Chapter 10 10.1 Atmospheric Basics Planetary Atmospheres: Earth - PDF document

Chapter 10 10.1 Atmospheric Basics Planetary Atmospheres: Earth and the Other Terrestrial Worlds • Our goals for learning • What is an atmosphere? • How does the greenhouse effect warm a planet? • Why do atmospheric properties vary with altitude? What is an atmosphere? Earth’s Atmosphere • About 10 km thick • Consists mostly of molecular nitrogen (N 2 ) and oxygen (O 2 ) An atmosphere is a layer of gas that surrounds a world An atmosphere is a layer of gas that surrounds a world Atmospheric Pressure Atmospheric Pressure • Pressure and density decrease with altitude because the weight of overlying layers is less • Earth’s pressure at sea level is Gas pressure Gas pressure Adding air Adding air Heating the air Heating the air depends on both depends on both molecules molecules also increases also increases – 1.03 kg per sq. meter density and density and increases the increases the the pressure. the pressure. – 14.7 lbs per sq. inch temperature. temperature. pressure in a pressure in a – 1 bar balloon. balloon. 1

Where does an atmosphere end? Where does an atmosphere end? • There is no clear upper boundary • Most of Earth’s gas is < 10 km from surface, but a small fraction extends to >100 km • Altitudes >60 km are considered “space” • Small amounts of gas are present even at > 300 km How does the greenhouse effect Effects of Atmospheres warm a planet? • Create pressure that determines whether liquid water can exist on surface • Absorb and scatter light • Create wind, weather, and climate • Interact with solar wind to create a magnetosphere • Can make planetary surfaces warmer through greenhouse effect Greenhouse Effect Planetary Temperature • Visible light passes • A planet’s surface through atmosphere temperature is and warms planet’s determined by surface balance between the energy of sunlight it absorbs and the • Atmosphere absorbs energy of outgoing infrared light from thermal radiation surface, trapping heat 2

Temperature and Distance Temperature and Rotation • A planet’s distance • A planet’s rotation from the Sun rate affects the determines the total temperature amount of incoming differences between sunlight day and night Temperature and Reflectivity “No Greenhouse” Temperatures • A planet’s reflectivity (or albedo ) is the fraction of incoming sunlight it reflects • Planets with low albedo absorb more • Venus would be 510°C colder without greenhouse sunlight, leading to effect hotter temperatures • Earth would be 31°C colder (below freezing on average) What do atmospheric properties Light’s Effects on Atmosphere vary with altitude? • Ionization: Removal of an electron • Dissociation: Destruction of a molecule • Scattering: Change in photon’s direction • Absorption: Photon’s energy is absorbed 3

Light’s Effects on Atmosphere Earth’s Atmospheric Structure • Troposphere: lowest layer of Earth’s • X rays and UV light can atmosphere ionize and dissociate molecules • Temperature drops with altitude • Molecules tend to scatter blue light more than red • Warmed by infrared light from surface and • Molecules can absorb convection infrared light Earth’s Atmospheric Structure Earth’s Atmospheric Structure • Stratosphere: Layer • Thermosphere: Layer above the troposphere at about 100 km altitude • Temperature rises with altitude in lower part, • Temperature rises with drops with altitude in altitude upper part • X rays and ultraviolet • Warmed by absorption light from the Sun heat of ultraviolet sunlight and ionize gases Earth’s Atmospheric Structure Why the sky is blue • Exosphere: Highest • Atmosphere scatters layer in which blue light from Sun, atmosphere gradually making it appear to fades into space come from different directions • Temperature rises with altitude; atoms can • Sunsets are red because escape into space red light scatters less • Warmed by X rays and UV light 4

