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第1段
1 .listen to part of a lecture in an astronomy class.
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2 .Last class, we were discussing Earth\'s global magnetic field.
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3 .Now, scientists still don\'t fully understand the source of Earth\'s magnetism
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4 .or the forces that make up this magnetic field surrounding our planet,
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5 .but they hypothesize that the motion of the fluid and Earth\'s outer core has a lot to do with it.
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6 .Yes, Fred,
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7 .you know, I just read something about magnetic fields of the moon.
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8 .The early astronauts detected magnetic fields there.
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9 .So does the moon have a global magnetic field too?
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10 .No, it doesn\'t, but it does have regions of magnetized rock that produce local magnetic fields.
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11 .Some are weak, but others are quite strong.
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12 .Local magnetic fields.
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13 .Yes, these local magnetic fields are scattered across the moon\'s surface.
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14 .The weak fields, we think these originated from volcanoes that erupted early in the moon\'s history.
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15 .See, after molten lava flowed out from a volcano,
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16 .it cooled and solidified into rock and became magnetized.
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17 .But the material composing lunar lava isn\'t highly magnetic,
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18 .so this hardened lava near the volcanoes only generates weak magnetic fields.
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19 .Now, the areas of strong magnetism are especially puzzling.
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20 .You see, they\'re not associated with any geologic features like volcanoes,
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21 .and we really can\'t explain why these areas have such highly magnetic rock
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22 .as I just mentioned, moon rock in general, just isn\'t very magnetic.
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23 .Scientists actually calculated that it would take a layer over 100 kilometers thick of typical moon rock to produce the strong magnetic fields that we\'re seeing in these areas,
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24 .but that exceeds the thickness of the entire lunar crust.
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25 .So the origin of these strong, local magnetic fields has us stumped,
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26 .and we\'ve been trying to explain this since 1969
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27 .since the early moon missions you alluded to
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28 .but a recent study may provide some answers.
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29 .A team of scientists noticed an important correlation between the largest grouping of strong local magnetic fields on the moon
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30 .and an area within the South Pole Aitken basin.
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31 .The South Pole Aitken basin is a huge crater
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32 .about half the size of the United States.
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33 .It was formed when an asteroid crashed into the moon.
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34 .And what do we know about the composition of asteroids? Laura,
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35 .they contain lots of iron,
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36 .yes, and other highly magnetic material.
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37 .And this huge basin is shaped sort of like an oval. It\'s kind of elongated in the north south direction,
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38 .which suggests that the asteroid hit the moon at an angle to form the basin.
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39 .Now, if this were the case, most of the magnetic material from the impact would land on one end of the basin,
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40 .and in fact, the largest collection of strong local magnetic fields is located on one end of the basin,
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41 .at the northern rim.
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42 .So we\'ve hypothesized that the rock that\'s generating these strong local magnetic fields was brought to the moon by the South Pole Aitken asteroid,
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43 .and the strong magnetic fields in areas further away from the impact basin
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44 .could be explained by materials from the asteroid that were ejected and dispersed following the impact.
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45 .And we know that large impact events were common during the early evolution of the solar system,
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46 .so this same scenario could also explain some of the other groupings of strong magnetic fields we see on the moon.
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47 .Okay, but wait, but that hypothesis doesn\'t explain
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48 .I mean, so how do those rocks from the volcanoes and asteroid, you know,
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49 .get magnetized? I mean, I remember learning that rocks can only become magnetized if there\'s already a magnetic field present,
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50 .but you said there\'s no global magnetic field on the moon.
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51 .So how did that happen?
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52 .Yes, it does remain a mystery how the rock that produced the weak and strong magnetic fields we see today became magnetized in the first place.
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53 .Okay, so this asteroid hypothesis only addresses the problem of how the strongly magnetic rock got to the moon,
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54 .right, and we\'re still left with how that material, and the weekly magnetic rock, for that matter, then became magnetized.
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55 .Fred, so,
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56 .maybe the moon used to have a global magnetic field like Earth\'s way back when the volcanoes erupted
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57 .and when the asteroid crashed into it.
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58 .Yes, we\'ve thought of that.
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59 .But in order to generate a global magnetic field,
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60 .we believe a planet or a moon needs to have a core, one like Earth has
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61 .now, the moon doesn\'t have such a core today.
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62 .But you know, we can\'t say for sure that it didn\'t have one long ago.
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