The Solar System

498 PART 4^ |^ THE SOLAR SYSTEM

they produce auroras 1000 times more luminous than those on
Earth. Th e auroras on Jupiter, like those on Earth, occur in rings
around the magnetic poles (■ Figure 23-4).
Th e four Galilean moons of Jupiter orbit inside the magne-
tosphere, and some of the heavier ions in the radiation belts
come from the innermost moon, Io. As you will see later in this
chapter, Io has active volcanoes that spew gas and ash. Because Io
orbits with a period of 1.8 days, and Jupiter’s magnetic fi eld
rotates in only 10 hours, the wobbling magnetic fi eld rushes past
Io at high speed, sweeping up stray particles, accelerating them
to high energy, and spreading them around Io’s orbit in a dough-
nut of ionized gas called the Io plasma torus.
Jupiter’s magnetic fi eld interacts with Io to produce a pow-
erful electric current (about a million amperes) that fl ows
through a curving path called the Io fl ux tube from Jupiter out
to Io and back to Jupiter. Small spots of bright auroras lie at the
two points where the Io fl ux tube enters Jupiter’s atmosphere
(Figure 23-4).
Fluctuations in the auroras reveal that the solar wind buff ets
Jupiter’s magnetosphere, but some of the fl uctuations seem to be
caused by changes in the magnetic dynamo deep inside the
planet. In this way, studies of the auroras on Jupiter may help
astronomers learn more about its liquid depths.

Solar wind

Axis of rotation

Magnetic

axis

Earth’s radiation

belts to scale

10 °

■ Figure 23-3

Jupiter’s magnetic fi eld is large and powerful. It traps particles from the solar
wind to form powerful radiation belts. The rapid rotation of the planet forces
the slightly inclined magnetic fi eld to wobble up and down as the planet
rotates. Earth’s magnetosphere and radiation belts are shown to scale.

Jupiter’s powerful magnetic fi eld is invisible to your eyes, but

the swirling cloud belts are beautifully visible in their complexity.

Jupiter’s Atmosphere

As you learned earlier, Jupiter is a liquid world that has no sur-

face. Th e gaseous atmosphere blends gradually with the liquid

hydrogen interior. Below the clouds of Jupiter lies the largest

ocean in the solar system—an ocean that has no surface and no

waves.

When you look at Jupiter, all you see are clouds. You can

detect a nearly transparent hydrogen and helium atmosphere

above the cloud tops by noticing that Jupiter has limb darkening,

just as the sun does (see Chapter 8). When you look near the

limb of Jupiter (the edge of its disk), the clouds are much dimmer

(look back at Figure 23-1) because it is nearly sunset or sunrise

along the limb. If you were on Jupiter at that location, you would

see the sun just above the horizon, and sunlight arriving there

would be dimmed by passing through the planet’s atmosphere. In

addition, sunlight refl ected from the clouds must travel back out

through the atmosphere at a steep angle to reach Earth, dimming

the light further. Jupiter is brighter near the center of the disk

because the sunlight shines nearly straight down on the clouds.

Study Jupiter’s Atmosphere on pages 500–501 and

notice four important ideas:

Th e atmosphere is hydrogen-rich, and the clouds are con-

fi ned to a shallow layer.

Th e cloud layers lie at certain temperatures within the atmo-

sphere where ammonia (NH 3 ), ammonium hydrosulfi de

(NH 4 SH), and water (H 2 O) can condense to form ice

particles.

Th e belt–zone circulation is driven by high- and low-pressure

areas related to those on Earth.

Finally, the major spots on Jupiter, although they are only

circulating storms, can remain stable for decades or even

centuries.

Circulation in Jupiter’s atmosphere is not totally understood.

Observations made by the Cassini spacecraft as it raced past Jupiter

on its way to Saturn revealed that the dark belts, which were

thought to be entirely regions of sinking gas, contain small rising

storm systems too small to have been seen in images by previous

probes. Evidently, the general circulation usually attributed to the

belts and zones is much more complex when it is observed in more

detail. Further understanding of the small-scale motions in Jupiter’s

atmosphere may have to await future planetary probes.

Th e highly complex spacecraft that have visited Jupiter are

examples of how technology can give scientists the raw data they

need to form their understanding of nature. Science is about

understanding nature, and Jupiter is an entirely new kind of planet

in your study. In fact, Jupiter has another feature that you did not

fi nd anywhere among the Terrestrial planets. Jupiter has a ring.

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