Neil F. Comins • William J. Kaufmann III Discovering the Universe

Neil F. Comins • William J. Kaufmann III Discovering the Universe
Eighth Edition CHAPTER 8 The Outer Planets The most complete visualization of Saturn and its ring system, taken by the Cassini spacecraft. This image is a combination of ultraviolet, visible light, and infrared observations. (Courtesy NASA/JPL-Caltech)

Key Questions…. How do we know what we do? What value is there to ask these questions, and build probes to answer them?

Essay Questions for the Final
Describe Jupiter's atmosphere. What are the similarities and differences between Jupiter and Saturn? Describe Jupiter's four largest satellites. Why are they similar, and why are they different?

Essay Questions for the Final
Describe Saturn and its Rings. How did they probably arise? How do we know? What do we now know about Titan? What were the scientific questions we wanted to answer by exploring Titan as we did? What are the properties shared by Uranus and Neptune? How are they unique?

In this chapter you will discover…
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 What Uranus and Neptune have in common and how they differ from Jupiter and Saturn

8.1 A Different Kind of Planet
Our goals for learning: What are jovian planets made of? What are jovian planets like on the inside? What is the weather like on jovian planets?

What are jovian planets made of?

Jovian Planet Composition
Jupiter and Saturn Mostly H and He gas Uranus and Neptune Mostly hydrogen compounds: water (H2O), methane (CH4), ammonia (NH3) Some H, He, and rock

Jovian Planet Formation
Beyond the frost line, planetesimals could accumulate ICE. Hydrogen compounds are more abundant than rock/metal so jovian planets got bigger and acquired H/He atmospheres.

Jovian Planet Formation
The jovian cores are very similar: ~ mass of 10 Earths The jovian differences are in the amount of H/He gas accumulated. Why did that amount differ?

Differences in Jovian Planet Formation
TIMING: The planet that forms earliest captures the most hydrogen and helium gas. Capture ceases after the first solar wind blew the leftover gas away. LOCATION: The planet that forms in a denser part of the nebula forms its core first. Like real estate sales, I guess…

Density Differences Uranus and Neptune are denser than Saturn because they have less H/He, proportionately.

Density Differences But that explanation doesn’t work for Jupiter.

Sizes of Jovian Planets
Adding mass to a jovian planet compresses the underlying gas layers.

Sizes of Jovian Planets
Greater compression is why Jupiter is not much larger than Saturn even though it is three times more massive. Jovian planets with even more mass can be smaller than Jupiter.

What are jovian planets like on the inside?

Interiors of Jovian Planets
No solid surface Layers under high pressure and temperatures Cores (~10 Earth masses) made of hydrogen compounds, metals, and rock The layers are different for the different planets — WHY?

Inside Jupiter High pressure inside of Jupiter causes the phase of hydrogen to change with depth. Hydrogen acts like a metal at great depths because its electrons move freely.

Inside Jupiter The core is thought to be made of rock, metals, and hydrogen compounds. The core is about the same size as Earth but 10 times as massive.

Comparing Jovian Interiors
Models suggest that cores of jovian planets have similar composition. Lower pressures inside Uranus and Neptune mean no metallic hydrogen.

Jupiter’s Magnetosphere
Aurora on Jupiter Jupiter’s strong magnetic field gives it an enormous magnetosphere. Gases escaping Io feed the donut-shaped Io torus.

What is the weather like on jovian planets?

Jupiter’s Atmosphere Hydrogen compounds in Jupiter form clouds.
Different cloud layers correspond to freezing points of different hydrogen compounds. Other jovian planets have similar cloud layers.

Jupiter’s Colors Ammonium sulfide clouds (NH4SH) reflect red/brown.
Ammonia, the highest, coldest layer, reflects white.

Saturn’s Colors Saturn’s layers are similar but are deeper in and farther from the Sun — more subdued.

