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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… 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 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 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. 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 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 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 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 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. 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 composed 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 giant 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.