Download 3 The Inner Planets

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CHAPTER 27
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Planets of the Solar System
3 The Inner Planets
SECTION
KEY IDEAS
As you read this section, keep these questions in mind:
• What are the basic characteristics of the inner planets?
• What are some similarities and differences between
the inner planets?
• What planetary features allow Earth to sustain life?
What Are the Inner Planets?
The inner planets are the planets closest to the sun.
These planets are Mercury, Venus, Earth, and Mars. They
are also called terrestrial planets, because they are
all rocky, like Earth. In Latin, terra means “earth.” The
terrestrial planets are made up mostly of solid rock, and
they have metallic cores. Their surfaces have bowl-shaped
depressions called impact craters. These craters formed
when other objects in space collided with the planets.
What Are the Characteristics of Mercury?
Mercury is the planet closest to the sun. It circles the
sun every 88 Earth days, and it rotates on its axis once
every 59 Earth days. Its surface has many craters, and
it has cliffs that are hundreds of kilometers long. These
cliffs may be wrinkles that developed in the crust when
the molten core cooled and shrank.
Mercury has almost no atmosphere. The lack of an
atmosphere and a slow rotation cause temperatures on
Mercury to vary widely. During the day, the temperature
may reach as high as 427 °C. At night, the temperature
may drop to –173 °C.
READING TOOLBOX
Organize As you read this
section, use a four-corner
fold to compare the inner
planets. Label the outer
flaps with the planet names.
Describe each planet
underneath the correct flap.
READING CHECK
1. Explain What are two
characteristics the terrestrial
planets have in common?
Mercury
Crust
Mantle
Characteristics of Mercury
Core
Diameter
4,880 km, or 38% of Earth’s diameter
Density
5.4 g/cm3, or 98% of Earth’s density
Surface
gravity
38% of Earth’s surface gravity
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The Inner Planets continued
Critical Thinking
2. Identify How long does
it take for Venus to revolve
around the sun?
What Are the Characteristics of Venus?
Venus is the second planet from the sun. It has an
orbital period of 225 Earth days. In some ways, Venus is
much like Earth. The two planets have similar size, mass,
and density. However, Venus rotates very slowly, only
once every 243 Earth days. Venus and Earth also differ in
other ways.
Venus
Crust
Mantle
Core
LOOKING CLOSER
3. Compare Which planet is
smallest—Mercury, Venus, or
Earth?
Characteristics of Venus
Diameter
12,100 km, or 95% of Earth’s diameter
Density
5.2 g/cm3, or 95% of Earth’s density
Surface gravity
91% of Earth’s surface gravity
VENUS’S ATMOSPHERE
READING CHECK
4. Identify Which two
features are responsible for
high temperatures on Venus?
Venus and Earth have very different atmospheres. The
atmospheric pressure of Venus is 90 times the pressure
on Earth. Venus’s atmosphere is composed of about
96% carbon dioxide. High carbon dioxide levels and the
planet’s nearness to the sun make temperatures on Venus
very high.
The large amount of carbon dioxide prevents most
of the radiation that enters Venus’s atmosphere from
escaping. The heat remains trapped at Venus’s surface,
raising temperatures. This process is called a greenhouse
effect. On Venus, a runaway greenhouse effect raises the
average surface temperature to 464 °C.
Venus also has sulfur dioxide droplets in its upper
atmosphere. These droplets form a cloud layer that
reflects sunlight. The reflection is so strong that Venus is
one of the brightest objects in the night sky.
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The Inner Planets continued
SURFACE FEATURES OF VENUS
Scientists have used probes and satellites to study
Venus. Data show that Venus has a rocky landscape. The
surface is made of basalt and granite. Scientists also
discovered landforms such as mountains, volcanoes, lava
plains, and sand dunes.
The surface of Venus has some craters. All the craters
are about the same age, and they are relatively young.
This evidence makes scientists think that volcanic
activity changes Venus’s surface. Energy inside the planet
heats the interior over time. Volcanoes erupt and cover
the surface of the planet with lava. The lava buries older
craters. Scientists think that this kind of event probably
won’t happen again for another 100 million years.
What Are the Characteristics of Earth?
Earth is the third planet from the sun. Its orbital period
is 365.25 days. Earth completes one rotation on its axis
about every 24 hours.
Earth has had a very active geologic history. Over the
last 250 million years, Earth’s continents separated from
a single landmass and moved to their present positions.
Weathering and erosion have changed (and continue to
change) the surface of Earth.
