Neil F. Comins • William J. Kaufmann III
Discovering the Universe
Ninth Edition
CHAPTER 8
The Outer Planets
WHAT DO YOU THINK?
1.
2.
3.
4.
5.
Is Jupiter a “failed star?” Why or why not?
What is Jupiter’s Great Red Spot?
Does Jupiter have continents and oceans?
Is Saturn the only planet with rings?
Are the rings of Saturn solid ribbons?
In this chapter you will discover…
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Jupiter, an active, vibrant, multicolored world
more massive than all of the other planets
combined
Jupiter’s diverse system of moons
Saturn, with its spectacular system of thin, flat
rings and numerous moons, including bizarre
Enceladus and Titan
Uranus and Neptune, ice giants similar to each
other and different from Jupiter and Saturn
Close-ups of Jupiter’s Atmosphere
The dynamic winds, rapid rotation, internal heating,
and complex chemical composition of Jupiter’s
atmosphere create its beautiful and complex banded
pattern. The white oval has persisted over 40 years.
Jupiter Unwrapped
Cassini images of Jupiter were combined and opened
to give a maplike representation of the planet. The
banded structure is absent near the poles.
Jupiter’s
Great
Red Spot
The Great Red Spot circulates counterclockwise with a
period around 6 days. The clouds that encounter the
spot are forced to pass around it, and when other oval
features are near it, the entire system becomes
particularly turbulent, like batter in a blender
Creating Red Spot Jr.
For 60 years prior to 1998, the three white ovals
traveled together at the same latitude on Jupiter.
Between 1998 and 2000, they combined into one white
oval, labeled BA, which …
Creating Red Spot Jr.
… became a red spot, named Red Spot Jr., in 2006.
The Great Red Spot on Jupiter is
analogous to what activity on Earth?
 a) a hurricane
 b) a volcano
 c) a forest fire
 d) a lake
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Temperature Profile of
Jupiter’s and Saturn’s Upper Layers
Original Model of Jupiter’s Belts and Zones
The light-colored zones and dark-colored belts in
Jupiter’s atmosphere were believed to move per the
arrows. Observations by the Cassini spacecraft on its
way to Saturn suggest that just the opposite may be
correct! In either case, Jupiter’s rapid differential rotation
shapes the rising and descending gas into bands of
winds parallel to the planet’s equator.
Cutaways of
Jupiter and Saturn
The interiors of both
Jupiter and Saturn are
believed to have the four
regions shown. Their
atmospheres are thin
layers above the normal
hydrogen, which boils
upward, creating the belts
and zones. Liquid
metallic hydrogen is the
key to their magnetic
fields.
Jupiter’s Magnetosphere
The ion-trapping regions of Jupiter’s magnetosphere
(orange, analogous to the Van Allen belts) extend into
the realm of the Galilean moons. The “magnetotail”
points away from the Sun, sometimes reaching Saturn.
Jupiter’s Magnetosphere Causes Aurorae
High-energy
particles, trapped in
Jupiter’s
magnetosphere,
excite gases in the
planet’s upper
atmosphere, causing
them to glow as
aurorae.
Comet Shoemaker-Levy 9 and
its Encounters with Jupiter –
Tidal Forces in Action
The comet was torn apart by the planet’s tidal
gravitational force on July 7, 1992, fracturing into at least
21 pieces. Its returning debris, shown here in May 1994,
struck the planet between July 16 and July 22, 1994.
Comet Shoemaker-Levy 9 Impacts Jupiter
Visible (left) and ultraviolet (right) images of Jupiter
taken by the Hubble Space Telescope after three
pieces of the comet struck the planet. The darkness of
the impact sites may have come from carbon
compounds in the comet debris.
Correct diameter of Ganymede: 5268 km
Io - Closest Moon – for Volcano Lovers
These Voyager images show both sides of Io. The
range of colors results from surface deposits of sulfur
ejected from Io’s numerous volcanoes.
Io – where the action is
The ongoing lava flow from this eruption at Tvashtar
Catena has considerably altered this region of Io.
Io
This is a Galileo
image of an
eruption of Pilan
Patera on Io.
Europa
Europa’s ice surface is covered by numerous streaks
and cracks that give the satellite a fractured
appearance. The streaks are typically 20 to 40 km wide.
