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Uranus & Neptune

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  1. Uranus & Neptune The ‘Small’ Giants 1

  2. Mass & Radius EarthUranusNeptune Mass1 14.5 17.1 Radius Equatorial 6378 25,559 24,764 Polar 6356 24,973 24,341 Oblateness0.003 0.023 0.017 2

  3. Uranus is 4.00 times Earth’s size Neptune is 3.88 times Earth’s size 3

  4. Density & Surface Gravity EarthUranusNeptune kg/m35515 1270 1638 100% 23% 30% gravity 1 G 0.9 G 1.1 G 4

  5. Uranus’ Magnetosphere • Uranus’ spin axis istilted 98o to the ecliptic. How did this come about? • The magnetic axis istilted 59o to the rotation, and it doesn’t pass directly through the center of Uranus. • It’s offset from the center by 0.3 Ru. • Larger, but 30% weaker than Earth’s field. 5

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  9. Movie showing the “corkscrewing” of Uranus’ magnetic field. http://csep10.phys.utk.edu/astr161/lect/uranus/uranus.mpg 9

  10. What’s the Source of the Mag. Field? • Neither Uranus nor Neptune have enough internal pressure to turn their hydrogen into its metallic form. • Then what conducts the electrical current to produce the magnetic field? 10

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  12. Internal Conductor • (Uranus & Neptune are constructed the same way inside.) • Slush of water, methane, ammonia • Dissolved salts • The mantle is the conducting layer, not the core. 12

  13. Conductive icy mantle Rocky, metallic core GaseousH & He Visible cloudsurface 13

  14. What’s missing? The ammonia & sulfur compounds that give Jupiter its bright colors and Saturn its yellows. 14

  15. Neptune is Similar • Neptune’s spin axis is only tilted 28o to the ecliptic. • The magnetic axis istilted 47o to the rotation, and it’s offset from the center by 0.5 RN. • Larger, but 50% weaker than Earth’s field. 15

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  17. Orbits & Rotation UranusNeptune Orbital Distance (AU) 19.2 30.0 (billions of km) 2.9 4.5 Orbital Period (years) 84 165 Eccentricity 0.046 0.011 Rotational Period (hrs) -17.4** 16.1 ** retrograde rotation because of axis tilted 98o (rotational period = rotation of magnetic field) 17

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  19. The tilt ofUranus’ axisaffects itssurfaceactivity. 19

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  21. When the Voyager 2 spacecraft passed by Uranus in 1986, the south pole was pointed at the sun. The sun will be directly over Uranus’ equator in December, 2007 Uranus is becoming more Neptune-like. 21

  22. Atmospheres UranusNeptune Atmos. Temperature 58 K 47 K Wind Speeds (km/hr) 700 2000 Sunlight (W/m2) 3.7 1.5 (earth = 1367 W/m2) 0.3% 0.1% 22

  23. Uranus, not Neptune! Cloud bands & a dark spot are appearing. 23

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  25. The original Great Dark Spot has disappeared in recent years. New storms form occasionally. 25

  26. Neptune’s Rotation, Cloud Bands, Storms http://hal.physast.uga.edu/~jss/1010/ch11/neptune.mpg 26

  27. Both Planets have Rings • Uranus has 13 faint rings • Epsilon Ring is brightest (outermost main ring) • Discovered as a star was occulted by Uranus in 1977. • 2 new outer rings discovered in 2003 • Ring particles are up to 10 m diameter. 27

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  30. Rings of Uranus detail Epsilon Ring 30

  31. Artwork by David Hardy 31

  32. Neptune’s Rings • Neptune only has 4 rings • Much thinner & darker than Uranus’ rings. • Not uniform in density. The rings appear to have denser arcs. • Composed of dust and larger particles. • Named after John Couch Adams, Urbain LeVerrier, Johann Galle. 32

  33. Rings of Neptune 33

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  35. Like Saturn’s rings, one of Uranus’ rings appears twisted or “braided” 35

  36. Uranus’ Moons • More dirty iceballs: • Miranda, Ariel, Umbriel, Titania, Oberon • 16 Small asteroidal moons (Puck, Juliet, Ophelia, Desdemona, etc.) • What is the origin of the moons’ names? • Larger moons (except for Miranda) were all discovered by William Herschel in 1787. 36

  37. Miranda • Miranda shows evidence of having been shattered and re-formed by gravity. • Extremely high fault cliffs (20 km) and twisted features are evidence. 37

  38. Miranda’s “chevron” 38

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  41. Ariel • Ariel is characteristic of the other moons • Made of ice (density of 1,670 kg/m3) • Very sooty (albedo 0.35) • Much evidence of tectonic activity, which is surprising for such a cold moon (60K) 41

  42. Extremely deep canyons and faults 42

  43. Umbriel The bright spot at the N pole is the “fluorescent cheerio” (a crater floor) 43

  44. Titania 44

  45. The largest moon – about 1500 km diameter A mountain about 6 km high 45

  46. Moons of Neptune • Different from moons of Uranus • Triton (a captured moon!) • Neried (a captured asteroid with the most eccentric orbit) • 1.3 million km to 9.6 million km • 6 small asteroidal moons 46

  47. Triton • Triton may be the coldest moon in the solar system. (34.5 K or -391oF) • Volcanically active • Volcanoes erupt liquid nitrogen laced with dark carbon. • Dark streaks on surface. • Icy cantaloupe terrain. • Retrograde orbit! • Triton formed elsewhere, was captured by Neptune’s gravity. 47

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  50. Liquid has flowed on Triton’s surface in the past 50