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Search for Life in the Universe

Chapter 4The Habitability of Earth

(Part 2)

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Outline

• Geology and Habitability• Climate Regulation and Change

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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

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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

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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

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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

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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

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Mantle Convection→ Plate Techtonics

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Plate Tectonics Over Time

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Cause of Aurora Borealis

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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

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Cause of Greenhouse Effect

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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

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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

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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

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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

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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