# The Moon's origin story doesn't add up

## Метаданные

- **Канал:** Howtown
- **YouTube:** https://www.youtube.com/watch?v=ghW7BEC0byg
- **Источник:** https://ekstraktznaniy.ru/video/34140

## Транскрипт

### Segment 1 (00:00 - 05:00) []

There's something wrong with this story. It goes, "Once upon a time, in a cozy orbit around a yellow dwarf star, a young planet was just starting to cool when out of nowhere, it was struck by Thea, a planet about half its diameter. A spray of molten rock formed a ring of debris that eventually came together to form a moon, our moon. This is the giant impact hypothesis. I think it's a very attractive story because it's dramatic and it's easy to explain. It's easy to visualize. But when you stop to ask, hey, how do they know that? Which is what we do on this channel, you discover that this simple story has one fatal flaw, one mystery that no one has been able to crack. A lot of people might be content with the simple story, but this video is for people who are curious about how science really works, how we can even begin to piece together what happened over 4 billion years ago. how we really ended up with our big weird perfect moon. All right, ready. So, we got about 5 minutes until we're going to take this important moon shot. We'll see if we can figure out how far away it is. Surprisingly, the distance to the moon or the radius of its orbit is one of the keys to understanding where the moon came from for reasons that I'll explain in a minute. And so, I tasked Adam and Charlie with finding that distance from the surface of the Earth. When two people look at the moon, it appears to be in a different place against the stationary background stars. The closer the moon is, the greater that difference. Normally, it's hard to get a picture with the moon and the stars because the moon is way too bright. But not during a lunar eclipse. So on March 3rd, Adam and Charlie gathered a global squad of amateur lunar photographers. — Melbourne, — you know, Nevada, — Yokohama, Japan. — 3 2 1. — Here's the photo Charlie took. Just 500 miles away in Adelaide, the moon is in a slightly different position. And as the observers get more separated, so does the apparent shift. We can take any two photos. And with just the locations of the observers and the difference in the moon position, get a rough calculation of the moon's distance. We did that for a bunch of pairs and came up with an average value of 380,239 km, just a fraction of a percent over the actual distance provided by NASA. This is what 380,000 km looks like. And this is how fast light travels between us. Using just simple ground observations, scientists in centuries past were able to estimate not just the distance, but the path of the moon's orbit and its mass. And their rough calculations are enough to reveal that our moon is rather odd. It's really large compared to the Earth, while most of the other moons in the solar system are relatively tiny. And our moon carries a weird amount of our systems angular momentum. If linear momentum tells you how hard it is to stop an object moving in a straight line, angular momentum tells you how hard it is to stop a wheel or an ice skater or a planet and its moon. It's determined by an object's mass, velocity, and distance from the center of rotation. Physicists confusingly abbreviate angular momentum as L. These two systems have the exact same angular momentum. This side is spinning slower, but the moon's mass is further out. Remember, we just calculated the real radius and the moon is actually way out here. And so, we can see that it's got a lot of angular momentum. It holds over 80% of the momentum of the system, much more than the spinning of Earth. That's also pretty strange. If you add up the angular momentum of all the moons around Jupiter, that makes up less than 7% of the systems total. What's more, the moon's tidal drag on Earth slows Earth's spin and shifts that angular momentum into the moon's orbit. The moon is inching farther away as the Earth slows down. And if you extrapolate the other direction in time, if you rewind billions of years to near the beginning of the solar system, Earth's day would only be about 4 or 5 hours long. And the moon would be much closer, huge in the sky. What does all of this have to do with the origin of the moon? Well, any good theory of moon formation has to end up with the physical system that we observe today with this very specific and strange angular momentum. And until about 50 years ago, every theory struggled to do so. In 1873, Eido Ro floated one story. Once upon a time, a gently spinning field of dust slowly condensed. Chunks of rock at the center of the swirl pulled together to form the Earth, and chunks on the outskirts pulled together to form the moon. This is called co-acretion, and it's how we think most tiny moons in the solar system formed. But physicists haven't been able to figure out how this gentle accreing cloud could end up with so much mass in the moon. We just see too much angular momentum to support this story. A few years later, Charles Darwin's son, George, half-heartedly proposed a different hypothesis. Once upon a time, a young planet was spinning so fast that a chunk of it was flung outward and formed the moon. That would require a

