Part 2: The Outer Planets

Transcription

1 The Solar System

2 Part 2: The Outer Planets

3 Size comparison of the eight planets and Pluto. Note the absence of Saturn s Rings

4 Vocabulary Review: Revolution/Orbital Period: Rotation/Rotational Period: Axis: Astronomical Unit (AU): Satellite:

5 Before we get there.. It s no easy ride from Mars to Jupiter. Along the way you will encounter the Asteroid belt

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7 What is an Asteroid? Asteroids are primordial objects left over from the formation of the Solar System. All asteroids (except Ceres) are currently classified as small solar system bodies. Some asteroids have moons or are found in pairs known as binary systems.

8 Most asteroids orbit the Sun between Mars and Jupiter in a "Main Asteroid Belt" that is centered around 2.7 AU s from the Sun.

9 Way to close.. Some asteroids have been found inside Earth's orbit, while others -- including burnt out or dormant comets, are located beyond Saturn's orbit. Many have orbits that cross Earth's path and while small asteroid fragments hit the Earth every day as meteorites, bigger asteroids are believed to have landed with impacts that killed off a significant share of life on the planet in times past.

10 Fireball over Northern Spain, 4th January Probably began as a 10 m sized object. Exploded at an altitude of 30 km.

11 Every so often the Earth is hit by an asteroid. This crater was created about 40,000 years ago and destroyed my mother-inlaws house. Unfortunately she was not home.. Meteor Crater in Arizona. It is approximately 1 km in diameter.

12 Uh-oh..

13 How Big? Most asteroids may be only the size of pebbles. 16 asteroids have a diameter of 240 km (150 miles) Ceres, the largest, has a diameter of about 914 km (568 miles). It has been estimated that the total mass of the Main Asteroid Belt were combined together, the resulting object would measure less than 1,300 to 1,500 km (810 to 930 miles) across, which is less than one half the diameter of the Earth s Moon.

14 While some have suggested that they are the remains of a protoplanet that was destroyed in a massive collision long ago, the prevailing view is that asteroids are leftover rocky matter that never successfully coalesced into a planet.

15 Stoney asteroid 243 Ida and its satellite Dactyl.

16 253 Mathilde is a main belt asteroid

17 The near-earth asteroid 433 Eros is made of material that has not been much altered since its accretion from the circum-solar dust disk 4.5 billion years ago. Eros is roughly the size of Manhattan, NYC, USA

18 NEAR Shoemaker captured this movie on December 3-4, 2000, while in orbit 200 kilometers (125 miles) from the center of 433 Eros.

19 Eros was one of the first asteroids to be visited by a spacecraft, and the first to be orbited and soft-landed on. NASA spacecraft NEAR Shoemaker entered orbit around Eros in 2000, and came to rest on its surface in February, 2001.

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21 The first 10 asteroids.. 1 Ceres 2 Pallas 3 Juno 4 Vesta 5 Astraea 6 Hebe 7 Iris 8 Flora 9 Metis 10 Hygiea

22 Asteroid 951 Gaspra's Best Face

23 Do you want an asteroid named after you? 7307 Takei (Named after George Takei from Star Trek)

24 Ceres A Planet no longer..

25 Ceres Facts Discovered by Giuseppe Piazzi in According to one NASA estimate, Ceres may have over one-third of the total mass of the Main Asteroid Belt. It is named after Ceres, the Roman goddess of plants and the harvest.

26 What am I???? The classification of Ceres has changed more than once. 1801: At the time of its discovery it was considered a planet s: It was reclassified as an asteroid. 2006: It was again reclassified, now as a dwarf planet.

27 What do you need to know? Revolution: 4.6 years Rotation: 9 Hours 4 minutes. MAXIMUM Surface Temperature: F Tilt on its axis: 4 0 Therefore, it has no seasons. Distance from the Sun: 2.8 AU s.? Miles Gravitational Pull: 27% of Earth.

28 More quick facts.. Because it is round, it is likely that Ceres may have a layered interior like those of the rocky inner planets such as Earth. The dwarf planet appears to have a thin and dusty dark crust that covers a thicker mantle of water ice and a rocky core. Based on its low density, Ceres may be made of 25 percent water ice.

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30 Ceres compared with Earth and Luna (The moon)

31 Rotating?

32 Best pictures so far

33 So when are you coming to visit? To date no space probes have visited Ceres. However, the DAWN Mission, launched in September, It explored the asteroid 4 Vesta in 2011 before heading to Ceres, when it will arrive in 2015.

34 4 Vesta, from the Dawn spacecraft.

35 A view of Vesta s south pole in 2011

36 Vesta, Ceres, and Luna, our Moon

37 More images of Vesta

38 Moving on After leaving the asteroid belt, we notice an enormous object dominating the space in front of us. It is truly immense, large enough that if it were hollow it could contain 1000 Earths. It seems to be a system of its own with many moons and a small ring system. It can only be one thing.the planet Jupiter.

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40 Jupiter

41 History Jupiter was named after the king of the gods in Roman mythology. In Greek mythology, he was known as Zeus, and was famous for hurling lightning bolts at earthbound mortals. Jupiter is the fourth brightest object in the sky, so it has been known to exist since prehistoric times.

42 Jupiter King of the Gods Planets Jupiter is the fifth planet from the Sun and is the largest one in the solar system. Orbital Period: Earth years Distance from the Sun: 482,546,000 miles 5.2 AU s Rotation: 9 hr 55 min 30 sec

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45 Jupiter s tilt is only 3 o. Therefore, it has no seasons.

