AOSC 637

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AOSC 637

• Lesson 24

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Uranus
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Uranus

• Has been visited by Voyager 2 in 1966.
• Plane spins on an axis almost parallel to the
  ecliptic plane. Polar regions can point directly
  at the sun.iu
• Uranus is however hotter at the equator than at
  the poles. Not known why.
• Bulk of planet composed largely of rock and
  various ices. Has about 15 hydrogen and a little
  helium.
• Has very dark rings. Composed of fairly large
  particles. (up to 10 meters in diameter) and fine
  dust.
• Has a magnetic field probably generated by
  motions at shallow depths.

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Uranus
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Uranus

• Blue color is the result of absorption of red
  light by methane in the upper atmosphere.
• Like the other gaseous planets, Uranus has bands
  of clouds that blow around rapidly. They are
  extremely faint.
• Pictures from the Hubble Space Telescope (HST)
  show an apparent seasonal effect

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Uranus
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Neptune
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Neptune

• Voyager 2, 1989.
• Bulk composition similar to Uranus various ices
  and rocks. About 15 hydrogen and a little
  helium.
• Blue coloration is the result of absorption of
  the red light from the sun by methane.
• Like Jupiter and Saturn, Neptune has an internal
  heat source it radiates more radiation than it
  receives from the sun.
• At the time of the Voyager encounter, the most
  prominent feature on Neptune was the Great dark
  Spot. Neptunes winds blew the spot westward at
  300 meters per second.
• However, HST observations in 1994 showed that the
  Dark Spot had disappeared. Later on a new dark
  spot was seen.

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Neptune
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Neptune
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Triton
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Triton

• Voyager 2 on Aug 25, 1989.
• Largest of Neptunes satellites.
• It has a retrograde orbit about Neptune. It is
  believed that Triton was formes elsewhere and the
  captured by Neptune.
• Tritons density (2.0) is slightly greater then
  that of Saturns icy moons. Triton is probably
  about 24 water ice, with the remainder rocky
  material.
• Few craters young surface. Almost the entire
  Southern hemisphere is covered with an ice cap
  of frozen nitrogen and methane
• Extensive ridges and valleys freezing and
  thawing cycles
• Ice volcanoes

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Triton
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Triton
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Pluto
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Pluto

• No spaecraft flybye. HST can only resolve largest
  features.
• Surface temperature varies from about 38 to 63 K.
• Bulk is probably a mixture of 70 rock, and 30
  water ice.
• Bright areas of the surface seem to be covered
  with ices of nitrogen with smaller amounts of
  solid methane, ethane and CO

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Atmospheric Chemistry

• Composition of the atmospheres of Uranus and
  Neptune is closer to that presumed for the sun,
  than that of Jupiter and Saturn.
• Neptune has winds that approach the speed of
  sound.
• The presence of H2 had been measured from Earth.
  Airglow and auroral emissions from Voyager have
  also shown the presence of atomic and molecular
  hydrogen up to high altitudes.
• Excitation is either from impact of low energy
  electrons fron the Sun, or from Rayleigh and
  resonance scattering of solar radiation.

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Atmospheric Chemistry

• Spectra show strong absorption bands of methane.
  As noted before the absorption in the red leads
  to the color of the planets.
• Voyager detected clouds of methane in both Uranus
  and Neptune.
• C2H2 and C2H6 is produced in a similar manner to
  that on Jupiter and Saturn. Synthesis of higher
  hydrocarbons is likely and aerosols may be
  formed. Changes in the brightness of Neptune are
  in antiphase with changes in solar activity
  suggesting that changes in solar radiation is
  connected withy changes in aerosol production
  which then changes the planetary albedo.

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Atmospheric Chemistry

• Condensation removes most of the ammonia on
  Uranus and Neptune in contrast to Jupiter and
  Saturn.
• The presence of CO in the atmosphere of Neptune
  is a puzzle. The observed mixing ratio is about
  three orders of magnitude larger than on Jupiter
  and Saturn, and 40 times larger than on Uranus.
• Measurements from HST show an almost constant
  mixing ratio throughout the troposphere and
  stratosphere, which indicates a source close to
  the surface.
• Neptune has a large internal heat source, which
  could provide the dynamical forcing for the
  mixing. By contrast Uranus does not have such an
  internal heat source.

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Atmospheric Chemistry

• Triton and Pluto have tenuous atmospheres
  according to ground-based measurements.
• Voyager fly-by of Triton shows strong airglow
  features from neutral N2 and ionized oxygen
  similar to Titan.
• Triton atmosphere made up predominately of N2.
  Methane is also present in the lower atmosphere.
  In addition further measurements have detected
  CO, CO2 and H. The H must be produced from
  methane, as on Titan, Uranus and Neptune.
• The observed surface temperature and pressure are
  consistent with a saturation equilibrium of N2 in
  the atmosphere.
• Concentration of methane is some 30 times less
  than equilibrium, indicating that it is removed
  by photochemical processes. Because nitrogen is
  present, this photochemistry should be similar to
  Titans.

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Atmospheric Chemistry

• The pressure at the surface of Pluto has been
  estimated to be 3 microbars.
• The experimental information comes mainly from
  ground-based instruments and IRAS (InfraRed
  Astronomical Satellite)
• Near IR spectrum dominated by bands from solid
  methane. Other measurements indicate a system
  with a surface frost. This surface ice is
  inferred to consist of solid N2 (98) with
  methane (1.5) and CO (0.5).