1. Earth: The Standard of Comparative Planetology Chapter 7

2. How the Earth was Formed?

3. The planets formed from the solar nebula 4.6 billion years ago, and now you are ready to visit the planets and get to know them as individuals. That will help you confirm your understanding of the origin of the planets and will reveal new principles of planetary evolution. You begin your study with Earth, and that means you must see your home world in a new way, not as a location for exciting vacations, international trade, and strategic political agreements, but as a planet. In this chapter, you will answer four essential questions: How does Earth fit among the terrestrial planets? How was Earth changed since it formed? What is the inside of Earth like? How was Earth’s atmosphere formed and evolved? Guidepost (1)

4. Thinking about Earth’s interior will help you answer an important question about science: How can scientists know about things they cannot see? Like a mountain climber establishing a base camp before attempting the summit, you will, in this chapter, establish your basis of comparison on Earth. In the following chapters you will visit worlds that are un-Earthly but, in many ways, familiar. Guidepost (2)

5. I.A Travel Guide to the Terrestrial Planets A. Five Worlds B. Core, Mantle, and Crust C. Atmospheres II. The Early History of Earth A. Four Stages of Planetary Development B. Earth as a Planet III. The Solid Earth A. Earth's Interior B. The Magnetic Field C. Earth's Active Crust IV.Earth’s Atmosphere A. Origin of the Atmosphere B. Human Effects on Earth’s Atmosphere Outline

6. I. A Travel Guide to the Terrestrial Planets

7. A Travel Guide to the Terrestrial Planets Similar internal structures: Liquid heavy- metal core Liquid mantle of lighter rocks Solid Crust Earth, Venus, and Mars have atmospheres, dominated by similar physical processes (e.g., greenhouse effect).

8. Chapter Summary (1) How does Earth fit among the terrestrial planets? Earth is the standard of comparative planetology in the study of the terrestrial planets because we know it best and because it contains all of the phenomena found on the other terrestrial planets. The terrestrial planets include Earth, the moon, Mercury, Venus, and Mars. Earth’s moon is included because it is a complex world and makes a striking comparison with Earth. The terrestrial worlds differ mainly in size, but they all have low-density crusts, mantles of dense rock, and metallic cores. Comparative planetology warns you to expect that cratered surfaces are old, that heat flowing out of a planet drives geological activity, and that the nature of a planet’s atmosphere depends on the size of the planet and its temperature.

9. Scientific Argument….. Why do you expect the inner planets to be high-density worlds? What made of those craters?

10. II. The Early History of Earth

11. The Early History of Earth Earth formed 4.6 billion years ago from the inner solar nebula. Four main stages of evolution: Most traces of bombardment (impact craters) now destroyed by later geological activity Two sources of heat in Earth’s interior: Potential energy of infalling material Decay of radioactive material

12. Chapter Summary (2) How has Earth changed since it formed? Earth formed rapidly from the solar nebula and was hot enough to be molten. Earth has passed through four stages as it has evolved: (1) differentiation; (2) cratering; (3) flooding by lava and water; and (4) slow surface evolution. Earth is peculiar in that it has large amounts of liquid water on its surface, and that water drives strong erosion that alters the surface geology. Earth is also peculiar in that it is the only known home for life.

13. Scientific Argument….. Why should you think Earth went through an early stage of cratering?

14. III. The Solid Earth

15. Earth’s Interior Direct exploration of Earth’s interior (e.g. drilling) is impossible. Earth’s interior can be explored through seismology: earthquakes produce seismic waves. Two types of seismic waves: Pressure waves: Particles vibrate back and forth Shear waves: Particles vibrate up and down

16. Seismology (Animation – From Book) (Animation – From Book) Seismic waves do not travel through Earth in straight lines or at constant speed. They are bent by or bounce off transitions between different materials or different densities or temperatures. Such information can be analyzed to infer the structure of Earth’s interior.

