Skip to content

Spark Science Physics

Why Is The Far Side Of The Moon So Different? | Science's Greatest Mysteries Episode 1

Spark's Science's Greatest Mysteries opens with the far side of the Moon, a hemisphere that looks nothing like the one we always see. Starting from Luna 3's shocking 1959 photographs, the film walks through the three great clues to the puzzle: the missing dark maria, the near side's KREEP chemistry from the Apollo rocks, and the GRAIL gravity map showing a crust about 30 kilometers thick on the near side and 50 kilometers on the far side. It then lays out three competing explanations that all begin with the giant impact that formed the Moon: uneven cooling next to a red hot young Earth, a second giant impact that flung the near side onto the far side, and a lost companion moon that merged with ours in a slow motion splat. It closes with the new evidence, frozen Apollo samples and China's Chang'e far side landings, that may finally settle which one is right.

Published Apr 9, 2025 51:57 video 33 min read Added Jul 7, 2026 Open on YouTube →

At a glance

On the 7th of October 1959 a small Soviet probe called Luna 3 swung around the back of the Moon and photographed a face no human being had ever seen. The pictures were so unlike the familiar side that some people in the United States accused the Russians of faking them. This first episode of Spark's Science's Greatest Mysteries follows the shock of that moment forward through 65 years of hard science, because the far side really is a different world. It is rugged where the near side is flat. It has almost none of the dark volcanic plains that give the near side its familiar face. Its crust is around 20 kilometers thicker. And its rocks are missing a radioactive chemical signature that soaks the near side.

The film builds the case in order, one discovery at a time. Luna 3 reveals the two faces. The Apollo rocks reveal the chemistry. NASA's GRAIL gravity mission reveals the lopsided crust. Then it lays out how the Moon was born, the giant impact hypothesis, and hands the microphone to three scientists who each think they can explain why that birth left the Moon so divided: a molten Moon baked unevenly by a red hot young Earth, a second giant impact that flung the near side onto the far side, and a long lost companion moon that merged with ours in a slow motion splat. None of them has won yet. The last act is about the evidence that could settle it, the frozen Apollo vaults, China's Chang'e landings on the far side, and the seismometers a future far side base might finally plant in the ground. By the end you have the whole case file, and a clear sense of why one grey rock in the sky is the best crime scene in the solar system.

Luna 3 and the photograph that shocked the world

The story opens with a machine and a date. On the 7th of October 1959 the Soviet probe Luna 3 passed over the lunar surface and its onboard camera took a series of pictures of a place no one had ever laid eyes on. When the images were beamed back to Earth, they crossed every border at once. As one scientist puts it, anything we know is there but cannot see always pulls at our curiosity, and here at last was the answer to the oldest of all lunar questions: what is on the other side.

The answer was a surprise. The photograph looked almost nothing like the front of the Moon. There were even people in the United States who suggested the Russians had faked it, that this could not be the back of the Moon because it did not resemble the front. But it was real, and scientists grasped very quickly just how different the far side was from the near side. That difference has never been fully explained. The far side, the narrator says, is one of science's greatest mysteries, and the scientists in the film do not disagree. One calls it the holy grail of planetary science. Another, Erik Asphaug, frames it as the opposite, one of the low hanging fruits, an accessible problem you could actually hope to crack in a single lifetime.

The episode signals from the start that we are living in a special window for this question. NASA locked away some of its most precious Moon rocks half a century ago and is only now releasing them, armed with instruments that did not exist in 1972. China has landed on the far side for the first time in history. In the next 10 to 20 years, everyone in the film agrees, our whole picture of the Moon is going to change.

Why we only ever see one face

Before you can ask why the two sides differ, you have to understand why there even are two fixed sides. The Moon, like most large satellites in the solar system, is tidally locked: it keeps one hemisphere turned toward its planet at all times. So there is a near side that is always near and a far side that is always far. For as long as humans have looked up, that far side stayed hidden, and the mind rushes to fill a hidden place with stories.

Some of those stories were wonderfully strange. There were tales that the Germans had built a base on the back of the Moon during the Second World War. Isaac Asimov wrote an amusing short story in which the first astronauts to round the far side discover it is nothing but stage props. The far side was a blank screen, and people projected onto it, right up until Luna 3 developed its film and replaced imagination with data.

Maria and a rugged far side

For a planetary scientist like Bill Hartmann, the Luna 3 picture was completely unexpected. Hold up the near side, he explains, and the eye goes straight to the big dark patches. Those are the lunar maria, vast plains of lava that erupted onto the surface and froze into smooth dark basalt. Something like a third of the near side is covered by these lava plains. They are the face we grew up with, the outline that reads as a man or a rabbit in the Moon.

The far side has almost none of them. The single most striking thing about the Russian photograph was that the dark patches were mostly gone. And there was more. The far side was hillier and more rugged, a battered highland terrain, in sharp contrast to the flatter, darker near side. Every lunar mission since has only confirmed the split in finer and finer detail. Two hemispheres, one Moon, wearing two completely different faces, and no one could say why.

Figure 1. The two faces of the Moon. The near side that we always see is flatter and darker, a third of it flooded by ancient lava plains, and its rocks carry a distinctive radioactive chemistry. The far side is a battered highland world with almost no maria and a crust roughly 20 kilometers thicker. Explaining that contrast is the whole mystery.

Apollo 1969: 382 kilograms of Moon

Pictures from orbit can only take you so far. As the film notes, we learn a lot from a spacecraft circling a world, but we learn far more when we can carry a piece of it home and put it under our instruments. In 1969 that finally happened. Across the Apollo missions, astronauts brought back 382 kilograms of lunar material: soils, rocks, and drilled core samples. Half a century later those samples are still working. Cosmochemist Jess Barnes is one of the few scientists on Earth studying them, and she stresses that they are under constant fresh study because our analytical techniques keep improving. Every new tool lets the same rock answer a question it could not answer before.

The KREEP anomaly

When the Apollo rocks were analyzed, they delivered a second surprise to sit beside the visual one. The two sides are not just shaped differently. They are made of different stuff. A large fraction of the returned samples carry a chemical fingerprint enriched in potassium, in rare earth elements, in phosphorus, and in radioactive elements. Scientists have a name for that exact cocktail: KREEP, from potassium (K), the Rare Earth Elements (REE), and phosphorus (P). Crucially, every sample that carries it comes from the near side.

We have never landed on the far side to collect and return rocks, so for the far side the film relies on random samplings delivered by meteorites knocked off the Moon. Compare the two and the contrast is stark. The KREEP signature is all over the near side and simply is not found on the far side. As Barnes says, this is not just a mystery of appearance. It is a geochemical anomaly on the near side that the far side does not share. Whatever divided the Moon divided its chemistry too.

GRAIL maps the invisible gravity field

The next clue came not from a camera or a rock hammer but from gravity. A few decades after Luna 3 and Apollo, NASA launched a mission to map something you can never photograph: the Moon's gravitational field. That field is worth mapping because it reads the interior. The way gravity varies from place to place tells you how mass is distributed below the surface, which is one of the only ways to see down into the crust and ask how thick it is. Earlier spacecraft had measured lunar gravity, but at hopelessly coarse resolution, and scientists realized that answering the big questions would need a far sharper map.

