2If you encountered another planet, what would you want to learn about it? Basic physical parametersHow old is the planet? How was it formed?How is it structured internally? Externally?
3Learning moreDoes it have a magnetic field? How strong is it? How is it structured? What does it tell us about the planet’s interior?How is its atmosphere structured? Of what is it made? What are its weather patterns? How does the atmosphere help control the planet’s energy budget?
4Learning more (2)Has the atmosphere always been the same as it is now? How has the atmosphere interacted with the surface?What kinds of physical processes have produced the planet’s landforms?Is there life on the planet? How does it interact with the various ecosystems?
5Physical Properties Diameter: 12,756 km at the equator Why specify “at theequator”?Because the polar diameter is only 12,697 km
6Physical Properties The bulging at the equator and flattening at The poles is called OBLATENESS.It’s due to the rotation of the planet.Earth is the most spherical (least oblate)of all the planets.
7Physical Properties Volume: 1.1 trillion cubic kilometers (km3) Mass: 5.97 x 1024 kilogramsMt. Everest is about than 2400 km3This is equal to about 81 moons!
8Physical Properties Density: 5500 kg/m3 or 5.5 g/cm3 We compare the density of materials like rocks & metals to the density standard: WATER!Water’s density is 1.0 g/cm3
9Physical PropertiesMost rocks have densities between 2.5 and 4.0 g/cm3Most metals have densities greater than 6.0 g/cm3Iron is 7.8 g/cm3Nickel is 8.9 g/cm3
10Physical Properties The density of an object reflects its composition. What does Earth’s density tell you about its composition?Earth must be made of acombination of rocksand metals.
11How old is the Earth? About 4.6 billion years. The oldest rocks are found on the north slope of Canada, the Canadian Shield. These rocks are 4.0 billion years old. They were dated from the radioactive decay of uranium into lead.The other 0.6 billion years is an estimate of how long it took the earth to form.
12How was the earth formed? AccretionDifferentiationInterior StructureEvidence, or “How sure are we?”
15A star ignites in the center and a temperature gradient forms.
16Solid chunks, called planetissimals, begin to condense close to the star.
17AccretionPlanetissimals (dust-sized to small moon-sized solid bodies) begin to form near the sun.Gravity attracts planetissimals into larger and larger bodies. Planets begin to grow.Most of the “stuff” of planetissimals is rock (silicates: Na, Ca, Mg, O, Si) and heavy metals (Fe, Ni).
18Differentiation As earth grows in size, its gravity grows too It begins to pull in other planetissimals, which impact on its surface.What happens when you repeatedly hit a piece of metal with a hammer?
20DifferentiationAll the heat generated by planetissimals impacting on the surface, plus the heat generated by radioactive decay in the young earth’s interior causes the entire earth to melt.
21DifferentiationWhen the entire earth is molten, the heavy elements (iron, nickel) sink to the interior.The lighter materials (granite-type rocks) rise to the surface.The medium density rocks (basalt-type) ends up in the middle.Layers form: core, mantle, crust.
22Molten Earth, heatedby impacts & radioactivedecay.Differentiated (layered)Earth after cooling.
23Interior StructureInner Core (kept solid by the immense pressure of all the material on top of it.)Outer Core (less pressure allows it to be a liquid.)MantleAsthenosphereLithosphereCrust
24Solid Inner Core2400 km diameterIron & NickelLiquid Outer Core2270 km thickIron & NickelCrustkm thickGranitic rocks:FeldsparsMantle2900 km thickBasaltic rocks:Olivine, PyroxeneInterior section of mantle is a thick fluid called theasthenosphere. The outer mantle + the crust are rigidand are collectively called the lithosphere.
25More about the layersThe difference between the mantle and the crust is based on chemical composition.The difference between the asthenosphere and the lithosphere is based on viscosity (the ability to flow under pressure.)