46 Interesting Facts: This enormous planet radiates twice as much heat as it absorbs from the Sun. It has an extremely strong magnetic field. It is slightly flattened at its poles and it bulges out a bit at the equator.

47 The gravitational pull of Jupiter is about 2.6 times more that of Earth. Therefore, a 100 pound person would weigh 260 pounds on Jupiter.

48 More Facts: Jupiter's diameter is 88,700 miles (142,800 km). This is a little more than 11 times the diameter of the Earth. Jupiter is so big that all the other planets in our Solar System could fit inside Jupiter (if it were hollow).

49 OK, we get it It s a big planet.

50 Jupiter s Atmosphere In 1995, The spacecraft Galileo dropped a probe into Jupiter's atmosphere. This is what we learned. Jupiter is essentially a gigantic ball of hydrogen gas with small amounts of helium and other gasses. One surprising find was that there is much less water in Jupiter's atmosphere than previously expected. Astronomers believe that Jupiter has no solid surface. The average temperature at the cloud top is -244 o F

51 The gases get thicker and denser as they get closer to the planet's center. The extreme pressure at Jupiter's core compresses the hydrogen into a rare liquid metallic state.

52 Jupiter's outer atmosphere is extremely dynamic. Colorful bands and spots can be see rotating and changing regularly. The colors in the bands are believed to be the result of chemical reactions in the atmosphere. The Galileo probe measured wind speeds exceeding 400 miles (643 km) per hour.

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54 One of the most famous features of Jupiter is the giant red spot. This spot is actually a gigantic storm system. It is a high-pressure region where the winds blow in a counter-clockwise direction at over 250 miles (402 km) per hour.

55 Red Spot Facts: This whirlwind varies in size and color from year to year. It is the biggest storm in the solar system so large 3 Earth s could fit inside. Jupiter's Great Red Spot was discovered in 1664 by Robert Hooke. It never slows or dies probably because it never goes over land; (on Earth, hurricanes loses most of their energy and die when they stray over land). The smaller white spots are also giant storms. The colors we see are the result of chemical reactions occurring in the atmosphere.

56 Turbulence caused by storms Smaller Storms

57 The Great Red Spot in Black and White. Note the layering details.

58 You want to talk moons? Jupiter has a total of sixty-seven (67) moons, ranging from tiny "moonlets" only a few miles in diameter up to giant Ganymede, which is larger than two of the planets in the solar system. The four largest moons are called the Galilean moons because they were discovered by Galileo Galilei when he started using his primitive telescope almost five hundred years ago.

59 the Jovian System. Jupiter s large number of moons creates a mini solar system known as the Jovian System. The Galileo mission to Jupiter sent back enough information about Jupiter and its moons to keep scientists busy for years to come. Since 2003, we have added 17 moons to the list. The following slides will discuss the more important moons in the Jovian system.

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61 Moons 1. Io 2. Europa 3. Ganymede 4. Callisto 5. Amalthea 6. Himalia 7. Elara 8. Pasiphae 9. Sinope 10. Lysithea 11. Carme 12. Ananke 13. Leda 14. Thebe 15. Adrastea 16. Metis 17. Callirrhoe 18. Themisto 19. Megaclite 20. Taygete 21. Chaldene 22. Harpalyke 23. Kalyke 24. Iocaste 25. Erinome 26. Isonoe 27. Praxidike 28. Autonoe 29. Thyone 30. Hermippe 31. Aitne 32. Eurydome 33. Euanthe 34. Euporie 35. Orthosie 36. Sponde 37. Kale 38. Pasithee 39. Hegemone 40. Mneme 41. Aoede 42. Thelxinoe 43. Arche 44. Kallichore 45. Helike 46. Carpo 47. Eukelade 48. Cyllene 49. Kore 50. S/2003 J2 51. S/2003 J3 52. S/2003 J4 53. S/2003 J5 54. S/2003 J9 55. S/2003 J S/2003 J S/2003 J S/2003 J S/2003 J S/2003 J S/2003 J S/2003 J S/2003 J24

62 The Galilean Moons The four largest moons of Jupiter are known as the Galilean moons and are named Callisto, Ganymede, Europa, and Io. They were discovered in 1610 by Galileo.

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64 Io is one of the most exotic places in the solar system. Io It is the most volcanic body in the SS. The volcanoes shoot out sulfur and sulfur dioxide. It has mountains much taller than those on Earth, reaching heights of 52,000 feet

65 The most surprising thing learned studying Io is that it has active volcanoes! Until we saw this picture, we had assumed that Earth was the only body in the solar system that had active volcanoes.

66 Volcanic Activity

67 A photo snapped by New Horizons as it passed on it s way to Pluto

68 Io is tidally locked to Jupiter meaning it spins as fast as it takes to orbit Jupiter 1.79 days.

69 Io is constantly being expanded and contracted by the gravitational pull of Jupiter, Europa and Ganymede. The continuous tugging on the small moon has many effects: First: the constant expansion and contraction generates a lot of heat. Second: the constant stretching of the surface actually cracks the surface of Io and may eventually break the small moon into pieces. Third : the cracks allow the almost constant volcanic eruptions we observe. Fourth: The surface is in constant change due to the volcanic activity.

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71 A close-up of Io s surface

72 Frozen, Watery Europa One of the most exciting moons in the solar system because it is believed to have a liquid ocean below its surface!