That map was GRAIL, the Gravity Recovery and Interior Laboratory. Mark Wieczorek was one of its lead scientists, and he calls it one of the highlights of his career. The goal was the most precise gravity map of the Moon ever made, and for the first time a good map of the far side, which had been very poorly constrained. GRAIL did it with two spacecraft flying in formation, playfully named Ebb and Flow. As they circled the Moon, tiny changes in the Moon's gravity tugged one satellite slightly ahead of or behind the other, and by measuring the minute fluctuations in the distance between the two, scientists reconstructed the field in extraordinary detail. Wieczorek says that after just the first month of data the map was already better than he had hoped to have at the very end of the mission, roughly 10 times better than anything before it.

A lopsided crust, 30 km against 50 km

Turn that gravity map into a map of crust thickness and the far side mystery gets a new and physical dimension. On GRAIL's color scale, blue marks a thin crust close to zero and red marks a thick one near 60 kilometers. The pattern is unmistakable. The near side crust averages around 30 kilometers. The far side crust averages around 50 kilometers. The presence of rugged highlands on the far side had already hinted the crust would be thicker there, but the precision of GRAIL turned a hunch into a hard number: a difference of more than 20 kilometers between the two hemispheres, with no explanation for why. As Wieczorek puts it, the signal was so strong you could not miss it. It was right in front of your face.

Figure 2. The lopsided crust GRAIL revealed. The Moon's rocky interior sits slightly off center, pushed away from Earth, so the crust is thin on the near side that faces us and much thicker on the hidden far side. Any successful theory of the far side has to explain not just its look and its chemistry but this stubborn 20 kilometer difference in crust thickness.

Three separate lines of evidence now pointed the same way. The Moon was not just two faced in appearance. Its two sides were chemically different, and its crust was lopsided. The cause of all of it was a single unsolved puzzle, and to solve it, scientists first had to answer an even older question: how did the Moon get here at all?

The giant impact hypothesis and the Hawaii conference

Bill Hartmann has been painting the solar system since he was a boy, and he offers the film its best working motto: a strange sentence popped into his head one day, that if you cannot draw it, you probably do not understand it. His life's work has been drawing the Moon well enough to understand it. He starts from what makes ours so odd. Most satellites are tiny next to their planets. Ours is about a quarter the size of Earth. However the Moon formed, it was clearly an unusual process.

As a young astronomer, Hartmann and his colleagues began to read the Moon's surface as a record of violence. The recognition of a huge impact basin helped settle a long argument, showing that the great crater like features are not volcanic but carved by big impacts. Interplanetary debris was hammering the Moon constantly. That reframing led Hartmann to tie the Moon's history to Earth's own. He proposed that as Earth was forming, a second smaller planet, later nicknamed Theia, formed in a similar orbit around the Sun. As the two circled the star their paths eventually crossed, with catastrophic consequences. The collision blew an enormous cloud of gas and dust into orbit around Earth. That superheated debris gathered, formed a molten body, and slowly cooled into the Moon.

In the early 1980s a conference was called to see whether the field could agree on a single origin story, and someone made the inspired choice to hold it in Hawaii. Everyone came, in flower leis and, Hartmann jokes, an astonishing number of beards. Having worked out the details with his colleague Don Davis at the Planetary Science Institute, Hartmann put the idea up for tough scrutiny at what has been called one of the most significant meetings in the history of planetary science. The field went through many competing theories, and the giant impact idea, in Hartmann's words, got the A+. It still reigns as the go to explanation for how the Moon was born, because it fits so much of what we know about the solar system.

But it has a blind spot. The sheer volume of debris from such a collision neatly explains the Moon's unusual size. It says nothing about why the radioactive KREEP elements cluster on the near side today, nor why the far side crust is so much thicker, nor why the two faces look so different. The giant impact tells you where the Moon came from. It does not tell you why the Moon is divided. That is where three modern theories pick up the thread, and where they part company.

Three theories, one shared beginning

All three of the leading explanations start from the same giant impact. They agree the Moon was born molten and battered out of a collision with Earth. They disagree entirely about what happened next to leave it with two such different sides.

Theory one: the near side heating theory

For Mark Wieczorek, the answer lies in the Moon's earliest, hottest moments. The collision liberated so much energy that the young Moon began in a very hot, most likely completely molten state, effectively a global ocean of magma. As it cooled it started to crystallize and grow a solid crust. Here is the key idea: if for some reason the far side crystallized faster than the near side, it would build its crust more quickly, and that crust could end up thicker. The observed asymmetry falls out of an asymmetry in cooling.

But why would the far side cool faster than the side that always faces us? Wieczorek's answer is Earth itself, acting as a second sun. Picture a still glowing, molten Earth hanging in the Moon's sky. It would bathe the near side hemisphere in heat while the far side, turned away, radiated its warmth into cold space. The near side stays molten longer; the far side freezes first and grows the thicker crust.

There is an obvious objection, and the film raises it honestly. At the Moon's present average distance of about 380,000 kilometers, even a red hot Earth could not deliver enough heat to matter. The rescue comes from a beautiful, well established measurement. Observatories on Earth fire powerful lasers at the Moon and time how long the pulse takes to bounce back, a technique called lunar laser ranging. Decades of these measurements show the Moon is drifting away from us at roughly 3.8 centimeters per year. It sounds trivial, but run the clock backward and it describes a completely different early system. Just after the giant impact the Moon sat only a couple of Earth diameters away, roughly 18,000 kilometers, more than 20 times closer than it is now. At that range a molten Earth really would pour enough energy onto the near side to keep it soft while the far side hardened. Some scientists extend the idea to the maria too: a thinner near side crust would fracture more easily under impacts and quakes, opening the way for the later volcanic eruptions that flooded the near side with dark lava. Wieczorek thinks the heat theory is plausible, and is careful to add that it needs much more research. On paper it is elegant, explaining both the thicker far side crust and the darker near side face with a single mechanism.

Theory two: the second impact

On the other side of the world, a very different kind of scientist attacks the same problem. The film introduces him as Meng Hua Zhu, a computer scientist who has been fascinated by the near side and far side divide for a decade and who tackles it with heavy numerical simulations. The Moon was not always his subject. In 2006 he was working happily at a computer company, and it was watching the news of China's early lunar missions that redirected his whole life. At 20 he started learning lunar science; he has now studied the Moon for about 15 years. His wife jokes that the Moon might as well be his second wife.

Zhu's idea is a second collision. His simulations show that in the Moon's earliest days there was a high probability of a large impact striking it. He proposes that such an impact hit the near side and excavated an enormous amount of material. Long after the giant impact that made the Moon, another big object came dangerously close to Earth and this time slammed into the Moon's near side, blasting a vast quantity of surface material outward. That debris rained back down and settled onto the far side, building up the highlands and the thicker crust we see there, while the scarred near side is consistent with what remote sensing shows. Zhu is scrupulous about uncertainty: he cannot say the scenario is correct until there is evidence, and he cannot rule out the others. But like the heat theory it fits the facts on the table. The relocated debris builds the far side's rugged, thick crust, and the violence of the impact could melt and churn the near side, accounting for its distinct KREEP chemistry.

