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Physics LibreTexts

7.10 Structure Within Ring Systems

What causes ring particles to cluster into ringlets with well-defined edges? What keeps particles out of the nearly empty gap in Saturn’s rings known as the Cassini Division? Why does Neptune's ring system show concentrations of material into arcs? Scientists are still debating some of these questions, but it seems clear that most of the structure in rings is caused by gravitational forces acting on the particles. These forces could be from nearby satellites, or from smaller moonlets within the rings themselves, some of which remain undiscovered. 


Saturn's faint ring structures come out during an eclipse. Click here for original source URL


Near Saturn's equinox, shepherd satellite Daphnis and its waves cast shadows on the A Ring. Click here for original source URL.

Satellites that confine rings into narrow zones are called shepherd satellites or shepherd moons. Scientists have studied their gravitational effects on the myriad nearby particles. It turns out that, like pairs of sheepdogs, they keep the flock of neighboring ring particles in an orderly group. An outer satellite will slow down any particle that strays outward, so the particle drops into a lower orbit. An inner shepherd satellite will speed up any particle that strays inward, boosting it into a higher orbit. So the thin rings of Uranus and Neptune, for example, are probably confined by small moonlets. The Voyager spacecraft confirmed this theory when it found a few such moonlets straddling narrow rings in the Saturn and Uranus systems.

Close-up of part of the rings of Saturn. Click here for original source URL.

A clear piece of evidence that satellites control rings comes from the biggest gap in Saturn's rings: the Cassini Division. Particles in that region find themselves orbiting around Saturn in half the orbital period of the nearest large satellite, Mimas. Particles in that region feel a repeated gravitational tug from Mimas, at the same position, every two trips around Saturn. This phenomenon, when one body has an orbital period equal to a simple fraction of the orbital period of another — such as 1/2, 1/3, or 2/3 — is called resonance. The regularly repeated gravitational tug of the larger body disturbs the orbit of the smaller body, often kicking the smaller body clear out of its original orbit.

Gravitational forces also explain the two kinds of spiral waves that propagate through rings. Density waves form where particles in elliptical orbits get “bunched up” at an orbital resonance. The same phenomenon, on a much larger scale, creates spiral arms in galaxies. If a satellite has an inclined orbit, its gravitational perturbations can induce ring particles to move up and down out of the ring plane. This forms another type of spiral wave called a bending wave. 

Dark?B Ring?spokes in a low-phase-angle?Cassini?image of the rings' unlit side. Left of center, two dark gaps (the larger being the?Huygens Gap) and the bright (from this viewing geometry) ringlets to their left comprise the?Cassini Division. Click here for original source URL.

While most structures within rings are related to gravitational forces, spokes are an example of a non-gravitational phenomenon. Spokes are dark features crossing Saturn’s rings perpendicular to their orbits. Voyager took a series of images as the spokes moved around the rings, and found that they rotate at the same rate as Saturn’s magnetic field. The spokes are believed to be clouds of micron-sized dust particles shadowing the rings. These tiny particles are kicked off rocky satellites from countless impacts. When the solar wind bombards them, they build up an electrostatic charge on their surfaces. Saturn’s magnetic field then affects their motions, perhaps even bringing them up or down out of the ring plane. Jupiter’s rings are also affected by that planet’s strong magnetic field — magnetic forces bring orbiting dust particles out of the ring plane, widening the ring into a donut-shaped “halo.”

A mosaic of 107 images showing 255? (about 70%) of the F Ring as it would appear if straightened out. The radial width (top to bottom) is 1,500?km. Click here for original source URL.

Besides the exact nature of the spokes, scientists have many other unanswered questions about the intricate structures in ring systems. Some rings, like Saturn’s F ring, appear braided, like two narrow rings twisted together. The material in this ring also clumps together in knots, instead of spreading out around the planet. These effects may be due to the complex gravitational influence of the ring’s two shepherding satellites, Prometheus and Pandora, which are in unpredictable, chaotic orbits. Or they may be the unexpected effects of still-undiscovered satellites. Data from the Cassini spacecraft should clarify some of these issues; new satellites have already been discovered within Saturn’s rings. However, Cassini has also returned images of previously unseen features, presenting more puzzles about the unexplained structure of ring systems.