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Mercury's Interior and Surface Mercury, the innermost planet of the solar system, is a little bigger than the Earth's Moon. The surface of the planet is covered with craters, like the Moon, but temperatures there can reach over 800oF because Mercury is so close to the Sun and rotates so slowly. Scientists believe that the interior structure of Mercury includes a metallic core, an intermediate rocky layer, and a thin brittle crust. The composition of Mercury is probably high in iron, although surface features indicate that volcanic activity once existed at the surface. There is little evidence of motions near the surface of the planet now, although at earlier times during Mercury's evolution the surface was much more active. We know relatively little about Mercury, compared to most of the other planets, because it is relatively difficult to see and only one spacecraft has studied the planet. An interesting fact is that Mercury has no natural satellites or moons. This picture shows Mercury. Mercury is the smallest planet in our Solar System. It is also the closest planet to the Sun. This picture was taken by the MESSENGER spacecraft in January 2008. This color picture shows Mercury pretty much as human eyes would see it. Mercury is mostly gray. If you look closely, however, you can see some shades of blue and brown. Image courtesy of NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington. This is an image showing the cratered surface of the planet Mercury. Surface of Mercury Images of the surface of Mercury obtained by Mariner 10 showed a planet covered with craters, looking very much like the Earth's Moon. During its three passes by the planet, Mariner 10 took pictures of about half the surface of the planet, so we don't know what the rest of the surface looks like. Mercury's impact craters were made early in the evolution of the solar system, nearly 4 billion years ago, by meteorites which hit the surface. The relatively smooth plains between craters indicates that at one point the surface was probably volcanic, as lava flows filled in after the impacts of large objects. It appears that these plains and heavily cratered regions are not uniformly scattered around the surface. Instead, craters are concentrated on one part of the planet while plains dominate another part. Wrinkle ridges or "rupes", a few km across and hundreds of km long, are also common on the surface. These features, which can reach several km in height, were probably created as the interior of the planet cooled and shrunk, causing the surface to collapse in and form ridges. Major surface features include ridges, plains, and numerous large craters, the largest (about 800 mi or 1300 km across) named the Caloris Basin. This is a mosaic of the caloris basin and its surrounding area. The Caloris Basin is closest to the sun when Mercury is at the closest point in its orbit to the sun. Image Courtesy of NASA This is an image of wrinkle ridges on the surface of Mercury. The photograph was taken by the Mariner 10 spacecraft. Courtesy of NASA. Structure of Mercury's Interior Mercury has a radius of 2439 km (1524 mi), and the metallic iron-nickel core is believed to make up about 75% of this distance. Measurements of the planet's magnetic field made by Mariner 10 as it flew by the planet indicates that this core is likely to be hot and fluid. In contrast with the other terrestrial planets, the rest of the planet is probably made up of a solid rocky layer topped with a thin crust about 100 km thick. The surface of Mercury is covered with a variety of craters formed from the impact of meteorites billions of years ago and other features that reveal information about the evolution of the planet. This diagram shows the internal structure of the planet Mercury. Scientists believe that the core of Mercury is hot, liquid iron-nickel. Mercury has no atmosphere and as a result, the surface is pocked by craters from collisions with asteroids over billions of years. Windows Original. Atmosphere of Mercury Mercury has almost no atmosphere. The planet's small size means that its gravity is too weak to hold down a normal atmosphere. There is a very thin atmosphere around the planet. Mercury's thin atmosphere is constantly being "blown away" into space by the pressure of sunlight and by the solar wind. Gases are constantly being added to Mercury's atmosphere, too. That's why it still has any atmosphere at all - even though that atmosphere is really, really thin. Mercury's atmosphere contains small amounts of hydrogen, helium, and oxygen. It also has even tinier amounts of sodium, potassium, calcium, and magnesium. Some of the gas particles come from the solar wind. Others are made by radioactive decay or when micrometeorites smash into the surface. All of these gases are soon carried away from Mercury by