Theory three: two moons and the big splat

In Arizona, a third detective comes at the mystery sideways. Erik Asphaug describes his method as looking at things everyone considers solved and hunting for the fly in the ointment. It does not win popularity contests, he admits, but the far side is exactly his kind of problem, a low hanging fruit you might crack in a lifetime. He arrived at his answer, fittingly, by not aiming at it. He and his colleagues had been studying asteroids and comets, many of which have a lumpy, piled up shape, plainly one body that crashed gently onto another, and they wondered whether the same trick could shape the Moon.

His theory again builds on the giant impact, but with a twist. That impact, he stresses, does not just make the Moon; it makes a whole disc of debris around Earth. So what if that disc formed not one moon but two? Asphaug proposes a smaller companion moon sharing our Moon's orbit. Two moons circling Earth in the same lane is an unstable arrangement, and eventually they are doomed to collide. The nature of that collision is everything. Fast collisions shatter and scatter; the Earth races around the Sun at about 30 kilometers per second, and typical impacts run at 10 to 20 kilometers per second. But two bodies already trapped in orbit around the same planet can only close on each other slowly. Asphaug's analogy is two race cars circling a track and gently nudging into each other. In his model the two moons meet at only about 2.5 kilometers per second, and at that speed they do not destroy each other, they merge. One simply splatters onto the other.

That slow splat spreads the smaller moon's material across one hemisphere, building the far side's extra crust and rugged highlands. And when Asphaug added a magma ocean beneath the crust to his model, it explained even more. As the companion plated its material onto the far side, it squeezed the underlying ocean of molten rock around to the near side. That displaced magma, rich in exactly the incompatible elements KREEP is made of, would leave its chemical stamp on the near side. In one stroke the big splat accounts for the far side's thick highlands and the near side's chemistry. Like the others, what it now needs is proof.

Theory	What happened after the giant impact	What it explains	The catch
Near side heating (Wieczorek)	A molten Moon, only 18,000 km from a red hot Earth, cools unevenly. The far side freezes first and grows a thicker crust; the softer near side later erupts as maria.	fits thicker far crust and the darker near side face from one mechanism.	weak spot hard to deliver enough heat; a Moon that close tends to spiral in and crash.
Second impact (Zhu)	A later large object slams the near side, excavating surface material that flies over and piles onto the far side.	fits the thick rugged far side, plus a churned near side chemistry.	weak spot we see no preserved giant near side scar; a hot young Moon would not keep one.
Two moons, big splat (Asphaug)	A second, smaller moon shares the orbit, goes unstable, and merges at ~2.5 km/s, plating the far side and squeezing the magma ocean to the near side.	fits thick far highlands and the near side KREEP layer at once.	weak spot needs two moons of near identical composition, which many find implausible.

Figure 3. The three leading explanations, side by side. All three start from the same giant impact birth and try to account for the same three facts: the far side's thicker crust, its rugged look, and the near side's KREEP chemistry. Each fits the evidence. None has been proven, and each has a genuine weak spot.

The skeptics answer back

The film is careful to let each theory take its punches, because that is how the science actually runs. Against the heat theory, the sharpest objection is the energy budget: how do you get enough heat to do the job? Push the Moon close enough to Earth to bake the near side and you push it so close that it tends to spiral inward and crash. Against the second impact theory, the problem is a missing scar. We do not easily see a big near side impact basin, and if the collision happened very early, while the Moon was still hot and soft, there is no reason such a basin would have been preserved. Against the two moons theory, Wieczorek names two doubts: the model asks the two moons to have nearly identical compositions, which he finds a little implausible, and it is not obvious it is plausible to have two moons in orbit in the first place.

Hartmann sums up the state of play with a baseball line. All these theories are on the table, but none of them has hit a home run. It is a genuine scientific battle, he says, and that is exactly what drives science: he will stand by his idea, others will try to pull it down, and in the end we will know who was right.

The Moon as a solar system time capsule

Why pour so much effort into one grey rock? Because, the scientists explain, the Moon is the best preserved evidence in the neighborhood. Planetary geologists think of themselves as detectives, using the clues around them to reconstruct the history of the solar system, and they treat the Moon as a crime scene: we have the finished product but not the story of how it got that way, so they read rocks the way a forensic team reads DNA and fingerprints.

The trouble with reading that history on Earth is that Earth erases its own past. Our planet has plate tectonics, a crust that constantly renews itself, and so almost no rocks survive from 4.5 billion years ago, the age of the solar system. The Moon is the opposite. It has no plate tectonics, so its surface has stayed essentially unchanged for billions of years, preserving, as Barnes puts it, a treasure trove or time capsule of everything that has happened since it formed. That makes it a record not only of itself but of Mars, Mercury, Venus, and the early Earth. Geologist Katie Joy, one of NASA's lead sample investigators, calls the Apollo rocks the gift that keeps on giving, because every new technique lets us put fresh questions to the same old stones. There is a catch, though. Every sample we have was collected from a geologically narrow patch of the near side. To truly understand the far side, we have to go back and gather rocks from a much more diverse region.

Artemis and the frozen Apollo vaults

Two paths lead to that new evidence, and the film follows both. The first is a return to the Moon with people. NASA's Artemis program aims to put humans back on the lunar surface in the coming years, and Jess Barnes is already preparing for the harvest. Before we go, she says, we have to work out how to curate and preserve samples so precisely that we lose none of that DNA and fingerprint level evidence for future generations.

To get ready, NASA has begun opening evidence it deliberately sealed away. After Apollo 17, scientists had the foresight to lock some samples in special storage for 50 years, betting that future tools would ask better questions, a program now known as the Apollo Next Generation Sample Analysis effort. That half century is up, and Barnes is one of the handful of people chosen to study these pristine rocks with methods that were impossible in 1972. One example: her team uses a particle accelerator to measure the oxidation state of sulfur in the samples, a genuinely forensic technique applied to lunar material only in the last few years. It has already surfaced a puzzle. Samples prepared 50 years ago hold noticeably more sulfur than samples that were kept frozen for those same 50 years. No one is sure why yet, but the answer matters enormously, because it tells us how to store the rocks we are about to bring back so that they do not quietly change before we can read them.

Chang'e lands on the far side

The second path is robotic, and it is being blazed by China. On the 2nd of January 2019, the China National Space Administration achieved what no one ever had: Chang'e 4 set its lander and rover down safely on the far side of the Moon. Zhu describes watching the television every minute, every second. It was a pure technological milestone, proof the thing could be done, and it opened a firehose of data from ground no spacecraft had ever touched. Zhu is one of the scientists analyzing it. Every month the Chinese agency releases images, radar returns, mineral spectra, and orbiter data, letting researchers derive the shallow subsurface structure of the landing site and begin to understand how those far side features formed.

The Chang'e program did not stop there. Chang'e 5 brought home about 1.7 kilograms of fresh rock, though from the near side, now being studied by Chinese lunar scientists. And the film points ahead to Chang'e 6, the mission meant to return the first samples ever collected from the far side, which Zhu hopes could finally settle the question of the Moon's two sides. Whatever the individual result, the momentum is unmistakable. As one scientist says, there has never been a better time to study the Moon; the gap is closing, and in the next 5 to 10 years we may answer these questions for real.

