Universe 1

Astronomy 20 1

EarthThere's no place like home.

Astronomy 20 2

Earth

Earth as a planet

Largest terrestrialplanet

Magnetic field Plate tectonics Near inner edge of

Habitable Zone Uniquely placed for life

Astronomy 20 3

Earth

Internal Structure

liquid

solid

Universe 2

Astronomy 20 4

Earth

Magnetic Field

Generated by currents in outer core Field flips at irregular intervals

—about every million yrs Protects us from solar wind

Astronomy 20 5

Earth

Surface Geology

Continents vs ocean basinsOceansErosion by water and icePlate tectonics

Astronomy 20 6

Earth

Plate tectonics Continental & oceanic plates Volcanos & earthquakes on plate boundaries

Universe 3

Astronomy 20 7

Earth

Plate Boundaries

Mid-ocean ridges: sea-floorspreading

Subduction zones: destruction ofsea floor

Astronomy 20 8

Earth

San Andreas Fault

Astronomy 20 9

Earth

Continental Drift World

continent:Pangaea

Atlantic oceanyoung

Universe 4

Astronomy 20 10

Earth

Atmosphere

Pressure = 1 bar

CompositionNitrogen 78%Oxygen 21%Argon 1%Water vapor 1-3%Carbon dioxide 0.03%

4

Astronomy 20 11

Earth

History

Microscopic life

Fish

DinosaursMammals & birds

First bacteria

4500 Myr

540 Myr

250 Myr65 Myr

O2

No O2 K-T extinction

Astronomy 20 12

Earth

Faint Young Sun Paradox

Sun only 75% as bright inthe beginning.

Oceans should have beenfrozen till 2 Gyr ago.

Oldest sedimentary rocks3.8 Gyr old.

Resolution: CO2 →greenhouse effect

Universe 5

Astronomy 20 13

Earth

Continuously Habitable Zone

Problem: CO2 must have gonedown in a precise way.

Hart: CHZ very narrow0.95 AU – 1.01 AU

Implication: habitable planetsare rare.

Astronomy 20 14

Earth

CO2 Cycle

Volcanos add CO2

Rain removes CO2

Astronomy 20 15

Earth

CO2 Thermostat

Rising temperature →More evaporation →More rainfall →Depletion of CO2 →Less greenhouse effect →Lower temperature

Universe 6

Astronomy 20 16

Earth

Continuously Habitable Zone

CO2 Thermostat → CHZ is wide

0.95 AU to 1.5 AU Habitable planets are common

Astronomy 20 17

Earth

Goldilocks & the 3 Planets

Mars: slowing of volcanic activity →loss of CO2 atmosphere

Venus: runaway greenhouse effect Earth: just right — barely!

Pika

What a close call!

Astronomy 20 18

Earth

The Far Future

Sun continues to brighten CO2 levels decline 900 Myr from now, too little CO2 for

plants →end of life on Earth 1700 Myr from now, T reaches

boiling → Earth ends up like Venus!