1. The thin blue line that keeps us alive
THE ATMOSPHERE
The thin blue line that keeps us alive

2. I. Theories of the History
A. Early Earth would have been very different
and inhospitable compared to the Earth
today. Why?

4. I. Theories of the History
A. Early Earth would have been very different
and inhospitable compared to the Earth
today. Why?
1.Hot
a.Why? - Primordial heat, collisions and
compression during accretion, decay of short lived radioactive elements
b.Consequences - Constant volcanism, surface
temperature too high for liquid water or life as we
know it, molten surface or thin, unstable basaltic
crust.

5. I. Theories of the History
B. Atmosphere - early atmosphere probably
completely different in composition from
today (H2, He)
C. Formation
1. Cooling
  a. Primordial heat dissipated to space
b. Condensation of water (rain), leading to accumulation of surface water.
c. Accumulation of new atmosphere due to volcanic out gassing
d. Conditions appropriate for evolution of life

6. I. Theories of the History
D. First Atmosphere
1. Composition - Probably H2, He
2. These gases are relatively rare on Earth compared to other places in the universe and were probably lost to space early in Earth's history.
WHY?

7. I. Theories of the History
a. Earth's gravity was not strong enough to hold lighter gases b. Earth still did not have a differentiated core (solid inner/liquid outer core) which creates Earth's magnetic field which deflects solar winds. c. Once differentiated, the heavier gases could be retained

8. I. Theories of the History
E. Second Atmosphere
1. Produced by volcanic out gassing. Gases produced then were probably similar to those created by modern volcanoes.
a. These gases include some of those found in Earth’s atmosphere today, like H2O, CO2 and N2
b. No free O2 at this time (not found in volcanic gases).
2. Ocean Formation - As the Earth cooled, H2O produced by out gassing could exist as liquid allowing oceans to form.

9. I. Theories of the History
F. Addition of O2 to the Atmosphere
1. Today, the atmosphere is ~21% free oxygen.
How did oxygen reach these levels in the atmosphere?
a. Photochemical dissociation - breakup of water molecules by UV rays from sun
b. Produced O2 levels approx. 1-2% current levels. At these levels O3 (ozone) can form to shield Earth surface from UV
c. Photosynthesis - O2 from photosynthesis
produced first by cyanobacteria, and
eventually higher plants - supplied the
rest of O2 to atmosphere.

10. II. The Modern Atmosphere
A. Atmosphere - Envelope of gases that surrounds the Earth. 1. Used by life as a reservoir of chemical compounds used in living systems. 2. Has no outer boundary, just fades into space 3. Densest part of atmosphere (97% of mass) lies within 30 km of the Earth (about same thickness as continental crust).

11. II. The Modern Atmosphere
B. Composition 1. What we think of as the atmosphere is really only the first of five layers. This layer is known as the troposphere. However, as it is the layer that affects us most on a daily basis, you should know that… a. It is a mainly nitrogen mixture b. Oxygen comprises approximately 1/5 of it c. It is where all of Earth’s weather happens

12. II. The Modern Atmosphere
Percentage composition of Earth’s atmosphere

13. II. The Modern Atmosphere
Percentage composition of Earth’s atmosphere

14. II. The Modern Atmosphere
C. Layers of the Atmosphere
3. Mesosphere - The mesosphere extends
from 260K to 280K ft (53 miles) above
Earth’s surface. Here is where most
meteors burn up upon entering the
atmosphere.
2. Stratosphere - extends from the tropopause
to about 170K ft. Temperature increases
with height due to increased absorption
of UV radiation by the ozone layer. The
stratopause, is found at 160 to 180K ft.
The pressure here is 1/1000 that of sea
level.
1. Troposphere - begins at Earth’s surface. It’s
thickness varies from 30K ft at the poles to
56K ft at the equator. This is the layer
where Earth’s weather happens and
contains 80% of the atmosphere’s mass.

15. II. The Modern Atmosphere
C. Layers of the Atmosphere
5. Exosphere - The outermost layer of Earth's
atmosphere extends from the top of the
thermosphere upward into space. It is mainly
composed of hydrogen and helium. Here, gas
particles can travel thousands of Km between
collisions.
4. Thermosphere – With very little gas and without
the protection of the ozone layer, temperatures
of this layer can rise to 1,500°C  (2,700 °F).
Here, the air so scarce that an individual gas
molecule travels an average of 1 kilometer
between collisions with other molecules. The
International Space Station orbits in this layer,
between 200 and 240 mi above the Earth. It is
also here that the interaction between solar
winds and the Earth’s magnetic field create the
Aurora Borealis.

17. II. The Modern Atmosphere
D. Other Atmospheric Features 1.Ozone layer - contained within the stratosphere. In this layer ozone concentrations are about 2 to 8 parts per million, which is much higher than in the lower atmosphere. It is mainly located in the lower portion of the stratosphere from about 49K to 110K ft. About 90% of the ozone in our atmosphere is contained in the stratosphere.

