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Planet Earth

Planet Earth

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Planet Earth

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  1. Planet Earth AST 2010: Chapter 7

  2. Basic Facts • The Earth is a medium-sized planet with a diameter of 13,000 km • It is one of the inner or terrestrial planets • It is composed primarily of heavy elements, such as iron, silicon, and oxygen • It has much less light elements, such as hydrogen and helium, than the outer planets • Earth's orbit around the Sun is nearly circular • The Earth is the only planet in our solar system that is neither too hot nor too cold • It is warm enough to support liquid water on its surface • It is “just right” to sustain life — at least life as we know it AST 2010: Chapter 7

  3. Some properties of the Earth Semi-major axis 1.00 AU Orbital period 1.00 year Mass 5.98 x 1024 kg Diameter 12,756 km Escape velocity 11.2 km/s Rotation period 23 h 56 m 4 s Surface area 5.1 x 108 km2 Atmospheric pressure 1.00 bar Basic Properties AST 2010: Chapter 7

  4. Earth's Interior (1) • The interior of the Earth is difficult to study even with today's amazing technology • Its composition and structure must be determined indirectly from observation made near or at the surface only • Earth’s skin or crust is a layer only a few kilometers deep • The Earth is composed largely of metals and silicate rock • Most of this material is in a solid state, but some of it is hot enough to be molten AST 2010: Chapter 7

  5. Earth's Interior (2) • The structure of the interior of the Earth has been probed in great detail by measuring the transmission of seismic waves through it • Seismic waves are waves that spread through the interior of the Earth from earthquakes or explosions • Seismic waves travel through Earth rather like sound waves through a struck bell • In a bell, the sound frequencies depend on what material the bell is made of and how it was constructed • Similarly, the way seismic vibrations behave depends on the composition and structure of the planet AST 2010: Chapter 7

  6. Earth’s Internal Layers (1) • The Earth is divided into four main layers: crust, mantle, core, and inner core • The crust is the top layer, the part we know best • The crust under the oceans, which covers 55% of the surface, is typically about 6 km thick and is composed of volcanic rocks called basalt • Basalts are produced by cooling volcanic lava and made primarily of silicon, oxygen, iron, aluminum, and magnesium • The continental crust, which covers 45% of the surface, is 20 to 70 km thick and is mainly composed of another class of volcanic rocks called granite • The crust makes up only about 0.3% of the Earth’s total mass AST 2010: Chapter 7

  7. Earth Internal Layers (2) • The mantle is the largest part of the solid Earth, stretching from the base of the crust down to a depth of 2,900 km • The mantle is more or less solid, but may deform and flow slowly due to its high pressures and temperatures • Below the mantle is Earth’s dense metallic core • In addition to iron, it contains nickel and sulfur, all compressed to a very high density • The core is 7,000 km in diameter • Its outer part is liquid • The inner core is 2,400 km in diameter and is probably solid AST 2010: Chapter 7

  8. Rocks (1) • Basalt & granites are two examples of a class of rocks called igneous rocks • They are rocks that have cooled from a molten state • All volcanically produced rocks are igneous • There are two other kinds of rocks • Sedimentary rocks are made of fragments of igneous rocks or the shells of living organisms deposited by wind or water and cemented without melting • Metamorphic rocks are produced when high temperature or pressure alters igneous rocks physically or chemically • These are commonly found on Earth, but not on other planets AST 2010: Chapter 7

  9. Rocks (2) • A fourth group of rocks are called primitive rocks • Their formation dates back to formation of the planet • They have largely escaped chemical modification by heating • Thus, they represent the original material out of which the planetary system was made • No primitive rock is left on the Earth because it was heated early in its history • Primitive rocks may be found in comets, asteroids, or small planetary satellites AST 2010: Chapter 7

  10. Differentiation • The separation of the earth interior into layers is an example of differentiation • Differentiation observed on Earth is evidence that it was once warm enough for the mantle rocks to melt • It allows the heavier metals to sink to the center and form a very dense core AST 2010: Chapter 7

  11. Earth’s Magnetic Field • Much about Earth's interior can be learned from the Earth's magnetic field • Earth behaves in some ways as if a giant bar magnet were inside it • The magnet is roughly aligned with the rotational axis of the planet • Earth’s magnetic field is generated by moving material in Earth’s liquid metallic core • The circulating liquid metal sets up an electric current, which in turn produces a magnetic field AST 2010: Chapter 7

