Listening audioscript

WHY DOES THE PROFESSOR SAY THIS: 
 (Professor) 
 
Let me just reiterate the importance of that for future reference. 
 
PASSAGE FOUR 
 
P
age 173
[ mp3 041-042] 
Questions 14 through 19. 
Listen to part of a lecture in an astronomy class.
(Professor) 

The Giant Impact Model, or Theory, as it is described in the media…. 
This hypothesis explains a lot of things about the Earth-Moon system 
and about the composition of the two bodies. Now, we’re going to delve a 
little deeper into the model itself in a minute, and I’ll put up the pictures, 
but let me just explain the model in brief and give you some of the 
evidence in support of the model. Then, I’ll provide you with evidence 
that does not support the model. That will finally lead me to my primary 
point in this lecture, which is why I’m calling this a model, and not a 
theory. 
 
So, the idea is that soon after our solar system formed, there were a lot 
of planet-sized objects orbiting around the sun, some of which were 
orbiting at the same distance from the sun. As planets began to acquire 
greater mass, their stronger gravitational fields began to disturb their 
orbits, and send some of them smashing into each other. The Giant 
Impact Model postulates that a planet the size of Mars, called Theia, was 
one of these doomed planets.

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LPREP IBT 3 E AudioScript 
29
 
Oh, and incidentally, Theia was named after the Greek Titan, believed to 
be the earth goddess of sight and the light of the sky, who gave birth to 
the moon goddess, Selene. So there’s a bit of trivia for you that I 
personally find very fitting. Whether you think it’s clever or not, you 
should be thankful. Certainly, the name Theia is more suggestive and 
memorable than the series of digits and letters that are sometimes used 
to name outer-space objects. 
So, anyway, back to what I was describing. And, you know what? Let 
me just get this picture up now, because it’ll make more sense. I was 
going to wait until we started talking about the math involved in the 
model, but you might as well see what I’m describing.
OK, so here you can see it. Theia crashed into the object that would 
become the Earth, the proto-Earth, at something like a 45 degree angle, 
vaporizing … um, completely burning off the outer layers of both planets 
and sending the iron core at the center of Theia into the core of the 
Earth. Then, very quickly, some of the material that had been vaporized 
and blown off came together again on the Earth and some also formed 
into a ball that became the moon.
So, both planets would then have been transformed into molten hot 
liquid balls, with the Earth retaining most of the iron cores of both the 
proto-Earth and Theia, and the moon being composed of the outer layers 
of rock from the crash and much less iron.
One of the first bits of evidence used for the Giant Impact Model was this 
difference in iron between the two. When the Apollo astronauts brought 
back samples of moon rocks and data, they indicated that the moon 
does, indeed have much less iron than the Earth. This evidence actually 
predates the Giant Impact Model, and it was this observation that got 
people thinking about what could explain this difference in the first place.
Now, why does the iron core matter? Well, it turns out that without this 
extra massive iron core the Earth would not have the strong magnetic 
field that it does. And it’s this magnetic field that deflects a lot of the sun’s 
harmful radiation away from the Earth’s surface. So, life on this planet 
would have developed quite differently, if at all, had it not been for this 
protective field. And the iron core matters for other reasons … but I’m 
getting more off topic than usual here.
Another piece of evidence is the computer model of the Giant Impact 
itself. The fact that you can calculate various collision scenarios using the 
laws of physics and tremendous number calculating power … um, I 
mean, that you can get exactly the results that this hypothesis predicts 
using the computer model … this is evidence in and of itself. Now, before 
you go swallowing this whole story hook, line, and sinker, let me qualify 
some of the evidence that might change your mind. And that will lead me 
to a point about theories. 
There is, in fact, evidence that does not support the Giant Impact Model. 
For one thing, there is no evidence that the entire Earth was ever a giant 
ocean of molten hot liquid rock, as this model predicts. Now … there are 
several other pieces of evidence that support the model, but I think it’s 
safe to say that the evidence is not overwhelming enough to call this a 
theory.

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LPREP IBT 3 E AudioScript 
30
But, um … the specific point I want to make right now has to do with the 
difference between a theory and a model or hypothesis as these terms 
are used by scientists. You see, most non-scientists think that a theory is 
a hypothesis, or a guess about the world. This is not true at all. In order 
for a hypothesis or model to qualify as a scientific theory, it must explain 
what has been observed and tested repeatedly. That does not mean that 
there are no problems at all, or…or no mysterious phenomena that 
seem to defy the theory. In general, minor irregularities do not disprove 
theories, but instead define gaps in our knowledge … gaps that may 
eventually be explained by later evidence or developments of the theory.
So, OK, to apply this to our current model. The Giant Impact Model… not 
Theory, though it may eventually be called that without question, explains 
a lot of what we understand about our Earth and moon. However, it does 
not explain everything. If we find further evidence that does not support 
the model, we may eventually be forced to reject it. On the other hand, if 
evidence continues to accumulate and further explanations are found for 
any irregularities or anomalies…eventually most people will accept the 
model as accurate and it will then, without doubt, bear the title of Theory.