Io has active volcanoes that constantly
replenish its surface. The source of
energy for these volcanoes is
 a) tidal heating due to Jupiter.
 b) heat from the Sun.
 c) heat within Io left over from its
formation.
 d) radioactive decay of elements, which
keeps Io hot.
Surface Features
on Europa
This false-color Galileo image of Europa combining
visible and infrared observations shows smooth plains
of ice, mineral ridges deposited by upwelling water, and
numerous fractures probably caused by tidal stresses.
Surface Features on Europa from
Subsurface Activity - probably Liquid Water
Surface Features on Europa
Lenticulae attributed to rising warmed ice and debris
travel up from the moon’s interior by convection,
arriving at and then leaking out at the surface.
Lava Lamp Illustrates Process
Ganymede
This side of Ganymede is dominated by the huge, dark, circular
region called Galileo Regio, which is the largest remnant of
Ganymede’s ancient crust. Darker areas of the moon are older;
lighter areas are younger, tectonically deformed regions. The light
white areas in and around some craters indicate the presence of
water ice. Large impacts create white craters, filled in by ice from
below the surface.
Two Surfaces of Ganymede
The older, rougher, more heavily cratered parts of Ganymede are
the dark terrain. These regions are surrounded by younger,
smoother, less cratered bright terrain. The parallel ridges suggest
that the bright terrain has been crafted by tectonic processes.
Callisto
The outermost Galilean satellite
is almost exactly the same size
as Mercury. Numerous craters
pockmark Callisto’s icy surface.
The series of faint, concentric
rings that cover much of this
image is the result of a huge
impact that created the impact
basin Valhalla. Valhalla
dominates the Jupiter-facing
hemisphere of this frozen,
geologically inactive world.
Callisto
The two insets in this Galileo mission image show spires that contain
both ice and some dark material. The spires were probably thrown
upward as the result of an impact. The spires erode as dark material in
them absorbs heat from the Sun.
Jupiter’s Irregularly Shaped Inner Moons
Jupiter’s Rings
The top picture is a cutaway diagram of
Jupiter’s rings, which are generated
from debris blasted off the inner moons
Adrastea, Metis, Amalthea, and Thebe.
The bottom image is a portion of
Jupiter’s faint ring system,
photographed by the New Horizons
spacecraft heading to Pluto. The outer
three bright rings are composed of
pebble- to rock-sized fragments. The
rest is mostly dust. The brightest portion
of the ring is about 6000 km wide. While
the outer edge of the ring is sharply
defined, the inner edge is somewhat
fuzzy. A tenuous sheet of material
extends from the ring’s inner edge all
the way down to the planet’s cloud tops.
Jupiter’s Torus
These are quarter images of Io’s and Europa’s tori (also called
plasma tori because the gas particles in them are charged;
that is, the gases are plasmas. Some of Jupiter’s magnetic
field lines are also drawn in. Plasma from tori flow inward
along these field lines toward Jupiter.
Belts and Zones on Saturn
(a) Cassini took this extremely high resolution image of Saturn in
2007. Details as small as 53 km (33 mi) across can be seen.
There is less swirling structure between belts and zones on
Saturn than on Jupiter. (b) Combining infrared and visible
images, Cassini took this view of a hexagonal pattern of clouds
that rotates much more slowly than the surrounding belts and
zones. The pattern’s origin is still under investigation.
Merging Storms on Saturn
This sequence of Cassini images shows two
hurricane-like storms merging into one on Saturn in
2004. Each storm is about 1000 km (600 mi) across.
Saturn as Seen from Earth
Saturn’s rings are aligned with its equator, which is
tilted 27° from the plane of Saturn’s orbit around the
Sun. Therefore, Earth-based observers see the rings at
various angles as Saturn moves around its orbit. The
Earth-based photographs show how the rings seem to
disappear entirely about every 15 years.
Numerous Thin Ringlets Constitute Saturn’s Inner Rings
This Cassini image shows some of the structure of
Saturn’s rings, including some of the moonlets orbiting in
them. As moons orbit near or between rings, they often
cause the ring ices to develop ripples, like the grooves in
a phonograph record.
Saturn’s Moon Pan
Orbiting in Encke’s division, Pan is the “shepherd”
moon that keeps the division clear of small debris. It is
the innermost known moon of Saturn.
