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LearningTranscript
00:00The Moon, Earth's nearest neighbor, but one of the most mysterious objects in the solar system.
00:07If you actually want to look at one of the greatest mysteries of the universe right now, just look up
00:12at the Moon.
00:12For decades, scientists were sure they'd solved many of the Moon's deepest secrets.
00:19But now, new evidence suggests that nearly everything we thought we knew about the Moon could be wrong.
00:26We might need to completely rethink our understanding of the formation of the Moon.
00:31Imagine if you could have looked up at the night sky and saw two moons. That would have been lit!
00:38Why is our Moon so big?
00:43Why does it look so strange?
00:46And why is the near side so totally different from the far side?
00:55Today, pioneering technology could finally reveal answers.
01:01Z is preparing to fire.
01:02This is the inside story of the race to unravel the mysteries of our Moon.
01:24The Moon. Since mankind first looked up to the sky, it's been there, looking back.
01:31It remains the only place beyond our home planet where humans have set foot.
01:37That's one small step for man, one giant leap for mankind.
01:46Without it, life as we know it probably wouldn't exist.
01:52The Moon has been the Earth's companion for 4.5 billion years.
01:56It drives the motions of the tides, and the motions of the tides are intricately linked to the cycle of
02:02life on Earth.
02:04The Moon is vital for life on Earth.
02:07The Moon is there in the sense of a satellite of planet Earth, but it's there also in the sense
02:13of allowing life as we know it to have the features that we experience.
02:18As important as the Moon is, scientists now know that it's also extremely weird.
02:26The first clue is its surprising size.
02:30If there were an alien astronomer from another solar system, I think they would come to our planet and look
02:35up at the Moon with astonishment.
02:38There's something about our Moon that makes it incredibly strange.
02:42It's just too darn big.
02:46Of all the major planets in the solar system, Earth has the biggest Moon relative to the size of the
02:52planet.
02:52And that's very, very strange.
02:55Large moons are rare amongst the rocky planets in our solar system.
03:02Mercury and Venus have no moons at all.
03:08Mars has two tiny moons, Dimas and Phobos.
03:13Many hundreds of times smaller than their host planet.
03:18It's thought they were once asteroids that Mars ensnared with its gravitational pull.
03:25But Earth's Moon is a freak.
03:28Over 2,100 miles across.
03:31Weighing in at 77 million trillion tons.
03:35It's more than a quarter the size of Earth.
03:38Far too big to be a captured asteroid.
03:41How did we get this giant cosmic companion?
03:47Scientists think that the Moon must have been formed by an event of extraordinary violence.
03:53When you look at the surface of the Moon, one thing you can see is that it's been hit a
03:57lot.
03:58But maybe it was created from the biggest hit.
04:02For over 30 years, one theory for the nature of this violent birth has ruled supreme.
04:10It's called the giant impact hypothesis.
04:14The main idea of how the Moon formed was that something came in, smacked the Earth,
04:20pits flew everywhere and they eventually recondensed forming the Moon as we see it today.
04:254.5 billion years ago, when Earth is still a scorched ball of hot rock,
04:31another young planet called Theia comes too close for comfort.
04:38As it strikes the edge of the Earth at an angle,
04:42traveling at more than 26,000 miles per hour,
04:46Theia rips off the top of the Earth.
04:49The debris forms a ring.
04:56And over time, the fragments fuse together into a ball of scorching rock
05:02that eventually becomes our Moon.
05:12Scientists want to investigate if this established theory is correct.
05:16If the giant impact hypothesis is true, then we should be able to find evidence.
05:22So they revisit clues first gathered over 45 years ago.
05:27In the 1960s and 70s, Apollo astronauts collected over 830 pounds of Moon rocks.
05:36That boat is going to roll.
05:38Man, that is hard.
05:41Just don't stub your toe.
05:43Scientists think that these rocks could hold the answer.
05:47It's orange.
05:48As to whether the conventional theory of how the Moon formed is right.
06:03Edward Young is a geochemist.
