Discovery of Uranus, Neptune, Pluto

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Discovery of Uranus, Neptune, Pluto

• The last 3 planets were discovered with
  telescopes, in the last 150 years or so (too
  faint to see with eyes)
• Studied (glimpsed) by Voyager 2, powered by
  gravity slingshot explored all Jovian planets
• Uranus was discovered accidentally by William
  Herschel
• Neptune was discovered mathematically!
  Following observations of perturbations in the
  orbit of Uranus by an unseen object, Adams and
  Levellier independently predicted its existence
• Pluto is not a (Jovian) planet, more like a
  moon, planetesimal, or asteroid was discovered
  in 1930 by Clyde Tombaugh

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Uranus
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View of Uranus from Earth(something strange ?)
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Orbit of Uranus around the Sun Rotation axis
tilted at 98 degrees !
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Uranus

• Uranus and Neptune are almost twins in size
  25,000 Kms radius
• Uranus is tilted on its side by 98 degrees each
  pole faces the sun for half the orbital period
  (84 years)
• Cause of tilt? Where on Uranus would you go to
  spend summer vacation ?
• Uranus rotates somewhat backwards like Venus
  rotation period 17hr 14m
• Weak magnetic field 0.74 x Earths, could be a
  very small metallic mantle
• Methane atmosphere, hence greenish hue
• H2O (ice) mantle

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Neptune
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Neptune and the Great Black Spot
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Methane ice clouds on Neptune
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Atmosphere of Neptune (c.f. Jupiter)
A pale imitation of Jupiter!
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Neptune

• Neptunes atmosphere very active, with
  convection currents
• Great Black Spot 10,000 Kms long, cyclonic
  storm like the GRS on Jupiter
• Internal energy source (gravity) like Jupiter
  emits 2.6 times the energy it receives from the
  sun
• Tilted at 30 degrees
• Strange moon Triton, which rotates backwards,
  probably a captured Kuiper Belt Object
• Magnetic field tilted 50 degrees to rotation
  axis mag poles not aligned with geo poles (Earth
  and Jupiter are about 10 degrees)

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Magnetic and geographical axes
Neptunes Magnetic axis is NOT centered
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Structures of Uranus and Neptune
Relatively thin atmospheres compared to Jupiter
and Saturn, but still quite extensive compared
to terrestrial planets
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Atmospheres of Uranus and Neptune

• More He than Jupiter and Saturn
• Insufficient gravity ? loss of H
• H is locked up in methane CH4, 10 x Jovian,
  (also more carbon) ? absorbs red, reflects blue
• Hydrocarbon smog mainly frozen methane haze
  and clouds with ethane, acetylene, etc.
• Discovery of Jupiters ring(s) by Voyager 2

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Discovery of Ring Systems in Jovian Planets

• Jupiter --------- Voyager 2
• Saturn --------- Galileo
• Uranus --------- Occultation
• Neptune --------- Occultation
• - Occultation ? Obscuration of starlight by
  rings
• - Jupiters ring is very thin and dusty material

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Occultation of starlight by rings of Uranus
Dusty rings of Uranus not seen from the Earth
occultation method used following the discovery
of Jupiters ring by Voyager 2
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Occultation Spectrum due to rings of Uranus
dimming of starlight blocked by the rings as seen
from the Earth
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Uranus rings and moons
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Uraniun system with rings
Note the rotation axis and position of rings
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Voyager 2 view of Rings of Uranus(dusty,
extended atmosphere makes rings unstable)
Relatively more extended atmospheres due to lower
gravity than Jupiter and Saturn
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Neptunes rings (similar to Uranus dusty
ambient atmosphere)
The slit in the middle blocks out light from the
main body of the planet, Thereby enabling the
faint rings to be seen
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Rings of Uranus and Neptune

• Both Uranus and Neptune have ring systems
• Made mainly of rocks, not ice like Saturn,
  therefore nearly invisible
• Owing to low gravity, the ring structures are
  diffuse and unstable extend into the moons
  (hence heavily cratered) or spiral into the
  planet
• Probably last no more than 10-100 million years

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Moons of Uranus and Neptune

• Uranus has 5 large moons, and 10 small ones, all
  heavily cratered
• Miranda shows large oval structures may have
  shattered several times and re-assembled
• Neptune has 5 small moons (lt 100 Kms) bunched
  near the rings followed by Proteus at 200 Kms,
  and then Triton
• Uranus and Neptune do not have large Galilean
  satellites did not form moons on their own as in
  Jupiter and Saturn

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Moons of Uranus heavily cratered due to
interaction with loose ring particles and dust
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Miranda Broken and re-assembled ?
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Triton captured by Neptune ?
Orbits opposite to direction of rotation of
Neptune
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Triton

• Triton is more dense, 2 g/cc, indicating rocky
  composition, than the moons nearer to Neptune
  which are mostly ice
• Reverse of the trend for Galilean satellites
• Triton might have been a Kuiper Belt Object,
  captured by Neptune
• Has a very thin atmosphere, 0.00002 bars
• T 38 K ? N and CH4 freeze and snow
• Polar caps extend to 25 degree latitude

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