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listen to part of a lecture in an astronomy class.

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Last class, we were discussing Earth\'s global magnetic field.

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Now, scientists still don\'t fully understand the source of Earth\'s magnetism

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or the forces that make up this magnetic field surrounding our planet,

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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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Yes, Fred,

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you know, I just read something about magnetic fields of the moon.

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The early astronauts detected magnetic fields there.

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So does the moon have a global magnetic field too?

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No, it doesn\'t, but it does have regions of magnetized rock that produce local magnetic fields.

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Some are weak, but others are quite strong.

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Local magnetic fields.

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Yes, these local magnetic fields are scattered across the moon\'s surface.

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The weak fields, we think these originated from volcanoes that erupted early in the moon\'s history.

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See, after molten lava flowed out from a volcano,

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it cooled and solidified into rock and became magnetized.

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But the material composing lunar lava isn\'t highly magnetic,

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so this hardened lava near the volcanoes only generates weak magnetic fields.

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Now, the areas of strong magnetism are especially puzzling.

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You see, they\'re not associated with any geologic features like volcanoes,

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and we really can\'t explain why these areas have such highly magnetic rock

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as I just mentioned, moon rock in general, just isn\'t very magnetic.

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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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but that exceeds the thickness of the entire lunar crust.

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So the origin of these strong, local magnetic fields has us stumped,

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and we\'ve been trying to explain this since 1969

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since the early moon missions you alluded to

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but a recent study may provide some answers.

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A team of scientists noticed an important correlation between the largest grouping of strong local magnetic fields on the moon

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and an area within the South Pole Aitken basin.

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The South Pole Aitken basin is a huge crater

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about half the size of the United States.

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It was formed when an asteroid crashed into the moon.

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And what do we know about the composition of asteroids? Laura,

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they contain lots of iron,

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yes, and other highly magnetic material.

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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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which suggests that the asteroid hit the moon at an angle to form the basin.

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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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and in fact, the largest collection of strong local magnetic fields is located on one end of the basin,

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at the northern rim.

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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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and the strong magnetic fields in areas further away from the impact basin

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could be explained by materials from the asteroid that were ejected and dispersed following the impact.

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And we know that large impact events were common during the early evolution of the solar system,

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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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Okay, but wait, but that hypothesis doesn\'t explain

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I mean, so how do those rocks from the volcanoes and asteroid, you know,

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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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but you said there\'s no global magnetic field on the moon.

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So how did that happen?

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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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Okay, so this asteroid hypothesis only addresses the problem of how the strongly magnetic rock got to the moon,

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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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Fred, so,

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maybe the moon used to have a global magnetic field like Earth\'s way back when the volcanoes erupted

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and when the asteroid crashed into it.

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Yes, we\'ve thought of that.

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But in order to generate a global magnetic field,

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we believe a planet or a moon needs to have a core, one like Earth has

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now, the moon doesn\'t have such a core today.

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But you know, we can\'t say for sure that it didn\'t have one long ago.

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