1959 Luna 3. The Soviet probe photographs the far side for the first time. It looks nothing like the near side, and the mystery is born.
1969 to 1972 Apollo. Astronauts return 382 kilograms of soil, rock, and core. Later analysis finds the near side is enriched in KREEP, a signature absent from the far side.
1984 The Kona conference. In Hawaii, the field weighs rival origin stories and the giant impact hypothesis gets, in Hartmann's words, the A+.
2011 The big splat, published. Asphaug and colleagues propose a second, smaller moon that merged with ours in a slow collision, thickening the far side.
2011 to 2012 GRAIL. The twin satellites Ebb and Flow map lunar gravity 10 times more sharply than ever, revealing a crust about 30 km thick on the near side and 50 km on the far side.
2019 Chang'e 4. China lands the first spacecraft on the far side and begins returning radar and mineral data from untouched terrain.
2020 Chang'e 5. About 1.7 kilograms of fresh near side rock come home for study.
2024 Chang'e 6. The first samples ever gathered from the far side are carried back to Earth, exactly the evidence the theories have been waiting for.
soon Artemis and a far side base. Crewed landings, the opening of Apollo samples frozen since the 1970s, and the dream of seismometers planted on the far side to listen for the answer.

Figure 4. Sixty five years of chasing the far side, from the photograph that started it to the samples that might finish it. Every rung added a fact the next theory had to explain.

A base on the far side, and listening for the seam

The film's last movement looks forward to a human presence on the Moon, and to the experiments that presence could finally run. Mahesh Anand argues the far side is worth going to precisely because we know so little about it, and imagines a protected shelter that works like an Antarctic research outpost, visited by astronauts from time to time. Getting there starts from orbit. Yang Gao shows off a CubeSat, a tiny craft packed with cutting edge instruments; her team wants to fly laser based tools to measure water content and map the quantity of useful materials, the reconnaissance that precedes any base.

Once resources are located, the trick is using them in place rather than hauling everything from Earth. Anand demonstrates an instrument that detects and measures water locked inside a lunar sample, water that future crews could tap. For the base itself the answer may be underfoot, in regolith, the fine dust that blankets the Moon. Real Moon dust is too precious to experiment with, so Anand works with a NASA developed synthetic simulant based on the Apollo samples, and he has managed to melt it into solid brick with nothing more than a household style microwave in a few minutes, a building material for a permanent outpost. Thinking outside the box, he says, is how you break new ground, and he is optimistic that within 10 to 20 years humans will be living and working on the Moon for extended stretches.

A far side base would do more than plant a flag. It could carry the one instrument that might crack the case: a seismometer. Understanding the Moon's interior almost demands one, and it is far easier for a human than a robot to set it precisely on the surface. Each theorist knows exactly what a seismic network could reveal. Asphaug is hunting for a specific echo: a reflective boundary about 30 kilometers deep on the far side, the buried seam where his two moons fused together. Zhu wants seismic data to test whether a giant near side impact really flung material across to build the far side. Wieczorek is the most sober of the three; his heat theory describes events so early in lunar history that most of the evidence has been wiped from the surface, so it may be very hard to prove either way. He is optimistic that in 20 years we will understand far more, though whether we ever truly solve it, he is honestly not sure.

Where it stands

Everything the film treats as established really is. The two hemispheres genuinely differ in appearance, in maria coverage, in crust thickness, and in near side KREEP chemistry, and those observations are not in dispute. What remains open is the cause. The giant impact hypothesis for the Moon's origin is the mainstream consensus, but the near side and far side asymmetry, often called the lunar dichotomy, has no agreed answer. The three explanations in the film are real, published, competing hypotheses, and the film is right that none has won. A reader should hold them as live candidates, not settled fact.

One honest update on the film's own timeline is worth adding. The episode speaks of Chang'e 6 as a mission still due to fly. In reality Chang'e 6 launched in May 2024 and returned the first far side samples that June, so the very evidence the scientists were waiting for is already on Earth and under study. Early results are refining, not overturning, the picture the film draws. The far side is still a mystery, but for the first time in history we are reading it from rocks we carried home from its surface, exactly as everyone in this film hoped.

Key takeaways

The Moon keeps one face turned to Earth forever because it is tidally locked, so there is a permanent near side and a permanent far side.
Luna 3 photographed the far side in 1959 and revealed it is rugged and nearly free of the dark maria that cover about a third of the near side.
The Apollo rocks showed the near side is chemically enriched in KREEP, potassium, rare earth elements, and phosphorus, a signature missing from the far side.
GRAIL mapped the Moon's gravity with the twin Ebb and Flow satellites and found the crust averages about 30 kilometers on the near side and 50 kilometers on the far side.
All three leading theories accept the giant impact hypothesis of the Moon's birth and disagree only about what divided it afterward.
Theory one: a molten Moon, once only about 18,000 kilometers from a red hot Earth, cooled unevenly, freezing the far side crust thicker.
Theory two: a later giant impact struck the near side and threw material onto the far side, building its highlands.
Theory three: a second smaller moon merged with ours in a slow 2.5 kilometer per second splat, plating the far side and squeezing the magma ocean toward the near side.
The Moon has no plate tectonics, so it preserves a 4.5 billion year record that Earth's own surface has erased, making it a time capsule for the whole solar system.
Fresh evidence is arriving from newly opened frozen Apollo samples and from China's Chang'e far side landings; a seismometer on the far side may be the instrument that finally settles the case.

Chapters

00:00 Luna 3 and the photograph that shocked the world 03:00 Why we only ever see one face 05:20 Maria and a rugged far side 08:00 Apollo 1969, 382 kilograms of Moon 10:30 The KREEP chemistry anomaly 13:00 GRAIL maps the invisible gravity field 16:20 A lopsided crust, 30 km against 50 km 18:40 The giant impact hypothesis and the Hawaii conference 23:20 Three theories, one shared beginning 24:20 Theory one, the near side heating theory 28:30 A receding Moon and a much closer past 31:00 Theory two, the second impact 35:00 Theory three, two moons and the big splat 39:40 The skeptics answer back 42:20 The Moon as a solar system time capsule 44:40 Artemis and the frozen Apollo vaults 47:20 Chang'e lands on the far side 49:40 A base on the far side, and listening for the seam 51:00 The testing ground for everything beyond

Notable quotes

"For me, it's one of these low hanging fruits, these accessible problems that you could hope to solve in your lifetime." (00:45, Erik Asphaug on why the far side is worth a career)

"That photograph was kind of strange to everybody because it didn't look much at all like the front side of the moon." (05:05, Bill Hartmann on the Luna 3 image)

"We see a geochemical anomaly on the near side that we don't see on the far side of the moon." (12:05, Jess Barnes on the KREEP signature)

"You could not miss the signal. It was just right in front of your face." (16:35, Mark Wieczorek on the GRAIL crust map)

"If you can't draw it, you probably don't understand it." (18:50, Bill Hartmann on how he works)

"A good analogy is two race cars racing around doing the laps and they bump into each other. That bump is quite slow." (37:20, Erik Asphaug on a slow motion lunar collision)

"None of them have hit a home run." (40:10, Bill Hartmann on the three competing theories)

"You can think of the moon as kind of like a crime scene. We have the end product, but we don't really know how it got to where it is today." (41:05, on reading the Moon like a forensic detective)

"If you can't go to the far side of the moon, you'll never go to Mars." (51:10, Erik Asphaug on the far side as a testing ground)