the solar wind and by Mercury's magnetic field. Atmospheric pressure at the planet's surface is less than one trillionth of Earth's (around one nanopascal or 10-14 bar). Temperatures at the surface range between 100 and 700 kelvins (-280° F to 800° F or -173° C to 427° C). Lead melts at 600 kelvins! This large range in surface temperature is possible because Mercury is so close to the Sun (a year is only 88 Earth days long) and does not have sufficient atmosphere present to moderate the range in surface temperature. Mercury's thin atmosphere contains hydrogen, helium, and oxygen. It also has smaller amounts of sodium, potassium, calcium, and magnesium. This picture shows sodium near Mercury. Red and green areas have the most sodium. Image courtesy of NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington. Venus Inside and Out Venus is the second planet from the Sun, and is Earth's closest neighbor in the solar system. Venus is the brightest object in the sky after the Sun and the Moon, and sometimes looks like a bright star in the morning or evening sky. The planet is slightly smaller than Earth, and its interior is similar to Earth. We can't see the surface of Venus from Earth, because it is covered with thick clouds that strongly reflect sunlight. However, space missions to Venus have shown us that its surface is covered with craters, over 1600 major volcanoes, mountains, large highland terrains, and vast lava plains. The surface of Venus is not where you'd like to be, with temperatures reaching more than 450C (approaching 900F - high enough to melt lead), an atmosphere 90 times heavier than our own, and clouds of sulfuric acid floating around to top it off! Like Mercury, Venus does not have any natural satellites or moons. This is a global view of the surface of Venus. NASA/JPL This image is a composite of the complete radar image collection obtained by the Magellan mission. It shows craters on the surface of about 300 to 500 million years old. NASA/JPL Inside Venus Venus is a slightly smaller than the Earth, with a diameter 95% that of Earth (12,103 km) and a mass 81% that of Earth. If we could walk around on the surface of the planet (without being killed by the toxic blast furnace of an atmosphere), gravity would be close to that on the surface of Earth. The interior of Venus is probably similar to Earth's interior. Venus, like Earth, is one of the terrestrial planets and is made of rock and metal. It probably has a partly molten metallic core, a rocky mantle, and a crust. The planet rotates very slowly, taking more than 243 Earth days to spin once on it's axis (even longer than the time it takes for Venus to orbit the Sun, about 225 Earth days). This may be the reason the planet doesn't have a magnetic field like many of the other planets, including Earth. The varied terrain of Venus, including volcanoes, mountains, craters, and lava flows, suggests that the planet was once, and perhaps still is, geologically active. But basic questions about the interior of Venus remain, such as the thickness of the lithosphere. This is an illustration of the interior layers of Venus. Windows Original. The Atmosphere of Venus This is an artist's rendition of a column of clouds on Venus. The temperature of the different layers is shown at the left. Windows to the Universe original image The atmosphere of Venus is very hot and thick. You would not survive a visit to the surface of the planet you couldn't breathe the air, you would be crushed on by the enormous weight of the atmosphere, and you would burn up in surface temperatures high enough to melt lead. The atmosphere of Venus is made up mainly of carbon dioxide, and thick clouds of sulfuric acid completely cover the planet. The atmosphere traps the small amount of energy from the sun that does reach the surface along with the heat the planet itself releases. This greenhouse effect has made the surface and lower atmosphere of Venus one of the hottest places in the solar system! If you were on the surface of the planet, the air above you would be about 90 times heavier than the Earth's atmosphere. This is like what a submarine experiences at 3000 ft below the surface of the Earth's ocean. The atmosphere is composed mainly of carbon dioxide (96%), 3.5% nitrogen, and less than 1% is made up of carbon monoxide, argon, sulfur dioxide, and water vapor. Why should Venus and not the Earth have a hot and thick atmosphere? Some scientists call it the Goldilocks phenomenon. Measurements made by probes which travelled through the atmosphere have shown that the atmospheric temperature remains nearly constant through the long dark night. Thus there are neither significant seasons, nor daily temperature changes in the atmosphere. Volcanic Rises Volcanic rises are more like islands than a continents. They are broad, sloping highlands over 1000 km across. Measurements