How Did The Moon Form?

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Speaker 1: You know, I look up at the sky at night, and one of the most amazing things to stare at as the moon, because it just seems so calm and peaceful, hanging up there in the sky and looking down at us. Yeah, moonlight is so calming, right and reassuring. Yes, especially if you're a vampire or a werewolf. I heard that the moon actually has this crazy, violent, cataclysmic past. Yeah, that's right, hanging up there in the sky acting all common nights. It turns out it may have been party to one of the greatest murder mysteries in the history of the Solar system, a very impactful event, that's right. And I love when there are things hanging out right there on our faces that give us clues as to great drama that took place in deep dark history. Hi. I'm Jorge. I'm a cartoonist, and I'm Daniel. I'm a particle physicist, And welcome to our podcast. Daniel and Jorge explained the universe in which we look around at anything in the universe and try to explain it to you. On today's program, that big white, shiny thing in the night sky, the thing you referred to as the moon, not any moon, Not a moon, The moon, the moon earth little sidekick. That's right. And if you're a person who wonders and thinks about the history of things, you like all of humanity must have looked up in the night sky wondered what are the stars? Where do they come from? But the biggest thing, the fattest thing out there in the night sky is the moon. And what a mystery that must have presented to ancient man and woman. Right, what is that giant disk and where did it come from? That's right, And that's a question that science is still trying to answer. We've gone to the moon, We've looked at the moon with crazy telescopes, people have walked across it and brought samples back. But we still don't know the answer to the question how did the Earth get its moon. That's a crazy idea to me that we don't know where the moon came from, you know, like we it could have just appeared out of door one day, or aliens could have dropped it off. Oh that's an option I haven't heard yet that like somebody opened a wormhole in the moon just look popped, there's a moon all of a sudden. That's an awesome idea. Yeah, how does something that big just come about right, so perfectly round and pretty smooth and bright. Yeah, but it turns out the Earth's moon is not like the other moon's. You know, one of these things is not like the other things, and the moon is kind of weird, which makes it hard to explain. Yeah, it's a weird moon. It's the only one we have, and so for a long time it's sort of defined the whole concept of moon. Right, But now that we've seen other planet and we've seen how many little moons they have, were like, jeez, our moon is kind of weird. Does my moon make me look fat? You know, because we've got a big, fat moon. Yeah. So this is an interesting question, and we wondered how many of you out there no or think they know the answer to the question where did the moon come from? So I walked around the campus of UC Irvine and accosted random strangers who are willing to talk to me and asked them this question, where do you think the moon came from? Yeah, so before you listen to these answers, think for yourself what would be your best guess. Here's what people have to say. I'm not entirely certain. I just know gravity plays a part in it. There was like a meteorite that hit the Earth which broke off some rocks or something like that. From what I remember, it might be that, well this is like the big Thing, but I don't really know, no, No, My best guess is that it either came from some sort of material that was already in the atmosphere and by some sort of a potational pull was brought in. Alright, So not a lot of strong ideas here. I think it's a pretty there's a good breath of ideas. Yeah. I like the people who answer where did the moon come from? And they would just say gravity, Like that's the answer too, pretty much everything there, or the or someone said the Big Bang. I'm like, yeah, of course everything came from the Big Bang. That is that is a good default physics answer that somebody asked you a physics question. The answer is always the Big Bang. How does the hig boson give mass to other particles? It's the Big Bang? Really, you can't go wrong. So could have see that person on the street figured out how to always be right? Well, let's talk about the moon. How big is the moon? Daniel? Or let's maybe take a step back. What is the moon? Right, So it's just a definitional thing, right. You have solar systems that have these hierarchies. You almost always have the main masses in the center. You have star where most of this has gathered, and then you have the planets orbiting around it. And then you know, you have need a name for the stuff orbiting around the planets. The theory this could go on forever. Right, you have the star with planets around it, and then moons around the planets, and then you can have things orbiting around the moon, and then things orbiting around those things, like moons can have moonies. Yeah, I think they're called moonlits moon or moon ETOs or something. Yeah. So so it's just it's just the name given to something floating around a planet. Yeah, exactly. If you're a blob of stuff floating around the planet, then we call you a moon. But it's interesting because you've got to be big enough, right, Like, if you're just specks of stuff, then we call you a ring. Right, if you're like distributed all the way around the planet, then we got then you've got rings, Like Saturn has rings. Saturn's got rings Jupiter's got rings, you know. Um, and it's in some ways a question of definition, Like there is stuff floating around the Earth into sort of a vague, hazy rings. So could you say the Earth has rings? You know, people argue about that kind of stuff, but that's just like that's arguing about the definition. It's not really arguing about the science generally. But it has it has to have a certain size to be called a moon, I think, so, yeah, otherwise it's just a rock rock rock or rock let or rocky rocky dough. Yeah, there must be some organization out there that's tasked with classifying what the moon and what's the moonlit, and what's just a piece of ring and with just random garbage in space. You know. Um, that doesn't sound a very glamorous job. So how big is our moon? Our Our moon is really big. Our moon is two thousand miles across, which is pretty big compared to the Earth, which you know, is only eight thousand miles across, and most of the other planets their moons are tiny in comparison to the size of the planet. So it's pretty big. So it's like from California to about like Arkansas or something, you're giving me the worries here, Like if you took the Moon and sort of gently put it down, how far would it would it look? Like? Why do you even imagine that? Like placing the Moon on the Earth just for scale, you know, like you were to walk across the Moon, how long would it take you? You know? Right? Yeah, um, well not that long because you could bounce because of the gravity is pretty low. But yeah, it's like two thousand miles, so it's not as far as l A to New York, for example. But it's pretty big, right, It's pretty big. And the interesting