TPO57听力Lecture2原文+MP3+题目+答案下载

TPO57听力Lecture2原文+MP3+题目+答案下载

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TPO57听力Lecture2原文：

Listen to part of a lecture in an astronomy class.

Professor:

Traditionally, astronomers worked out how old geologic features of planets and moons are by the number of marks on the surface.The more craters in the surface, say, on a lava flow, the more asteroids and comets that place encountered over time, so the older it must be.This seems to make sense for relative age.That is, a surface feature with fewer craters is younger than one with more craters.

But absolute age, actual age, is trickier. We have to know exactly how old one surface is.For example, we do have a very clear idea of the ages of some surfaces of the moon from rocks we brought back and then this information can allow us to extrapolate the age of another surface that has a similar concentration of craters. That's the traditional way to calculate it.

But two developments have brought this traditional way into question.For one, a recent study of the craters on one of Jupiter's moons, Europa, suggests that at least 95% of its small craters were formed by secondary impacts.Ok, secondary impacts. They're the impacts of the chunks of rock or ice that break off as a result of the primary impact.The primary impact refers to the impactor itself, asteroid, comet, hitting the planet or moon, and when that happens,pieces of rock or ice break off and go flying and when those chunks come back down and smash into the planet, those are the secondary impacts.

So using the old way we would have assumed that the surface of Europa is much older than it might actually be.And it's conceivable that a very large strike from an impactor might throw out some fairly large chunks, ones that are larger than some of the smaller direct strikes.So we can't use size to determine if a crater is the result of a primary impact or a secondary one. And of course impactors come in different sizes, though actually we think there are fewer small ones than there used to be.

。。。。余下原文省略

TPO57听力Lecture2题目：

1、What is the lecture mainly about? 

A The formation and physical characteristics of craters 

B Methods for determining the age of impact craters 

C Differences between craters on planets and moons in the inner solar system 

D Problems with using craters to date surfaces of planets and moons

2、Why does the professor mention rocks from Earth's moon? 

A To compare the surface of Earth's moon to the surface of a moon of Jupiter 

B To stress the need for further exploration of space 

C To explain a way of calculating the absolute age of surfaces of planets or moons 

D To show how to identify a secondary impact on the surface of Earth's moon

3、According to the professor, what are secondary impacts? 

A Impacts of fragments thrown outward by a previous impact 

B Impacts of objects from outside the asteroid belt 

C Impacts of smaller asteroids in the asteroid belt 

D Impacts in areas of a moon or planet other than those primarily studied

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