73 Europa is very different from the other moons of Jupiter. Instead of a rocky, cratered surface like Callisto and Ganymede, it instead has a smooth outer surface of cracked ice. There are very few sign of craters on Europa. In fact, only three large craters have been found. This indicates that Europa's surface is very young and active. In photographs sent back by the Voyager and Galileo probes, the surface resembles sea ice on Earth.

74 Europa both spins on it s axis and orbits Jupiter once every 3.55 days.

75 Astronomers believe that beneath a layer of ice may exist an ocean, kept liquid by the moon's internal heat. This liquid ocean could be as much as 30 miles deep.

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77 The existence of life at deep ocean floor vents on Earth have led some scientists to speculate that there could be a similar life on Europa. Around these deep sea vents are life forms that do not need sunlight to survive. They instead feed on bacteria that get their nutrients from chemicals seeping up from the ocean floor. This process is known as chemosynthesis. This could also be the case on Europa! Tube worms at deep ocean vents

78 The surface of Europa

79 Ganymede

80 Cold, Rocky, Huge Ganymede This moon is Jupiter s largest. It is also the largest in the solar system bigger than Mercury and Pluto!

81 Ganymede is another example of the wealth of information sent back by the Galileo mission. The spacecraft has told us that this giant moon has a very liquid iron core, surrounded by a large layer of material that probably is some kind of silicon-based substance with a thin outer layer of ice. The giant moon also has many craters, much like our own Moon, but the craters are, mysteriously, not very deep and appear to be smoothed out, as opposed to the craters on our Moon. The Galileo mission also taught us that, at one time, there were active volcanoes on Ganymede, but there are no volcanoes now. The giant moon also has fault lines, like the famous San Andreas fault in California. The lines tell us that at one time the moon's surface was flexing and changing like Io's does now, although that activity seems to have come to a stop.

82 Facts: Ganymede has a weak magnetic field the only moon that does. Ganymede spins and orbits Jupiter once every 7 days. Ganymede probably has a liquid iron core and a mantle made of layers of salt water and ice. Ganymede has a thin basic atmosphere.

83 Frozen, Cratered Callisto The last of the Galilean moons is Callisto. Scientists think that Callisto may also have a thin ocean below the surface!

84 With a diameter of 2880 miles, Callisto is quite a bit larger than our own Moon, but is still much smaller than giant Ganymede. Callisto is without a doubt the most heavily cratered object in the known solar system. This suggests that the surface is very old and inactive.

85 Facts: Callisto orbits Jupiter and spins on it s axis once every 16.7 days. It may have and Antifreeze ocean layer deep under the crust. Callisto has a thin atmosphere of CO 2

86 In the Future.. In 2003 Callisto was chosen as a possible site for colonization. It was chosen because it is far enough from Jupiter that it is not affected by it s radiation, and it is geologically stable. Starbase Callisto?

87 It appears that Callisto's surface has undergone very little change since its formation over four billion years ago. Because its surface is so geologically inactive, astronomers believe that Callisto may represent what many of the moons in the Solar System may have looked like at a much earlier stage in their development.

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89 The surface of Callisto.

90 IO Europa Ganymede Callisto Cutaway views of the possible internal structures of the Galilean satellites.

91 Jupiter with Io and Europa

92 The less important children.. Jovian satellite Metis, imaged by the Galileo spacecraft Very little is known about these moons, most are probably captured asteroids and/or comets. This image of Thebe was taken by NASA's Galileo spacecraft on January 4, 2000, at a range of 193,000 kilometers.

93 Surprise!! Voyager 1 found that Jupiter has rings, much like Saturn. But unlike Saturn's rings, Jupiter's rings are very dark, thin, delicate and simple. They are too thin to be seen from Earth. They are probably formed by dust created bymeteor impacts on the Galilean moons.

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96 Jupiter Assaulted

97 Comet Shoemaker-Levy From July 16 through July 22, 1994, pieces of an object designated as Comet P/Shoemaker-Levy 9 collided with Jupiter. This is the first collision of two solar system bodies ever to be observed, and the effects of the comet impacts on Jupiter's atmosphere have been simply spectacular and beyond expectations. Comet Shoemaker-Levy 9 consisted of at least 21 seperate fragments with diameters estimated at up to 2 kilometers.

98 This is an infrared of Jupiter taken on July 21, Io, the closest of the Jovian moons, can be seen crossing the planet in the northwest of the image (top right).

99 This is an ultraviolet image of Jupiter taken on July 21, Io, the closest of the Jovian moons, can be seen crossing the planet.

100 Image of Jupiter with the Hubble Space Telescope Planetary Camera. Eight impact sites are visible.

101 One of the strangest facts about this comet is the fact that it was orbiting Jupiter, not the Sun. Studies indicate that the comet was captured away from the sun by Jupiter in the late 1960 s or early 1970 s. Since then, it has gotten closer and closer to Jupiter until finally, the collision took place. Since the impact of SL9, two further very small comets have been found to be orbiting Jupiter. Studies have shown that the immense planet can capture comets from solar orbit into Jovian orbit rather frequently.

102 The visible scars from the impacts could be seen on Jupiter for many months after the impact. They were extremely prominent, and observers described them as more easily visible even than the Great Red Spot.

