OFFICIAL35 What does the professor mainly discuss?

[00:00.00]Narrator: Listen to part of a lecture in an Earth science class.
[00:04.96]MALE PROFESSOR: Let's review something from last week. [00:07.42]We talked about an event that happened 65 million years ago. [00:10.97]Anyone?[00:11.76]FEMALE STUDENT: An asteroid hit Earth. [00:13.11]Um...well, we think an asteroid hit Earth, near the Yucatan Peninsula, in Mexico, and that wiped out all the dinosaurs.[00:20.22]MALE PROFESSOR: Right. I wouldn't say that we've got 100% proof, but there's very strong evidence that this is why that mass extinction occurred. [00:28.45]Okay. But did you know there was an earlier extinction far greater than the one that killed off the dinosaurs? [00:35.32]It was what we call the Permian Extinction.[00:38.56]Now, way back about 290 million years ago, at the beginning of the Permian Period, there was just one big continent, a super continent. [00:49.27]And as the climate warmed up, plant and animal species began to diversify profusely. [00:55.01]So life during the Permian Period was abundant and diverse.[00:59.19]But about 250 million years ago, the Permian Period ended with a rapid mass extinction, something happened that wiped out 75% of the land animals and over 95% of ocean life. [01:13.56]So what was it? [01:15.21]What could have caused this. [01:16.77]Well, with the all the evidence that it was an asteroid that led to the dinosaur extinction, we began asking ourselves: is it possible that another asteroid much earlier caused the Permian Extinction? [01:29.42]And so researchers have been looking for an impact crater.[01:33.44]FEMALE STUDENT: I thought the Permian Extinction was caused by a decline in sea water oxygen levels. [01:38.72]Isn't that what's in the textbook?[01:40.55]MALE PROFESSOR: But don't forget the textbook makes it very clear that's only a theory.[01:44.93]FEMALE STUDENT: And it mentions something about volcanic eruptions too.[01:48.02]MALE PROFESSOR: It does, but now this new theory has led to a search for evidence of an asteroid impact.[01:53.71]And one place of interest is a region called Wilkes Land in eastern Antarctica.[01:59.71]A few years ago, a researcher reported a strange anomaly beneath the ice in Wilkes Land. [02:05.10]Evidence of what may be a mascon.[02:07.50]That's just short for mass concentration. [02:11.06]When an asteroid hits Earth, when it slams into Earth's crust, we think that causes molten rock from deep below the surface to rise up into the impact area. [02:20.98]Sort of like if you bump your head, you get a big lump under the skin. [02:24.48]Fluid makes the area swell. [02:26.57]Anyway, the material flowing up from below the crust is more dense than the crust itself. [02:32.06]So that's how we get a mascon, a spot in the crust with newer crust material that's more dense than the material all around it. [02:39.69]There're lots of mascons on the moon too, where a mascon's density causes a small increase in the local gravity that can be measured and mapped by orbiting spacecraft. [02:50.03]And where do these mascons tend to be found? [02:53.11]In the centers of impact craters on the moon's surface. [02:57.45]But back to Wilkes Land. [02:59.38]We're not certain that the mascon there...what might be a mascon ...was actually caused by the impact of an asteroid, but there does seem to be evidence. [03:09.83]Researchers notice a gravity anomaly similar to those on the moon. [03:14.59]And the spot where the gravity readings are especially high...this is right in the middle of a 500-kilometer wide, circular ridge, what could be part of an old impact crater. [03:25.09]And if there was an asteroid impact there in Wilkes Land, the next question is: did it happen 250 million years ago?[03:33.29]Because that would put it when in geologic history?
[03:36.61]FEMALE STUDENT: At the end of the Permian Period? Right when those animals went extinct.[03:40.24]MALE PROFESSOR: Exactly.
[03:41.65]FEMALE STUDENT: But can't researchers figure that out by studying the rocks there in Wilkes Land...where this impact supposedly took place?[03:49.70]MALE PROFESSOR: Well, to get to anything from that long ago, we would have to drill down to about a mile, about 1.6 kilometers of solid ice that covers the area today. [03:59.23]And that's not likely to happen. [04:01.11]But speaking of rocks, I should mention that Wilkes Land is not the only place of interest here. [04:07.43]There's another called the Bedout High off the coast of Australia. [04:11.82]And we have rock samples from the Bedout High. [04:15.19]Some apparently have extraterrestrial origin. [04:17.96]I mean, they show the effects of extreme temperatures and pressures, the level of extremes produced only by an impact. [04:25.80]And as for their age, well, they do in fact, date back to about 250 million years ago.