Terrestrial atmospheres

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Terrestrial atmospheres
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Overview

• Most of the planets, and three large moons (Io,
  Titan and Triton), have atmospheres

• Venus
• Very thick
• Mostly CO2
• Some N2
• Sulfuric acid clouds

• Mars
• Very thin
• Mostly CO2
• Some N2, Ar
• Winds, dust storms

• Earth
• Mostly N2, O2
• Some H20, Ar
• Only 0.03 CO2.
• Water clouds

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Secondary atmospheres

• Can calculate how many volatiles had to be added
  to the atmosphere to get present surface
  conditions
• Not just current atmosphere content, but also the
  oceans and CO2 locked up in rocks and shells.

• Percent (by weight) added to atmosphere by
  volcanic outgassing

Gas Deep eruptions Continental geysers Required amount
H2O 57.8 99.4 92.8
CO2 23.5 0.33 5.1
Cl2 0.1 0.12 1.7
N2 5.7 0.05 0.24
S2 12.6 0.03 0.13
Others lt1 lt1 lt1
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Atmospheric compositions

• Comparison of total volatile content on Venus,
  Earth and Mars shows better agreement.

Volatile Venus Earth Mars
H2O
Atmosphere 60 3 0.02
Oceans/caps 0 250,000 5000?
Crust 160,000? 30,000 10,000?
Total 160,000? 280,000 15,000?
CO2
Atmosphere 100,000 0.4 50
Oceans/caps 0 0 10
Crust 0 100,000 gt900?
Total 100,000 100,000 gt1000?
N2
Atmosphere 2,000 2,000 300
40Ar
Atmosphere 4 11 0.5
Table 11.2 mass fraction of volatiles (x109).
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Physical Structure

• Use the equation of hydrostatic equilibrium to
  determine how the pressure and density change
  with altitude, in an isothermal atmosphere. You
  may neglect the change in gravitational force
  with altitude.

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Physical Structure

• Pressure decreases with increasing altitude

• Atmospheres are compressible, so density
  decreases with altitude

• Compare with the pressure structure of the
  oceans, where the density remains approximately
  constant.

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Physical Structure

• Surface temperatures and pressures are very
  different for the three terrestrial planets
• But the pressure scale heights are similar

Venus Earth Mars
Tequil (K) 238 263 222
Tsurf (K) 733 288 215
Psurf (bar) 92 1.013 0.0056
rsurf (kg/m3) 65 1.2 0.017
H(km) 16 8.5 18
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Thermal structure

• The thermal structure of the terrestrial
  atmospheres are similar
• There is at least one temperature minimum
• Caused by heat being trapped by cloud layers
• Temperature gradient depends on heat transport
• Radiation depends on the opacity of the
  atmosphere
• Convection
• Conduction important near the surface

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Chemical Structure

• Earth
• water concentrated near surface
• CO2 locked in rocks, shells
• Leads to oxygen-rich atmosphere

• Mars
• Water and CO2 clouds

• Venus
• Dominated by sulfur, CO2
• Clouds of sulfuric acid at 48-58 km

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Atmospheres and Water

• Lots of evidence that liquid water existed on
  Mars surface
• Liquid water requires higher temperatures and
  pressures
• Must have been a much denser atmosphere at one
  point

• Venus too hot for liquid water to exist
• Water evaporates, H and O dissociate
• H is lost, and O forms CO2, sulfuric acid.

Present-day Mars
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Opacity

• Consider a completely transparent atmosphere
• No radiation (sunlight) is absorbed
• Optical sunlight hits the ground and heats it up
• Earth reradiates this energy in the infrared
• No effect on atmosphere

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Opacity

• Earths atmosphere is not transparent
• Ozone (high altitude) strongly absorbs UV
  radiation
• Water and CO2 (lower altitude) strongly absorb
  infrared radiation

• Upper atmosphere heated by incoming solar
  radiation
• Outgoing radiation from ground heats lower
  atmosphere

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Earth transparency

• Earths atmosphere is opaque at infrared
  wavelengths

optical
infrared
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Greenhouse effect

• Optical radiation strikes the Earth and heats it
  up
• Infrared radiation is absorbed by lower
  atmosphere and reradiated in all directions
• Including back to the ground, heating it further
• Surface and lower atmosphere heat up until the
  amount of IR radiation escaping the atmosphere is
  equal to the amount of solar radiation coming in

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Greenhouse Effect

• Assume a simple model of the greenhouse effect,
  where a cloud layer (with same surface area as
  the Earth) is completely transparent to optical
  radiation, and completely opaque to infrared
  radiation. Calculate by how much the surface
  temperature increases when the cloud layer is
  present.

Solar heating
Heat radiated away
Heat returned to surface
Clouds heated from below
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Break
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Convection

• Solar heating alone would cause global
  circulation, as hot air rises and cool air sinks

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Rotation and the Coriolis force

• Recall in a rotating reference frame, objects do
  not move in a straight line.
• Applet

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Rotation and Coriolis force
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Winds

• Coriolis force splits the Hadley cells in each
  hemisphere into three cells.

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Diversion Australian toilets

• Does the Coriolis force influence the direction
  in which a vortex forms?

The acceleration due to the Coriolis force is
where w is the angular velocity of rotation, and
v is the velocity of the moving particle
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Upper atmospheres
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Upper atmospheres

• The thermosphere is heated by energetic photons
  from the Sun
• Low density means light atoms are able to float
  to the top

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Escape of atmospheres

• Particles with velocity greater than the escape
  velocity will leave the atmosphere (if the
  density is low enough that they will not collide
  with another atom)
• What is the average velocity of a particle at
  temperature T? How high a T do you need for a
  particle with this velocity to escape?

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Escape of atmospheres

• Particles with velocity greater than the escape
  velocity will leave the atmosphere (if the
  density is low enough that they will not collide
  with another atom)
• At a given temperature, particles of a given mass
  have a Maxwellian velocity distribution, with a
  long tail to high velocities

• At T2000 K (top of thermosphere), how much
  faster than the average velocity must a hydrogen
  atom be moving to escape?

• How about for a Neon atom?

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Venus

• CO2 rich atmosphere
• Leads to a strong greenhouse effect, at high
  surface temperatures
• Clouds made mostly of sulfuric acid

• Motion of upper atmosphere is due to convection,
  as a result of the strong temperature gradient.
• Almost no wind or weather at the surface, due to
  the slow rotation of Venus

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Mars

• Weather dominated by dust storms
• Very small dust particles (1 micron diameter) can
  be carried by strong winds (gt180 km/h)

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Titan

• N2 rich atmosphere
• Little greenhouse effect, cold surface
  temperatures
• Smog-colour from interactions of solar radiation
  and methane in atmosphere

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Next Lecture

• The Giant Planets