Speaker 1: Imagine for a second that you're in the most pristine
laboratory you can think of. You've got instruments that are just,
you know, perfectly tuned to pick up the slightest vibration,
a tremor, a crack, anything.
Speaker 2: Right, the quietest room in the universe exactly.
Speaker 1: Now, what if you decide to run the most powerful
controlled experiment you can possibly imagine? You take a massive
two and a half ton spacecraft and you deliberately crush
it right into your subject.
Speaker 2: Well, if that subject were Earth, you'd expect a pretty
clear result. You'd get a big shockwave. Sure, it'd rattle
a few size.
Speaker 1: Mix stations, and then it would die down.
Speaker 2: And then it would die down pretty quickly. Actually, yeah,
our planet is just it's messy. It's full of heat
and water and friction. It's really really good at dampening energy.
It cracks, it settles, it moves on.
Speaker 1: But the Moon, our ancient, silent neighbor, it did something
completely different. It defied every single expectation. When NASA slammed
the Apollo twelve lunar modulus sent stage into the This
was November twentieth, nineteen sixty nine. The seismic event that
followed didn't last for a few minutes even close. It
just kept going for nearly an hour. It was this low,
persistent hum that I'm sure both terrified and absolutely exhilarated
the scientists back and mission control.
Speaker 2: Oh for sure. And that one event, I mean, it
produced this phrase that just it exploded. It became part
of space history, but it also fueled five decades of
conspiracy theory. Famous quote, famous quote. NASA reported that the
moon rang like a bell. It didn't crack like a rock.
It resonated like a musical instrument.
Speaker 1: And that profound mystery, why did the moon ring? That's
our mission today on the Deep Dive. We're going to
go so much deeper than just that quote. We're going
to investigate the origins of this this whole pseudo scientific
spaceship moon hypothesis that the event launched.
Speaker 2: And then we're going to use the hard data, the
really hard one data from the Apollo seismic experiments all
the way up to the super precise gravity map from
the Grail mission, and.
Speaker 1: We're going to reveal the actual complex and differentiated structure
of our Moon, which it turns out, is a reality
that is far stranger and honestly, far more fascinating than
the fiction.
Speaker 2: It really is. So it's time to settle the score.
Is the Moon some kind of engineered artifact or is
it just an extraordinary natural phenomenon. Let's dive deep into
that seismic serenade.
Speaker 1: So the Apollo program, I mean everyone remembers it for
the human achievement for one giant leap, but it was
also maybe even more importantly about laying the foundation for
lunar geology.
Speaker 2: Oh absolutely, we had to know what the Moon was
made of, what was going on under the surface, and
for that, Apollo twelve gave us the first and most
critical setup.
Speaker 1: That's right. So when the crew Pete Conrad and Alan
Bean landed in the Ocean of Storms this is November
sixty nine, a huge part of their mission on the
surface was setting up the Apollo Lunar Surface Experiments package.
And the centerpiece of that package, at least for our
discussion today, is the Passive Seismic Experiment or PSC.
Speaker 2: That's the one.
Speaker 1: This thing wasn't just some off the shelf instrument they
picked up, right, This was I mean bespoke, high precision
engineering sent a quarter of a million miles from Earth.
What exactly went into this unit.
Speaker 2: It was serious hardware. It weighed about eleven point five
kill grands or twenty five pounds, and it was housed
in this specially designed thermally controlled box.
Speaker 1: To protect it from the temperature swings exactly.
Speaker 2: The lunar day and night cycle is just brutal. We're
talking hundreds of degrees of difference, so it had to
be protected. They used a lot of brilliant in its
construction because it's super lightweight but also incredibly rigid. But
the real genius, the real magic, was the sensor array itself.
Speaker 1: Okay, so tell us about the sensors. What were they
listening for.
Speaker 2: The main goal was to map the inside of the moon,
so they needed sensors they could hear, you know, both
the quick, sharp things happening on the surface and the deep,
slow groaning stresses.
Speaker 1: From within a pretty wide range.
Speaker 2: Wide range. So they had three matched long period seismometers
LP seismometers.
Speaker 1: And they were aligned how.
Speaker 2: Orthogonally, so one vertical, two horizontal. These LPs were tuned
to pick up very very slow moving waves, the kind
of thing you'd get from say, deep moonquicks caused by
the title pull from Earth or even you know, rumblings
from the core and mantle.
Speaker 1: So that's three sensors. What was the fourth one.
Speaker 2: The fourth was a short period or SP seismometer. It
was focused only on vertical movement, but it was sensitive
to much higher frequencies up to twenty hertz.
