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The Earth and its Moon 10 The Earth Basic Properties • Terrestrial planet (the original!) • Composition – rocky (heavy elements: oxygen, silicon, iron) – very different from sun and Jovian planets • Orbit – nearly circular • Climate – Not too hot, not too cold: JUST RIGHT! • only planet with liquid water • water necessary for life General Features Mass: MEarth = 6 x 1024 kg Radius: REarth = 6,378 km Density: = 5,500 kg/m3 Age: 4.6 billion years Liquid Water! Atmosphere! Question • Why does the Earth have an atmosphere, while the moon does not? Escape Speed • Earth: 11 km/s • Moon: 2.4 km/s • Explains why moon has no atmosphere! Convection Convection also occurs when you boil water, or soup. Think of Earth's surface as a boiling pot! Question • Neglecting other effects, the Earth should reradiate all of the energy received from the Sun, giving the Earth a surface temperature of 250 K = -23o C. • Why is the average temperature of Earth so much (about 40o C) higher? The Greenhouse Effect Main greenhouse gases are H2O and CO2 . If no greenhouse effect, surface would be 40 oC cooler! Questions • What are the major layers in the interior of the Earth? • How does density change with depth? • What about temperature? Formation of Earth • Link to formation of Earth • Link to video Questions • What does the differentiation exhibited by the Earth tell us about its past? – (Hint: The Earth was much hotter in the past.) • What were the two main sources of heating experienced by the Earth early in its history? Earth's Interior Structure Average density 5,500 kg/m3 Crust Mantle Core 3,000 kg/m3 5,000 kg/m3 11,000 kg/m3 Density increases with depth => "differentiation" Earth must have been molten once, allowing denser material to sink, as it started to cool and solidify. Heating due to (i) bombardment and (ii) radioactivity Bombardment by what? What important result do we see today? Density Stratification •Density = Mass/Volume •As Earth condensed its internal heat became so intense that it became molten. •Denser elements sank towards the core, lighter floated to the surface causing density stratificationmaking layers of the Earth’s internal structure according the density. •These layers can be characterized by either their chemical or physical composition. Chemical Composition • Chemically the Earth can be divided into 3 layers. 1.Crust – a thin 20 mi. layer composed of low-density rocks made of silica minerals -two types: oceanic and continental crust 2.Mantle -1800 mi. thick made of high density iron and magnesium silicate rock 3.Core- 1800 mi. made of higher density metal ( Ni and Fe) Crust Mantle Most volume Structure of the earth video Core Earth's Internal Structure Mantle is semi-solid rock. Cracks allow material to rise => volcanoes. Core temperature is 6000 K. Outer - Molten. Inner - solid. Metallic How do we obtain information about the structure of the Earth's interior? Like all waves, seismic waves bend or refract. S-waves are unable to travel in liquid. Measurement of seismic wave gives info on density of Earth's interior and which layers are solid/molten. Interior of Earth Activity • http://www.classzone.com/books/earth_sci ence/terc/content/investigations/es0402/es 0402page03.cfm Plate Tectonic Question • What is the driving force behind the motion of the plates? • What types of geological features or processes would you expect near plate boundaries? Plate Tectonics •Volcanic and earthquake activity occurs on plate boundaries What causes the drift? Convection! Plates ride on top of convective cells. One cycle takes millions of years => heat loss is extremely slow. Mantle Convection • • • • Heat escapes core; heats mantle Hot mantle rises Convection Cold mantle sinks Convection flow carries crust sideways Rift Zone Subduction Subduction Three Types of Boundaries 1. Transform -A transform boundary occurs where two plates slide against each other. 2. Divergent –Plates Move Apart 3. Convergent- Plates move together Transform Boundary • A transform boundary occurs where two plates slide against each other. But rather than sliding smoothly, the plates build up tension, then release the tension with a spurt of movement. This movement is felt as an earthquake. Transform plate boundary video Video Divergent Boundary Also known as spreading boundary, a divergent boundary occurs where two plates move apart, allowing magma, or molten rock, to rise from the Earth's interior to fill in the gap. The two plates move away from each other like two conveyor belts moving in opposite directions. For more on divergent boundary, go to The Sea Floor Spread. Rift Valley in Iceland • National Geographic Video • East African Rift Valley Convergent Boundary Also known as subduction boundary, a convergent boundary occurs where one plate slides under another as the two are pushed together. If there is land at the edge of one of these plates, the ocean plate will subduct, or slide under that plate. For more information, go to The Continental Slide. 3 Subtypes of Convergent Boundaries • Ocean-Ocean • Ocean-Continental • Continent-Continent • Animation Animation Collisional Boundary • A collisional boundary occurs where two land masses on plates are pushed together. Trying to occupy the same space, the land masses buckle and fold, creating mountain ranges. For more information, go to The Continental Crush Uplift: Building Mountain Ranges 1. Plate Tectonic activity 1 Present Repaving the Earth • Ocean floor renewed in few 100 million yrs • Continents last longer – Surface features erased by erosion (water and wind) • Evidence of craters erased in time – Oldest rocks on earth about 3.8 billion yrs old • Radioactive dating Volcanoes are usually found in places where: A. the low pressure of the atmosphere pulls the lava/magma to the surface. B. earthquakes occur from oceanic plates colliding with continental plates. C. deep-rooted mountains have cracked Earth’s crust. D. Earth’s rotation has caused weak spots in its crust. The change in position of the continents over time is primarily