  12. Earth’s Magnetosphere (1) • The Earth's magnetic field extends into surrounding space and traps small quantities of electric charges, such as electrons, that roam about the solar system • Within this region, called the magnetosphere, Earth’s field dominates over the weak interplanetary magnetic field extending outward from the Sun Cross-sectional view of Earth’s magnetosphere as revealed by spacecraft missions AST 2010: Chapter 7

  13. Earth’s Magnetosphere (2) • It was discovered in 1958 by instruments on the first U.S. Earth satellite, Explorer 1 • This satellite recorded the ions (charged particles) trapped in the inner part of the magnetosphere • This region has a fairly complex structure • It is composed of more than one layer or part • The layer discovered in 1958 is called Van Allen Belts after the physicist who built the instrumentation for Explorer 1 and correctly interpreted its measurements AST 2010: Chapter 7

  14. Solar Wind • Charges trapped in the magnetosphere flow outward from the Sun • Phenomenon is called solar wind • Flow of charged particles from the Sun is large • Trapped by the Earth’s magnetosphere • Their flow produces a deformation of magnetic field lines • Elongation far beyond Earth pointing away from the Sun • Magnetosphere typically extends  to 60000 km - 10 earth radii - from the earth - towards the Sun • Away from the sun, sizeable magnetic fields are measurable at a distance as large as the Moon's AST 2010: Chapter 7

  15. Geology • Study of processes that shape the crust • Although a fairly mature science, it is not until very recently that geologists were successful in understanding how landforms are created AST 2010: Chapter 7

  16. Plate Tectonics (1) • A theory that explains how slow motions of the earth mantle move large segments of the crust • Resulting in slow drifting of the continents • Formation of mountains and other large scale geological features • Earth's crust and mantle divided into ~12 major plates fit together like the pieces of a puzzle • Plates observed to move slowly relative to one another • In some places, such as the Atlantic ocean, the plates are moving apart, elsewhere they are forced together

  17. Plate Tectonics (2) • Driving power of the plates motion is provided by slow convection of the mantle • Convection: a process by which heat escapes from the interior of the mantle and produces and upward of warmer materials while cooler materials found above slowly sink down AST 2010: Chapter 7

  18. Plate Motion • The plates’ motion brings them to collide into one another and brings about dramatic changes on the surface of the Earth • Basically four types of interactions are observed between the crustal plates: • They can pull apart • One plate can burrow under another • The can slide alongside each other • They can jam into each other AST 2010: Chapter 7

  19. Rift Zones • Plates pull apart from each other along rift zone • An important rift zone is found in the Mid-Atlantic ridge • Few rift zones are also found on land • E.g. central African rift - these rifts shall eventually break apart the African continent • Most of the rift zones are however found in the oceans AST 2010: Chapter 7

  20. Subduction Zones • The point of contact where two plates come together is called subduction zone • Continental masses cannot be subducted but the thinner oceanic plates can be “easily” pushed down into the upper mantle • Subduction zones often marked by an ocean trench • Subducted plates forced down into regions of high temperature+pressure, eventually melts several hundred kilometers below the surface AST 2010: Chapter 7

  21. Crust Regeneration • Calculations of the rate at which the sea floor is spreading reveal the approximate age of oceanic crust • 60000 km of active rifts identified • Average separation of 4 cm per year • Correspond to an added area of 2 km2 per year • Enough to renew the entire oceanic crust in about 100 million years • Less than 3% the age of the planet • Oceans are a fairly recent feature of the planet AST 2010: Chapter 7

  22. Fault Zones • Crustal plates slide parallel to each another  along much of their lengths • Boundaries so formed lead to the formation of cracks or faults • Along active fault zones, the motion of one plate relative to the other may amount to several centimeters per year - basically the same as the spreading along the rifts AST 2010: Chapter 7

  23. San Andreas Fault • On the boundary between the Pacific and North American plates • Runs from the Gulf of California to the Pacific Ocean northwest of San Francisco • Pacific plate (west side) moves north carrying along Los Angeles, San Diego, and parts of Southern California • In a few million years, LA will be an island off the coast of San Francisco AST 2010: Chapter 7

  24. Beware of Faults! • Plates slide roughly alongside each other • The creeping motions of the plates builds up stresses in the crust • The stresses are eventually released in sudden, violent slippages, a.k.a. earthquakes • Average motion of the plates is constant • The longer the interval between earthquakes • the greater the stress and the larger the energy released when the surface finally moves AST 2010: Chapter 7

