Rings

Uranus Neptune

H Interiors of Jupiter, Saturn, Uranus and Neptune.These are from model calculations, and are expressed relative to each planet's radius.The numbers at each boundary are estimated temperatures.

Jupiter, Saturn, Uranus and Neptune have ring systems. Saturn's, shown in Fig. 25.14, has been known since Galileo, while those of Jupiter, Uranus and Neptune are recent discoveries. We first review the basic properties of the rings, and then consider the effects that are important in shaping them.

Fig 25.14.

Saturn's rings. (a) Voyager 1 image of the unlit side, so the B ring appears black. (b) Voyager 1 mosaic. Cassini's division appears to the right of center as five bright rings with a substantial dark gap. (c) Spokes in the B ring (Voyager 1 image). [NASA]

Fig 25.14.

Saturn's rings. (a) Voyager 1 image of the unlit side, so the B ring appears black. (b) Voyager 1 mosaic. Cassini's division appears to the right of center as five bright rings with a substantial dark gap. (c) Spokes in the B ring (Voyager 1 image). [NASA]

25.4.1 Basic properties

Saturn's rings (Fig. 25.14) shine by reflected sunlight. Therefore, the brighter areas are those with more material to reflect the light. Therefore, the brighter areas are those of higher optical depth, meaning a greater amount of reflected sunlight.

Three main rings are apparent. The A ring is the farthest from the planet. It has a width of 20 000 km, ranging from 2.02 to 2.27 times the planetary radius. Its thickness is less than 200 m. Approximately half of the light striking this ring is reflected back. The B ring is in the middle and is the brightest. It extends from 1.52 to 1.95 planetary radii. The A and B rings are separated by a gap, called Cassini's division. The innermost ring is the C ring. It is the darkest. It extends from 1.23 to 1.52 planetary radii.

Additional rings have been found from the ground and from spacecraft. The D ring is a faint ring inside the C ring (1.11 to 1.23 radii). The E ring is a very faint ring, outside the A ring (3 to 8 radii). The F ring is just beyond the A ring (2.37 radii), and was discovered by Pioneer 11. Finally, the G ring is a faint ring 2.8 radii from the planet.

The rings are composed of individual particles, rather than being solid structures. We can tell this from the spectra of the rings. The Doppler shifts vary across the rings, as shown in Fig. 25.15. These

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