103 Missions to Jupiter: Pioneer 10 and Pioneer 11 (They are heading off into interstellar space, the first craft ever to do so. Both carry a graphic message in the form of a 6- by 9-inch gold anodized plaque bolted to the spacecraft's main frame.) Voyager 1 & Voyager 2 (Both have traveled out of the solar system and if no unforeseen failures occur, we will be able to maintain communications with both spacecraft until at least the year Both Voyagers have plenty of hydrazine fuel -- Voyager 1 is expected to have enough propellant until 2040 and Voyager 2 until 2034 ) Galileo (Galileo was deliberately crashed in to Jupiter in 2003 to prevent any possibility that it might crash into Europa and contaminate any life that might be there. ) New Horizons Spacecraft (arrived in Feb and is moving on to Pluto)

104 The Galileo spacecraft's 14-year odyssey came to an end on Sunday, Sept. 21, 2003 when the spacecraft passed into Jupiter's shadow then disintegrated in the planet's dense atmosphere

105 Voyager

106 Moving on.. After leaving the Jovian system, we need to travel over 372, 000, 000 to reach our next planet. As we first enter the Saturnian system, we are presented with a sight of sheer beauty. It is a giant gas planet with a delicate system of silky rings. This is the planet Saturn.

107 Saturn

108 History Saturn was named after the Roman god of agriculture and the harvest. His Greek counterpart was Chronus, the youngest of the Titans. Many consider Saturn to be the most beautiful object in the Solar System. It is one of the more bright objects in the night sky, and has been known since prehistoric times.

109 What to Remember: Saturn is the sixth planet from the sun and is the second-largest planet. It has beautiful rings. Distance from the Sun: 884,740,000 miles 9.5 AU Orbital Period: A year on Saturn takes Earth years Rotational Period: Saturn rotates once every 10.2 hours

110 Tilt! Saturn's equator is tilted 27 degrees, very similar to the 23-degree tilt of the Earth. As Saturn moves along its orbit, first one hemisphere, then the other is tilted towards the Sun meaning it has seasons.

111 Much of what is known about the planet is due to the Voyager explorations in The mean temperature on Saturn (at the cloud tops) is 88 K (-185 C; -290 F). One very noticeable feature is that Saturn is visibly flattened at the poles as a result of the very fast rotation of the planet on its axis.

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114 The Atmosphere Saturn is a gas giant with a chemical composition very similar to that of Jupiter. It has75% hydrogen and 25% helium, with traces of methane, water, and ammonia. Saturn's interior is probably similar to that of Jupiter, perhaps with a core of rock and ice, surrounded by a layer of liquid metallic hydrogen. It is likely that there is no solid surface on Saturn. Instead, the atmospheric gases get denser and thicker as they approach the planet's center.

115 This picture illustrates the internal structure of Saturn. The outer layer is primarily composed of molecular hydrogen. As we go deeper, the gas starts to resemble a hot liquid. Deeper still, hydrogen changes into a new state of metallic hydrogen. In this state it resembles a molten metal. This metallic hydrogen state occurs at about half of Saturn's radius. Below this is a layer dominated by ice where "ice" denotes a soupy liquid mixture of water, methane, and ammonia under high temperatures and pressures. Finally at the center is a rocky or rocky-ice core.

116 A 100 pound person would only weigh 108 pounds on Saturn.

117 Some Facts: Saturn is also the least dense of all the planets. Density :0.7g/ml If Saturn could be placed into a giant sea of water, it would actually float! Saturn is like Jupiter with high winds and cloud banding, and storms. Winds at the equator can reach an unbelievable 1,100 miles per hour. The Hubble space telescope has also revealed that storms occasionally erupt in Saturn's turbulent atmosphere.

118 Without a doubt, the most striking feature of Saturn is its rings. Saturn's rings have been known to exist since 1659.

119 The Rings can be seen from Earth! This is due to the fact that the ring system is quite large and bright. It can easily be seen from Earth with a small telescope. The view of Saturn's rings from Earth actually changes from year to year. This is caused by the Earth passing through the plane of Saturn's rings. Every few years, the rings are seen edge-on and actually seem to disappear completely. This is because the rings are only slightly more than a mile thick.

120 Saturn has three main ring bands. They are identified as the A, B, and C-rings. A large gap can be seen between the A and B rings. This is known as the Cassini division, and was named after Giovanni Cassini, the discoverer the gap.

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122 Ring Facts: The origin of Saturn's rings is not well understood. They are not solid. They are composed of millions of tiny particles of ice and rock and dust, each in its own orbit around the planet. The particle size ranges from dust grains to as giant icebergs.

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124 Astronomers once believed that the rings could be the debris of one or more moons that were pulverized by asteroid or comet impacts. New observations by NASA's Cassini spacecraft indicate the rings of Saturn, once thought to have formed during the age of the dinosaurs, instead may have been created roughly 4.5 billion years ago when the solar system was still under construction. Scientists once believed Saturn's rings were relatively youthful and likely created by a comet that shattered a large moon, perhaps 100 million years ago. But ring features seen by instruments on Cassini indicate the rings were not formed by a single cataclysmic event. The ages of the different rings appear to vary significantly and the ring material is continually being recycled. The evidence is consistent with the picture that Saturn has had rings all through its history

125 The larger rings are actually composed of many smaller ringlets. The Voyager probes also found strange radial, spoke-like structures in the rings. These are believed to be caused by dust particles. Saturn's innermost ring, the F-ring, appears to be knotted or braided. Astronomers are not sure yet what causes this strange phenomenon.

126 Not an actual photo Artist s conception

127 Note the accretion of particles smaller ones combining to form larger ones.

128 An artist concept of a close-up view of Saturn's ring particles. The blue particles are composed mostly of ice and clump together to form elongated, curved aggregates, continually forming and dispersing. The space between the clumps is mostly empty. The largest individual particles shown are a few yards across. Image courtesy NASA/JPL/University of Colorado.

