1. Earth Astronomy 311 Professor Lee Carkner Lecture 12

3. Terra -- The Earth Goddess  Greeks and Romans personified the Earth as a mother goddess

4. Earth Facts  Size: 12700 km diameter  largest terrestrial planet  Orbit: 1 AU (1.5 X 10 8 km)  Description: wet, temperate, inhabited

5. Earth’s Celestial Motions  Earth is tilted on its axis by 23 1/2 degrees  Produces seasons  Earth has large satellite, The Moon  Moon is very large compared to Earth  Produces tides  May stabilize tilt of axis

6. Earth’s Atmosphere  Composition:  77% N 2  21% O 2  small amounts of water vapor, CO 2 other gasses  Very different from other atmospheres  Inner planets: mostly CO 2  Outer planets: mostly H 2

7. Early Atmosphere  Where did the original atmosphere come from?  Small amounts of volatiles in the planetesimals that formed the Earth  Bombardment of icy planetesimals (comets)  Early composition:  Carbon dioxide (CO 2 )  Water (H 2 O)  Methane (CH 4 )  Ammonia (NH 3 )

8. Formation of Atmosphere  Start with CO 2, H 2 O, CH 4, NH 3  Ultraviolet light breaks up some molecules (four key elements: C,H,O,N)  C, H, O form H 2 O, CO 2  CO 2 dissolves in H 2 O, H 2 O rains out to form oceans, N 2 left behind  Life (plants) forms and produces O 2  Final atmosphere -- O 2 and N 2

9. Earth’s Atmosphere

10. Atmosphere and Temperature  Planets with no atmosphere have a temperature determined by radiation balance:  Absorb radiation from the Sun  Emit radiation back into space  For a steady temperature, both rates must be equal  Using this method we get an average temperature of 250 K for the Earth  K stands for Kelvin, a temperature scale where 0 K is absolute zero  The coldest anything can get  To convert K to F:  T F = 1.8T K - 460

11. The Habitable Zone  In order to support life a planet must be in the habitable zone  Region around star where water is liquid on planetary surface (T~273 K, 0 C, 32 F)  Width of zone determined by the effectiveness of the carbonate-silicate cycle  Inner Edge -- where water is lost by UV dissociation  Outer Edge -- where CO 2 clouds block sunlight  For our solar system habitable zone width is about 0.95-1.37 AU

12. Hypothetical Habitable Zone Too hot, water is destroyed can’t remove CO 2 Too cold, try to warm up with more CO 2 but CO 2 forms clouds and blocks sunlight Just right, temperature kept stable at ~273 K (water is liquid)

13. Why is the Earth Habitable?  It has the right temperature for liquid water (~273 K) because:  Large enough to hold an atmosphere  Not too close or too far from the Sun  Has both water and plate tectonics for carbonate-silicate cycle

14. Climate  The atmosphere circulates due to convection (hot thing rise, cool things sink)  Two basic sources of convection  Bottom of atmosphere is warmer than top  Equator is warmer than poles  The Earth’s rotation breaks the atmosphere up into convection cells that keep the air circulating, producing global weather patterns

15. The Earth’s Surface  What shapes the Earth’s surface?  Plate tectonics  Water erosion  Also, volcanoes and cratering  Resurfacing is cyclical and constant  Plate tectonics raise up mountains, water erodes them down, the silt collects on the ocean floor and forms the rock to make the next mountains  Unlike other planets the Earth does not have many craters  Craters on Earth are largely obliterated by erosion

16. Plate Tectonics  The two top layers of the Earth are the crust and the mantle  Upper mantle is soft and plastic  Crust is hard and rigid  Crust is broken up into plates that float on the upper mantle  Plates move around and crash into each other forming trenches and mountains  Plates move a few inches a year

17. How Plate Tectonics Work

18. Plate Boundaries

19. Water  The Earth has more liquid water on its surface than any other planet  Carbonate-silicate cycle keeps temperature at point where water is liquid (273 K)  Water is present on Earth in all three phases (ice, liquid water, vapor) and continuously cycles between them

20. Water on the Earth

21. Plate Collision -- The Himalayas

22. The Grand Canyon

23. Meteor Crater

24. The Earth’s Interior  The crust and atmosphere are very thin compared the rest of the planet (like the skin of an orange)  We cannot view the inner layers directly (can’t drill deep enough)  We learn about them by studying the seismic waves from earthquakes

25. Seismic Waves  Types of waves:  P waves: pressure or compression wave  example: pslinky  S waves: shear waves  example: string  The different densities of the inner earth refract the waves  just like light through a lens  When an earthquake occurs we can measure the strength of S and P waves all over the Earth

26. Earthquake Studies of the Earth’s Interior

27. Seismic Waves and the Earth’s Interior  No S waves detected on opposite side of Earth  S waves cannot penetrate liquid  Core must be liquid  There is a shadow zone where no P or S waves are detected  Core mantle boundary refracts wave out of this zone  Very faint P waves detected in shadow zone  Refracted by solid inner core

28. Structure of the Earth

29.  Crust:  surface to 35 km  composed of silicates  Mantle  35-2900 km  composed of silicates and heavier material  Outer core  2900-5100 km  composed of liquid iron  Inner core  5100-6400 km  composed of solid iron  Density increases toward the center

30. Summary  Earth is unique for at least two reasons  Large amounts of liquid water  constantly reshapes the surface  Large amounts of free oxygen  produced by life  Earth has liquid water and life because it is in the habitable zone

31. Summary: Atmosphere  Earth’s initial atmosphere composed of CHON  H and O form water -- oceans  C and O form carbon dioxide -- rock  N stays in atmosphere  Plants produce oxygen  Mild temperature maintained by carbonate-silicate cycle

32. Summary: Surface  Solid iron inner core, liquid iron outer core, solid mantle and crust  Crust is broken up into plates which slide around on the upper mantle  Plate tectonics and erosion constantly alter surface