suggest that there is hot magma deep beneath these rises that helps to buoy them up and support their mountains, as shown in this picture. There are three particular types of volcanic rises: • Rift-Dominated Rises These are rises that contain two or more deep rift valleys. Atla Regio and Beta Regio are rift-dominated rises. • Volcano-Dominated Rises These rises contain one or more large shield volcanoes. Western Eistla Regio and Imdr Regio are volcano-dominated rises. • Corona-Dominated Rises These rises lack large rift valleys, like volcano-dominated rises. But instead of being dominated by shield volcanoes, their surfaces are dominated by coronae. Coronadominated rises include Eastern Eistla Regio and Themis Regio. This is a 3-D image of part of the Western Eistla Regio, Lava flows extend for hundreds of kilometers across the fractured plains shown in the foreground. NASA/JPL Earth's Interior and Surface Earth, the largest and densest rocky planet, was formed about 4.5 billion years ago. The Earth's interior is divided into four layers, which is typical of rocky planets. Each layer has different characteristics and is made of different elements and minerals. There are many different types of features on Earth’s surface due to the complexity of our planet. The surface is unique from the other planets because it is the only one which has liquid water in such large quantities. Water forms some features of Earth's surface such as rivers, oceans, beaches and lakes. Other surface features, such as mountains, earthquakes and volcanoes, are formed when large pieces of the Earth’s outer layer move slowly by plate tectonics. This is an image of the Grand Canyon in Arizona. It is 277 miles long and up to 18 miles across. The Grand Canyon is the largest canyon in the world and was cut by the Colorado River over millions of years. Some parts are almost a mile deep. Aris Multimedia Entertainment, Inc. 1994 Mean sea-surface salinity. Grey shaded areas (land) exceed 36 psu (practical salinity units). Image courtesy of NOAA Structure of the Interior of Earth This is an image of Earth and the interior layers. Windows Original. Earth has a diameter of about 12,756 km (7,972 mi). The Earth's interior consists of rock and metal. It is made up of four main layers: 1) the inner core: a solid metal core made up of nickel and iron (2440 km diameter) 2) the outer core: a liquid molten core of nickel and iron 3) the mantle: dense and mostly solid silicate rock 4) the crust: thin silicate rock material The temperature in the core is hotter than the Sun's surface. This intense heat from the inner core causes material in the outer core and mantle to move around. The movement of material deep within the Earth may cause large plates made of the crust and upper mantle to move slowly over the Earth’s surface. It is also possible that the movements generate the Earth's magnetic field, called the magnetosphere. The Earth's Moon The Earth's one natural satellite, the Moon, is more than one quarter the size of Earth itself (3,474 km diameter). Because of its smaller size, the Moon's gravity is one-sixth of the Earth's gravity, as we saw demonstrated by the giant leaps of the Apollo astronauts. While there are only two basic types of regions on the Moon's surface, there are many interesting surface features such as craters, mountain ranges, rilles, and lava plains. The structure of the Moon's interior is more difficult to study. The Moon's top layer is a rocky solid, perhaps 800 km thick. Beneath this layer is a partially molten zone. Although it is not known for certain, many lunar geologists believe the Moon may have a small iron core, even though the Moon has no magnetic field. By studying the Moon's surface and interior, geologists can learn about the Moon's geological history and its formation. The footprints left by Apollo astronauts will last for centuries because there is no wind on the Moon. The Moon does not possess any atmosphere, so there is no weather as we are used to on Earth. Because there is no atmosphere to trap heat, the temperatures on the Moon are extreme, ranging from 100° C at noon to -173° C at night. The Moon doesn't produce its own light, but looks bright because it reflects light from the Sun. Think of the Sun as a light bulb, and the Moon as a mirror, reflecting light from the light bulb. The lunar phase changes as the Moon orbits the Earth and different portions of its surface are illuminated by the Sun. Earth's Atmosphere The atmosphere is a mixture of nitrogen (78%), oxygen (21%), and other gases (1%) that surrounds Earth. High above the planet, the atmosphere becomes thinner until it gradually reaches space. It is divided into five layers. Most of the weather and clouds are found in the first layer. The atmosphere is an important part of what makes Earth livable. It blocks some of the Sun's dangerous rays