18. Earth’s Interior (2) Basic structure: Solid crust Solid mantle Liquid core Solid inner core Earth’s interior gets hotter towards the center. Earth’s core is as hot as the sun’s surface; metals are liquid. Melting point = temperature at which an element melts (transition from solid to liquid) Melting point increases with increasing pressure towards the center => Inner core becomes solid

19. Earth’s Magnetic Field (Video)Video Convective motions and rotation of the core generate a dipole magnetic field Earth’s core consists mostly of iron + nickel: high electrical conductivity

21. The Role of Earth’s Magnetic Field (Video) (Video) Earth’s magnetic field protects Earth from high-energy particles coming from the sun (solar wind). Surface of first interaction of solar wind with Earth’s magnetic field = Bow shock Region where Earth’s magnetic field dominates = magnetosphere Some high-energy particles leak through the magnetic field and produce a belt of high-energy particles around Earth: Van Allen belts

23. The Aurora (Polar Light) - VideoVideo As high-energy particles leak into the lower magnetosphere, they excite molecules near the Earth’s magnetic poles, causing the aurora

24. The Active Earth About 2/3 of Earth’s surface is covered by water. Mountains are relatively rapidly eroded away by the forces of water.

25. Tectonic Plates: VideoVideo Earth’s crust is composed of several distinct tectonic plates, which are in constant motion with respect to each other  Plate tectonics Evidence for plate tectonics can be found on the ocean floor … and in geologically active regions all around the Pacific

26. Plate Tectonics: VideoVideo Tectonic plates move with respect to each other. Where plates move toward each other, plates can be pushed upward and downward  formation of mountain ranges, some with volcanic activity, earthquakes Where plates move away from each other, molten lava can rise up from below  volcanic activity

27. Active Zones Resulting from Plate Tectonics Volcanic hot spots due to molten lava rising up at plate boundaries or through holes in tectonic plates

28. Earth’s Tectonic History -

30. History of Geological Activity Surface formations visible today have emerged only very recently compared to the age of Earth.

31. Chapter Summary (3) What is the inside of Earth like? Seismic waves generated by earthquakes can be detected by seismographs all over the world and can reveal Earth’s internal structure. Pressure (P) waves can travel through a liquid, but shear (S) waves cannot. Observations show that S waves cannot pass through Earth’s core, and that is evidence that the core is liquid. Earth’s magnetic field is generated by the dynamo effect in the liquid, convecting, rotating, conducting core. The magnetic field shields Earth from the solar wind by producing a bow shock and a magnetosphere around the planet. Radiation belts called the Van Allen belts and auroras are also produced by the field.

32. Chapter Summary (4) Earth is dominated by plate tectonics that breaks the crust into moving sections. Plate tectonics is driven by heat flowing upward from the interior. Tectonic plates are made of low-density, brittle rock that floats on the hotter plastic upper layers of the mantle. Volcanism and earthquakes are common along the edge of the plates. The continents are drifting slowly on the plastic mantle, and their arrangement changes with time. Where they collide, they can form folded mountain ranges. Most geological features on Earth, such as mountain ranges and the Grand Canyon, have been formed recently. The first billion years of Earth’s geology are almost entirely erased by plate tectonics and erosion.

33. Scientific Argument…. What evidence indicates that Earth has a liquid metal core? List two evidences (indirect).

34. IV. Earth’s Atmosphere

35. The Atmosphere - Video Video Atmospheric composition was severely altered (  secondary atmosphere) through a combination of two processes: 1) Outgassing: Release of gasses bound in compounds in the Earth’s interior through volcanic activity Earth had a primeval atmosphere from remaining gasses captured during formation of Earth 2) Later bombardment with icy meteoroids and comets

37. The Structure of Earth’s Atmosphere The ozone layer is essential for life on Earth since it protects the atmosphere from UV radiation Composition of Earth’s atmosphere is further influenced by: Chemical reactions in the oceans, Energetic radiation from space (in particular, UV) Presence of life on Earth The temperature of the atmosphere depends critically on its albedo = percentage of sun light that it reflects back into space Depends on many factors, e.g., abundance of water vapor in the atmosphere

38. Human Effects on Earth’s Atmosphere 1) The Greenhouse Effect - VideoThe Greenhouse Effect - Video Earth’s surface is heated by the sun’s radiation. Heat energy is re-radiated from Earth’s surface as infrared radiation. CO 2, but also other gases in the atmosphere, absorb infrared light  Heat is trapped in the atmosphere. This is the Greenhouse Effect. The Greenhouse Effect occurs naturally and is essential to maintain a comfortable temperature on Earth, but human activity, in particular CO 2 emissions from cars and industrial plants, is drastically increasing the concentration of greenhouse gases.

39. Do You Know What is Global Warming?

40. Global Warming Human activity (CO 2 emissions + deforestation) is drastically increasing the concentration of greenhouse gases. As a consequence, beyond any reasonable doubt, the average temperature on Earth is increasing. This is called Global Warming - VideoGlobal Warming - Video Leads to melting of glaciers and polar ice caps (  rising sea water levels) and global climate changes, which could ultimately make Earth unfit for human life!

