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Search for Life in the Universe. Chapter 4 The Habitability of Earth (Part 2). Outline. Geology and Habitability Climate Regulation and Change. Origin of the Continents. Seafloor crust (and volcanoes): Basalt: high-density igneous rock 5 10 km thick Radiometric dating: < 0.2 byr old

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Search for Life in the Universe


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    1. Search for Life in the Universe Chapter 4 The Habitability of Earth (Part 2) AST 248, Spring 2007

    2. Outline • Geology and Habitability • Climate Regulation and Change AST 248, Spring 2007

    3. Origin of the Continents • Seafloor crust (and volcanoes): • Basalt: high-density igneous rock • 510 km thick • Radiometric dating: < 0.2 byr old • Continental crust: • Granite: lower-density igneous rock • 2070 km thick • Radiometric dating: up to 4.0 byr old • Floats like an iceberg: higher and deeper • Plate tectonics: • Recycles seafloor crust • Continually add to continental crust AST 248, Spring 2007

    4. AST 248, Spring 2007

    5. Internal Heat and Active Geology • Geological activity: • Volcanic eruptions • Earthquakes • Source of energy today: radioactivity • Loss of Energy: • Smaller bodies lose energy faster per unit mass • Earth and Venus active • Moon and Mercury inactive • Mars low level of activity AST 248, Spring 2007

    6. AST 248, Spring 2007

    7. Mantle Convection and the Lithosphere • Even rock can flow, albeit slowly • Heat at the bottom  instability • Convection cells: • Bottom Limit: solid inner core • Top limit: lithosphere, solid upper mantle and crust • Rotation period: ~200 myr • Plate tectonics: • Cause: friction between lithosphere and mantle • Direction: that of the underlying convection cell AST 248, Spring 2007

    8. Plate Tectonics (1) • Wegener (18801930): proposed continental drift, no mechanism • Seafloor spreading: • Mantle material erupts at mid-ocean ridges • Continents move away from each other • Subduction: • Ocean trenches: dense seafloor  under less dense continents • Subducting seafloor crust heats  volcanoes  continental growth • Collision: • Himalayas: two continental plates pushing against each other • San Andreas Fault: plates sliding against each other • Rockies: past collision of continental plates AST 248, Spring 2007

    9. AST 248, Spring 2007

    10. AST 248, Spring 2007

    11. Plate Tectonics (2) • Lithosphere divided into ~ dozen plates • Earthquakes: readjustment along plate boundaries • Motion: few cm/yr  Atlantic Ocean in 200 myr • Pangaea: all continents together ~ 200 myr ago • Earlier motion: estimated with difficulty to 750 myr ago; unknown beyond that • Subduction zone 2.7 byr old found in Canada • Theory: • Mantle convection as long as Earth is differentiated • Earlier radioactivity stronger  stronger convection AST 248, Spring 2007

    12. AST 248, Spring 2007

    13. Mantle Convection→ Plate Techtonics AST 248, Spring 2007

    14. AST 248, Spring 2007

    15. AST 248, Spring 2007

    16. AST 248, Spring 2007

    17. AST 248, Spring 2007

    18. AST 248, Spring 2007

    19. Plate Tectonics Over Time AST 248, Spring 2007

    20. AST 248, Spring 2007

    21. AST 248, Spring 2007

    22. Cause of Aurora Borealis AST 248, Spring 2007

    23. Greenhouse Effect (1) • Without atmosphere: average Earth temperature today 17C • Actual global average: +15C • Zero-age Sun: 30% dimmer than today • Greenhouse effect: • Solar visible light penetrates atmosphere • Earth absorbs visible light • Earth emits infrared light • Escaping infrared light trapped by CO2 H2O and CH4 in the atmosphere • Earth temperature rises until energy outflow equals energy inflow AST 248, Spring 2007

    24. AST 248, Spring 2007

    25. Cause of Greenhouse Effect AST 248, Spring 2007

    26. AST 248, Spring 2007

    27. AST 248, Spring 2007

    28. Greenhouse Effect (2) • Early Earth: more CO2 warmer temperature (85C?, favoring thermophiles), in spite of dimmer Sun • Where is the CO2?: • Dissolved in ocean water: 60 times more than in the atmosphere • Locked up in carbonates: 170,000 times more than in the atmosphere • If all the CO2 were in the atmosphere: • The oceans would boil • Venus: surface temperature 470 C AST 248, Spring 2007

    29. Inorganic CO2 Cycle • CO2 dissolves in ocean water • Rain erodes silicate rocks  oceans • Silicates + CO2 in oceans  carbonate minerals that sink to the bottom • Subduction: carbonates  mantle, where they break up, releasing CO2 • CO2 outgassed by volcanoes AST 248, Spring 2007

    30. CO2 Cycle as a Thermostat • CO2 cycle sensitive to temperature  thermostat controlling the Earth temperature: • Earth warms: carbonates form more rapidly  lower CO2 content in the oceans  more atmospheric CO2 dissolving in the oceans  less greenhouse  cooling • Earth cools: carbonates form more slowly  higher CO2 content in the oceans  less atmospheric CO2 dissolving in the oceans  more greenhouse  warming • Thermostat adapted to changing solar luminosity AST 248, Spring 2007

    31. Long-Term Climate Change • Observed timescales for change: • CO2 feedback timescale today: 400,000 yr • Solar change: tens to hundreds of myr • Continent motion: hundreds of myr • Ice ages (wobble of Earth’s rotation axis): 41,000 yr • Snowball Earth: • Glaciers to the equator: 750580 myr ago • Oceans freeze to a depth ~ 1 km • Ice reflectivity 90%: prevents heating • CO2 outgassing continued  finally melting the oceans • Liquid reflectivity 5%: quick warming with liquid ocean AST 248, Spring 2007

    32. AST 248, Spring 2007

    33. AST 248, Spring 2007

    34. Short-Term Global Warming • Burning fossil fuels: CO2 in atmosphere increase 20% in last 50 years • No regulation by CO2 cycle: much too fast • Global warming unavoidable: eventually • Scales of decades to centuries: • Evaporation  less sunlight • But: clouds (H2O) also trap infrared radiation • Net short-term effect uncertain • Observed: temperature rose 1C 19002000 AST 248, Spring 2007