Methane on Uranus and Neptune
Methane gas on Neptune and Uranus absorbs red light but transmits blue light. Blue light reflects off methane clouds, making those planets look blue.

Jupiter’s Great Red Spot
A storm twice as wide as Earth Has existed for at least 3 centuries

Weather on Jovian Planets
All the jovian planets have strong winds and storms.

Thought Question Jupiter does not have a large metal core like the Earth. How can it have a magnetic field? The magnetic field is left over from when Jupiter accreted. Its magnetic field comes from the Sun. It has metallic hydrogen inside, which circulates and makes a magnetic field. That’s why its magnetic field is weak.

What have we learned? What are jovian planets made of?
Jupiter and Saturn are mostly made of H and He gas. Uranus and Neptune are mostly made of H compounds. What are jovian planets like on the inside? They have layered interiors with very high pressure and cores made of rock, metals, and hydrogen compounds. Very high pressure in Jupiter and Saturn can produce metallic hydrogen.

What have we learned? What is the weather like on jovian planets?
Multiple cloud layers determine the colors of jovian planets. All have strong storms and winds.

8.2 A Wealth of Worlds: Satellites of Ice and Rock
Our goals for learning: What kinds of moons orbit the jovian planets? Why are Jupiter’s Galilean moons geologically active? What geological activity do we see on Titan and other moons? Why are jovian planet moons more geologically active than small rocky planets?

What kinds of moons orbit the jovian planets?

Sizes of Moons Small moons (< 300 km)
No geological activity Medium-sized moons (300–1,500 km) Geological activity in past Large moons (> 1,500 km) Ongoing geological activity

Medium and Large Moons Enough self-gravity to be spherical
Have substantial amounts of ice Formed in orbit around jovian planets Circular orbits in same direction as planet rotation

Small Moons Far more numerous than the medium and large moons
Not enough gravity to be spherical: “potato-shaped”

Why are Jupiter’s Galilean moons geologically active?

Io’s Volcanic Activity
Io is the most volcanically active body in the solar system, but why?

Io’s Volcanoes Volcanic eruptions continue to change Io’s surface.
Io Volcanoes IR

Tidal Heating Io is squished and stretched as it orbits Jupiter.
But why is its orbit so elliptical?

The tugs add up over time, making all three orbits elliptical.
Orbital Resonances Every 7 days, these three moons line up.

Europa’s Ocean: Waterworld?

Tidal Stresses Crack Europa’s Surface Ice

Tidal stresses crack Europa’s surface ice
Tidal flexing closes crack, grinds up ice Tidal flexing opens crack, leaving two ridges

Europa’s Interior Also Warmed by Tidal Heating
Based on Galileo spacecraft measurements of the strength of gravity over different positions of Europa and theoretical modeling of the interior.

Ganymede Largest moon in the solar system
Clear evidence of geological activity Tidal heating plus heat from radio-active decay? Largest moon Figure might work better if there were one with out the callouts.

Callisto “Classic” cratered iceball
No tidal heating, no orbital resonances But it has magnetic field !?

Thought Question How does Io get heated by Jupiter? Auroras
Infrared light Jupiter pulls harder on one side than the other Volcanoes

What geological activity do we see on Titan and other moons?

Titan’s Atmosphere Titan is the only moon in the solar system which has a thick atmosphere. It consists mostly of nitrogen with some argon, methane, and ethane.

Titan’s Surface The Huygens probe provided a first look at Titan’s surface in early 2005. It had liquid methane, “rocks” made of ice.

Titan’s “Lakes” Radar imaging of Titan’s surface has revealed dark, smooth regions that may be lakes of liquid methane.

Medium Moons of Saturn Almost all show evidence of past volcanism and/or tectonics.

Ongoing Activity on Enceladus
Fountains of ice particles and water vapor from the surface of Enceladus indicate that geological activity is ongoing.

