The far side of the Moon, which has always faced away from Earth due to tidal locking, was first photographed in 1959 and revealed a completely different landscape than the near side—characterized by a thicker crust, no dark lava plains, and the largest impact scar in the inner solar system (South Pole-Aitken Basin). China's Chang'e 4 mission in 2019 discovered that the far side's mantle had been buried under 4 billion years of dust and rock, solving the 'missing mantle problem' that had puzzled scientists for decades. The far side was never dark or hidden—it simply faced the other way, waiting silently for humanity to finally look up and see it.
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What China Found on the Far Side of the Moon | Cosmic Lullaby
Added:Good evening. Or perhaps it isn't evening where you are. Perhaps it's the small hours, the ones that belong to no one, and you found your way here because sleep has been keeping its distance.
Either way, you're welcome. There's nothing to do now, nothing to solve. You can set it all down. Tonight, [music] we're going to talk about the moon. Not the one you know, not the pale familiar face that has followed you home on winter evenings, that hangs above the rooftops, that has been photographed and painted and sung about for as long as there have been people [music] to look up. We're going to talk about the other one, the half that turns away. Because here is something strange and it took humanity a very long time to notice how strange it is. The moon shows us one face always. It has shown that same [music] face to every human being who has ever lived. Your grandmother saw the same craters you see. So did the people who built Stonehenge. So did whoever first thought to call it beautiful. And behind that face, turned away, patient, silent, is a landscape that no human eye saw until 1959.
A place that was for the entire span of our species simply absent, not hidden, not secret, just facing the other way.
And when we finally went there, when a small machine settled onto that hidden ground and looked around, it found a world that did not match the one we thought we knew. Different crust, different history, different chemistry.
down in the deep places where [music] the rock remembers almost as if we had been living all this time beside a stranger and only ever seen the mask. So settle [music] in. Let your shoulders drop away from your ears. Let the day you've had begin to loosen its grip. It doesn't need you anymore. It's finished with you. We have a hundred things to notice tonight and there's no hurry with any of them. If you drift away in the middle of one, that's not a failure.
That's the [music] whole point. I'll keep going. The moon will still be turning and you'll have gone somewhere gentler. Let's begin at the [music] edge of the light, at the place where the near side end and the other one begins.
The shadow we had never seen one. For most of human history, the far side of the moon was not unknown. It was something stranger than unknown. It was unimaginable. Think about that for a moment. To wonder about a place, you first have to know it [music] exists.
And nothing in the sky told us it did.
The moon simply was its [music] face, round, marked, complete. The idea that it had a back, a hidden half, an entire hemisphere of geography turned permanently away from us. That took a very particular kind of imagination.
And it wasn't answered until October of 1959 when a Soviet spacecraft called Luna 3 swung around behind it and [music] sent home 29 grainy, ghostly photographs.
blurred, streable, and in them humanity saw, for the first time in 300,000 years of looking up the other half of the thing it had been looking at. The pictures were poor, but what they showed was unmistakable, and it made no [music] sense at all. Two, the near side of the moon, the one you know, is covered in dark, smooth planes.
The old astronomers thought they were oceans and gave them ocean names. The sea of tranquility, the ocean of storms. They were wrong about the water, but the names [music] stuck. And they were right about one thing. Those planes are flat [music] and dark and vast. They're ancient lava, basaltt, floods of molten rock that welled up from inside the moon and spread out across the lowlands and then cooled [music] and then stayed. They cover roughly a sixth of the lunar surface. And almost all of that sixth is on the side that faces us. Now turn the moon around. those dark seas almost gone. The far side is pale, cratered, rough, piled with highlands. Where the near side has smooth, dark plains, the far side has almost none, just a scattering, a few small patches like puddles where you expected an ocean.
Same world, same age, same distance from the sun, and two completely different faces. [music] Nobody knows for certain why. We have ideas and we'll come back to them, but hold that image for now. One hemisphere drowned in ancient lava, the other one dry. [music] It's a very old question, and it doesn't have a tidy answer yet.
Three, there's a phrase people use, the dark side of the moon, and it's lovely and it's wrong. Gently, completely wrong. The far side is not dark. It gets [music] exactly as much sunlight as the near side does. Two weeks of blazing day, two weeks of night, over and over, the way the near side does. The sun doesn't care which way the moon is facing. [music] It shines on everything.
What the far side is is unseen. Not dark, just turned away. And I think there's something in that worth sitting with. The thing we called dark for centuries wasn't dark at all. It was simply outside our view. The shadow was in us, [music] in where we stood, in what we could reach, never in the thing itself. The far side has been in sunlight this whole time. We just weren't there to see it for. So why does the moon only show us one face? Why doesn't it turn the way the Earth turns and let us see all of it in time? Here's the surprise. It does turn. The moon is rotating right now. It has never stopped. It's just that it rotates at exactly the speed it orbits. One spin, one lap, perfectly matched. Imagine walking a slow circle around a friend standing in a field and you keep your eyes on them the whole way. By the time you've come back to where you started, you've turned all the way around.
[music] You've made one full rotation.
But your friend standing in the middle never saw the back of your head. Not once. That's the moon [music] turning always and always showing us the same face. Five. This is called tidal locking. And it isn't a coincidence.
It's the end state. [music] It's where things settle. The Earth pulls on the moon a little harder on the near side than the far side. It's closer, so the pull is stronger. Over vast stretches of time, that uneven grip acted like a hand slowing a spinning coin. Whatever rotation the young moon had, [music] the Earth wore it down gradually, patiently over hundreds of millions of years until the spinning stopped mismatching the orbit until they locked into step. And here's what I find quietly astonishing about that. It means the moon was not always like this. There was a time early, molten, wild, when the moon turned freely, when a person standing on the earth, if there had been anyone, would have watched the whole surface roll past week after [music] week. Every crater, every plane, all of it. The face we know is not the moon's face. It's just where the spinning happened to stop. Six. Tidal locking is not rare.
It's not some quirk of our particular moon. It's what gravity does to almost everything given enough time. Most of the large moons of Jupiter are locked to Jupiter. Most of Saturn's are locked to Saturn. Wherever a small body has been circling a large one for long enough, the same slow grinding has happened. and the same quiet result has settled in.
There's something almost restful in that. It isn't a strange fate that befell our moon. It's simply the shape that time makes. Given long enough, the wobbling stops. The spinning slows.
Things come to rest, facing the thing they orbit. The whole solar system is full of small worlds, staring forever at the large ones that hold them. Patient, settled, done with turning. Seven. Now, here's a detail that [music] most people never hear, and it's a lovely one. We don't actually see 50% of the moon. We see about 59%.
The moon's orbit isn't a perfect circle.
It's slightly stretched, so the moon speeds up when it's closer to us and slows down when it's further away. But its rotation stays perfectly steady, which [music] means the two fall very slightly out of step and then back in over the course of a month. The result is a gentle rocking. The moon nods. It sways a few degrees this way, a few degrees that. It's called libration. And it means that over time we peak a little way around the edges. A thin crescent of the far side appearing and disappearing at the rim. So we've always been seeing a little more than we were supposed to.
Just a sliver, a glimpse past the curve.
Not enough to know what was there. but enough perhaps to wonder. 8, which brings us to January 3rd, 2019, and to a place with a name most people have never spoken, Von Carmon crater. On that day, a Chinese spacecraft called Chang Air 4 came down out of the black and landed on the floor of that crater slowly, carefully with no one watching from Earth because on the far side, no one can watch. Radio signals don't bend around a world. The moon's own body sits between that ground and every antenna we have. Changa 4 landed alone, out of contact with the planet that built it, speaking through a small satellite parked far out in the dark beyond the moon, precisely so that it could speak at all. It was the first time anything from Earth had ever touched that ground.
not flown over, not photographed from orbit, touched 4 and a half billion years of silence and then softly contact nine. Now, the name on that crater, it's worth pausing over because it's one of the strangest coincidences I [music] know of and almost nobody notices it.
Von Cararman Crater is named for Theodore Vonarman, a Hungarian engineer who spent his career at Caltech in Pasadena and who founded the Jet Propulsion Laboratory, the place that would go on to build so much of America's exploration of the solar system. And von Carman had a student, a brilliant one, a young Chinese engineer named Kan Schwessen, [music] who helped found that same laboratory and who became one of the architects of American rocketry. And then in the 1950s, in a wave of suspicion, America accused him of communist sympathies, stripped his clearance, held him under watch for years, [music] and finally in 1955 deported him. He went home to China, and he built their rocket program from nothing. An American official said many years later that it was one of the stupidest things the country had ever done. So here is what happened on that January morning. A spacecraft built by the program that Chiian Shuen founded [music] came down out of the dark and landed inside a crater named after the teacher he had been forced to leave behind. Neither of them lived to see it.
