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AST 111 Lecture 18

AST 111 Lecture 18. (Terrestrial) Planetary Atmospheres I. Atmosphere. Atmosphere: Layer of gas that surrounds a world Thin for terrestrial planets 2/3 of air within 10 km of Earth’s surface. Atmospheres and Planets. So what do atmospheres do? Pressure allows liquid phase of water

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AST 111 Lecture 18

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  1. AST 111 Lecture 18 (Terrestrial) Planetary Atmospheres I

  2. Atmosphere • Atmosphere: • Layer of gas that surrounds a world • Thin for terrestrial planets • 2/3 of air within 10 km of Earth’s surface

  3. Atmospheres and Planets • So what do atmospheres do? • Pressure allows liquid phase of water • Absorb and scatter light • “Radiation shield” • Ozone in Earth’s atmosphere absorbs UV radiation • Wind and weather • Can trap heat and warm the planet

  4. Molecular Motion • Molecules move fast and collide • 500 m/s on Earth • They therefore push on surfaces • Aside: Why don’t they travel across a room that fast?

  5. Pressure • Consider how gravity acts on a bunch of molecules in motion • They “pile up” toward the surface • The atmosphere below supports the atmosphere above

  6. Atmospheres and Planets • Planets are able to hold onto their atmospheres longer if: • They are large (stronger gravity) • The temperature of the atmosphere is low • Molecules don’t try as hard to escape

  7. Factors That Determine Planetary Climate • Distance from the Sun • Albedo: Reflectivity of surface and atmosphere • Greenhouse Effect: Trapping a planet’s emitted radiation

  8. Albedo • Sunlight rejected by planet • Low Albedo: • Darker: absorbs more • Soil, trees, etc. • High Albedo: • Lighter: reflects more • Cloud, ice caps, etc. • If the sunlight is reflected, it can’t warm the planet

  9. Albedo • Different materials respond differently to different frequencies of light! • Clouds reflect visible light. They do not reflect UV.

  10. Albedo Does albedo warm or cool a planet?

  11. Greenhouse Effect • Visible light from Sun absorbed by the ground • Ground returns absorbed radiation as a continuous spectrum. • Peaks in the infrared • Greenhouse gases absorb these infrared photons • Water Vapor • CO2 (Carbon dioxide) • CH4 (Methane)

  12. Greenhouse Effect • Keeps the lower atmosphere and ground warm • Energy from the photons can be “exchanged” for kinetic energy through collisions • Cloudy nights can be warmer than clear nights!

  13. Greenhouse Effect Does the greenhouse effect warm or cool a planet?

  14. Greenhouse Effect • Be thankful for it… • The infrared radiation emitted by Earth would escape straight back into space if not for the greenhouse effect. • Earth would be at 3 oF if not for the greenhouse effect. • We wouldn’t have liquid water.

  15. Examples • Venus has a high albedo and reflects 75% of incoming light. Why is it so hot (800 oF)? • Do Mercury and the Moon have a greenhouse effect? Why or why not? • Earth’s atmosphere is mostly diatomic nitrogen and oxygen (poor infrared absorbers). How would the temperature change if they were good infrared absorbers?

  16. Models for Planetary Climates • Planetary climates are modeled as follows: • Calculate Effective Temperature • Assumes planet absorbs all radiation, emits freely • Calculate Albedo Temperature • Assumes that planet can reflect incident radiation • Calculate Atmospheric Temperature • Assumes atmosphere can inhibit radiation emission by the planet

  17. Atmospheric Structure • Variation of temperature with height • Due to how atmospheric gas interacts with sunlight

  18. Interaction of Light and Atmosphere • X-rays: • Can remove electrons from atoms(Ionizes them) • Can dissociate (break apart) molecules

  19. Interaction of Light and Atmosphere • Ultraviolet: • Can dissociate (break apart) molecules

  20. Interaction of Light and Atmosphere • Visible light: • Usually transmitted, sometimes scattered

  21. Interaction of Light and Atmosphere • Infrared light: • Absorbed by molecules • Causes rotation and vibration in molecules

  22. Interaction of Light and Atmosphere X-rays (Page 304) Ultraviolet Visible Infrared

  23. Visible Light and the Surface • The atmosphere scatters visible light • Think in terms of light rays • If no scattering, would see stars with Sun in view • Blue light scattered more than red • Red sunsets

  24. Infrared Light and the Troposphere • Troposphere gets the infrared light emitted by Earth • Temperature drops farther from surface • Has convection and storms • Dense air • Surface heat

  25. Ultraviolet Light and the Stratosphere • Infrared not significant here • UV light absorbed by ozone here • UV light from Sun • The top layer absorbs more than the bottom • Gets hotter with height to a point • No convection, no weather

  26. X-Rays and the Thermosphere • Most gases absorb X-rays • They get absorbed by the first dense gas they encounter • Exosphere not dense enough • This is the thermosphere • Gets hotter higher up

  27. The Exosphere • Very low density gas • Faster molecules escape • Boundary between atmosphere and space • Gas very hot, but you wouldn’t feel it (low density)

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