from reaching Earth. It traps heat, making Earth a comfortable temperature. And the oxygen within our atmosphere is essential for life. Over the past century, greenhouse gases and other air pollutants released into the atmosphere have been causing big changes like global warming, ozone holes, and acid rain. Planet Structure The uniquely red global surface of Mars is marked by many interesting features - some like those on the Earth and others strangely different. The reddish color is caused by rust (iron oxide) in the soil. Some of these features are; volcanoes, canyon systems, river beds, cratered terrain, and dune fields. Of these features, the most interesting includes the apparently dead volcano Olympus Mons, which rises 23 km (~75,000 ft) above the surrounding plains and is the highest known peak in the Solar System. Valles Marineris is a giant canyon system that runs about 2,500 miles across the surface of the planet and reaches depths of 6 km or 4 miles (for comparison, the Grand Canyon is not more than 1 mile deep). This is a mosaic of Olympus Mons, the highest volcano on Mars. NASA This is an image of Valles Marineris. Image from: NASA NASA/JPL Martian Moons Mars has two moons Deimos and Phobos. Lower Atmosphere The atmosphere of Mars is much thinner than that of Earth, with a surface pressure averaging 1/100th that at the surface of the Earth. Surface temperatures range from -113oC at the winter pole to 0oC on the dayside during summer. Although the length of the Martian day (24 hours and 37 minutes) and the tilt of its axis (25 degrees) are similar to those on Earth (24 hours and 23.5 degrees), the orbit of the planet about the Sun affects the lengths of the seasons the most. The atmosphere is composed mainly of carbon dioxide (95.3%), nitrogen (2.7%), and argon (1.6%), with small amounts of other gases. Oxygen, which is so important to us on earth, makes up only 0.13% of the atmosphere at Mars. There is only one-fourth as much water vapor in the atmosphere. Although small, this is thought to be enough to allow water ice to be frozen into the surface of the planet. With so little water, clouds are rarely seen in the Martian sky. The possible role in the past of liquid water in forming the dry river beds which we can see is still unknown, particularly because water ice is not plentiful on the surface of the planet. This drawing represents several features of the atmosphere on Mars including: dust storms, an atmospheric pressure much lower than found on Earth, and a composition primarily of carbon dioxide. The Interior of Mars, from Mars Pathfinder Many missions have returned data on the shape of Mars, a measurement which contributes to an understanding of a planet's interior structure. These data first showed that Mars had a huge bulge, now called the Tharsis Ridge. Mars Pathfinder took better data on the shape of Mars by passing over the Martian poles instead of the around the middle. These data suggest that Mars has a large, and solid core, as shown in the figure to the left. (The figure is a rough drawing and is not meant to show the detailed shape of the core). These findings help address questions which remain about Mars, and contribute to the overall results returned by the Mars Pathfinder mission. Scientists used to think that because the Martian surface contained so much iron, and the Martian magnetic field was weak, that the early history of Mars was not sufficiently warm for Mars to differentiate, and form a large solid iron core, such as that of the Earth. A large molten iron core would generate a strong magnetic field. Thus, iron must have stayed mostly on the outside layers of Mars, making the surface red with rust. Recently, Mars Global Surveyor returned the first definitive measurement of a Martian magnetic field. Therefore, if the core of Mars is large and composed of iron as the Mars Pathfinder measurement shows, then this theory about the evolution of Mars may not be completely correct. This is a image of the Tharsis Montes volcano region showing Olympus Mons and a few other volcanoes in red/yellow. Image from: NASA Martian Volcanoes On this map of Mars, the lightly cratered Tharsis Ridge is shown, as well as the heavily cratered Martian highlands (near the bottom of the picture), and Valles Marineris to the right. The volcanoes are the yellow and red dots in the sea of blue in this image. The Tharsis Ridge is the home of several volcanoes, including Olympus Mons. Olympus Mons is the leftmost volcano in the image. The three large volcanoes which accompany Olympus Mons on the Tharsis Ridge are Arsia Mons, Pavonis Mons, and Ascraeus Mons. Measurements returned by Mars Global surveyor demonstrate the very large size of these volcanoes. Other volcanos named Tharsis Tholus, Ceraunius Tholus, Elysium Mons, and Albov Tholus are found in the right-hand side of the large topographic