David A. Rothery Teach Yourself Planets, Chapter 9, pp. 107-39, Hodder Education, 2000, 2003.
Copyright © David Rothery
Planetary rings consist of hordes of particles sharing orbits in their planet's equatorial plane, and occur around each of the four giant planets. Their width varies from planet to planet, but in general their thickness is no more than a few tens of kilometres. They generally lie close to their planet, inside what is known as the Roche limit where no large solid body could exist because it would be pulled apart by tidal forces. Therefore rings may represent material from that part of an orginal circumplanetary disc that was inside the Roche limit and so could not aggregate into a large body. Alternatively they may owe their origin to a satellite or comet that strayed within the Roche limit and was ripped apart. Some less substantial rings may be supplied by dust sprayed from a nearby satellite into space by impacts or volcanic activity.
The rings of Saturn have been known since the dawn of the telescopic era, although Galileo Galilei himself could not make out what they were. Jupiter's far less spectacular rings were not discovered until Voyager 1's arrival in 1979. Part of the reason for their inconspicuousness is that even the main one is far narrower than Saturn's. Another important factor is that Jupiter's rings reflect less than 5 percent of the incident sunlight (in other words their albedo is <0.05), presumably because they are made of silicate- or carbon-rich material, whereas Saturn's are icy and reflect up to 80 per cent (albedo up to 0.8).
Jupiter's rings are easier to see in forward-scattered light, i.e. when looking at them towards the Sun (Figure 9.8), than when seen in back-scattered light, which is the only view we ever get of them from Earth. This reveals that they are made mostly of very small particles, about a micrometre in size. There could be plenty of larger chunks too, but the total volume of matter in the rings is probably no more than required to make a single object about a kilometre across.
Micrometre-sized particles can survive in a ring for only about 1000 years before radiation pressure and interactions with the planet's magnetic field disperse them. Therefore either Jupiter's rings are young and ephemeral (which seems improbable) or there is a continual supply of fresh particles.
There are three components to Jupiter's ring system. The main ring is no thicker than 30 km and has an inner radius of 123,000 km and an outer radius of 129,000 km. Its outer edge coincides with the orbit of the tiny satellite Adrastea, whereas the innermost known satellite, Metis, actually orbits within the ring at a radius where the ring shows a marked drop in brightness (not perceptible in the oblique view of Figure 9.8). Apparently unique to Jupiter, there is a 20,000-km-thick halo above and below the main ring that is believed to consist of small particles forced out of the ring plane by electromagnetic forces. Beyond the rim of the main ring lies the so-called gossamer ring that is tenuous in the extreme, but is even thicker than the halo and contains the orbits of the satellites Amalthea and Thebe which are presumably the sources of the particles making up this ring.
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