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130 Shepherd Moons The Voyager probes revealed the existence of small moons in and near the rings. They are called "shepherd satellites" because they are believed to help keep the rings in place and stop the particles from building up into larger ringlets. Without the gravitation of these moons, the ring particles might fly off into space or crash into the planet and the rings would be lost.

131 Mimas

132 Ring particles, being closer to the planet than the satellite, are moving faster. If a particle drifts out of the ring, when it overtakes the satellite, the satellite's gravity will drag the particle back, slow it down, and cause it to fall back into the ring.

133 Stay to the right, single file!!

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135 The Many Moons of Saturn

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137 That s a lot of moons. To date, 62 moons have been found, with 52 officially named. Some moon names are: Aegir, Albiorix, Anthe, Atlas, Bebhionn, Bergelmir, Bestla, Calypso, Daphnis, Dione, Enceladus, Epimetheus, Erriapus, Farbauti, Fenrir, Fornjot, Greip, Hati, Helene, Hyperion, Hyrokkin, Iapetus, Ijiraq, Janus, Jarnsaxa, Kari, Kiviuq, Loge, Methone, Mimas, Mundilfari, Narvi, Paaliaq, Pallene, Pan, Pandora, Phoebe, Polydeuces, Prometheus, Rhea, Siarnaq, Skadi, Skoll, Surtur, Suttung, Tarqeq, Tarvos, Telesto, Tethys, Thrym, Titan and Ymir. Astronomers keep finding new moons, both using ground-based observatories and cameras onboard Cassini.

138 The dozens of moons orbiting Saturn vary drastically in shape, size, age and origin. Some of these moons have rocky surfaces, while others are porous, icy bodies. Many have craters, ridges and valleys, and some show evidence of tectonic activity. Some appear to have formed billions of years ago, while others appear to be pieces of a bigger, fragmented body. The most interesting one is Titan, the biggest of them all. Larger than Earth's Moon, Titan even has its own thick atmosphere -- the only natural satellite in the Solar System with such a luxury.

139 Titan Saturn s Largest Moon

140 Titan is the fifteenth of Saturn's known satellites and the largest. It was discovered 3/25/1655. Titan s composition is about half water ice and half rocky material. It is larger than Luna. The Cassini orbiter has studied Titan. In January 2005, the Huygens probe actually landed on the surface of Titan and sent back images from the surface.

141 What s so special about Titan? Alone of all the satellites in the solar system, Titan has a significant atmosphere. It is composed primarily of molecular nitrogen (as is Earth's) with a small amount of argon and methane. Interestingly, there are also trace amounts of at least a dozen other organic compounds The organics are formed as methane, which dominates in Titan's upper atmosphere, is destroyed by sunlight. The result is similar to the smog found over large cities, but much thicker.

142 In many ways, this is similar to the conditions on Earth early in its history when life was first getting started. But it is this thick hazy atmosphere that makes it so hard to see Titan's surface. Scientists think Titan is a good model of Earth a few million years ago.

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144 A picture from the surface of Saturn s moon Titan

145 Liquid Methane lakes on Titan

146 As we continue to study Titan, we find that we still have a lot of work ahead of us. PIA07236: Mosaic of River Channel and Ridge Area on Titan

147 The view from the Huygens probe as it descended through the Titanian atmosphere.

148 Other Moons of Interest. The following slides show a few of Saturn s many moons and some interesting facts.

149 More strangeness Trojan Moons Saturn has moons that share the same orbit. These moons all orbit at exactly the same distance from Saturn, but never meet.

150 Trojan Moons Telesto and Calypso bracket Tethys. Helene and Polydeuces bracket Dione. Telesto Calypso Tethys

151 Epimetheus and Janus share almost the same orbit and switch places with each other every 4 years. Janus Epimetheus

152 Mimas (7) Mimas reveals a striking resemblance to the popular film Star Wars Death Star. It is for that reason nick named by astronomers "The Death Star" moon The large crater which you can see is named Herschel. It is so big compared to the size of Mimas, that what ever hit the moon probably just about tore it apart. Mimas was discovered in 1789 by William Herschel. This moon was named after one of the Titans slain by Hercules.

153 Saturn's moons Pan and Atlas (pictured above) may have formed in two stages - their cores may be remnants of the breakup of a large icy body early in the solar system's history and their ridges may have formed later, as the cores swept up material from Saturn's rings. The scenario might explain why the ridges appear smooth and the polar regions rough

154 Enceladus (8) The eighth moon orbiting Saturn is Enceladus. Discovered in 1789 by William Herschel, Enceladus is the brightest object besides the Sun in the Solar System. Of course on Earth we can not see it at all without a telescope. This is because it is small, and far away. The Cassini probe discovered a water-rich plume venting from the moon's south polar region. This discovery, along with the presence of escaping internal heat and very few (if any) impact craters in the south polar region, shows that Enceladus is geologically active today.

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156 Image credit: NASA/JPL/Space Science Institute In this image, Enceladus s geysers are viewed against the skyscape of Saturn s nightside and its rings. Saturn is dimly illuminated by light reflecting off the rings.

157 Tethys (9) Tethys is like a giant ice cube in the sky. It is almost pure water Ice. At one time long ago it was liquid. It would have been a giant ocean, with no land, and no sea bed. Or like a giant blob of water floating in the sky.

158 Dione (12) Rhea (14) Rhea is somewhat similar to Dione. They both have similar composition, albedo features varied terrain and synchronous rotations.

