The Solar System

CHAPTER 23 | COMPARATIVE PLANETOLOGY OF JUPITER AND SATURN 513

proved mathematically that solid rings would be unstable.
Saturn’s rings, he concluded, had to be made of separated par-
ticles. In 1867, Daniel Kirkwood demonstrated that gaps in the
rings were caused by resonances with some of Saturn’s moons.
Spectra of the rings eventually showed that the particles were
mostly water ice.
Study The Rings of Saturn on pages 514–515 and
notice three points and a new term:

Th e rings are made up of billions of ice particles, each in its

own orbit around the planet. But, just as for Jupiter’s rings,

the particles observed now in Saturn’s rings can’t have been

there since the planet formed. Th e rings must be replenished

now and then by impacts on Saturn’s icy moons or by the

disruption of a small moon that moves too close to the

planet.

Th e gravitational eff ects of small moons called shepherd satel-

lites can confi ne some rings in narrow strands or keep the

edges of rings sharp. Moons can also produce waves in the

rings that are visible as tightly wound ringlets.

Th e ring particles are confi ned to a thin layer in Saturn’s

equatorial plane by the gravity of small moons. Th e small

moons in turn are controlled by gravitational interactions

with larger, more distant moons. Th e rings of Saturn, and

the rings of the other Jovian worlds, are created from and

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2

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controlled by the planet’s moons. Without the moons, there

would be no rings.

Modern astronomers fi nd simple gravitational interactions pro-

ducing even more complex processes in the rings. Where particles

orbit in resonance with a moon, the moon’s gravity triggers spiral

density waves in much the same way that spiral arms are produced

in galaxies. Th e spiral density waves spread outward through the

rings. If the moon follows an orbit that is inclined to the ring

plane, the moon’s gravity causes a diff erent kind of wave—spiral

bending waves—ripples extending above and below the ring

plane, which spread inward. Both of these kinds of processes are

shown in the inset ring image on page 515.

Many other processes occur in the rings. Specks of dust

become electrically charged by sunlight, and Saturn’s magnetic

fi eld lifts them out of the ring plane. Small moonlets embedded

in the rings produce gaps, waves, and scallops in the rings. Th e

Cassini spacecraft has recorded dramatic images (■ Figure 23-16)

of the Saturn ring system, including two faint outer rings (E and

G) that are rarely detectable from Earth, and these also appear to

be related to moons.

Th e word “particle” in colloquial language connotes tiny specks,

but in the context of Saturn’s rings, astronomers use that term to

refer to any object from snowlike powder grains up to building-sized

icy mini-moons (look again at page 514). Th e larger objects are

understood to be aggregates of the smaller ones. Th e subtle colors of

■ Figure 23-16

The Cassini spacecraft recorded this image as it passed through Saturn’s shadow. Earth is visible as a faint blue dot just inside the G ring,
and jets of ice particles vented from the moon Enceladus are visible at the left extreme of the larger E ring. Two faint rings associated
with small moons were discovered in this image. (NASA/JPL/Space Science Institute)

Visual-wavelength image

Enceladus

Earth