Resources mentioned

Spark, Science's Greatest Mysteries, Episode 1, the source documentary on the Spark channel.
Luna 3, the 1959 Soviet probe that first photographed the far side.
The Apollo program and Apollo 17, source of the 382 kilograms of returned lunar samples.
GRAIL, NASA's Gravity Recovery and Interior Laboratory, with its twin Ebb and Flow satellites.
Chang'e 4, Chang'e 5, and Chang'e 6, China's far side landing and sample return missions.
NASA's Artemis program, the crewed return to the lunar surface.
The Apollo Next Generation Sample Analysis program, opening samples frozen since the 1970s.
The giant impact hypothesis and the proposed impactor Theia.
KREEP, the potassium, rare earth element, and phosphorus signature concentrated on the near side.
Lunar maria, the dark volcanic plains, and the far side's largest impact structure, the South Pole to Aitken basin.
Tidal locking and lunar laser ranging, which shows the Moon receding at 3.8 centimeters per year.
The lunar magma ocean and plate tectonics, the process that erased Earth's oldest rocks.
Lunar regolith and CubeSats, the raw material and the scout craft for a future base.
William K. Hartmann and Don Davis of the Planetary Science Institute, authors of the giant impact hypothesis.
Erik Asphaug, whose companion moon big splat theory was published in 2011.
Mark Wieczorek, GRAIL scientist and author of the near side heating theory.
Meng Hua Zhu, whose simulations propose a second near side impact.
Jess Barnes and Katie Joy, sample scientists studying the Apollo collection.
Mahesh Anand and Yang Gao, working on lunar water, resources, and base construction.
Isaac Asimov, whose short story imagined stage props on the far side.

