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

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

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.

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

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?

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

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.

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.

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).

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

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

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

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.

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.

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.

II. The Modern Atmosphere 2. 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. 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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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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.

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