Chapter 9 Planetary Geology Earth and the Other Terrestrial Worlds

1. Chapter 9Planetary GeologyEarth and the Other Terrestrial Worlds

2. 9.1 Connecting Planetary Interiors and Surfaces Our goals for learning: • What are terrestrial planets like on the inside? • What causes geological activity? • Why do some planetary interiors create magnetic fields?

3. What are terrestrial planets like on the inside?

4. Earth’s Interior • Core: Highest density; nickel and iron • Mantle: Moderate density; silicon, oxygen, etc. • Crust: Lowest density; granite, basalt, etc.

5. Terrestrial Planet Interiors • Applying what we know about Earth’s interior to other planets helps us determine what their interiors are probably like.

6. Why are there layers? • Differentiation • Gravity pulls high-density material to center • Lower-density material rises to surface • Material ends up separated by density

7. Lithosphere • A planet’s outer layer of cool, rigid rock is called the lithosphere. • Contains the crust and upper part of the mantle. • It “floats” on the warmer, softer rock that lies beneath.

8. 3476 km Strength of Rock and Shape • Rock stretches when pulled slowly but breaks when pulled rapidly. • The gravity of a large object (>500 km) will slowly shape it into a sphere within about 1 billion years. • Larger objects will become spherical more quickly. • Molten objects will become spherical more quickly.

9. Thought Question What is necessary for differentiation to occur in a planet? a)It must have metal androck in it b) It must be a mix of materials of different density c) Material inside must be able to flow d) All of the above e) b and c

10. What causes geological activity?

11. Heating of Interior • Heat of Accretion: • Conservation of Energy Gravitational Potential Energy Thermal Energy at Impact Kinetic Energy • Heat from differentiation: • when planets were young. Denser Materials Sink Friction Increases Temperature Less Dense Materials Rise • Heat from Radioactive decay: • Most significant source of heat today.

12. Heating of Interior

13. Cooling of Interior • Convection transports heat as hot material rises and cool material falls. • Conduction transfers heat from hot material to cool material. • Radiation sends energy into space. • Note: Convection and Conduction only transport heat to the surface. The planet only loses heat through Radiation.

14. Role of Size • Smaller worlds cool off faster and harden earlier • Moon and Mercury are now geologically “dead” • Why?

15. Surface Area to Volume Ratio • Heat content depends on volume • Loss of heat through radiation depends on surface area • Time to cool depends on surface area divided by volume • Larger objects have smaller ratio and cool more slowly

16. Why do some planetary interiors create magnetic fields?

17. Not … because there is a big bar magnet in the center of the Earth • The core far exceeds the Curie temperature for iron • Curie temperature: Temperature at which a permanent magnet loses its magnetism • The highest known value for this is 1400K (Cobalt) • The core temperature is estimated at 6000K

18. Magnetic Domains

19. Magnetic Domains = groups of atoms with aligned poles Magnets can be temporary (like the needle used in the compass). This nail has its atoms aligned, but the effect is only temporary.  You can get this affect by rubbing the nail on a magnet. Neat fact:  Hitting the nail can demagnetize it, you are basically scrambling the atoms.

20. Sources of Magnetic Fields • Motions of charged particles are what create magnetic fields

21. Maxwell’s Equations of Electromagnetism Gauss’ Law for Electrostatics Gauss’ Law for Magnetism Faraday’s Law of Induction Ampere’s Law

22. Sources of Magnetic Fields • A world can have a magnetic field if charged particles are moving inside • 3 requirements: • Molten interior • Convection • Moderately rapid rotation

23. Requirements for a strong magnetic field • Convecting charged particles • The charge particles need to be moving • Convection requires a heat source – there needs to be an energy source to drive the motion and produce a net electric current • Moderately rapid rotation • Without rotation, there is no clear direction for the current to be flowing in, and no net field.

