THE ATMOSPHERE The thin blue line that keeps us alive.

THE ATMOSPHERE

The thin blue line that keeps us alive

The Historical Development of Earth’s Atmosphere

Describe how the atmosphere of Earth developed into its modern form.

Essential Question:

A. Early Earth would have been very different

and inhospitable compared to the Earth today. Why?

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.


B. Atmosphere - early atmosphere probably completely different in composition from today (H2, He)

C. Formation1. Cooling

a. Primordial heat dissipated to spaceb. Condensation of water (rain), leading to

accumulation of surface water.c. Accumulation of new atmosphere due to volcanic out gassingd. Conditions appropriate for evolution of life


D. First Atmosphere1. 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?


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


E. Second Atmosphere 1. Produced by volcanic outgassing. 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, CH4 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.


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.


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 (approx. 80% of

mass) lies within 30 km of the Earth (about same thickness as continental crust).

The Modern Atmosphere

B. Composition1. 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 itc. It is where all of Earth’s weather

happens



Percentage composition of Earth’s atmosphere

C. Layers of the AtmosphereThe Modern 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. Then we pass through the mesopause into the…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.

C. Layers of the AtmosphereThe Modern 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 and the temperature drops as the layer fades into deep space.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.

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• temp rises w/altitude

troposphere• layer closest to surface• temp drops with altitude

Reasons for Atmospheric Structure Electromagnetic wave interactions are responsible for the

structure we see. Troposphere

• absorbs IR photons from the surface (heat t’fer via radiation)• temperature drops with altitude• hot air rises and high gas density causes storms (heat t’fer via

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• coldest layer; gas extremely rarified; provides noticeable drag on

satellites

Other Atmospheric FeaturesOzone 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 atmospheric ozone is contained in the stratosphere.


Other Atmospheric FeaturesIonosphere- the part of the atmosphere that is ionized by solar radiation, stretches from 160K to 3,300K ft above the surface of the Earth and typically overlaps both the exosphere and the thermosphere.


Working in conjunction with the magnetosphere (the field created by the Earth’s polarity) the ionosphere has practical importance because it influences radio, TV and cell phone propagation on the Earth. Alternately, deflection and other interactions with solar winds produces auroras.


Importance of the Magnetosphere

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

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Additional vocab…

III. Planetary AtmospheresEarth and the Other Terrestrial Worlds

Ulf Merbold (1941 – )German Astronaut

“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.”

The Atmospheres of Other Planets

Describe the general atmospheric properties of each of the five terrestrial worlds.

Essential Question:

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

Comparing Terrestrial Atmospheres

The Greenhouse Effect and Planetary Temperature

How does the presence of an atmosphere affect a planet?

Essential Question:

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 atmospheric pressure• can allow water to exist as a liquid (at the right temperature)• measured with a barometer

creates wind and weather• promotes erosion of the planetary surface

creates auroras• interaction with the Solar wind when magnetic fields are

present

The Greenhouse Effect

Visible Sunlight passes through a planet’s atmosphere.

Some of this light is absorbed by the planet’s surface.◦ Amount determined by

the albedo, or reflectivity of the planetary surface. The higher (more reflective) the albedo, the less IR heat is absorbed.

The Greenhouse Effect

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

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.

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

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 0oC without

it

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!

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 with a barometer. 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.

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.

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.

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.



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.

THE ATMOSPHERE The thin blue line that keeps us alive.

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