06:06He's got his hands on a precious sample of Moon rock.
06:13We get samples like this from Johnson Space Center.
06:16We request a milligram or less.
06:19That's all we need, really, to make an analysis.
06:21This particular sample comes from the Apollo 16 mission.
06:24It was brought back by the astronauts.
06:28If the conventional theory is correct, and Theia collided with the Earth in a glancing blow,
06:34then the Moon should be mostly made out of Theia.
06:38It means that Moon rock should be chemically different than Earth rock.
06:43Ed designs experiments to test this prediction.
06:47In his lab, Ed has the largest and highest resolution mass spectrometer of its kind in the world.
06:56The machine analyzes the oxygen the rocks contain.
07:01Every solar system body has its fingerprint of molecular weight of oxygen.
07:05Theia should have been different from Earth.
07:07And if Theia is mostly in the Moon, we should expect to see the Moon having a slightly different molecular
07:13weight of oxygen than the Earth.
07:17Ed's colleague uses a laser to prepare a sample for analysis.
07:22The laser melts the rock and causes it to react with a chemical within the chamber, releasing oxygen gas.
07:30What most people don't think about is that rock is mostly oxygen.
07:34It's 90% by volume oxygen, and it's about half by weight oxygen.
07:39And so when we heat the sample and melt the sample with the laser beam,
07:44that leaves us, when we're all done, with just oxygen remaining as oxygen gas.
07:49And the oxygen gas is what we analyze.
07:52The oxygen acts as a unique identifier, a chemical fingerprint of the rock.
08:00Ed compares the chemical fingerprint of the Moon rocks to Earth rocks.
08:04The big question, are they different as predicted?
08:08Well, in this plot we're looking at our results for samples from the Earth and samples from the Moon.
08:15And on this axis of this plot, we're seeing the real fingerprint of the oxygen that makes up these bodies.
08:28If the collision that formed the Moon left more Theia in the Moon than it did in the Earth, we
08:34would expect there to be an offset.
08:35And what we're seeing on this plot is that there's no resolvable difference between the terrestrial samples from the Earth
08:41and the lunar samples.
08:44The result is surprising.
08:47The Moon and Earth rocks have virtually identical chemical fingerprints.
08:52It flies in the face of the conventional theory of how the Moon formed.
09:05You have this object, Theia, that impacts with this other object, Earth, and then in the end, you have two
09:12objects, the Earth and the Moon, that are now combinations of Earth and Theia.
09:18The problem is, is that we can't find evidence of Theia.
09:22It's like stumbling upon a site of a massive collision between what you know as two cars, but you can't
09:28find the other car.
09:30So is the entire Theia hypothesis just plain wrong?
09:34Or does the standard theory just need reworking for the Earth and the Moon to end up with virtually identical
09:42chemical fingerprints?
09:47One scientist might just have the answer.
09:51Nothing comes close to realizing that you've found something that no one has ever thought of before.
09:58And she's ready to put a radical new idea to the test.
10:15The Moon, one of the strangest objects in the solar system.
10:23Astronomers are in a race to unlock the secrets of its mysterious origin.
10:29They are convinced the conventional model for how the Moon formed is flawed.
10:35But is it completely wrong?
10:41Astrophysicist Sarah Stewart believes that the basic idea of the giant impact hypothesis is still correct, but with one crucial
10:49difference.
10:50Instead of hitting the Earth in a glancing blow, Theia could have slammed right into it.
10:58The energy of the collision partially vaporizes the two planets, mixing them together to create a superheated cloud of liquefied
11:07rock and gas called a synestia.
11:10As the seething mass cools, droplets of molten rock fuse together and gradually grow bigger.
11:17Over just a few decades, material from the two vaporized planets keeps merging and mixing and the synestia shrinks until
11:26it gives birth to the Moon.
11:28And crucially, a rebuilt Earth, made from the same material.
11:35There are scientists now that think when two planets collide, the first thing that you're left over with is a
11:40giant donut of extremely hot rock.