Neither of them could possibly have imagined it back in a dry canyon outside Pasadena [music] in the 1930s running small rocket tests in the dust.
But their two names are up there now on the far side of the moon, sitting in the same crater in [music] the same silence for as long as the moon lasts.
10. The lander carried a rover and the rover was called U2 [music] jade rabbit after a creature from an old Chinese story. A rabbit who lives on the moon and grinds medicine beneath a cassia [music] tree. Small, solar powered, six wheels, moving slowly enough that you could outwalk it at a stroll without breaking stride. It was built to last 3 months. It has now been working for more than 70 lunar days.
Waking each time the sun comes back.
Sleeping each time the long cold night arrives and the temperature outside falls past 290° below zero. Waking again. Nobody expected that. Nobody designed for it.
It [music] simply kept getting up. And there is one more thing on that lander.
quieter and sadder [music] and I think the most human detail in this entire story. Sealed inside a small canister the size of a coffee tin, the engineers had placed [music] some seeds, cotton, potato, a little soil, a little water, a little air, a tiny sealed world. [music] And on the 7th of January, a photograph came back showing that one cotton seed had put out a shoot. The first living thing ever to grow on the surface of another world. It [music] lived about 9 days. The canister had no heater. And when the lunar night came, as it always comes, as it has come every 14 days for 4 billion years, the cold got in and the small green chute died the way anything would. Left outside in a winter with no door to go back through. It's still there tonight, sealed in its tin on the floor of a crater named for a man whose student was sent away on the side of the moon that never sees the Earth. We're going to go deeper next down into the basin beneath all of this. The largest scar on the moon and one of the largest in the solar system.
>> [music] >> But rest here a moment first with the rabbit machine that wouldn't stop waking up and the seed that only managed nine days and grew anyway. The deepest hole on the moon 11. Von Carmon crater is about 115 mi across, roughly the distance from Boston to New York. a hole in the ground you could not walk across in [music] a week and could not see across on the clearest day imaginable.
And it is a small feature. That's the part that takes a moment. It isn't a large crater that happens to sit in an interesting place. On the map of where it is, it barely registers. It's a dimple, a pock mark on the floor of something so much larger that the mind refuses to hold the two of them at the same scale. The South Pole Atkin Basin is about 1,500 m across. That is New York to Denver. That is most of the width of a continent. And it is a hole in a world one quarter the size of ours.
Sit with that for a second. Not the number, the shape of it. A bowl gouged into a small world, wide enough that you could lay the United States across it and still have room at the edges. 12.
Here's what that actually means. If you were standing in it, [music] you would not know. That's the strange part. If you were set down in the exact center of that basin on the floor [music] and you looked around, the rim would be over the horizon in every direction. You would see gray ground curving gently away from you and scattered across it ordinary craters of ordinary size.
Nothing that read as a wall. [music] Nothing that read as an edge. nothing that told you that you were inside anything at all. You would simply think you were standing on the moon and you would be miles below where the moon's surface is supposed to be. There's something in that I keep returning to the largest wound on that entire world is invisible from inside it. You have to be far away and looking down before it takes any shape at all. Up close, it's just ground. I don't think that's only true of craters.
13. The floor of that basin sits about 8 mi below the highlands that surround it.
Let me give you something to measure that against. The Mariana Trench, the deepest place in Earth's oceans, the bottom of the Pacific, a place so deep that the water above would crush a submarine like a paper cup, [music] is about 7 mi down. So the moon is carrying a wound deeper than the deepest place on our entire planet. On a body one quarter our size with no oceans to fill it, no weather to soften it, no moving crust to fold it back under. [music] And it has been carrying that wound open, unhealed, exactly as it was for something like 4 billion years. Nothing has happened to it. Nothing was ever going to on the moon. A hole is permanent. There is no rain to wear the edges down, no river to fill it in, no continent [music] to slide over the top of it and swallow it whole. It happened once and then it simply stayed. 14. Something made that whole. And we can work backward from the size of it to the size of the thing that did it. And the answer is not comfortable. The object that struck there was at least 120 mi across. For comparison, [music] the asteroid that ended the age of the dinosaurs, the one that hit the shallow sea off the Yucatan [music] and closed the Cretaceous and darkened the sky and cleared the way for every mammal that has ever lived, including [music] you, including me, including everyone you have ever loved. That one was about 6 to9 mi across. The thing [music] that hit the moon was not a bigger asteroid.
It was [music] a different category of object entirely. It was not a mountain falling out of the sky. It was a small world. 15. The energy released in that collision comes out to something like 10 to the 26th jewels. That number is meaningless. Nobody can feel a number like that. So, let me try it a different way. The largest nuclear weapon ever detonated on Earth was a Soviet device tested over the Arctic in 1961.
A thing so large that the crew who dropped it were not expected to survive and were given only a 50/50 chance of outrunning their own blast. This impact released roughly half a billion times more energy than that. It melted thousands of cubic miles of rock in seconds, not over years. In seconds, it drove a shock wave through the entire body of the moon in one side and out the other and rang the whole world like a struck bell. and it dug a hole so deep that according to every model we have ever built, [music] it should have punched clean through the crust. 16. I want to slow down here because this next part is the whole reason the story matters. Every rocky world has layers.
It sorted itself out long ago when it was still molten. Light rock floating upward, [music] heavy rock sinking down. The way oil settles above water if you leave it alone long enough and give it no reason to mix. So on the outside there's a crust, thin, pale, light minerals that floated to the top of an ocean of magma [music] and cooled there. And underneath there's the mantle, denser, darker iron and magnesium. The deep interior of a world [music] rock that sank when everything was still liquid and settled and has been down there in the dark ever since.
On the moon, the crust runs about 20 to 25 mi thick. beneath that basin. In places it may be less than six, which means the impact should have opened the moon up. It should have torn the shell off and thrown the inside across the surface. It should have left the mantle, [music] the actual buried interior of another world. rock that had never in 4 and a half billion years seen the sun lying out in the open in daylight where anything with the right instrument could simply look at it. 17. So we looked for decades [music] we looked. Clementine looked. In 1994, Luna Prospector looked. Four years after that, India sent Chandrean. Japan sent Kaguya. NASA sent the Lunar Reconnaissance Orbiter, which is still up there now, still working, still watching. All of them carried spectrometers, instruments that read minerals from a distance using nothing but reflected sunlight because different minerals absorb light at different wavelengths.
And that pattern of absorption is a fingerprint. You don't have to touch [music] a rock to know what it is. You just have to look at it carefully enough. Every one of those spacecraft looked at the floor of that basin and every one of them found the wrong thing.
18. What they found was an orthosite.
Pale rock feld sparri light minerals crust. ordinary unremarkable crust sitting in the bottom of the deepest hole on the moon in the exact place where the crust should have been stripped away entirely [music] and thrown to the horizon. They called it the missing mantle problem and it sat there quietly unresolved for the better part of 30 years. The largest impact anywhere in the inner solar system had struck the thinnest part of the moon's shell. And somehow [music] the shell was still there, intact, as if nothing had happened at all. It's a strange kind of puzzle, not a mystery of something hidden exactly. More like arriving at a house where you know a fire has burned and finding the walls standing and the windows unbroken and no smoke anywhere and having to ask very carefully what you have actually misunderstood.
19 There was an answer on the table. It just could not be tested. The idea was burial. After the impact, the floor of that basin would have been a sea of molten rock, miles of it, glowing, [music] slowly cooling under a black sky. And as it cooled, it would have hardened into dark basult. A lid, a new floor laid down over the exposed [music] interior, sealing it away. And then the rain began.
Not water, rock. For 4 billion years, smaller impacts came down on that surface. Thousands of them. Millions of them. Each one shattering stone. Each one grinding what [music] was already broken into something finer. Until the top of the moon became what it is now, a layer of powder, crushed rock, dust that no wind has ever moved. If the mantle was down there, it was under all of that. Under the basalt lid, under the churned dust, under 4 billion years of patient destruction, invisible to anything looking down from above. 20.