Medium Moons of Uranus Varying amounts of geological activity occur.
Moon Miranda has large tectonic features and few craters (episode of tidal heating in past?).

Neptune’s Moon Triton Similar to Pluto, but larger
Evidence for past geological activity

Why are jovian planet moons more geologically active than small rocky planets?

Rocky Planets vs. Icy Moons
Rock melts at higher temperatures. Only large rocky planets have enough heat for activity. Ice melts at lower temperatures. Tidal heating can melt internal ice, driving activity.

What have we learned? What kinds of moons orbit jovian planets?
Moons of many sizes Level of geological activity depends on size Why are Jupiter’s Galilean moons geologically active? Tidal heating drives activity, leading to Io’s volcanoes and ice geology on other moons.

What have we learned? What geological activity do we see on Titan and other moons? Titan is the only moon with a thick atmosphere. Many other icy moons show signs of geological activity. Why are jovian planet moons more geologically active than small rocky planets? Ice melts and deforms at lower temperatures enabling tidal heating to drive activity.

8.3 Jovian Planet Rings Our goals for learning:
What are Saturn’s rings like? Why do the jovian planets have rings?

What are Saturn’s rings like?

What are Saturn’s rings like?
They are made up of numerous, tiny individual particles. They orbit over Saturn’s equator. They are very thin.

Earth-Based View

Spacecraft View of Ring Gaps
Voyager

Artist’s Conception of Close-Up

Gap Moons Some small moons create gaps within rings.

Why do the jovian planets have rings?

Jovian Ring Systems All four jovian planets have ring systems.
Others have smaller, darker ring particles than does Saturn.

Why do the jovian planets have rings?
They formed from dust created in impacts on moons orbiting those planets. How do we know this?

How do we know? Rings aren’t leftover from planet formation because the particles are too small to have survived this long. There must be a continuous replacement of tiny particles. The most likely source is impacts with the jovian moons.

Ring Formation Jovian planets all have rings because they possess many small moons close-in. Impacts on these moons are random. Saturn’s incredible rings may be an “accident” of our time.

What have we learned? What are Saturn’s rings like?
They are made up of countless individual ice particles. They are extremely thin with many gaps. Why do the jovian planets have rings? Ring systems of other jovian planets are much fainter with smaller, darker, less numerous particles. Ring particles are probably debris from moons.

FIGURE 8-1 Jupiter as Seen from a Spacecraft
This view was sent back from Voyager 1 in Features as small as 600 km across can be seen in the turbulent cloud tops of this giant planet. The complex cloud motions that surround the Great Red Spot are clearly visible. Also, clouds at different latitudes have different rotation rates. The inset image of Earth shows its size relative to Jupiter. (NASA/JPL; inset: NASA)

FIGURE 8-2 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. (a) A Voyager 2 southern hemisphere image showing a white oval that has existed for over 40 years. (b) A Voyager 2 northern hemisphere image showing a brown oval. The white feature overlapping the oval is a high cloud. (NASA)

FIGURE 8-3 Jupiter Unwrapped
Cassini images of Jupiter combined and opened to give a map like representation of the planet. The banded structure is absent near the poles. The Web link will take you to a movie version of this and related images. In them, you will see that the light and dark regions slide by one another, continually moving eastward or westward. (Courtesy NASA/JPL-Caltech)

FIGURE 8-4 The Great Red Spot
This true color image of the Great Red Spot, taken by Galileo in 1996, shows what this giant storm would look like if you were traveling over it in a spacecraft. The counterclockwise circulation of gas in the Great Red Spot takes about 6 days to make one rotation. The clouds that encounter it are forced to pass around it, and when other oval features are near it, the entire system becomes particularly turbulent, like the batter in a two-bladed blender. (Courtesy NASA/JPL-Caltech)

