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Search for Life in the Universe Chapter 4 The Habitability of Earth (Part 2) 5/23/2017 AST 248, Spring 2007 1 Outline • Geology and Habitability • Climate Regulation and Change 5/23/2017 AST 248, Spring 2007 2 Origin of the Continents • Seafloor crust (and volcanoes): – Basalt: high-density igneous rock – 510 km thick – Radiometric dating: < 0.2 byr old • Continental crust: – – – – Granite: lower-density igneous rock 2070 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 5/23/2017 AST 248, Spring 2007 3 5/23/2017 AST 248, Spring 2007 4 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 5/23/2017 AST 248, Spring 2007 5 5/23/2017 AST 248, Spring 2007 6 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 5/23/2017 AST 248, Spring 2007 7 Plate Tectonics (1) • Wegener (18801930): 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 5/23/2017 AST 248, Spring 2007 8 5/23/2017 AST 248, Spring 2007 9 5/23/2017 AST 248, Spring 2007 10 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 5/23/2017 AST 248, Spring 2007 11 5/23/2017 AST 248, Spring 2007 12 Mantle Convection → Plate Techtonics 5/23/2017 AST 248, Spring 2007 13 5/23/2017 AST 248, Spring 2007 14 5/23/2017 AST 248, Spring 2007 15 5/23/2017 AST 248, Spring 2007 16 5/23/2017 AST 248, Spring 2007 17 5/23/2017 AST 248, Spring 2007 18 Plate Tectonics Over Time 5/23/2017 AST 248, Spring 2007 19 5/23/2017 AST 248, Spring 2007 20 5/23/2017 AST 248, Spring 2007 21 Cause of Aurora Borealis 5/23/2017 AST 248, Spring 2007 22 Greenhouse Effect (1) • Without atmosphere: average Earth temperature today 17C • Actual global average: +15C • 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 5/23/2017 AST 248, Spring 2007 23 5/23/2017 AST 248, Spring 2007 24 Cause of Greenhouse Effect 5/23/2017 AST 248, Spring 2007 25 5/23/2017 AST 248, Spring 2007 26 5/23/2017 AST 248, Spring 2007 27 Greenhouse Effect (2) • Early Earth: more CO2 warmer temperature (85C?, 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 5/23/2017 AST 248, Spring 2007 28 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 5/23/2017 AST 248, Spring 2007 29 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 5/23/2017 AST 248, Spring 2007 30 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: 750580 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 5/23/2017 AST 248, Spring 2007 31 5/23/2017 AST 248, Spring 2007 32 5/23/2017 AST 248, Spring 2007 33 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 1C 19002000 5/23/2017 AST 248, Spring 2007 34