Download 3 Satellites of Other Planets

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CHAPTER 28
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Minor Bodies of the Solar System
SECTION
3 Satellites of Other Planets
KEY IDEAS
As you read this section, keep these questions in mind:
• How are the two moons of Mars similar, and how are
they different?
• How did scientists discover volcanoes on Io?
• What is one special characteristic of each of the
Galilean moons?
• How do the rings of Saturn differ from the rings of
the other outer planets?
What Are the Characteristics of Mars’s
Moons?
Mars has two tiny moons named Phobos and Deimos.
They revolve around Mars relatively quickly. Unlike most
other moons, Phobos and Deimos are not spherical.
Instead, they are chunks of rock with irregular shapes.
Scientists think these moons are actually asteroids that
were captured by Mars’s gravitational pull. At its longest,
Phobos is about 27 km across. Deimos is about 15 km
across.
Both Phobos and Deimos are dark, like the maria on the
surface of Earth’s moon. Phobos and Deimos both have
many craters. These craters indicate that many asteroids
and comets have hit the moons. Scientists think the large
number of craters means the moons are quite old.
What Are the Characteristics of Jupiter’s
Moons?
In 1610, Galileo Galilei discovered four moons orbiting
Jupiter. Since then, scientists have discovered that
Jupiter has many more than 60 moons. Most of Jupiter’s
moons are relatively small. The largest of the moons
are those that Galileo discovered. These four moons are
called the Galilean moons. They are
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READING TOOLBOX
Summarize Before you
read this section, create a
two-column table with six
rows. In the first column,
list the names of the six
main objects described in
this section (Mars, Jupiter,
Saturn, Uranus, Neptune, and
Pluto). In the second column,
describe the moons and
other characteristics of each
object.
READING CHECK
1. Explain Why do scientists
think Phobos and Deimos
are very old?
READING CHECK
2. Identify What are the four
Galilean moons?
Io
Europa
Ganymede
Callisto
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Satellites of Other Planets continued
IO
READING CHECK
3. Describe How was Io’s
volcanism discovered?
Io orbits closest to Jupiter. It is the first body other
than Earth on which scientists saw evidence of active
volcanoes. An engineer discovered the volcanoes while
studying images of Io that were taken by the Voyager
spacecraft. Volcanoes on Io eject thousands of metric
tons of material each second. The lava that erupts on Io
is even hotter than lava on Earth.
Parts of Io’s surface are yellow-red. This color is
evidence that volcanic material on Io is mostly sulfur and
sulfur dioxide. Data collected by the Galileo spacecraft
show that Io has a giant iron core and may have its own
magnetic field.
Io is one of the most
volcanically active bodies
in our solar system.
Critical Thinking
4. Compare Describe two
differences between Io and
Europa.
EUROPA
Europa is the second closest Galilean moon to
Jupiter. Europa is about the same size as Earth’s moon.
Astronomers think Europa has a rocky core that is
covered with ice up to 100 km deep. Spacecraft have
made observations of Europa. From the observations,
scientists have concluded that liquid water may exist
under the ice. If liquid water exists, it is possible that life
might also exist on Europa.
GANYMEDE
Ganymede is the third Galilean moon from Jupiter. It
is also the largest moon in the solar system. However,
the mass of Ganymede is relatively small because the
moon is made up mainly of ice mixed with rock. Images
of Ganymede show that it has dark, crater-filled areas. It
also has light areas that scientists think are long ridges
and valleys. Ganymede is the only moon in our solar
system that has its own magnetic field.
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Satellites of Other Planets continued
CALLISTO
Of the Galilean moons, Callisto is the farthest from
Jupiter. Callisto is similar to Ganymede in size, density,
and composition. However, the surface of Callisto is
much rougher than the surface of Ganymede. Its surface
has a very large number of craters.
Callisto has more craters
per square kilometer than
most other moons in the
solar system.
LOOKING CLOSER
5. Infer How did Callisto’s
craters form?
What Are the Characteristics of Saturn’s
Moons?
Saturn has dozens of moons. Many of them are small,
icy bodies that have many craters. However, five of
Saturn’s moons are quite large. Saturn’s largest moon,
Titan, has a diameter of 5,000 km.
TITAN
Unlike other moons in our solar system, Titan has
a thick atmosphere. Its atmosphere contains so many
hydrocarbons that smog hides most of Titan’s surface.
In 2005, the Huygens probe gathered data about
Titan’s atmosphere. Scientists are using the data to find
clues about how Titan and its atmosphere formed. The
probe sent back images of Titan’s surface as well. The
images show signs of a flowing liquid, which scientists
think is liquid methane.
READING CHECK
6. Identify What is the main
difference between Titan and
other moons in our solar
system?