FIGURE 8-5 Creating Red Spot Jr.
(a–d) For 60 years prior to 1998, the three white ovals labeled FA, DE, and BC traveled together at the same latitude on Jupiter. Between 1998 and 2000, they combined into one white oval, labeled BA, which (e) became a red spot, named Red Spot Jr., in (a–d: NASA/ JPL/WFPC2; e: NASA, ESA, A. Simon-Miller [NASA/GSFC], and I. de Pater [University of California Berkeley])

FIGURE 8-6 Jupiter’s and Saturn’s Upper Layers
These graphs display temperature profiles of (a) Jupiter’s and (b) Saturn’s upper regions, as deduced from measurements at radio and infrared wavelengths. Three major cloud layers are shown in each, along with the colors that predominate at various depths. Data from the Galileo spacecraft indicate that Jupiter’s cloud layers are not found at all locations around the planet; there are some relatively clear, cloud-free areas.

FIGURE 8-7 Original Model of Jupiter’s Belts and Zones
The light-colored zones and dark-colored belts in Jupiter’s atmosphere were believed, until recently, to be regions of rising and descending gases, respectively. In the zones, gases warmed by heat from Jupiter’s interior were thought to rise upward and cool, forming high-altitude clouds. In the belts, cooled gases were thought to descend and undergo an increase in temperature; the cloud layers seen there are at lower altitudes than in the zones. Observations by the Cassini spacecraft on its way to Saturn suggest that just the opposite may be correct (stay tuned)! In either case, Jupiter’s rapid differential rotation shapes the rising and descending gas into bands of winds parallel to the planet’s equator. Differential rotation also causes the wind velocities at the boundaries between belts and zones to move predominantly to the east or west.

FIGURE 8-8 Cutaways of Jupiter and Saturn
The interiors of both Jupiter and Saturn are believed to have four regions: a terrestrial rocky core, a liquid “ice” shell, a metallic hydrogen shell, and a normal liquid hydrogen mantle. Their atmospheres are thin layers above the normal hydrogen, which boils upward, creating the belts and zones.

FIGURE 8-9 Jupiter’s Magnetosphere
Created by the planet’s rotation, the ion-trapping regions of Jupiter’s magnetosphere (in orange, analogous to the Van Allen belts) extend into the realm of the Galilean moons. Gases from Io and Europa form tori in the magnetosphere. Some of Io’s particles are pulled by the field onto the planet. Pushed outward by the Sun, the magnetosphere reaches all the way to Saturn.

FIGURE 8-10 Comet Shoemaker-Levy 9 and its Encounter with Jupiter
(a) The comet was torn apart by Jupiter’s gravitational force on July 7, 1992, fracturing into at least 21 pieces. This comet originally orbited Jupiter, and its returning debris, shown here in May 1994, struck the planet between July 16 and July 22, (b) Shown here are visible (left) and ultraviolet (right) images of Jupiter taken by the Hubble Space Telescope after three pieces of Comet Shoemaker-Levy 9 struck the planet. Astronomers had expected white remnants (the color of condensing ammonia or water vapor); the darkness of the impact sites may have come from carbon compounds in the comet debris. Note the auroras in the ultraviolet image. Auroras and lightning are common on Jupiter, due, in part, to the planet’s strong magnetic fields and dynamic cloud motions. (a: H. A. Weaver, T. E. Smith, STScI and NASA; b: NASA)

FIGURE 8-11 The Galilean Satellites
The four Galilean satellites are shown here to the same scale. Io and Europa have diameters and densities comparable to our Moon and are composed primarily of rocky material. Ganymede and Callisto are roughly as big as Mercury, but their low average densities indicate that each contains a thick layer of water and ice. The cross-sectional diagrams of the interiors of the four Galilean moons show the probable internal structures of the moons, based on their average densities and on information from the Galileo mission. (NASA and NASA/JPL)