map of Mars, as well as the very large version of the map to the left. These volcanoes came into being early in Martian history. Martian Floods Separate from the Martian outflow channels, or the river valley networks, are large Martian lakes (600 km, or ~1000 miles across) which exhibit evidence of a periodic and catastrophic release of water in the form of floods. This evidence is found with an examination of the shorelines surrounding the lakes. An analysis of the combined evidence from the Martian outflow channels, the river valley networks, and catastrophic flooding suggests two possibilities for the global Martian water cycle. This is an image showing evidence of Martian flooding. COURTESY OF NASA The Composition of Jupiter's Interior The composition of Jupiter's interior is mostly the simple molecules hydrogen, in the form of liquid. Under the cloud layers, when the pressure of the interior becomes high enough, the hydrogen of which Jupiter is made changes to liquid hydrogen, which gradually changes further to liquid metallic hydrogen. The core of Jupiter is made out of heavier, rocky and metal elements. The Structure of Jupiter's Interior There is no surface to the giant planets, only a gradual change from the atmosphere, as shown in this drawing. The gases which Jupiter is mostly made of change to liquid inside Jupiter, but the change is very gradual. Therefore the giant planets do not have strict layers, as the earth-like planets do. The liquid sections of Jupiter form by far the largest portions of the planet, and penetrate very deep into the planet. The first liquid layer inside Jupiter, immediately under the atmosphere, is the liquid hydrogen layer. Under the liquid hydrogen layer is a liquid metallic hydrogen layer. At the deepest part of Jupiter is the core, which is probably the size of planet Earth. Jupiter's Moons and Rings Jupiter has 63 moons and a ring system. The four Galilean satellites; Io, Europa, Ganymede, and Callisto, are among the most interesting of all solar system bodies, particularly Io, with its active volcanism, and Europa with the possibility of a water environment friendly to life. The Galileo spacecraft explored the moons while orbiting Jupiter for eight years: 1996-2003. Other moons, in order of their distance from Jupiter, are: Metis, Adrastea, Amalthea, Thebe, Themisto, S/2003 J6, S/2003 J1, Leda, Himalia, Lysithea, Elara, S/2000 J11, S/2003 J20, S/2003 J3, S/2003 J12, S/2001 J10, S/2003 J18, S/2003 J16, S/2001 J7, Harpalyke, Praxidike, S/2001 J9, Ananke, S/2001 J3, Iocaste, S/2001 J2, S/2003 J15, S/2003 J17, S/2003 J11, S/2003 J9, Carme, S/2003 J19, S/2001 J6, S/2002 J1, S/2001 J8, Chaldene, Isonone, S/2001 J4, S/2003 J4, Erinome, Taygete, Pasiphae, S/2001 J11, Kalyke, Sinope, Magaclite, S/2003 J7, S/2001 J5, S/2003 J13, S/2003 J5, S/2001 J1, Callirrhoe, S/2003 J10, S/2003 J8, S/2003 J14, and S/2003 J2. These moons are part of a class of moons called the "Small Moons". Jupiter's moons are named after mythological figures. The Rings of Jupiter Jupiter has a series of rings circling it! Unlike Saturn's rings, which are clearly visible from Earth even through small telescopes, Jupiter's rings are very difficult to see. So difficult, in fact, that they weren't discovered until a few years ago. Jupiter's rings were first found by the Voyager 1 spacecraft in 1979. There are three parts to Jupiter's rings. The innermost, cloud-like ring is called the Halo Ring. The next one out is the Main Ring, which is quite narrow and thin. Beyond the Main Ring is the wispy, nearly transparent Gossamer Ring. As shown in the diagram, the Gossamer Ring has two parts: the Amalthea Gossamer Ring (closer to Jupiter) and the Thebe Gossamer Ring. Saturn's rings are mostly made of ice. Jupiter's rings are different - they are very dark and difficult to see. They are made up of small bits of dust. The Galileo spacecraft helped us discover where that dust comes from. Meteors striking the surface of Jupiter's small, inner moons kick up dust which then goes into orbit around Jupiter, forming the rings. An Overview of Jupiter's Atmosphere The king of planets is aptly named because it not only has the most dymanic atmospheric motion, but also the most riveting cloud patterns and storms, and the most majestic appearance of the giant planets. The dramatic appearance of Jupiter stems partially because the composition of Jupiter's atmosphere includes complicated molecules such as ammonia and methane, as well as simple molecules such as helium, hydrogen, and sulfur. The composition also includes exotic molecules such as germain. The atmosphere of Jupiter is only a narrow surface layer, compared to the vast interior of the planet. The three clouddecks of Jupiter are to be found at different levels in the troposphere, while hazes of smog can be found higher in the atmosphere. Jupiter is not much changed