Which means to answer the question, you would have to go, not orbit, go. You would have to land on the floor of that basin. You would have to find a small crater, a fresh one, young enough that its impact had punched through the lid and thrown up whatever was underneath, and recent enough that the rocks it exposed hadn't yet been ground back down into dust. You would have to drive to the rim of it. [music] You would have to put an instrument close enough to touch the stone, and you would have [music] to take a reading. And to do any of that, you would have to land on the far side of the moon, where nothing from Earth had ever landed and where no signal from Earth can ever reach. Which is exactly why nobody [music] did it. For 30 years, the question sat open, not because it was unanswerable, because answering it required going somewhere nobody had a good enough reason to go. And then one morning in May of 2019, 5 months after it landed, moving at a pace slower than you walk, a small six- wheeled machine came over a gentle rise on the basin floor. And there, in front of it, was a crater 70 m across, fresh, its rim [music] scattered with broken rock, thrown up from somewhere below.
Nobody had put it there. Nobody had planned for it. [music] It was simply in the way. We'll go to that crater next and to the two absorption bands that came back from it.
But rest here first in the great invisible bowl on the floor of the deepest hole on the moon where the horizon curves away [music] and there is nothing at all to tell you what you are standing inside of. What the rabbit found. 21. The crater had no name. It still doesn't. [music] It's 70 m across, about the length of a football field, [music] goalpost to goalpost. And on the moon, that is nothing. That is beneath the threshold at which anybody bothers.
There are millions like it. They don't go on maps. They don't get named after Hungarian engineers or Chinese goddesses. They are just holes of which the moon has an uncountable number. And yet this is the one. Out of everything on that basin floor, out of 1,500 miles of gray ground, the entire story comes down to this one anonymous dent that a rover happened to drive [music] past.
There's no reason it was that crater. It just happened to be where the rover was.
22. But it was the right kind of crater.
[music] And that part wasn't luck. That part was somebody knowing what to look for because the team needed a crater that was young. On the moon, young is a strange word. It might mean a few million years. It might mean a few tens of millions. What it means practically is recent enough that the damage hasn't been repaired [music] yet. And you can tell by looking. A fresh lunar crater has sharp edges, [music] a clean, crisp rim. An old one is soft, rounded, slumped. Because the moon has its own kind of erosion, and it works with nothing but time and a constant, whisper fine rain of micrometeorites.
Each one no bigger than a grain of dust.
Each one arriving at tens of thousands of miles an hour. There is no wind up there, no water, nothing you would recognize as weather. And [music] yet the moon still wears things down slowly, grain by grain, the way a river rounds a stone, except without the river, just the patience.
23. This crater was sharp. Its rim was clean. The rocks scattered around it were angular, fractured, their broken faces still [music] bright. They had not been sitting in the sun long enough to darken. They had not been buried again.
They had not been ground into anything.
[music] They had come from below recently, and they were still lying where the impact had thrown them, which meant whatever was under the lid, [music] this small crater had punched through it and brought some of it up.
The rover stopped at the rim and it lifted its head. 24. The instrument on U22's mast is a spectrometer and it's small. The opening is only a few [music] cm across. It doesn't take a photograph in any sense you'd recognize. It doesn't produce an image at all. It measures light. Reflected sunlight split apart into its wavelengths from the visible blue all the way out into the near infrared. And when sunlight lands on a mineral, the chemical bonds inside that crystal absorb certain wavelengths [music] and not others. Specific ones, always the same ones. So what comes back isn't a picture. It's a curve, a line of brightness against [music] wavelength with dips in it. And the dips are the fingerprint. Olivine absorbs broadly around 1 micrometer. Pyroxine absorbs in two narrow bands near 9/10 and 1.9.
Feldspar, the pale stuff, the crust, absorbs somewhere else entirely and far more weekly. The rover pointed at the rock and it read [music] the light coming off it. 25. The curve came back to Beijing to a control room at the National Astronomical Observatories where a team had been waiting, some of them for most of their careers, to find out what was under the floor of that basin. And on the screen, the curve had two deep absorption bands, one paired trough near 9/10 and 1.9 micrometers, low calcium pyroxine, and one broad absorption centered near 1.05 olivine, not feldspar, not crust, not an orthocite. The other thing, the heavy thing, the thing that sinks. 26. I want to be precise about why that mattered because the significance is easy to state and easy to [music] skate past.
Olivine and low calcium pyroxine are mantle minerals. They are dense. When the moon was young and molten, a worldsized ocean of liquid rock, these are the crystals that formed and sank.
They went down. They settled [music] out beneath the crust hundreds of millions of years before anything on Earth was alive. And they have been down there ever since. They have no business on the surface. Under ordinary circumstances, that material sits under 20 m of the [music] very crust it sank beneath. And here it was on the rim of a nameless crater in sunlight, which meant that something once had torn the moon open deep enough to reach it. 27. The lead scientist on Changanger 4's payload is a man named Lee Chunlai. And by all accounts, he is not given to dramatic statements. He is careful. He speaks the way people speak when they know the data will outlast whatever words they wrap around it. When he described that spectrum, he used one word, unambiguous.
That's a small word. It is a deliberately small word. But in a field where almost everything is a probability and almost every claim comes hedged in three directions, unambiguous is about as loud as anyone gets. The missing mantle problem had an answer after 30 years.
28. And the answer was not that the mantle had never been there. The answer was that it had been there all along, exactly where the models said it should be, thrown up by the impact, scattered across the basin floor precisely as predicted, and then buried [music] under lava, under dust, under 4 billion years of quiet, patient destruction.
The burial hypothesis had been right the whole time. It just couldn't be proven from above because the very thing that buried the mantle also hid [music] it from every instrument that came looking.
It took landing. It took wheels on the ground. It took a small crater that happened to be in the way and a machine slow enough and patient enough to stop and look at it. [music] 29. Nature published the finding in May of 2019.
The title was as restrained as the man who led the work. No exclamation, no claim of triumph, just an initial spectroscopic identification of mantle derived material on the lunar far side.
And what it meant beyond that one basin is this. We have never held a piece of the moon's deep interior. Not once.
Everything we know about what the moon is made of down where the rock is old, all of it is inference.
Gravity models, seismic echoes from four instruments the Apollo astronauts left on the near side. Assumptions stacked on assumptions.
And that matters because the deep interior of the moon is the best surviving evidence we have about where the moon came from.
about the collision that made it, about the earliest, most violent chapter of our own planet's history. For the first time, we were looking at the real thing, not a model of it, the rock itself in sunlight on the rim of a crater no one had bothered to name. 30. The rover didn't stay long. It took its readings.
It sent them up to the relay satellite and the satellite sent them home and then it moved on because there were other rocks, other craters, other ground it hadn't [music] looked at yet. It has never gone back. That crater is still there tonight. Its rim still sharp.
[music] The broken rocks still lying where the impact threw them slowly darkening in the sunlight. the way everything on the moon eventually darkens. In a [music] few tens of millions of years, there'll be dust like everything else, and the crater's edges will have softened, and there will be nothing to distinguish it from the millions of others. It waited 4 billion years to be [music] looked at. It was looked at for one afternoon, and then the machine drove on. We're going to leave the far side for a while now and go to the other side of the moon [music] and to a different Chinese spacecraft and to a question [music] about fire about whether the moon ever really went cold but rest here first on the rim of the crater with no name where the oldest rock on the moon sat in the dark for 4 billion years and then sat in the light for a few million more And [music] then for one single afternoon was seen.
The fire that should have gone out.
1931.
Small worlds cool fast. That's not a rule anyone invented. It falls out of geometry. A body loses heat through its surface and it stores heat in its volume. And [music] when you make something smaller, the volume shrinks faster than the surface does. So a small world has too much skin for the amount of warmth inside [music] it. It bleeds heat away and it cannot replace it. A cup of coffee goes cold before a bathtub does. Same physics, same reason. The moon is a small world about a quarter of Earth's diameter, roughly 180th of its mass. So the moon should have gone cold early and it should have stayed cold. 32 For a long time, the samples agreed.
Apollo brought [music] back rock. The Soviet lunar landers brought back rock.
Meteorites knocked off the moon by later impacts arrived here on their own. And we picked those up, too. And every volcanic sample anyone dated. Every piece of bassalt, every frozen remnant of lava that once flowed across those dark seas came back at roughly the same age. 2.8 billion years. 2.9 somewhere in there. And nothing younger.