26How do we know?What evidence do we really have that the interior of the earth is the way we think it is?Deep mines are hot!Heat and molten material escape from volcanos and geysers.Earthquake waves
27Earthquake WavesWhen an earthquake occurs it produces 2 types of waves:P or primary waves. These are waves of compression of the rock. They travel fastest.S for secondary or shear waves. The rock moves up & down or sideways.
29Earthquake wavesP waves are capable of traveling through both liquids and solids, so they travel through mantle and cores.S waves can’t travel through liquids, so they stop when they hit the outer core.Look closely at the next diagram.
31How big’s that core?With the right placement of seismometers around the earth’s surface, we get a good estimate of the size of the outer core.The size of the inner core is calculated from theory.
32Earth’s Magnetic Field Why does Earth have a global or world-wide magnetic field, while other similar planets either have no magnetic fields or very different kinds of fields?Why should we care about Earth’s magnetic field? What does it do for us?
33Earth’s Magnetic Field Magnetic fields are made wherever there is an electric current, that is the movement of electrons.In a regular bar magnet, the magnetic field comes from the electrons orbiting around the nuclei of the iron atoms. All the electrons orbit in the same direction.
35Earth’s Magnetic Field In the earth, electrical currents run through the molten iron core. Friction within the molten, flowing iron knocks electrons off iron atoms.The molten iron flows at about 0.8 inches per second, but the electrical currents can flow faster.
36Earth’s Magnetic Field The electrical currents within the earth cause earth to act like a gigantic electromagnetic generator.This is called the Dynamo Theory of magnetic field generation.
38A little more info…The earth’s magnetic field isn’t strong enough for us to feel, but many animals can sense it and even use it to navigate. It’s only about 0.4 Gauss, much weaker than a small magnet you can hold in your hand.On average, the North & South poles “flip” every 390,000 years. There have been 9 flips in the past 3.5 million years.
39The poles “flip” ?No one knows how long the process takes, maybe a few years, maybe a few minutes.Every so often, what was the North magnetic pole suddenly becomes the South magnetic pole.Lava that cools quickly on the sea floor records these flips and lets us date them.
40Stripes of different magnetic polarity form in the rocks as the lava from the mid-ocean ridge cools.
41Strange Things Going On Earth’s magnetic field is NOT aligned with its rotational axis.The magnetic field is tilted 12o to the rotational axis, and doesn’t even pass directly through the center of the earth.Does this mean that the electrical currents don’t flow evenly and uniformly inside the earth? Is there turbulence inside?
42Magnetic Fields in Space Earth’s magnetic field extends 7-10 times the earth’s diameter outward from the earth.The earth’s magnetic field would be spherical, but the solar wind compresses it on the side closest to the sun, and stretches it out into a long tail on the side opposite the sun.Overall, it’s kind of tadpole shaped.
43Magnetic Field Structure The whole magnetic field is called the magnetosphere.On the side closest to the sun, where the solar wind compresses the field, there is a “bow shock”, just like a boat pushes some water out of the way at its bow as it sails forward.
46Why do we care?Earth’s magnetic field isn’t just “there” with no purpose. Without it, you and I and every living thing on this planet would be dead (including the cockroaches!)The magnetic field channels away the solar wind.It also prevents erosion of the atmosphere.
47Solar Wind So what is the solar wind anyway? It’s radiation: extremely hot, high-energy, fast-moving charged particles (ions) given off by the sun. Most of these particles are protons.If you were exposed to it for just a few hours without protection, your skin and every organ in your body would be burned, and you’d have a fatal dose of radiation poisoning.
48How does the magnetic field protect us? The magnetic field captures the solar wind and channels much of it into a donut of radiation around the earth.This donut (actually 2 layers – one inside the other) is called the Van Allen Radiation Belt (V.A.R.B.)
50Van Allen Radiation Belts Satellites must orbit either below or above the V.A.R.B., or their electronics would be fried.The problem is even worse when we send manned missions into space. The ship must pass through the radiation belts as quickly as possible or the crew is toast !
