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Lesson 3a

Lesson 3a. Properties of the Moon. Comparison to Earth. The Moon’s radius is 27% that of the Earth (about one-fourth as big) The mass of the Moon is 1.2% the Earth’s mass (about 100 times less massive) If you spread the Moon out on the Earth it would roughly fill in the Pacific Ocean.

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Lesson 3a

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  1. Lesson 3a Properties of the Moon

  2. Comparison to Earth • The Moon’s radius is 27% that of the Earth (about one-fourth as big) • The mass of the Moon is 1.2% the Earth’s mass (about 100 times less massive) • If you spread the Moon out on the Earth it would roughly fill in the Pacific Ocean. • The average density of the Moon is 3.34 gm/cm3 • The Moon has no atmosphere • The same side of the Moon always faces the Earth

  3. How can the Moon’s radius be 27% of the Earth, but the mass be 1.2% that of Earth?

  4. . • The moon is made up of much less dense material • The moon must be inflated in some way • The moon’s volume is 64 times smaller than the Earth’s

  5. Volume = (4/3)πR3 • RE = 4RM • VE/VM = (RE3)/(RM3) • VE/VM = (43RM3)/(RM3) • VE/VM = 64 • VE = 64VM • If the Moon had the same composition as the Earth, the Earth would be 64 times more massive. (Moon 1.6% of Earth’s mass)

  6. What can you conclude from the Moon’s density being 3.34 gm/cm3 compared to the Earth’s density of 5.5 gm/cm3?

  7. Moon has a very small iron core

  8. Seismic information from Apollo • The moon has small “moon-quakes” which are likely caused by Earth’s gravity. • The interior of the Moon is not fully stratified or differentiated like the Earth. • It has a small solid inner core of iron, a possible partially molten outer core and a mantle that is almost 1000 km thick.

  9. GRAIL – Gravity Recovery and Interior Laboratory

  10. GRAIL was launched on Sep 11, 2011 • As twin spacecraft orbit the Moon, they will exchange radio signals. This will allow them to compute the distance between the space craft. • As one spacecraft passes over a different density region of the Moon it will either speed up or slow down because the gravity will increase or decrease.

  11. This will allow mapping or Moon’s interior

  12. Mantle is not uniform

  13. Nearside of the Moon

  14. Large impacts and Maria formation

  15. Large asteroid impacts made the Lunar crust very thin in some locations. • These impacts occurred when the Moon was still molten inside. (around 4 billion years ago) • Over time (hundreds of millions of years) the thin and weakened crust allowed magma to extrude up to the surface through fault fractures • The mare filled with lava and later impacts caused craters in the Maria.

  16. Lava filled large impact craters and also spread across low- lying surface

  17. Farside of the Moon

  18. Why are there very few mare on the far-side of the Moon? • Far-side: Maria covers 2.5% of surface • Near-side: Maria covers 31.2% of surface

  19. Why are there very few mare on the far-side of the Moon? • Far-side: Maria covers 2.5% of surface • Near-side: Maria covers 31.2% of surface • The crust on the far-side is thicker (~80 km) than on the near-side (~60 km). • It is currently unclear why this is the case but a controversial idea may explain this.

  20. Two moon hypothesis – Aug 2011 • The Earth may have had two moons. The smaller moon (diameter ~ 750 miles) orbited with our Moon. • Slowly they merged together. • Since they had nearly the same speed and direction it was a slow motion event that spread the smaller moon all over the far side of our Moon.

  21. GRAIL will give us the answer

  22. Lunation and Libration

  23. Things to notice • The same side of the Moon always faces the Earth. • The Moon grows and shrinks in apparent size as it orbits. • The Moon wobbles (libration) as it orbits the Earth.

  24. Lunation and Libration

  25. Given the same side always faces the Earth, does the Moon rotate on its axis and if so how long does it take to complete one rotation?

  26. . • To keep the same side toward us the Moon can not rotate on its axis • It has to rotate once every 24 hours • It has to rotate once every orbit

  27. Time to discuss gravity and orbits.

  28. The International Space Station (ISS)

  29. Why are the astronauts in the ISS, weightless? • There is no gravity in outer space • There is no gravity in a vacuum. • The ISS is to far away for Earth gravity to effect it • The astronauts are too small to be effected • The astronauts are falling

  30. Before answering the question: Why are the astronauts in the ISS weightless? Let’s look at a little physics.

  31. If you were to drop a heavy hammer and a feather at the same time, which one will hit the ground first? This is assuming that there is no air to make the feather slow up.

  32. Please make your selection... • The hammer • The feather • They hit at the same time.

  33. Galileo • A feather and a hammer

  34. Let’s examine our answers • There is no gravity in outer space • Question: Why is the ISS going around (orbiting) the Earth? Because the force of gravity is holding it in orbit.

  35. If there is no force of gravity in outer space then Newton’s first law tells us the ISS will travel in a straight line. But it doesn’t

  36. Let’s examine our answers • There is no gravity in outer space • Question: Why is the ISS going around (orbiting) the Earth? Because the force of gravity is holding it in orbit. • Earth is so far away that gravity is very weak • There is no gravity in a vacuum • Question: Why is the ISS orbiting the Earth? Clearly Gravity works in a vacuum and at the distance of the ISS.

  37. Now a little more thinking • They (the astronauts) are too small for gravity to affect them.

  38. Now a little more thinking • They (the astronauts) are too small for gravity to effect them. • Question: Gravity is holding the ISS in orbit. What does Newton’s First Law tell us should happen to the astronauts if gravity isn’t effecting them? Would they be weightless?

  39. If the astronauts are too small for gravity to affect them then this should happen… Not weightless, pressed against side

  40. The correct answer is… • The astronauts are falling. The ISS is falling around the Earth and (from Galileo) we know that the astronauts are falling at the same rate as the ISS.

  41. Newton’s big realization

  42. Newton’s Law of Gravity • F = GMm/r2 Where G is a proportionality constant called the universal gravitational constant. In MKS units it has the value of G = 6.67 x 10-11 meters3/kg x seconds2 M and m are the masses of the two objects and r is the distance between them.

  43. How light spreads out

  44. D – circular orbit • E & F are elliptical orbits. • The object doesn’t drop fast enough given its speed.

  45. So, • If your speed is too slow you will fall too rapidly and you will hit the surface of the Earth. • If your speed is too fast you will move in an elliptical orbit • If you move at just the right speed (~17,000 MPH) then you will drop at the same rate as the Earth curves around under you. A circular orbit.

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