Search for Life in the Universe

04/22/23 AST 248, Spring 2007 1

Search for Life in the Universe

Chapter 4The Habitability of Earth

(Part 2)

04/22/23 AST 248, Spring 2007 2

Outline

• Geology and Habitability• Climate Regulation and Change

04/22/23 AST 248, Spring 2007 3

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

04/22/23 AST 248, Spring 2007 4

04/22/23 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

04/22/23 AST 248, Spring 2007 6

04/22/23 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

04/22/23 AST 248, Spring 2007 8

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

04/22/23 AST 248, Spring 2007 9

04/22/23 AST 248, Spring 2007 10

04/22/23 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

04/22/23 AST 248, Spring 2007 12

04/22/23 AST 248, Spring 2007 13

Mantle Convection→ Plate Techtonics

04/22/23 AST 248, Spring 2007 14

04/22/23 AST 248, Spring 2007 15

04/22/23 AST 248, Spring 2007 16

04/22/23 AST 248, Spring 2007 17

04/22/23 AST 248, Spring 2007 18

04/22/23 AST 248, Spring 2007 19

Plate Tectonics Over Time

04/22/23 AST 248, Spring 2007 20

04/22/23 AST 248, Spring 2007 21

04/22/23 AST 248, Spring 2007 22

Cause of Aurora Borealis

04/22/23 AST 248, Spring 2007 23

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

04/22/23 AST 248, Spring 2007 24

04/22/23 AST 248, Spring 2007 25

Cause of Greenhouse Effect

04/22/23 AST 248, Spring 2007 26

04/22/23 AST 248, Spring 2007 27

04/22/23 AST 248, Spring 2007 28

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

04/22/23 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

04/22/23 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

04/22/23 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: 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

04/22/23 AST 248, Spring 2007 32

04/22/23 AST 248, Spring 2007 33

04/22/23 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 1C 19002000