11:43We're talking rock that's been vaporized.
11:47And out of this one object, this one big donut of gaseous rock, two things formed.
11:53In the very core of this was the Earth, basically the remnant of these two planets that collided.
11:58And farther out in this hot donut of gas, that was where the Moon formed.
12:07At Sandia National Laboratories, Sarah uses the Z machine to put the new synestia idea to the test.
12:18The Z machine produces short bursts of electrical energy of phenomenal intensity, found nowhere else on Earth.
12:26It was originally built to study the physics of nuclear weapons.
12:37Sarah wants to use the power of the Z machine to prove that a big enough impact could turn Earth
12:43and Theia into a giant churning cloud of vaporized rock.
12:51In this chamber, we are recreating planet formation in the lab.
12:59The experiments are extremely energetic, and so part of the massive chamber is to contain all of the debris from
13:08the explosions.
13:08Sarah smashes materials together inside the Z machine and measures the point at which they vaporize.
13:19Earth is represented by a tiny sample of mineral that's mounted onto a block.
13:25Theia is represented by a metal plate.
13:29That strikes the target, that generates a shock wave that travels through the sample, Earth, and we can determine when
13:39the material begins to vaporize.
13:49When the Z machine fires, it releases a pulse of electricity 1,000 times greater than a lightning bolt.
13:57Attention building 983, the Z is preparing to fire.
14:06The electrical energy generates a force that smashes the sample and target together at more than 100 times the speed
14:13of sound.
14:15This creates a mini earthquake that shakes the entire site.
14:21Will the results seal the fate of Sarah's new idea?
14:29The hot-off-the-press result is that 20 to 30 percent of the Earth was vaporized enough to generate
14:37a synestria as we predicted.
14:40The data indicates that Theia, colliding with Earth, could have created a single churning cloud of vaporized rock, a synestria.
14:52It's in this highly vaporized synestria that the Moon forms and leads to the chemical similarity between the Earth and
15:02the Moon.
15:07The synestria idea is that instead of just throwing this stuff up into orbit after the impact,
15:13the rock on the surface of the Earth actually vaporized and the Earth was spinning very rapidly and this wound
15:18up forming sort of a donut shape.
15:22Now this actually solves a lot of problems.
15:24It actually shows how the rocks on the Moon could be similar in many ways to the Earth.
15:28They all formed out of the same cloud of debris.
15:31The new synestria idea is gaining significant traction because it may also help to explain another of the Moon's strangest
15:40mysteries.
15:44Since the Moon landings, scientists have known that Moon rocks have mysteriously low amounts of particular elements, such as sodium
15:52and potassium.
15:54Planets like the Earth and Mars are rich in these chemicals, so why not the Moon?
16:01The Earth and the Moon are almost chemically identical, but not quite.
16:06The Moon has less potassium and sodium in it than the Earth does, and that's really weird. You expect them
16:12to be the same.
16:18Geoscientist Catherine Macris thinks that the synestria theory could finally solve this decades-old mystery.
16:26I like to go in the lab and sort of create my own Moon.
16:32And with the help of the mini-Moon she created, she's ready to put the synestria theory to the test.
16:51Geoscientist Catherine Macris is trying to finally solve a decades-old mystery.
16:57If the Moon is formed from the same cloud of debris as the Earth, why does it have low levels
17:03of certain elements?
17:05Catherine thinks a curious quirk of the synestria theory may have allowed the Earth to steal the Moon's sodium and
17:12potassium.
17:14The quirk has all to do with temperature and the order in which the Earth and the Moon form.
17:22Hidden deep inside the swirling clouds of the synestria, the embryonic Moon takes shape first.
17:30As it grows, its gravity sucks in more and more of the superheated molten rock around it.
17:37The surface of the young Moon becomes so hot, sodium and potassium evaporate back into the cloud.
17:44They drift away into the synestria in a trail of vapor.
17:52As the synestria cloud cools, it shrinks, eventually forming the Earth, complete with the Moon's missing sodium and potassium.