18. II. The Modern Atmosphere
2. Ionosphere - the part of the atmosphere that is ionized by solar radiation, stretches from 160K to ,300K ft above the surface of the Earth and typically overlaps both the exosphere and the thermosphere. It forms the inner edge of the magnetosphere. It has practical importance because it influences, for example, radio propagation on the Earth. It is responsible for auroras.

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20. III. Planetary Atmospheres Earth and the Other Terrestrial Worlds
“For the first time in my life, I saw the horizon as a curved line. It was accentuated by a thin seam of dark blue light – our atmosphere. Obviously this was not the ocean of air I had been told it was so many times in my life. I was terrified by its fragile appearance.”
Ulf Merbold (1941 – )
German Astronaut
© 2005 Pearson Education Inc., publishing as Addison-Wesley

21. Our goals for learning:
11.1 Atmospheric Basics
Our goals for learning:
Describe the general atmospheric properties of each of the five terrestrial worlds.
What is atmospheric pressure?
Summarize the effects of atmospheres.
© 2005 Pearson Education Inc., publishing as Addison-Wesley

22. Comparing Terrestrial Atmospheres
© 2005 Pearson Education Inc., publishing as Addison-Wesley

24. Effects of an Atmosphere on a Planet
greenhouse effect
makes the planetary surface warmer than it would be otherwise
scattering and absorption of light
absorb high-energy radiation from the Sun
scattering of optical light brightens the daytime sky
creates pressure
can allow water to exist as a liquid (at the right temperature)
creates wind and weather
promotes erosion of the planetary surface
creates auroras
interaction with the Solar wind when magnetic fields are present
© 2005 Pearson Education Inc., publishing as Addison-Wesley

25. 11.2 The Greenhouse Effect and Planetary Temperature
Our goals for learning:
What is the greenhouse effect?
How would planets be different without the greenhouse effect?
Compare the greenhouse effect on Venus, Earth, and Mars.
© 2005 Pearson Education Inc., publishing as Addison-Wesley

26. The Greenhouse Effect
Visible Sunlight passes through a planet’s atmosphere.
Some of this light is absorbed by the planet’s surface.
Planet re-emits this energy (heat) as infrared (IR) light.
planet’s temperature lower than Sun
IR light is “trapped” by the atmosphere.
its return to space is slowed
This causes the overall surface temperature to be higher than if there were no atmosphere at all.
© 2005 Pearson Education Inc., publishing as Addison-Wesley

27. Greenhouse Gases
Key to Greenhouse Effect…gases which absorb IR light effectively:
water [H2O]
carbon dioxide [CO2]
methane [CH4]
These are molecules which rotate and vibrate easily.
they re-emit IR light in a random direction
The more greenhouse gases which are present, the greater the amount of surface warming.
© 2005 Pearson Education Inc., publishing as Addison-Wesley

28. What Determines a Planet’s Surface Temperature?
Greenhouse Effect cannot change incoming Sunlight, so it cannot change the total energy returned to space.
it increases the energy (heat) in lower atmosphere
it works like a blanket
In the absence of the Greenhouse Effect, what would determine a planet’s surface temperature?
the planet's distance from the Sun
the planet’s overall reflectivity
the higher the albedo, the less light absorbed, planet cooler
© 2005 Pearson Education Inc., publishing as Addison-Wesley

29. Greenhouse Effect on the Planets
Greenhouse Effect warms Venus, Earth, & Mars
on Venus: it is very strong
on Earth: it is moderate
on Mars: it is weak
avg. temp. on Venus & Earth would be freezing without it
© 2005 Pearson Education Inc., publishing as Addison-Wesley

30. Structure of Earth’s Atmosphere
pressure & density of atmosphere decrease with altitude
temperature varies “back and forth” with altitude
these temperature variations define the major atmospheric layers
exosphere
low density; fades into space
thermosphere
temp rises w/altitude
mesosphere
temp drops with altitude
stratosphere
troposphere
layer closest to surface

31. Atmospheres Interact with Light
X rays
ionize atoms & molecules
dissociate molecules
absorbed by almost all gases
Ultraviolet (UV)
dissociate some molecules
absorbed well by O3 & H2O
Visible (V)
passes right through gases
some photons are scattered
Infrared (IR)
absorbed by greenhouse gases
© 2005 Pearson Education Inc., publishing as Addison-Wesley

32. Reasons for Atmospheric Structure
Light interactions are responsible for the structure we see.
Troposphere
absorbs IR photons from the surface
temperature drops with altitude
hot air rises and high gas density causes storms (convection)
Stratosphere
lies above the greenhouse gases (no IR absorption)
absorbs heat via Solar UV photons which dissociate ozone (O3)
UV penetrates only top layer; hotter air is above colder air
no convection or weather; the atmosphere is stratified
Thermosphere
absorbs heat via solar radiation which ionizes all gases
contains ionosphere, which reflects back human radio signals
Exosphere
hottest layer; gas extremely rarified; provides noticeable drag on satellites
© 2005 Pearson Education Inc., publishing as Addison-Wesley

