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NARRATOR:Listen to part of a lecture in an Earth science class.
MALE PROFESSOR:Let's review something from last week.We talked about an event that happened 65 million years ago... Anyone?
FEMALE 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.
MALE PROFESSOR:Right, uh, I wouldn't say that we've got 100 percent proof, but there's very strong evidence that this is why that mass extinction occurred.OK, 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 supercontinent.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 percent of the land animals and over 95 percent of ocean life.
So what was it?What could have caused this?Well, with 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.
FEMALE STUDENT:I thought the Permian extinction was caused by a decline in seawater oxygen levels.Isn't that what's in the textbook?
MALE PROFESSOR:But don't forget, the textbook makes it very clear-that's only a theory.
MALE STUDENT:And it mentions something about volcanic eruptions, too.
MALE 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 are 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 noticed 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?
At the end of the Permian period, right when all those animals went extinct.
Exactly.
But... can't researchers figure that out by studying the rocks there in Wilkes Land... where this impact supposedly took place?
Well, to get to anything from that long ago, we'd have to drill down through 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 of 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!
旁白：请听一段地球科学课的讲座的节选。
教授：我们先复习一下上周讲过的东西。我们讲到了6500万年前发生的一个事件...有人记得吗？
学生：小行星撞击了地球。嗯……我们认为是有一个小行星撞击了地球，在墨西哥的尤卡坦半岛（Yucatan Peninsula）附近，这个事情使得恐龙灭绝了。
教授：对的。我不能说我们能百分百确定，但是的确有强有力的证据证明这就是大灭绝发生的原因。好的。但是你们知道吗，在恐龙灭绝的那场大灭绝之前，还有一场更大的灭绝。它被我们称为二叠纪大灭绝。
现在，追溯到29000万年前，在二叠纪的开端，当时只有一个大洲，一个超级大洲。然后随着气候变暖，动植物开始变得多样化。所以二叠纪的物种是数量丰富且种类繁多的。
但是在25000万年前，一场迅速的物种大灭绝终结了二叠纪某些事情一次性使得75%的陆地动物和95%的海洋生物灭绝。
所以这到底是什么事情呢？什么导致了这场灭绝呢？所有的证据显示，小行星撞击地球导致了恐龙的灭绝。我们就开始问自己了：可不可能是更早时候也有一颗小行星撞击了地球，导致了二叠纪的灭绝？研究者们就开始寻找陨石坑。
学生：我以为二叠纪大灭绝是由海水氧气含量降低导致的。这不是课本上写的吗？
教授：但是别忘了，课本上也写了这仅仅是个理论。
学生：课本上也提到了一些关于火山爆发的事情。
教授：是的，但是现在的新理论导致了一场寻找陨石坑证据的活动。有趣的是，一个南极洲东边叫做威尔克斯区(Wilkes Land)的地方。
几年前，一个研究者报告说威尔克斯区(Wilkes Land)下面的冰层有异常情况，可能是一个质量密集现象。这是质量浓度的简写。
当小行星撞击地球时，它撞入地球的外壳，我们认为它会导致地球内部的熔化的岩石上升冲入陨石坑,就像是你撞了头，你的皮肤下面就会出现肿块，液体使得那块地方水肿了。不管怎样，从地球内部流上来的液体比地壳本身密度更高。我们这样就得到了一个陨石坑，它就是一个地壳上的地点，比周围的物质更新，密度更高。
月球上也有很多的陨石坑,这些陨石坑导致其周围地区的引力变大，进而可以通过沿轨道运行的航天器来测量和定位。那么哪里可以找到这些陨石坑呢？可以在月球表面的陨石坑中心找到它们。
让我们回到威尔克斯区(Wilkes Land)。我们并不确定有陨石坑……不确定什么可能是一个陨石坑……不确定这个坑确实是因为小行星撞击形成的，但是我们的确有一些证据,研究者们注意到了类似于月球上的重力异常现象。在那些重力特别大的地点……刚好在500千米的一个圆形山脊的中间，那就可能是一个老的陨石坑了。
如果真的在威尔克斯区（Wilkes Land）里有个陨石坑，那么下一个问题就是：它是在25000万年以前形成的吗？因为这个问题会帮我们判断它所处的地质历史位置。
在二叠纪末期？正好是在那些动物灭绝的时候。
确实。
但是研究者不能通过研究威尔克斯区（Wilkes Land)里面的岩石来确定这个时间……这个小行星撞击发生的时间？
为了从那么久远以前发现一些东西，我们需要深入地下大约一英里，大致是1.6公里，冲破覆盖这片地区的坚硬冰层去探索。这个是不大可能的。
但是说起岩石，我必须提到，威尔克斯区(Wilkes Land)并不是唯一让研究者感兴趣的地点。还有一个叫做Bedout High的地方，远离澳大利亚的海岸。
我们有那里的岩石样本,这些岩石样本里有一些来自地球外的物质。我的意思是，它们展现出极度高温和高压的影响，这种情况只会在小行星撞击地球的时候出现。至于它们的年龄嘛，实际上可以追溯回25000万年前。
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