18:06Catherine wants to test this idea.
18:09Her experiment recreates the high temperatures in the synestria to see how this affects the chemical makeup of the Moon.
18:19We have a 400-watt CO2 laser. This is the laser right here. It's very big and powerful.
18:26The laser is actually pointing straight up right now, but I have some mirrors or lenses here that actually send
18:34it straight down here so that it's shooting right inside of this chamber here.
18:39Catherine puts a miniature synthetic Moon into the metal chamber.
18:44The sample's chemical makeup is similar to that of the embryonic Moon when it starts to grow in the synestria.
18:54When it melts, it's going to be a little tiny ball of magma basically floating and spinning.
19:04The embryonic mini-moon levitates on a jet of gas, just like it's on an air hockey table.
19:12And Catherine uses the laser to heat it up.
19:18As the temperature rises, the embryonic mini-moon starts to glow.
19:26We can see that it's getting hotter and hotter.
19:30It reaches over 3,600 degrees Fahrenheit.
19:35This is the same temperature as the outer edge of the synestria cloud.
19:39A trail of vapor suddenly bursts from the surface of the mini-moon.
19:45Catherine thinks this smoke may include the vital sodium and potassium missing from the real Moon.
19:55It started out being a really dark brown, and we can see that it changed color.
20:01Right now what we're seeing is that it's very, very, very pale yellow.
20:05And it's also gotten smaller.
20:09Further tests confirm that the chemical makeup of the miniature embryonic Moon has been altered.
20:16It contains less sodium and potassium.
20:20The red line represents potassium.
20:23The blue line represents sodium.
20:25And what we see is that the longer we heat the melt, the more of this material is evaporated and
20:35then lost.
20:36And so what we are left with is a Moon that has less of those elements.
20:46One of the amazing things about this idea is that the Moon would have formed before the Earth.
20:52In the very outer parts of the cloud, the smaller Moon formed.
20:55And maybe as much as a thousand years later, things finally settled down in the core of the cloud to
21:00make our planet.
21:03So in the Sinestia debris cloud, the Earth and the Moon would have had slightly different formation times, with the
21:09Moon forming just prior to the Earth.
21:11And because they would have formed at different times, the Earth is able to steal the sodium and potassium from
21:16the Moon.
21:17And this could explain their chemical differences.
21:22The brand new Sinestia theory could be scientists' best explanation for how Earth's Moon was born 4.5 billion years
21:31ago.
21:32It explains why the Moon and Earth share almost identical chemical fingerprints.
21:38And crucially, why the Moon has less potassium and sodium.
21:44But scientists still want to know why the Moon looks the way it does.
21:50The familiar dark patches that cover the near side of the Moon are unique in the Solar System.
21:57What are they? And what forces created them?
22:02And is it possible that the Earth once had two moons in the sky?
22:08Imagine if you could have looked up at the night sky and saw two moons.
22:12That would have been lit!
22:27A new generation of scientists are challenging the established theory of how the Moon was born.
22:34But one mystery remains unsolved.
22:39The familiar dark splotches that cover the near side of the Moon are unique in the Solar System.
22:45What made the man and the Moon?
22:49And why don't we see similar features on other moons?
22:56We think of our Solar System as just being eight planets, right?
22:59But in reality, many of those planets have many moons, and those moons happen to be some of the most
23:05fascinating places in the Solar System.
23:06You've got Io, you know, the Solar System's greatest hellscape.
23:11You've got Enceladus, which has beautiful plumes.
23:14You've got Titan, which rains liquid methane.
23:17And then you have our Moon.
23:19Our Moon is unique.
23:20If you've ever seen the man and the Moon, right, that face that you see on the near side of
23:24the Moon,
23:25that doesn't happen on all moons in the Solar System.
23:29Even more mysteriously, the dark rocks that make up the man and the Moon are almost completely absent from the
23:37far side.
23:39All of those dark splotches that we see on the near side, they're not on the other side.