159 Hyperion (16) Saturn's Sixteenth moon was discovered by Bond and Lassell in Hyperion is the largest object in the Solar System that is not ball shaped. its size and shape tells scientists that it probably used to be part of a larger rounder moon, which somehow broke apart.

160 Iapetus (17) A really strange moon Iapetus is another world made up almost entirely of water ice. This ice cube does not however follow the same path that Saturn's other moons follow. Instead it orbits Saturn from top to bottom. Another strange feature of this moon is that one side is completely black and the other is almost white! No one knows why it is this way.

161 Another view of Iapetus.

162 Phebe Phebe orbits opposite all of the other moons (West- East) and is likely a captured Kuiper belt object.

163 Visits Pioneer 10 Voyagers 1 & 2 Cassini / Huygens

164 As our exploration of the Saturian Family comes to a close.. we prepare for another long Journey to a planet no one knew about until the 1700 s.

165 Uranus

166 History of Uranus The first planet discovered in modern times. Discovered by William Herschel on March 13, Uranus is named for the ancient Greek god of the Heavens, the only planet with a Greek, not Roman name.

167 The Important Stuff Uranus is the seventh planet from the sun in our solar system. It is the 3 rd largest. Distance from the Sun: 1.8 billion miles, A.U. Orbital Period: it takes Earth years for Uranus to orbit the sun once. Rotation: Each day on Uranus takes 17.9 Earth hours.

168 Earth vs. Uranus

169 A 100-pound person on Uranus would weigh 91 pounds.

170 What s inside? Uranus is a frozen, gaseous/icy planet with a rocky core. Uranus' atmosphere consists of 83% hydrogen, 15% helium and 2% methane.

171 Unlike Jupiter and Saturn, Uranus, (and Neptune), are better called ice giants than gas giants. The mantle is believed to be a mixture of icy water and ammonia.

172 The mean temperature on the surface of Uranus' cloud layer is -350 F

173 Tilted Over? Uranus has the strangest tilt of all of the planets. Its tilt is 97.9 o! That means that Uranus rolls along in its orbit on its equator.

174 This tipped rotational axis gives rise to extreme seasons on Uranus.

175 Its unusual position is thought to be the result of a collision with a planet-sized body early in the solar system's history.

176 More Rings In 1977, the first nine rings of Uranus were discovered. During the Voyager encounters, these rings were photographed and measured, as were two other new rings and ringlets. Uranus' rings are distinctly different from those at Jupiter and Saturn. All rings seem to contain a very thin distribution of fine dust. The outermost epsilon ring is composed mostly of ice boulders several feet across.

177 This dramatic Voyager 2 picture reveals a continuous distribution of small particles throughout the Uranian ring system. All the previously known rings are visible here; however, some of the brightest features in the image are bright dust lanes not previously seen.

178 Ring Facts Most of the rings are not quite circular. Most are not exactly in the plane of the equator. The rings vary in brightness. The rings are very narrow (some only a few kilometers across) and no material can be detected in the regions between the rings. The rings possess small shepherd satellites, as discussed with Saturn's rings.

179 Voyager 2 has discovered two 'shepherd' satellites associated with the rings of Uranus. The two moons -- Cordelia (bottom) and Ophelia (top)-- are seen here on either side of the bright epsilon ring; all nine of the known Uranian rings are visible. The image was taken Jan. 21, 1986

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181 Voyager 2 is the only spacecraft to study Uranus.

182 Another Large Family Like the other gas giants, Uranus has a large group of satellites in orbit 27 to date. Five of these moons are quite large.

183 The five largest moons of Uranus

184 The many moons of Uranus 1. Cordelia 2. Ophelia 3. Bianca 4. Cressida 5. Desdemona 6. Juliet 7. Portia 8. Rosalind 9. Mab 10. Belinda 11. Perdita 12. Puck 13. Cupid 14. Miranda 15. Francisco 16. Ariel 17. Umbriel 18. Titania 19. Oberon 20. Caliban 21. Stephano 22. Trinculo 23. Sycorax 24. Margaret 25. Prospero 26. Setebos 27. Ferdinand All named for characters created by Shakespeare and Alexander Pope.

185 Cordelia (1) and Ophelia (2) The closest moon to the Surface of the planet Uranus is Cordelia and the next is Ophelia. These moons are called sheperd rings because they help maintain the shape of the first two Uranian rings.

186 Bianca (3) Bianca was discovered by Voyager 2 in It is a small icy world, and like many of Uranus' moons, probably a captured asteroid, or comet.

187 Miranda (11) The eleventh of Uranus' moons is the world Miranda. Miranda is a very interesting world. It is made up half of rocky material, and half of ice. In one place Miranda has a huge cliff that is 5 km high. Miranda was discovered in 1948 by Kuiper.

188 Ariel (12) Discovered in 1851 by Lassell, Ariel is the twelfth moon in Uranus' system. As you can see by looking at the picture, Ariel is made up of a system of large interconnecting valleys. These valleys probably formed as Ariel froze, and cracked. The smooth surface of this world tells scientists that it has been re-surfaced some time in the recent past.

189 Umbriel (13) Umbriel's heavily catered surface gives scientists a lot of information about this world. It means that Umbriel has not been resurfaced for a long time, probably since it was first formed.

190 Quick Vocab Lesson Re-surfacing is when the old surface of a world is covered up by a new surface. This happens on Earth by volcanic eruptions. Re-surfacing can sometimes happen if a hot object hits a world. That object might melt the surface, forming a new smoother surface in its place.