24. When a planet cools • If there was a magnetic field • There may be a remnant of the field locked into the rocks, minerals, and possibly the core after it cools • Much weaker than the dynamic field of a planet with a molten interior driving the field

25. Minor difficulties • To have a magnetic field we need a current • To have a current we need a potential difference to drive the electrons and protons in different directions • What can be the source of such a potential difference that can keep the electrons and protons in a conducting liquid core separate? • It is not clear (at least to me, and lots of others), how this really works

26. Table of planetary values

27. Why do we care about the Earth’s magnetic field? http://ny.pbslearningmedia.org/resource/nvsl.sci.space.magnetic/earths-magnetic-shield/ Are we in for trouble? https://uk.news.yahoo.com/scientists-tracking-dangerous-weakening-earths-223256543.html What about Mars? https://video.search.yahoo.com/yhs/search?fr=yhs-mozilla-001&hsimp=yhs-001&hspart=mozilla&p=video+magnetic+field+protect#id=23&vid=ad8608992b6ace32aefd1a5ae2c18498&action=view

28. How do we learn about the Earth’s core, mantle, and crust? • Deep drilling • Seismic waves • We know earth’s density from its size and applying Kepler’s 3rd law on the moon • All of the above • A and B

29. The inside of the earth is filled with molten lava • True • False • It is only molten in a very narrow region underneath the lithosphere.

30. How do we know that the Earth is geologically active? • Volcanoes • Seismology • Measuring the temperature inside • All of the above • A and B

31. What is the lithosphere? • Another name for a planet’s crust • The crust plus the mantle • A relatively rigid outer layer of rock that floats on more molten rock below • The boundary between the core and mantle

32. Where did the Earth’s (interior) heat come from? • Volcanoes • Impacts as the Earth was accreting • Radioactivity • All of the above • B and C

33. What is necessary for a differentiation to occur in a planet? • It must have metal androck in it • It must be a mix of materials of different density • Material inside must be able to flow • All of the above • B and C

34. What is the source of the Earth’s magnetic field? • Magnetic rocks • Magnetized iron in the Earth’s crust • Magnetized iron in the Earth’s core • Molten metal circulating inside the Earth, moving electrons like in a wire

35. Why are smaller terrestrial bodies such as Mercury or the Moon “geologically dead”? • They don’t have volcanoes • They cooled off faster than the Earth did • They don’t have erosion • They were hit by fewer meteorites • They are made of different materials than the Earth

36. What is convection? • Heat radiates from a planet into space • Heat travels from atom to atom from inside a planet to the outside • Hot material inside a planet rises, and cool material sinks towards the center • Metal conducts energy throughout the earth’s core

37. What have we learned? • What are terrestrial planets like on the inside? • Core, mantle, crust structure • Denser material is found deeper inside • What causes geological activity? • Interior heat drives geological activity • Radioactive decay is currently main heat source • Why do some planetary interiors create magnetic fields? • Requires motion of charged particles inside planet

38. 9.2 Shaping Planetary Surfaces Our goals for learning: • What processes shape planetary surfaces? • Why do the terrestrial planets have different geological histories? • How do impact craters reveal a surface’s geological age?

39. What processes shape planetary surfaces?

40. Processes that Shape Surfaces • Impact cratering • Impacts by asteroids or comets • Volcanism • Eruption of molten rock onto surface • Tectonics • Disruption of a planet’s surface by internal stresses • Erosion • Surface changes made by wind, water, or ice

41. Impact Cratering • Most cratering happened soon after solar system formed (First 600-700 mil. years). • Asteroids and comets impact the surface at speeds between 40,000 and 250,000 km/hr (10-70 km/s) • Craters are about 10 times wider than object that made them. • Craters are about 10-20% as deep as they are wide. • Impact angle not a significant factor on shape of crater. • Small craters greatly outnumber large ones.

42. Impact Craters Meteor Crater (Arizona) Tycho (Moon)

43. Impact Craters on Mars “standard” crater impact into icy ground eroded crater

44. Volcanism • Volcanism happens when molten rock (magma) finds a path through lithosphere to the surface. • Molten rock is called lava after it reaches the surface.

45. Lava and Volcanoes Runny lava makes flat lava plains. Slightly thicker lava makes broad shield volcanoeslike in Hawaii (Not very steep). Thickest lava makes steep stratovolcanoes(tall and steep).

46. Outgassing • Volcanism also releases gases from Earth’s interior into atmosphere (H2O, CO2, N2, H2S and SO2).

47. Tectonics • Convection of the mantle creates stresses in the crust called tectonic forces. • Compression forces make mountain ranges. • Valley can form where crust is pulled apart (mid-oceanic ridge).

48. Plate Tectonics on Earth • 200 million years ago, the Earth was covered by one super-continent called Pangaea. • Pangaea broke up into Earth’s continents which slide around on separate plates.

50. Erosion • Erosion is a blanket term for weather-driven processes that break down or transport rock through the action of ice, liquid or gas. • Processes that cause erosion include • Glaciers (ice) • Rivers (liquid) • Wind (gas)