### Segment 2 (05:00 - 10:00) [5:00]

much faster spinning Earth than we see, — right? — In order to achieve this fish, the system would need to have started with way more angular momentum than it currently has. A third hypothesis was discussed by engineer and inventor William Samuel Hensen. Once upon a time, a giant sphere of rock flew past Earth, got caught in its gravity, and slowly settled into a stable orbit. But a passing body would have so much energy, it would be hard to tame. It's seen as an improbable thing that you would have this foreign body kind of show up, slow down, and — make it into stable orbit, too. Yeah. a stable, like quite round orbit. and also not just continue on its way or crash into Earth. Which leads us back to where we began, the giant impact hypothesis. The impact would take care of some of Thea's excess energy, turning it into heat and melting rock. And a collision could conceivably create the kind of angular momentum that we currently see in the system. So, the impact theory lines up pretty well with what we know about the physics. But there's another line of evidence that messes everything up. Real quick, I've got a question for you, and it's it might seem a little weird, but I promise that it relates back to all this. Why do they keep adding blades to razors? — I mean, I assume you just get a closer shave. The more blades, the better, right? — The idea is that, yeah, if the first blade doesn't cut cleanly, then the second or third blade is going to do the job. But in practice, that usually means there's more blade on skin contact, which is a big driver of irritation, razor burn. And so, this machine shop up in Canada decided to create something better. This used to be an aerospace shop. They made a component for this Mars rover. — And someone there was just like, I have another problem I'd like to solve. — Yeah. They designed this razor. It's got a single blade that's held stable at a precise angle. So, when you can control the geometry of the shave, you don't need more blades. You can get a close shave with less irritation. And they named their company Henen Shaving after William Samuel Henson who not only proposed the capture hypothesis of the moon but he also invented the T-shaped razor. — That's the seamless integration that these advertisers are looking for. — Exactly. This is the tie-in that will really sell some razors. You can visit hensenshaving. com/howtown or use code howtown to receive a 100 free blades with the purchase of a razor. That's a two plus year supply. — Wow. 100 free blades. — 100 free blades. You got to put both the blades and the razor in the cart for the code to take effect. — In 1967, a group of American tourists in Iceland were stranded on a snow field and had to pull their bus up a slope. What makes this photo remarkable is that many of these men would later walk on the surface of the moon. They were visiting Iceland's lava fields to learn how to collect lunar rock samples. This is David Scott. Four years later, he would be scooting around the moon on the lunar rover, collecting the rocks that tell us what the moon is made of. Sample 15555 was named Great Scott in his honor. Compared to similar Earth rocks and meteors, it's low in elements associated with iron. That lines up well with the fact that the moon doesn't seem to have much of an iron core. It's much less dense than Earth. That could happen with a giant impact. This thing kind of hits Earth at an angle that whacks the rocky mantle off of Earth. The cores merge and stay with Earth. So you would get this kind of iron depleted moon. And it's another knock against the co-acion theory. Why would you have this fluffy moon that's not as dense as the Earth if they kind of formed out of the same things? Sample 60025 is thought to be a piece of the moon's first crust. It's an almost pure chunk of plagiio clays, a mineral that's common in Icelandic lava flows. What makes it weird is how pure it is. Plagial clays can rise to the top of magma and concentrate. So, a rock like this suggests the moon was once molten. Sample 1221 is a piece of pigeonite basaltt. It's very low in zinc, potentially because volatile elements like it were essentially boiled off at some point. So that suggested that there was some big hot thing that had happened early in the moon's formation, which is definitely consistent with a huge impact. The Apollo missions returned with 382 kg of lunar material. Some ended up as gifts for other countries. Some were stolen by a NASA intern so he could have sex on top of them. Why did you do it? — The simple answer is to say that I did it for love. One chunk was placed in a stained glass window at the US National Cathedral, but most were endlessly analyzed. Their compositions relentlessly checked and double-checked, and they reinforced the findings from those three rocks I mentioned. — There were many planetary scientists saying, "Hey, we went to the moon to learn about the moon and how it formed. How come we don't know how it formed yet? Let's have a conference on that. " — William Hartman was one of the four people who proposed giant impact theory in the 70s. So where can we have a