SATURN’S OTHER MOONS
Saturn’s icy moons resemble Jupiter’s icy Galilean
moons. Enceladus has erupting geysers, so scientists
think this moon may have underground water near the
surface. The presence of water could mean Enceladus
might have conditions needed for life. Saturn’s other
smaller moons have irregular shapes.
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LOOKING CLOSER
7. Identify Name three of
Saturn’s moons other than
Enceladus and Titan.
Telesto
Calypso Helene
Enceladus
Phoebe
Hyperion
Tethys
Dione
Rhea
Titan
Iapetus
Saturn
This image shows some of Saturn’s largest moons. Distances are not to scale.
What Are the Characteristics of the Moons
of Uranus and Neptune?
READING CHECK
8. Explain How did
scientists discover most of
Uranus’s smaller moons?
Talk About It
Model With a partner, create
a model to represent the
motions of Pluto and Charon.
Together, talk about how
to use your model to figure
out whether you could see
Charon from all points on
Pluto’s surface, or whether
Charon would be visible only
from certain places.
By the mid-1800s, scientists had discovered Uranus’s
four largest moons: Oberon, Titania, Umbriel, and Ariel.
A fifth moon, Miranda, was discovered in 1948. Scientists
have recently discovered many smaller moons using
spacecraft and observatories such as the Hubble Space
Telescope. Scientists now know that Uranus has at least
24 small moons.
Neptune has at least 13 moons. Triton, which is a large
icy moon, is unusual. It revolves around Neptune in a
backward, or retrograde, orbit. Some scientists think
Triton formed somewhere else in the solar system and
was captured later by Neptune’s gravity. Triton’s diameter
is 2,705 km, and its atmosphere is thin.
What Are the Characteristics of Pluto’s
Moons?
Scientists no longer consider Pluto a planet. However,
Pluto has at least three moons. Pluto’s largest moon,
Charon, is almost half the size of Pluto. Because Charon
and Pluto are so close in mass, both bodies orbit a
common balance point called a barycenter that is located
between both bodies.
Charon orbits Pluto in 6.4 days, which is the same
length of time as one day on Pluto. Thus, Charon stays
in the same place in Pluto’s sky. In the same way that
one side of Earth’s moon always faces Earth, one side of
Pluto always faces Charon.
Pluto’s other two moons, Nix and Hydra, are much
smaller. Nix and Hydra also orbit the barycenter between
Pluto and Charon.
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Critical Thinking
Pluto
Charon
Nix
Hydra
9. Infer Why do you think
scientists had to use very
strong telescopes to discover
Nix and Hydra?
Scientists discovered Nix and Hydra in 2005 using the Hubble Space Telescope.
What Are the Characteristics of the Rings of
the Gas Giants?
Each of Saturn’s rings is divided into hundreds of
smaller rings, or ringlets. The ringlets are made up of
billions of pieces of rock and ice. The pieces range in
size from particles the size of dust to chunks as big as a
house. Each piece follows its own orbit around Saturn.
Scientists once thought that the material in Saturn’s
rings was as old as Saturn itself. Evidence now shows
that the rings are much younger. Scientists think the
particles came from a large cometlike body that entered
Saturn’s orbit and broke apart.
The other gas giants also have rings. These rings are
relatively narrow, and thus harder to detect. For instance,
Saturn’s rings were discovered more than 300 years ago,
but Jupiter’s ring was not discovered until 1979. The table
below describes the rings of the gas giants.
Characteristics of the Rings of Gas Giants
READING CHECK
10. Identify What are
Saturn’s rings made of?
LOOKING CLOSER
Planet
Description of Rings
Saturn
• hundreds of small ringlets forming larger rings
• very thin ring system
Jupiter
• single thin ring
• made up of particles that may have come from Io or
another one of Jupiter’s moons
Uranus
• 12 thin rings
Neptune
• a relatively small number of rings
• clumpy rather than thin and uniform
11. Compare How are
Neptune’s rings different
from the rings of the other
gas giants?
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Minor Bodies of the Solar System
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Section 3 Review
SECTION VOCABULARY
Galilean moon any one of the four largest
satellites of Jupiter—Io, Europa, Ganymede,
and Callisto—that were discovered by Galileo
in 1610
1. Describe Complete the table below to describe the special characteristics of the
Galilean moons.
Moon
Special Characteristics
Io
covered with a thick layer of ice; may have liquid water under the ice
largest moon in the solar system; has its own magnetic field
Callisto
2. Compare How are Io’s volcanoes different from volcanoes on Earth?
3. Describe Give one similarity and one difference between Phobos and Deimos.
4. Compare What is the main difference between Saturn’s rings and the rings of
other planets?
5. Identify How is Triton different from most other planets in the solar system?
6. Describe How do scientists think Saturn’s rings formed?
7. Infer Scientists did not discover Jupiter’s ring until 1979. What is the most likely
reason it took scientists so long to discover Jupiter’s ring?
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Minor Bodies of the Solar System