FIGURE 8-12 Io (a) This true-color view was taken by the Galileo spacecraft in The range of colors results from surface deposits of sulfur ejected from Io’s numerous volcanoes. Plumes from the volcano Prometheus rise up 100 km. Prometheus has been active in every image taken of Io since the Voyager flybys of (b) Galileo image of an eruption of Pilan Patera on Io. (c) Photographed in 1999 and then 2000 (shown here), the ongoing lava flow from this volcanic eruption at Tvashtar Catena has considerably altered this region of Io’s surface. (a: Moses Milazzo, PIRL, LPL, NASA; b: Galileo Project, JPL, NASA; c: University of Arizona/JPL/NASA)

FIGURE 8-13 Europa Imaged by the Galileo
spacecraft, 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. (NASA/JPL)

FIGURE 8-14 Surface Features on Europa
(a) 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 believed to be caused by tidal stresses. (b) This region of Europa’s surface shows the jumbled, stressed features common to the surface, as well as direct indications of liquid water activity underground. (c) 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. The white domes are likely to be rising material that has not yet reached the surface. (d) A lava lamp in which warmed material rises through cooler liquid. The rising material in Europa is analogous to the rising motion of the blobs in a lava lamp, except that on Europa, the motion is through ice. (a, b: NASA/JPL; c: NASA/JPL/University of Arizona/University of Colorado; d: Bianca Moscatelli)

FIGURE 8-15 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. (NASA/JPL)

FIGURE 8-16 Two Surfaces of Ganymede
The older, rougher, more heavily cratered parts of Ganymede, the dark terrain, are surrounded by younger, smoother, less cratered bright terrain. The parallel ridges suggest that the bright terrain has been crafted by tectonic processes. (NASA)

FIGURE 8-17 Callisto The outermost Galilean satellite is almost exactly the same size as Mercury. Numerous craters pockmark Callisto’s icy surface. (a) 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. (b) 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. (a: Courtesy of NASA/JPL; b: NASA/JPL/Arizona State University)

FIGURE 8-18 Irregularly Shaped Inner Moons
The four known inner moons of Jupiter are significantly different from the Galilean satellites. They are roughly oval-shaped bodies. Although craters have not yet been resolved on Adrastea and Metis, their irregular shapes strongly suggest that they are cratered. All four moons are named for characters in mythology relating to Jupiter (Zeus, in Greek mythology). (NASA/JPL, Cornell University)

FIGURE 8-19 Jupiter’s Ring and Torus
(a) A portion of Jupiter’s faint ring system, photographed by Voyager 2. The ring is probably composed of tiny rock fragments. The brightest portion of the ring is about 6000 km wide. The outer edge of the ring is sharply defined, but 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. (b) Quarter images of Io’s and Europa’s tori (also called plasma tori because the gas particles in them are charged—plasmas). Io is visible in its torus (green), while Europa is visible in its torus (blue). Some of Jupiter’s magnetic field lines are also drawn in. Plasma from tori flow inward along these field lines toward Jupiter. (a: NASA/JPL, Cornell University; b: NASA/JPL/Johns Hopkins University Applied Physics Laboratory)

FIGURE 8-20 Saturn Combined visible and ultraviolet images from the Hubble Space Telescope reveal spiral arcs of aurora near Saturn’s south pole. The image of Earth shows its size relative to Saturn. Note that there is much less contrast between Saturn’s clouds than those of Jupiter. (NASA, ESA, J. Clarke [Boston University], and Z. Levay [STScl]; inset: NASA)

FIGURE 8-21 Belts and Zones on Saturn
(a) Cassini took this extremely high resolution image of Saturn in Details as small is 53 km (33 mi) across can be seen. There is less swirling structure between belts and zones on Saturn than on Jupiter. (b) In 2006, Cassini’s infrared imager took this view of a hexagonal pattern of clouds that rotates much more slowly than the surrounding belts and zones. Its origin is still under investigation. (a: NASA/JPL/Space Science Institute; b: NASA/JPL/University of Arizona)