### Segment 3 (10:00 - 15:00) [10:00]

conference that many people will come to? I know Hawaii. — In October 1984, as parts of the US faced blizzard conditions, over a 100 lunar scientists gathered at a hotel to try to come to a consensus. Before the conference, a bunch of them had been surveyed about a handful of theories, and not a single scientist ranked any of the theories as probable. — We have three theories of the origin of the moon, and none of them work. So the moon must not exist. — The giant impact hypothesis wasn't even on the survey. It wasn't really a big part of the conversation until the Hawaii conference. They really hashed it out. Came out of that conference sort of crowning giant impact as the leading theory, the least bad theory for the origin of the moon. — Was that like one of those things where like after the conference where there were like big headlines like this just in moon product of the impact and then we all put that in the textbooks and the science continued but the rest of the world was like we did it — 100%. That's exactly what happened. There was a book that came out called The Origin of the Moon, which I was able to see when I visited Hartman. — Solving the mystery of the moon's origin was built as a major goal of lunar exploration. Neither the Apollo astronauts, the Luna vehicles, nor all of the king's horses, and men could assemble enough data to explain the circumstances of the moon's birth. — The book has a chapter that kind of reviews all of the theories that have been proposed so far. It has a report card where it kind of gives different grades based on how well they meet the constraints. The grade impact theory did the best in that report card. Although several of the categories were graded incomplete. In other words, they didn't have enough information to judge this hypothesis. A giant impact could fit with the observed angular momentum. But there wasn't rock solid evidence that this must be what happened. — There hasn't been a theory that seems to be any better than that. So, I mean, that's where we are. — You got to start that way. with like here's our best guess and now let's go out and see if that guess is supported by like tests we can do. — Yeah. And this is where it gets uh more complicated. In 2001, Robin Canup produced a simulation that had 30,000 particles, which was what the state-of-the-art was at the time. They give the particles a starting condition and just play it forward and see what they get. And they were basically just like we checked a lot of different versions of impacts and this is the one that explains what we see the best. — Big Mars size thing hits the Earth and then does it all get mushed up together or does like part of it shoot off and then the rest becomes part of the Earth or is it like it dislodges part of Earth? What happened to that Mars-ized thing? you're really getting at the heart of this issue because what they found was that in order to get an orbiting disc with the angular momentum that we see today, you end up with a disc that's more than 70% made out of thea rather than made out of the earth. — Most of the material that ends up in the disc is going to be originating from the impactor itself. Most of it would be foreign to earth. This conclusion was a big deal and a huge problem for the giant impact hypothesis because it conflicted with one last piece of evidence from those lunar rocks. — When we look at the samples that we brought from the moon, we see that the moon is very earthlike. — Earthlike in what way? Well, in our universe, elements can exist at a few different weights. These are different weights of oxygen, different isotopes. And the relative amounts of these isotopes in a rock give that rock a specific signature, a flavor. You can take any sample and plot the relative abundance of the different isotopes on a graph like this. Meteorites from Mars all fall on this line. Rocks from the asteroid Vesta And Earth rocks fall consistently on this line. Every new source we test seems to have a distinct flavor except for rocks from the moon. What you see is that the Earth and the Moon fall on the exact same line, which means they have the same ratios. And as testing became more and more precise, and to this day, they're publishing papers taking those rocks out of storage again, running them again through even more precise instrumentation, they're identical to at the scale of parts per million, like two or three parts per million. This could make sense if the moon was mostly formed from Earth rocks. But if, as the physics predicts, the moon is mostly made from a passing alien planet, why would Earth and the moon taste exactly the same? And it's not just oxygen isotopes, the signatures of a bunch of different elements also look identical. When you layer on all of these matching fingerprints, it becomes less and less likely that two separate independent bodies would have had them matching prior to impact. — Ah, the flot thickens. Wow, that's cool. So, it basically would have to be like an Earth twin that hit us. Like, it would have to be so similar. — really, really similar. And I think most researchers think it's just too unlikely. And that's