Speaker 1: And that one was for what surface impacts.
Speaker 2: Exactly, meteoroid strikes, any kind of surface level tectonic event.
The SP unit was vital for recording those. So in short,
they had this incredibly comprehensive array designed to turn the
entire Moon into a giant sensitive microphone.
Speaker 1: Okay, so you have your giant microphone setup. Now you
need to test it. You need a sound source. And
this brings us to that famous event, the calibrated impact.
It just sounds so reckless, crashing a piece of expensive
hardware like that. But this was essential science, wasn't it.
Speaker 2: It was absolutely crucial. Think about it. The scientists needed
a known input. They needed to know the exact amount
of kinetic energy delivered at a very specific point.
Speaker 1: So they could measure how the waves traveled from that.
Speaker 2: Point precisely to understand how the lunar crust transmits energy.
I mean, you could just wait for a natural event
like a random meteoroid impact. But that's that's haphazard. You
don't know the mass, you don't know the velocity.
Speaker 1: It's junk data, right, And it couldn't have been easy
to guarantee the kind of precision you'd need to call
a spent rocket stage a calibrated source. I mean they
had to hit a specific spot to get the best
readings for the seismometer.
Speaker 2: The precision was just yeah, it was astonishing. It was
a real testament to NASA's command of orbital mechanics at
the time. They used the spent lunar module ascent stage.
Speaker 1: So this is the part that carried Conrad and being
back up from the surface to the command module.
Speaker 2: That's the one They knew its exact mass down to
the gram, and they could calculate its velocity at the
moment of impact with incredible accuracy. For Apollo twelve, that
LM hit the surface at a staggering six thousand, forty
eight kilometers per hour.
Speaker 1: Wow, let me just translate that. That is nearly three
thy eight hundred miles per hour. That impact had some serious,
serious power.
Speaker 2: Oh yeah, it released energy equivalent to almost a ton
of TNT. It blasted a crater that they estimated it
was about nine meters or thirty feet wide. And here's
the really crucial detail for the experiment, the location. The location.
They managed to position the impact just one hundred and
thirty five kilometers or about eighty four miles away from
where the Apollo twelve sizic station was sitting.
Speaker 1: Close enough for a really strong signal.
Speaker 2: A strong, clear signal. That was the whole point.
Speaker 1: So the planning is perfect, the instruments are running, the
impact happens one hundred and thirty five kilometers away. What
was the first sign that they had just hit something
that was not a normal solid planet.
Speaker 2: The data started streaming back and it was immediately obvious
that something was fundamentally wrong or at least profoundly unique.
How so well on Earth, when a major shockwave hits
a seismometer, the seismogram spikes, You get this huge, sharp peak,
and then the energy just decays really rapidly. Within a
few minutes, maybe ten for a really massive earthquake, it's
basically gone.
Speaker 1: And the signal they got from the Moon.
Speaker 2: Was the total opposite. It must have been absolutely riveting
in Houston.
Speaker 1: I can only imagine it was just surreal.
Speaker 2: The signal didn't spike and then crash. It started small,
it slowly grew in size to reach its peak amplitude,
and then instead of dying, it just persisted for how long?
The initial reverberation from that Apollo twelve LM impact lasted
for over fifty five minutes, just shy of.
Speaker 1: An hour an hour from one impact.
Speaker 2: An hour, and some of the records from later larger
impacts like the mighty Apollo thirteen SIVB Booster crash was
a much bigger object. They suggested that the residual evaperations
went on for over three hours.
Speaker 1: Three hours of a low, continuous humming. That's what led
to the legendary quote.
Speaker 2: Right.
Speaker 1: NASA scientists were reporting that even after an hour had passed,
the smallest residual reverberations still hadn't completely stopped.
Speaker 2: And that, you know, journalistically, led to the perfect analogy.
The moon rang like a bell.
Speaker 1: And we should be really clear here because this is
where the misunderstanding starts. They didn't mean it made a
sound like a church bell's tone, right.
Speaker 2: It's about the physics, not acoustics. They meant that it's
physical structure resonated, it sustained that elastic wave energy for
a completely unprecedented amount of time. It was an extremely
slow decay of a seismic signal.
Speaker 1: A phenomenon that geophysicists call the seismic coda.
Speaker 2: The seismic coda, Yeah, the long tail of the vibration.
Speaker 1: So just to be one hundred percent clear for everyone listening,
because this gets so twisted when we talk about ringing.
We're talking about seismic waves, vibrations moving through rock. Right,
there's no atmosphere, so you couldn't actually hear anything.