41. What would happen if Earth were to be 6 degrees WARMER?

42. Human Effects on the Atmosphere (2) 2) Destruction of the Ozone Layer - VideoDestruction of the Ozone Layer - Video Ozone (= O 3 ) absorbs UV radiation, (which has damaging effects on human and animal tissue). Chlorofluorocarbons (CFCs) (used, e.g., in industrial processes, refrigeration and air conditioning) destroy the Ozone layer. Destruction of the ozone layer as a consequence of human activity is proven (e.g., growing ozone hole above the Antarctic); Must be stopped and reversed by reducing CFC use, especially in developed countries!

47. Video: Ozone 2004-2010 (NASA OMI daily image animation)

48. Several Common Misconceptions…. The greenhouse effect is bad. Human output of CO2 is minor compared to natural sources such as volcanoes. The observed warming of Earth is due to natural causes rather than the greenhouse effect. All Earth will warm at the same rate if there is global warming. Ozone is bad because people hear it mentioned as a pollutant of city air, produced by auto emissions. Global warming and ozone depletion are two names for the same thing….

49. Scientific Argument…. Why does Earth’s atmosphere contain little carbon dioxide and lots of oxygen?

50. Chapter Summary (1) How does Earth fit among the terrestrial planets? Earth is the standard of comparative planetology in the study of the terrestrial planets because we know it best and because it contains all of the phenomena found on the other terrestrial planets. The terrestrial planets include Earth, the moon, Mercury, Venus, and Mars. Earth’s moon is included because it is a complex world and makes a striking comparison with Earth. The terrestrial worlds differ mainly in size, but they all have low-density crusts, mantles of dense rock, and metallic cores. Comparative planetology warns you to expect that cratered surfaces are old, that heat flowing out of a planet drives geological activity, and that the nature of a planet’s atmosphere depends on the size of the planet and its temperature.

51. Chapter Summary (2) How has Earth changed since it formed? Earth formed rapidly from the solar nebula and was hot enough to be molten. Earth has passed through four stages as it has evolved: (1) differentiation; (2) cratering; (3) flooding by lava and water; and (4) slow surface evolution. Earth is peculiar in that it has large amounts of liquid water on its surface, and that water drives strong erosion that alters the surface geology. Earth is also peculiar in that it is the only known home for life.

52. Chapter Summary (3) What is the inside of Earth like? Seismic waves generated by earthquakes can be detected by seismographs all over the world and can reveal Earth’s internal structure. Pressure (P) waves can travel through a liquid, but shear (S) waves cannot. Observations show that S waves cannot pass through Earth’s core, and that is evidence that the core is liquid. Earth’s magnetic field is generated by the dynamo effect in the liquid, convecting, rotating, conducting core. The magnetic field shields Earth from the solar wind by producing a bow shock and a magnetosphere around the planet. Radiation belts called the Van Allen belts and auroras are also produced by the field.

53. Chapter Summary (4) Earth is dominated by plate tectonics that breaks the crust into moving sections. Plate tectonics is driven by heat flowing upward from the interior. Tectonic plates are made of low-density, brittle rock that floats on the hotter plastic upper layers of the mantle. Volcanism and earthquakes are common along the edge of the plates. The continents are drifting slowly on the plastic mantle, and their arrangement changes with time. Where they collide, they can form folded mountain ranges. Most geological features on Earth, such as mountain ranges and the Grand Canyon, have been formed recently. The first billion years of Earth’s geology are almost entirely erased by plate tectonics and erosion.

54. Chapter Summary (5) How has Earth’s atmosphere formed and evolved? Because Earth formed hot, it never had a primeval atmosphere. Because Earth formed in a molten state, its first atmosphere was probably mostly carbon dioxide, nitrogen, and water vapor. Most of the carbon dioxide was dissolved in seawater, and plant life has added oxygen. The greenhouse effect can warm a planet if gases such as carbon dioxide in the atmosphere are transparent to light but opaque to infrared. The natural greenhouse effect warms Earth and makes it comfortable for life, but greenhouse gases added by industrial civilization are responsible for global warming.

55. Chapter Summary (6) The ozone layer high in Earth’s atmosphere protects the surface from ultraviolet radiation, but certain chemicals called chlorofluorocarbons released in industrial processes attack the ozone layer and thin it. This is allowing more harmful ultraviolet radiation to reach Earth’s surface.