Habitability of EarthASTR 1420

Lecture #7

Sections 4.4 & 4.5

Formation of Stars and Planets

Formation & evolution of Earth

Interior Structureof the Earth

• Earth’s long-term stability depends on volcanism, plate tectonics we need to know the internal structure!

• How do we know?Overall density versus surface rockGravity probe? Magnetic field?But, mostly from seismic waves!

•Interior structureo Core : Nickel and Iron. Inner core (solid),

outer core (liquid!)o Mantle : rocky material (silicate

minerals)o Crust : lowest-density rock.

Lithosphere : a layer of cold, rigid rocks above warmer, softer rocks.

Differentiation• By the time of the Moon creating impact,

Earth was already differentiated!

• How did it happen so fast?

Earth was molten (or at least nearly molten) throughout its interior…

• Heat sources of the meltingo Impact heato Potential energy of sinking heavy materialo Radioactive decay energy

All terrestrial worlds in out solar system had similar melting and differentiating…

Materials that are separated according to their densities…

Continental Drift

• Continental Drift was proposed by meteorologist and geologist Alfred Wegener in 1912

• The idea was inspired by the puzzle-like fit of South America and Africa, indicating these two continents were part of a single “supercontinent” about 200 million years ago

Plate Tectonics : Wilson Cyclehttp://www.youtube.com/watch?v=ZLJLFYXp-0Q

Plate Tectonics• Internal heat makes hot material expand and rise

while cooler material on top contracts and falls• Plate tectonics produces and recycles seafloor crust

(that is why the seafloor crust is usually less than 200 million years old). At ocean trenches, the seafloor crust pushes under the less dense continental crust, returning the seafloor crust to the mantle and may cause volcanic eruptions.

At plate boundaries…

Hot Spots • Sometimes, localized plume of mantle material get uplifted…

Hawaiian islandsYellow Stone Park

On-going…

• In ~100 Myrs, California will slide northward to Alaska, the Mediterranean Sea will become mountains, Australia will merge with Antarctica

Magnetic Field

• Charged particles from solar flares interact with earth’s magnetic field and atoms and molecules in Earth’s atmosphere and produce aurora

Cause of Magnetic Field• Requirements for a magnetic fieldo Electrically conducting fluido Convectiono Rotation of the planet

Electromagnet

Magnetic field as a shield!High velocity electrified gas (plasma) blows into the Earth at speed of ~250 miles/second!http://www.gsfc.nasa.gov/gsfc/spacesci/pictures/20020509imagessu/magneticfield_mpeg.mpg

Auroras : northern & southern lights

Aurora seen at

UL : WashingtonUR : CanadaLL : Texas• Aurora Borealis :

• Aurora Australis :

Aurora in Georgia!Oct. 29, 2003. Dahlonega, GA

Triggered by a coronal mass ejection…

Other Planets, too!Jupiter seen in X-ray (Chandra satellite)

Greenhouse effect

• Greenhouse gases (H2O, CO2, CH4): effectively “trap” infrared light

• Without the greenhouse effect, global average temperature of the Earth would be 30°F versus current value of 59°F!

• for a very similar size and distance from the Sun compared to those of Earth, Venus’ global average temperature is 878°F! Why??

CO2 Cycle

• On Earth, CO2 content in the air is regulated by the Carbon Dioxide Cycle (the carbonatesilicate cycle, or the inorganic carbon cycle) to avoid runaway Greenhouse effect

• Carbonate rocks : limestone, calcite, dolomite, chalk, …

CO2 in the Earth : atmosphere 1, ocean 60, rocks 170,000

Regulation of Earth’s Climate• The cycle is about 400,000 years• The CO2 cycle acts as a thermostat• For the warm Earth, carbonate minerals forms in the oceans at a more rapid rate, and

takes away more CO2

What global warming? We don’t need to worry about it since the CO2 cycle will take care of it!!

Really?? runaway greenhouse effect!

Ice Ages• Different configuration of continents can

cause a climate change (hundreds of million years cycle).

• But, cycles of ice ages are much faster!

• Why?

• Plate tectonics!!

Milankovitch cycles

• Serbian scientist:o Precessiono Obliquityo Eccentricity

http://www.youtube.com/watch?v=wLAYRdSnRSI

Earth's most common mechanism for climate change

Snowball Earth

Earth’s long-term Habitability

• Several Key Factorso Volcanic outgassing atmosphereo Magnetic field shield atmosphereo Moderate greenhouse effecto CO2 cycle as a climate regulatoro Milankovitch cycles and ice ageso Some extreme cases (snowball Earth and

hothouse)

? Should we expect to find plate tectonics and CO2 cycle on other planets?

In summary…

Important Concepts• Plate Tectonics• CO2 cycle and climate regulation• Greenhouse effect• Runaway greenhouse effect• Snowball/Hothouse Earth

Important Terms• Core, mantle, crust (+ lithosphere)• Plate tectonics• Milankovitch cycles• Differentiation• Precession, obliquity, eccentricity

Chapter/sections covered in this lecture : 4.4 & 4.5Threats to Life : next class!