Speaker 2: That's a vital distinction. Yeah, there's virtually no atmosphere, So
acoustic waves sound as we know it, can't travel. An
audible ding dong is completely out of the question.
Speaker 1: But seismic waves are a form of sound.
Speaker 2: They are they're longitudinal. In sheer waves, it's energy moving
through solid material, which we detect as vibrations. So the
ringing is just the perfect word to describe how those
seismic waves are bouncing and reverberating all through the Moon's interior.
It was acting more like a finely tuned, extremely rigid
tuning fork than a damp composite rock like Earth.
Speaker 1: Yeah, that makes sense. If you smash a piece of
say dry, rigid glass, you might get a high frequency ping,
but the energy dissipates really fast as it cracks, right,
But if you hit a massive, solid, rigid iron sphere,
it rings, it hums. The fact that the moon kept
humming for nearly an hour that immediately suggested an underground
structure that was fundamentally unlike anything we'd ever measured, and
that difference, of course, is what led to all the speculation.
Speaker 2: The moment that seismic data hit the public, especially when
it was packaged with that incredibly dramatic ring like a
bell analogy, it just created a vacuum, and it was
a vacuum that was perfect for pseudoscientific theories.
Speaker 1: Science had a genuine mystery on its hands, and the
fringe theories had an explanation ready to go. The quote
was everywhere. I remember seeing it in Popular Science magazine
in early nineteen seventy and in the public imagination. The
logic seems pretty simple. If a massive solid object resonates
uniformly like that instead of just breaking.
Speaker 2: And going silent, then it must not be solid.
Speaker 1: It must be empty inside. It must be hollow, and.
Speaker 2: That logical leap, as you put it, is the engine
that drives the entire hollow moon hypothesis, and that very
quickly evolved into the much more outlandish spaceship moon conjecture.
Speaker 1: The timing here is just so key, isn't it. This
is nineteen seventy We're at the absolute peak of the
Cold War. The space race is still hot, We're constantly
looking up wondering who's ahead.
Speaker 2: And we're consuming massive amounts of science fiction. The idea
of alien visitation was just it was in the air,
it was normalized.
Speaker 1: So this theory, the spaceship moon theory, it didn't actually
originate in the US, did it.
Speaker 2: No, Surprisingly, it came from the Soviet Union. In nineteen
seventy two Soviet authors Michael Volsen and Alexander Shcherbakov. They
published an article in Sputnik magazine.
Speaker 1: It was just sort of like a Soviet international propaganda digest, that's.
Speaker 2: A good way to put it. Yeah, And the title
of the article was is the Moon the creation of
alien intelligence?
Speaker 1: Well, that leaves very little to the imagination. What was
their central claim beyond just you know, the moon is hollow?
Speaker 2: Their core thesis was that the Moon is an ancient artifact,
a colossal spaceship or maybe some kind of observation satellite
that was placed very precisely into Earth's orbit by an
extremely advanced extraterrestrial civilization.
Speaker 1: And the ringing was their proof.
Speaker 2: The ringing was their proof of a massive manufactured outer
shell surrounding a huge central void. They even tried to
explain why it was there, suggesting it was some kind
of long term observation post or a supply station.
Speaker 1: But to make a theory like that compelling, you can't
just rely on one weird seismic anecdote. They had to
build a bigger case, and they did.
Speaker 2: They were clever. They synthesized several known but slightly anomalous
scientific observations about the Moon, and if you don't have
the deep geophysical context, these points seem highly highly compelling.
Speaker 1: So what were the main oddities they used to build
their argument.
Speaker 2: They leveraged four key points. The first one and the
most critical for their argument, was the Moon's low bulk density.
Speaker 1: Okay, let's just engage with the layman's logic here for
a second. The Moon's average density is about three point
three grams per cubic centimeter. Earth's average is five point.
Speaker 2: Five a huge difference.
Speaker 1: So why isn't that low number the ultimate smoking gun
for emptiness? From their perspective, it seems obvious.
Speaker 2: Well. Their argument is simple. If the Moon was formed
from the same stuff as Earth, it should be much
denser since it's so disproportionately light for its size. They
argue that a massive internal emptiness avoid is the only
way to account for the low mass.
Speaker 1: And to a casual absord the math seems to check out.
It suggests there's missing volume on the inside.
Speaker 2: Right, And they're not wrong that the Moon is light
compared to Earth. That difference is real and it does
need explaining.
Speaker 1: Okay, So that's point one. The second point they love
to bring up was the cosmic coincidence of the perfect eclipse.