Saturn’s F Ring and One of Its Shepherds
Two tiny satellites, Prometheus and Pandora, orbit
Saturn on either side of the F ring. The gravitational
effects of these two shepherd satellites confine the
particles in the F ring to a band about 100 km wide.
Saturn’s
Complex
Ring
System
Saturn’s Ring System
Saturn’s Rings and Moons
Titan (off image on right) is 1.2 million km
(750,000 mi) from the center of Saturn.
Saturn and Its Outer, Giant Ring
This artist’s rendition of Saturn’s giant ring is drawn to
scale with an infrared image of Saturn and the rings we
normally see. The giant ring spans the region from 6 million
km (3.7 million mi) to 18 million km (11.1 million mi) beyond
Saturn. Put another way, the giant ring is as wide as 30
Saturns placed side by side.
Spokes in Saturn’s Rings
Believed to be caused by Saturn’s magnetic field
moving electrically charged particles that are lifted
out of the ring plane, these dark regions move
around the rings like the spokes on a rotating wheel.
Saturn’s Diverse Moons
(a–c) These Voyager 1, Voyager 2, and Cassini images show the
variety of surface features seen on five of Saturn’s seven spherical
moons. They are not shown to scale (refer to the diameters given
below each image). (d) In comparison, this Cassini image shows
the nonspherical moon Phoebe, almost as dark as coal, carrying
many craters, landslides, grooves, and ridges. Phoebe is barely
held in orbit by Saturn. Astronomers believe that it was captured
after wandering in from beyond the orbit of Neptune.
Saturn’s Diverse Moons
Two particularly intriguing moons are (e) Iapetus and (f) Hyperion. The
ridge running along the equator of Iapetus is believed to have developed
as the moon formed. Apparently Iapetus cooled so rapidly that the ridge
did not have time to settle away. Perhaps the most bizarre object
photographed in the solar system, Hyperion, shows innumerable impact
craters. These features are different from craters seen in other objects in
that the crater walls here have not filled in the bottom of the craters. This
moon’s low gravity and the pull of nearby Titan may explain this unusual
phenomenon.
Surface Features on Titan
(a) These are Voyager images of Titan’s smoggy atmosphere.
(b) This is a Cassini image of Titan (diameter 5150 km) showing
lighter highlands, called Xanadu, and dark, flat, lowlands that
may be hydrocarbon seas. Resolution is 4.2 km (2.6 mi).
Which statement about Saturn's rings is
most accurate?
 a) Saturn's rings are solid ribbons
 b) Saturn's rings are typically composed
of centimeter to meter-sized pieces
 c) Saturn's rings are uniformly smooth all
around
 d) Saturn's rings are several thousand
kilometers thick
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Surface Features on Titan
Riverbeds meandering across the Xanadu
highlands of Titan. These are believed to have
formed by the flow of liquid methane and ethane.
Surface Features on Titan
The Cassini Mission
to Saturn included
the Huygens Probe
which landed on
Titan and relayed
information about
its atmosphere and
its surface
The lake on the left is likely filled with liquid methane and ethane
found at Titan’s north pole. The Huygens probe took the image
on the right at Titan’s surface on January 14, 2005. What appear
like boulders here are actually pebbles strewn around the
landscape. The biggest ones are about 15 cm (6 in.) across.
Rhea
Rhea (diameter 1530 km) is heavily cratered. The bluish regions on the
inset are believed to be ices uncovered as a result of impacts.
Enceladus
(a) Cassini view of the two distinct landscapes on Enceladus, one
heavily cratered, the other nearly crater-free. The blue “tiger stripes”
are believed to be due to upwelling of liquid that froze at the surface.
(b) The crater-free region near the south pole. The ridges are thought
to be created by tectonic flows. The inset shows ice boulders.
Enceladus
Icy particles ejected from Enceladus may
be continually coming out of the moon.
Exaggerated Seasons on Uranus
Uranus’s axis of rotation is tilted so steeply that it lies nearly in
the plane of its orbit. Seasonal changes on Uranus are thus
greatly exaggerated. For example, during midsummer at
Uranus’s south pole, the Sun appears nearly overhead for
many Earth years, during which time the planet’s northern
regions are subjected to a long, continuous winter night. Half
an orbit later, the seasons are reversed.
