Chapter 5: The Earth

1. Chapter 5: The Earth

2. Barringer Crater Crater 1.2 km across 0.2 km deep Meteoroid 50 m across mass 200 k tons

3. An asteroid impact like the one that killed off the dinosaurs is expected once every: A: year B: hundred years C: thousand years D: hundred thousand years E: hundred million years Clicker Question:

4. Question 4 a) beyond the orbit of Neptune. b) between Earth and the Sun. c) between Mars and Jupiter. d) in the orbit of Jupiter, but 60 degrees ahead or behind it. e) orbiting the jovian planets in captured, retrograde orbits. Most asteroids are found

5. The Asteroid Belt is located between 2.1 and 3.3 A U from the Sun. Question 4 a) beyond the orbit of Neptune. b) between Earth and the Sun. c) between Mars and Jupiter. d) in the orbit of Jupiter, but 60 degrees ahead or behind it. e) orbiting the jovian planets in captured, retrograde orbits. Most asteroids are found

6. Pale Blue Dot Earth as seen from Voyager 1, when it was 6 billion km from home.

7. General Features Mass: MEarth = 6 x 1027 g Radius: REarth= 6378 km Density: p = 5.5 g/cm3 Age: 4.6 billion years

8. Earth's Internal Structure How do we know? Earthquakes. See later Crust: thin. Much Si and Al (lots of granite). Two-thirds covered by oceans. Mantle is mostly solid, mostly basalt (Fe, Mg, Si). Cracks in mantle allow molten material to rise => volcanoes. Core temperature is 6000 K. Metallic - mostly nickel and iron. Outer core molten, inner core solid. Atmosphere very thin

9. gas is ionized by solar radiation ozone is O3 , which absorbs solar UV efficiently, thus heating stratosphere commercial jet altitudes Earth's Atmosphere Original gases disappeared. Atmosphere is mostly due to volcanoes and plants! 78% Nitrogen 21% Oxygen temperature on a cool day

10. Ionosphere Particles in the upper reaches of the atmosphere are ionized by the sun. Radio signals below ~20 MHz can “bounce” off the ionosphere allowing Communication “over the horizon” (or mountains)

11. Earthquakes They are vibrations in the solid Earth, or seismic waves. Two kinds go through Earth, P-waves ("primary") and S-waves ("secondary"):

12. How do they measure where Earthquakes are centered? seismic stations * * *

13. Like all waves, seismic waves bend when they encounter changes in density. If density change is gradual, wave path is curved. S-waves are unable to travel in liquid. Thus, measurement of seismic wave gives info on density of Earth's interior and which layers are solid/molten. No P waves too: they must bend sharply at core boundary Zone with no S waves: must be a liquid core that stops them But faint P waves seen in shadow zone, refracting off dense inner core Curved paths of P and S waves: density must slowly increase with depth

14. Earth's Interior Structure Average density Crust Mantle Core 5.5 g/cm3 3 g/cm3 5 g/cm3 11 g/cm3 Density increases with depth => "differentiation" Earth must have been molten once, allowing denser material to sink, as it started to cool and solidify.

15. Earthquakes and volcanoes are related, and also don't occur at random places. They outline plates. Plates moving at a few cm/year. "Continental drift" or "plate tectonics"

16. When plates meet... 1) Head-on collision (Himalayas) 2) "Subduction zone" (one slides under the other) (Andes) 3) "Rift zone" (two plates moving apart) (Mid-Atlantic Ridge) 4) They may just slide past each other (San Andreas Fault) side view top view => mountain ranges, trenches, earthquakes, volcanoes

17. The Mid-Atlantic Ridge is a rift zone.

18. What causes the drift? Convection! Mantle slightly fluid and can support convection. Plates ride on top of convective cells. Lava flows through cell boundaries. Earth loses internal heat this way. Cycles take ~108 years. Plates form lithosphere (crust and solid upper mantle). Partially melted, circulating part of mantle is asthenosphere.

19. Pangaea Theory: 200 million years ago, all the continents were together!

20. Convection Earth's surface heated by Sun. What would happen if it couldn't get rid of the energy as fast as it gets in? Convection also occurs when you boil water, or soup. Think of Earth's surface as a boiling pot! Convection causes both small-scale turbulence and large scale circulation patterns. It also occurs within Earth, on other planets, and in stars.

21. The Greenhouse Effect Main greenhouse gases are H2O and CO2 . If no greenhouse effect, surface would be 40 oC cooler!

22. The Moon Mass = 7.4 x 1025 g = 0.012 MEarth = 1738 km = 0.27 REarth Radius Density = 3.3 g/cm3 (Earth 5.5 g/cm3) = 1/6 that of Earth Gravity

23. We always see the same face of the Moon. This means: period of orbit = period of spin Why? Tidal Locking The tidal bulge in the solid Moon elongates it slightly (2-3 km) along an axis pointing to Earth. If orbit period faster than spin period, tidal bulge would have to move around surface of Moon, creating friction, which slows the Moon’s spin down until tidal bulge no longer migrates around. Top view of Moon orbiting Earth Earth

24. Far side of the moon

25. Tides A feature of oceans (but solid material has small tides too). Two high and two low tides per day. Tides are due to Moon's gravitational pull being stronger on side of Earth closest to it (Sun causes smaller tides). Earth-Moon gravity keeps them orbiting each other. But side of Earth closest to Moon has slightly stronger pull to Moon => bulges towards it. Other side has weaker pull => bulges away compared to rest of Earth. The Earth spins once a day while the bulge always points towards and away from the Moon => high and low tides.

26. Tides

27. Tides

28. Extreme Tides – Bay of Fundy • Highest tide change from low tide to high tide of 56 feet • Shares title of most extreme tides with Ungava Bay ( near Hudson Strait in Canada)

29. The Lunar Surface - Large, dark featureless areas: "maria" or "seas". - Lighter areas at higher elevation: "highlands". - Loads of craters (due mostly to meteorite impacts). No winds to erode them away. - Highlands have 10x the crater density of maria. highlands maria

30. Lunar Volcanism (long ago) Remember: volcanism is a way of losing internal heat Evidence: - Maria: result of old, widespread lava flows (filled in largest, early impact craters) - "Rilles": ditches indicating old lava flows - Linear chains of craters (not formed by impacts), probably marks ancient fault, collapsed lava domes

31. Cratering - Impact speeds several km/sec - "Ejecta blanket" of pulverized rock surrounds crater - Impacts => "regolith": ~20 m thick layer of pulverized rock covering Moon.

32. Cratering Rates Small meteroids common, large ones rare. So same true for craters: Crater sizeOccurrence 10 km 1 m every 10 million years every month If no other processes (erosion, lava flows) change the surface, the number of craters in an area tells you the age of the surface.

33. Moon's History Age: 4.5 billion years 3.9 billion years ago: heaviest meteoritic bombardment ended 3.9 - 3.2 billion years ago: volcanism created maria. Maria are just the largest craters, filled in. 3.2 billion years -> present no volcanism, cratering continued at lower rate, geologically dead!

34. Earth and the Moon • Moon stabilizes Earth’s rotational axis, maintains 23.5 degree tilt=> gives us stable climate, seasons, etc. • Earth’s rotational period is slowing down – day is getting longerMoon is moving away from earth => 3.8 cm or 1.5 inches per year