Habitability of Earth ASTR 1420 Lecture #7 Sections 4.4 & 4.5

Formation of Stars and Planets

Formation & evolution of Earth

Interior Structure of the Earth Earth’s long-term stability depends on volcanism, plate tectonics  we need to know the internal structure! Earth’s long-term stability depends on volcanism, plate tectonics  we need to know the internal structure! How do we know? How do we know? Overall density versus surface rock Overall density versus surface rock Gravity probe? Magnetic field? Gravity probe? Magnetic field? But, mostly from seismic waves! But, mostly from seismic waves! Interior structure Interior structure o 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! By the time of the Moon creating impact, Earth was already differentiated! How did it happen so fast? How did it happen so fast?  Earth was molten (or at least nearly molten) throughout its interior… Heat sources of the melting Heat sources of the melting o Impact heat o Potential energy of sinking heavy material o 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 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 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

Plate Tectonics Internal heat makes hot material expand and rise while cooler material on top contracts and falls 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. 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… Sometimes, localized plume of mantle material get uplifted…  Hawaiian islands  Yellow Stone Park

On-going… In ~100 Myrs, California will slide northward to Alaska, the Mediterranean Sea will become mountains, Australia will merge with Antarctica 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 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 field Requirements for a magnetic field o Electrically conducting fluid o Convection o 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!

Auroras : northern & southern lights Aurora seen at UL : Washington UR : Canada LL : Texas Aurora Borealis : Aurora Borealis : Aurora Australis : Aurora Australis :

Aurora in Georgia! Oct. 29, Dahlonega, GA Triggered by a coronal mass ejection…

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

Greenhouse effect Greenhouse gases (H 2 O, CO 2, CH 4 ): effectively “trap” infrared light Greenhouse gases (H 2 O, CO 2, CH 4 ): effectively “trap” infrared light Without the greenhouse effect, global average temperature of the Earth would be 30°F versus current value of 59°F! 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?? for a very similar size and distance from the Sun compared to those of Earth, Venus’ global average temperature is 878°F! Why??

CO 2 Cycle On Earth, CO 2 content in the air is regulated by the Carbon Dioxide Cycle (the carbonatesilicate cycle, or the inorganic carbon cycle) to avoid runaway Greenhouse effect On Earth, CO 2 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, … Carbonate rocks : limestone, calcite, dolomite, chalk, … CO 2 in the Earth : atmosphere 1, ocean 60, rocks 170,000

Regulation of Earth’s Climate The cycle is about 400,000 years The cycle is about 400,000 years The CO 2 cycle acts as a thermostat The CO 2 cycle acts as a thermostat For the warm Earth, carbonate minerals forms in the oceans at a more rapid rate, and takes away more CO 2 For the warm Earth, carbonate minerals forms in the oceans at a more rapid rate, and takes away more CO 2 What global warming? We don’t need to worry about it since the CO 2 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). Different configuration of continents can cause a climate change (hundreds of million years cycle). But, cycles of ice ages are much faster! But, cycles of ice ages are much faster! Why? Why? Plate tectonics!! Plate tectonics!!

Milankovitch cycles Serbian scientist: Serbian scientist: o Precession o Obliquity o Eccentricity Earth's most common mechanism for climate change

Snowball Earth

Earth’s long-term Habitability Several Key Factors Several Key Factors o Volcanic outgassing  atmosphere o Magnetic field shield  atmosphere o Moderate greenhouse effect o CO 2 cycle as a climate regulator o Milankovitch cycles and ice ages o Some extreme cases (snowball Earth and hothouse) ? Should we expect to find plate tectonics and CO 2 cycle on other planets?

In summary… Important Concepts Plate Tectonics CO 2 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 & 4.5 Threats to Life : next class!