Full transcript

On the 7th of October 1959, a Russian space probe, Luna 3, was about to attempt something remarkable: taking the first ever photographs of something no one had seen before. Anything that we know is there and we can't see it always attracts the sense of curiosity. As it passed over the lunar surface, the onboard camera took a series of incredible pictures. What was captured sent shock waves across the globe: the first ever images of the far side of the moon. It was amazing. The scientists saw those pictures and realized pretty quickly just how different the far side was from the near side. There were actually people in the United States who suggested that the Russians were faking it. Now, scientists are racing to explain why the familiar near side of the moon is completely different from the side that we never see. The far side of the moon is one of those long-standing cases, unsolved cases. So we really need to crack that history. Everyone working in lunar science has tried to explain why the two hemispheres are different. For many planetary scientists, solving this perplexing riddle is the holy grail. For me, it's one of these low-hanging fruits, these accessible problems that you could hope to solve in your lifetime. Half a century ago, NASA locked away precious moon rocks and are only now releasing them. We're able to do things that just weren't possible 50 years ago. And with the launch of audacious new missions, the Chang'e 4 mission shows extraordinary achievement. This is the first time we could get some information from the far side. We're on the brink of an exciting phase of lunar science. Because we know so little about the far side, it is a good idea that we plan to explore the far side of the moon in the near future. In the next 10, 20 years, we're going to be in a whole new space in terms of what we think about the moon. The far side of the moon is one of science's greatest mysteries. And I remember as a kid discovering that there were maps of the moon and there were mountains and there were names of places and it wasn't just a light up in the sky. It was a place that really got me interested in what is the surface like, what are the conditions like? For as long as we have gazed to the heavens, the moon's far side has been hidden from us, keeping scientists in the dark. Oddly enough, for the moon and most other satellites in the solar system, they all keep one side toward the planet that they're going around. So you have a near side, which is always the near side, and a far side, which is always a far side. Before the Luna 3 probe, no one had ever seen the mysterious far side, a world full of unknowns. It fueled some outlandish ideas. There were stories like the Germans might have established a base on the back side of the moon during World War II. There's a very amusing Isaac Asimov short story about first astronauts going around on the far side of the moon and it's all stage props there on the back side. When the Luna 3 images were beamed back down to Earth, it was a shocking revelation. In '59, out of nowhere, the Russians suddenly had a photograph of the far side of the moon. That photograph was kind of strange to everybody because it didn't look much at all like the front side of the moon. For planetary scientists like Bill Hartmann, this picture was completely unexpected. So this is the front side of the moon and these dark patches are big lava planes that erupted on the moon. Something like a third of the front side is covered by these lava planes. But there's a striking difference on the mysterious far side. The big difference from the far side, and this is the Russian far side picture, is that there just are not as many patches of dark lava on the far side. But that wasn't all. Luna 3 also revealed a hillier, more rugged far side, contrasting dramatically with a much flatter, darker near side. There were actually people in the United States who suggested that the Russians were faking it, that this couldn't be a back side of the moon picture because it didn't look very much like the front. Then you're curious about, well, what's the rest of the story? What are we going to find out? And as we began to learn more about the moon, in a way, it got more interesting and more complex. Luna 3's remarkable images of the far side revealed an unmistakable difference between the moon's two faces. And subsequent lunar missions have confirmed the contrast between the rugged far side and the flatter, darker near side in even more detail. Ever since, scientists have puzzled over what in the moon's past might have caused its Jekyll and Hyde nature. But the mystery of the far side of the moon was about to deepen, because the next historic mission brought back more than just pictures. We can learn a lot remotely from spacecraft orbiting a planet, but we gain even more if we can bring samples back and analyze them on Earth. In 1969, the desire to study the moon up close was finally realized. The Apollo missions, they're really important because we brought back with those missions 382 kilograms of lunar material, soils, rocks, and core samples. Today, Jess Barnes is one of the few scientists on Earth studying this valuable resource. These samples are still undergoing vigorous study because we are constantly improving our analytical techniques. So we're constantly able to address past questions that maybe weren't answered, but also pose new ones. Analysis of the Apollo rocks has revealed another puzzling difference between the sides. They are composed of different materials. When we study the chemistry of samples returned by the Apollo missions, what we realized is that a lot of the samples have signatures that are enriched in potassium, rare earth elements, phosphorus, radioactive elements, and they're all from the near side. And at the moment, we haven't been to visit the far side and collect samples and bring them back. So we rely on random samplings from meteorites. When the two samples are compared chemically, there's a striking difference. This potassium, rare earth element and phosphorus rich signature is seen in samples that we've collected from the near side. But that signature is specific to the near side and it's not found on the far side. This unexpected finding was another twist in the moon's enigmatic story. The near side far side mystery is not just specific to appearance. It's also to do with chemistry. We see a geochemical anomaly on the near side that we don't see on the far side of the moon. The Luna 3 probe shocked the world with its revelation of the moon's two-sided nature, and chemical analysis of the moon rocks returned by the Apollo missions established even more differences between its two faces. But while scientists were looking for answers to the mystery that began with Luna 3, new technology was about to raise yet more questions. You can trace the origins of astronomy back to Galileo and even beforehand. But for people who study the moon, Luna 3 is really the first mission and that's what started planetary science. A few decades after the Luna 3 and Apollo revelations, the far side mystery deepened as a new mission was launched to map an otherwise invisible aspect of the moon, its gravitational field. The gravity field tells us about how mass is distributed below the surface. So it's one way where we can actually see below the surface of the moon to determine things like how thick was the crust. We've had spacecraft that measured the gravity field of the moon, but the resolution was very, very poor. And as time went on, scientists realized that if we wanted to answer some of these fundamental questions about the moon that we would actually need to have a higher resolution gravity map. Mark Wieczorek was one of the lead scientists of GRAIL, NASA's Gravity Recovery and Interior Laboratory mission. He dissected the data as it beamed back down to Earth. The GRAIL mission was one of the highlights of my scientific career. The objective was to make the most precise gravity map of the moon that we've ever had. Not only the near side gravity of the moon, but also the far side gravity, which was very poorly constrained at that time. Two satellites, Ebb and Flow, orbited the moon together in a fixed formation. By analyzing minute fluctuations in the distance between them, scientists calculated changes in the moon's gravity. Honestly, after the first month of data collection, this map was better than I anticipated that we'd have at the end of the mission. It was really 10 times better than anything we've ever had for the moon beforehand. It was just very clear. And just like previous lunar revelations, the findings would cause ripples throughout the scientific world. Here we have the thickness of the crust of the moon. And this color scale here gives you the exact value of the thickness with blues indicating low thicknesses close to zero and red corresponding to very thick values close to 60 kilometers. And what we can easily see is that the average thickness of the crust is somewhere around 30 kilometers on the near side of the moon. But on the far side of the moon, it's about 50 kilometers. The presence of highlands on the far side already suggested the crust would be thicker than the near side. But the incredible resolution of the GRAIL maps revealed a stunning difference in thickness of more than 20 kilometers between the moon's two sides. And there was no explanation why. It's long been known that the near side and far side hemispheres of the moon are very different. And using the GRAIL data, we found out that it was even more different than we thought. You could not miss the signal. It was just right in front of your face. The moon wasn't just two-faced, and its two sides weren't just chemically different. Its lopsided crust was yet another peculiar and unexplained discovery. The cause of all these differences was one of science's greatest mysteries. In order for scientists to solve the puzzle of the moon's two-sided nature, they first needed to know how its story began. I had a strange sentence pop into my mind a few years ago and the sentence was, if you can't draw it, you probably don't understand it. Bill Hartmann has been painting the intricate wonders of our solar system since he was a young boy. His life's work has been dedicated to solving the mysteries of our strange moon. There's many satellites in the solar system, but ours is unique in the sense of being big compared to the planet Earth. It's about a quarter the size of its parent planet. And so, however the moon formed, it was an unusual process. Discovering exactly how the moon formed is key to finding out why it has two such distinct sides. And as a young astronomer, Bill and his colleagues thought the answer could lie in what they saw on the moon's surface. We discovered this big impact basin on one side of the moon, which really seemed to solidify the idea that these crater-like features are not volcanic. These are formed by big impacts. So you have lots of interplanetary debris hitting the moon all the time. These impacts gave Bill the idea that the history of the moon is tied to that of our own planet. He suggested that as the Earth was forming, another smaller planet might have formed in a similar orbit around the sun. As the planets circled our star, their paths inevitably crossed with catastrophic consequences. The resulting collision caused an enormous explosion, throwing up a vast cloud of gas and dust around our planet. This superheated debris then combined, forming a new molten body, one that slowly cooled to form our moon. In the early 1980s, a conference was proposed to see if scientists could agree on a story for the moon's origins. It was decided, let's have that conference in Hawaii. Sure enough, everybody wanted to come to the conference. All of a sudden, everybody's wearing their Hawaiian flower leis. It's amazing how many people were wearing beards in those days. Having worked out the details with his colleague Don Davis, it was time for Bill's theory to face some tough scrutiny. And that meeting has been sometimes described as one of the most significant meetings in planetary science. We went through lots of different theories. And the giant impact theory was the one that got the A+. The giant impact hypothesis still reigns supreme as the go-to theory of