Neil F. Comins • William J. Kaufmann III

Discovering the UniverseDiscovering the UniverseTenth EditionTenth Edition

CHAPTER 8CHAPTER 8The Outer PlanetsThe Outer Planets

In this chapter, you will discover…In this chapter, you will discover… Jupiter, an active, vibrant, multicolored world Jupiter, an active, vibrant, multicolored world

more massive than all of the other planets more massive than all of the other planets combinedcombined

Jupiter’s diverse system of moonsJupiter’s diverse system of moons Saturn, with its spectacular system of thin, flat Saturn, with its spectacular system of thin, flat

rings and numerous moons, including bizarre rings and numerous moons, including bizarre Enceladus and TitanEnceladus and Titan

Uranus and Neptune, ice giants similar to each Uranus and Neptune, ice giants similar to each other and different from Jupiter and Saturnother and different from Jupiter and Saturn

Jupiter is the largest planet in the solar system.Jupiter is the largest planet in the solar system. It emits more energy than it receives from the Sun.It emits more energy than it receives from the Sun. A cloud cover is continually moving and confined to narrow A cloud cover is continually moving and confined to narrow

bands of latitude that move faster at the equator than the bands of latitude that move faster at the equator than the poles. This causes an effect called differential rotation. poles. This causes an effect called differential rotation.

The dark reddish bands are called belts, and the light-The dark reddish bands are called belts, and the light-colored ones are called zones.colored ones are called zones.

Belts and zones flow eastward or westward, displaying a Belts and zones flow eastward or westward, displaying a zonal flow much different than winds found on Earth. zonal flow much different than winds found on Earth.

Jupiter’s belts and zones provide a framework for turbulent Jupiter’s belts and zones provide a framework for turbulent swirling cloud patterns, as well as rotating storms similar to swirling cloud patterns, as well as rotating storms similar to hurricanes.hurricanes.

Jupiter’s Appearance

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! In either case, Jupiter’s rapid differential rotation shapes the rising and descending gas into bands of winds parallel to the planet’s equator.

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.

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.

The Great Red Spot

This image of the Great Red Spot shows the counterclockwise circulation of gas in the Great Red Spot that takes about six days to make one rotation. 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 two-bladed blender.

Creating Red Spot Jr.

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…

Creating Red Spot Jr.

… became a red spot, named Red Spot Jr., in 2006.

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.

Jupiter’s atmosphere is mostly hydrogen and Jupiter’s atmosphere is mostly hydrogen and helium with traces of methane, ammonia, water helium with traces of methane, ammonia, water vapor, and other gases.vapor, and other gases.

There are three major cloud layers. There are three major cloud layers. The uppermost layer is made of frozen ammonia The uppermost layer is made of frozen ammonia

crystals. crystals. The middle layer is ammonium hydrosulfide. and The middle layer is ammonium hydrosulfide. and

the bottom is mostly water vapor.the bottom is mostly water vapor. Below its cloud layer, Jupiter’s mantle is entirely Below its cloud layer, Jupiter’s mantle is entirely

liquid, with no definite boundary between the liquid, with no definite boundary between the gaseous atmosphere and its liquid mantle.gaseous atmosphere and its liquid mantle.

Jupiter’s Atmosphere

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.

Beneath Jupiter’s clouds, we find liquid molecular Beneath Jupiter’s clouds, we find liquid molecular hydrogen and helium.hydrogen and helium.

The heat and pressure of the atmosphere transforms The heat and pressure of the atmosphere transforms hydrogen into liquid metallic hydrogen, making it behave hydrogen into liquid metallic hydrogen, making it behave like a metal.like a metal.

Electric currents running through this rotating metallic Electric currents running through this rotating metallic region generate a magnetic field. region generate a magnetic field.

The Nice theory suggests a rock and metal protoplanet The Nice theory suggests a rock and metal protoplanet formed out beyond the snowline and is the source of formed out beyond the snowline and is the source of Jupiter’s core. Jupiter’s core.

Water, carbon dioxide, methane, and ammonia probably Water, carbon dioxide, methane, and ammonia probably existed as ice on the surface of the protoplanet and were existed as ice on the surface of the protoplanet and were likely compressed with it as the atmosphere formed. likely compressed with it as the atmosphere formed.

Jupiter’s Interior and Magnetosphere

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 (doughnut-shaped regions) in the magnetosphere. Some of Io’s particles are pulled by the field onto the planet. Pushed outward by the Sun, the magnetosphere has a “magnetotail” pointing away from the Sun. The magnetotail sometimes reaches all the way to Saturn.

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.

Jupiter’s Magnetosphere

High-energy particles, trapped in Jupiter’s magnetosphere, excite gases in the planet’s upper atmosphere, causing them to glow as aurorae. The magnetosphere and cloud motion also lead to lightning on Jupiter.

Comet Shoemaker-Levy 9 and Its Encounter with Jupiter

The comet, originally orbiting Jupiter, was torn apart by the planet’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, 1994.

Comet Shoemaker-Levy 9 and Its Encounter with Jupiter

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 aurorae in the ultraviolet image.