This is probably the most emotionally appealing argument for a
non scientist.
Speaker 2: It's the ultimate piece of circumstantial evidence, isn't it. They
cite the astronomical fact, and even a scientist is revered
as Isaac Asimov called it a sheer astronomical accident that
the Moon is precisely fourteen hundredth the size of the.
Speaker 1: Sun and coincidentally it's also fourteen hundredth thus far away
from us exactly, which means which means, during a total
solar eclipse, the Moon perfectly snugly covers the Sun's disc.
It's a mathematically perfect fit, and it's unique in our
solar system.
Speaker 2: And the theorists argue that for a body of the
Moon's size to be at the exact orbital distance required
to pull off this spectacular optical illusion, well, that suggests
deliberate artific placement, not cosmic chance.
Speaker 1: That's a classic argument from design. It implies control planning, which,
if you're already thinking about alien intelligence, just seems to
validate the whole theory.
Speaker 2: Their third piece of so called evidence was more psychological.
It played on human emotion and the sheer strangeness of
this new frontier. They used astronaut anecdotes.
Speaker 1: So things the Apollo cruse said.
Speaker 2: Right, these astronauts were venturing into this stark, utterly unfamiliar world,
and they often used very evocative language to describe it.
Neil Armstrong called it magnificent desolation. Buzz Aldrin once mentioned
that the lunar surface had a strange mechanical feel under
the landing struts of the eagle.
Speaker 1: Ah. I see. So the emotional poetic descriptions of astronauts
experiencing a completely alien environment were taken literally stripped of
all contexts.
Speaker 2: Yeah, they were repurposed as circumstantial proof that they were
walking on a manufactured, engineered structure. Any mention of the
Moon being unnatural or alien was treated as hard evidence
of an artifact, And the.
Speaker 1: Final pillar of their argument was the compositional anomalies. This
ties back to the whole idea of an engineered outer shell.
Speaker 2: This is where they brought in the armour plating idea.
The basalt rocks brought back by the Apollo missions at
surprisingly high concentrations of titanium, often in a mineral called ilmanite.
Speaker 1: And titanium is incredibly strong and heat resistant exactly.
Speaker 2: So the proponents of the theory suggested that this was
evidence of an artificial metal rich outer layer, a protective
hull designed to withstand cosmic debris and extreme heat, just
like a battleship is armored.
Speaker 1: So when you put it all together, it's a pretty
compelling story. You start with a mysterious noise, you layer
in some genuinely unusual data points, little density, perfect eclipses,
and you wrap it all up with the story of
alien engineers. It's better than most science fiction, and.
Speaker 2: That connection to science fiction is so important. Decades before
Vasen and Scherbakoff, you had H. G. Wells writing about
the Selenites living inside a.
Speaker 1: Hollow moon right in the First Men in the Moon.
Speaker 2: And then later writers like Edgar Reice Burrows and more
recently David Weber. They all played with the idea of
a manufactured or inhabited moon. The popular imagination was already
primed and ready to accept the idea that our closest
companion could be an intentional creation.
Speaker 1: But I think the time has come to shift from
that captivating narrative of fiction and speculation back to the rigorous,
sometimes messy reality of planetary science, because the real explanation
for why that bell rang is far more intricate and
frankly way cooler than any alien spaceship.
Speaker 2: I agree, the scientific explanation for that prolonged reverberation is
one of the most elegant takedowns of a conspiracy theory
that I know of. The moon didn't ring because it
was hollow. It rang because it is an almost perfectly
preserved and natural object with two very unique characteristics, extreme
dryness and extreme fracturing.
Speaker 1: Okay, let's start with the dryness and how that affects
energy loss. We need to talk about seismic wave attenuation
and something called the quality factor or CUE.
Speaker 2: Right. So, attenuation is basically a measure of how quickly
seismic wave energy is loss converted into heat as the
wave travels through a material. It's like internal friction. Okay,
and the quality factor Q is just the number we
use to quantify this damping. A low Q factor means
high energy loss, the wave dies out vast. A high
Q factor means very low energy loss. The wave forssists
for a long time.
Speaker 1: So why does Earth have such a low coup factor?
Why does our planet silence itself so quickly?
Speaker 2: Because Earth is hot, it's dynamic, and most importantly, it's wet.
Water and other volatile compounds like innhydrated minerals. They act
as incredibly effective.
Speaker 1: Damping agents, like a shock absorber.
Speaker 2: It's exactly like a shock absorber. Think of the difference
between hitting a damp sponge versus hitting a dry piece
of ceramic. The internal friction that's generated when seismic waves
push through water or through regions of partial melt in
the upper mantle, it just dissipates the energy very, very rapidly.
