Origin and Evolution of Saturn's Ring System

Abstract

ABSTRACT:The origin and long-term evolution of Saturn's rings is still an unsolved problem in modern planetary science. In this chapter we review the current state of our knowledge on this longstanding question for the main rings (A, Cassini Division, B, C), the F Ring, and the diffuse rings (E and G). During the Voyager era, models of evolutionary processes affecting the rings on long time scales (erosion, viscous spreading, accretion, ballistic transport, etc.) had suggested that Saturn's rings are not older than 10 8 years. In addition, Saturn's large system of diffuse rings has been thought to be the result of material loss from one or more of Saturn's satellites. In the Cassini era, high spatial and spectral resolution data have allowed progress to be made on some of these questions. Discoveries such as the "propellers" in the A ring, the shape of ring-embedded moonlets, the clumps in the F Ring, and Enceladus' plume provide new constraints on evolutionary processes in Saturn's rings. At the same time, advances in numerical simulations over the last 20 years have opened the way to realistic models of the rings' fine scale structure, and progress in our understanding of the formation of the Solar System provides a better-defined historical context in which to understand ring formation. All these elements have important implications for the origin and long-term evolution of Saturn's rings. They strengthen the idea that Saturn's rings are very dynamical and rapidly evolving, while new arguments suggest that the rings could be older than previously believed, provided that they are regularly renewed. Key evolutionary processes, timescales and possible scenarios for the rings' origin are reviewed in the light of these recent advances.

An Evolving View of Saturn’s Dynamic Rings

“…The Voyager-era (1980s) perspective was that today's planetary ring systems cannot be primordial but must be continuously regenerated from their local arrays of moonlets, through vigorous evolutionary processes (3,4). To create Uranus' narrow rings, or the diffuse rings of Jupiter or Neptune, merely requires destroying a 1-to 10-km-diameter moonlet by impact with a heliocentric interloper.…”

The Origin of the Natural Satellites

“…In contrast, prior ring origin models produce an initial ring comparable in mass to the current rings (Charnoz et al, 2009;Dones, 1991). Salmon et al (2010) found that such dense rings can evolve to something resembling Saturn's ring by viscous spreading after O(10 8 ) years, but it is not clear how the ignored MMRs with the closest satellites such as Mimas would affect this evolution.…”

“…Here, the stripped material accumulates in a pure ice moon whose final mass is m o 10 25 g. Canup estimated that the location of synchronous orbit around a newly formed Saturn would be located at about 3R S due to the slower rotation of the more distended planet, so that this ice moon eventually spirals inward due to planetary tides, but at a slower rate than the original satellite due to its smaller mass. For planetary tidal parameters (k 2 /Q) P (with likely values 10 À6 k 2 /Q P 10 À5 ; e.g., Charnoz et al, 2009), the ice moon takes $10 6 years [5 Â 10 À6 /(k 2 /Q) P ] [10 25 g/m o ] to decay within the ice Roche limit, where it disrupts into a massive ice ring. This timescale is long enough that the ring forms after the remnant satellite has collided with Saturn and when the circumplanetary gas disk has essentially dissipated.…”

Detecting ring systems around exoplanets using high resolution spectroscopy: the case of 51 Pegasi b

“…Values of Q as high as 10 7 have been proposed for stars (Penev et al 2012), and this value could lead to τ ∼ 10 8 yr if we assume k 2 ∼ 0.1, as expected for giant planets (Yoder 1995). Ring systems are believed to have several possible origins: the result of impact events (e.g., Tiscareno 2013), captured objects or satellites that are tidally destroyed (e.g., Charnoz et al 2009b;Canup 2010), or even remnants from planet formation (though this last hypothesis is less likely - Charnoz et al 2009a). In all cases, they settle in a special plane around the planet, called the Laplacian plane (e.g., Lehébel & Tiscareno 2015).…”