how our moon was formed. It seems to fit so many criteria of what we know about the solar system that I think it's still a major part of planetary science. Bill and Don's theory explains a lot about our unusual moon. The sheer amount of debris thrown up by such a vast collision would lead to its unusual size. But the theory offers no explanation why the radioactive elements found in the Apollo samples are more common on the near side today. And it doesn't explain the far side's much thicker crust or even the surprising differences in appearance revealed by the Luna 3 probe. Today, in the race to explain the mystery, three competing theories stand out. All three begin with the giant impact hypothesis. But it's what happened next to cause the moon's two-sided nature where they all disagree. The first theory picks up in the aftermath of the moon's violent birth. When the moon formed, a lot of energy was liberated. And because of this, the moon just started out in a very hot state and most likely in a completely molten state. For Mark Wieczorek, the behavior of this early molten moon is key to a theory he thinks best explains the mystery. The moon starts to cool and after a small period of time begins to crystallize. Now, if for some reason the far side of the moon was crystallizing faster than that of the near side, you would grow the crust more quickly and conceivably the thickness of the crust would actually be greater as well. But why should the far side of the moon cool faster than the side which always faces towards us? The Earth acted as a heat source. So you can imagine that if you have the Earth here acting as the sun, it will be heating one hemisphere of the moon and not the other. And because of that, the far side of the moon crystallized more quickly and that can promote growing a thicker crust. But at an average distance of around 380,000 kilometers, even a red-hot molten Earth would struggle to apply enough heat to the moon's surface. The answer may lie in a well-established but surprising discovery. On Earth, we have observatories with very big lasers. We shoot them towards the moon and we time how long it takes for the laser pulse to come back. And that gives us the distance between the Earth and the surface of the moon. And by making these measurements over several decades, we have found that the moon is receding from the Earth at roughly 3.8 centimeters per year. It may seem like a trivial amount, but it reveals an early moon Earth system very different from today's. So when the moon formed shortly after this giant impact, most people believe that the moon was very close to the Earth, just a couple Earth diameters away. At about 18,000 kilometers from Earth, it could have been over 20 times closer than it is now. The Earth would be emitting lots of energy and this is one of the reasons that we think that the far side might have perhaps crystallized more quickly and grown a thicker crust. It's a neat theory and some planetary scientists have added to the idea. They suggest that impacts and seismic activity could have broken up the crust more easily on the thinner near side and this may have later resulted in more volcanic eruptions on the near side creating the dark lava planes we see today. Even though I personally think this heat theory is a plausible solution to this problem, we definitely need more research to investigate this in more detail. The near side heating theory neatly fits the evidence. The faster cooling on the far side explains the thicker crust found by the GRAIL mission and the increased volcanic eruptions on the near side explain its darker appearance. But while Mark waits for crucial evidence, the field remains open to other equally compelling theories. On the other side of the Earth, another moon enthusiast is taking a different approach to solve this long-standing mystery. I know this difference between the near and far side for almost 10 years. Computer scientist Meng-Hua Zhu tackles problems using complex simulations. But the moon was not always his focus. At 2006, I work at a computer company and my life was great. But at that time, I watch news about China's Luna missions. And it was China's quest to the moon that inspired a dramatic change in his career. So at 20 years old I started to learn the lunar science. I studied the moon for almost 15 years until now. My wife said you almost time together with the moon. You know, the moon might be your wife. I'm not the first one, you know. Dedicated the last half decade to tackling the problem of the moon's radically different sides. How to form the far side is very impressed to me and five years ago I start to do the numerical simulations to see if I could solve these questions. We found on the moon at the very early times they have a very high probability for the impact occurred on the moon. We propose if the impact occurred on the near side, it could have excavated a huge material. According to Zhu's theory, long after the impact that formed the moon, another large object came dangerously close to the Earth, but this time violently colliding with the moon's near side, ejecting a vast quantity of the moon's surface. This material will deposit on the far side, form the highland, and impacted on the near side could be consistent with the remote sense observation. Zhu thinks the ejected near side debris would eventually settle on the far side, explaining the thicker crust and rugged terrain we see today. I cannot say this scenario is correct or not before we found some evidence and I cannot rule out other scenarios. Just like the heat theory, Zhu's second impact idea seems to fit the evidence. The displaced debris from the collision would create the far side's rugged surface and thicker crust. And the energy of this impact could melt and churn up the near side, accounting for its unique chemical signature. But although Zhu's theory is a strong contender, another idea is gaining momentum. In Arizona, a third planetary detective thinks his theory best answers the mystery of the moon. My approach to science has always been looking at things that are solved and finding the fly in the ointment. You know, this doesn't make any sense. And that's not a way to win popularity contests necessarily, but I think it's important. And the far side of the moon for me, it's one of these low-hanging lifetime fruits. Eric Asphaug came at the problem from a different angle, looking at a type of collision that usually occurs on a much smaller scale. What's funny about science is you sometimes get to the right answer better by not trying to go directly straight ahead. And the approach that we were taking at the time was to understand asteroids and comets. They tend to have these almost piled up shapes, one thing that crashed onto another. And we started pursuing that idea for moon formation. Once again, Eric's hypothesis builds on the giant impact story of the moon's formation. One of the important things to understand about the giant impact theory is it doesn't just produce the moon, it produces a disc around the Earth. And we started to wonder, what if you form the moon and you form another moon? What's going to be their long-term fate? Eric believes that not one but two moons were formed in the aftermath of the giant impact. And it's these two moons circling the Earth that set the scene for what happened next. Eventually that system becomes unstable. And now you have two moons orbiting the Earth in the same orbit without any constraint and they're doomed to collide. And it's the unique nature of this lunar collision that is key to Eric's theory. The idea of planets colliding is not a new idea. This has been thought of for centuries, that planets would grow by collisions and develop bigger planets and then sometimes destroy each other, scattering their pieces all over the place. We're thinking of things that are quite a bit slower because with a slower collision, they don't destroy each other. They actually merge. Instead of creating a whole new planet out of the two objects, you end up with one splattered onto the other. But could two massive moons really merge slowly instead of colliding catastrophically? Most collisions in the solar system are very fast collisions. The Earth's orbiting the sun at 30 kilometers per second. So typical impact velocities are 10, 20 kilometers per second. But if you have two objects that are trapped in orbit around the Earth, the collision velocity is quite constricted. And a good analogy is two race cars racing around doing the laps and they bump into each other. That bump is quite slow. In Eric's model, the two moons approach each other at a relatively slow speed of around 2.5 kilometers per second before colliding. The resulting slow motion collision would spread the material from the smaller second moon out over the far side of our moon. And when Eric developed his model further with a partially molten moon, he discovered it could explain more than just the geographical differences. When we added a magma ocean layer underneath the crust of the moon to our model of a splat, the splat that plates material onto the far side squished the magma ocean onto the near side. This displaced ocean of molten rock would explain the chemical differences between the moon's two sides. And the material from the second moon would explain the rugged highlands and thicker crust on the far side. But just like the other theories, what it needs now is proof. Ever since the moon's two-faced nature was revealed by the Luna 3 probe, scientists have proposed explanations for the mystery of the far side. Now, three rival explanations lead the pack. Perhaps when the moon was very young, the heat from the still molten Earth caused the near side to solidify more slowly. But this heat theory is an idea that some have issues with. One of the problems with the heat theory, probably the most important one, is how do you get enough heat to get this to happen? If you make the moon too close to the Earth so that there's enough heating, the moon will actually tend to spiral into the Earth and crash. Or maybe a second violent collision propelled material over from the near side to the far side causing the thicker crust. This second collision theory also has its skeptics. It's challenged because we don't really easily see a big impact to scar very well. The problem we've got is if this occurred very early on in the moon's history when the moon was still hot, there's no reason why we should preserve a big impact basin. And finally, a slower impact from a second moon might have deposited a thick layer of debris onto the far side. But the two moons theory doesn't convince everyone. I see several problems with this theory. One is that the authors proposed that these two moons would have identical compositions. And personally, I think this is a little implausible. And the second is just whether it's plausible to have two moons in orbit in the first place. I think that all these theories are on the table, but none of them have hit a home run. And it's an interesting kind of a scientific battle that's going on today. But I think that's what drives science is I'm going to stand by my idea and others are going to try to pull it down and then in the end we're going to know who was right. All three theories offer a possible explanation for the moon's two different faces. But to solve the mystery of the far side of the moon, what they all need is decisive evidence proving them to be true. Finding that though is easier said than done. The quest to crack the mystery of the far side of the moon continues to attract lunar scientists across the globe because exploring the moon's history offers a unique insight into a much bigger story. Planetary scientists and geologists tend to think of themselves as detectives. We're using the clues and evidence all around us to piece together the history of the solar system. The far side of the moon is one of those long-standing unsolved cases. And so you can think of the moon as kind of like a crime scene. We have the end product, but we don't really know how it got to where it is today. So we use rocks like you would find DNA or fingerprints to piece together that history. So here we have some rocks from this local area. You can see that they're full of lots of different colors and different types of