33. So that became the picture, and it was a reasonable picture. The moon had been hot once, violently so, molten all the way through. Then it cooled and it cracked, and lava rose through the cracks and flooded the lands. Those dark plains you can see from any back garden on any clear night. And then roughly 3 billion years ago, it ran out of heat.
The eruption stopped. The lava froze where it lay. And the moon has been a dead world ever since. A stone, a fossil, something that finished happening a very long time ago and has simply been sitting there since, being hit by things. That was the story. It was in the textbooks. It was taught [music] 34. There was one loose thread. If you count craters on a surface, you can estimate its [music] age because craters accumulate at a roughly steady rate. The way footprints accumulate in a field nobody walks around. More craters, older ground. Fewer craters, younger ground.
And there were places on the near side, patches of dark basaltt where the crater count came out low, suspiciously low.
Low enough to suggest those lava planes might be 1 and a half billion years old, maybe even one, which was impossible because the moon was supposed to have been dead by then. But crater counting is calibrated against dated [music] samples. And every dated sample we had came from a handful of Apollo landing sites, all clustered on the near side, all of them old. So the method was being asked to measure something it had never been anchored against. And nobody [music] could say whether the low counts meant young lava or whether the method had simply drifted out past the edge of where it was reliable.
Nobody could say because nobody had a rock from those planes. 35. In December of 2020, a Chinese spacecraft called Chang Air5 landed in a place called Oceananis Proarum, the ocean of storms. It landed near a volcanic dome called Mons Roomka, chosen deliberately out of everywhere on the moon because the crater counts there said the basaltt was young. It was the first mission to bring lunar material back to Earth since 1976, 44 years. Two full generations of scientists had trained, worked, and retired in the gap. It scooped and drilled [music] and gathered about 1.7 kg of rock and soil, less than 4 lb. It lifted off the surface. It rendevued in lunar orbit automatically with nobody flying it and it came home. 36. The samples went into a laboratory [music] in Beijing, opened under nitrogen so no breath of Earth's air would ever touch them. And then the basaltt fragments were dated using lead isotopes which work as a clock because uranium decays into lead at a rate that has never varied and cannot be hurried. You measure the ratio. The ratio tells you how long ago the rock crystallized. It is one of the most precise instruments in all of science, and it does not care what anybody expected. The answer came back 2.03 billion years. 37 800 million years younger than anything anyone had ever dated from the moon. Let me try to hold that open for a moment because the number is easy to hear and hard to feel. 800 million years is longer than there has been complex life on Earth. When that lava was still liquid, still moving, still glowing, still spreading out across the floor of the ocean of storms. There was nothing on this planet but single cells, no plants, no animals, no bones, no eyes, no shells, nothing that could have looked up. The moon was supposed to have been a dead rock for a billion years by then. It was still burning 38. And that is a genuine problem, not a small correction. A problem because the physics hasn't changed. The moon is still small. It still should have cooled. The heat should have been gone.
The usual explanation for late vulcanism [music] anywhere is radioactive heating. There's a class of material rich in potassium [music] and rare earth elements and phosphorus that concentrates radioactive elements and keeps generating warmth long after everything else has cooled.
Geologists call it creep. If you had a pocket of that under the crust, it could keep the rock soft enough to melt. So the team looked for it in the samples, in the chemistry [music] of the rock itself, and it wasn't there. The Chang 5 basaltt came from a source that was poor in exactly the material that was supposed to explain it. 39.
So, the honest answer is we don't fully [music] know. There are proposals.
Perhaps the melting happened shallower than we thought in a layer that was easier to melt, not hotter. Perhaps the moon's interior overturned long ago, and moved fusible material into a place where it could melt at lower temperatures.
Perhaps a subcrustal layer heated the mantle from above, downward rather than from within. Every one of those is a serious idea with serious people behind it. None of them is settled. What we know is the fact. The rock [music] is 2 billion years old. It was liquid. The moon was still warm. And nobody has a clean explanation for why. I find that I like that there's something restful in a question that stays open, that resists us quietly and goes on being true whether or not we have understood it yet. 40. There's one more thing that sample did and it's the part that will outlast the rest. Remember crater counting, the method that ages every surface in the solar system by counting the marks on it. [music] It needs anchors, real rocks of known age, from known ground. And there was a hole in the anchors, a gap running from about 3 billion years ago down to about one. Nothing dated, nothing to calibrate against. Every age estimate that [music] fell inside that window on Mars, on Mercury, on the moons of Jupiter and Saturn, on worlds we will never touch, was an extrapolation across empty space.
The Chong F5 basaltt landed in the middle of that [music] gap. 2.03 03 billion years right in the center of the emptiness. Which means that a few pounds of rock scooped [music] from a plane on the near side of the moon quietly adjusted the age of surfaces across the entire solar system. Places no one has ever been. Places no one will ever go.
[music] That is what a sample does. It doesn't just answer the question you asked it. 41. And here is where the story turns. Because Chunga 5 landed on the near side, the familiar side, the half we have been looking at forever, the half Apollo went to, the half every dated rock in every collection on Earth had come from. Every single thing we knew about the inside of the moon, its chemistry, its heat, its history, its fire, came from one hemisphere. And the other hemisphere, as we've already seen, does not look the same. Different crust, different thickness, no dark seas, a thousand-mile scar. So a question sits there unavoidable and it had never once been tested. Is the far side of the moon the same world underneath? Or have we spent 400 years studying one face and quietly assuming the rest of it matched?
We'll go and find out next. There's a spacecraft that went and got the answer and it came home in 2024.
But rest here first in the ocean of storms, where the lava was still moving, when the earth had nothing on it but the smallest possible life, where the fire went out so long ago that nothing remembers it, [music] and where a small machine came down and picked up the ashes and carried them home to be read.
The fire that should have gone out, 1931.
Small worlds cool fast. That's not a rule anyone invented. It falls out of geometry. A body loses heat through its surface and it stores heat in its [music] volume. And when you make something smaller, the volume shrinks faster than the surface does. So a small world has too much skin for the amount of warmth inside [music] it. It bleeds heat away and it cannot replace it. A cup of coffee goes cold before a bathtub does. Same physics, same reason. The moon is a small world, about a quarter of Earth's diameter, [music] roughly 180th of its mass. So, the moon should have gone cold early, and it should have stayed cold, 32.
For a long time, the samples agreed.
Apollo brought back rock. The Soviet lunar landers brought back rock.
Meteorites knocked off the moon by later [music] impacts arrived here on their own. And we picked those up, too. And every volcanic sample anyone dated.
Every piece of bassalt, [music] every frozen remnant of lava that once flowed across those dark seas came back [music] at roughly the same age. 2.8 billion years. 2.9 somewhere in there and nothing younger, 33. So that became the picture, and it was a reasonable picture. The moon had been hot once, violently so, molten all the way through. [music] Then it cooled and it cracked and lava rose through the cracks and flooded the lands. Those dark plains you can see from any back garden on any clear night. And then roughly 3 billion years ago, it ran out of heat.
The eruption stopped. The lava froze where it lay. And the moon has been a dead world ever since. A stone, a fossil, something that finished happening a very long time ago [music] and has simply been sitting there since, being hit by things. That was the story.
It was in the textbooks. It was taught.
34. There was one loose thread. If you count craters on a surface, you can estimate [music] its age. Because craters accumulate at a roughly steady rate, [music] the way footprints accumulate in a field nobody walks around. More craters, older ground.
Fewer craters, younger ground. And there were places on the near side, patches of dark basalt, where the crater count came out low, suspiciously low. Low enough to suggest those lava planes might be 1 and a half billion years old, maybe even one, which was impossible because the moon was supposed to have been dead by [music] then. But crater counting is calibrated against dated samples. And every dated sample we had came from a handful of Apollo landing sites, all clustered on the near side, all of them old. So the method was being asked to measure something it had never been anchored against. And nobody could say whether the low counts meant young lava or whether the method had simply drifted out past the edge of where it was reliable.
Nobody could say because nobody had a rock from [music] those planes. 35. In December of 2020, a Chinese spacecraft called Chang Air5 landed in a place called Oceananis Proelarum, the ocean of storms. It landed near a volcanic dome called Mons Roomka, chosen deliberately out of everywhere on the moon because the crater counts there said the basaltt was young. It was the first mission to bring lunar material back to Earth since 1976, 44 years. Two full generations of scientists had trained, worked, and retired in the gap. It scooped and drilled and gathered about 1.7 kg of rock and soil, [music] less than four pounds. It lifted off the surface. It rendevued in lunar orbit automatically with nobody flying it and it came home.