  25. More about San Andreas • The San Andreas Fault, near Parkfield, has slipped every 22 years during the past century • moving an average of about 1 m each time • In contrast, the average interval between major Earthquakes in the Los Angeles region is about 140 years • the average motion is about 7 m AST 2010: Chapter 7

  26. Mountain Building • When two continental masses are brought together by the motion of the crustal plates, they are forced against each other  under great pressure • The surface buckles and folds forcing  some of the rock deep below the surface and others to raise to large heights (sometimes many kilometers!) • This is how mountain ranges are formed on Earth • The Alps result from the interaction of the African Plate with the European plate • We will see, however, that other mechanisms lead to formation of mountains on other planets AST 2010: Chapter 7

  27. Volcanoes • Volcanoes mark the location where molten rock, called magma, rises from the upper mantle through the crust • Volcanoes are formed numerously along oceanic rift zones where rising hot material pushes plates away from one another • Volcanic activity is also observed in subduction zones • In both cases, the volcanic activity brings to the surface large amount of materials from the upper mantle AST 2010: Chapter 7

  28. More about Volcanoes • Volcanic activity also found near mantle "hot spots" areas • far from plate boundaries • but where heat rises from the interior of the planet.  • Best known hot spot lies under Hawaii • Supplies in magma three active volcanoes - two of which are on land, and the third in the ocean.  • It is estimated that the Hawaiian hot spot has been active for at least 100 million years. • Shaping the Pacific plate, the hot spot has generated a 3500-km long chain of volcanic islands AST 2010: Chapter 7

  29. Earth’s Atmosphere • Provides the air we breathe • The air of the atmosphere exerts a constant pressure (on the ground) • The atmosphere pressure at sea level is used to define the pressure unit called bar • Humans have existed mostly at sea level and are thus accustomed to such a pressure • The total mass of the atmosphere is ~ 5x1018 kg • Although this sounds like a lot, it constitutes only one millionth of the total mass of the Earth • Yet it composition is quite vital to us humans and other living creatures on the surface of this Earth AST 2010: Chapter 7

  30. Structure of Earth’s Atmosphere AST 2010: Chapter 7

  31. Troposphere • Altitude range:   • Sea level - 9 miles • Densest area of the atmosphere • Most weather occurs and almost all aircraft fly in this region • Temperatures drop as elevation increases • Warm air, heated on the surface, rises and is replaced by descending currents of cooler air • The circulation generates clouds and other manifestations of weather • As one rises through the troposphere, one finds the temperature drops rapidly with increasing elevation • The temperature is near 50oC below freezing at the top of the troposphere AST 2010: Chapter 7

  32. Stratosphere • Altitude range:  • 9 - 31 miles • Dry and less dense • The air in this layer moves horizontally and does not move up and down within it • Temperatures here increase with elevation • Near the top of the stratosphere, one finds a layer of ozone (O3) • Ozone is a good absorber of ultraviolet light • It thus protects the surface from the sun's ultraviolet radiation and makes it possible for life to exist on the planet AST 2010: Chapter 7

  33. Mesosphere • Altitude range:   • 31 - 62 miles (50 - 100 km) • Temperatures fall as low as -93 degrees Celsius in this region • Chemicals are in an excited state, as they absorb energy from the sun AST 2010: Chapter 7

  34. Ionosphere • Altitude range:  62 - 124 miles. • This region is characterized by the presence of plasma. • Its boundaries vary according to solar activity. AST 2010: Chapter 7

  35. Thermosphere • Altitude range:  • 124 - 310 miles (200 - 500 km) • Temperatures increase with altitude due to the sun's energy, reaching as high as 1,727 degrees Celsius • Auroras, caused by the sun's particles striking the earth's atmosphere, occur at this level AST 2010: Chapter 7

  36. Exosphere • Altitude range:   • 310 - 434 miles (500 - 700 km) • The region begins at the top to the thermosphere and continues until it merges with interplanetary gases, or space • The prime components, hydrogen and helium, are present at extremely low densities AST 2010: Chapter 7