minerals welded together and they're really important for piecing together Earth's geological history. But we're kind of unfortunate on Earth in that we have plate tectonics. So we don't have rocks that are 4.5 billion years old or the age of the solar system on Earth because they've been destroyed through the action of plate tectonics. With its crust constantly renewing itself, the Earth's distant geological past has been lost. But the moon is a different story. The moon is vital and important and unique because it doesn't have plate tectonics. And so it preserves as like a treasure trove or time capsule everything that happened since it formed. Because the moon's surface has remained essentially unchanged for billions of years, it offers unique evidence for cosmic detectives around the world. Like Jess Barnes, geologist Katie Joy is fascinated by the far side mystery. Because people hadn't seen the far side of the moon, we just made the assumption that the far side was the same as the near side. So Luna 3 turned our ideas on its head, and those ideas were still being turned on our head. Katie is one of NASA's lead investigators, studying the precious Apollo samples to better understand the moon's mysterious past. Because the moon's surface is very old, it can tell us a lot about history of the solar system's past. Can tell us about Mars, Mercury, Venus, and the Earth. Every time there's new technology, we can ask these rocks new questions. And that's why they're sort of the gift that keeps on giving because they're always going to be available for us to look at with new ideas. The Apollo samples continue to teach us about the moon, the Earth, and the rest of the system. But when it comes to solving the far side mystery, there's a problem. With the lunar missions that have been to the surface and collected samples, we've only visited a very geologically restricted area on the near side of the moon. So in order to fully understand the moon and the far side composition, we really need to go back and collect samples from a more diverse region. And with NASA's Artemis mission, aiming to return humans to the lunar surface in the coming years, Jess is preparing for a new batch of evidence. We want to preserve that DNA, fingerprints, rocks as precisely as possible so that we don't lose any of that evidence. So before we go back to the moon, we really need to understand what's the best way to curate and keep samples safe and preserve them for future generations. To get ready for these upcoming missions, NASA is releasing some crucial untouched evidence from its vaults. After Apollo 17, people soon realized that it would be beneficial to future generations to lock some of the Apollo samples away in special storage for 50 years. And now we're at the point in time when NASA has decided things have improved so much that we're now at a position to open those samples, use new technologies on them to answer past and new questions. Now armed with techniques impossible 50 years ago, Jess is one of the handful of people chosen by NASA to analyze these precious samples. One of the things that we've been doing is using a particle accelerator to measure the oxidation state of sulfur in our samples. We're really looking at these samples forensically. And this is a really new technique that's been applied to lunar samples only in the last few years. This tool allows Jess to piece together the sample's history and establish how new evidence might be better preserved in the future. We find that samples that were prepared 50 years ago contain a lot more sulfur than we see in samples that were actually stored frozen for 50 years. We're not quite sure why that is, but this is one of the things that we want to find out. Do we need to do something differently for samples coming back from the moon in the future? It's a problem that must be addressed before we return for more evidence. Something Jess knows is critical if we're ever to finally solve the mystery of the moon's far side. It's possible that with Artemis samples that we will learn a lot more about the far side. So I'm really looking forward to Artemis and the other missions to go back to the lunar surface, collect samples, and bring them back. For many, the only way to finally answer the riddle of our two-faced moon is to collect samples from the far side. And on the 2nd of January 2019, hopes of doing just that received a welcome boost as the Chinese space agency became the first ever team to land there safely. When Chang'e 4 land, I was very excited. I look at the TV every day and every minute, you know, every second. Chang'e was a technological achievement proving that this can be done. It was an amazing feat and there is some interesting data coming back from the Chang'e lander that operate on the far side. Zhu eagerly watched as the Chinese rover started sending back data from this previously unexplored terrain. We do not have any data until now. You know, if we can study the structure of far side, its composition, its minerals, this would be very interesting. Zhu is one of the lead scientists analyzing the new information. Every month, China space agency release the data from the images and the radars and mineral spectrums and also some orbiter datas. This is the first time we could look at the moon very clear. We could use these radar data to derive the shallow structures and we combine the image datas and the mineral spectrums and we could understand what happened about the landing side and why they formed these structures. But this iconic Chinese mission doesn't end with Chang'e 4. We've just had the Chang'e 5 samples that have been collected from the near side of the moon where 1.7 kilos of moon rock are now sitting over in China where they're being worked on by our Chinese lunar science colleagues. And with Chang'e 6 due to return samples from the far side for the first time, Zhu hopes it will finally settle the mystery of the moon's two sides. But the achievement of the Chang'e missions has already sparked a new era of lunar exploration. There's no better time to study the moon than right now. The Chinese missions are returning the first samples from the lunar far side. And it's finally feeling like the gap is closing and that in the next five years, maybe 10 years at the most, we're going to answer these questions for real. A mystery that began with the remarkable achievement of the Luna 3 probe has only deepened as scientific advances teach us more about the moon's strange two-sided nature. And while different theories still battle to explain the differences, perhaps the clue that finally closes the case of the far side of the moon will be found by the most audacious mission yet. The next big thing that's going to happen is that humans will be returning to the moon and potentially stay there for a much longer period than we have ever did before. What Mahesh Anand proposes could change the way we explore the solar system forever. Because we know so little about the far side, it is a good idea that actually we plan to explore the far side of the moon in the near future. You could build a shelter so that astronauts can be protected and it could become just like an outpost we have in Antarctica where astronauts could visit from time to time. And these plans aren't just idle speculation. Scientists across the globe are working to make them a reality. Orbital missions by satellites is going to be the precursor of the construction of the moon base. So here we have a model of a cubesat spacecraft. As you can see, it's a very small sized object jam-packed with a lot of cutting edge technologies. Yang Gao hopes her team will be among the first utilizing such a craft to find resources for the lunar base. We are going to fly laser-based instruments that can help measure the water content and also try to map and understand the quantity of some of the materials. Once these vital resources are located, the next step is to extract them. Mahesh Anand is an expert when it comes to utilizing materials from moon rock. Here we have an instrument that we use in our pioneering research for detecting water in lunar samples and the process starts with a piece of a lunar sample. Once it is inside the machine, it allows us to determine how much water is present and might be available to humans rather than carrying all the resources with us. But to build the lunar base, we'll need a lot more than just water. The answer may lie with regolith, the dust found on the moon's surface. We started thinking, how could you actually build your own lunar base? So how do you actually make bricks out of lunar regolith? Real moon dust is a precious commodity here on Earth. So Mahesh works with NASA developed synthetic moon dust based on the Apollo samples. We have been able to melt lunar simulants using microwaves within a few minutes. And it's just amazing how when you think outside the box, you often end up making new discoveries and breaking new grounds. By melting the lunar simulant, Mahesh has succeeded in creating the building material for a permanent base. I'm very optimistic that in the 10 or 20 years time, we would have humans actually going to the moon and staying there and doing scientific research over extended periods of time. A base on the far side of the moon could be our best hope of solving the mystery of the moon's two-sided nature. In planetary science, there's always been a debate as to whether it's better to do things with robots or with humans. And it's much easier for a human to place an instrument on the surface. Seismometers are probably the most useful if you want to understand the interior structure of the moon. So that would be one of the key instruments that we'd like to see on a future mission to the far side of the moon. These seismometers might provide Eric Asphaug with the evidence he needs to prove that the near side far side disparity was caused by a slow motion collision with a second moon. We're looking for a seismic indication that there is a contact boundary between the second moon and the primary moon. There should be this reflective boundary about 30 kilometers deep on the far side of the moon that would represent where the two moons got stuck together. Zhu hopes for evidence confirming his theory that a much higher velocity impact propelled some of the near side surface over to the far side. If we have some seismic data, we can compare with our models and see if this impact really formed the near side and far side asymmetries. But for the theory that the heat from a molten Earth caused the near side of the moon to solidify more slowly, Mark Wieczorek sees a long road ahead. Unfortunately, it's going to be very, very hard to prove this because this is a process that happened very early in lunar history and most of the evidence has been wiped clean from the surface. Nevertheless, I'm optimistic that in the next 20 years, we will have a better understanding of what's possible and what's not possible. Whether we ever solve this mystery, I'm not sure. Since Luna 3's remarkable mission in 1959, the mystery of the far side of the moon has captivated scientists because solving this riddle may shine a light on much more than just our cosmic neighbor. Understanding how the far side near side dichotomy actually arose could help us when we try to understand what's happening elsewhere in the solar system. It could even be the vital stepping stone to venture beyond our home planet. I think of it as the testing ground for all that comes later. If you can't go to the far side of the moon, you'll never go to Mars. You'll never get beyond this little realm of space that we're proud of having explored. But it's really just a tiny little fraction of what's out there. More than half a century ago, the tiny Luna 3 probe took a photograph that completely changed our understanding of the moon. And those who've dedicated their lives to the moon's secrets won't give up the chase anytime soon. I'm sure that once we address those questions, more questions will pop up. And that's just the nature of science. The more you learn, the more questions you have because scientists are curious and will never stop exploring. With the answer to the riddle of the far side of the moon closer than ever, we could soon be on the brink of solving one of science's greatest mysteries.