The Solar System

CHAPTER 23 | COMPARATIVE PLANETOLOGY OF JUPITER AND SATURN 497

cannot be anything like the rock you know on Earth. Th e center
of Jupiter is fi ve or six times hotter than the surface of the sun
and is prevented from exploding into vapor only by the tremen-
dous pressure. If there is a core, it is “rocky” only in the sense that
it contains heavy elements.
How do astronomers know Jupiter is hot inside? Infrared
observations show that Jupiter is glowing strongly in the infrared,
radiating 1.7 times more energy than it receives from the sun.
Th at observation, combined with models of its interior, provides
an estimate of its internal temperature. You will learn later in this
chapter that Saturn resembles Jupiter in this respect.
You can tell that Jupiter is mostly a liquid just by looking at
it. If you measure a photograph of Jupiter, you will discover that
it is slightly fl attened; it is a bit over 6 percent larger in diameter
through its equator than through its poles. Th is is referred to as
Jupiter’s oblateness. Th e amount of fl attening depends on the
speed of rotation and on the rigidity of the planet. Jupiter’s fl at-
tened shape shows that the planet cannot be as rigid as a
Terrestrial planet and must have a liquid interior.

Earth

Jupiter

Moon

Io

384,400 km

422,000 km

Orbital velocity

1.02 km/s

Orbital velocity

17.09 km/s

■ Figure 23-2

It is obvious that Jupiter is a very massive

planet when you compare the motion

of Jupiter’s moon Io with Earth’s moon.

Although Io is 10 percent farther from

Jupiter, it travels 17 times faster in its

orbit than does Earth’s moon around Earth.

Clearly, Jupiter’s gravitational fi eld is much

stronger than Earth’s, and that means

Jupiter must be very massive.

b hi lik h k k E h Th

■ Table 23-1 \ Composition of Jupiter and

Saturn (by Number of Molecules)

Jupiter Saturn

Molecule (%) (%)

H 2 86 93

He 13 5

H 2 O 0.1 0.1

CH 4 0.1 0.2

NH 3 0.02 0.01

Basic observations and the known laws of physics can tell

you a great deal about Jupiter’s interior. Its vast magnetic fi eld

can tell you even more.

Jupiter’s Magnetic Field

As early as the 1950s, astronomers detected radio noise coming

from Jupiter and recognized it as synchrotron radiation. Th at

form of radio energy is produced by fast electrons spiraling in a

magnetic fi eld, so it was obvious that Jupiter had a magnetic

fi eld.

In 1973 and 1974, two Pioneer spacecraft fl ew past Jupiter,

followed in 1979 by two Voyager spacecraft. Th ose probes found

that Jupiter has a magnetic fi eld about 14 times stronger than the

Earth’s fi eld. Evidently the fi eld is produced by the dynamo eff ect

operating in the highly conductive liquid metallic hydrogen as it

is circulated by convection and spun by the rapid rotation of the

planet. Th is powerful magnetic fi eld dominates a huge magneto-

sphere around the planet. Compare the size of Jupiter’s fi eld with

that of Earth in ■ Figure 23-3.

Jupiter’s magnetic fi eld defl ects the solar wind and traps

high-energy particles in radiation belts much more intense than

Earth’s. Th e radiation is more intense because Jupiter’s magnetic

fi eld is stronger and can trap and hold more particles, and

higher-energy particles, than Earth’s fi eld can. Th e spacecraft

passing through the radiation belts received radiation doses

equivalent to a billion chest X-rays—at least 100 times the lethal

dose for a human. Some of the electronics on the spacecraft were

damaged by the radiation.

You will recall that Earth’s magnetosphere interacts with the

solar wind to produce auroras, and the same process occurs on

Jupiter. Charged particles in the magnetosphere leak downward

along the magnetic fi eld, and, where they enter the atmosphere,