What Rings Tell Us About The Nearby Moons

Spiral bending waves (corrugations) and density waves (alternate crowding and separating of ring particles in a radial direction) are due to gravitational interactions with nearby satellites, especially in the rings of Saturn and Uranus (Figure 1.5). Sharp edges to ring boundaries or gaps within otherwise continuous rings are generally also indications of ring particle interactions with natural satellites. Still other effects—like wakes along an otherwise sharp edge or condensations within a ring, or chaotic appearance at the outer or inner boundaries of a ring—are due to interactions with planetary satellites.

It is therefore natural, when such ring features are observed, to try to associate the features with known satellites. Occasionally such associations with known satellites are not possible, or the ring features are of such a nature that they are more likely to be associated with nearby, as yet unseen, satellites. In such circumstances, the ring features often lead to predictions about the precise location of a potential new satellite, and additional observations (or poring over existing images) can result in the discovery of the proposed satellite. The former procedure led to the discovery of a small satellite (Daphnis) orbiting within the Keeler gap near the outer edge of Saturn's A ring. The latter is exemplified by Pan, discovered nearly eight years after the Voyager swingbys of Saturn from Voyager images of the Encke gap in Saturn's A ring [5].

The nature and magnitude of the gravitational interactions between ring particles and the satellites responsible for the individual features can also lead to a rough estimate of the mass of the perturbing satellite. When these are combined with images of sufficient resolution to permit size and shape estimates for the relevant satellite, rough density estimates are possible. Those density estimates can further lead to composition estimates, relatively easily distinguishing between solid rocky satellites

Figure 1.5. A pair of wave trains in Saturn's A ring near the distance at which ring particles circle the planet five times for every three orbits of the moon Mimas. At the left is a spiral density wave. At the right is a bending wave, where the ridges cast shadows in the adjacent troughs. The distance between the two wave trains is about 400 km. (Voyager FDS 43993.50)

Figure 1.5. A pair of wave trains in Saturn's A ring near the distance at which ring particles circle the planet five times for every three orbits of the moon Mimas. At the left is a spiral density wave. At the right is a bending wave, where the ridges cast shadows in the adjacent troughs. The distance between the two wave trains is about 400 km. (Voyager FDS 43993.50)

(with densities greater than 2 grams per cubic cm, abbreviated gcm~3), solid icy satellites (densities between about 1 and 1.5gcm~3), or loose "rubble piles'' of icy material (densities significantly less than 1 gcm~3).

Finally, radial ring density variations and, to some extent, vertical ring structure, has led ring scientists to speculate that the source of nearby ring material is from a particular satellite. Saturn's expansive E ring was brightest (densest) near the orbit of Enceladus, and one of the goals of the Cassini Mission was to determine if evidence can be found to substantiate geologically active processes on Enceladus that might generate E-ring particles. Similarly, Jupiter's Gossamer ring has a vertical structure that has led scientists to speculate that it is being fed by particles being blasted (most likely by meteoroid bombardment) from the surfaces of Thebe and Amalthea, each of whose orbits is slightly inclined to Jupiter's equatorial plane. The Gossamer ring particles thus generated seem to migrate only toward Jupiter's atmosphere and not outward. That behavior and possible explanations for it will also be discussed in later chapters.

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