  37. About Ozone • Increasing evidence atmospheric ozone is being destroyed • Agents of destruction are industrial compounds called CFCs (chlorofluorocarbons) • Each year, a large ozone forms above the Antarctic continent • By now, the ozone loss has progressed into the temperate zone • The production of CFCs has been banned by international agreement • These chemicals are however destroyed slowly and are still  often released in the atmosphere • One can thus expect further reduction of the ozone layer in the next century AST 2010: Chapter 7

  38. Weather and Climate • All planets with atmospheres have weather • Weather is simply the name given to the circulation of air through the atmosphere • Climate is a term used to describe the evolution of weather through long periods of time: decades or centuries • Changes in climate are typically difficult to detect over short periods of time. However, their accumulating effects can be sizeable and sometimes quite dramatic AST 2010: Chapter 7

  39. About the Weather • The energy that power this motion is derived primarily from the sunlight that heats the Earth's surface  • As the planet rotates, and orbits the Sun, the slower seasonal changes cause variations in the amount of heat of sunlight striking the different parts of the planet • The heat then proceeds to redistribute itself from warmer to cooler areas giving rise to various weather patterns AST 2010: Chapter 7

  40. Hurricane Elena • In the Gulf of Mexico on Sept 1, 1985 • Wind speeds were in excess of 110 miles per hour • Eventually made landfall near Gulfport, Mississippi

  41. Origin of Life • Early Earth atmosphere is believed to contain abundant carbon dioxide but no oxygen gas  • In the absence of oxygen, many complex chemical reactions are possible that lead to the production of amino acids, proteins, and many other chemical building blocks of life • Genetic studies of the many million species that now live on Earth suggest that they are related to one another • This led to the idea that all terrestrial life descends from a single common microbial ancestor AST 2010: Chapter 7

  42. Evolution of Life • Blue-algae consume carbon dioxide and produce oxygen as a waste product  • They use the energy from sunlight, in a process called photosynthesis to develop and grow • They are thought to have proliferated and eventually evolved into what we know today as plants AST 2010: Chapter 7

  43. Appearance of Oxygen in the Atmosphere • Studies suggest that oxygen started to accumulate in the atmosphere some 2 billion years ago • Led to formation of the Earth's ozone layer • Layer produced a shield under which more complex life could evolve and develop • Life is believed to arise from the vast oceans and venture into solid grounds • In this scenario, as animals evolved in environment increasingly rich in oxygen, they were able to develop techniques for breathing oxygen directly from the atmosphere (primitive lungs appeared) AST 2010: Chapter 7

  44. Role of Carbon Dioxide (CO2) • Sunlight striking the surface is absorbed • heats the surface layers • re-emitted as infrared/heat radiation • Atmospheric CO2 transparent to visible light • does not impede sunlight to reach the surface • CO2 opaque to infrared energy • behaves as a blanket, trapping the heat in the atmosphere and impeding the flow back of energy back to space • This is called the greenhouse effect AST 2010: Chapter 7

  45. Greenhouse Effect • On average, as much heat reaches Earth’s surface from the atmospheric greenhouse effect as from direct sunlight • This explains why • night-time temperatures are only slightly lower than daytime temperatures • life is actually possible on this planet AST 2010: Chapter 7

  46. Global Warming (1) • Estimated that greenhouse effect elevates the surface temperature by about 23°C on the average • Without this effect, the average temperature would be below freezing • Earth would be covered with ice • Global ice age • An increase of atmospheric CO2  implies that the atmospheric temperature would rise to much higher average values • and then endanger life on our planet AST 2010: Chapter 7

  47. Global Warming (2) • Modern society increasingly depends on energy • Energy production is accomplished by burning fossil fuels which when burned release carbon dioxide • The problem is exacerbated by ongoing destruction of tropical forests in Asia, Africa, and South America • Atmospheric CO2 has increased by about 25% in the last 100 years • It is rising at a frightening pace of 0.5% per year • CO2 level will soon reach twice the value it had before the industrial revolution • Consequences are complex, not completely known • Sophisticated and elaborate computer models are used • Conclusions are not firm at this point AST 2010: Chapter 7

  48. Craters Why is there no clear evidence of craters on Earth?

  49. Suggested Answer • Geological activity!

  50. Earth Craters • Evidence of fairly recent impacts can be found on our planet's surface • The best studied case took place on June 30, 1908, near the Tunguska River in Siberia, Russia  • 8 km above the ground • Flattened more than a thousand square kilometers of forest • Blast wave spread around the world and was recorded by instruments designed to record changed in atmospheric pressure AST 2010: Chapter 7