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Transcript
The Earth: Cradle of Humankind Structure Interior Magnetic Field Atmosphere • Audience member: “The earth is a flat disc, held up on the shell of a giant turtle.” • Scientist: “Then what’s the turtle standing on?” • Audience member: “You’re very clever, but it’s turtles all the way down!” Basic Earth Properties • Roughly spherical, R = 6378 km – Bulges slightly at equator (“oblate spheroid”) • Mass ~ 6 x 1024 kg • Composition (in order of decreasing abundance) – Oxygen, Silicon, Aluminum, Iron, Calcium, Magnesium, Sodium, Potassium, trace heavier elements + some naturally radioactive ones – Silicates (Silicon compounds like SiO2) • Average Density ~ 5.5 g/cm3 – About twice density of surface rock… What else has to do with this bulge??? Earth’s Interior • Because Earth’s outer layers are about twice density of normal rock, conclude that interior must be heavier • When Earth first formed, was homogeneous mixture of Iron, Nickel, and Silicates – Gravity pulls heavier materials toward center, leaving lighter silicates near surface • “Differentiation” • Like letting chocolate chip ice cream melt…chips sink to bottom. Why the difference in shape? • Inner core: solid Iron & Nickel • Outer core: liquid Iron & Nickel • Mantle: solid, but malleable silicates (magma) • Crust: solid silicates How Do We Know This? • Seismology – Like ultrasound sonograms, interior can be probed by analyzing sound waves created by earthquakes • P-waves: compression waves (move through solids AND liquids) • S-waves: “wiggle” waves (cannot move through liquid core) Ultrasound: creates high-frequency pressure waves in soft tissues and fluid Strength and angles of reflection allow a computer to create an image of any surfaces Heating in the Core • Two mainstream theories – Impact heating – Radioactive heating • Important: the Earth does not generate heat like the Sun does; it has been slowly cooling since its formation. Impact Heating • Heat generated by collisions with other, small, rocky bodies… • Surface eventually cools down, but heat trapped in interior (keeps interior molten) Radioactive Heating • Radioactive materials (Uranium, Plutonium, Iridium, etc.) – Can produce large amounts of heat to melt interior rock… – Like a nuclear reactor • Larger objects hold on to heat longer – Baked potato vs. large iron ingot – Really surface/volume ratio – So heat accumulated during birth of Earth is still stored in the interior • The heat is NOT being being GENERATED in core • Slowly losing heat Plate Tectonics • Hot, molten material can rise to the surface, where it spreads out underneath the crust • This motion drags parts of the crust with it, creating the crustal plate motions we see – Subduction creates mountain ranges – Sea-floor spreading – Earthquakes generated by “sticking” plates • Pressure builds until rock breaks and crust “lurches” forward – Volcanoes • Frictional heating of subsurface material until it rises to surface and breaks through • Magma turns into lava flows Earth’s Atmosphere • Currently on 3rd atmosphere – First was H & He (basic building blocks) • Rapidly escaped into space due to high surface temperature and low mass/gravity – Second was water vapor, CO2, and N • Spewed out by newly-forming volcanoes – Third is O2, N2, plus trace components • As planet cooled, water vapor condensed into our oceans • Single-celled organisms (algaes & bacteria mostly) consume CO2 and convert it into O2 – Precursors to modern-day photosynthetic plant-life • Over last couple billion years, the VAST majority of the CO2 was scrubbed into O2 • Other theories: was our atmosphere delivered here? – Gas release by impacts with planetesimals (small rocky bodies) • Release Oxygen trapped within rock – Vaporization of frozen water and gases during comet impacts with infant Earth • Might “life” have been brought to the newborn planet by comets/asteroids? – Spectra analysis and mass spectrometer impact fragment analysis show presence of complex & organic molecules – “Panspermia” : “seeds of life everywhere” – Raises some very interesting questions… Atmosphere • Greenhouse Effect – Sunlight reaches surface, converted to heat – Surface re-radiates this heat (IR) , SOME of which is absorbed by atmosphere (recall atmospheric window) – Atmosphere warmed up by increasing concentrations of greenhouse gases (CO2, Methane) trapping IR light – DOES NOT GENERATE HEAT…merely slows down it’s escape into space • Much like a blanket keeps you warm… – The blanket does not generate heat, just keeps it trapped closer to your body… This process is hotly-debated here on Earth, but on Venus it is an established fact. Venus’ atmosphere of CO2, Methane, Ammonia, and other acidic components have turned its surface into a 1000 °F nightmare! • “Global Warming” – Based on temperature measurements – Skewed data: only certain parts of whole timeseries shown – Skewed data: only certain locations shown – Urban island effect – extra heat – Keeling curve (1960-present) – Concentration of CO2 – Glaciers & polar ice announced to be retreating, but advancing glaciers get no attention • Global Warming • A.K.A. “Climate Change” • The atmosphere is a dynamic entity. It will never reach a nice, quiet, calm, steady state. Climate change happens regardless of human activity. The question is: how much worse can we possibly make it? • Circulation – Air and ocean currents deflected by rotation of the Earth – “The Coriolis Effect” – Responsible for spinning air currents into spiral storm systems that develop into hurricanes – Also influences Jet Stream & Trade Winds Magnetic Field • Like a bar magnet, Earth has roughly dipolar magnetic field – North & South poles • Example – “Mass” generates “Gravitational Field” – “Electric Currents” generate “Magnetic Field” • Coil, wrapped around nail, attached to battery • Where do these currents come from? – Combination of rotation and convection of molten iron in liquid outer core • Very complex motions of high speed positive & negative charges – But we don’t know for sure…this is the only mechanism that we can currently describe that can generate Earth’s field… Effects of Magnetic Field • Navigation – Compass needle lines up with Earth’s magnetic north direction • But poles wander and eventually reverse polarity • Protection – Deflects high-energy particles in solar wind that are damaging to living tissues How do we know that the Earth’s magnetic field completely changes its polarity? • Simulated fly-by of Earth’s magnetic field • Magnetic Pole Reversal • Aurorae – p+ & e- are deflected and funneled onto the poles where they collide with atmospheric gases • “excites” electrons in Ozone and Nitrogen molecules to higher energy levels • When they fall back down, they emit light seen as the Northern and Southern Lights • Van Allen Radiation Belts – Regions of trapped p+ and e- that constantly spiral around the field lines, moving back and forth between N and S poles – “Space is radioactive!” • Confirmed during first space flights NEXT TIME • The Moon