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The Moon

The Moon . 3.344 g/cm 3. The Moon: The View from Earth. From Earth, we always see the same side of the moon. Moon rotates around its axis in the same time that it takes to orbit around Earth:. Tidal coupling:. Earth’s gravitation has produced tidal bulges on the moon;.

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The Moon

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  1. The Moon

  2. 3.344 g/cm3

  3. The Moon: The View from Earth From Earth, we always see the same side of the moon. Moon rotates around its axis in the same time that it takes to orbit around Earth: Tidal coupling: Earth’s gravitation has produced tidal bulges on the moon; Tidal forces have slowed rotation down to same period as orbital period

  4. Lunar Surface Features Two dramatically different kinds of terrain: • Highlands: Mountainous terrain, scarred by craters • Lowlands: ~ 3 km lower than highlands; smooth surfaces: • Maria (pl. of mare): Basins flooded by lava flows

  5. Highlands and Lowlands Sinuous rilles = remains of ancient lava flows May have been lava tubes which later collapsed due to meteorite bombardment. Apollo 15 landing site

  6. The Highlands Saturated with craters Older craters partially obliterated by more recent impacts … or flooded by lava flows

  7. Impact Cratering Impact craters on the moon can be seen easily even with small telescopes. Ejecta from the impact can be seen as bright rays originating from young craters

  8. History of Impact Cratering Rate of impacts due to interplanetary bombardment decreased rapidly after the formation of the solar system. Most craters seen on the moon’s (and Mercury’s) surface were formed within the first ~ 1/2 billion years.

  9. Meteoroid impacts have been the only significant “weathering”agent on the Moon • The Moon’s regolith, or surface layer of powdered and fractured rock, was formed by meteoritic action

  10. Apollo Landing Sites First Apollo missions landed on safe, smooth terrain. Later missions explored more varied terrains. Apollo 17: Taurus-Littrow; lunar highlands Apollo 11: Mare Tranquilitatis; lunar lowlands

  11. All of the lunar rock samples are igneous rocks formed largely of minerals found in terrestrial rocks The lunar rocks contain no water They differ from terrestrial rocks in being relatively enriched in the refractory elements and depleted in the volatile elements High-lands anorth-osite Mare basalt Impact breccia

  12. What about the other side?

  13. The Moon’s airless, dry surface is coveredwith plains and craters • The Earth-facing side of the Moon displays light-colored, heavily cratered highlands and dark-colored, smooth-surfaced maria • The Moon’s far side has almost no maria

  14. How can we explain the difference? Why do we have more maria on one side than the other? Maria are lava flows. Why would you get more volcanics on one side? Explore the interior of the moon.

  15. Moonquake Earthquake

  16. The Moon has no global magnetic field but hasa small core beneath a thick mantle

  17. The History of the Moon Moon is small; low mass rapidly cooling off; small escape velocity no atmosphere unprotected against meteorite impacts. Moon must have formed in a molten state (“sea of lava”); Heavy rocks sink to bottom; lighter rocks at the surface No magnetic field small core with little metallic iron. Surface solidified ~ 4.6 – 4.1 billion years ago. Alan Shepard (Apollo 14) analyzing a moon rock, probably ejected from a distant crater. Heavy meteorite bombardment for the next ~ 1/2 billion years.

  18. Formation of Maria Impacts of heavy meteorites broke the crust and produced large basins that were flooded with lava

  19. The maria formed after the surrounding light-colored terrain, so they have not been exposed to meteoritic bombardment for as long and have fewer craters

  20. Virtually all lunar craters were caused by space debris striking the surface • There is no evidence of plate tectonic activity on the Moon

  21. Formation of Maria (2) Major impacts forming maria might have ejected material over large distances. Apollo 14 Large rock probably ejected during the formation of Mare Imbrium (beyond the horizon!)

  22. Origin of Mare Imbrium Terrain opposite to Mare Imbrium is jumbled by seismic waves from the impact.

  23. Similar Processes to Earth?  • Volcanism • Water • Cratering • Winds • Earthquakes • Dynamo Effect     

  24. How did the Moon form? • What are the constraints? • BIG (for a moon) • NO IRON CORE • WAS CLOSER TO US IN PAST • IGNEOUS ROCKS LACKING VOLATILES • SIMILAR COMPOSITION TO EARTH’S MANTLE • CAN IT HAPPEN?

