The Solar System

512 PART 4^ |^ THE SOLAR SYSTEM

Like Jupiter, Saturn’s atmosphere is rich in hydrogen and
displays belt–zone circulation, which appears to arise in the same
way as the circulation patterns on Jupiter. Th e light-colored
zones are higher clouds formed by rising gas, and the darker belts
are lower clouds formed by sinking gas.
Notice, however, that the zones and belt clouds are not very
distinct on Saturn compared with Jupiter (■ Figure 23-15a).
Measurements from the Voyager and Cassini spacecraft indicate
that Saturn’s atmosphere is much colder than Jupiter’s—something
you would expect because Saturn is twice as far from the sun and
receives only one-fourth as much solar energy per square meter.
Th e clouds on Saturn form at about the same temperature as the
clouds on Jupiter, but those temperature levels are deeper in
Saturn’s cold atmosphere. Compare the cloud layers in
Figure 23-15b with those shown in the diagram on page 501.
Because they are deeper in the atmosphere, the cloud layers look
dimmer from your viewpoint outside, and a
high layer of haze formed by methane crystals
makes the cloud layers even more indistinct.
Th e atmospheres of Jupiter and Saturn are actu-
ally very similar once you account for the fact
that Saturn is colder.
One dramatic diff erence between Jupiter
and Saturn concerns the winds. On Jupiter,
winds form the boundaries for each of the
belts and zones, but on Saturn the pattern is
not the same. Saturn has fewer such winds, but
they are much stronger. Th e eastward wind at
the equator of Saturn, for example, blows at
500 m/s (1100 mph), roughly fi ve times faster
than the eastward wind at Jupiter’s equator.
Th e reason for this diff erence is not clear.

Saturn’s Rings

Looking at the beauty and complexity of Saturn’s rings, an

astronomer once said, “Th e rings are made of beautiful physics.”

You could add that the physics is actually rather simple, but the

result is one of the most amazing sights in our solar system.

In 1609, Galileo became the fi rst to see the rings of Saturn;

but perhaps because of the poor optics in his telescopes, he did

not recognize the rings as a disk. He drew Saturn as three

objects—a central body and two smaller ones on either side. In

1659, Christian Huygens (pronounced approximately, Howk-gins)

realized that the rings form a disk surrounding but not touching

the planet.

Understanding Saturn’s rings has required human ingenu-

ity continuing to the present day. In 1859, James Clerk

Maxwell (for whom the large mountain on Venus is named)

■ Figure 23-15

(a) Saturn’s belt–zone circulation is not very distinct at vis-

ible wavelengths. These images were recorded when Saturn’s

southern hemisphere was tipped toward Earth. (NASA and

E. Karkoschka) (b) Because Saturn is colder than Jupiter, the

clouds form deeper in the hazy atmosphere. Notice that the

three cloud layers on Saturn form at about the same tempera-

tures as do the three cloud layers on Jupiter.

Temperature (°F)

Ultraviolet

Visible

a Infrared

Ammonia

Ammonia hydrosulfide

Water

100

• 100


• 200

200

• 300 – 200 – 100 0 100 212

0

Altitude (km)

Temperature (K)

b

100 200 300 400

Clear hydrogen

atmosphere

Methane haze

Saturn

Jupiter