Uranus and Neptune

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Uranus and Neptune

• Astronomy 311
• Professor Lee Carkner
• Lecture 19

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Compared to the clouds of Jupiter, the clouds of
Saturn are,

• Much brighter
• More spread out in depth
• More likely to have water in them
• Flowing in only one direction
• At a higher temperature

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Which best describes the structure of Saturns
rings?

• A single disk of uniform density
• Denser away from Saturn and less dense near
  Saturn
• Large particles in the center and smaller
  particles at the edges
• A radial series of rings and sub-rings
• Chaotic with no regular structure

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Uranus -- God of the Sky

• He gives his name to Urania the Greek muse of
  astronomy

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Discovery of Uranus

• Uranus was discovered in 1781 by William Herschel
  while surveying the sky
• Herschel had built a very high quality telescope
  and was systematically observing the brighter
  stars when he found Uranus

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Observing Uranus

• Most of our information about Uranus comes from
  Voyager 2 and HST
• No future missions planned

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Uranus Facts

• Size 4 Earth diameters
• Orbit 19.22 AU
• Description blue-green, featureless, tilted on
  its side

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The Rotation of Uranus

• The tilt of Uranuss axis is 98 degrees
• Extreme tilt may be due to a large impact when
  Uranus was forming
• The large tilt produces seasons where half of the
  planet is in sunlight and half in darkness for
  long periods of time

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Seasons on Uranus
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Composition of Atmosphere

• Hydrogen 84
• Helium 14
• Methane (CH4) 2
• The large amount of methane gives Uranus its
  bluish color

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Structure of Atmosphere

• Ammonia, Ammonium hydrosulfide and water have
  frozen out in the lower atmosphere where we cant
  see them
• Careful observations have determined that Uranus
  does not have alternating zones and bands
• Winds mostly blow east

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Uranuss Rings

• As Uranus moved past a star, the star dimmed
  several times before being occulted by the planet
  
• Rings are composed of dark material
• They reflect very little light and are difficult
  to observe at optical wavelengths

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The Moons of Uranus

• Uranus has 5 major moons and 22 minor moons
• Moons are composed of mixture of ice and rock
• Two of the moons shepherd the Epsilon ring
• The other rings may also have shepherd moons that
  are too small to see

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Radiation Darkening

• It is cold enough at the orbit of Uranus to form
  solid methane (CH4) ice
• Impacts by high energy electrons from the
  magnetosphere break off the carbon atoms

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Magnetic Fields Fields on Uranus and Neptune

• Voyager data indicate that Uranus and Neptune
  have magnetic fields
• The directions of the magnetic fields are tilted
  with respect to the rotation axis
• How can the dynamo effect produce an off-centered
  field?

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Rotation and Magnetic Axis
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Determining Mass

• How are the masses of planets determined?
• We can measure the period and the orbital radius
  of a moon or spacecraft
• The relationship between them depends on mass

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Neptune -- God of the Sea

• The name is appropriate due to Neptunes
  blue-green color

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The Discovery of Neptune

• Was an undiscovered planet altering the orbit?
• In 1846 J. G. Galle used Le Verriers
  calculations to find Neptune after a 30 minute
  search

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Observing Neptune

• Neptune shows no detail from Earth
• Best images from Earth from the Hubble Space
  Telescope

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Neptune Facts

• Size 4 Earth diameters
• Orbit 30.11 AU
• Description more distant, cloudier Uranus

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Neptunes Atmosphere

• Neptune has visible storms like Jupiter, but they
  appear to be short lived
• The white features in Neptunes atmosphere are
  high altitude methane clouds
• All seem to be moving east rather than in
  opposite directions

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Composition and Heating

• Atmospheres composition similar to Uranuss
• 84 Hydrogen
• 14 Helium
• 2 Methane
• Neptune may be still contracting

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The Rings of Neptune

• Neptunes rings were discovered via occultation
• Voyager 2 revealed that the outer ring has places
  called arcs with more particles
• Inner narrow ring has shepherd moons

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The Moons of Neptune

• Neptune has 12 small satellites and Triton
  (diameter 2700 km)
• Tritons orbit is unstable, in 100 million years
  it will be inside of the Roche limit giving
  Neptune a spectacular ring system

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The Interiors of Uranus and Neptune

• We can model each planet with a similar interior
• Mantle of water and ammonia (Windex)
• The two planets have relatively more heavy
  elements and less hydrogen than Jupiter and
  Saturn
• They also do not have enough gravity to produce
  liquid metallic hydrogen

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The Interiors of Uranus and Neptune
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The Formation of Uranus and Neptune

• Uranus and Neptune formed more slowly than
  Jupiter and Saturn
• By the time they formed much of the hydrogen and
  helium was dispersed

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Next Time

• Read Chapter 11.5 and 12.5

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Summary

• Information comes from Voyager and HST
• Blue-green in color with white clouds
• Caused by methane
• Radiation darkening produces dark, soot colored
  rings and moons
• Interior composed of rocky core, water and
  ammonia mantle and hydrogen atmosphere
• Offset magnetic field
• Formed more slowly than Jupiter and Saturn and so
  captured less gas

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Summary Uranus

• Discovered by survey (1781)
• Faint cloud patterns
• Due to low internal heat
• Tilted on its axis
• Causing non-uniform solar illumination

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Summary Neptune

• Discovered through use of Newtons laws (1846)
• Most distant gas giant
• Has more internal heat and stronger cloud
  features than Uranus