Planetary Atmospheres

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Planetary Atmospheres
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Observing planetary atmospheres

• Usually by means of spectroscopy of reflected
  sunlight
• Separate absorptions by the planetary atmosphere
  from solar and telluric lines

Using the nature of the transitions or the
Doppler shift
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Pressure structure

• Atmospheres are in hydrostatic equilibrium
• Ideal gas law
• Exponential decrease of pressure with height
• Pressure scale height

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Surface / Cloud top Scale Heights
Surface gravity
Molecular weight
Temp.
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Features of scale heights

• Typically, 10 km ? atmospheres are thin
  relative to their lateral extent
• Depend on temperature (Venus is a prime example)
• Depend on the weight of the gas one common H as
  long as the gas is well mixed, but different
  gases separate in the upper layers (heterosphere)

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Atmospheric energy budget

• In general, most of the heating is at the bottom
  (Venus is an exception)
• Transport mechanisms
• - Conduction
• - Radiation
• - Convection

The occurrence of convection depends on whether
?T is larger or smaller than ?AT
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The dry adiabatic lapse rate

• In an adiabatic process, energy is neither gained
  nor lost
• Pressure-density relationship
• Ideal gas law ?
• Temperature gradient

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Dry adiabatic lapse rates
Adiabatic index
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Features of ?AT

• It depends on g? small gradients for small
  objects (Mars, Titan) and atmospheres with light
  gases (giant planets)
• If T drops faster with z than ?AT, the atmosphere
  becomes unstable to convection ? ?T? ?AT
• Reasons for ?Tlt ?AT radiative heat transport
  (thin atmospheres) or cloud formation (latent
  heat)

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Greenhouse effect (1)

• The atmosphere is more or less transparent to the
  Suns visual light but very opaque to the IR
  thermal radiation from the surface
• Thus, the heat is trapped and the surface gets
  hotter than otherwise
• The greenhouse effect was likely important to
  keep water liquid on the early Earth and Mars

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Greenhouse effect (2)

• Energy absorption thermal emission ?
  Equilibrium temperature
• Earth surface is 40 K warmer
• Venus surface is 500 K warmer (very high IR
  opacity due to CO2 absorption)
• Mars has too thin atmosphere Titan has mostly N2
  (not a greenhouse gas)

Assuming an isothermal planet
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Earths temperature profile

• Ozone absorption at 20-60 km causes temperature
  inversion Stratosphere
• Photoionization at 100-300 km causes another
  inversion Thermosphere
• At gt 500 km the temperature is almost constant
  due to efficient conduction

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Venus, Mars, Titan
thermal profiles
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Day-night temperature contrast

• Amount of solar energy absorbed per day
• Amount of energy needed to raise the air
  temperature by ?T
• Fractional temperature variation
• Very small for Earth and Venus (s) but large for
  Venus (c) and Mars

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Clouds

• Usually formed by a minor constituent (Earth,
  Venus, Titan, giant planets)
• High albedo, often rather opaque (Venus)
• Local heating (due to release of latent heat) and
  greenhouse effect
• Major influence on (T,p) structure and thus on
  wind circulation
• Special case dust clouds on Mars

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Cloud formation

• Below its saturation pressure, a substance occurs
  only in gaseous form
• If more is added after the saturation pressure is
  reached, all extra molecules go into the liquid
  or solid phase
• psat grows very rapidly with T
• psat decreases faster with z than p(z) ?
  condensation ? further rise of rising air ?
  further condensation

Clausius-Clapeyron equation
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Wet adiabatic lapse rate

• The adiabatic relation needs to account for the
  release of latent heat due to condensation
• This leads to a reduced value of ?, and thus a
  smaller ?AT
• The effect can be substantial (Earth)

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Atmospheric dynamics

• Maximum insolation at equator ? convective wind
  pattern called Hadley circulation
• On fast rotating planets, the Coriolis force
  causes strong westerly deflection at
  mid-latitudes
• On Venus the Hadley cell proceeds to the polar
  region

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Zonal winds on giant planets

• The giant planets are very fast rotators
• The Coriolis deflection of Hadley cells is very
  important
• Large number of fast zonal winds in alternating
  directions
• This correlates with visual appearance

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Geostrophic/Cyclostrophic balance

• A thermally driven N-S wind is deflected by the
  Coriolis force into a zonal wind with velocity u
• This wind is also subject to a Coriolis force
  with a N-S horizontal component ? Geostrophic
  balance
• If the rotation is slow, the centrifugal force of
  the zonal wind may dominate ? Cyclostrophic
  balance

Wind equations
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Geostrophic/cyclostrophic wind speeds
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Upper atmospheric layers (1)

• Turbopause (z100 km) winds and turbulence
  below vertical separation above
  (homosphere/heterosphere)
• Photochemistry important in Earths stratosphere
  and ionosphere, caused by solar UV radiation

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Upper atmospheric layers (2)
Concentration vs height of a photoproduced constit
uent
Earths main ionospheric layers
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Exosphere

• The escape probability of a molecule moving at
  vgtve goes quickly from 0 to 1 over a range ?zH
  this defines the exobase (base of the exosphere)
• The exobase is 25-30 scale heights above the
  thicker atmospheric layers (500 km for the Earth)

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Jeans escape
Thermal escape from the exosphere
Assume a Maxwellian velocity distribution at
temperature Tex
Compare the mean kinetic energy kTex with the
gravitational potential energy at the exobase
Escape parameter
For atomic hydrogen
Escape rate proportional to exp(-?esc) very slow
for heavy gases!
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Other erosion mechanisms

• Solar wind sweeping - on planets without a
  magnetosphere (impinging solar ions)
• Hydrodynamic escape (atmospheric blowoff) - the
  bulk of a light gas below the exobase moves away
  dragging the rest of the atmosphere along
• Impact erosion - for impactors larger than H, the
  shock heated air forms a hot bubble that escapes
  from the planet