Uranus

1
Uranus

• Discovery
• All previous planets known since antiquity
• Easily visible to the naked eye.
• Uranus is at limit of best human eyesight from
  darkest sights
• Uranus discovered
• On March 13, 1781
• By William Herschel
• With telescope

2
Uranus Interior Structure

• Composition
• Contains less H and He than Jupiter
• By percentage contains more volatiles, especially
  water
• Surface composition is mostly H and He with
  traces of NH3, CH4, and water
• Differentiated with silicates and metals in
  central core

3
Uranus Interior Structure

• Core
• Contains iron/nickel, silicates
• Solid form due to the high pressure and
  composition
• Inner Mantle
• High pressure liquid water with dissolved
  minerals and/or NH3 and CH4
• Solution is very good electrical conductor

4
Uranus Interior Structure

• Outer Mantle
• High pressure liquid hydrogen and helium

5
Uranus Interior Structure

• Atmosphere
• Top layer gaseous due to pressure
• Mostly H and He with traces of NH3 and CH4
• Appears generally featureless due to low
  temperatures (haze) and low UV flux for organic
  colorations
• High contrast images show belt-zone structure
  similar to Jupiter and Saturn
• Parallel to equator
• Overall blue-green color due to methane
  absorption of yellow/red light

6
Uranus Magnetic Field

• Little weaker than Earths
• Not at the center of planet
• Produced 30 of way from center in the liquid
  water inner mantle
• Tipped 60 to the rotation axis
• Interacts with solar wind

7
Uranus Rings

• Very tenuous set of 9 rings
• Composed of very dark dust particles
• Either darkened ice or rocky particles
• Extremely narrow, so must have shepherd
  satellites like Saturn
• Voyager 2 detected two inside and outside
  outermost ring
• Origin of rings unknown, but are inside Uranus
  Roche limit, so may be disrupted satellite

8
Uranus Satellites

• 20 satellites plus ring particles
• Satellites range from 1550 km to 20 km in size
  with a composition of ice and rock
• Satellites orbit in plane of Uranus equator
• Impact which tipped Uranus must have occurred
  early in protoplanet phase so that material out
  of which satellites formed was also tipped.

9
Neptune

• Discovery
• not bright enough to see with naked eye ? not
  known it ancient times
• Unlike Uranus, existence and position was
  predicted
• discovered Sept 23, 1864

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Neptunes Interior Structure

• Almost identical to Uranus
• likely contains a bit more water, NH3, and CH4 by
  percentage (or less H and He)
• rocky iron/nickel core
• liquid water inner mantle
• liquid hydrogen outer mantle
• gaseous atmosphere composed mostly of H and He
  with some CH4 and NH3
• Has a belt/zone structure, but is generally bland
  blue color
• Occasional spots on surface

11
Neptunes Magnetic Field

• Half as strong as Earths
• Produced in liquid water mantle
• Offset from center of planet, about halfway to
  the surface
• Inclined 47 to axis of rotation

12
Neptunes Rings

• Composed of very dark dust particles
• Either darkened icy particles or rocky particles
• Similar to Uranus ring particles
• Distribution of particles in each ring is not
  uniform
• Believed to be arcs
• Voyager 2 showed that they were continuous

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Neptunes Satellites

• 8 known satellites not counting ring particles
• Two largest have strange characteristics
• Nereid is an icy and rocky moon about 300 km in
  diameter
• In a very elliptical orbit with a period of
  almost 1 Earth year
• Triton, 3/4 size of Moon

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Triton

• On retrograde orbit
• Tenuous atmosphere
• Composed of N2 and CH4
• 1/100,000 density of Earths atmosphere
• N2 seems to have seasonal cycles of vaporizing at
  one pole and solidifying at the other

15
Triton

• Relatively young surface
• Very few craters
• Evidence of geologic activity
• Heat source may be tidal heating (if orbit was
  more elliptical in past)
• OR Triton may have a solid state version of the
  greenhouse effect (transparent solid nitrogen)
• Composed of ices of N2, CH4, CO2, and CO

16
Pluto

• Discovered 2/18/1930
• Too dim to see even with small telescope
• By Clyde Tombaugh
• Blink comparator plates
• Position predicted by Percival Lowell based on
  predictions from Neptunes orbit
• Deviations were not gravitational, but
  observational error

17
Charon

• Plutos only known satellite
• About half the size of Pluto
• ? 1.8 g/cc
• More ice and less rock than Pluto
• Pluto and Charon tidally locked to each other
• Period of Charons orbit rotation period of
  both bodies 6.4 days

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Plutos Atmosphere and Surface

• Very tenuous atmosphere of N2 and CO
• Similar to Tritons
• No close-up images of Plutos surface, but should
  be similar to Tritons
• Composed mostly of N2 ice with traces of CO
• Poles are brighter than equatorial regions
• N2 gas ? N2 ice cycle

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Charons Atmosphere and Surface

• Composed mostly of water ice
• Pluto and Charon must have lots of water ice
• Water ice covered by N2 ice on Pluto, whereas
  Charon has no atmosphere

20
Ice Dwarfs

• Formed in outer solar system
• 20 small (a few 100 km) icy objects have been
  detected between the orbit of Saturn (10AU) and
  60AU.
• Probably planetesimals leftover from formation of
  solar system
• If more abundant in past, could explain some
  collisions (Uranus, Titan)
• May be the inner edge of the Kuiper Belt of
  comets
• Pluto Charon possible examples