23:44There's a huge difference in geology between the far side and the near side of the Moon, and that is
23:49really peculiar.
23:57Noah Petro works at NASA's Goddard Space Flight Center.
24:02He's dedicated his career to solving the Moon's greatest mysteries.
24:10Noah is a project scientist on NASA's Lunar Reconnaissance Orbiter mission.
24:16Launched in 2009, this robot spacecraft has been orbiting the Moon ever since, mapping its surface.
24:27Noah thinks that the data from this lunar probe could help explain how the man and the Moon was created.
24:37What we have up here is a view of what's happening with the Lunar Reconnaissance Orbiter.
24:41And so what we have in this display is a sense of what the spacecraft is seeing, what the spacecraft
24:45is doing.
24:46Right now, LRO is just about as close to the South Pole as it's going to get.
24:50We're passing about two degrees away from the South Pole, and in a moment, LRO will start passing over the
24:55far side of the Moon.
24:57Noah's spacecraft is equipped with a special instrument that scans the Moon in infrared,
25:03a type of radiation invisible to our eyes.
25:09The scans show that the man on the Moon is made of rocks that are rich in iron, different from
25:15the rest of the white lunar crust.
25:19The areas in dark red and orange are areas that have lots of iron at the surface.
25:23The areas in blue have little or no iron at the surface.
25:26The iron-rich rocks are a type of solidified lava known as basalt.
25:32The results confirm what's been known since the Apollo moon landings.
25:38The near side of the Moon is covered in these massive pools of volcanic rock that have erupted and flowed
25:44onto the lunar surface, covering much of the near side.
25:50The Earth's most active volcanoes spew iron-rich lava onto the surface.
25:58The Moon's dark patches are made in just the same way, but on a whole different scale.
26:07Some of these lava floods are the size of an entire country.
26:13It's an extraordinary revelation.
26:16For hundreds of millions of years, one side of the Moon burned with volcanic fire.
26:23While the other side was almost completely undisturbed.
26:28Why?
26:30Those dark patches on the Moon, those are pools of hardened, cooled lava that flowed up 4 billion years ago.
26:37And they're largely confined to one side of the Moon, the side that faces the Earth.
26:42And this is a real mystery.
26:45These lava pools that are hundreds of miles across are weird.
26:50It's not clear where they come from.
27:01Peter Schultz looks for clues in the Moon's geological scars.
27:07Well, I've been interested in the Moon since I was a kid.
27:11With my telescope seeing the craters, I didn't understand how come there were so many craters on the Moon.
27:18Peter thinks that a giant asteroid collision may have led to the lava plains on the near side of the
27:23Moon.
27:25And to prove it, he's taking aim.
27:30With one of NASA's largest guns.
27:34We're going to fire a projectile down here about 22 times the speed of sound.
27:39We do this with a combination of gases and gunpowder.
27:42The gunpowder explodes, sends a piston down, compresses hydrogen gas up to about a million times the atmospheric pressure of
27:51the air we breathe.
27:52When that happens, and it's let loose, it sends a projectile down this tube into the chamber.
28:00That's when we see the magic happen.
28:04Peter thinks that a single asteroid strike could have weakened one side of the Moon, making it more prone to
28:11volcanic floods.
28:12But the asteroid would have had to be huge, hundreds of miles across.
28:23Peter believes he's found evidence of one asteroid strike big enough to fit the bill.
28:31But there's a problem.
28:33It's on the wrong side of the Moon.
28:48Planetary scientist Peter Schultz is hoping to solve one of the Moon's greatest mysteries.
28:54What led to the dark areas of lava that cover the near side of the Moon?
28:58And why are they confined to only that side?
29:02The answer, he believes, is tied to evidence of a giant asteroid strike found on the opposite side.
29:10Here's the Moon.
29:11This is the near side.
29:12This is the side that has all these lava planes covering it.
29:15Very smooth.
29:16Not very many craters.
29:18But there's only one big impact on the Moon that we know today.
