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We should move on now from our discussion on Mars and speak a bit about Mercury and the moon. There are a lot of features that we can compare between the two . . . that′s probably the best way to understand them.

The second - I mean, the first thing I want to talk about today is a comparison between the, uh, the composition of - wait, not the composition – I mean the surfaces of the two bodies, in terms of some of the easily perceivable physical features. We′ll start with the moon. Many of you have probably viewed the moon through a telescope. You′d have seen large dark areas that appear in many places on the surface. These were previously thought to be oceans - like our oceans. They were even given the name maria, which is Latin for seas. They′re actually not oceans at all. They′re flat areas that were caused by lava flow long ago in the moon′s history.

Now, on the other hand, Mercury doesn′t really have any of these maria to speak of. So what does this suggest? Well - and this is more of a side note - um, it suggests that lava flow wasn′t really very routine in Mercury′s history . . . at least not to the extent that we see it on the moon. Any questions about that? OK, now moving on to a similarity, well, this is again something you′ve probably noticed about the moon: craters - craters being the big circular marks . . . the holes on the surface of the moon. At first glance, Mercury and the moon are very similar in this sense. Both surfaces are simply covered in craters. But why? Well, it all has to do with the lack of an atmosphere.

Um, I really don′t want to get into too much depth on this, but here on Earth, if a meteorite enters our atmosphere, it usually burns up before it can collide with the surface. Well, Mercury and the moon don′t have atmospheres like the Earth does. So, well, there′s really nothing to stop the meteorite from slamming into the surface. Don′t worry too much about that - it′s a different discussion. Just remember that Mercury and the moon look quite similar because of their craters.

 

Earlier, I mentioned the maria on the surface of the moon that you don′t really see on Mercury. Well, to be fair to poor Mercury, we should mention something that it has that the moon doesn′t. One such feature is something called a scarp. You can think of a scarp as a cliff - uh, maybe a raised area is a better way to put it - well, OK, the best way to understand it is to think of a wrinkle on your skin. That′s what these look like. And, well, the moon doesn′t really have anything analogous to that. So, why would these form on Mercury? Well, scientists don′t know for certain, but they suggest that after Mercury formed, the cooling process the planet undergo, I mean, underwent, caused the surface to shrink. This shrinking caused these wrinkles to form . . . just like loose skin, actually.

1. What is the main topic of the lecture?
A. A comparison of the surfaces of a planet and the moon
B. A comparison of the cause of craters on two planets
C. A comparison of how two celestial bodies formed
D. A comparison of the atmospheric content of two planets
Explain:
2. What aspect of the moon′s surface does the professor mainly discuss?
A. The raised cliffs and how they were formed
B. Two of the most visible physical features
C. The flat areas caused by meteorite impacts
D. How it affects the moon's atmosphere
Explain:
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