Cutaways of Uranus and Neptune
The interiors of both Uranus and Neptune are believed to have
three regions: a terrestrial rocky core surrounded by a liquid
water mantle, which is surrounded, in turn, by liquid hydrogen
and helium. Their atmospheres are thin layers at the top of their
hydrogen and helium layers.
We know that Neptune contains a larger
proportion of heavier elements than
Saturn because Neptune :
 a) is smaller than Saturn.
 b) is larger than Saturn.
 c) has a higher density than Saturn.
 d) has a lower density than Saturn.
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The Magnetic Fields of Five Planets
This drawing shows how the magnetic fields of Earth, Jupiter,
Saturn, Uranus, and Neptune are tilted relative to their rotation
axes. Note that the magnetic fields of Uranus and Neptune are
offset from the centers of the planets and are steeply inclined to
their rotation axes. Jupiter, Saturn, and Neptune have north
magnetic poles on the hemisphere where Earth has its south
magnetic pole.
The Rings and Moons of Uranus
(Right) This is a full-scale image of Uranus and its inner and outer rings.
(Center) This image of Uranus, its rings, and eight of its moons was taken
by the Hubble Space Telescope. (Left) This close-up of part of the ring
system was taken by Voyager 2 when the spacecraft was in Uranus’s
shadow looking back toward the Sun. Numerous fine dust particles
between the main rings gleam in the sunlight. The short streaks are star
images blurred because of the spacecraft’s motion during the exposure.
Discovery of the Rings of Uranus
Uranus’ rings were discovered from Earth by “accident”
while studying its occultation of a star
Miranda
The patchwork appearance of Miranda in this mosaic of Voyager 2
images suggests that this satellite consists of huge chunks of rock and ice
that came back together after an ancient, shattering impact by an asteroid
or a neighboring Uranian moon. The curious banded features that cover
much of Miranda are parallel valleys and ridges that may have formed as
dense, rocky material sank toward the satellite’s core. At the very bottom of
the image—where a “bite” seems to have been taken out of the satellite—
is a range of enormous cliffs that jut upward as high as 20 km, twice the
height of Mount Everest.
Neptune’s Banded Structure
Several HST images at different wavelengths were combined to create
this enhanced-color view of Neptune. The dark blue and light blue areas
are the belts and zones, respectively. The dark belt running across the
middle of the image lies just south of Neptune’s equator. White areas are
high-altitude clouds, presumably of methane ice. The very highest clouds
are shown in yellow-red, as seen at the very top of the image. The green
belt near the south pole is a region where the atmosphere absorbs blue
light, probably indicating some differences in chemical composition.
Neptune
This view from Voyager 2 looks down on the southern
hemisphere of Neptune in 1989. The Great Dark Spot’s
longer dimension at the time was about the same size
as Earth’s diameter. It has since vanished. Note the
white, wispy methane clouds.
Neptune’s Rings
Two main rings are easily seen in this view alongside overexposed
edges of Neptune. In taking this image, the bright planet was hidden
so that the dim rings would be visible, hence the black rectangle
running down the center of the figure. Careful examination also reveals a
faint inner ring. A fainter-still sheet of particles, whose outer edge is
located between the two main rings, extends inward toward the planet.
Triton’s South Polar Cap (Neptune’s moon)
Approximately a dozen high-resolution Voyager 2 images were
combined to produce this view of Triton’s southern hemisphere.
The pinkish polar cap is probably made of nitrogen frost. A notable
scarcity of craters suggests that Triton’s surface was either melted
or flooded by icy lava after the era of bombardment that
characterized the early history of the solar system.
A Frozen Lake on Triton
Scientists think that the feature in the center of this
image is a basin filled with water ice. The flooded
basin is about 200 km across.
The Capture and Destruction of Triton
Disadvantage
of a retrograde
orbit
(a) Triton was captured by Neptune in a retrograde orbit. (b) The
tides that Triton then created on the planet caused that moon’s
orbit to become quite circular and (c) to spiral inward. (d) Triton
will eventually reach Neptune’s Roche limit and (e) be pulled apart
to form a ring.
The Outer
Planets:
A Comparison
Summary of Key Ideas
Jupiter and Saturn
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Jupiter is by far the largest and most massive planet in
the solar system.