36. The samples went into a laboratory in Beijing, opened under nitrogen so no breath of Earth's air would ever touch them. And then the basalt fragments were dated using lead isotopes which work as a clock because uranium decays into lead at a rate that has never varied and [music] cannot be hurried. You measure the ratio. The ratio tells you how long ago the rock crystallized. It is one of the most precise instruments in all of science. And it does not care what anybody expected. The answer came back 2.03 billion years. 37 800 million years younger than anything anyone had ever dated from the moon. Let me try to hold that open for a moment because the number is easy to hear [music] and hard to feel. 800 million years is longer than there has been complex life on Earth. When that lava was still liquid, still moving, still glowing, still spreading out across the floor of the ocean of storms. There was nothing on this planet but single cells, no plants, no animals, no bones, no eyes, no shells, nothing that could have looked up. The moon was supposed to have been a dead rock for a billion years by then.
It was still burning 38. And that is a genuine problem, not a small correction.
A problem because the physics hasn't changed. The moon is still small. It still should have cooled. The heat should have been gone. The usual explanation for late vulcanism anywhere is radioactive heating. There's a class of material rich in potassium and rare earth elements and phosphorus that concentrates radioactive elements and keeps generating warmth [music] long after everything else has cooled.
Geologists call it creep. If you had a pocket of that under the crust, it could keep the rock soft enough to melt. So the team looked for it [music] in the samples, in the chemistry of the rock itself, and it wasn't there. The Chonger 5 basaltt came [music] from a source that was poor in exactly the material that was supposed to explain it. 39.
So, the honest answer is we don't fully know. There are proposals. Perhaps the melting happened shallower than we thought in a layer that was easier to melt, not hotter. Perhaps the moon's interior overturned long ago, and moved fusible material into a place where it could melt at lower temperatures.
Perhaps a subcrustal layer heated the mantle from above downward rather than from within. Every one of those is a serious idea with serious people behind [music] it. None of them is settled.
What we know is the fact. The rock is 2 billion years old. It was liquid. The moon was still warm. And nobody has a clean explanation for why. I find that I like that [music] there's something restful in a question that stays open, that resists us quietly and goes on being true, whether or not we have understood it yet. 40. There's one more thing that sample did, [music] and it's the part that will outlast the rest.
Remember crater counting? The method that ages every surface in the solar system by counting the marks on it.
[music] It needs anchors. Real rocks of known age from known ground. And there was a hole in the anchors. [music] A gap running from about 3 billion years ago down to about one. Nothing dated, nothing to calibrate against. Every [music] age estimate that fell inside that window on Mars, on Mercury, on the moons of Jupiter and Saturn, on worlds we will never touch, was an extrapolation across empty space. The Chong F5 basaltt landed in the middle of that gap, 2.03 03 billion years right in the center of the emptiness. Which means that a few pounds of rock scooped from a plane on the near side of the moon quietly adjusted the age of surfaces across the entire solar system. Places no one has ever been. Places no one will ever go. That is what a sample does. It doesn't just answer the question you asked it. [music] 41. And here is where the story turns. Because Chunga 5 landed on the near side, the familiar side, the half we have been looking at forever, the half Apollo went to, the half every dated rock in every collection on Earth had come from. Every single thing we knew about the inside of the moon, its chemistry, its heat, its history, its fire, came from one hemisphere. And the other hemisphere, as we've already seen, does not look the same. Different crust, different thickness, no dark seas, a thousandm scar. So a question sits there unavoidable and it had never once been tested. [music] Is the far side of the moon the same world underneath? Or have we spent 400 years studying one face and quietly assuming the rest of it matched? We'll go and find out next. There's a spacecraft that went and got the answer and it came home in 2024.
But rest here first in the ocean of storms, where the lava was still moving, when the earth had nothing on it but the smallest possible life, where the fire went out so long ago that nothing remembers it, and where a small machine came down and picked up the ashes and carried them home to be read. Two halves of a stranger, 1942.
On the 1st of June 2024, a spacecraft called Changair 6 came down onto the floor of Apollo Crater. Apollo Crater sits inside the Great Basin, the enormous invisible bowl we spent the second part of tonight standing in. It is about 330 mi across, which is to [music] say it would be one of the most significant features on the moon if it weren't sitting inside something five times larger. The name is a coincidence, more or less. It was named by International Convention after the American missions long before anyone imagined a Chinese lander would set down there. But there is something in it anyway. A crater named for the program that took 12 men to the near side, receiving at last a visitor on the side those 12 men never touched. 43.
The landing had to be perfect, [music] and there was no one to help. Nobody could fly it. The signal delay is too [music] long and the far side has no line of sight to Earth at all. Every burn, every correction, every moment the spacecraft measured its own altitude against the ground, [music] rushing up at it, all of that had been written in advance and loaded into its memory before it ever left the Earth. In the control room in Beijing, the engineers sat and watched numbers arrive. a little after the fact. They could not intervene. They could not steer. They could only wait to be told by a machine 400,000 [music] km away whether the thing they had spent years building had put itself down safely on ground that no human eye had ever seen from close range. It had 44.
Then it did something no spacecraft had ever done. [music] It drilled. It scooped. It gathered just under 2 kg of rock and soil about 4 lb from the surface and from 2 m down. And then it took off again. An ascent vehicle lifted off the top of the lander and climbed away from the floor of Apollo crater into black sky. the first launch in the history of the solar system from the far side of the moon. It found the orbiter [music] automatically with no one flying either one. It handed over its small sealed container and then it was discarded and left behind and it is still up there somewhere now going around and around. 3 weeks later, a capsule came down under a parachute [music] onto the grasslands of Inner Mongolia. Recovery teams were already standing in the field, waiting for it.
Inside were the first pieces of the far side of the moon ever to touch the Earth. 45. Here is what they wanted to know. For 400 years, everything we understood about the moon, its chemistry, its interior, its heat, its history, came from one face. Apollo landed on the near side. Luna landed on the near side. Chunga 5 landed on the near side. Every dated rock in every collection on Earth came from the same hemisphere. And the far side does not look like that hemisphere. Different crust, thicker, no dark seas, a wound 1,500 miles wide. So, is it the same world underneath, or has the moon been two different things all along, and we only ever examined one of them? 46.
The first answer came from water. Not liquid water. There is none. And there never was. What we mean is water bound inside the minerals themselves.
Molecules of it locked into crystal structures trapped there when the rock first cooled and carried ever since. You can measure it. And the amount tells you something about the deep interior the rock came from. Because a magma that rises from a wetter mantle [music] carries more water with it, and a magma from a drier one carries less. The near side had been measured. We had numbers.
Nobody had ever measured the far side.
It was not possible. [music] There was nothing to measure. 47. The Chunga 6 basult came back dry. The mantle source it came from works out to something like 1 to 1 and 1/2 microgram of water for every gram of rock. That is not much.
That is a trace. But the point is the comparison because it appears to be drier than the equivalent source on the near side which is a strange thing for a world to be. The two hemispheres of the moon are not two planets. They form together from the same material in the [music] same event. There is no ocean between them. No boundary, nothing separating one from the other except the fact that one of them faces us. And yet the water inside them is not the same.
48. The second answer was stranger and it goes deeper. The rock was also examined for its isotopes, strontium and neodymium. These are the kind of measurement that lets you look past the rock itself and see the source, the region [music] of mantle it melted out of hundreds of miles down billions of years ago. And the source came back extraordinarily depleted. That word has a specific meaning here. It means the mantle down there had been stripped.
Something had already pulled out of it all the elements that melt easily, [music] drawn them off, taken them away, leaving behind a residue, a mantle that had already given everything it had to give.
It is one of the most depleted mantle sources ever measured anywhere on the moon. 49. which raises the question of what did the stripping and there is a candidate sitting directly overhead the impact. If the collision that carved the great basin was violent enough and everything we said in the second part suggests it was it may not have simply excavated the crust. It may have melted the mantle beneath it hundreds of kilometers down. And that melt once formed would have risen [music] and separated and carried away everything easily melted with it. [music] Leaving behind exactly what we found, a mantle that had been drained. If that is right, then the scar on the far side is not only a hole in the surface. The impact reached inside. [music] It changed what the moon is made of down where nothing can be seen. And 4 billion years later, a small spacecraft scooped up 2 kg of the residue and carried it back to be measured. 50. The basalt was also dated [music] 2.8 8 billion years, which puts farside vulcanism squarely in the same era as the near sides. And combined with an older fragment in the same hall, dated to around 4.2 [music] billion, it means the far side was erupting lava across a span of more than a billion years. So, it wasn't dead.