51Where does the radiation go? Since the sun continually supplies new solar wind, where does the solar wind go that the earth has already captured?The magnetic field channels some of it into our atmosphere at the north & south poles. Here it ionizes oxygen and nitrogen atoms, causing the beautiful northern and southern lights.
52Northern Lights?The northern lights are properly called the “aurora borealis.” They’re nothing more than a very large fluorescent light display (without the fluorescent tube!)The northern lights are sometimes seen as far south as Florida, especially when the sun is very active.
54This aurora was photographed in Tennessee in October, 2002.
55Where does the rest of the radiation go? Much of it flows through the magnetic field, around the earth, and “drips” off the tail of the magnetic field. The tail is called the “magnetotail.”Without our “Teflon-coating” of magnetic field, the earth would have been cooked many billions of years ago.
56Switch Gears…!Let’s switch topics, from the magnetic field to the atmosphere.Earth’s atmosphere is unlike any other planet’s in chemical composition, but it is like every other planet’s in the processes that go on within it.
57Chemical Composition Our current atmosphere is: 78% nitrogen (N) 21% oxygen (O)1% argon (Ar), helium (He), carbon dioxide (CO2), water vapor (H2O), and about 20 other rare gases.
58Chemical CompositionThe % of water in the atmosphere can vary from near 0% over deserts to 0.5% in the tropics.The % of carbon dioxide has doubled in the past 300 years, from 150 parts per million (ppm) to about 340 ppm today.This means that our atmosphere is evolving! Could it have evolved in the past?
59Atmospheric PressurePressure is the downward push of the column of air above you.At earth’s surface, the air (barometric) pressure is 14.7 pounds / square inch.Other units are inches of mercury in a barometer, and 1013 millibars.
61The Atmosphere’s Structure Earth’s atmosphere has both vertical and horizontal structure.Vertically, the atmosphere is divided into 4 layers.Horizontally, the atmosphere is divided into 6 circulation cells, 3 in the northern hemisphere & 3 in the southern.
624 LayersTroposphere, the weather layer. From the earth’s surface to 10 km up. It gets colder the higher up you go within this layer.Stratosphere, the circulation layer. The jet stream and ozone layer that protects us from UV light are in this layer. Extends from 10 to 40 km up. Temperature rises as you go up within this layer.
634 Layers continuedMesosphere, a middle layer, up to 75 km. Here the air pressure is only 1/10,000th of the pressure at the earth’s surface. The temperature again falls as you go up within this layer. Thermosphere, the hot layer, up to 120 km. This is the outer edge of earth’s atmosphere. Here, the temperature equalizes with the temperature of the hot solar wind. This is where auroras form.
65What sets off one layer from the next is the way the temperature varies within it.
66How does the atmosphere affect the surface? …in 4 ways:35% of the sunlight that hits the atmosphere is reflected back into space by clouds.The % of visible light reflected by a planet is called its albedo. Earth’s albedo is 0.35.Clouds, ice, deserts all increase albedo.A high albedo generally means that the planet has a cold surface (lots of ice.)
68How does the atmosphere affect the surface? 33% of the sunlight is absorbed by gases and dust, but then is re-radiated as infrared (heat). Much of this infrared light goes back into space and is lost.The absorption of light is called attentuation or extinction (just like the dinosaurs!)Greenhouse gases (water vapor, carbon dioxide, methane or CH4) help to trap the heat and prevent it from going back into space.Without the greenhouse gases, earth’s surface would be about -18oC.
70How does the atmosphere affect the surface? Dust in the atmosphere also causes reddening, a process where the blue light is scattered, but red light is allowed to pass straight through.We see the scattered blue light as the blue of our sky. We also see the red of sunset as it passes straight through the atmosphere.Reddening also happens in space when starlight passes through dusty nebulas.By the way, only 32% of the sunlight makes it to the surface.