from its early evolution out of the early solar nebula, and in fact, may still be forming! This is a three-color filter image of Jupiter and Io. It was taken on June 10, 1979 by Voyager 2. Dr. Micheal Belton An Overview of Jupiter's Atmospheric Composition Jupiter's atmosphere is mostly made of the simple molecules hydrogen and helium. Sulfur is present there, as well as nitrogen and oxygen. Nitrogen and oxygen make up the air that we breath every day on Earth. These molecules combine to make clouds of complex molecules, such as clouds of water and smog. A Look at Jupiter's Clouds Jupiter shows a pattern of clouds of white, brown, and orange. And then there is the Great Red Spot. The Great Red Spot is the largest of the clouds of Jupiter. Other cloud shapes include eddy shapes, white ovals, brown ovals, and brown barges. The eddies and white ovals are outlined in this picture, when shown at full size. These clouds form in stripes and move across the face of Jupiter. The stripes are similar to those found on all the giant planets. There are three layers of clouds on Jupiter, and each one is composed of different molecules. At one level there are clouds of ammonia, at another level there are clouds made of ammonia and sulfur, and at a third level there are clouds of water (H2O). Hazes of smog on Jupiter are to be found at very high altitudes above the clouds of Jupiter. The Structure of Saturn's Interior There is no surface to the giant planets, only a gradual change from the atmosphere, as shown in this drawing. The gases which Saturn is mostly made of change to liquid inside Saturn, but the change is very gradual. Therefore the giant planets do not have strict layers, as the earth-like planets do. The liquid sections of Saturn form by far the largest portions of the planet, and penetrate very deep into the planet. The first liquid layer inside Saturn, immediately under the atmosphere, is the liquid hydrogen layer. Under the liquid hydrogen layer is a liquid metallic hydrogen layer. At the far interior is the core of Saturn. Motions in the interior of Saturn contribute in a very special way to the development of the powerful and extensive magnetosphere of Saturn. Heat generated within Saturn contributes to the unusual motions of the atmosphere The Composition of Saturn's Interior Saturn's composition is primarily that of simple molecules such as hydrogen and helium, but there is also ice of ammonia, ice of methane and water ice. Under the cloud layers, when the pressure of the interior becomes high enough, the hydrogen of which Saturn is made changes to liquid hydrogen, which gradually changes further to liquid metallic hydrogen. Compared to Jupiter, Saturn does not have as much metallic hydrogen, but there is more ice. Because the magnetosphere comes from the metallic layer, this means that Saturn has a smaller magnetosphere than does Jupiter. At the far interior of Saturn is found the core, which is made of rocky and metal elements. An Overview of Saturn's Atmosphere The dramatic appearance of Saturn stems mainly from the spectacular rings. The atmosphere looks much less dramatic. The clouds of Saturn are much less colorful than those of Jupiter. This is because the composition of Saturn's atmosphere includes more sulfur. This adds to Saturn's overall yellow appearance. When you look closely, however, Saturn's atmosphere is just as fierce as that of Jupiter! The atmosphere of Saturn, like Jupiter, is only a narrow region, compared to the vast interior of Saturn. The three clouddecks of Saturn are to be found mostly low in the atmosphere, while hazes of smog can be found higher up. Saturn is not much changed from its early evolution out of the primitive solar nebula, and in fact, may still be forming! Motions in the cloud patterns indicate that, like Jupiter, the basic weather of Saturn can be described as a striped Saturn Clouds, overview This image of Saturn makes use of false color to show the cloud pattern. The clouds form in bands which move across the disk of Saturn. The banded pattern of clouds, or stripes, is similar to those found on all the giant planets, particularly in Jupiter's belts and zones. The similarity among all the giant planets, even Uranus, suggests that there may be a common way this pattern is created. Cloud shapes of Saturn include eddy shapes, white ovals, and brown ovals, just like on Jupiter. A row of swirling eddies can be seen in the very middle of this image in white. There are three clouddecks on Saturn, and each one is composed of different molecules. There is a clouddeck of ammonia clouds, a clouddeck of ammonia hydrosulfide clouds, and a clouddeck of water clouds (H2O). Hazes of smog on Saturn are to be found at very high altitudes above the clouds of Saturn. Saturn's Moons and Rings The gas giant planet Saturn has a large group of 62 moons. Saturn's moon Titan is one