That's why our seismic code only lasts for a few minutes.
Speaker 1: And the Moon, the early reports from NASA described it
as being incredibly rigid, like a piece of dry stone.
Speaker 2: The Moon is the polar opposite of Earth in this respect,
bone dry. It's almost entirely free of water and other
volatile compounds below the surface. It's also relatively cool and
very rigid throughout its shallow.
Speaker 1: Depths, So all those things that act like shock absorbers
here they just aren't there.
Speaker 2: They're just not there. This almost total lack of internal
damping agents means that seismic energy is conserved with phenomenal efficiency.
The Moon has an exceptionally high Q factor we estimated
to be as high as five thousand in the upper mantle,
and on Earth, the Earth's crustal Q factors rarely get
above a few hundred, so it's a massive difference.
Speaker 1: So the energy is conserved so well that once you
pump it in, it just doesn't have a way to
die out quickly. That explains the duration of the signal exactly.
Speaker 2: That explains why the Moon doesn't absorb the energy. But
that rigidity and dryness alone, that doesn't explain the complexity
of that hour long, slowly peaking signal. For that, you
need the second key factor, which is extreme scattering and heterogogeneity.
Speaker 1: Okay, and this is where the relentless pounding the Moon
has taken for billions of years comes into play. You're
talking about the megaregolith.
Speaker 2: The megaregolis is absolutely vital to this whole process. See.
Unlike Earth, where tectonic activity and erosion are constantly smoothing
out and recycling the surface, the Moon has basically preserved
a record of every single major impact for four point
five billion years.
Speaker 1: So it's just been shattered over and over again.
Speaker 2: Over and over and this has resulted in a really
deep layer the crust and the upper mantle down to
maybe two hundred kilometers. That is a highly fractured, heterogeneous structure.
It's a jumble of shattered bedrock, broken and re welded rock,
and fine dust a regulith.
Speaker 1: So it's not a uniform solid structure at all. It's
an almost unbelievably complex mash of different rocks with random
variations in density and composition at every turn.
Speaker 2: It is the ultimate scattering medium. Imagine you launch a
billiard ball across a perfectly smooth table. It follows a nice, straight,
predictable path. That's sort of what seismic waves do on
Earth for the most part. Now, imagine putting that same
billiard ball into a giant box that's filled with thousands
of irregularly shaped hard objects. It's going to bounce everywhere.
That's the Moon's megaregulith.
Speaker 1: It's like the energy gets trapped in a seismic pinball machine.
Speaker 2: That is a perfect analogy, or you could think of
it as a vast hall of mirrors. For seismic waves.
The energy, which we've already established is being conserved really well.
Instead of following a short direct path from the impact
of the sensor.
Speaker 1: It just bounces off everything.
Speaker 2: It's continuously reflected, trapped, and diffused throughout the entire medium.
The ways end up taking these myriad, long, winding indirect
paths to the seismometer, and all those different path lengths
arriving at different times exponentially stretches out the time it
takes for the entire signal to decay.
Speaker 1: And that gives you the prolonged seismic that lasts for hours.
Speaker 2: That's exactly right, and this is why in geophysics we
have to talk about single versus multiple scattering theory. Oh wait,
explain that on Earth. Because the high attenuation kills the
wave so quickly, we can generally get away with using
single scattering theory. We assume the energy travels more or
less directly, maybe hits one or two boundaries, and then
it dies simple. On the Moon, the waves are scattered
so intensely and their energies can serve so well that
you have to model it using multiple scattering or diffusion theory.
The energy literally diffuses throughout the Moon's near surface volume,
almost like light diffusing through thick fog or a glass
of milk.
Speaker 1: So the complex persistent signal, the very evidence that was
cited to prove the Moon is a smooth, uniform engineered shell,
is actually the perfect acoustic signature of an ancient, fractured, messy,
and extremely dry natural world.
Speaker 2: It's the ultimate irony of the ringing like a bell quote,
isn't it?
Speaker 1: It really is. It proves the exact opposite of what
the theory claims it does.
Speaker 2: It captures the imagination perfectly. But the detailed science tells
us that if the Moon were an engineered artifact, say
a smooth, homogeneous metal sphere, it would probably produce a
very clean, simple, high frequency tone that decayed very quickly
like a tuning fork. The messy complex prolonged decay that
is the signature of natural complexity.
