Our Home, The Earth Size And Shape Of The Earth

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10/9/2008Tuesday, 10/7/08 Project Part 1 (due 10/16) Return tests Chapter 5Chapter 5The EarthCopyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Our Home, The Earth Earth’s beauty is revealed from space through blue seas,green jungles, red deserts, and white clouds. From our detailed knowledge of Earth, astronomershope to understand what properties shape other worlds Earth is a dynamic planet with its surface andatmosphere having changed over its lifetime. Slow and violent motions of the Earth arise from heatgenerated within the planet Volcanic gases accumulate over billions of yearscreating an atmosphere conducive to life, which in turntogether with water affects the air’s compositionSize and Shape of the Earth In simple terms, the Earthis a huge, rocky spherespinning in space andmoving around the Sun at aspeed of about 100 milesevery few seconds Earth also has a blanket ofair and a magnetic fieldthat protects the surfacefrom the hazards ofinterplanetary spaceComposition of the EarthSize and Shape of the Earth The Earth is largeenough for gravity tohave shaped it into asphere More precisely, Earth’sspin makes its equatorbulge into a shapereferred to as an oblatespheroid – a result ofinertia The most common elements ofthe Earth’s surface rocks are:––––––oxygen (45.5% by mass),silicon (27.2%),aluminum (8.3%),iron (6.2%),calcium (4.66%), andmagnesium (2.76%) Silicon and oxygen usuallyoccur together as silicates Ordinary sand is the silicatemineral quartz and is nearlypure silicon dioxide1

10/9/2008Density of the Earth– Density is a measure of how much material (mass) ispacked into a given volume– Typical unit of density is grams per cubic centimeter– Water has a density of 1 g/cm3, ordinary surface rocks are3 g/cm3, while iron is 8 g/cm3– For a spherical object of mass M and radius R, its averagedensity is given byM43π R3– For Earth, this density is found to be 5.5 g/cm3– Consequently, the Earth’s interior (core) probably is iron(which is abundant in nature and high in density)A Sonogram of the Earth!The Earth’s Interior Earthquakes generate seismic waves thatmove through the Earth with speedsdepending on the properties of the materialthrough which they travel These speeds are determined by timing thearrival of the waves at remote points on theEarth’s surface A seismic “picture” is then generated of theEarth’s interior along the path of the waveProbing the Interior of the Earth Seismic waves are of twotypes: S and P This is the only way we have to probe the Earth’sinterior!Interior Structure Observations show P waves but no S waves atdetecting stations on the opposite side of the Earthfrom the origin of an Earthquake the Earth has a liquid core!– P waves compress materialand travel easily throughliquid or solid– S waves move materialperpendicular to the wavedirection of travel and onlypropagate through solidsInterior Structure of the Earth A solid, low-density and thincrust made mainly of silicates A hot, thick, not-quite-liquidmantle with silicates A liquid, outer core with amixture of iron, nickel andperhaps sulfur A solid, inner core of ironand nickel2

10/9/2008Layers of the EarthDifferentiation The Earth is layered in such a fashion that the densestmaterials are at the center and the least dense at thesurface – this is referred to as differentiation– Differentiation will occur in a mixture of heavy and lightmaterials if these materials are liquid for a long enough time ina gravitational field– Consequently, the Earth must have been almost entirely liquidin the past The Earth’s inner core is solid because it is under suchhigh pressure (from overlying materials) that thetemperature there is not high enough to liquefy it – thisis not the case for the outer liquid coreTemperature Inside the EarthTemperature Inside the Earth Heating the Earth’sCore– The estimatedtemperature of theEarth’s core is 6500 K– This high temperature isprobably due to at leastthe following two causes: Heat generation from theimpact of small bodiesthat eventually formed theEarth by their mutualgravitation The radioactive decayof radioactive elementsthat occur naturally inthe mix of materials thatmade up the EarthAge of the Earth In either case, the thermal energy generated istrapped inside the Earth’s interior due to the longtime it takes to move to the surface and escapeAge of the Earth Radioactive potassium has a half-life of 1.28billion years and decays into argon, which is a gasthat is trapped in the rock unless it melts Radioactive decay used to determine the Earth’sage– Assume rock has no argon when originally formed– Measuring the ratio of argon atoms to potassium atomsgives the age of the rock– This method gives a minimum age of the Earth as 4billion years– Other considerations put the age at 4.5 billion years– Radioactive atoms decay into daughter atoms– The more daughter atoms there are relative to theoriginal radioactive atoms, the older the rock is3

10/9/2008ConvectionMotion in the Earth’s Interior– Heat generated by radioactive decay in the Earthcreates movement of rock– This movement of material is called convection– Convection occurs because hotter material will beless dense than its cooler surroundings andconsequently will rise while cooler material sinks Convection in the Earth’s interior– The crust and mantle are solid rock, although whenheated, rock may develop convective motions– These convective motions are slow, but are the causeof: earthquakes, volcanoes, the Earth’s magnetic field,and perhaps the atmosphere itselfSubductionPlate Tectonics Subduction Rifting– Hot, molten material rises from deep in the Earth’s interiorin great, slow plumes that work their way to the surface– Near the surface, these plumes spread and drag the surfacelayers from below– The crust stretches, spreads, and breaks the surface in aphenomenon called riftingPlate Tectonics– Where coolmaterial sinks, itmay drag crustalpieces togetherbuckling themupward intomountains– If one piece ofcrust slips underthe other, theprocess is calledsubductionPlate Tectonics The shifting of large blocks of the Earth’ssurface is called plate tectonics Rifting and subduction are the dominant forcesthat sculpt the landscape – they may also triggerearthquakes and volcanoes– Early researchers noted that South America andAfrica appeared to fit together and that the twocontinents shared similar fossils– It was later proposed (1912) that all of thecontinents were once a single supercontinentcalled Pangea– The Earth’s surface is continually building upand breaking down over time scales of millionsof years4