Atmospheres of Other Planets Earth’s Magnetosphere • Earth is only planet with a stratosphere because of UV- absorbing ozone molecules (O 3 ). • Those same molecules protect us from Sun’s UV light. • Magnetic field of Earth’s atmosphere protects us from charged particles streaming from Sun (solar wind) No- -greenhouse temperatures greenhouse temperatures No What have we learned? Aurora • What is an atmosphere? – A layer of gas that surrounds a world • How does the greenhouse effect warm a planet? – Atmospheric molecules allow visible sunlight to warm a planet’s surface but absorb infrared photons, trapping the heat. • Why do atmospheric properties vary with altitude? – They depend on how atmospheric gases interact with sunlight at different altitudes. • Charged particles can enter atmosphere at magnetic poles, causing an aurora What creates wind and weather? 10.2 Weather and Climate • Our goals for learning • What creates wind and weather? • What factors can cause long-term climate change? • How does a planet gain or lose atmospheric gases? 5

Weather and Climate Global Wind Patterns • Global winds blow in • Weather is the ever-varying combination of distinctive patterns wind, clouds, temperature, and pressure – Local complexity of weather makes it difficult to predict – Equatorial: E to W – Mid-latitudes: W to E – High-latitudes: E to W • Climate is the long-term average of weather – Long-term stability of climate depends on global conditions and is more predictable Circulation Cells: No Rotation Coriolis Effect • Heated air rises at equator • Cooler air descends at poles • Without rotation, these motions would • Conservation of angular momentum causes a ball’s produce two large apparent path on a spinning platform to change circulation cells direction Coriolis Effect on Earth Coriolis Effect on Earth • Air moving from • Conservation of pole to equator is angular momentum going farther from causes large storms to axis and begins to swirl lag Earth’s rotation • Direction of circulation • Air moving from depends on hemisphere equator to pole goes – N: counterclockwise closer to axis and – S: clockwise moves ahead of Earth’s rotation 6

Circulation Cells with Rotation Prevailing Winds • Coriolis effect deflects north-south winds into east-west winds • Deflection breaks each of the two large “no-rotation” cells breaks into three • Prevailing surface winds at mid-latitudes blow from smaller cells W to E because Coriolis effect deflects S to N surface flow of mid-latitude circulation cell What factors can cause long-term Clouds and Precipitation climate change? Solar Brightening Changes in Axis Tilt • Sun very gradually grows brighter with time, • Greater tilt makes more extreme seasons, while increasing the amount of sunlight warming planets smaller tilt keeps polar regions colder 7

Changes in Axis Tilt Changes in Reflectivity • Small gravitational tugs from other bodies in solar system cause Earth’s axis tilt to vary between 22° and 25° • Higher reflectivity tends to cool a planet, while lower reflectivity leads to warming How does a planet gain or lose Changes in Greenhouse Gases atmospheric gases? • Increase in greenhouse gases leads to warming, while a decrease leads to cooling Sources of Gas Losses of Gas Thermal escape Sweeping by Thermal escape Sweeping by of atoms solar wind of atoms solar wind Impacts of Outgassing Outgassing Evaporation of Evaporation of Impacts of particles and particles and from volcanoes from volcanoes surface liquid; surface liquid; sublimation of photons eject photons eject sublimation of Condensation Condensation Chemical Chemical Large impacts Large impacts surface ice small amounts small amounts surface ice onto surface onto surface reactions with reactions with blast gas into blast gas into surface surface space space 8

What have we learned? Thermal Escape • What creates wind and weather? – Atmospheric heating and Coriolis effect • What factors can cause long-term climate change? – Brightening of Sun – Changes in axis tilt – Changes in reflectivity – Changes in greenhouse gases What have we learned? 10.3 Atmospheres of Moon and Mercury • How does a planet gain or lose atmospheric gases? • Our goals for learning – Gains: Outgassing, evaporation/sublimation, • Do the Moon and Mercury have any and impacts by particles and photons atmosphere at all? – Losses: Condensation, chemical reactions, blasting by large impacts, sweeping by solar winds, and thermal escape Do the Moon and Mercury have Exospheres of Moon and Mercury any atmosphere at all? Moon Moon Mercury Mercury • Sensitive measurements show Moon and Mercury have extremely thin atmospheres • Gas comes from impacts that eject surface atoms 9