Surface temperatures on Earth range from −90 °C
to 60 °C. Earth’s average temperature is 15 °C. Earth’s
unique atmosphere and distance from the sun allow
water to exist in a liquid state here. Mercury and Venus
are so close to the sun that any liquid water would boil.
The other planets are so far from the sun that any water
would freeze. Earth is the only planet scientists know of
that has oceans of liquid water.
READING CHECK
5. Describe What evidence
do scientists have that
volcanic activity completely
changes the surface of
Venus?
Earth
Lithosphere
Asthenosphere
Mesosphere
Characteristics of Earth
Outer
core
Inner
core
Diameter
12,756 km
Density
5.515 g/cm3
Surface gravity
9.8 m/s2
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The Inner Planets continued
Critical Thinking
6. Apply Concepts What
would have happened
if Earth’s oceans did not
absorb carbon dioxide in the
atmosphere?
LIFE ON EARTH
Earth is the only known planet that has the proper
combination of water, temperature, and oxygen to
support life. Scientists think that as oceans formed on
Earth, carbon dioxide from the atmosphere dissolved
in the oceans. This process kept carbon dioxide from
building up in the atmosphere. With less carbon dioxide
in the atmosphere, less heat was trapped at Earth’s
surface.
As a result, Earth maintained the moderate
temperatures living things need to survive. Plants and
cyanobacteria added oxygen to the atmosphere. As
oxygen levels increased, living things that breathe
oxygen began to develop.
What Are the Characteristics of Mars?
READING CHECK
7. Explain Why do Mars and
Earth have similar seasons?
Mars is the fourth planet from the sun. It has an
average distance of about 228 million kilometers from the
sun. Its orbital period is 687 Earth days, and it rotates on
its axis every 24 h 37 min. Mars’s axis tilts at almost the
same angle that Earth’s axis does. Thus, Mars’s seasons
are similar to those on Earth.
Massive volcanoes on the surface of Mars are evidence
that the planet was once geologically active. A system of
deep canyons also covers part of the surface. Scientists
think that the canyons are cracks that formed in the crust
as the planet cooled. Water may have then further eroded
the cracks.
Mars
Crust
Critical Thinking
8. Apply Prior Knowledge
On which planet would you
weigh more—Earth or Mars?
Explain your answer.
Mantle
Core
Characteristics of Mars
Diameter
6,800 km, or 53% of Earth’s diameter
Density
3.9 g/cm3, or 71% of Earth’s density
Surface gravity
38% of Earth’s surface gravity
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The Inner Planets continued
MARTIAN VOLCANOES
Tharsis Montes is one of several volcanic regions on
Mars. Volcanoes in this region are 100 times as large as
Earth’s largest volcano. The biggest volcano on Mars is
Olympus Mons. It is about three times as tall as Mount
Everest. Its base is about the size of Nebraska.
Scientists think that Olympus Mons has grown so large
because Mars has no moving tectonic plates. As a result,
the mountain may have built up around a steady magma
source for millions of years.
Scientists are not yet sure whether Martian volcanoes
are still active. However, they have observed certain
events that produced seismic waves. These events are
called marsquakes.
WATER ON MARS
The pressure and temperature on Mars are too low
for water to exist as a liquid on the planet’s surface.
However, several NASA missions have found evidence
that liquid water did exist on the surface in the past.
Many surface features look like water eroded them. Other
features might be evidence of flood plains produced by
massive volumes of water.
The surface temperatures on Mars can range from
20 °C to –130 °C, depending on location and season.
Although most of the water on Mars is trapped in polar
icecaps, frozen water also exists below the surface.
If liquid water exists below Mars’s surface, the odds
of finding life on the planet would increase. However,
scientists have not yet found evidence for life on Mars.
READING CHECK
9. Explain Why do scientists
look at Mars for signs of life?
Scientists think liquid water possibly formed these gullies on Mars.
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Section 3 Review
SECTION VOCABULARY
terrestrial planet one of the highly dense
planets nearest to the sun; Mercury, Venus,
Mars, and Earth
1. Identify Relationships What is the relationship between the terms inner planet
and terrestrial planet?
2. Summarize Use the graphic organizer below to describe the characteristics of the
inner planets.
Mercury
Orbital Period
Rotation Period
Temperature Range
88 Earth days
59 Earth days
–173 °C to 427 °C
Venus
Size Ranking (1 is
largest, 4 is smallest)
up to 464 °C
Earth
1
Mars
3. Identify What characteristics of Earth make it possible for life to exist here?
4. Describe Why does Mercury have such a large daily temperature range?
5. Explain Why does Venus have such a high surface temperature?
6. Explain Why do scientists think that the volcanoes on Mars may still be active?
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Planets of the Solar System