caused by A. continental plates floating on the ocean. B. mantle material circulating inside Earth. C. Earth’s slow shrinking as it cools. D. global wind patterns and sustained ocean currents. Earth’s Atmosphere • Composition – Mostly nitrogen, N2 (78%) – Some oxygen, O2 (21%) – A little water, H2O – Teeny, tiny bit of carbon dioxide CO2 • Atmospheric Pressure – 14.7 pounds per square inch (1 bar) – Produced by weight of atmosphere above us Atmospheric Protection • Ozone, O3 – Ozone absorbs ultraviolet light – Protects us from sunburn – Ozone destroyed by chlorine • NOTE: Ozone destruction NOT same problem as global warming! Ozone and CFCs 2000 Ozone hole Over Antarctica Change in CFC-11 Last 20+ years First Atmosphere Composition - Probably H2, He • These gases are relatively rare on Earth compared to other places in the universe and were probably lost to space early in Earth's history because – Earth's gravity is not strong enough to hold lighter gases – Earth still did not have a differentiated core (solid inner/liquid outer core) which creates Earth's magnetic field (magnetosphere = Van Allen Belt) which deflects solar winds. • Once the core differentiated the heavier gases could be retained Second Atmosphere • Produced by volcanic out gassing. Gases produced were probably similar to those created by modern volcanoes (H2O, CO2, SO2, CO, S2, Cl2, N2, H2) and NH3 (ammonia) and CH4 (methane) • No free O2 at this time (not found in volcanic gases). • Ocean Formation - As the Earth cooled, H2O produced by out gassing could exist as liquid in the Early Archean, allowing oceans to form. – Evidence - pillow basalts, deep marine sediments in greenstone belts. – Link to Goldilocks principle activity Greenhouse Effect • Sunlight absorbed by ground • Heat emitted using infrared light • Infrared light trapped by gasses in Earth’s atmosphere – Greenhouse gasses: • Carbon Dioxide (CO2) • Water (H2O) • Heat cannot escape • Earth’s surface gets hotter – about 23 degrees C (40 degrees Global Warming Atmospheric Protection • Meteors (“shooting stars”) – Small rocks melt/burn up before hitting surface – Larger objects can get through 1833 Leonid Storm 1998 Leonids 1997 Leonids Seen by MSX Meteor Showers Causes of Meteor Showers Peekskill Meteorite 1992 Perils from Space Effects of Impacts • Where are the craters on Earth? – erased by geologic activity • volcanoes, earthquakes, plate tectonics – erased by erosion • weather, rain, wind • Evidence for impacts? – some craters still exist (Meteor Crater, AZ) – geologic evidence of old craters – occasional impacts today • Effects of impacts? – global climate changes • mass extinctions? (dinosaurs) – water in oceans? Questions • What causes the tides? • When are the highest (and lowest) tides seen? The Moon Mass = 0.012 MEarth Radius = 0.27 REarth Density = 3,300 kg/m3 (Earth 5,500 kg/m3) Gravity = 1/6 that of Earth Tides Tides are due to Moon's gravitational pull being stronger on side of Earth closest to it (Sun also influences tides). Two high and two low tides per day. Why? Tides (cont.) Lunar Structure Moon composed of material very similar to Earth's mantle! The Lunar Surface Dark areas: "maria" ●More recent lava flows. ● Lighter areas at higher elevation: "highlands". ● Many craters (due to meteorite impacts). Only important source of erosion! ● Highlands have 10x the crater density of maria => Highlands are older! ● maria highlands The Moon • Mass 1/80 of Earth’s mass • Gravity 1/6 of Earth’s • Atmosphere – no real atmosphere – few volatiles (elements that evaporate at relatively low temperatures; e.g. water) Apollo 17 Dec 1972 Lunar Surface • Dominant Features – Craters – Maria = “seas” • Dark features • Radioactive dating (moon rocks) – age 3.3 - 4.4 billion yrs – older than rocks on Earth • Earth-Moon system about 4.5 billion years old Geological Features • Surface dominated by impacts • Highlands – heavily cratered, light colored • oldest parts of surface – silicate rocks • Maria – 17% of surface, mostly on near side – dark material, fewer craters • youngest parts of surface – volcanic plains of basalt • lava-filled impact basins Composition & Structure • Average density – 3.3 g/cm3 • lower than Earth’s mantle (5.5 g/ cm3) • Similar to Earth’s crust • Composition – mostly lighter silicates – depleted in iron – similar to Earth’s crust • Mantle – solid – little seismic activity Not geologically active (now) • Core – Small, possibly iron rich – solid and cold Differentiation Moon’s interior molten in Impact Craters • Not erased by erosion (no atmosphere) – preserved record of impacts – indicator of solar system history • Crater Origin – not volcanic – meteor impacts • surface “explosions” • creates circular craters – typical characteristics • bowl-shaped • turned up rims • central peaks Crater Counts • Number of craters indicates age – many craters = old – few craters = young • Ongoing impacts – during last 3.8 billion yrs • Compare: highlands vs. maria – maria formed 3.8 billion years ago • Based on number of craters – highlands have many more craters • highlands older • Conclude: – Period of heavy bombardment • prior to 3.8 billion years ago A moon covered with numerous and very old craters created by meteorite impacts likely a) b) c) d) has no ocean to cover the craters. orbits a large Jupiter sized planet. has a cold, solid interior. has no protective magnetic field. How did the Moon form? We're not quite sure! Three older theories: 1) "Fission": The material that would be the Moon was thrown off the Earth. Problem: Earth not spinning fast enough. 2) "Coformation": The Moon and Earth formed out of the same material at the beginning of the Solar System. Problem: Moon has different density and composition. 3) "Capture": The Moon was a stray body captured into orbit around Earth. Problem: an extremely unlikely event. So now, Impact theory preferred: Early in Solar System, a Mars-sized object hit the forming Earth, ejecting material from the mantle which coalesced to form Moon. Computer simulations suggest this is plausible. Formation of Moon Giant Impact Theory