  25. The Origin of Earth’s Moon Early hypotheses: Break-up of Earth during early period of fast rotation Fission hypothesis: Problems: No evidence for fast rotation; moon’s orbit not in equatorial plane capture hypothesis: Capture of moon that formed elsewhere in the solar system Problem: Requires succession of very unlikely events Sister hypothesis: Condensation and accretion at time of formation of Earth Problem: Different chemical compositions of Earth and moon

  26. Modern Theory of Formation of the Moon The Large-Impact Hypothesis • Impact heated material enough to melt it consistent with “sea of magma” • Collision not head-on Large angular momentum of Earth-moon system • Collision after differentiation of Earth’s interior Different chemical compositions of Earth and moon

  27. The Moon probably formed from debris castinto space when a huge planetesimal struckthe proto-Earth • The collisional-ejection theory holds that the proto-Earth was struck by a Mars-sized protoplanet and that debris from this collision coalesced to form the Moon • This theory successfully explains most properties of the Moon • The Moon was molten in its early stages, and the anorthositic crust solidified from low-density magma that floated to the lunar surface • The mare basins were created later by the impact of planetesimals and filled with lava from the lunar interior

  28. Lunar rocks reveal a geologic history quite unlike that of Earth • The anorthositic crust exposed in the highlands was formed between 4.0 and 4.3 billion years ago • The mare basalts solidified between 3.1 and 3.8 billion years ago • The Moon’s surface has undergone very little change over the past 3 billion years

  29. Quiz Questions 3. How do we know that Copernicus is a young impact crater? a. It is on the side of the Moon that faces Earth. b. It has a central peak and raised rim. c. It has scalloped slopes along its inner crater walls. d. Blocks of material in its ejecta formed secondary craters. e. It has bright rays that extend onto the surrounding maria.

  30. Quiz Questions 4. How do we find the relative ages of the Moon's maria and highlands? a. By counting the number of impact craters. b. By measuring the depth of the lunar regolith. c. By measuring the lunar latitude and longitude. d. By measuring the size of the smallest impact craters. e. By measuring variations in the Moon's gravitational field.

  31. Quiz Questions 5. Why do almost all impact craters have a circular shape? a. High-speed projectiles vaporize explosively upon impact, sending out spherical compression waves. b. The impacting projectiles have a spherical shape and thus punch out circular penetration holes. c. Erosion has reduced the irregular craters to circular shapes. d. Most impacts occur from directly overhead. e. A circle is the most perfect form.

  32. Quiz Questions 7. Why do we suppose that the Moon formed with a molten surface? a. The Moon is covered with volcanic craters of all sizes. b. Samples from the maria regions are basalt, a common igneous rock. c. The oldest lunar rock samples are about 4.4 billion years old and composed of anorthosite, a mineral that crystallizes and rises to the top of a lava ocean. d. Both a and b above. e. All of the above.

  33. Quiz Questions 12. Why does the Moon have large maria on the Earth-facing side, yet no large maria on the opposite side? a. The maria regions are the same on both sides; we normally don't see those on the far side. b. The late heavy bombardment only occurred on the Earth-facing side. c. The maria on the far side are not as dark as those on the near side. d. The Moon's crust is thicker (or elevations higher) on the far side. e. No large impact basins exist on the Moon's far side.

  34. Quiz Questions 14. For what reasons do we reject the condensation (double planet, sister) hypothesis of the Moon's origin? a. The Moon has a much lower density than Earth. b. The Moon is very low in volatiles, compared to Earth. c. The Moon is much smaller and less massive than Earth. d. Both a and b above. e. All the above.

  35. Quiz Questions 15. How does the large impact hypothesis explain the Moon's lack of iron? a. The impact occurred before either planetesimal had differentiated and formed an iron core. b. The ejected orbiting material that formed the Moon was initially at a high temperature. c. Both planetesimals were differentiated, and the two iron cores went to Earth. d. The impacting planetesimal was not differentiated and thus had no iron core. e. The Moon's lack of iron is the major problem of the large impact hypothesis.

  36. Answers 1. e 2. b 3. e 4. a 5. a 6. d 7. c 8. c 9. b 10. d 11. e 12. d 13. e 14. d 15. c 16. d 17. e 18. b 19. a 20. e

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