FIGURE 8-22 Merging Storms on Saturn
This sequence of Cassini images shows two hurricane-like storms merging into one on Saturn in Each storm is about 1000 km (600 mi) across. (NASA/JPL/Space Science Institute)

FIGURE 8-23 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 plane of Saturn’s rings and equator keeps the same orientation in space as the planet goes around its orbit, just as Earth keeps its 231⁄2° tilt as it orbits the Sun. The accompanying Earth-based photographs show how the rings seem to disappear entirely about every 15 years. (Top left: E. Karkoschka/U. of Arizona Lunar and Planetary Lab and NASA; bottom left: AURA/STScI/NASA; bottom right and top right: A. Bosh/Lowell Obs. and NASA)

FIGURE 8-24 Numerous Thin Ringlets Constitute Saturn’s Inner Rings
This Cassini image shows that Saturn’s rings contain numerous ringlets. Inset: As moons orbit near or between rings, they cause the ring ices to develop ripples, often like the grooves in an old-fashioned record. (NASA/JPL/Space Science Institute)

FIGURE 8-25 The F Ring and One of its Shepherds
Two tiny satellites, Prometheus and Pandora, each measuring about 50 km across, orbit Saturn on either side of the F ring. Sometimes the ringlets are braided, sometimes parallel to each other. In any case, the passage of the shepherd moons causes ripples in the rings. The gravitational effects of these two shepherd satellites confine the particles in the F ring to a band about 100 km wide. (NASA)

FIGURE 8-26 Saturn’s Outer Ring System
(a) Photographed with the Sun behind Saturn, the inner and intermediate regions of Saturn’s ring system are seen to be very different from each other. Beyond the F ring, the particles are dust and pebble-sized. (b) Another view of the inner and intermediate rings, where subtle color differences are indicated. (c) Superimposed on this Cassini image are labels that indicate how far the rings extend into the moon system of Saturn. Titan (off image on right) is 1.2 million km (750, 000 mi) from the center of Saturn. (a, b: NASA/JPL/Space Science Institute; c: NASA/JPL)

FIGURE 8-26 Saturn’s Outer Ring System (a) Photographed with the Sun
behind Saturn, the inner and intermediate regions of Saturn’s ring system are seen to be very different from each other. Beyond the F ring, the particles are dust and pebble-sized. (b) Another view of the inner and intermediate rings, where subtle color differences are indicated. (c) Superimposed on this Cassini image are labels that indicate how far the rings extend into the moon system of Saturn. Titan (off image on right) is 1.2 million km (750, 000 mi) from the center of Saturn. (a, b: NASA/JPL/Space Science Institute; c: NASA/JPL)

FIGURE 8-27 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. (NASA)

FIGURE 8-28 Saturn’s Diverse Moons
(a–e) 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). The ridge running along the equator of Iapetus (e) has not yet been explained. (f) Perhaps the most bizarre object photographed in the solar system, Hyperion, shows innumerable impact craters. They are different from craters seen in other objects in that the crater walls here have not filled in the bottom of the craters. The moon’s low gravity and the pull of nearby Titan may explain this unusual phenomenon. (g) This Cassini image of Phoebe, an irregular moon of Saturn almost as dark as coal, shows how the smaller moons have nonspherical shapes, along with 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. (a, e, f: NASA/JPL/ Space Science Institute)

FIGURE 8-29 Surface Features on Titan (diameter 5150 km)
(a) Voyager images of Titan’s smoggy atmosphere. (b) Cassini image of Titan of lighter highlands, called Xanadu, and dark, flat, lowlands that may be hydrocarbon seas. Resolution is 4.2 km (2.6 mi). (c) Riverbeds meandering across the Xanadu highlands of Titan. These are believed to have formed by the flow of liquid methane and ethane. (d) Lake likely filled with liquid methane and ethane found at Titan’s north pole, with Lake Superior, a Great Lake on Earth, for comparison. (e) The Huygens spacecraft took this image at Titan’s surface on January 14, What appear like boulders here are actually pebbles strewn around the landscape. The biggest ones are about 15 cm (6 in.) and 4 cm (1.5 in.) across. (a: NASA/JPL/Space Science Institute; b: NASA/JPL/University of Arizona; c: NASA/JPL; d: NASA/JPL/GSFC; e: NASA/JPL/ESA/University of Arizona)