10/9/2008Continental DriftContinental PlatesThe Earth’s AtmosphereComposition of the Earth’sAtmosphere Veil of gases aroundEarth constitutes itsatmosphere Relative to otherplanetary atmospheres,the Earth’s atmosphereis unique However, studying theEarth’s atmosphere cantell us aboutatmospheres in generalOrigin of the Earth’s Atmosphere Several theories toexplain origin ofEarth’s atmosphere– Release of gas(originally trappedwhen the Earthformed) byvolcanism orasteroid impacts– From materialsbrought to Earth bycomet impacts The Earth’s atmosphere is primarily nitrogen(78.08% by number) and oxygen (20.95% bynumber) The remaining gases in the atmosphere (about 1%)include: carbon dioxide, ozone, water, and argon,the first three of which are important for life This composition is unique relative to the carbondioxide atmospheres of Mars and Venus and thehydrogen atmospheres of the outer large planetsThe Early Atmosphere Early atmosphere different than today– Contained much more methane (CH4) andammonia (NH3)– Solar UV was intense enough to break out Hfrom CH4, NH3 , and H2O leaving carbon,nitrogen, and oxygen behind while the Hescaped into space– Ancient plants further increased the levels ofatmospheric oxygen through photosynthesis5

10/9/2008The Ozone Layer Oxygen in the atmosphere provides a shield against solarUV radiation O2 provides some shielding, but O3, or ozone, providesmost of it Most ozone is located in the ozone layer at an altitude of25 km Shielding is provided by the absorption of UV photonsby oxygen molecules (both O2 and O3) and theirresultant dissociation Single O atoms combine with O and O2 to replenish thelost O2 and O3 It is doubtful that life could exist on the Earth’s surfacewithout the ozone layerStructure of the Earth’s Atmosphere Atmosphere extends to hundreds of kilometersbecoming very tenuous at high altitudes The atmosphere becomes less dense withincreasing altitude Half the mass of the atmosphere is within the first4 kilometers The atmosphere eventually merges with thevacuum of interplanetary spaceThe Earth’s Magnetic Field Magnetic fields alsohave polarity – adirection from a northmagnetic pole to asouth magnetic pole Magnetic fields aregenerated either bylarge-scale currents orcurrents on an atomicscaleThe Greenhouse Effectinfrared wavelengths Visible light reaches theEarth’s surface and is This reduces the rate of heatconverted to heatloss and makes the surface As a result, the surface radiateshotter than it would beinfrared energy, which isotherwisetrapped by the atmosphere atThe Earth’s Magnetic Field Magnetic forces arecommunicated by amagnetic field – directphysical contact is notnecessary to transmitmagnetic forces Magnetic fields aredepicted in diagrams bymagnetic lines of force– Each line represents thedirection a compass wouldpoint– Density of lines indicatestrength of fieldOrigin of the Earth’s Magnetic Field The magnetic field of the Earth is generated bycurrents flowing in its molten iron core The currents are believed to be caused byrotational motion and convection (magneticdynamo) The Earth’s geographic poles and magnetic polesdo not coincide Both the position and strength of the poles changeslightly from year to year, even reversing theirpolarity every 10,000 years or so6

10/9/2008Magnetic Effects in the UpperAtmosphere Earth’s magnetic fieldscreens the planet fromcharged particles emittedfrom the Sun As the charged solarparticles stream pastEarth, they generateelectrical currents in theupper atmosphere These currents collidewith and excite molecules As the molecules deexcite, light photons aregiven off resulting inaurora The Earth’s magnetic fielddeflects the charged particlesinto spiral trajectories andslows them downThe Magnetosphere Region of the Earth’senvironment where theEarth’s magnetic fieldaffects particle motion iscalled the magnetosphereAurora Within the magnetospherecharged particles are trappedin two doughnut shaped ringsthat encircle the Earth and arecalled the Van Allen radiationbeltsAir and Ocean CirculationMotions of the Earth Rotational and orbitalmotions define the dayand year and cause theseasons But our planet’s motionshave other effectsThe Coriolis Effect Responsible for: In the absence of anyforce an object will movein a curved path over arotating object This apparent curvedmotion is referred to asthe Coriolis effect– The spiral patternof large storms aswell as theirdirection ofrotation– The trade windsthat move fromeast to west in twobands, one northand one south ofthe equator The direction of the jet streams,narrow bands of rapid, highaltitude winds The deflection of ocean currentscreating flows such as the GulfStream7

10/9/2008The Coriolis Effect Also – The atmospheric band structure of the rapidlyrotating Jupiter, Saturn, and NeptunePrecession As the Earth moves around the Sun over long periodsof time, the direction in which its rotation axis pointschanges slowly This changing in direction of the spin axis is calledprecession Precession is caused by the Earth not being a perfectsphere – its equatorial bulge allows the Sun and Moonto exert unbalanced gravitational forces that twist theEarth’s spin axis The Earth’s spin axis precesses around once every26,000 years Currently the spin axis points at Polaris – in A.D.14,000 it will point nearly at the star Vega Precession may cause climate changesPrecession8

Our Home, The Earth Earth’s beauty is revealed from space through blue seas, green jungles, red deserts, and white clouds. From our detailed knowledge of Earth, astronomers hope to understand what properties shape other worlds Earth is a dynamic planet with its

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