TOEFL Listening: ETS-TOEFL听力机经 - QU3X4SY4BB3P1E4OQ$

Lecture: Magnetic field: Narrator: Listen to part of a lecture in an astronomy class. Professor: Last class, we were discussing Earth's global magnetic field. Now, scientists still don't fully understand the source of Earth's magnetism or the forces that make up this magnetic field surrounding our planet. But they hypothesize that the motion of the fluid in Earth's outer core has a lot to do with it. Uh, Yeah. Fred? Fred: You know, I just read something about magnetic fields of the Moon. The early astronauts detected magnetic fields there. So does the Moon have a global magnetic field too? Professor: No, it doesn't. But it does have regions of magnetized rock that produce local magnetic fields. Some are weak, but others are quite strong. Fred: Local magnetic fields? Professor: Yes. These local magnetic fields are scattered across the Moon's surface. The weak fields, we think these originated from volcanoes that erupted early in the Moon's history. See, after molten lava flowed out from a volcano, it cooled and solidified into rock and became magnetized. But the material composing lunar lava isn't highly magnetic, so these hardened lava near volcanoes only generates weak magnetic fields. Now the areas of strong magnetism are especially puzzling. See they're not associated with any geologic features, like volcanoes, and we really can't explain why these areas have such highly magnetic rocks. As I just mentioned. Moon's rock, in general, just isn't very magnetic. Scientists actually calculated that it would take a layer over one hundred kilometers thick of typical Moon's rock to produce the strong magnetic fields we're seeing in these areas, but that exceeds the thickness of the entire lunar crust. So the origin of these strong local magnetic fields has us stumped. And we've been trying to explain this since 1969, since the early Moon missions you alluded to. But a recent study may provide some answers. A team of scientists noticed an important correlation between the largest grouping of strong local magnetic fields on the Moon and an area within the south pole Aken basin. The south pole Aken basin is such a huge crater, about half the size of the United States. It was formed when an asteroid crashed into the Moon. And what we know about the composition of asteroids? Lorra? Lorra: They contain lots of iron. Professor: Yes, and other highly magnetic materials. And this huge basin is shaped sort of like an oval. It's kind of elongated in the north south direction, which suggests that the asteroid hit the Moon at an angle to form the basin. Now if this were the case, most of the magnetic materials from the impact would land on one end of the basin. And in fact, the largest collection of strong local magnetic fields is located on one end of the basin at the northern rim. So we hypothesized that the rock that's generating the strong local magnetic fields was brought to the Moon by the south pole Aken asteroid. And the strong magnetic fields in areas further away from the impact basin could be explained by materials from the asteroid that were ejected and dispersed following the impact. And we know that large impact events were common during the early evolution of the solar system. So this same scenario could also explain some of the other groupings of strong magnetic fields we see on the Moon. Lorra: Okay. But ... wait ... but that hypothesis doesn't explain ... I mean, so how do those rocks from the volcanoes an asteroid, you know, get magnetized? I mean, I remember learning that rocks can only become magnetized if there's already a magnetic field present. But you said there's no global magnetic field on the Moon. So how did that happen? Professor: Yes, it does remain a mystery how the rock that produced the week and strong magnetic fields we see today became magnetized in the first place. Lorra: Okay, so this asteroid hypothesis only addresses the problem of how the strongly magnetic rock got to them. Professor: Right. And we're still left with how that material and the weakly magnetic rocks that matter then became magnetized. Fred? Fred: So, maybe the Moon used to have a global magnetic field like Earth, way back when the volcanoes erupted and when the asteroid crashed into it? Professor: Yes, we thought of that. But in order to generate a global magnetic field, we believe a planet or a Moon needs to have a core, one like Earth has. Now the Moon doesn't have such a core today, but you know, we can't say for sure that it didn't have one long ago.