The purpose of this chapter has been to set the stage for the spectacular data on Saturn's rings returned by the Cassini Orbiter. In Chapter 10 we report on some of the preliminary findings of this important international space mission. Hopefully, the data we include in this book will whet the appetites of interested readers sufficiently to cause them to search for the later results that will already have appeared or will shortly appear that further expand on our understanding of the Saturn ring system.
The rings of Saturn have been observed for centuries; none of the other ring systems was even known to exist prior to 1977. They have progressed from ignominy to passing familiarity, primarily as a consequence of the Voyager 1 and 2 encounters in 1980 and 1981. By the time the Cassini ring data are fully absorbed and digested, they will be old friends, but like the best of our friends, there will remain many things about them that are still strange and seemingly idiosyncratic. In part, it is the difficulty associated with delving into their complexity, and the thrill of recognition that they are slowly but surely helping us to understand the ways of Nature, that endear them to us.
The Saturnian rings are evolving on a number of timescales ranging from days to eons. Those changes are not apparent to observers with small to intermediate telescopes, where the bejeweled planet is sometimes seen to have atmospheric storms that come and go, but appears to be surrounded by an unchanging set of rings. But, to a trained observer using Earth-orbiting telescopes or large ground-based telescopes equipped with resolution-enhancing adaptive optics or sophisticated robotic spacecraft that carry our eyes and ears to Saturn, the changes in the rings are unmistakable. It is the understanding of those changes that will eventually enable space scientists to extend that understanding to the asteroid belt, to the trans-Neptunian Kuiper Belt, to protoplanetary disks around other Milky Way stars, to spiral structure in the Milky Way Galaxy and other galaxies, and possibly even to clusters of galaxies.
Now let us examine briefly each of Saturn's rings, summarizing some of the known and unknown characteristics of each.
We know the radial location of the outer edge of the D ring, but its inner edge is indistinct, and the forces which maintain those boundaries are poorly understood. We see radial structure within the D ring, but we do not know what causes that structure, and Voyager and Earth-based data are insufficient to show us changes in that structure. Dust-sized particles likely dominate its population, judging by its brightness in forward scattering, but we do not know the thickness of the ring or whether its particles are affected by Lorentz resonance. The mass of the ring is also unknown.
The C-ring inner and outer boundaries are known, but the processes which give rise to the sharp drop in ring brightness and optical depth in the B-ring to C-ring transition are unknown. Semi-regular radial structure within the ring remains unexplained, and the vertical thickness is apparently comparable with that of the A and B rings. The ring appears to have a preponderance of particles in excess of 10 cm in radius, and the fractional abundance of water ice is much smaller than for the A and B rings. Within the C ring are five known gaps, and two of those gaps, named Colombo and Maxwell, contain eccentric ringlets, possibly associated with undiscovered satellites. The ring mass is estimated to be about 0.000000002 times that of Saturn.
Saturn's densest, brightest, and most massive ring is its B ring. It is also the most extensive ring radially other than the tenuous and dusty E ring. While the confining mechanism at the inner boundary is unknown, the outer boundary is caused by a 2: 1 resonance with the satellite Mimas, which also gives that boundary a two-lobed shape which turns at the orbital rate of Mimas. There are no clear gaps within the B ring, although the Huygens gap, which contains an eccentric ringlet, is at its outer edge. Water ice is the dominant constituent of the ring, but other material provides coloration. The outer half of the ring is denser and brighter than the inner half for unknown reasons. Before Cassini, there were no complete, high-resolution optical depth profiles for the outer half of the ring. In the middle of this B-ring outer half, ghostly radial spokes occasionally form, possibly as a result of meteoroid impact with ring particles. Material in the spokes becomes electrically charged, and they appear to be electrostatically suspended above the main ring and rotate at the rate of the magnetic field for a time period of from less than one Saturn rotation to nearly three rotations. The outer half of the B ring also contains a myriad of radial structures of an irregular nature, the cause of which is yet to be deciphered. From the sharpness of the ring's outer edge, its thickness has been estimated to be as small as 10 meters! The ring mass is estimated to be 0.00000005 times that of Saturn.