29:22And that's on the other side.
29:24And this is the South Pole Lake and Basin.
29:27Almost larger than the radius of the Moon.
29:30The South Pole-Aitken Basin.
29:331,550 miles wide and almost five miles deep.
29:38It is the largest impact crater on the Moon and one of the largest in the solar system.
29:44Peter wants to investigate if the giant asteroid that created this enormous crater could also critically damage the Moon on
29:52the opposite side.
29:56He uses the NASA Ames gun to recreate the birth of the massive South Pole-Aitken Basin.
30:06Here is what we're going to hit.
30:09And we're going to hit it with this tiny glass sphere.
30:14We're going to put it on just like a golf tee and slam it at really high speeds.
30:23The team loads the gun with 100 grams of gunpowder.
30:28And place the surrogate Moon in position.
30:33With the gun locked and loaded, a pump sucks air out of the chamber to recreate the conditions of outer
30:39space.
30:40And it's ready to fire.
30:42There we go.
30:525.32 K.
30:54Very good.
30:58Footage from the high speed camera reveals what happened.
31:04The impact to the bottom of the sphere creates a shockwave that travels right through to the other side.
31:12Where the wave fronts converge, they crack the body of the sphere.
31:17So here comes the projectile.
31:19It hits right here.
31:21And the shockwave begins and then the shockwave converges and it's inside the sphere.
31:31And nothing is on the surface on the opposite side.
31:34It's all on the inside.
31:36And then everything begins to fail.
31:39And you get this crack that comes all the way up.
31:50Peter thinks that this shockwave is crucial.
31:56It explains how a massive impact on one side of the Moon could have created deep cracks on the opposite
32:02side.
32:04Look at that.
32:07The shockwave started here, propagated through, and then caused all this damage that's on the inside, but they went from
32:16the inside out.
32:17So that when you look at the back side, you can see this whole portion just popped right off.
32:26But the South Pole-Aitken impact alone can't explain the full story of the man and the Moon.
32:33Because it took another cosmic event to turn the cracks into floods of lava.
32:404.3 billion years ago, a huge asteroid strikes the far side of the Moon.
32:48Violent shockwaves travel through the Moon.
32:52Where the shockwaves converge, they cause the inside of the Moon to crack.
32:57But the man on the Moon doesn't yet show his face.
33:02A few hundred million years later, asteroids rain down, puncturing the weakened crust and releasing oceans of lava.
33:14As the lava cools, it solidifies, finally turning into the dark basalt rock that makes up the man and the
33:23Moon.
33:30Collisions within our solar system are the way that we have a Moon at all.
33:33We think that the Moon was formed by a giant collision, and that perhaps collisions are part of how the
33:38Moon evolves subsequently.
33:40Without collisions, we wouldn't have a Moon, and certainly not a Moon that looks the way it does.
33:44Scientists solve the mystery of why the near side of the Moon looks the way it does.
33:50But what about the side of the Moon that we can't see from Earth?
33:58New evidence suggests that the far side may hide an even darker past.
34:05For the longest time, the far side of the Moon was just a matter of a sci-fi.
34:10We had no idea what was on the far side of the Moon. We couldn't see it.
34:14And so, of course, everyone's mind always goes to aliens. That's clearly where they were hiding.
34:19I think the far side of the Moon has always had this wonderful poetic aspect to it.
34:23It's very intriguing. People even called it the dark side of the Moon.
34:26And it turns out there was a mystery waiting for us there the entire time.
34:30Is this so-called dark side the secret burial site of the Moon's long-lost sibling?
34:51Scientists are edging closer to revealing the secret history of our Moon.
34:57But perhaps the biggest mystery of all is the one that lies hidden from Earth, the far side of the
35:04Moon.
35:06The far side of the Moon has always been intriguing, mostly because we couldn't see it from the Earth.
35:12An interesting thing about the Moon is that it always keeps the same side facing the Earth.
35:17And this is a normal result of close orbiting bodies.
