Download The outer solar system has four giant planets.

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KEY CONCEPT
The outer solar system
has four giant planets.
BEFORE, you learned
NOW, you will learn
• Planets formed along with
the Sun
• Vast distances separate planets
• The gravity of a terrestrial
planet may be strong enough
to hold the heavier gases
• About the four giant planets in
the solar system
• What the atmospheres of giant
planets are like
• About the rings of giant planets
VOCABULARY
THINK ABOUT
gas giant p. 734
ring p. 737
What is Jupiter like inside?
Most of Jupiter’s huge mass is
hidden below layers of clouds.
Scientists learn about Jupiter by
studying its gravity, its magnetic
field, its motions, and its radiation.
Scientists also use data from other
space bodies to make models,
from which they make predictions. Then they observe Jupiter
to test their predictions. What might it be like under Jupiter’s clouds?
The gas giants have very deep atmospheres.
VOCABULARY
Remember to draw a
word triangle when you
read a new term.
You have already read about the four rocky planets in the inner solar
system, close to the Sun. Beyond Mars stretches the outer solar system,
where the four largest planets slowly orbit the Sun. The gas giants —
Jupiter, Saturn, Uranus (YUR-uh-nuhs), and Neptune—are made
mainly of hydrogen, helium, and other gases.
When you think of gases, you probably think of Earth’s air, which
is not very dense. However, the giant planets are so large and have
such large amounts of these gases that they have a lot of mass.
The huge gravitational force from such a large mass is enough to pull
the gas particles close together and make the atmosphere very dense.
Inside the giant planets, the gases become more dense than water.
The outermost parts are less dense and more like Earth’s atmosphere.
check your reading
734 Unit 5: Space Science
Why are the gas giants dense inside?
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The atmosphere of a giant planet
Interior of a Giant Planet
is very deep. Imagine traveling into
one. At first, the atmosphere is thin
Jupiter
and very cold. There may be a haze
hydrogen—gas
of gases. A little lower is a layer of
and liquid
clouds that reflect sunlight, just like
hydrogen—liquid
clouds on Earth. There are strong
metal
winds and other weather patterns.
dense, hot core
Lower down, it is warmer and there
are layers of clouds of different
materials. As you go farther, the
atmosphere gradually becomes
dense enough to call a liquid. It also
gets thousands of degrees hotter as
you get closer to the center of the
planet. The materials around you
become more and more dense until they are solid. Scientists think
Jupiter
that each of the four gas giants has a solid core, larger than Earth,
Jupiter’s colorful stripes
deep in its center.
are produced by clouds at
different levels in Jupiter’s
deep atmosphere.
Jupiter is a world of storms and clouds.
Jupiter is the largest planet in the solar system. It is more than 10
times larger than Earth in diameter and more than 1200 times
larger in volume. A jet plane that could circle Earth in
about 2 days would take 23 days to circle Jupiter.
If you could weigh the planets on a cosmic
scale, all the other planets put together
would weigh less than half as much
as Jupiter.
Mass 318 Earth masses
Diameter 11 Earth diameters
Average distance
from Sun 5.2 AU
Orbits in 12 Earth years
Rotates in 9.9 hours
Jupiter is more than five times
farther from the Sun than Earth is.
It moves more slowly through space
than Earth and has a greater
distance to travel in each orbit.
Jupiter takes 12 Earth years to go
once around the Sun.
Even though it is big, Jupiter
takes less than 10 hours to turn once
on its axis. This fast rotation produces
fast winds and stormy weather. Like
Earth, Jupiter has bands of winds that
blow eastward and westward, but Jupiter
has many more bands than Earth does.
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Great Red Spot
Stripes of cold clouds form along the bands.
The clouds look white because they are made of crystals
that reflect sunlight. The crystals in these high white
clouds are frozen ammonia rather than frozen water,
as on Earth. Between Jupiter’s white bands of clouds,
you can see down to the next layer. The lower clouds
are brown or red and made of different chemicals.
