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官方真题Official 35-L4

Narrator:

Listen to part of a lecture in an Earth science class.

Professor:

Let's review something from last week. We talked about an event that happened 65 million years ago. Anyone?

Student:

An asteroid hit Earth. Um...well, we think an asteroid hit Earth, near the Yucatan Peninsula, in Mexico, and that wiped out all the dinosaurs.

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.

Okay. But did you know there was an earlier extinction far greater than the one that killed off the dinosaurs? It was what we call the Permian Extinction.

Now, way back about 290 million years ago, at the beginning of the Permian Period, there was just one big continent, a super continent. And as the climate warmed up, plant and animal species began to diversify profusely. So life during the Permian Period was abundant and diverse. 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.

So what was it? What could have caused this?

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? And so researchers have been looking for an impact crater.

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? And so researchers have been looking for an impact crater.

Student:

I thought the Permian Extinction was caused by a decline in sea water oxygen levels. Isn't that what's in the textbook?

Professor:

But don't forget the textbook makes it very clear that's only a theory.

Student:

And it mentions something about volcanic eruptions too.

Professor:

It does, but now this new theory has led to a search for evidence of an asteroid impact. And one place of interest is a region called Wilkes Land in eastern Antarctica.

A few years ago, a researcher reported a strange anomaly beneath the ice in Wilkes Land. Evidence of what may be a mascon. That's just short for mass concentration.

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. Sort of like if you bump your head, you get a big lump under the skin. Fluid makes the area swell. Anyway, the material flowing up from below the crust is more dense than the crust itself. 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.

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. And where do these mascons tend to be found? In the centers of impact craters on the moon's surface.

But back to Wilkes Land. 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. Researchers notice a gravity anomaly similar to those on the moon. 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.

And if there was an asteroid impact there in Wilkes Land, the next question is: did it happen 250 million years ago? Because that would put it when in geologic history?

Student:

At the end of the Permian Period? Right when those animals went extinct.

Professor: Exactly.

Student:

But can't researchers figure that out by studying the rocks there in Wilkes Land...where this impact supposedly took place?

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. And that's not likely to happen.

But speaking of rocks, I should mention that Wilkes Land is not the only place of interest here. There's another called the Bedout High off the coast of Australia. And we have rock samples from the Bedout High. Some apparently have extraterrestrial origin. I mean, they show the effects of extreme temperatures and pressures, the level of extremes produced only by an impact. And as for their age, well, they do in fact, date back to about 250 million years ago.

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