Speaker 1: So even with that really elegant explanation of low attenuation
and diffusion scattering, we still have to definitively knock down
the other cornerstones of the spaceship moon theory, and the
big one is the claim that the Moon is just
too light to be solid, that it must contain a
massive void.
Speaker 2: Right, and this is where the concept of planetary differentiation
and especially modern gravitational mapping provides the final definitive physical refutation.
We can now confirm that the Moon is a fully
differentiated solid body with a complex layered interior, a crust,
a mantle, and a dense core.
Speaker 1: And this structure formed billions.
Speaker 2: Of years ago, about four point five billionyears. Yeah, long
before any potential alien engineers would have shown up on
the scene.
Speaker 1: Okay, so let's tackle that density issue head on. Why
is the low bulk density of three point three grams
per cubic centimeter not proof that.
Speaker 2: It's hollow, because that number is entirely consistent with the
leading theory of how the Moon formed in the first place,
the giant impact hypothesis or GIH.
Speaker 1: This is the idea that a Mars sized object, which
we sometimes call THEA, slammed into the very early Earth.
Speaker 2: That's the one The GIH posits that the Moon formed
from the debris that was flung out into orbit after
that cataclysmic collision. But the crucial detail here is the
state of the proto Earth before it got hit.
Speaker 1: It had already started to differentiate.
Speaker 2: Exactly by the time THEA struck Earth had already undergone
significant differentiation. Most of Earth's dense metallic iron, which is
the stuff that makes our planet's average density so high
at five point five, had already sunk down and segregated
into its core.
Speaker 1: So when the impact happened and blasted all that material
out into space, that.
Speaker 2: Material came primarily from Earth's iron depleted rocky mantle and
from the impactor itself. So the cloud of debris that
eventually coalesced to form the Moon was naturally inherently iron
pore from the very beginning.
Speaker 1: So the Moon is light because of its birth circumstances,
not because it's a hollow shell.
Speaker 2: Precisely, that single point completely eliminates the fundamental need to
invent a giant vacuum cavity to explain the low density.
Speaker 1: But the proof that the Moon is solid comes from
deep inside. Let's go back to the Apollo seismic data
for a minute. What do the moonquakes tell us about
the mantle?
Speaker 2: The Apollo passive seismic experiments, which it's amazing to remember,
ran all the way until nineteen seventy seven.
Speaker 1: Wow, for years after the last astronauts left for.
Speaker 2: Years, and they provided conclusive proof of a massive solid mantle.
They recorded thousands of deep moonquakes. These things were originating
between seven hundred and one thousand kilometers below the surface.
Speaker 1: And what causes those.
Speaker 2: The tidal stresses exerted by Earth's gravity. As the Moon
orbits us, it's constantly being squad eased and stretched.
Speaker 1: One thousand kilometers deep. That is a huge distance for
a seismic wave to have to travel to reach the surface.
Speaker 2: It's massive, and the fact that the seismic waves generated
by these deep quakes can propagate hundreds and hundreds of
kilometers upward through the tholid mantle and still be detected
by the instruments on the surface. That conclusively proves the
physical existence of a massive solid medium occupying all that
volume below the crust.
Speaker 1: So there's no way it's a hollow shell just under
the surface.
Speaker 2: No way the internal void, if it existed, would have
to start at best way down near the very center
of the Moon.
Speaker 1: And now we get to the really definitive proof of
that central mass. This is thanks to modern data and
a concept called the moment of inertia or mo.
Speaker 2: The moment of inertia is maybe the most powerful tool
of planetary geophysicists has because it tells you exactly how
the mass is distributed inside a rotating body.
Speaker 1: And how do we measure it.
Speaker 2: For the Moon, we calculate it by observing the Moon's
gravity field with extreme precision and by watching how it's
slightly wobbles or libraids in its orbit. That data has
been refined for decades by the lunar laser ranging experiments.
Speaker 1: Where they bounce lasers off the reflectors the Apollo astronauts
left behind.
Speaker 2: That's right, and that gives us these incredibly precise measurements.
Speaker 1: So what are the theoretical benchmarks for MOI? How does
it tell us if something is hollow or solid?
Speaker 2: Okay? So think of it like this. A perfectly hollow
sphere with all of its mass concentrated right on the
outer rim, would have an AMI factor of point sixty seven.
Speaker 1: Okay, high number for hollow.
Speaker 2: A body with perfectly uniform density all the way through,
like a solid, undifferentiated ball of rock, would have an
AMOY of point four lower. And if a body has
a dense core and a lighter exterior, like all differentiated
worlds do, its MOY factor has to be less than
point four because more of the mass is concentrated at
the center of rotation.