Between the B and A rings is the Cassini Division, once thought to be empty, but now known to be filled with material totaling about half the mass of the C ring. It contains six known gaps, the innermost (Huygens gap) and outermost (unnamed) of which are more than 200-km wide and contain ringlets. Its inner boundary is controlled by a 2: 1 Mimas resonance, but the source of the sharp increase from the outer Cassini Division to the A ring is not well understood. Like the C ring, the Cassini Division appears to be more polluted with non-icy material than the A and B rings.
The outermost of Cassini's main rings is the A ring. Its outer boundary is at the radial distance of a 7: 6 Janus resonance and is seven-lobed. There are two gaps within the A ring, the 325-km wide Encke gap and the 35-km wide Keeler gap. Pan and several partial ringlets (arcs?) occupy the Encke gap. Most of the known density waves and bending waves lie within the A-ring boundaries. Like the B ring, its dominant chemical constituent is water ice. Between the Keeler gap and the outer edge of the A ring, the structure is irregular, both radially and longitudinally. While some of that structure may be related to the gravitational resonance with Janus that is responsible for the seven-lobed outer A-ring edge, the reasons for and detailed processes operative within that irregular structure are not well understood.
Saturn's mysterious F ring is time-variable, both radially and azimuthally. Although the whole width of the ring was seen in stellar occultation data, only the central core was detected by the radio astronomy occultation experiment, implying that there are few particles more than 1-10 cm in radius away from that core, at least at the longitude sampled by that experiment. Bright knots within the F ring may be larger bodies that serve as sources for the F-ring material. At one time, it was thought to be radially confined by the two satellites which flank it, Prometheus and Pandora, but that now seems unlikely. Its mass and vertical thickness are unknown.
The G ring, about 13,000-km wide, does not have well-defined radial boundaries or small-scale radial structure. Its mass is unknown, but its vertical thickness is estimated to be between 100 and 1,000 km. Spacecraft data are not consistent with a narrow range of particle sizes, and the ring has a reddish hue, so the G ring is unlike the E ring in thickness, particle sizes, and composition. Beyond these facts, very little is known about the G ring, including, among other things, its source and its age.
The tenuous and extended E ring has been found to have a bluish color, a very narrow range of particle sizes, and a vertical extent that varies with distance from the planet. Its inner edge is near the orbit of Mimas, but it is both densest and thinnest in vertical extent (about ± 1,000 km) near the orbit of Enceladus, which is believed to supply the icy material of which it is composed. The volume density of particles diminishes with increasing radial distance beyond Enceladus. The vertical thickness also increases, reaching a vertical half-thickness of at least 15,000 km near the radial distance of Dione. Although they become optically undetectable before it reaches the orbit of Rhea (about 8 RS), E-ring particle concentrations centered near Saturn's equatorial plane may possibly exist nearly to the orbit of Titan (about 20 RS).
Estimates of the age of the Saturn ring system vary from the 4.5 x 109-year age of Saturn to as little as a few tens of millions of years. It is difficult to understand how a ring system as massive as the satellite Mimas could be so finely divided through natural processes in time periods from tens to hundreds of millions of years, particularly in the absence of an interplanetary meteoroid flux that is thought to have been almost depleted well before that time period. Meteoritic bombardment has other consequences that affect the present composition and structure of the Saturn ring system as well as estimates of its age and evolution. If such bombardment has been ongoing for a large fraction of the age of the solar system, then the composition of the rings should be similar to that of the meteorites, which are not composed of water ice. Even for a moderate ring age, it seems likely that the water-ice particles which largely compose the A and B rings should have been coated with dark meteoritic material, but they remain bright and highly reflective. Meteoritic bombardment should also scatter debris across the ring system, yet there remain what appear to be significant radial variations in the colors of the rings. Other processes also act to deplete the rings of material, either ejecting it from the Saturn system, or, alternatively, causing it to fall into the planet. These and other considerations are discussed in Chapter 10 in light of preliminary findings from the Cassini Orbiter.
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