What Causes Ring Systems

Between 1610, when Galileo first glimpsed Saturn's ring, and 1977, when the ring of Uranus was discovered, the solar system, for that matter the universe, contained only a single example of a planetary ring system—namely, the ring of Saturn. Astronomers speculated on which of two mechanisms gave birth to that ring: (1) Was it the remnant material from Saturn's formative years that was prevented from coalescing into individual satellites? or (2) Was the ring material the debris from one or more former satellites that somehow wandered too close to Saturn and were torn apart by gravitational tides or were shattered by a collision with an interplanetary interloper? It is now abundantly clear that the first of these choices is no longer tenable, primarily because ring scientists have come to understand that the present rings cannot have existed for more than a small fraction of the age of the solar system.

Does that then once and for all answer the question about the origins of Saturn's rings and, by analogy, the origins of planetary ring systems in general? Unfortunately (or perhaps fortunately, depending on one's viewpoint), continued observations of planetary ring systems have suggested that a number of different mechanisms help to generate rings and define their shape. As is often the case in scientific investigations, more detailed examination of ring characteristics and phenomena revealed a wholly unsuspected complexity and diversity associated with planetary rings and ring systems.

Let's examine briefly the second mechanism proposed by astronomers: that the rings of Saturn formed as one or more satellites wandered too close to Saturn. Most individuals are familiar with the concept of tides, especially Earth's tides, which are caused primarily by the gravitational pull of Earth's only natural satellite, the Moon. What may be less familiar is that the Earth also exerts tidal forces on the Moon. The Moon has no liquid on its surface, but the solid surface is deformed as a result of these tidal forces. The Moon keeps its same face toward the Earth, so the deformation results in a slightly non-circular equator for the Moon, whose equatorial diameter in the line of sight from Earth is more than 600 meters larger than its equatorial diameter along its orbit. That equatorial deformation is only about 0.002% of the equatorial diameter.

French mathematician Edouard Albert Roche (1820-1883) recognized that, for a satellite which is closer than a distance which has become known as the "Roche limit'', such tidal forces can become stronger than the forces which hold the satellite together. In such circumstances the satellite will be torn asunder. For an icy satellite circling Saturn, the Roche limit is at a distance from Saturn's center of about 2.4 times the radius of the planet. All of Saturn's main rings (D, C, B, A, and F) are within this distance, so it seems likely that they are primarily the product of tidal breakup of former moons of Saturn. The approximate distance (from the center of the respective planet) of the Roche limit for Earth and the four giant planets is given in Table 1.1. The equatorial radius of each planet is also shown. The main rings of Saturn and Uranus and all the rings of Jupiter and Neptune are within their respective Roche limits. Earth's Moon is at a mean distance from Earth's center of 384,400 km, more than twenty times Earth's Roche limit.

Table 1.1. Roche limit (in kilometers and miles) for Earth and the four giant planets.

Planet

Radius (km)

Roche limit (km)*

Roche limit (mi)*

Earth

6,378

l8,47O

ll,47O

Jupiter

7l,492

l75,OOO

lO8,OOO

Saturn

6O,268

l47,OOO

92,OOO

Uranus

25,559

62,OOO

39,OOO

Neptune

24,764

59,OOO

37,OOO

* Roche limit distances are referenced to the center of the respective planet.

* Roche limit distances are referenced to the center of the respective planet.

The extended G and E rings of Saturn lie exterior to the main rings and also exterior to the Saturn Roche limit. They are therefore unlikely to have been formed by the breakup of natural satellites or gravitationally captured asteroids. The origin of the G ring remains unexplained, but the origin of the E ring has been attributed to icy eruptions from Saturn's satellite Enceladus. More will be said about that in Chapter 10, which covers early findings on the ring system from the Cassini Orbiter.

Once a large quantity of ring particles has been created, mutual collisions between the particles cause them to spread in all directions from the point of breakup. Polar flattening of the planet and the gravitational influence of satellites, nearly all of which orbit the planet near its equatorial plane, tend to flatten the distribution of the ring particles over time. Given enough time, the azimuthal distribution of ring particles becomes almost uniform unless the gravitational interactions with nearby satellites prevent such a uniform distribution. Radial spreading is also relatively efficient, but such spreading is inhibited or modified by two mechanisms. Those particles that spread inward toward the planet may eventually fall into the atmosphere of the planet and are lost from the ring system. Both outward and inward movement of particles can also be inhibited by the gravitational influence of nearby satellites. These two effects will be discussed in more detail in later chapters.

Jupiter's Main and Gossamer rings are unlikely to be the result of either satellite breakup or eruptions from satellite surfaces. In some ways, Jupiter's enormous gravity acts like a magnet for large numbers of meteoroids in the solar system. These meteoroids occasionally strike the surfaces of small satellites and launch surface dust and debris into orbit around Jupiter. The Gossamer ring actually has characteristics that enable scientists to determine relatively unambiguously the source satellite for the ring material. More will be said about this in Chapters 4 and 6.

Jupiter's Halo ring and tiny particles that form radial spokes in Saturn's B ring are affected by still another mechanism. If the particles are tiny enough and can also become electrically charged, either by the action of sunlight or by other mechanisms, such particles can be deflected from normal near-equatorial orbits around Jupiter or Saturn by the respective planet's magnetic field, which tends to drag them along at the angular speed of rotation of the planet itself. For Jupiter's Halo ring, the result is a donut-shaped torus of ring particles interior to Jupiter's main ring. For Saturn's Bring spokes, the result is ghostly structures that seem to defy the laws of orbital mechanics.

It is likely that a variety of other mechanisms create and shape planetary rings, and it is partially the pursuit of ring studies to discover and explain these effects and to use them as predictors of ring system evolution.

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