Jupiter has about 67 moons, with the four Jupiter has about 67 moons, with the four largest (Galilean satellites) probably forming largest (Galilean satellites) probably forming the same way as the inner planets.the same way as the inner planets.

The others are either captured planetesimals The others are either captured planetesimals and smaller pieces of space debris.and smaller pieces of space debris.

The inner and six outer moons orbit in the The inner and six outer moons orbit in the same direction Jupiter rotates (prograde). same direction Jupiter rotates (prograde). The rest display retrograde orbit.The rest display retrograde orbit.

Jupiter’s Moons

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.

Io

These Voyager images show both sides of Io. 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 1979.

Io

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.

Io

• This is a Galileo image of an eruption of Pilan Patera on Io.

• As it orbits Jupiter, the gravitational pull on Io causes changes in its distance from the planet.

• The resulting tidal forces stretch and squeeze it and are probably the source of hot molten material (magma).

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.

Europa

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 believed to be caused by tidal stresses.

Surface Features on Europa

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.

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. The white domes are likely to be rising material that has not yet reached the surface.

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.

• Largest satellite in solar system

• Magnetic field• Thin atmosphere

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.

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).

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.

Saturn’s atmosphere lacks the colorful contrast Saturn’s atmosphere lacks the colorful contrast visible on Jupiter but still has belts and zones.visible on Jupiter but still has belts and zones.

Saturn’s atmosphere is not as compressed and Saturn’s atmosphere is not as compressed and slightly different in composition.slightly different in composition.

A smaller mass results in less hydrogen being A smaller mass results in less hydrogen being converted to liquid metallic hydrogen in Saturn.converted to liquid metallic hydrogen in Saturn.

Saturn’s rings are composed of fragments of ice Saturn’s rings are composed of fragments of ice and ice-coated rocks making them appear brighter and ice-coated rocks making them appear brighter than the other gas giants.than the other gas giants.

Saturn’s aurorae form spirals instead of rings. Saturn’s aurorae form spirals instead of rings. Saturn’s satellites are different from Jupiter’s.Saturn’s satellites are different from Jupiter’s.

Saturn Is Different than Jupiter

Belts and Zones on Saturn

(a) Observing in the infrared, 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. (b) Observed in visible light, the north pole is covered by and encircled by huge storms, imaged by Cassini in 2012. (a: NASA/JPL/University of Arizona; b: NASA/JPL-Caltech/SSI)

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.

This storm swept around Saturn’s northern mid-latitudes throughout 2011.

Storm Sweeping Across Saturn

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 23½° tilt as it orbits the Sun. The accompanying Earth-based photographs show how the rings seem to disappear entirely about every 15 years.

Saturn’s Ring System

Viewing Saturn’s night side, the smaller pieces of debris scatter lightfrom the Sun forward toward the Cassini spacecraft, including dust-sized particles in the Cassini division. Note that the Cassini division is bright in this image, while the B ring is quite dark. (NASA/JPL/Space Science Institute)

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.

The 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.

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, which lead to the formation of large snowballs in them. The gravitational effects of these two shepherd satellites confine the particles in the F ring to a band about 100 km wide.

Saturn’s Ring System

Photographed with the Sun behind Saturn, the inner and intermediate regions of Saturn’s ring system are shown to be very different from each other. Beyond the F ring, the particles are dust- and pebble-sized.

Saturn’s Ring System

This is another view of the inner and intermediate rings, where subtle color differences are indicated.

Saturn’s Ring System

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.

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.

These Voyager 1, Voyager 2, and Cassini images show the variety of surface features seen on three of Saturn’s seven spherical moons. They are not shown to scale (diameters given below each image). In comparison, the Cassini image on the far right shows the nonspherical moon Phoebe, almost as dark as coal, carrying many craters, grooves, landslides, 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

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).

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 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.

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.

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.

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

(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.

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

• (a) 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.

• (b) Methane clouds above the belts and zones.

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.

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 hiddenso that the dim rings would be visible, hence the black rectanglerunning 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

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

This series of drawings depicts how (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.

Summary of Key IdeasSummary of Key Ideas

Jupiter and Saturn Jupiter is by far the largest and most massive planet in Jupiter is by far the largest and most massive planet in

the solar system.the solar system. Jupiter and Saturn probably have rocky cores surrounded Jupiter and Saturn probably have rocky cores surrounded

by a thick layer of liquid metallic hydrogen and an outer by a thick layer of liquid metallic hydrogen and an outer layer of ordinary liquid hydrogen and helium. Jupiter’s layer of ordinary liquid hydrogen and helium. Jupiter’s core may be dissolving. Both planets have an overall core may be dissolving. Both planets have an overall chemical composition very similar to that of the Sun.chemical composition very similar to that of the Sun.

