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 Cycle
http://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!