URANUS Discovered in 1781 by William Herschel.

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5 URANUS Discovered in 1781 by William Herschel.

6 NEPTUNE Discovered in 1846, using Newton s Laws.

7 Jupiter and Saturn consist mainly of hydrogen and helium. Jupiter: Escape speed = 60 km/sec Air temperature = 165 K (-160 o F) Saturn: Escape speed = 35 km/sec Air temperature = 93 K (-290 o F) Earth: Escape speed = 11 km/sec Air temperature = 290 K (60 o F)

8 Atmospheres of Jupiter and Saturn retain H, He. Because Jupiter and Saturn are cold and have high escape speed, they hang onto hydrogen and helium. Jupiter s atmosphere is 75% hydrogen, 24% helium. Saturn s atmosphere is 92% hydrogen, 6% helium. Question: Where is Saturn s helium? Answer: Saturn is so cold, its helium condenses and rains downward.

9 Jupiter and Saturn radiate away more energy than they receive from the Sun. What is the source of the extra energy? Mostly, it is heat left over from when the planets formed. (Big objects cool more slowly). In addition, the helium rain of Saturn generates heat as it falls.

10 As objects fall, gravitational potential energy is converted to other forms of energy. Water at the base of Niagara Falls is 0.13 o C warmer than at the top.

11 Jupiter and Saturn have belts and zones of clouds, plus circular storms. Air heated from above by the Sun, from below by internal heat: Strong winds and large storms. We see clouds of ammonia (NH 3 ), colored by complex compounds.

12 Jupiter s atmosphere is divided into light-colored zones and darkcolored belts. High-speed winds blow eats or west at the boundaries between them.

13 Saturn has a similar pattern of zones and belts. They are less dramatic than Jupiter s: Saturn s clouds are buried deep in its atmosphere and are blurred by haze.

14 The Great Red Spot of Jupiter An enormous circular storm (up to 40,000 km across) in the southern hemisphere of Jupiter.

15 Clouds are usually made of different stuff from the air they float in. Venus: carbon dioxide atmosphere, sulfuric acid clouds. Earth: nitrogen atmosphere, water clouds. Mars: carbon dioxide atmosphere, water & carbon dioxide clouds. Jupiter & Saturn: hydrogen atmosphere, ammonia clouds.

16 Jupiter and Saturn are differentiated.

17 Moons of Jupiter & Saturn

18 Jupiter has ~70 known moons. ~60 are small (<300 km across) and irregular (similar to Phobos and Deimos). Many of the outer moons have retrograde orbits; they are probably captured asteroids.

19 Moons of Jupiter: orbits.

20 4 of the moons of Jupiter are large (> 3000 km across) and spherical (like our Moon). These are the four Galilean moons: Io, Europa, Ganymede, Callisto

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22 Galilean Moons: A miniature analog to the Solar System Revolution counterclockwise, on nearly circular orbits, in nearly the same plane. Io and Europa: mostly rock Ganymede and Callisto: rock and ice

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24 Galilean Moons: Interiors

25 Callisto: Jupiter s CRATERED moon Surface of Callisto is dust-covered ice. Impact craters break through dusty layer to clean ice below.

26 Ganymede: Jupiter s LARGE moon Ganymede is larger in diameter than the planet Mercury: However, Ganymede density = 1900 kg/m 3 ; Mercury s density = 5400 kg/m 3. Ganymede is less massive than Mercury.

27 Europa: Jupiter s SMOOTH moon Europa has almost no craters, no hills more than a few hundred meters tall. Dark lines on a white icy surface.

28 Spectrum of Europa s surface shows it is almost pure water ice. Fracture lines reveal the icy crust is broken into many small plates : But density of Europa is 3000 kg/m 3. => Thin ice layer over rocky interior.

29 Close-ups reveal that Europa s ice is fractured into rafts and floes, like those of the Arctic Ocean: Thin ice layer atop liquid water ocean atop rocky interior.

30 Io: Jupiter VOLCANIC moon Io has no craters, but MANY active volcanoes. Garish colors come from sulfur and sulfur compounds.

31 Voyager and Galileo spacecraft saw volcanoes in the process of erupting: Impact craters are covered by sulfur, sulfur compounds, and lava at a rate of 1 meter per century!

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33 Io: a cartographer s nightmare (or a dream). Two pictures taken five months apart: Io is volcanically hyperactive. WHY?

34 What heats Io s interior? Io experiences strong tides because it is close to Jupiter. Io s orbit is constantly changing, thanks to an orbital resonance with Europa. Close to Jupiter: larger tidal bulges. Far from Jupiter: smaller bulges. Tidal flexing of Io heats its interior.

35 Terrestrial planets: Internal heat determined by size of planet. Galilean moons: Internal heat determined by proximity of Jupiter. Near Jupiter (Io): lots of tidal activity, much volcanic activity, no ice. Far from Jupiter (Callisto): little tidal heating, no volcanic activity, lots of ice.

36 Saturn has 56 known moons. 49 are small (<300 km across) and irregular (like small moons of Jupiter). 6 are mid-sized ( km in diameter) and spherical. One of Saturn s moons is the giant moon Titan.

