THE ATMOSPHERE The thin blue line that keeps us alive

 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?

 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 (H 2, 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

 D. First Atmosphere 1. Composition - Probably H 2, 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 H 2 O, CO 2, CH 4 and N 2 b. No free O 2 at this time (not found in volcanic gases). 2. Ocean Formation - As the Earth cooled, H 2 O produced by out gassing could exist as liquid allowing oceans to form.

F. Addition of O 2 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 O 2 levels approx. 1-2% current levels. At these levels O 3 (ozone) can form to shield Earth surface from UV c. Photosynthesis - O 2 from photosynthesis produced first by cyanobacteria, and eventually higher plants - supplied the rest of O 2 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).

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

Percentage composition of Earth’s 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. 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 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.

 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

 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 (O 3 ) 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 Features 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 atmospheric ozone is contained in the stratosphere.

Other Atmospheric Features Ionosphere- 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.

 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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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.”

 Describe the general atmospheric properties of each of the five terrestrial worlds. Essential Question:

 Venus, Earth, & Mars received their atmospheres through outgassing. most common gases: H 2 O, CO 2, N 2, H 2 S, SO 2  Chemical reactions caused CO 2 on Earth to dissolve in oceans and go into carbonate rocks (like limestone.) this occurred because H 2 O could exist in liquid state N 2 was left as the dominant gas; O 2 was exhaled by plant life as the dominant gas on Venus, CO 2 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

 How does the presence of an atmosphere affect a planet? Essential Question:

 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

 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.

 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  Key to Greenhouse Effect…gases which absorb IR light effectively: water [H 2 O] carbon dioxide [CO 2 ] methane [CH 4 ]  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.

 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 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 0 o C without it

 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!

 Describe the general atmospheric properties of each of the five terrestrial worlds. Moon and Mercury: essentially airless with very little atmospheric gas. Venus: thick CO 2 atmosphere, with high surface temperature and pressure. Mars: thin CO 2 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 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.

 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.