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Search for Life in the Universe
Chapter 4
The 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
– 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
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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 (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
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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 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
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Cause of Greenhouse Effect
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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
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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: 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
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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 1C 19002000
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