The Solar System

CHAPTER 23 | COMPARATIVE PLANETOLOGY OF JUPITER AND SATURN 503

rotation, coupled with the outward fl ow of heat from its hot

interior, drives a dynamo eff ect that produces a powerful mag-

netic fi eld. Th at vast magnetic fi eld traps high-energy particles

from the solar wind to form intense radiation belts and auroras.

Th e rapid rotation and large size of Jupiter cause belt–zone

circulation in its atmosphere. Heat fl owing upward from the

interior causes rising currents in the bright zones, and cooler gas

sinks in the dark belts. As on Earth, winds blow at the margins of

these regions, and large spots appear to be cyclonic disturbances.

Internal heat has been escaping since Jupiter formed, so you can

guess that Jupiter’s atmospheric circulation and storms were

stronger in the distant past and will diminish in the future.

Although the age of planet building is long past, debris in the

form of meteorites and occasional comets continue to hit Jupiter,

as it does all the planets. Any debris left over from the formation

of Jupiter would have been blown away by sunlight and the solar

wind or destroyed by other processes long ago, so the dust trapped

in Jupiter’s thin ring must be young. It probably comes from

meteorites hitting and eroding the innermost moons.

Your study of Jupiter has been challenging because it is so

unlike Earth. Most of the features and processes you found on the

Terrestrial planets are missing on Jupiter, but, as the prototype of

the Jovian worlds, it earns its place as the ruler of the solar system.

Giant Impacts in the Outer
Solar System

Comets are very common in the solar system, and Jupiter prob-
ably is hit by comets more often than most planets due to its
strong gravity. No one had ever seen it happen until 1994, when
fragments from a comet disrupted by Jupiter’s tidal forces a few
years earlier looped back and smashed into the planet. Th ose
impacts are covered more fully, including images, in Chapter 25
within the context of a discussion of the eff ects on Earth of
comet and asteroid impacts.
Th e comet collision with Jupiter was an astonishing specta-
cle, but what can it tell you about Jupiter? In fact, the separate
impacts were revealing in two ways. First, astronomers used the
impacts as probes of Jupiter’s atmosphere. By making assump-
tions about the nature of Jupiter’s atmosphere and by using
supercomputers, astronomers created models of a high-velocity
projectile penetrating into Jupiter’s upper atmosphere. By com-
paring the observed impacts with the models, astronomers were
able to fi ne-tune the models to better represent Jupiter’s atmo-
sphere. Th is method of comparing models with reality is a critical
part of science.
Second, the spectacle is a reminder that asteroids and com-
ets, the leftover construction debris from the formation of the
solar system, are expected to hit all the planets and their moons
occasionally. In 2009, Australian amateur astronomer Anthony
Wesley discovered a dark spot that appeared suddenly on Jupiter,
and alerted the world scientifi c community. Th e spot is appar-
ently the scar of a new impact by an asteroid or comet.
Astronomers estimate that Jupiter probably is hit by an object
larger than 1 km several times per century. With no solid surface,
Jupiter itself does not display its history of impacts, but its
moons do. Th e fact that Jupiter’s moons are observed to have
radically diff erent crater counts is a clue that their geologic histo-
ries have diff ered.

The History of Jupiter

Your goal in studying any planet is to be able to tell its story—to
describe how it got to be the way it is. While you can understand
part of the story of Jupiter, there is still much to learn.
If the solar nebula theory for the origin of the solar system
is correct, then Jupiter formed from the colder gases of the outer
solar nebula, where ices of water and other molecules were able
to condense. Th us, Jupiter grew rapidly and became massive
enough to capture hydrogen and helium gas from the solar
nebula and form a deep liquid hydrogen envelope. Models are
uncertain as to whether a heavy element core survives; it may
have been mixed in with the convecting liquid hydrogen enve-
lope, and astronomers estimate that the mass of Jupiter’s heavy
element core is between 0 and 10 Earth masses.
In the interior of Jupiter, hydrogen exists as liquid metallic
hydrogen, a very good electrical conductor. Th e planet’s rapid

SCIENTIFIC ARGUMENT

How do astronomers know Jupiter is hot inside?

A scientifi c argument is a way to test ideas, and sometimes it is

helpful to test even the most basic ideas. You know that something

is hot if you touch it and it burns your fi ngers, but you can’t touch

Jupiter. You also know something is hot if it is glowing bright

red—it is red hot. But Jupiter is not glowing red hot. You can tell

that something is hot if you can feel heat when you hold your hand

near it. That is, you can detect infrared radiation with your skin.

In the case of Jupiter, you would need greater sensitivity than the

back of your hand, but infrared telescopes reveal that Jupiter is a

source of infrared radiation; it is glowing in the infrared. Sunlight

would warm Jupiter a little bit, but it is emitting 70 percent more

infrared than it should. That means it must be hot inside. From

models of the interior, astronomers conclude that the center must

be fi ve or six times hotter than the surface of the sun to make the

surface of the planet glow as much as it does in the infrared.

Astronomical understanding is usually based on simple

observations, so build an argument to answer the following simple

question. How do astronomers know that Jupiter has a low

density?

Jupiter’s Family

of Moons

How many moons does Jupiter have? Astronomers are fi nding

many small moons, and the count is now over 60. (You will have

to check the Internet to get the latest fi gure because more moons

are discovered every year.) Most of these moons are small and

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