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So how do we know
that there is a core, and that the core is made
up of a liquid outer core and a solid inner core? And the answer there comes
from the same technique that we saw Mohorovicic use
in 1909 to essentially see the behavior, or when you
measure the seismic waves, or whether you can even
measure the seismic waves, the different distances
from an earthquake. So if there's an
earthquake right here. We're calling that zero degrees. Let's remember a
couple of things here. Let's remember that P-waves
can travel through anything. They can travel through solid or
liquid or air for that matter. So they can travel
through anything. But S-waves, S for secondary,
these are the transverse waves, these can only travel
through solids. So it turns out that if
an earthquake happens at zero degrees, and you
had seismograph stations all over the world, and
these are extremely sensitive in order to be able to measure
earthquakes that are happening thousands of kilometers
away, it turns out that there's something called
an S shadow, an S-wave shadow. If these are S-waves you
can measure them here. You can measure them here. They can go all
the way over here. They can go over here. They can go over there. You can measure them over here. So you could measure them
at all of these points, but then all of a
sudden at 105 degrees, and so we're measuring
zero degrees here and we're going
outwards like that, all of a sudden at 105
degrees and further you stop measuring S-waves. For some reason you would
think that some of the S-waves would get over here, maybe they
would be a little bit weaker, but they would be able to
get all the way over here. But they just abruptly stop. No more S-waves. So in this whole area right
over here you get no S-waves. And obviously I could
flip this picture over and you would see
a symmetric thing on the other side of the globe
that all of this area over here you also would not see S-waves. You'd only see them from 105
degrees in this direction and 105 degrees
in that direction. And the only reasonable
explanation that we can give is that there must be some material
that an S-wave cannot travel through that it would have to
travel through to get to these points beyond 105 degrees. And we know that S-waves
only travel in solids. So the assumption there is
that at some point beyond 105 degrees it's hitting liquid. So that's what tells
us that this right here is probably a liquid. It's hitting some
layer that is liquid. So that tells us
that there's a core, and at least the outer part
of that core is liquid, enough to stop S-waves. So the S-waves, because
it only travels in solids it leads to this S-wave shadow. And this tells us
that we have a core. And that core, at least
the outer part, is liquid. We don't know yet whether the
inner part is liquid or solid. Now, the next point
of evidence is how do we know that
there's an inner core? And we can use P-waves for that. A P-wave can travel
through anything, but remember, in general for
the same type of material if you get denser material
it's going to move faster, so it's going to
refract outwards like we've seen over here. But if it goes into a liquid,
in general, sound waves, or I should say P-waves, seismic
waves move slower in liquids. And so the refraction
patterns we get when we do measure
from seismograph stations around the world is that it
looks like the P-waves are kind of doing what you would
expect in the mantle, but then they're
getting refracted as if they're going
to a slower medium as they go through
the outer core. And we see that right over here. And then they get
refracted again to get to some point
on the other side. Now, that is just what you would
expect if it was all liquid, but if you go to stations
that are even further out it looks like, if you just look
at the refraction patterns, and you can now model
this with fancy computers and get all the data
points, but you could say, well, the only way that reality
can fit the data that we get based on when
things reach here is if the P-waves are
being first refracted through the outer core, but
then they're refracted in a way that they're going through
denser material, significantly denser material
than the inner core. And then they're just
continuing to refract the way you would expect. So it's really the refraction
pattern of the P-waves. And frankly, the
fact that there's this what you call
a P-wave shadow. The P-wave shadow
by itself, all that tells you is that kind
of roughly crazy things are happening
someplace in the core. But the real way to know that
we have an inner core that's solid, as opposed to the
whole thing being liquid, is that the P-waves
is the pattern of when and how
the P-waves reach essentially the other
side of the globe. And then you can kind
of, based on modeling how waves would travel
through different densities and different types of
mediums, you could say, well, there's got to be an
inner core right over here. And obviously, it's a lot
more math than I'm going into. But if you do the math
based on the shadow, and you know the
speed of the material, and all of that type
of thing, then you can figure out the depth at
which these transitions occur. We know that we have a
transition from mantle to outer core here. And then a transition from
outer core to core there. So hopefully that
satiates your questions about how do we know what
the composition of the earth is without ever having
to dig down there, because we've never even
gotten below our crust.