Sometimes there are clear patches in the brown clouds, where the next
layer of bluish clouds shows through.
moon
shadow
This image shows one of
Jupiter’s moons casting a
shadow on Jupiter. If you
were in that shadow,
you would experience a
solar eclipse.
check your reading
What are Jupiter’s white stripes?
Storms can form between bands of winds that blow in opposite
directions. Because Jupiter has no land to slow the storms, they can last
for a long time. The largest of these storms is the Great Red Spot, which
is twice as wide as Earth and at least 100 years old. Its clouds rise even
higher than the white ammonia-ice clouds. Scientists are trying to find
out which chemicals produce the spot’s reddish color.
Saturn has large rings.
reminder
Density is the amount of
mass in a given volume. An
object of low density can
still have a great total mass
if it has a large volume.
The sixth planet from the Sun is Saturn. Saturn is only a little smaller
than Jupiter, but its mass is less than one-third that of Jupiter. Because
there is less mass, the gravitational pull is weaker, so the gas particles
can spread out more. As a result, Saturn has a much lower density
than Jupiter. The storms and stripes of
clouds form deeper in Saturn’s
atmosphere than in Jupiter’s, so
the details are harder to see.
Saturn
Saturn has an average density less than that of liquid water
on Earth. The diameter of Saturn’s ring system is almost as
great as the distance from Earth to the Moon.
Mass 95 Earth masses
Diameter 9 Earth diameters
Average distance from Sun 9.5 AU
736 Unit 5: Space Science
Orbits in 29 Earth years
Rotates in 11 hours
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Saturn was the first planet known to have rings. A planetary ring is a wide, flat zone of small particles that orbit a
planet. All four gas giants have rings around their equators.
Saturn’s rings are made of chunks of water ice the size of a
building or smaller. Larger chunks, considered to be tiny
moons, orbit within the rings. Saturn’s main rings are very
bright. The outermost ring is three times as wide as the planet,
but it is usually too faint to see. Saturn’s rings have bright and
dark stripes that change over time.
gap between
rings
shadow
You can use Saturn’s rings to see the planet’s seasons. Like Earth’s
axis of rotation, Saturn’s axis is tilted. The angle is 27 degrees. When the
image on this page was taken, sunlight shone more on the northern
hemisphere, so the north side of the rings was bright. The shadow of
the rings fell on the southern hemisphere. Winter started in Saturn’s
northern hemisphere in May 2003 and will last more than seven Earth
years. Saturn is almost ten times farther from the Sun than Earth is, so
Saturn takes almost 30 Earth years to go around the Sun once.
Sunlight shines from the
upper right of this image.
The rings cast shadows
on Saturn‘s clouds.
Giant Planets
Why do Saturn’s rings seem to change size?
SKILL FOCUS
Observing
PROCEDURE
1
Poke the stick through the plate and cut off the plate’s rim. Shape the clay
onto both sides of the plate to make a model of a planet with rings.
2 Model Saturn’s orbit for your partner. Stand between your partner and the
classroom clock. Point one end of the stick at the clock. Hold the model at
the same height as your partner’s eyes. Have your partner watch the model
with just one eye open.
3 Move one step counterclockwise around your partner and
point the stick at the clock again. Make sure the model is as
high as your partner’s eyes. Your partner may need to turn to
see the model.
MATERIALS
•
•
•
•
ice-cream stick
disposable plate
scissors
clay
TIME
20 minutes
4 Continue taking steps around your partner and pointing the
stick at the clock until you have moved the model all the
way around your partner.
5 Switch roles with your partner and repeat steps 2, 3, and 4.
WHAT DO YOU THINK?
• How did your view of the rings change as the model planet
changed position?
• How many times per orbit do the rings seem to vanish?
CHALLENGE How do Saturn’s axis and orbit compare with
those of Earth?
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Uranus and Neptune are extremely cold.
The seventh and eighth planets from the Sun are Uranus and Neptune.