The visible features of Jupiter exist in the outermost The visible features of Jupiter exist in the outermost 100100  km of its atmosphere. Saturn has similar features, but km of its atmosphere. Saturn has similar features, but they are much fainter. Three cloud layers exist in the they are much fainter. Three cloud layers exist in the upper atmospheres of both Jupiter and Saturn. Because upper atmospheres of both Jupiter and Saturn. Because Saturn’s cloud layers extend through a greater range of Saturn’s cloud layers extend through a greater range of altitudes, the colors of the Saturnian atmosphere appear altitudes, the colors of the Saturnian atmosphere appear muted.muted.

Jupiter and Saturn The colored ovals visible in the Jovian The colored ovals visible in the Jovian

atmospheres are gigantic storms, some of which atmospheres are gigantic storms, some of which (such as the Great Red Spot) are stable and (such as the Great Red Spot) are stable and persist for years or even centuries.persist for years or even centuries.

Jupiter and Saturn have strong magnetic fields Jupiter and Saturn have strong magnetic fields created by electric currents in their metallic created by electric currents in their metallic hydrogen layers.hydrogen layers.

Jupiter and Saturn Four large satellites orbit Jupiter. The two inner Galilean Four large satellites orbit Jupiter. The two inner Galilean

moons, Io and Europa, are roughly the same size as our moons, Io and Europa, are roughly the same size as our Moon. The two outer moons, Ganymede and Callisto, Moon. The two outer moons, Ganymede and Callisto, are approximately the size of Mercury.are approximately the size of Mercury.

Io is covered with a colorful layer of sulfur compounds Io is covered with a colorful layer of sulfur compounds deposited by frequent explosive eruptions from volcanic deposited by frequent explosive eruptions from volcanic vents. Europa is covered with a smooth layer of frozen vents. Europa is covered with a smooth layer of frozen water crisscrossed by an intricate pattern of long cracks.water crisscrossed by an intricate pattern of long cracks.

The heavily cratered surface of Ganymede is composed The heavily cratered surface of Ganymede is composed of frozen water with large polygons of dark, ancient crust of frozen water with large polygons of dark, ancient crust separated by regions of heavily grooved, lighter-colored, separated by regions of heavily grooved, lighter-colored, younger terrain. Callisto has a heavily cratered ancient younger terrain. Callisto has a heavily cratered ancient crust of frozen water.crust of frozen water.

Jupiter and Saturn Both Jupiter and Saturn have rings. Saturn is circled by a Both Jupiter and Saturn have rings. Saturn is circled by a

system of thin, broad rings lying in the plane of the system of thin, broad rings lying in the plane of the planet’s equator. Each major ring is composed of a great planet’s equator. Each major ring is composed of a great many narrow ringlets that consist of numerous fragments many narrow ringlets that consist of numerous fragments of ice and ice-coated rock. Jupiter has a much less of ice and ice-coated rock. Jupiter has a much less substantial ring system.substantial ring system.

Saturn’s moon Titan has a thick atmosphere of nitrogen, Saturn’s moon Titan has a thick atmosphere of nitrogen, methane, and other gases, as well as lakes of methane methane, and other gases, as well as lakes of methane and ethane.and ethane.

Saturn’s moon Enceladus has areas with very different Saturn’s moon Enceladus has areas with very different surface features: an older, heavily cratered region and a surface features: an older, heavily cratered region and a newer, nearly crater-free surface created by tectonic newer, nearly crater-free surface created by tectonic activity.activity.

Uranus and Neptune Uranus and Neptune are quite similar in appearance, mass, Uranus and Neptune are quite similar in appearance, mass,

size, and chemical composition. Each has a rocky core size, and chemical composition. Each has a rocky core surrounded by a thick, watery mantle topped by a layer rich in surrounded by a thick, watery mantle topped by a layer rich in hydrogen and helium; the axes of their magnetic fields are hydrogen and helium; the axes of their magnetic fields are steeply inclined to their axes of rotation; and both planets are steeply inclined to their axes of rotation; and both planets are surrounded by systems of thin, dark rings.surrounded by systems of thin, dark rings.

Uranus is unique in that its axis of rotation lies near the plane of Uranus is unique in that its axis of rotation lies near the plane of its orbit, producing greatly exaggerated seasons on the planet.its orbit, producing greatly exaggerated seasons on the planet.

Uranus has five moderate-sized satellites, the most bizarre of Uranus has five moderate-sized satellites, the most bizarre of which is Miranda.which is Miranda.

Triton, the largest satellite of Neptune, is an icy world with a Triton, the largest satellite of Neptune, is an icy world with a tenuous nitrogen atmosphere. Triton moves in a retrograde orbit tenuous nitrogen atmosphere. Triton moves in a retrograde orbit that suggests it was captured into orbit by Neptune’s gravity. It that suggests it was captured into orbit by Neptune’s gravity. It is spiraling down toward Neptune and will eventually break up is spiraling down toward Neptune and will eventually break up and form a ring system.and form a ring system.

Key TermsKey TermsA ringB ringbeltC ringCassini divisiondifferential rotationEncke divisionF ringGalilean moon (satellite)Great Dark SpotGreat Red Spothydrocarbon

liquid metallic hydrogenoccultationpolymerprograde orbitresonanceretrograde orbitringletRoche limitshepherd satellite (moon)spokezone (atmospheric)