RINGS

Key Concepts

(1) All the Jovian planets have rings, but only Saturn's are broad and bright.

The extreme brightness of Saturn's rings relative to those of the other Jovian planets can be deduced from a chronology of ring discoveries:

A brief definition of a `stellar occultation' is probably in order. A stellar occultation occurs when a planet (such as Uranus or Neptune) passes in front of a star as seen from Earth. On March 10, 1977, for instance, Uranus passed in front of the star SAO 158687. Before the star disappeared behind Uranus, however, it was briefly dimmed 5 times, as it passed behind the five thickest rings of Uranus. Then, after the occultation was over, the star dimmed 5 more times, as it passed behind the rings again.

All the Jovian planets have rings; none of the terrestrial planets do (although the Earth is rapidly acquiring an artificial ring of satellites and space junk). The rings of the Jovian planets, however, differ widely from each other in their properties.

Saturn has very broad, bright rings, pictured below in a Voyager image.

The broad, bright rings of Saturn are separated by narrow gaps.

Saturn's rings lies very close to the planet itself. The bright portion of the rings (what is termed the `A' and `B' rings) stretches from 92,000 kilometers from Saturn's center (1.5 times the equatorial radius of Saturn) to 137,000 kilometers from Saturn's center (2.3 times the equatorial radius). The rings of Saturn are thus 274,000 kilometers in diameter, less than the distance from the Earth to the Moon.

The rings of Saturn are amazingly thin in the direction perpendicular to the ring plane. Measurements by the Voyager spacecraft show that they have a typical thickness of only 30 meters (about 100 feet). Compare this to their diameter of 274,000,000 meters, and you realize that the rings are very thin indeed. A scale model of Saturn's rings, made out of a sheet of paper 0.1 millimeters thick, would have to be almost a kilometer in diameter.

Uranus & Neptune have narrow, dark rings, separated by broad gaps. The Hubble Space Telescope image below shows Uranus circled by its most prominent ring.

Jupiter, the largest Jovian planet, has the wimpiest ring. It has a single dark ring, made of fine dust particles.

The rings of Jupiter, Uranus, & Neptune are made of dark rock or of dirty ice. In contrast with the shiny white ice chunks that make up the rings of Saturn, they reflect only a few percent of the light that strikes them.
The rings of Jupiter, Uranus, & Neptune are low in mass. They could all be swept up and compressed into a single rocky & icy moon about 10 kilometers across.
The rings of Jupiter, Uranus, & Neptune are narrow, averaging 10 kilometers in width (to use round numbers).

(2) Rings may be the remains of a moon pulled apart by tidal forces.

All the ice chunks in all the rings of Saturn, if they were collected in one place, would make an icy moon about a hundred kilometers across. Where did all that ice come from, and why is it spread out in a ring?

The leading hypothesis for the origin of Saturn's rings states that the rings actually once were a single moon. The moon was ripped apart into chunks by the tidal force of Saturn and gradually spread out into a ring circling the planet.

Consider a moon which is gradually coming closer to a planet. As the moon comes closer and closer, the tidal forces stretching the moon become stronger and stronger. Eventually, the moon reaches the Roche limit. The Roche limit of a planet is defined as the distance at which the tidal force pulling a moon apart are equal to the gravitational force holding the moon together. The exact value of the Roche limit depends on the relative density of the planet and the moon. (A very dense moon is held strongly together by its own gravity and can approach closer to the planet before being ripped apart by the tidal force.) For a planet and moon of equal density, the Roche limit is equal to 2.4 times the planet's radius.

The Jovian planets, which have large radii, also have large Roche limits as a consequence, and can tidally disrupt moons out to a larger radius. This probably explains why Jovian planets have rings and the puny, tidally-challenged terrestrial planets don't. The ring of Saturn, it is hypothesized, had its origin in an icy moon which spiraled inward towards Saturn until it was torn apart by tides.

(3) Narrow rings are kept from spreading by the gravitational effect of shepherd moons.

A closeup view of the rings of Saturn (as in the case of this false-color Voyager image) reveals that the bright rings of Saturn are divided into an intricate structure of thousands of narrow ringlets, each only a few kilometers wide. In the ordinary course of events, this elaborate substructure should be wiped out as ring particles constantly collide with each other and go careening off in random directions (just as the intricate patterns formed by a marching band would be wiped out if the band members all started colliding with each other and staggered off in random directions).

The intricate substructure of Saturn's rings is created and maintained by orbital resonances with the moons of Saturn. As an example, consider the Cassini division, a large gap in the rings of Saturn which has existed since at least the seventeenth century, when it was discovered by Cassini. The Cassini division is caused by an orbital resonance with the moon Mimas (that's the moon that looks like the Death Star).

Thus, if a chunk of ice happened to wander into the Cassini division, every time it went twice around Saturn, it would encounter Mimas in the same location, and would get a tug in the direction of Mimas. These repeated tugs, always in the same direction, would eventually tug the ice chunk right out of the Cassini division. Saturn has a lot of moons, and there are a lot of potential resonances; hence the elaborate pattern of ringlets and gaps.

Gravitational interactions between moons and ring particles also explain why many rings, such as the rings of Uranus, are so thin. Narrow rings tend to spread, thanks to the collisions between ring particles. However, narrow rings are kept from spreading by the gravitational pull of shepherd moons. The image below shows shepherd moons in action. Just outside the main, bright rings of Saturn lies a very narrow ring, called the F ring. Just outside the F ring lies a small moon called Pandora. Just inside the F ring lies another small moon, this one called Prometheus. (Please click on the image below to see a larger picture of the F ring of Saturn and its shepherd moons, Pandora and Prometheus, the two small white dots in the image.)

The gravitational interaction among the outer shepherd, the inner shepherd, and the ring particles tends to add angular momentum to particles that stray inward (thus causing them to move back out to the ring). Similarly, the gravitational interaction subtracts angular momentum from particles that stray outward (thus causing them to move back in to the ring). The shepherd moons are well named; they return straying ring particles to the fold. The biggest of Uranus's rings has a pair of shepherd moons called Ophelia and Cordelia, which were just barely resolved by the Voyager 2 spacecraft. Presumably the fainter rings also have shepherds; if so, they were too small to be seen by the Voyager cameras.

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