These planets are similar in size—both have diameters roughly one-third
that of Jupiter. Unlike Jupiter and Saturn, Uranus and Neptune are
only about 15 percent hydrogen and helium. Most of the mass of each
planet is made up of heavier gases, such as methane, ammonia, and
water. As a result, Uranus and Neptune are more dense than Jupiter.
Uranus is a smooth bluegreen in visible light.
The small infrared image
shows that the pole facing
the Sun is warmer than
the equator.
Uranus looks blue-green, and Neptune appears deep blue. The
color comes from methane gas, which absorbs certain colors of light.
Each planet has methane gas above a layer of white clouds. Sunlight
passes through the gas, reflects off the clouds, then passes through the
gas again on its way out. The gas absorbs the red, orange, and yellow
parts of sunlight, so each planet’s bluish color comes from the remaining
green, blue, and violet light that passes back out of the atmosphere.
rings
Uranus
Uranus is about twice Saturn’s distance from the Sun. The farther
a planet is from the Sun, the more slowly it moves along its orbit.
The greater distance also results in a larger orbit, so it takes Uranus
84 Earth years to travel around the Sun.
pole
Like the other gas giants, Uranus has a system of rings and moons
around its equator. The ring particles and moons orbit Uranus in the
same direction as the planet’s spin. Unlike the other planets, Uranus
has an axis of rotation that is almost in the plane of its orbit.
As a result, Uranus seems to spin on its side. During a
solstice, one pole of Uranus points almost straight
toward the Sun.
Image not available for use
on this CD-ROM. Please
refer to the image in the
textbook.
Some scientists think that there was a
large collision early in Uranus’s history.
The result left the planet and its system
spinning at an unusual angle.
Uranus
Each pole of Uranus experiences more than 40 years
of sunlight and then more than 40 years of darkness
as the planet orbits the Sun.
Mass 15 Earth masses
Diameter 4 Earth diameters
Average distance from
Sun 19 AU
738 Unit 5: Space Science
Orbits in 84 Earth years
Rotates in 17 hours
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Neptune
Neptune orbits about 10 AU farther from the Sun than Uranus,
so you would expect it to be colder. However, Neptune has
about the same outside temperature as Uranus because it is
hotter inside.
Uranus is usually one smooth color, but light and dark areas
often appear on Neptune. Clouds of methane ice crystals
can form high enough in the atmosphere of
Neptune to look white.
Storm systems can appear in darker
shades of blue than the rest of the
planet. One storm, seen during the
flyby of the Voyager 2 spacecraft in
1989, was named the Great Dark
Spot. Unlike the huge storm on
Jupiter, the Great Dark Spot did
not stay at the same latitude. It
moved toward Neptune’s equator.
The winds there may have broken
up the storm. Images of Neptune
obtained a few years later with the
Hubble Space Telescope showed no
sign of the Great Dark Spot.
check your reading
What are the white patches
often seen on Neptune?
Neptune
Neptune has a large moon
that orbits in a direction
opposite to Neptune’s rotation. Scientists think a giant
collision might have occurred
in Neptune’s past.
Mass 17 Earth masses
Diameter 4 Earth diameters
Average distance
from Sun 30 AU
Orbits in 164 Earth years
Rotates in 16 hours
High clouds cast shadows
on the layer below.
cloud
shadow
KEY CONCEPTS
CRITICAL THINKING
1. Which planet has a greater
mass than all the other planets
put together?
4. Compare and Contrast
Why do Jupiter and Saturn
show a lot of white, while
Uranus and Neptune are
more blue in color?
2. What do you see instead of a
solid surface when you look at
an image of a giant planet?
3. Which planets have rings?
CHALLENGE
6. Apply If Uranus had areas
of ice crystals high in its
atmosphere, how would its
appearance change?
5. Analyze Most of Saturn is
much less dense than most
of Earth. Yet Saturn’s mass is
much greater than Earth’s
mass. How can this be so?
Chapter 21: Our Solar System 739