191 Titania (14) Discovered in 1787 by William Herschel, Titania is the largest of Uranus' moons. It is believed that long ago Titania had a huge liquid water ocean. As Titania cooled, the surface froze over. Later as the interior froze, the surface expanded, and cracked. These cracks formed the giant valleys found on Titania today.

192 Oberon (15) Uranus's fifteenth moon, and second largest is Oberon. Oberon was discovered in 1787 by William Herschel. Early in Oberon's history it was a very active world, but today it is frozen solid. Oberon is about half rock, and half frozen water.

193 Little is known about the other moons, as no spacecraft have visited them since their discovery.

194 Bidding the Uranian system goodbye, we travel another incredible distance, only to come to a planet that seems to be almost the same as the one we left.

195 Neptune

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197 History Neptune was named after the Roman god of the seas. It was discovered in 1846 by Johann Gottfried Galle of the Berlin Observatory. Neptune was the first planet whose existence was predicted mathematically (the planet Uranus's orbit was perturbed by an unknown object which turned our to be another gas giant, Neptune).

198 The Facts: Neptune is the eighth planet and outermost of the gas giants. Distance from the sun: 2,798,116,000 miles 30.06AU Orbital Period: Neptune orbits the Sun once every 165 years. (This is the first year since it s discovery that Neptune has completed one revolution around the sun!!) Rotational Period: A day on Neptune is 16 hours and 7 minutes.

199 The sun as seen from Neptune

200 A 100- pound person would weigh 119 pounds on Neptune.

201 Seasons in the sun.. Neptune's rotational axis is tilted 30 degrees. This gives Neptune seasons. Each season lasts 40 years; the poles are in constant darkness or sunlight for 40 years at a time.

202 The first two thirds of Neptune is composed of a mixture of molten rock, water, liquid ammonia and methane. The outer third is a mixture of heated gases comprised of hydrogen, helium, water and methane. Methane gives Neptune its blue cloud color.

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206 Since Neptune was known to be so similar to Uranus, astronomers were expecting to find another bland and featureless world when Voyager 2 arrived there in August of Much to the surprise of everyone on the Voyager team, Neptune would turn out to be anything but bland. The most startling feature to be discovered was a giant blue spot in the planet's southern hemisphere. It was very similar in appearance to the famous red spot on Jupiter, and about half the size.

207 In 1994, the Hubble space telescope revealed that the giant blue spot has disappeared. Recently, a new blue spot was observed in the planet's northern hemisphere. Astronomers are not sure why the spot on Neptune was relatively short-lived, while the great red spot on Jupiter has bee present for hundreds of years. This indicates that the Neptune's atmosphere is highly active and prone to sudden and severe changes.

208 Neptune s cloud temperature is F. The core is thought to be F. This difference is what drive the many storms seen on Neptune.

209 Neptune has a set of four rings which are narrow and very faint. The rings are made up of dust particles thought to have been made by tiny meteorites smashing into Neptune's moons. From ground based telescopes the rings appear to be arcs but from Voyager 2 the arcs turned out to be bright spots or clumps in the ring system. The exact cause of the bright clumps is unknown.

210 Visitations: Neptune was visited by NASA's Voyager 2 in August, Before this visit, virtually nothing was known about Neptune.

211 Neptune's rings were first detected in star occultation experiments from Earth in 1983, but they were very difficult to study before the data from Voyager 2. The following image (Ref) shows the faint rings of Neptune (the light from the body of the planet is blocked off in the black region to allow the rings to be seen clearly).

212 Another view of Neptune s rings.

213 Going..Going..Gone?? New Earth-based observations announced in 2005 appeared to show that Neptune's rings are much more unstable than previously thought. In particular, it seems that the Liberté ring might disappear in as little as one century. The new observations appear to throw our understanding of Neptune's rings into considerable confusion.

214 Again with the moons! Neptune possesses thirteen confirmed moons. The largest of these moons is Triton. 1. Triton 2. Nereid 3. Naiad 4. Thalassa 5. Despina 6. Galatea 7. Larissa 8. Proteus 9. Halimede 10. Psamathe 11. Sao 12. Laomedeia 13. Neso

215 The first four moons of Neptune, Naiad, Thalassa, Despina, and Galatea, are so close to Neptune that they orbit within its ring system. Little is known about them. The next one out, Larissa, was actually discovered in 1981, when it blocked a star.

216 Proteus is the second-largest moon in orbit around Neptune. It is so close to the planet that Earthbound telescopes cannot see it.

217 The last five moons were discovered in the first few weeks of and throughout Very little is yet known about them.

218 Triton

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220 Is it stolen?? Triton orbits Neptune in what is known as a retrograde orbit. This means that it orbits Neptune a direction opposite the planet's rotation. It is the only large moon in the Solar System to do this. Astronomers are not quite sure of the reason for this. It is likely that Triton was captured by Neptune's gravity. In fact, many astronomers have noticed that the surface features of Triton, as well as its size, are very similar to what they believe the planet Pluto to look like. Some even wonder if there is some connection between Triton's features and the fact that Pluto actually crosses Neptune's orbit from time to time.

221 Triton is the coldest known object in the Solar System. Its surface temperature averages only -391 F. This is caused by the moon's high albedo. Very little sunlight is absorbed by the surface. Triton's axis of rotation is tilted 157 degrees with respect to Neptune's axis. This causes the moon's polar and equatorial regions to be alternately pointed towards the Sun. This causes extreme seasonal changes as Triton's orientation changes. This uneven heating and cooling could account for some of the moon's curious surface features. Triton has an extremely thin atmosphere composed mainly of nitrogen and methane.

