METO 637

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METO 637

• Lesson 22

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Jupiter
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Jupiter

• Jupiter and Saturn are known as the gas planets
• They do not have solid surfaces, their gaseous
  materials get denser with depth.
• What we see is the top of the clouds at about I
  atmosphere pressure.
• Jupiter probably has a core of rocky material
  amounting to about 15 Earth masses.
• The main bulk of the planet is in the form of
  liquid metallic hydrogen. This implies a pressure
  of greater than 4 million bars.
• Metallic hydrogen is an electrical conductor and
  the source of Jupiters magnetic field

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Jupiter

• The outermost layer is composed of molecular
  hydrogen and helium.
• The helium is liquid in the interior and gaseous
  in the outer layer.
• Has high velocity winds which are confined to
  wide bands of latitude. Winds blow in opposite
  directions in adjacent bands.
• Evidence is that the winds are driven by
  Jupiters internal heat sources, and not by the
  sun.
• Vivid colors seen in the clouds are the result of
  chemical reactions within the clouds probably
  involving sulfur compounds.

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Saturn
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Saturn

• Is the least dense of the planets density of
  0.7 is less than that for water.
• Like Jupiter, Saturn is about 74 hydrogen, 25
  helium, and trace amounts of water, methane,
  ammonia and rocks.
• This composition is similar to the promordial
  Solar Nebula from which the solar system was
  formed.
• As for Jupiter, Saturns interior consists of
  rocky core, a liquid metallic hydrogen layer and
  a molecular hydrogen layer. Traces of various
  ices are also present.

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Saturn

• The core of Saturn (and Jupiter) are hot (12,000
  K). This high temperature is due to the slow
  gravitational compression of the planet
  (Kelvin-Helmholtz mechanism)
• Jupier and Saturn have a rapid rotation - 10
  hours. This causes oblateness, although Saturn is
  affected the most (10)
• Saturn has prominent rings. These are quite thin
  about one kilometer.
• Ring particles seem to be mainly composed of
  water ice.

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Jupiter and Saturn

• Atmospheric composition of Jupiter was
  investigated by several instruments on the
  Voyager spacecraft. The following instruments
  were flown
• (1) IRIS Infrared radiation
• (2) UVS Ultraviolet Spectrometer
• (3) PPS Photo-Polarimetry aerosols
• (4) RSS Radio Science ions
• As expected they found that the bulk of the
  atmosphere was composed of hydrogen and helium.
• The fractional abundance of He is markedly
  smaller than that for the solar ratio (0.16)
  indicating gravitational separation from hydrogen
  within the interior of the planets.

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Thermal emission spectra from Jupiter IRIS
(Voyager 1)
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Jupiter and Saturn

• Deep in the atmosphere thermal chemistry yields
  compounds which are mainly in thermo-chemical
  equilibrium.
• Photochemistry can convert CH4 to heavier
  hydrocarbons and NH3 to N2H4
• Some of the chemical compounds formed are
  condensable. The temperature profile shows a
  distinct minimum at about 100 mb, which can act
  as a cold trap.
• This limits the mixing ratios of condensable
  gases above the minimum.
• On Jupiter NH3 is limited to a mixing ratio of
  about 10-7.

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Jupiter and Saturn

• The condensates remain as aerosols.
• On Jupiter dense water clouds form at 270K,
  while near the 200K level H2S is thought to react
  with NH3 to form a cloud of solid NH4SH
  particles.
• White crystals of ammonia precipitate out at
  154K, to produce the visible upper layer cloud.
• Above the clouds photochemistry can take place.

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Jupiter and Saturn

• The chemistry of the atmospheres of Jupiter and
  Saturn is greatly influenced by the reaction of
  other species with H and H2
• The atomic hydrogen is formed photochemically
  from the abundant molecular hydrogen.
• Hydrides such as CH4, NH3 and PH3 also undergo
  photolysis to produce intermediate compounds such
  as CH2, CH, NH2 and PH2. These then participate
  in further reactions.

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Hydrogen

• As noted before, molecular hydrogen is the
  dominant constituent. It dissociates at
  wavelengths less than 100nm in a dissociation
  continuum that begins at 84.5nm.
• It also has an ionization continuum at 80.4 nm.
  Absorption in his continuum leads to the
  production of hydrogen atoms
• H2 H2 ? H3 H
• H3 e ? H2 H (or 3H)
• There is a net downward flow of atomic hydrogen
  from the ionosphere to lower altitudes.
• Methane photolysis requires photons below 145nm,
  and ammonia requires photons below 160 nm.

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Synthesis of organic compounds

• We noted before that Lyman alpha radiation from
  the sun is very intense. This can dissociate
  methane
• CH4 h? ? CH3 H
• ? 1CH2 H2
• ? 1,3CH2 2H
• ? CH H H2
• The methylene radical (CH2) can then react to
  form observed products
• CH2 H2 ? CH3 H
• CH3 CH3 M ? C2H6 M

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Synthesis of organic compounds

• Ethylene is formed by the reaction
• CH CH4 ? C2H4 H2
• The ethylene is then photolyzed to acetylene
• C2H4 h? ? C2H2 H2
• Acetylene is photochemically stable because its
  products C2H and C2 react with H2 to regenerate
  C2H2.
• Higher hydrocarbons, even polymers, can be formed
  by reactions of C2H2 with other species, for
  example
• 1CH2 C2H2 M ? CH3C2H (methylacetylene)
• This product has also been observed in both
  atmospheres,

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Ammonia and Phosphine

• The primary process is
• NH3 h? ? NH2 H
• Followed by
• NH2 NH2 M ? N2H4 M (hydrazine)
• NH2 H M ? NH3 M
• Analogous reactions are found for phosphine
• PH3 h? ? PH2 H
• PH2 PH2 M ? P2H4 M
• PH2 H M ? PH3 M
• P2H4 (a solid) is probably formed as a
  condensation product.
• The concentrations of both ammonia and phosphine
  decrease rapidly above the tropopause.