[music] It was never dead. It simply produced almost nothing to show for it.
The near side flooded, [music] the far side didn't. Same fire, same age, same world. And on one hemisphere, the lava filled the lowlands and made [music] those dark seas that every human being who has ever lived has looked up at, and on the other, it barely reached the surface at all. 51. So what made the difference? The leading explanation is boring and it is probably right and I find something restful in that. The crust on the near side it's thin [music] around 20 m. On the far side it's thicker around 25 and in places more.
And lava is not strong. It rises because it's buoyant. [music] And it stops when the weight above it is more than it can lift. A few miles of extra rock. That's all. Not a different chemistry, not a different history, not some deep asymmetry in what the moon is. Just a lid that was slightly heavier on one side. [music] And so on one face of the moon, the lava broke through and spread out and made the dark seas and the man in the moon and the rabbit and every shape that every culture on earth has ever seen up there. And on the other face, it rose and pressed against the underside of the crust and did not have quite enough strength to finish. 52. I keep thinking about that. [music] Every face humanity has ever imagined in the moon. Every myth, every story, every child lying on the grass making shapes out of the shadows. All of it is made of lava that happened to break through. And on the other side, the same lava rose and stopped a few miles short. If the crust had been thinner over there, the far side would have dark seas, too. It would have its own shapes, its own faces, [music] its own creatures. Nobody would ever have seen them. 53. There is one more thing in those samples, and it may be the most remarkable of all. Among the soil, researchers found grains of olivine. very magnesiumrich, very primitive. Primitive is a technical word and it means early. It means this crystal may be among the first solid things that ever formed on the moon, condensing out of the magma ocean when the moon was still a sphere of liquid rock with no surface at all, no crust, nothing you could stand on. Some of these grains may have crystallized before the moon had a face, and they were carried up, entrained in rising magma, hauled from hundreds of miles down and left on the surface, where they sat and waited until something came from the third planet and picked them up.
We're going to go somewhere very cold next to the South Pole and to craters where the sun has never once shone [music] and to what has been quietly collecting there since the beginning but rest here first with the two halves of a world that were never quite the same and with a crystal older than the moon's own face sitting now in a laboratory in Beijing having waited 4 and a2 billion years for someone to come and look at it before there was a moon. 64.
Everything we've talked about tonight, the far side, the basin, the ice, the fire, the dry mantle, the crystal older than the moon's own face. All of it rests on one question that has never been fully answered. Where did the moon come from? And I want to be honest with you about this because it would be easy to give you a tidy story and let you fall asleep on it. The tidy story exists. Most people have heard some version of it, but it is not settled. It has never been settled and the arguments about it are still going on in journals right now between people who are very good at this. 65.
Start with the strangeness because the moon is strange and it took us a long time to notice how strange it is enormous. Not in absolute terms but relative to the thing it orbits. It is more than a quarter of Earth's diameter.
[music] There is nothing else like that among the rocky planets. Mars has two moons and they are potato-shaped lumps a few miles across. captured asteroids barely worth the name. And the moon has almost no iron core. Earth has a huge one, a ball of molten metal at the center, generating the magnetic field that keeps the solar wind off your skin. The moon has a tiny one. Proportionally, almost nothing. It's the wrong size and it's made of the wrong stuff. For a long time, we had three explanations, and every single one of them broke. 66. The first was capture. The moon formed somewhere else, wandered too close, and Earth's gravity caught it. Except that capturing something that large intact without either destroying it or flinging it back out into space requires a set of approach speeds and angles so precise that orbital [music] mechanics essentially never hands them out. It's not impossible. It's just implausible enough that nobody could make it work.
The second was fishing. The young earth was spinning so fast that it flung a piece of itself off. The way a lump of wet clay flies off a potter's wheel, turning too quickly. Except that nobody could find a spin rate that would actually do this without tearing the whole planet apart in the process. And the third was co-acretion. [music] The sister theory. Earth and Moon simply formed side by side out of the same cloud of material at the same time.
Clean, simple, no violence required.
[music] Except that if they formed from the same stuff, the moon should have the same proportion of iron that Earth does.
And it doesn't. Not remotely. Three explanations.
Each one solved part of the puzzle and shattered on a different [music] piece of it. 67. The answer when it came was violence. In 1975, two American scientists proposed that the moon was not gently formed at all.
It was debris. Something enormous [music] roughly the size of Mars hit the young earth. Not headon. a glancing blow and that collision threw an immense quantity of rock into orbit and out of that ring of shattered material over time the moon slowly gathered itself together they gave the impactor a name later Thea after the Titan of Greek myth who was the mother of the moon goddess and it explains so much it explains why the moon is so large It explains why it has almost no iron. A glancing impact strips away mantle, the outer rock, and leaves the metal cores of both bodies to merge and sink into Earth. It explains why the Earth moon system spins the way it does. [music] 68. But here's the part I love because it isn't how you imagine science working. For nearly a decade, the giant impact idea sat there alongside the other three, and nobody could decide.
[music] There was no killer experiment, no decisive measurement, just four ideas and a field that couldn't agree. So, in the early 1980s, three scientists issued something closer to an ultimatum than an argument. There would be a conference in Kona in Hawaii in October of 1984.
You have 18 months. Go back to the Apollo data. Go back to your models.
Work out what you actually believe because showing up with no position is no longer going to [music] be an option.
69. And that's what broke it. Not a discovery, a deadline. At that conference, the giant impact hypothesis emerged as the clear favorite, [music] and it has been the standard explanation ever since. I think about that more than I probably should. that one of the foundational facts about our own moon, a thing you have looked at your entire life got settled not by a telescope [music] or a rock or a flash of insight, but because a few people got tired of the argument and set a date. Consensus is a human process. It happens in rooms between tired people on schedules. The moon didn't change. only we did 70 and then slowly a problem emerged and it has never gone away. If the moon is mostly made of thea, a different body formed somewhere else out of different material, then the moon should be chemically different from Earth.
Measurably, obviously, every world in the solar system has its own isotopic signature, a specific ratio of oxygen isotopes, of titanium, of chromium. It's like a fingerprint of where and when a thing formed. Mars has one. The asteroids have theirs. They're all different. So, we measured the moons and it matched Earth's almost exactly to a degree that seemed impossible. 71.
Which should not happen. If the moon is Thea's debris, it should carry Thea's fingerprint. it carries ours instead. So either thea happened by pure coincidence to be made of material almost identical to earth's which is possible but a remarkable coincidence to lean on or something homogenized them something mixed the two bodies so thoroughly that by the time the moon condensed out [music] there was no longer any meaningful difference between what came from thea and what came from earth. And that second idea produced one of the strangest objects ever proposed in planetary science.
72. The idea is that the impact wasn't just violent. It was catastrophically violent. [music] Energetic enough that both bodies briefly stopped being bodies at all. Not a planet with a debris ring around it. Something else. A vast spinning glowing structure of vaporized and liquid rock turning too fast and carrying too much energy to [music] settle into any shape a planet could hold. Wider than the Earth by many times, part vapor, part droplets, all of it churning. It didn't fit any category [music] that existed. So they had to invent a name for it. They called it a sinestia. And in the model, the Earth and the Moon both condensed out of that structure, which is why they match, because for a brief and unimaginably violent period, there was no difference between them. [music] There was no earth and no Thea. There was one thing, and it was too hot to have a surface, and everything inside it was mixed. The Earth and the Moon are not parent and child [music] in this picture. They are twins condensed out of the same cloud a few thousand years apart. We're going to sit with that for a while because it is not the accepted answer. It's one of several [music] and there is a serious argument against it that we'll come to.
But rest here first in a structure with no name until someone gave it one. Where the earth you're lying on and the moon outside your window were [music] for one brief and terrible moment the same glowing thing. The argument that isn't over. 73. The sinestia is a beautiful idea and beauty is not evidence. So people went looking for a way to test it. And the test is elegant once you see it. If the Earth and Moon really did condense out of one homogenized cloud, then every element in both of them should match, not just oxygen, everything. [music] Because if they were thoroughly mixed, there was nothing left to be different about. So the way to break the theory is to find one element that doesn't match. 74.