222 There are very few craters visible on the surface, indicating that the moon is very young and probably highly active. There are high ridges and deep valleys all over the moon's surface. Voyager 2 discovered that Triton has ice volcanoes. Voyager photographed a plume of frozen material being ejected from the moon's surface. Astronomers believe this material to be composed of liquid nitrogen or methane.

223 The Mystery Continues

224 As we move on from Neptune, we leave more questions than answers. However, if we think that is bad, we find many unanswered questions at the Dwarf planet, Pluto

225 Pluto A Dwarf Planet or

226 How Pluto was discovered.

227 The best photos we have of Pluto & Charon

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231 An outcast. As we leave behind us the system of Neptune and its moons, we leave the world of the gas giants and approach the extreme outer limits of our solar system. This region is inhabited by a small, rocky world called Pluto. Once called a planet, it was demoted in the summer of 2006 because of the discovery of many other pluto-like objects. (See the Outer Solar System Show)

232 What makes a Dwarf a Dwarf? A dwarf planet is defined as being a celestial body which fulfills the following four criteria: it must orbit the Sun, it must have sufficient mass that its gravity causes it to be round or almost round in shape, it must not be another object's satellite, and it must not have cleared the neighborhood around its orbit. In other words, a dwarf planet is an object orbiting the Sun which is not quite big enough to be a planet, but which is bigger than a mere asteroid or Kuiper Belt Object. Also, it is important to note that a 'dwarf planet' is a totally separate category of object to 'planet', thus a dwarf planet is not a subcategory of 'planet', but a different class of object in its own right.

233 History Pluto was named after the Roman god of the underworld. Charon, its companion, was named after the mythological figure who ferried the dead across the river Styx to Hades. Pluto was discovered in 1930 by Clyde W. Tombaugh at Lowell Observatory in Arizona. It was much too small to be seen by most telescopes of the time.

234 Alone? Not Quite. For many decades, Pluto was thought to be alone. In 1978, Jim Christy discovered that Pluto had a companion. This moon, Charon, is more than half the size of its parent planet. In fact, many astronomers have referred to Pluto and Charon as "twin planets" because they are so close to each other in size. Because Charon is so large, it does not actually orbit around Pluto. Rather, the two bodies actually orbit around a common center of gravity somewhere between them. Pluto and Charon are also tidally locked in a synchronous orbit. meaning thy keep the same sides facing each other. Both Pluto and Charon may even share an atomosphere!

235 The facts: Pluto and Charon are one set of many Dwarf Planets beyond the orbit of Neptune. Distance from the Sun: 3,673,537,000 miles or 39.4 AU Orbital Period: Both orbit the sun once in 248 years. Rotational Period: Both spin around an axis between them every 6.3 days.

236 What the Sun and Charon probably look like from the surface of Pluto.

237 Say that again?? Pluto and Charon orbit around each other every 6.3 days. Picture two people holding hands and spinning around a rock on the ground. That is the way these two move.

238 Both Pluto and Charon orbit around this center point every 6.3 days.

239 Tilt Unlike most of the eight classical planets, but similar to Uranus, Pluto spins on its side that is, with its poles almost in its orbital plane. This extreme axial tilt, together with the high orbital tilt, makes for pronounced seasonal climatic changes though, of course, it is always very cold!

240 Pluto's orbit is so highly eccentric that is actually crosses inside the orbit of Neptune. Because of this, Pluto is actually closer to the Sun than Neptune for 20 years of its 248-year orbit. The 17- degree inclination of Pluto's orbit means that it there is no chance that it will eventually collide with Neptune.

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242 Pluto is the only major body in our SS that has not been visited by a spacecraft. It is so tiny, and so far away that even the Hubble space telescope has trouble resolving any surface features on the planet. Pluto s composition is believed to be much like Triton, (Neptune s Moon) In fact, both are believed to have originated from the Oort Cloud. Charon, is believed to me mostly water ice, with less than 30% rock.

243 What s inside? (Pluto)

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245 Was Pluto's moon created by cosmic hit-and-run? Pluto s moon, Charon, may have been blasted off the planet in a large collision early in the solar system's formation, new research suggests. The process is similar to that thought to have formed the Earth's own moon. The object that hit Pluto probably measured between 1600 and 2000 kilometers in diameter, and struck the planet at a speed of 1 kilometer per second. The object may have come from the Kuiper Belt the ring of icy rocks on the fringes of the solar system where Pluto also resides.

246 This collision may have been responsible for other moons as well. Nix and Hydra were discovered in Styx and Kerberos were found in All of Pluto s moons have names that reference the underworld.

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248 Pluto is expecting a visitor in 2015 The New Horizions Spacecraft passed the orbit of Uranus in March of 2011.

249 This artist's rendering illustrates a giant impact scenario similar to one that likely resulted in the two, newly discovered moons of Pluto. A team led by Southwest Research Institute reports on this finding in a paper published Feb. 22, 2006, in Nature.

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251 Scientists think that Pluto might even have rings left over from this collision.

252 What lies beyond Pluto? Recent studies have revealed that icy objects, similar to Pluto and Charon, exist within the Kuiper belt. The Kuiper belt is the region beyond Neptune in the plane of the solar system. It stretches from 30 A.U. to roughly 500 A.U. from the Sun, and is shaped like a stepped-on bagel. In 1951, the astronomer Gerard Kuiper reasoned that if there are no Jovian planets beyond Neptune, then the icy planetesimals that formed in the solar nebula should still be running around loose in the Kuiper belt.

253 Coming Soon: A visit to the Kuiper Belt!