72Atmospheric Circulation Earth’s atmosphere has 3 circulation cells in each hemisphere (called Hadley cells on other planets).The northern-most is the polar cell, from 90o to 60o north latitudeWe live in the temperate cell, from 60o to 30o north latitude.The southern-most is the tropical cell, from 30o north to the equator.
73Atmospheric Circulation How does the air circulate?Warm air rises at the equator, cools off at high altitude, then falls back to the surface at 30o north latitude. It eventually circulates back to equator.The polar cell operates by cold air falling at the north pole, flowing away from the pole, warming and rising at about 60o north latitude.The temperate circulation cell is just caught in the middle between the tropical & polar cells.
75Atmospheric Circulation If the earth didn’t rotate, the air in the circulation cells would simply move north and south.However, the earth’s rotation causes the Coriolis Effect. This causes moving wind to turn or deflect to the right in the northern hemisphere. The circulation cells turn into tubes, allowing the winds to move all the way around the globe.
76Click here for an animation of the Coriolis Effect
77Other PlanetsHow are other planet’s atmospheres different? Other planets rotate faster or slower, are hotter or cooler.How would rotating faster affect the atmosphere? It might turn circulation cells into bands, where the winds simply move from west to east or east to west.Hotter temperatures could be expected to make the wind speeds higher.
78East – west bands of winds result from a very rapid rate of rotation.
79Where did the atmosphere come from? Some water and gases were contributed by comets, meteors, and other planetissimals impacting on earth’s surface.
81Where did the atmosphere come from? But most of the atmosphere came from volcanic outgassing. Volcanoes release over 100 billion kilograms of water vapor and gases into the atmosphere every year.Over 4.5 billion years, 5 x 1020 kilograms of water and gases have been released. This is enough for the atmosphere and the oceans!
83Did the atmosphere change? Earth has had 3 atmospheres.The first atmosphere was hydrogen (H) and helium (He) from the original solar nebula.Since these gases are light and the earth was hot way back then, most of the H and He was eventually lost to space.
842nd atmosphereAfter the H and He escaped into space, only the gases that were too heavy to be lost were left behind: nitrogen (N2) and carbon dioxide (CO2).We still have the N2 today, but where did the CO2 go?Most of it dissolved in the oceans, combined with calcium (Ca) and was turned into limestone. Some was absorbed by photosynthetic bacteria.
85The layer of limestone below formed by the chemical equation: CO2 + CaO CaCO3
863rd atmosphereAs the photosynthetic bacteria began to use the CO2, it began to produce oxygen (O2).Some of the bacteria evolved into photosynthetic plants which increased the rate of O2 production.Our present atmosphere is N2 and O2 in about a 4:1 ratio.
87First came the photosynthetic bacteria, then the green plants First came the photosynthetic bacteria, then the green plants. Both addedoxygen (O2) to our atmosphere.
88Earth’s GeologyEarth’s surface changes by processes that are similar to some of the other planets:Wind blows particles that cause erosion and build up structures like sand dunes and dust fields.Flowing water cuts canyons and river beds, and transports material. Flood can erode huge channels.Flowing ice or lava can act much like flowing water.Subsurface movements cause hills, mountains, volcanoes, huge cracks and rift valleys.
89Earth’s GeologyEarth also has larger-scale processes that other planets don’t have: plate tectonics.Earth’s crust is divided up into about 20 large pieces or plates of rigid crust that float on top of flowing mantle.Where these plates come together or pull apart is where we get mid-ocean ridges, chains of volcanoes and mountains, and earthquakes.
92Earth’s GeologyThe movement of plates is driven by hot currents of magma welling up from deep within the mantle of the earth.Look at the following diagrams closely. Look especially for places where the plates are separating or crushing together, because we’ll be looking for similar features on other planets too!
93A plume of hot magma is welling up from the deep mantle. Notice how it pushes up the crust. A mid-ocean ridge may form here.