In 2021, a team measured venadium. Venadium is an unglamorous metal. It's in your kitchen knives. Nobody thinks about it. But its isotopes are useful precisely because nothing exotic happens to them. They don't get scrambled by the processes that scramble other elements. And the moon's venadium did not match Earth's.
It was offset, small, a fraction of a part per thousand, but real and measured carefully, and it sat outside the error bars, which if you take it at face value, is a problem for the sinestia, [music] because in a fully mixed cloud, there is no room for an offset. There is nothing left to be offset from. 75.
The team's interpretation was that the offset points [music] back toward the older, simpler picture, a classic glancing impact. [music] Incomplete mixing. Most of the moon built from Thea, a body that happened to have formed from material broadly similar to Earth's from the same neighborhood of the young solar system, which is why the other elements match so well. Not one glowing cloud, just two objects that were always somewhat alike, colliding, and not blending perfectly.
So, settled then, no. 76.
Because other measurements say [music] the opposite, calcium isotopes were measured carefully in 2023. [music] They matched. magnesium matched, iron matched, and in 2025, a study of refractory elements, the ones that stay solid at absurd temperatures [music] and so preserve the deepest memory of what a body is made of, found the Earth and Moon so nearly identical that the authors argued a classic impact simply cannot produce that. Only thorough mixing can. [music] So, one line of evidence says the bodies didn't fully blend. Several others say they did. Both sets of measurements are good. Both were done by careful people. They cannot both be right. 77. And I want to stay here for a moment because this is what science actually looks like most of the time. Not a revelation, not a moment where someone shouts. A slow, patient disagreement between honest people [music] conducted in journals over decades with the evidence pointing several directions at once. We do not know how the moon formed. We know it was violent. We know something enormous hit the earth. Beyond that, the details, how violent, how thoroughly things mixed, what Thea was actually made of, [music] whether it was even one impact or two, remain genuinely open. There is a thing hanging in your sky and it has been there every night of your life, and we have not finished working out where it came from. 78. And then in 2025, something new arrived and it's one of the most remarkable results in this whole field. They found out where Thea came from. Not what it was made of exactly, where it formed, which region of the young solar system it [music] condensed in. The technique is iron isotopes.
Because the solar system when it was forming was not uniform. The inner part [music] close to the sun, hot, dry, carried one isotopic signature. The outer part the frost line carried another. And those signatures never mixed. They're still detectable today [music] in meteorites 4 and a half billion years later. 79. They measured the iron in lunar rock. They measured it in Earth's rock. And they measured it across the meteorites. The surviving fragments of both the inner and outer solar system. And the Earth and the Moon sat together at one end of the range, the inner end, the hot end, close to the sun. The was not a wanderer from the outer dark. It was not some icy visitor that fell inward and struck us by chance. It formed near where we formed, and by some readings of the data, it may have formed even closer to the sun than the Earth did. 80. So, the object that destroyed the young Earth and made the moon was, in a real sense, a neighbor.
It condensed out of the same disc of dust in the same region at more or less the same time. It grew the way Earth grew, sweeping up rock, colliding with smaller things, getting larger. It was a planet or nearly one. It had a core and a mantle and presumably a surface, and its orbit was not quite stable, and ours was not quite empty. And so one day, roughly 4 and a half billion years ago, two worlds that had grown up in the same neighborhood found each other. One of them ended, and the other one is under you right now. 81.
There is one more piece, and it changes how you should look at the moon tonight.
The impact left both bodies molten.
[music] The moon in particular came out of it as a sphere of liquid rock, an ocean of magma, hundreds of miles deep with no surface at all, nothing you could stand on, nothing that would hold you. And as it cooled, it sorted itself. The light minerals, feldspar, crystallized and floated. They rose through the liquid rock and gathered at the top and cooled there and that became [music] the crust.
The pale highlands, the bright ground, the heavy minerals, olivine, pyroine crystallized and sank down through the melt, settling out beneath the crust into the dark that became the mantle.
And that is why everything in tonight's story fits together. That is why finding Olivine on the surface meant something.
That is why the missing mantle problem was a problem at all. Because the moon separated itself once when it was young and liquid, light on top, heavy below, and it has stayed that way ever since.
And the only thing that has ever brought the deep material back up into the light is violence. an impact large enough to reach in and tear it [music] open. We're going somewhere quieter next to something that is happening right now slowly while you lie there and that will not stop but rest here first in the magma ocean before there was a surface before there was a face before there was anything to see [music] when the moon was still deciding what it would be. It is leaving 19 ice 82. The moon is moving away from us. Not dramatically, not in any way you will ever see or feel, but steadily, measurably, without pause. And it has been doing this for as long as it has existed, about an inch and a half a year. That's the width of your thumb.
every year quietly while you sleep, while you work, while everyone you have ever known lives out their entire life and dies. You will never notice it.
Nobody has ever noticed it. It is far too slow for a human life to register.
And [music] it is completely relentless and it has been going on since before there was anything on this planet with eyes. 83. We know this with absurd precision. And the reason why is one of my favorite things about the whole Apollo program. They left mirrors up there. Small panels of reflectors set out on the lunar dust by the astronauts and simply abandoned when they climbed back into the module and went home.
They're built from corner cubes, a shape with the peculiar property that any light entering it goes straight back the way it came, no matter what angle it arrives at. So for more than 50 years, observatories on Earth have been firing lasers at the moon and catching the light when it comes back. You send a pulse, you wait, [music] you time the round trip. Light travels at a speed that never varies and [music] the distance falls out of the arithmetic.
The measurement is accurate to within a fraction of an inch across a quarter of a million miles. 84. And those mirrors are still up there, still working tonight. Nobody has maintained them.
Nobody has touched them [music] since the 1970s.
There is no power source, no electronics, nothing to break. They are just polished glass sitting in the dust doing the one passive thing they were shaped to do. The men who placed them are mostly gone now. The mirrors will still be there in a million years. And in a 100 million, they will still be pointing at the Earth, still ready to send a pulse of light back [music] to a planet that may by then have nobody left on it who knows why they're there.
Changa 6 carried one too, incidentally, a small passive reflector [music] contributed by Italy sitting now on the floor of a crater on the far side. So, there is one more mirror up there than there used to be. Waiting for a laser that hasn't been fired yet. 85.
So, why is it leaving? And here is the surprise. The reason is the ocean. The moon pulls on the sea. It raises a bulge of water on the side of the earth facing it. That's the tide. And that much everyone knows. But the earth is spinning underneath that bulge and spinning much faster than the moon travels around us. So the spinning earth drags the bulge forward. It carries the tide out ahead of where the moon actually is. Which means there is now a great mass of water slightly ahead of the moon tugging on it, pulling it forward, adding energy to it, speeding it up. And an object that gains energy in orbit does not fall closer. It climbs. 86. And there is a price. There is always a price. Energy does not come from nowhere. Every inch the moon climbs, it takes from the spin of the earth. We are slowing down. The tides are a break. [music] an immense soft unrelenting break made of seawater applied continuously to the rotation of an entire planet and it has never once been released. The day is getting longer about 2,000th of a second per century.
That is nothing. It is beneath measurement in a human lifetime. It is also on the scale of the earth's history one of the most consequential facts about this planet because it [music] never stops. 87 which means the past was genuinely different not metaphorically physically. If the day is lengthening now it was shorter before and the moon which is climbing now was closer before and we can prove it. We can read it out of stone. There are rocks called tidal rhythmmites, sediments laid down in ancient shallow seas where each thin band of silt corresponds to a single tide coming in. Count the bands. Count how many fall between the monthly cycles. And you can recover from a slab of rock how many days there were in a month 600 million years ago. There are fossil corals that do something similar.
Corals lay down fine daily growth bands and coarser annual ones. Count the daily lines between the yearly ones and the rock tells you how many days the year had. Living corals give you about 365.
Deonian corals 400 million years old give you around 400 88 400 days in a year. Not because the year was longer.
The Earth's orbit hasn't changed. The year has always been the year. There were simply more days in it because each one was shorter. And you can push further back. around 1 and a half billion years ago. The day was roughly 18 hours long. Go back to 1.6 billion and it drops to something closer to 17 and 3/4. Sit with that for a moment.