of the few moons in the Solar System with much of an atmosphere. It is also one of the largest moons in the Solar System. Some of Saturn's moons are odd. A giant crater on Mimas makes it look like the "Death Star" from the Star Wars movies. Iapetus is quite dark on one side and very bright on the other. Hyperion is shaped like a garbage can. Prometheus and Pandora are called the "shepherd moons" because they herd particles into Saturn's "F ring". Phoebe is very dark and moves in a strange orbit. It may be an asteroid or Kuiper Belt Object that Saturn's strong gravity captured long ago. The other medium-sized moons of Saturn are Rhea, Dione, Tethys, Enceladus, Janus and Epimetheus. Some of Saturn's moons are icy moons. Most of the others are small moons which are basically large rocks in space. Some stories from old myths include characters that some of Saturn's moons were named after. This is a color image of Saturn's satellite Rhea taken by Voyager 1 on November 12, 1980. NASA/JPL Saturn's Rings Many people like Saturn's rings. Although Saturn isn't the only planet with rings, it is the only planet famous for them. Almost every image or drawing of the planet has the rings included. But few people know much about them or why they are there. Saturn's rings are made mostly of ice and rock pieces. It looks like one big band, but is actually many smaller bands combined. The particles range in size from a couple centimeters to over a kilometer in size. The rings are very thin. Although they reach diameters in the hundred thousands kilometers, they are no more than 1.5 km thick. So how can such a thin layer of ice pieces be so beautiful? The ice creates a rainbow effect much like a sprinkler does in the sun. The Sun's rays are refracted by the frozen water, giving us a colorful display! An gigantic new "ring" was discovered in 2009. The Phoebe Ring is about 100 times bigger than the main ring system. Scientists think the ice and dust in the ring comes from the strange moon Phoebe. Particles from this ring might cause the strange coloration of the surface of Iapetus. This image of Saturn and its rings was taken by Voyager 1 in 1980. Courtesy of NASA An Overview of Uranus' Interior The Giant planets do not have the same layered structure that the earthlike planets do. Their evolution was quite different than that of the earthlike planets, and they have much more gas and ice inside. Uranus's interior is primarily made of methane ice. Motions in the interior of Uranus help form the magnetosphere of Uranus. Heat generated within Uranus helps form the unusual winds of the atmosphere. This is a diagram of the interior of Uranus. Image from: The New Solar System The Structure of Uranus' Interior There is no surface to the giant planets, only a gradual change from the atmosphere, as shown in this drawing. Therefore the giant planets do not have strict layers, as the earth-like planets do. Ice begins forming in the atmosphere of Uranus and keeps increasing until there is slush, and then solid ice. There is not much difference between the atmosphere and the ocean on Uranus, unlike the earth, where there is a definite surface between the air and the ocean. An Overview of Uranus' Atmosphere The plain aquamarine face of Uranus confirms the fact that Uranus is covered with clouds. The sameness of the planet's appearance shows that the planet's atmosphere is mostly composed of one thing, methane. The planet appears to be blue-green because the methane gas of the atmosphere traps red light and does not allow that color to escape. Beside clouds of methane crystals low in the atmosphere, smog, composed of ethane (the same product that can provide fuel for automobiles), is also present high in the atmosphere. The cloud particles constantly recycle themselves, first creating then destroying the heaviest crystals. This is an indication that Uranus' atmosphere is still evolving from its formation out of the solar nebula. Because Uranus lies on its side, Uranus has very strange seasons. Motions in the cloud patterns indicate that, like Jupiter and Saturn, the basic weather of Uranus can be described as a striped pattern of winds. This means that, even though the pattern is hard to see, Uranus is striped, just like Jupiter and Saturn. Uranus Clouds, Overview The clouds of Uranus, composed of methane crystals, are found very low in the atmosphere, and are difficult to see below the smog haze s of the planet. False color is used, in the image of Uranus to the right, to better show the pattern the clouds make. In the image, the atmosphere displays a pattern of concentric circles (that is circles inside of circles). This is because the center o f the image is the north pole. The winds of Uranus blow clouds counterclockwise in the picture, as opposed to left to right as they would in an image of Jupiter. Because Uranus lies on its