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Neil F. Comins • William J. Kaufmann III Discovering the Universe Eighth Edition CHAPTER 8 The Outer Planets Jupiter as Seen from a Spacecraft This view was sent back from Voyager 1 in 1979. 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. 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 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. 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. 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. 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. Differential rotation also causes the wind velocities at the boundaries between belts and zones to move predominantly to the east or west. Jupiter’s Four Largest Moons Io This true-color view was taken by the Galileo spacecraft in 1999. 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 Galileo image of an eruption of Pilan Patera on Io. 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. 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. 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. 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. 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. 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. 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. 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. Saturn’s Outer Ring System Another view of the inner and intermediate rings, where subtle color differences are indicated. Saturn’s Outer 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. 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. Surface Features on Titan Voyager images of Titan’s smoggy atmosphere. Surface Features on Titan 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). 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 Lake likely filled with liquid methane and ethane found at Titan’s north pole, with Lake Superior, a Great Lake on Earth, for comparison. Surface Features on Titan The Huygens spacecraft took this image 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.) and 4 cm (1.5 in.) across. 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. Inset shows ice boulders. Enceladus Icy particles leaving Enceladus. 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. 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. 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. 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 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 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 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 Outer Planets: A Comparison