Saturn’s F ring and shepherd satellites a natural outcome of satellite system formation

Abstract

Saturn's F ring is a narrow ring of icy particles, located 3,400 km beyond the outer edge of the main ring system. Enigmatically, the F ring is accompanied on either side by two small satellites, Prometheus and Pandora, which are called shepherd satellites 1-3 . The inner regular satellites of giant planets are thought to form by the accretion of particles from an ancient massive ring and subsequent outward migration 4-7 . However, the origin of a system consisting of a narrow ring and shepherd satellites remains poorly understood. Here we present N-body numerical simulations to show that a collision of two of the small satellites that are thought to accumulate near the main ring's outer edge can produce a system similar to the F ring and its shepherd satellites. We find that if the two rubble-pile satellites have denser cores, such an impact results in only partial disruption of the satellites and the formation of a narrow ring of particles between two remnant satellites. Our simulations suggest that the seemingly unusual F ring system is a natural outcome at the final stage of the formation process of the ring-satellite system of giant planets.Saturn's F ring was discovered in 1979 by Pioneer 11 (refs 1,8). Its unusual structures have been revealed through observations by the two Voyager spacecraft 2,3 , other telescopes 9,10 , and in far more detail by the Cassini spacecraft 11-13 . Owing to the planet's tidal force, even low-velocity collisions between particles in the ring do not necessarily lead to gravitational accretion 14,15 , and easily result in disruption 16,17 . Observations by Cassini also suggest ongoing accretion and disruption of aggregates within the ring 18-20 , but the ring's origin has been poorly understood. Dust generation due to collisions between unseen small moonlets 21,22 or meteoroid impacts onto such small bodies 23 has been proposed as the origin of the F ring, but detailed studies have not been carried out, and the relationship between the ring-formation process and the shepherd moons has not been clarified. Whereas the principal regular satellites of giant planets, such as Saturn's Titan and Jupiter's Galilean satellites, were probably formed in circumplanetary gas disks 24 , recent models show that the increasing masses of the inner regular satellites of Saturn, Uranus and Neptune with increasing distance from the central planet can be explained by satellite accretion from particles spreading from an ancient massive ring beyond the Roche limit (the distance beyond which particles can gravitationally accrete) and subsequent outward migration 4-6 . As the surface density of the particle disk decreases as a result of radial spreading, the mass of satellites formed from the disk as well as their rate of outward migration due to gravitational interaction with the ring decreases 7 . Thus, multiple small satellites tend to coexist near the disk's outer edge for a significant period of time during the final stage of satellite system formation.We perform N -body simulations of impacts betw...

Rings in the Solar System: A Short Review

“…Thus, outgassing of dust particles from the surface may occur (Pan & Wu 2016). Also, a partial disruption of a primordial differentiated object through an extreme close encounter with a giant plant may directly form debris and satellites around the remnant objects (Hyodo et al 2016) and a long-term dynamical evolution may shape the debris to the observed narrow structures (Hyodo & Ohtsuki 2015). Either of the proposed scenario may apply to one of the observed ringed small objects, but further observations and investigations are required to constrain more about their physical properties and their origins.…”

On the tidal environment of an outwardly migrating F ring

“…Currently it is unclear if the F ring and its shepherd moons formed at this radial location, because it is close to the Roche limit or purely a coincidence. Hyodo & Ohtsuki (2015) showed that the F ring and its shepherd moons were a natural outcome of a collision between two satellites at its current location. If such a collision occurred at a larger radial distance than the current location of the F ring, it would place it in a lower tidal environment but still with nearby shepherd moons.…”

The peculiar shapes of Saturn’s small inner moons as evidence of mergers of similar-sized moonlets

“…Our study also implies that hit-and-run collision between similar-sized objects were frequent throughout the formation of the small inner moons. Such an event was proposed to generate the F ring 6 . Although a hit-and-run event between Prometheus and Pandora was previously considered to explain the origin of the F ring, in our model the F ring material could originate from the hit-and-run collision of the precursors of Prometheus, then be trapped in the stability area between Pandora and Prometheus after the merging of the precursors.…”