FIGURE 8-30 Enceladus (diameter 500 km)
(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. Inset shows ice boulders. (c) Icy particles leaving Enceladus. (a–c: NASA/JPL/Space Science Institute)

FIGURE 8-31 Uranus, Earth, and Neptune
Images of Uranus, Earth, and Neptune are to the same scale. Uranus and Neptune are quite similar in mass, size, and chemical composition. Both planets are surrounded by thin, dark rings, quite unlike Saturn’s, which are broad and bright. The clouds on the right of Uranus (false color pink) are each the size of Europe. (NASA)

FIGURE 8-32 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.

FIGURE 8-33 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.

FIGURE 8-34 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 steeply inclined to their rotation axes. Jupiter, Saturn, and Neptune have north magnetic poles on the hemisphere where Earth has its south magnetic pole.

FIGURE 8-35 The Rings and Moons of Uranus Image of Uranus, its rings, and eight of its moons was taken by the Hubble Space Telescope. Inset: Close-up of part of the ring system 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. Uranus’s rings are much darker than Saturn’s, and this long exposure revealed many very thin rings and dust lanes. The short streaks are star images blurred because of the spacecraft’s motion during the exposure. (NASA [inset])

FIGURE 8-36 Discovery of the Rings of Uranus
(a) Light from a star is reduced as the rings move in front of it. (b) With sensitive light detectors, astronomers can detect the variation in light intensity. Such dimming led to the discovery of Uranus’s rings. Of course, the star vanishes completely when Uranus occults it.

FIGURE 8-37 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. (NASA)

FIGURE 8-38 Neptune’s Banded Structure
Several Hubble Space Telescope 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. (Lawrence Sromovsky, University of Wisconsin-Madison and STScI/NASA)

FIGURE 8-39 Neptune This view from Voyager 2 looks down on the southern hemisphere of Neptune. 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. (NASA/JPL)

FIGURE 8-40 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. (NASA)

FIGURE 8-41 Triton’s South Polar Cap
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. (NASA/JPL)

FIGURE 8-42 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. (NASA)

The Outer Planets: A Comparison

FIGURE 8-43 The Capture and Destruction of Triton
This series of drawings depicts how Triton was captured by Neptune in a retrograde orbit. The tides that Triton then created on the planet caused that moon’s orbit to become quite circular and to spiral inward. It will eventually reach Neptune’s Roche limit and be pulled apart to form a ring.

A Barren Landscape With a thin atmosphere and an abundance of lighter elements, such as silicon (silicon dioxide is sand), Lithia might resemble a barren desert here on Earth. (Photri)

Summary of Key Ideas

Jupiter and Saturn 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. 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 The colored ovals visible in the Jovian atmosphere 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 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 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 Uranus and Neptune are quite similar in appearance, mass, size, and chemical composition. Each has a rocky core surrounded by a thick, watery mantle; 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 liquid metallic hydrogen B ring occultation belt
Cassini division C ring 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

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.

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.

WHAT DID YOU THINK? Does Jupiter have continents and oceans?
No. Jupiter is surrounded by a thick atmosphere primarily of hydrogen and helium that gradually becomes liquid as you move inward. The only solid matter in Jupiter is its core.

WHAT DID YOU THINK? Is Saturn the only planet with rings?
No. All four outer planets (Jupiter, Saturn, Uranus, and Neptune) have rings.

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.

Neil F. Comins • William J. Kaufmann III Discovering the Universe

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Neil F. Comins • William J. Kaufmann III Discovering the Universe

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