There was a time on this planet when the sun came up, crossed the sky, went down, and came up again. And the whole thing took less time than the gap between one of your birthdays and [music] lunch the following day. Nothing was watching.
There was no eye on Earth capable of seeing a sunrise. Just single cells in warm shallow water under a sky that turned faster than any sky has turned [music] since. And the moon hung closer, larger, brighter, [music] pulling harder on a sea with nothing living in it worth speaking of. 89. Now, here is the part that genuinely surprised me when I first learned it.
The moon has not been leaving at a steady rate.
>> [music] >> You'd assume it would be constant, a slow, unvarying drift, [music] the same inch and a half year after year all the way back. It isn't, not remotely, around 300 to 200 million years ago, the moon was retreating at roughly 7 cm a year, almost twice the current rate. And in the long stretch before that, through the Cambrian, through the Carboniferous, across hundreds of millions of years, it was slower than today, around 2.2 cm.
So, it crept. Then, it sprinted. Then, it settled back toward the pace it keeps now. The moon has been speeding up and slowing down in its departure all along.
[music] And nobody standing on the earth could have known. 90. And the reason is the shape of the oceans. Tides are not really about the moon. They're about the water [music] and about how water sloshes. Every ocean basin has a natural rhythm, a frequency it prefers, the way a bathtub does if you rock it back and forth. Get the rocking close to that rhythm, and the water piles higher and higher [music] with almost no extra effort. So when the tide's timing falls near the ocean's own preferred rhythm, the tides swell, the breaking strengthens, the moon flees faster, [music] and ocean basins are not permanent.
Continents drift, they close seas and tear open new ones. And every single time they do, they retune the tides of the entire planet. Which means that the pace at which the moon leaves the sky is being set has always [music] been set by the slow wandering of continents on a world 400,000 km away from it. Around the time pangia existed, something in that arrangement brought the tides into a stronger resonance and the moon ran.
The drift of continents you cannot feel is governing the departure of the moon you cannot reach. 91. It will keep going, not forever and not in any way that should trouble you. The sun will swell [music] and make the earth unlivable long before the moon gets far enough away to matter to anyone. Nobody needs to lose sleep over this, least of all tonight. But it is going [music] an inch and a half a year and it will not stop. And there is nothing that could ever be done about it. And there is one thing it will take with it. Total solar eclipses exist because of a coincidence so precise it barely seems possible. The moon is 400 times smaller than the sun and it sits 400 times closer. And so from this planet in this era, the two of them appear exactly the same size in the sky. Nowhere else in the solar system does this happen. No other moon fits its sun so perfectly and it is temporary. As the moon retreats, its disc will shrink and one day it will no longer be quite large enough to cover the sun completely. There will be a last total eclipse. Somewhere out there, hundreds of millions of years from now, on one particular afternoon, the shadow will sweep across the ground for the final time, and then it will never happen again. Nobody will know it was the last one. That is the thing about last times.
They almost never announce themselves.
We're going back to the far side now one last time. Back to where we began. But rest here first on a planet that is slowing down under a moon that is quietly letting go an inch and a half every year. Patiently without malice [music] and without ever once looking back. The face that never turns. 92. We started the night on the far side. So, let's go back there down onto the floor of Von Carman Crater where a small machine has been sitting for years now.
It is night there at the moment. Or it will be soon. The lunar night arrives every 14 days and lasts another 14 and there is no way to hurry it. U22 is folded up, panels closed, instruments off, waiting. Outside, the temperature has fallen past 290° below zero. There is no wind. There is no sound. There is nothing at all except the starlight and the ground and a machine that has done this more than 70 times and is preparing once again to survive it. 93 Somewhere behind it on ground it crossed years [music] ago.
There is a small crater with no name, 70 m across, sharp rimmed, utterly unremarkable. And on its rim there are rocks olivine low calcium pyroxine that formed perhaps a 100 kilometers down inside the moon [music] in the dark when the moon was still liquid and still deciding what it would become. They sank there. They stayed there for hundreds of millions of years while the young solar system finished its worst violence.
Then something 120 mi across struck this hemisphere and tore the crust open and threw them out onto the surface and lava covered them and dust buried them and they lay there sealed through 4 billion years in which absolutely nothing happened [music] to them. And then a few million years ago, or perhaps a few tens of millions, a small rock struck the ground nearby and opened a hole 70 m wide and brought them back into the light where they sat and waited a little longer until a six- wheeled machine came over a rise and stopped [music] and looked at them. 94. That's the shape of it. That's what tonight has been circling around. A piece of the inside of a world which waited 4 billion years to be seen and traveled no distance at all to get there. It didn't go anywhere.
[music] It never moved. It was simply exposed and then buried [music] and then exposed again by nothing but accident by the arithmetic of falling rocks. And then one afternoon it was read. 95. And there is the seed sealed in its small canister on the lander a few hundred meters away. a cotton sprout that lived 9 days in January of 2019 and then froze when the night came. The first living thing to grow on the surface of another world and the first to die there. [music] It is still in the tin. It will be in the tin in a million years. There is nothing on the moon that will ever break it down. No bacteria, no rot, no rain, no chemistry of any kind.
Decay is a thing living planets do.
[music] It requires life to perform it.
That cotton chute will simply remain freezedried, intact, on the floor of a crater named for a man whose most gifted student was accused and stripped of his clearance and sent away, and who went home and built the program that put it [music] there. 96 And the whole time that ground has never seen the earth. I want to say that plainly because it's easy to hear and hard to absorb. If you stood on the floor of Von Carmon and looked up, there would be no earth in the sky. Not small, not distant, not a pale dot on the horizon. Nothing. The Earth has never risen over that ground. Not once in 4 and a half billion years. Not for a moment, at any angle, in any season, at any point in the entire history of the solar system. Every photograph ever taken of the Earth from the surface of the moon, every [music] earthrise, every blue marble hanging above the gray was taken from the near side because on the far side there is no shot to take. 97.
And that is because of tidal locking, the same slow grinding process we described at the very beginning of tonight. The Earth's gravity pulling harder on one side of the moon than the other over hundreds of millions of years, wearing down the spin until it matched the orbit. And once that happened, once the two locked into step, half of the moon turned toward us and half of it turned away forever. Not by design, not for any reason, [music] just because that is where the spinning stopped. 98. And here is the thing I keep coming back to [music] and the reason we've spent the night on this.
The two halves are not the same.
Different crust, thicker on the far side, different volcanic history.
[music] The near side flooded with dark seas. The far side barely erupting at all. A drier mantle underneath. A source rock stripped and depleted in a way the near side isn't. The largest scar in the inner solar system on the half we can't see. 400 years of astronomy, every telescope ever built, every myth, every face, every rabbit anyone has ever seen in the moon. And all of it was one hemisphere. We knew the moon the way you know someone's face across a room completely in every detail and not at all. 99. The far side was never hidden.
That's what I'd want you to take from tonight. If you take anything, it was never dark and it was never secret and it was never withholding anything from us. It has been sitting in full sunlight for 4 and a half [music] billion years.
Exactly as it is, waiting for nothing, [music] expecting no one. It simply faced the other way. And we were on the wrong side of a locked rotation, standing on a planet that couldn't see it, calling it dark because we could not think of a better word for a place we had no way to reach. The shadow was never on the moon. 100. It is still night on the far side. You2 is folded up in the cold doing nothing, waiting the way it has waited 70 times before. In a few days, the sun will come around and the panels will warm and the mast will lift and the cameras will take a photograph of a plane that has not changed at all since the last time it looked. And then it will drive on a few more meters, slower than you walk. Above it, the sky is black and full of stars and completely empty of the earth. Out past the rim of the basin, the relay satellite is still there, still turning in its slow loop, still listening for a voice that won't [music] come until sunrise.
And underneath all of it, 4 billion years deep, the moon's interior is still exactly where it settled. Heavy on the bottom, light on the top, sorted once when everything was liquid, and never disturbed since. None of it was waiting for us. It didn't know we were coming.
It did nothing at all for 4 billion years because nothing on the far side of the moon has ever had any reason to. But we came anyway slowly across 60 years of photographs and relays and one small patient rover that outlived every estimate anyone made of it. And we are still only partway through reading what is there. Sleep well. The moon will be turning all night, showing us the one face it has always shown us. And the other one, bright, sunlit, silent, will be facing out into the dark [music] the way it always has. Good night.
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