35:23The Moon is tidally locked to the Earth, and that means that as it rotates around our planet,
35:28it always shows the same face towards the planet as it goes around.
35:35Scientists get their first glimpse of the far side of the Moon in the late 1950s.
35:44Although the Americans landed a human on the Moon first, it was actually the Soviets who were able to explore
35:49the Moon really thoroughly before the Americans could.
35:52They actually sent a probe around to the far side of the Moon, the side we can never see from
35:56Earth, and discovered something really weird.
36:02In stark contrast to the lowlands of the near side, the far side is deeply cratered and mountainous.
36:10It's an alien landscape.
36:17Data from NASA's Moon mission GRAIL reveals something even more mysterious.
36:25On the lowlands of the near side of the Moon, the crust is little more than 15 miles deep.
36:35But on the mountainous far side...
36:40It's up to two and a half times thicker.
36:43That means that the Moon is lopsided.
36:46It's an astonishing revelation that only adds to the mystery.
36:51What created the mountainous far side of the Moon...
36:55...and gave it its super thick crust?
37:08Eric Aspok is a planetary geologist.
37:13He believes he can solve the mystery.
37:16His idea is inspired by a comet.
37:26So this here is comet 67P.
37:30It's mostly dirt and ice mixed together.
37:33And what you can see is it's really two objects stuck together.
37:37There's the neck.
37:39There's the head.
37:41There's the body.
37:42There's layers all over this object.
37:48Comet 67P is made from two comets that collided to become one.
37:56Is it possible that the Moon is also an amalgamation of two different bodies?
38:03Eric's idea is incredible.
38:05For tens of millions of years, the Earth may have had two moons.
38:10A large moon and a smaller twin sister.
38:14Eric thinks that this smaller moon could have merged with its larger sibling...
38:19...to create the far side of the Moon's thick crust.
38:25But why didn't two moons colliding smash each other to pieces?
38:34Eric carries out numerical modeling to investigate.
38:39He thinks the speed of the collision is key.
38:43This kind of collision that we modeled, it's basically two objects orbiting the Earth in the same orbit...
38:51...falling into each other.
38:52So they don't come in with a lot of speed.
38:54It's a completely different physics.
38:56And that's what we found out with our modeling.
38:59You can think of it almost as a gargantuan landslide.
39:06Eric thinks that the two moons collide at an incredibly slow speed.
39:10About a tenth of the speed of a typical asteroid collision.
39:16Instead of blasting out a crater, the small sister moon flattens out like a cosmic pancake.
39:27Spreading itself across the surface and making the crust much thicker.
39:33If this two-moon theory is right, then the thick crust of the mountain's far side...
39:38...is the remnants of Earth's long-lost second moon.
39:51If you had a time machine and went back to a hundred or fifty million years after the Earth formed...
39:56...and you looked up, you would see a big moon in the sky.
40:01And a littler moon.
40:02And eventually, that little moon would smash into it.
40:05I can't imagine, I can't imagine how awesome that would be to see that up in the sky.
40:18When you hear the word collision, you think of something very violent and dramatic and over in an instant.
40:24And instead, if there really was a smaller moon colliding slowly with our moon...
40:29...it was more of a gravitational snuggle up.
40:32It happened over a long period of time.
40:36Scientists are beginning to shed light on the many mysteries of the moon.
40:45Explaining its violent birth and its equally violent history.
40:51Amazingly, something as close and obvious as the moon...
40:55...is actually one of the most mysterious objects in the entire universe.
40:58It's very rare for one big idea to explain everything, so we add more to it.
41:03And now we have a much more layered and nuanced understanding of how the moon formed.
41:07But there are still some weird things about the moon that we have to explain.
41:14Born when a planet the size of Mars crashes into Earth...
41:18...a second companion moon slams into its surface...
41:24...to create the mountainous far side.
41:30...and a massive asteroid collision marks the beginning of what becomes the man in the moon.
41:38The moon may have one of the most exciting stories in the universe.
41:43...
42:02In the universe...
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