METO 637

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METO 637

• Lesson 23

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Titan
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Titan

• A satellite of Jupiter.
• Titan has a bulk composition of about half water
  ice and half rocky material.
• Although similar to the other satellites of
  Jupiter it is denser because it is so large that
  its gravity slightly compresses the interior.
• Titan has no magnetic field. Hence the solar wind
  can ionize and carry away some of the molecules
  from the top of the atmosphere.
• There is some evidence for precipitation,
  erosion, mechanical abrasion, and other fluvial
  activity.
• Few if any craters visible surface must be
  young. However it could be that oceans have
  filled some of the voids.

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The atmosphere of Titan

• Has an atmosphere that is largely composed of
  Nitrogen.
• The temperature at the surface is 94.5K and the
  surface pressure is 1.5 bar.
• The instrument IRIS on Voyager detected a suite
  of hydrocarbons and nitrogen compounds in
  addition to methane.
• Satellite is covered by colored clouds. Clouds
  have been identified as arising from the gaseous
  organic compounds Titans equivalent of
  photochemical smog!
• Clouds extend from the surface to 200 km.
• Above this is a thinner haze layer of aerosol
  particles. Heating in this layer leads to a
  temperature inversion.

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Altitude profile of temperature and total number
density on Titan (Model)
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Titans emission spectra in the ultraviolet
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Composition of Titans atmosphere
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Schematic of temperature profile on Titan
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Photochemistry of Titan

• The minimum temperature at the tropopause is
  about 70 K.
• For many of the organic compounds found on Titan
  the measured abundance is above the saturated
  vapor pressure at the tropopause.
• The source of these species cannot therefore be
  the troposphere, but must be the stratosphere.
  Must be derived from volatile parent molecules.
• The absence of H2 and the presence of abundant N2
  modify the chemistry considered previously for
  Jupiter and Saturn. On the planets radicals such
  as CH3 or NH2 derived from CH4 and NH3 react by
  abstraction of H from H2 or by three body
  combination with H-atoms to return to the parent
  molecule.
• So-called do nothing cycle

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Photochemistry of Titan

• This cannot happen on Titan, so the less
  hydrogen-rich hydrocarbons are favored.
• Mixing ratios of C2H6 are four times greater on
  Titan than on Saturn, those of C2H2 are 27 times
  greater, and C2H4, which cannot be detected on
  Saturn, is clearly detected on Titan.
• The key differences between Titan and the planets
  are the absence of back-reactions involving H and
  H2, the presence of processes involving N and N,
  and the quenching of 1CH2 to 3CH2 by N2
• This leads to the formation of C2H2, C2H4 and
  C3H4 from the triplet state.

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Atmospheric chemistry on Titan
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Photochemistry of Titan

• CH4 h? N2 ? 3CH2 H2 (or 2H) N2
• 3CH2 3CH2 ? C2H4H2 (or2H)
• Followed by
• C2H2 h? ? C2H H
• C2H CH4 ? C2H2 CH3
• These then lead to the formation of the other
  organic compounds
• 3CH2 CH4 ? C2H4 H
• CH3 CH3 M ? C2H6 M
• C2H C2H6 ? C2H2 C2H5
• C2H5 CH3 M ? C3H8 M

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Theoretical altitude profiles of H2 and
hydrocarbons on Titan
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Theoretical altitude profiles of H2 and
hydrocarbons on Titan
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Altitude profiles of carbon species and H (Model)
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Io
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Io

• Very few, if any, impact craters on the surface.
  Surface is young
• Hundreds of volcanic calderas. Some are still
  active. Striking photographs have been taken from
  Voyager 1 of actual eruptions.
• Vapor from the vents of the volcanoes appears to
  be SO2 or S. Optical emissions have been observed
  from atomic sulfur and oxygen
• Atmosphere is tenuous pressure at surface about
  10-7 atmospheres.

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Io, Sulfur saturation vapor pressure
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Io

• Sulfur dioxide has a vapor pressure of 10-9 bar
  in the polar regions (lt98K) and on the
  night-side. But at the sub-solar point the
  pressure could be as high as 10-7 bar, 130K).
• Simplistic view of the atmosphere is a relatively
  dense atmosphere near the volcanoes and the
  sub-solar point, which becomes thin near the
  poles and on the dark side.
• Microwave observations show 4-35x10-9 bar of SO2
  covering 3-18 of the surface consistent with
  SO2 being in equilibrium with the surface
  temperature.
• It has been suggested that O2 at a pressure of
  20x10-9 also exists, but there is no direct
  evidence.

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Io photochemistry

• Primary path for the dissociation of SO2 is as
  follows
• SO2 h?(?lt221 nm) ? SO O
• SO2 h?(?lt221 nm) ? S O2
• Followed by
• SO SO ? SO2 S
• S O2 ? SO O

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SO2 number density (N) and temperature (T) for Io
(Model)
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Distribution of major constituents on Io (Model)
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Europa
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Europa

• Satellite of Jupiter
• Similar in composition to Io, but unlike Io has a
  thin outer layer of ice.
• Very few craters on Europa, suggesting a young
  and active surface. Images of Europa surface
  strongly resemble images of sea ice on Earth.
• Has a very tenuous atmosphere 10-11 bar, composed
  of oxygen. Almost certainly not of biogenic
  origin. Most likely source is the bombardment of
  the icy surface by UV radiation, and charged
  particles in the solar wind.
• Has a weak magnetic field, which varies
  periodically as Europa moves through Jupiters
  massive magnetic field.
• Interpreted as signifying that Europa has a
  conducting layer beneath the surface probably a
  salty ocean.

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Callisto
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Callisto

• The satellite of farthest from Jupiter.
• Surface is covered entirely with craters. Is very
  old. Callisto has the oldest and most cratered of
  any body yet discovered in the solar system (4
  billion years).
• Has a very tenuous atmosphere composed of carbon
  dioxide.
• Has a weak magnetic field.
• Little evidence of tectonic activity