37 Moons of Saturn: orbits.

38 Titan: Saturn s ATMOSPHERIC moon Nearly the same size as Ganymede: escape speed is the same. Twice as far from the Sun as Ganymede: temperature is lower. Titan, alone among moons, has a substantial atmosphere.

39 Titan s atmosphere: About 90% nitrogen (N 2 ), some methane and other hydrocarbons. Surface air pressure is 50% higher than Earth s. Surface air temperature is 95 Kelvin, cool enough for hydrocarbon clouds to form. Surface is hidden by a layer of smog.

40 On January 14, 2005, the Huygens probe launched by the Cassini spacecraft landed on Titan. What chemistry is going on in Titan s atmosphere? Does Titan have hydrocarbon oceans? Does Titan have complex organic compounds?

41 Some press releases: NASA Reports That Methane Drizzles on Saturn's Moon, Titan July 27, 2006 Cassini Finds Lakes on Titan's Arctic Region July 27, 2006 Scientists Solve the Mystery of Methane in Titan's Atmosphere March 1, 2006 Stormy Weather on Saturn's Moon Titan October 20, 2005 Organic Materials Spotted High Above Titan's Surface April 25, 2005

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43 Uranus and Neptune are nearly identical in their internal structure. Uranus and Neptune are planetary twins, like Earth and Venus. Uranus is 3% larger in radius but 15% smaller in mass than Neptune.

44 Interiors of Uranus and Neptune 1) Gaseous atmosphere: hydrogen, helium, methane 2) Liquid outer layer: hydrogen, helium 3) Liquid or slushy mantle: water, ammonia 4) Solid core: rock, metal

45 The rotation axis of Uranus is tilted by about 90 o, causing extreme seasons. Axis tilts: Jupiter = 3 o Saturn = 27 o, Neptune = 30 o Uranus = 98 o

46 The seasons of Uranus Winter solstice (AD 1985): north perpetual dark, south perpetual sun. Vernal equinox (2006): sun rises once every 17 hours. And so forth

47 Near the winter solstice (1986), the atmosphere of Uranus was boring: (left: visible light) No storms; very faint belts & zones. (right: ultraviolet) Heat flow from summery south to wintery north smeared out storms.

48 As the vernal equinox approaches, Uranus is becoming stormy: Heating of northern hemisphere produces storms, made visible by clouds of methane.

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50 Uranus has 27 known moons. 22 are small (< 200 km across) and irregular. 5 are mid-sized (400 => 1600 km in diameter) and spherical. No giant moons.

51 Neptune has surprisingly strong storms, Temperature at Neptune s cloud level is 55 Kelvin (-360 o F). This is as warm as Uranus, although Neptune is much further from the Sun. driven by internal heat.

52 Neptune s atmosphere is being warmed from below by internal heat. This extra heat drives large circular storms, like the Great Dark Spot seen above.

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55 Neptune has 13 known moons. 12 are small (< 500 km across) and irregular. One is the giant moon Triton. Proteus Triton Nereid

56 Triton: Neptune s Frosty Moon Surface temperature = 38 Kelvin. Covered with frost at poles: frozen methane, frozen nitrogen.

57 Triton has craterfree plains; perhaps the frozen calderas of extinct ice volcanoes? Density of Triton = 2100 km/m 3 ; ice mantle over rocky core.

58 Triton has geysers 5 miles high driven by vaporized nitrogen. Plumes are swept downwind in the very tenuous nitrogen atmosphere.

59 A farewell to the Solar System: Voyager 2 looks back at Neptune and Triton.

60 PLUTO Jan. 23, 1930 Jan Discovered in 1930, by sheer luck. Orbital period = 248 years Rotation period = 6.4 days

61 ERIS (a.k.a. Xena ) Oct. 21, 2003 Discovered in 2005, systematic search. Orbital period = 557 years Rotation period =??

62 Orbits of Neptune, Pluto and Eris Do not worry they will not collide. The orbits are not in the same plane.

63 Pluto s moon, CHARON, was discovered in From the ground, Charon looked like a lump on Pluto s side.

64 Hubble Space Telescope

65 Pluto and its moon Charon are icy worlds Diameter of Pluto = 2300 km Diameter of Charon = 1200 km that resemble Triton. Separation = 20,000 km

66 Very little is known about the surface of Pluto and Charon. Hubble Space Telescope images suggest white icecaps are present, as well as dark patches (dusty?) and light patches (clean ice?). Albedo varies.

67 Pluto and Charon have many properties in common with Neptune s moon Triton. Cold surfaces (about 40 Kelvin) Icy mantles and rocky cores (about 2000 kg/m 3 ) Pluto has a thin atmosphere (like Triton); Charon has none.

68 At last, a mission to Pluto: NEW HORIZONS Launch: January 2006 Jupiter flyby: February 2007 Pluto flyby: July 2015 Then onward into the outer solar system.

69 Eris ( Xena ), the troublemaker. Discovered in 2005 by Mike Brown and collaborators. It has a moon. It is BIGGER than Pluto! Trouble!