Jupiter and Saturn probably have rocky cores
surrounded by a thick layer of liquid metallic hydrogen
and an outer layer of ordinary liquid hydrogen and
helium. Both planets have an overall chemical
composition very similar to that of the Sun.
The visible features of Jupiter exist in the outermost
100 km of its atmosphere. Saturn has similar features,
but they are much fainter. Three cloud layers exist in the
upper atmospheres of both Jupiter and Saturn. Because
Saturn’s cloud layers extend through a greater range of
altitudes, the colors of the Saturnian atmosphere appear
muted.
Jupiter and Saturn
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The colored ovals visible in the Jovian
atmospheres are gigantic storms, some of which
(such as the Great Red Spot) are stable and
persist for years or even centuries.
Jupiter and Saturn have strong magnetic fields
created by electric currents in their metallic
hydrogen layers.
Jupiter and Saturn
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Four large satellites orbit Jupiter. The two inner Galilean
moons, Io and Europa, are roughly the same size as our
Moon. The two outer moons, Ganymede and Callisto,
are approximately the size of Mercury.
Io is covered with a colorful layer of sulfur compounds
deposited by frequent explosive eruptions from volcanic
vents. Europa is covered with a smooth layer of frozen
water crisscrossed by an intricate pattern of long cracks.
The heavily cratered surface of Ganymede is composed
of frozen water with large polygons of dark, ancient crust
separated by regions of heavily grooved, lighter-colored,
younger terrain. Callisto has a heavily cratered ancient
crust of frozen water.
Jupiter and Saturn
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Saturn is circled by a system of thin, broad rings lying in
the plane of the planet’s equator. Each major ring is
composed of a great many narrow ringlets that consist of
numerous fragments of ice and ice-coated rock. Jupiter
has a much less substantial ring system.
Titan has a thick atmosphere of nitrogen, methane, and
other gases, as well as lakes of methane and ethane.
Enceladus has areas with very different surface features:
an older, heavily cratered region and a newer, nearly
crater-free surface created by tectonic activity.
Uranus and Neptune
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Uranus and Neptune are quite similar in appearance, mass,
size, and chemical composition. Each has a rocky core
surrounded by a thick, watery mantle topped by a layer rich in
hydrogen and helium; the axes of their magnetic fields are
steeply inclined to their axes of rotation; and both planets are
surrounded by systems of thin, dark rings.
Uranus is unique in that its axis of rotation lies near the plane
of its orbit, producing greatly exaggerated seasons on the
planet.
Uranus has five moderate-sized satellites, the most bizarre of
which is Miranda.
Triton, the largest satellite of Neptune, is an icy world with a
tenuous nitrogen atmosphere. Triton moves in a retrograde
orbit that suggests it was captured into orbit by Neptune’s
gravity. It is spiraling down toward Neptune and will eventually
break up and form a ring system.
Key Terms
A ring
B ring
Belt
C ring
Cassini division
differential rotation
Encke division
F ring
Galilean moon (satellite)
Great Dark Spot
Great Red Spot
hydrocarbon
liquid metallic hydrogen
occultation
polymer
prograde orbit
resonance
retrograde orbit
ringlet
Roche limit
shepherd satellite (moon)
spoke
zone (atmospheric)
WHAT DID YOU THINK?
Is Jupiter a “failed star”? Why or why not?
 No. Jupiter has 75 times too little mass to
shine as a star.
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WHAT DID YOU THINK?
What is Jupiter’s Great Red Spot?
 The Great Red Spot is a long-lived, oval
cloud circulation, similar to a hurricane on
Earth.
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WHAT DID YOU THINK?
Does Jupiter have continents and oceans?
 No. Jupiter is surrounded by a thick
atmosphere primarily composed of
hydrogen and helium that gradually
becomes liquid as you move inward. The
only solid matter in Jupiter is its core.
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WHAT DID YOU THINK?
Is Saturn the only planet with rings?
 No. All four giant planets (Jupiter, Saturn,
Uranus, and Neptune) have rings.
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WHAT DID YOU THINK?
Are the rings of Saturn solid ribbons?
 No. Saturn’s rings are all composed of
thin, closely spaced ringlets consisting of
particles of ice and ice-coated rocks. If
they were solid ribbons, Saturn’s
gravitational tidal force would tear them
apart.
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