side, with the north pole facing the sun, the seasons, and weather of Uranus should be very strange. These are two images of Uranus. The one on the left is true image and the one on the right is a false image. a Uranus' Moons and Rings Uranus has 27 fascinating moons and a complicated ring system. The ring is a completely different form of ring than the one around Saturn or Jupiter. At Uranus there is a very obvious partial ring, or "ring arc". Many moons are icy moons with fascinating surface features. These icy moons have no atmosphere nor magnetosphere. The interiors of these moons are not active, and there is not much possibility for life. The moons are, in order; Cordelia, Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Belinda, and Puck. These moons are part of a group called the "Small Moons". Icy moons of Uranus are; Miranda, Ariel, Umbriel, Titania, Oberon, Caliban, and Sycorax. In 1999, four more Uranian moons were found. They include Prospero, Setebos, Stephano and 1986 U 10. Moons of Uranus from left to right: Miranda, Puck, and Titana. Icy Moon Icy moons are large or small moons which are mostly made of ice. These moons are unlike the earth's moon, which is made of silicate rock. Perfect examples of icy moons are 3 of the Galilean satellites, Europa, Ganymede, and Callisto. Except maybe for Europa and Triton, these moons have no atmosphere. But the surfaces of these moons, especially Ganymede, show that in their history, something may have happened inside which changed the way the surface of the moon looks today. Activity in the interior could also provide an environment suitable for life. An Overview of Neptune's Interior The Giant planets do not have the same kind of layers inside that the Earth-like planets do. Their evolution was quite different than that of the Earth-like planets, and they have much more gas and ice inside. Neptune's interior make-up is primarily methane ice, just as Uranus' is. Motions in the interior of Neptune help form the magnetosphere of Neptune. Heat generated within Neptune helps form the unusual winds of the atmosphere. The Structure of Neptune's Interior There is no surface to the giant planets, only a gradual change from the atmosphere, as pictured in this drawing. Therefore the giant planets do not have strict layers, as the earth-like planets do. Ice begins forming in the atmosphere of Neptune and keeps increasing until there is slush, and then solid ice. There is not much difference between the atmosphere and the ocean, unlike the earth, where there is a surface. An Overview of Neptune's Atmosphere Neptune's atmosphere shows a striped pattern of clouds. This cloud pattern is very similar to that of Jupiter and Saturn. Neptune even has a Great Dark Spot similar to Jupiter's Great Red Spot. The history of Neptune's atmosphere is similar to that of the other Giant planets. The composition of Neptune clouds is thought to be methane molecules. Motions in the cloud patterns give clues about Neptune's weather, which is similar to that of Jupiter and Saturn. Neptune's Great Dark Spot of 1989 Unlike Jupiter's Great Red Spot, the Great Dark Spot of Neptune is thought to be a hole, similar to the hole in the ozone layer Earth, in the methane cloud deck of Neptune. The white clouds shown in the picture are above the "hole". In many images of Neptune, the Great Dark Spot can be seen to change size and shape. on This is a close-up view of the Great Dark Spot taken on the Voyager in August 1989. NASA Neptune's Moons and Rings Neptune has 13 moons. It also has rings, but its rings are different from Saturn's. Neptune's largest moon is named Triton. Triton is much larger than any of the planet's other moons. Triton is a very cold place, so the moon is covered with ice. Even though Triton is cold there is a lot going on there. It has geysers like the ones at Yellowstone Park on Earth. The geysers shoot ice 8 km (5 miles) high into Triton's thin atmosphere! There may be water under the ice at Triton. It is even possible that there might be life in that water. The interior of Triton is probably geologically active. The rest of Neptune's moons are much smaller than Triton. Nereid was discovered by Gerard Kuiper in 1949. Despina, Galatea, Larissa, Naiad, Proteus and Thalassa were found by the Voyager 2 spacecraft in 1989. Five more small moons have been found recently. Three of those were discovered in 2002 and two more were found in 2003. The newest moons don't have official names yet. Neptune's rings are much darker than Saturn's bright rings. Saturn's rings are made of ice, which reflects lots of light. Neptune's rings are probably made of rocks and dust. Rocks and dust don't reflect as much light. This is a color image of Triton taken by Voyager 2 in August 1989. Triton is Neptune's largest satellite. Surface Structures Internal Structures Atmosphere Moons and Ring Systems