Search for Life in the Universe

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

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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

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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

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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

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

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19. Plate Tectonics Over Time AST 248, Spring 2007

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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

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25. Cause of Greenhouse Effect AST 248, Spring 2007

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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

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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