33. Magnetospheres
The Sun ejects a stream of charged particles, called the solar wind.
it is mostly electrons, protons, and Helium nuclei
Earth’s magnetic field attracts and diverts these charged particles to its magnetic poles.
the particles spiral along magnetic field lines and emit light
this causes the aurora (aka northern & southern lights)
this protective “bubble” is called the magnetosphere
Other terrestrial worlds have no strong magnetic fields
solar wind particles impact the exospheres of Venus & Mars
solar wind particles impact the surfaces of Mercury & Moon
© 2005 Pearson Education Inc., publishing as Addison-Wesley

34. Gain/Loss Processes of Atmospheric Gas
Ways to lose atmospheric gas:
condensation – gas turns into liquids or ices on the surface when cooled
chemical reactions – gas is bound into surface rocks or liquids
stripping – gas is knocked out of the upper atmosphere by Solar wind particles
impacts – a comet/asteroid collision with a planet can blast atmospheric gas into space
thermal escape – lightweight gas molecules are lost to space when they achieve escape velocity
gas is lost forever!
© 2005 Pearson Education Inc., publishing as Addison-Wesley

35. Origin of the Terrestrial Atmospheres
Venus, Earth, & Mars received their atmospheres through outgassing.
most common gases: H2O, CO2, N2, H2S, SO2
Chemical reactions caused CO2 on Earth to dissolve in oceans and go into carbonate rocks (like limestone.)
this occurred because H2O could exist in liquid state
N2 was left as the dominant gas; O2 was exhaled by plant life
as the dominant gas on Venus, CO2 caused strong greenhouse effect
Mars lost much of its atmosphere through impacts
less massive planet, lower escape velocity
© 2005 Pearson Education Inc., publishing as Addison-Wesley

36. What have we learned? (or at least should know…
Describe the general atmospheric properties of each of the five terrestrial worlds.
Moon and Mercury: essentially airless with very little atmospheric gas. Venus: thick CO2 atmosphere, with high surface temperature and pressure. Mars: thin CO2 atmosphere, usually below freezing and pressure too low for liquid water. Earth: nitrogen/oxygen atmosphere with pleasant surface temperature and pressure.
What is atmospheric pressure?
The result of countless collisions between atoms and molecules in a gas. Measured in bars (1 bar = Earth’s pressure at sea level.)
Summarize the effects of atmospheres.
Atmospheres absorb and scatter light, create pressure, warm the surface and distribute heat, create weather, and interact with the Solar wind to make auroras.
© 2005 Pearson Education Inc., publishing as Addison-Wesley

37. What have we learned? (or at least should
What is the greenhouse effect?
Planetary warming caused by the absorption of infrared light from a planet’s surface by greenhouse gases such as carbon dioxide, methane, and water vapor.
How would planets be different without the greenhouse effect?
They would be colder, with temperatures determined only by distance from the Sun and reflectivity.
Compare the greenhouse effect on Venus, Earth, & Mars.
All three planets are warmed by the greenhouse effect, but it is weak on Mars, moderate on Earth, and very strong on Venus.
© 2005 Pearson Education Inc., publishing as Addison-Wesley

38. What have we learned? (or at least should know…
Describe the basic structure of Earth’s atmosphere.
Pressure and density decrease rapidly with altitude. Temperature drops with altitude in the troposphere, rises with altitude in the lower part of the stratosphere, and rises again in the thermosphere and exosphere.
How do interactions with light explain atmospheric structure?
Solar radiation heats and ionizes gas in the thermosphere. Solar ultraviolet is absorbed by molecules such as ozone, heating the stratosphere. Visible light warms the surface (and colors the sky), which radiates infrared light that warms the troposphere.
© 2005 Pearson Education Inc., publishing as Addison-Wesley

39. What have we learned? (or at least should know…
Contrast the atmospheric structures of Venus, Earth, and Mars.
Venus and Mars lack and ultraviolet-absorbing stratosphere. Both contain higher percentages of greenhouse gases
What is a magnetosphere?
Created by a global magnetic field, it acts like a protective bubble surrounding the planet that diverts charged particles from the Solar wind, channeling some to the magnetic poles where they can lead to auroras. Only present on Earth.

40. What have we learned? (or at least should know…
Describe four factors that can cause long-term climate change.
The gradual brightening of the Sun over the history of the Solar System. Changes in a planet’s axis tilt. Changes in a planet’s reflectivity. Changes in a planet’s abundance of greenhouse gases.
Describe the processes by which an atmosphere can gain and lose gas.
Gains come from outgassing, evaporation/sublimation, or bombardment, but the latter only if there’s very little atmosphere. Gases can be lost by condensation, chemical reactions with surface materials, stripping from the upper atmosphere by small particles or photons, being blasted away by impacts, or by achieving thermal escape velocity.
© 2005 Pearson Education Inc., publishing as Addison-Wesley