s Roche limit — the distance from a planet where its gravitational tidal forces will tear a moon apart — which is why particles in Saturn’s main ring system spread out instead of coalesceing through their own gravity to form new moonlets.

“This is why the rings remain as rings,” says Ohtsuki.

However, eventually some particles move beyond from Saturn’s Roche limit, at which point they can coalesce under their own gravity and form moonlets.

“The most exciting thing is that in order to form the F ring by collision, the location of the collision needed to be around where F ring currently exists,” says Hyodo.

“At this F ring location, the gravity of Saturn is not too strong nor too weak. Thus, the collision outcome at this location could become partial destruction where two remnant moons are left and fragments form a narrow ring between those two moons.”


Ohtsuki and Hyodo say that their finding applies to other ringed systems, including Uranus’s Epsilon ring and shepherd satellites Cordelia and Ophelia.

Read the entire study at Nature Geoscience: “Saturn’s F ring and shepherd satellites as a natural outcome of satellite system formation.”


Email the author at and follow him at @dvorsky. Top image by NASA/Cassini