### Segment 4 (15:00 - 20:00) [15:00]

the big problem that undermines the giant impact hypothesis. If we follow the physics, then we expect a moon with the different makeup than Earth. But the chemistry shows them to be identical. So, to solve this riddle, we'll need to explore some messier theories and maybe collect some more data from space. If you've made it this far, please support our little team of three by joining our Patreon, where we have behindthe-scenes videos and ad free episodes. Now, back to the show. So, is there like a theory waiting in w in the wings that's just like been waiting for its moment to shine, or is it now it's just completely wide open? — Well, I wouldn't say it's totally wide open. I think that everyone wants to preserve uh an impactbased explanation. Maybe it's like such a high energy collision that both bodies turn into complete vaporized rock. This is called the sinestia. It's one of the more interesting proposals. Both the earth and the moon form out of this kind of homogenized cloud of stuff. This story does solve the identical flavor problem. In these types of ink mats, you can get more mixing and therefore create moons that are relatively um more earthlike. But now you're left with another problem. In order to create enough mixing, most of the scenarios that they were proposed include twice or even more the anger momentum that we currently have. How do you remove the anger momentum? — Well, the people behind the Sinestia hypothesis have an idea. Maybe there's some way that the sun's influence can kind of transfer momentum out of this system. The physics is a bit too complicated for us to explain here, but the basic idea is that when the moon's orbit lines up just so with the sun, some of its angular momentum could be absorbed by the sun earth system, which is called ection resonance. And as far as I can tell, they are actively debating whether this is a plausible thing to have happened. What do you think? — Um, the likelihood is very small. I'm not saying it's not possible. I'm just saying that it's less likely. Some have proposed that there were multiple impacts, not just one big one. A bunch of little hits might lead to more mixing and more earth material ending up in the debris disc. you have more chances to create a moon that is isly more similar. But there are also challenges with this theory and that is more likely scenarios that you're going to scatter them and you're not going to grow enough. — Here's another way to solve the paradox. Assume that thea did have the exact same flavor as Earth. Maybe the impactor that formed the moon maybe was earthlike uh to begin with because it had accreted in the same uh neighborhood as Earth. — Some very recent papers have said sure Mars and asteroids have different flavors because they formed in different parts of the solar system. But inner solar system bodies like Earth, Thea, and Venus could have been made in the same general area at the same time out of the same stuff. One way to check this is going to be very hard and that is to get samples from Venus. — We've tried visiting Venus before. There are only six pictures of its surface. The longest surviving probe lasted just 2 hours and 7 minutes because the surface is over 460° C. We haven't invented a robot that could survive those conditions long enough to retrieve or analyze rock. There is one body in the inner solar system that holds more clues and that we know we can reach. the moon. The Apollo missions gathered samples from six sites, Soviet probes from three more. But it would be nice to check rocks from other places to see if they are still Earthlike, especially from the South Pole. They think a giant meteor strike down here sent chunks of the moon's inards to the surface. NASA is talking about landing astronauts in this region to grab some of those rocks. And you know what that means? It's time to practice in Iceland once again. A big theme of this whole story is how do you evaluate a theory? Really, we tend to reject theories that seem the most improbable. But that could lead us astray. We might have a selection effect where we're not talking about some random Earth Moon system. We're talking about the Earth Moon system that has creatures who do science. And if the moon is a filter for our existence as observers, then we should have a bigger tolerance for like a pretty improbable event. It's worth wondering if we're here in part because of our big weird moon. Imagine a universe with trillions of solar systems. In most moons are small. Where big moons form or are captured, they are usually torn apart or flung away or collapse into their planets. But one

### Segment 5 (20:00 - 20:00) [20:00]

random planet in a cozy orbit around a yellow dwarf managed to keep its big moon. And maybe that moon helped shield the young planet's atmosphere or stabilize its wobbles, creating a less chaotic climate. Moon-driven tides might have created the kind of wet, dry cycles that facilitated the chemistry of the first life. And eons later, here we are wondering why the moon is so special. Because it is so special. — I love the moon. I you know like I think the moon is so great. I don't know how that place — I just love that it's we've just got like this friend that's in the sky. We all get to look at it. — My mom has always told me because I moved away from home when I was, you know, 18 years old and never moved back. Um that if I can always look at the moon because she will leave love there for me and we all share it and it's just like so special.