Speaker 1: So the big question where does the moon land?
Speaker 2: The modern data derived from decades of laser ranging and
the incredibly precise measurements from the geomission can strains the
Moon's MOY factor two point three ninety four plus or
minus point zero zero.
Speaker 1: Two point three nine four. Okay, So that is exceptionally
close to the uniform density number of ono point four.
Speaker 2: It's very close, yeah, which tells us the Moon's density
gradient isn't super steep, but critically it.
Speaker 1: Is lower, and that small deviation is everything.
Speaker 2: It is everything. The fact that the MOI is less
than point four is definitive physical proof that the mass
is concentrated towards the center. It confirms that the Moon
is not only not hollow, which would require a point
sixty seven, but it is a fully differentiated body with
a denser core than its surrounding material. That single highly
precise number just completely destroys the hollow hypothesis.
Speaker 1: So we've established the Moon must have a dense center.
But the absolute final nail in the coffin comes from
the g RAIL mission, the Gravity Recovery and Interior Laboratory.
This is what allowed us to actually map the structure
of that core.
Speaker 2: Dreil was a masterpiece of Geodyssy flew from twenty eleven
to twenty twelve. It was two identical spacecraft flying in
this tight tandem format orbiting the Moon, and.
Speaker 1: They were measuring the distance between each other constantly.
Speaker 2: They measured the minute changes in distance between them caused
by subtle variations in the Moon's gravity on the surface below.
When the lead satellite passed over an area of higher
mass concentration, you would be pulled forward slightly, increasing the distance,
and they could measure that change down to the micrometer.
Speaker 1: That's insane. And that data allowed them to build the
most accurate gravity map of the Moon ever conceived by far.
Speaker 2: And this map, when you combine it with a re
analysis of all that old Apollo seismic data, it finally
allowed scientists to peel back the layers and see exactly
what's down there in that central space.
Speaker 1: So what do they find.
Speaker 2: They confirmed a distinct, multilayered core and it entirely occupies
the space that the void Hyposteris claims is empty. At
the very center, there's a solid inner core made of
pure iron, extending out to a radius of about two
hundred and forty kilometers.
Speaker 1: And round that solid core.
Speaker 2: Surrounding that is a liquid outer core. This is made
of molten iron alloy, probably mixed with some lighter elements
like sulfur and nickel. And this liquid core extends out
to a radius of about four hundred and eighty kilometers
and it's dense, very dense, around five grams per cubic centimeter.
Speaker 1: Structure is huge a radius of four hundred and eighty kilometers.
That means the core occupies about twenty percent of the
Moon's entire diameter.
Speaker 2: It does.
Speaker 1: You absolutely cannot have a solid, high density iron core
and a massive liquid iron outer core and still pretend
there's a giant vacuum cavity in there. The space is
physically occupied.
Speaker 2: And what's more, the existence of those distinct layers was
confirmed by a really sophisticated reanalysis of the original Apollo
moonquake data, specifically by observing how different types of seismic
waves behaved as they pass through the center.
Speaker 1: Okay, tell us how they can distinguish between solid and
liquid just by using waves.
Speaker 2: Well, they are two main types of seismic body waves.
You have P waves, which are compressional waves, and S waves,
which are sheer waves. TA waves can travel through both
solid and liquid MATERIALS wave, however, physically cannot propagate through
a liquid They just stopped debt.
Speaker 1: So if they recorded P waves traveling all the way
through the center, but they saw S waves disappearing at
a specific radius, they've just mapped the boundary between the
solid mantle and the liquid core.
Speaker 2: That's exactly it. The researchers were able to map the
Moon's core boundaries with precision by tracing the S wave
shadow the region where S waves cease to be transmitted.
This is fundamental geophysical evidence. It proves the existence of
a high density molten outer core surrounding an even denser
solid inner core. No amount of wishful thinking can erase
that physical reality.
Speaker 1: Okay, so let's just quickly address that last piece of
circumstantial evidence. The conspiracy theorists used the high titanium rocks.
Why isn't that a sign of artificial armour plating.
Speaker 2: The high concentration of titanium mainly in that mineral ilmanite
is explained entirely by a natural but incredibly dramatic geological
process called mantle overturn.
Speaker 1: And this happened very early in the Moon's history.
Speaker 2: Early around four point two billion years ago, right after
the global magma ocean had mostly solidified. As that magma
ocean crystallized, some of the last materials to solidify included
these really dense minerals like ilmanite.
Speaker 1: So they formed a layer near the top.
Speaker 2: They formed a layer in the uppit part of the mantle.