70 Eris and Pluto: Spectra

71 Q: Are Pluto and Eris planets? A: That depends on how you define planet. As of August 2006, new category of dwarf planet.

72 The International Astronomical Union defines "planet" as a celestial body that, within the Solar System, (a) is in orbit around the Sun; (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape; and (c) has cleared the neighborhood Planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune

73 The International Astronomical Union defines a "dwarf planet" as a celestial body that, within the Solar System, (a) is in orbit around the Sun; (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium shape; (c) has NOT cleared the neighborhood around its orbit; and (d) is not a satellite Dwarf Planets (so far): Ceres, Pluto, Eris

74 The Kuiper belt, beyond Neptune, contains small, icy, Pluto-like objects. The Kuiper belt lies close to the ecliptic plane, and stretches from 30 A.U. to 50 A.U. from the Sun. Named after Gerard Kuiper, who predicted it should be full of planetesimals. Over 800 Kuiper belt objects (a subset of trans- Neptunian objects) have been discovered in the belt since 1992.

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76 The icy Kuiper Belt Objects are leftover planetesimals. Kuiper Belt Objects have colors (and spectra) consistent with them being icy. They are probably planetesimals that formed within the Kuiper belt. The Kuiper belt Objects are scattered too thinly for them to have accreted into a larger body.

77 Studies of the Outer Solar System continue. More objects like Pluto and Eris (or even bigger) might exist. They are hard to find: Very dim; Very slow moving. Perhaps more easily detected from their gravitational influence? From studies of Neptune s orbit: No more Jovian (massive) planets within 200 A.U. of the Sun.

78 Sedna: Even further away! Discovered in 2003 by Brown et al.: Eccentricity: Semi-major axis: A.U. Perihelion: A.U. Aphelion: A.U. Orbital period: years! Might be added as a dwarf planet.

79 Sedna: Sedna the most distant object known in the Solar System.

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81 Comets

82 Comets are big, dusty snowballs. If a comet comes close to the Sun, the ice is vaporized, and the dust is freed. Thus, comets in the inner Solar System are surrounded by dust clouds. If the Earth passes through the dust, a meteor shower results.

83 Naked-eye comets are spectacular: for much of history they were unpredictable. Adoration of the Magi, Giotto, AD 1304

84 18 th Century: Edmund Halley found that a comet in AD 1607 had the same orbit as a comet in AD It is the same comet! Now called comet Halley; its most recent return was in 1986.

85 Comet Halley has an orbit which is highly eccentric, highly inclined, and retrograde. Orbital period of 76 years.

86 Comets are dirty snowballs : ice mixed with dust and carbon compounds. Strip away their tails, and comets are just snowballs several km across. A comet contains: frozen water, frozen carbon dioxide, ammonia, dust & rocks, carbon, complex carbon compounds

87 Some comets, like Comet Halley, have orbital periods of less than 200 years. However, other comets, like Comet Hale- Bopp, have orbital periods of millions of years.

88 Most comets are in the Kuiper belt or the Oort cloud, far from the Sun. Comets with short orbital periods come from the Kuiper belt, A.U. from the Sun. We know the Kuiper belt is full of icy objects we have seen them!

89 Comets with long orbital periods come from the Oort cloud, ,000 A.U. from the Sun. Oort cloud is a swarm of comets that stretches one-fifth the way to the nearest neighboring star.

90 It is estimated there are several trillion comets in the Oort cloud. Total mass of Oort cloud = 100 times mass of Earth. We only see those rare comets on extremely eccentric orbits.

91 A comet or asteroid packs quite a wallop. An object 10 kilometers across, traveling at 10 km/sec, will upon impact release as much energy as a million 100 megaton bombs. It will blast out a crater at least 100 kilometers across.

92 A comet or asteroid impact probably caused the extinction of the dinosaurs. In the Cretaceous mass extinction, 65 million years ago, 70% of all species were killed off including all dinosaurs. The extinction was rapid, geologically speaking.

93 The Chicxulub impact crater, 180 km across, buried under thick sediment. Estimated age: million years.

94 Few closing questions: 1) How do we know the mass of Ceres? 2) Can asteroids have atmosphere? 3) Do asteroids have volcanoes? 4) Do asteroids have craters? 5) Do asteroids have satellites? 6) How do we know the density of asteroids?

95 1) Name the major differences and similarities between Uranus and the Earth. 2) Name the major differences between Uranus and Neptune. 3) Is there magnetic field around Uranus? 4) Are there volcanoes on Neptune? 5) Are there quakes on Neptune? 6) Is there a ring around Uranus?

96 1) Does Pluto have an atmosphere? 2) Name the major differences between Pluto and Eris. 3) What is Pluto made of? 4) Is there a ring around Pluto? 5) Does Pluto have a moon? 6) Can there be another large planet out there? (planet X )

97 1) Name the major differences and similarities between Jupiter and the Earth. 2) Name the major differences between Jupiter and Saturn. 3) Is there magnetic field around Saturn? 4) Are there volcanoes on Jupiter? 5) Are there quakes on Saturn? 6) Could Mars have a ring?