But here's the dramatic part. This layer of dense ilmanite
rich rock was sitting on top of less dense material
underneath it. It was gravitationally unstable, like.
Speaker 1: Having an ocean where the heavier water is somehow floating
on top of the lighter water.
Speaker 2: Perfect analogy. It couldn't last, so the Moon in a
way literally turned itself inside out. Wow, that dense ilminite
rich layer began to sink and overturn, causing this enormous
vertical mixing within the mantle, and Grail's gravity maps actually
confirmed this. They revealed these long linear anomalies, vast submerged
features that are the geological scars of this sinking material
as it cascaded downward.
Speaker 1: So that complex natural process accounts for all the compositional
weirdness they found.
Speaker 2: Every bit of it. It completely eliminates the need for
an engineered hull. And plus we know the crust itself
is highly asymmetrical. It's much thinner on the near side
facing us and thicker on the far side. That kind
of irregularity is the signature of a messy natural evolution,
not uniform engineering. The geophysical evidence for a solid, internally
dynamic moon is just it's.
Speaker 1: Overwhelming, So we can pretty confidently conclude that the Moon's
famous ringing wasn't the sound of a holospace ship.
Speaker 2: Not at all. It was the extraordinary acoustic signature of
a natural, highly rigid, and extremely fractured solid world. The
sound persists because there's almost no friction to dampen the energy.
Speaker 1: And the signal is so complex because that energy gets
trapped and bounced around in that deep layer of shattered rock.
Speaker 2: The core takeaway here really is that the scientific truth,
based on the hi Q factor diffusion scattering and the
definitive gravitational proof of a dense, multi layered iron core,
is just so much more complex and so much more
compelling than the simple idea of alien engineering.
Speaker 1: And refuting the hollow moon theory doesn't make the Moon boring,
not by a long shot. It still holds these profound,
real world mysteries that are driving the entire next generation
of space exploration.
Speaker 2: I think that's the most exciting part. While the deep
structure is solid, we're now finding that the crust is
full of natural voids that could be absolutely crucial for
a future human habitat.
Speaker 1: You're talking about the lava tubes.
Speaker 2: The collapse lava tubes. Yeah, recent high resolution images have
confirmed these natural lunar pits and caves, especially in regions
like the Sea of Tranquility. Some of them are huge.
We're talking eighty meters long and forty five meters wide.
Speaker 1: And why are these natural voids so important for something
like the Artemis program.
Speaker 2: They are critical because they offer natural shielding. Inside those
lava tubes, the temperatures are stable. They hover around a
relatively comfortable minus twenty degrees.
Speaker 1: Celsius compared to the extreme three hundred degree swing on
the surface exactly.
Speaker 2: But even more importantly, they would shield future astronauts and
their equipment from the constant rain of micrometeoroid impacts and
from the lethal doses of solar and cosmic radiation. So
we might not be living in some alien bunker.
Speaker 1: But we could be settling in these ancient natural geological
bunkers precisely.
Speaker 2: It's amazing to think that the same geological complexity that
gave us the titanium anomalies and the asymmetrical crust also
created the perfect natural shelters for humanity's return.
Speaker 1: And even with all this incredible data from the Apollo
seismometers running for eight years, to the beautiful gravity maps
from geoplanetary scientists still describe our understanding of the Moon's
deep interior as being well rudimentary.
Speaker 2: It's true the fine details of that mantle overturned the
exact composition of the inner core. There's still so much
to learn. The Artemis program is planning to deploy new
seismic instruments like the Farcite Seismic Suite to refine our models.
The story of how the Moon became so lopsided is
still being written deep below the surface.
Speaker 1: So we've definitively established why the Moon rings. It's the
sound of conserved energy in a dry, highly fractured solid world.
That the emotional power of that original quote rang like
a bell. It allowed the spaceship Moon theory to persist
for decades. It just goes to show how easily a
poetic phrase can supersede complex science in the popular mind.
Speaker 2: It really speaks to our profound, very human desire for mystery.
Speaker 1: So we'll leave you with this thought to ponder. We
know the scientific facts. We know exactly why the Moon
really rang, thanks to the immense rigor of panetary science,
But even knowing the physics of the low density and
the MUAI confirmation. What soundgiving detail about the Moon. Maybe
it's the sheer astronomical coincidence of that perfect solar eclipse,
or the incredible scale of that ancient mantle overturn. What
detail still makes you pause and wonder if our closest
neighbor is still stranger than we can fully grasp.
Speaker 2: Let us know your thoughts in the comments. We're back
next time for another deep dive