thing is that it's at the same scale as other planets in the Solar System. Okay, so I looked it up, uh, and I did some quick calculations because I'm a train engineer and I can do some math here, and so like if the Earth was the size of a basketball, the Moon would be a little bit under the size of a tennis ball. I prefer I preferred fruit based analogy. So I'm gonna go with a watermelon and an apple. Watermelon, and then are you hungry, Daniel? Do we need to take a break here and break for a while? A right, as a watermelon and an apple. Except what this interesting? The distance is maybe a lot larger than most people think. So, like you would have to put a watermelon down and then walked about twenty five ft and then set down that apple. And that's about the distance between the Earth and the Moon. Yeah, it's crazy. These things seem big, but they're tiny compared to the distances between them, right, which is sort of a larger lesson for everything in space, right, Like the Sun and the Earth seem huge, but they're really far apart compared to their diameters. And our solar system is far from the next star and and it's incredible the distances between stuff and space. Yeah, okay, so that's the moon. Um. The crazy thing is that we don't know where the moon came from. That's right. People have been trying to figure out how do you get the Moon this big and this weird around the planet this close to the Sun, and they can't figure it out. You know, they have simulations and theories, and you know, this is what scientists do. They say, can we explain what we see? And they start with an idea and they see does that work? Right? Can I take that idea and end up with the situation I see in front of me, and they have some ideas, and we'll talk about them, we'll dig into them. But the bottom line is that none of the ideas we currently have completely work. They all have problems, which suggests that the answer is something we haven't yet thought of, or some weird twist on one of the current ideas. All right, bring it down for us. How what are the different ways that a planet can get a moon? Like? If I wanted a moon, what would I need to do? You just go on Amazon, man, you can order anything. Well, you can probably get a moon. Tomort. You want a moon, I get you a moon by tomorrow afternoon. That's what Walter from the Big Labowski would saying. Can I just get a slightly a shorter person to just follow me around and turn around me? That's not a moon, that that's an assistant. You want somebody in your orbit, right who protects you from stuff? Right, cleans out all the space junk that's coming at you. Um. Yeah, So how does the planet get a moon? Well? One option is that it's formed when the planet is formed. Right, Let's remember how our planets formed, and it's um not from the Big Bang, right, Like our professional physicist from earlier. Directly, it is from gravity though, So you're saying that the planet can get a moon at the same time that it's forming, kind of like a little twin brother. Yeah, exactly. So imagine how is the planet formed. Well, it starts from a big poof of gas and dust and rocks, right, and then gravity coalesces it together gradually, slowly, slowly into a big clump. And you might wonder, why doesn't it all form into one big clump, right? Why do you get little bits left over as moons or other stuff? Um? And this sort of two answers to that, like most of it goes into the big clump. Right. First of all, like most of the stuff goes into the planet. Um. The reason it doesn't all is that some of it's traveling really fast, and so rather than getting pulled down into the central clump, essentially ends up in orbit. Right. So what we call in physics, we call it angular momentum. Supported you know, it's the reason that the Earth doesn't just fall into the Sun. Right, It's because it's moving fast. Oh. I see, So there's a bunch of stuff that came together, But some of that stuff was a little bit out on the periphery and didn't quite make it into the main planet clump exactly. And it's all spinning, right, the whole thing is spinning and it's going and so it starts out spinning around and it's moving too fast to get sucked in. Um. And the other part of the story is that none of this happens in isolation, right. The Earth is not in the middle of totally empty space forming quietly. This stuff going on all the time, And in the early soul or system, we think things were pretty crazy and so um. It might have been that the early blob that formed the Earth could have formed a single planet with no moon and no other objects or whatever. But it probably got perturbed a lot, you know, things crash into it, things bump into it. Even just the tugging of the Sun and other planets coming nearby keep that stuff from really settling, and so some of it ends up still out in space opening around the planet. Okay, it formed at the same time as the planet. Yeah, that happens in some cases, we think, um, so sorry it's too late for you there. Or if you wanted a moon from the from zero, you've missed your window. I need to go back to the to the womb. That's right, the womb moon the moon. That's a that's a tongue twist right there. Um yeah. But the better way, I think, the more popular way the planets gives moves. All the cool planet at least get moons by basically interacting with other objects. You know, like, um, something smashes into them, or something flies by and they capture it. Um, that kind of stuff. Wait wait wait wait, um, so those are two other things. You can either capture a moon, Like if the Earth was flying around and suddenly there was a little bit of rock out there, it would it could like bring it into its orbit. It could bring it into its orbit. That's not very easy to do because much more likely is that something comes and smashes into the Earth and then the resulting debris floats up into space and then turns into a system of rings and then coalesces into a moon. That's much more likely. It's because it's hard to capture something entirely. Either it deflects right, it bounces off into space, or it hits the Earth getting captured. It means you have to be exactly the right speed at exactly the right angle at exactly the right orbit. It's a difficult thing to do. It's like getting a perfect pool shot, you know, jumping the eight ball or something. You're saying, that's harder than getting hit by something out in space. Yeah, because the Earth has gravity, right, So if something comes close by, it's most likely just going to spiral in and smash into the Earth, or it's going to bounce off into flect. So coming in from somewhere else and then ending up in orbit is pretty trick. Okay, So I think the most likely way to get a moon is for something to smash into the planet and the debris to float up and then coalesce into a moon. Alright, let's get into this smashing idea, but first let's take a quick break. All right. So you're saying that one way to get a moon, and maybe our moon came this way, is that something smashed into the Earth and kind of like broke off a piece that then became the moon. That's right, Yeah, And that's a that's the way we think some of the moon's have occurred in the Solar System. But it's tricky to explain our moon, right, Um, and because our moon is so big, right, Our moon is huge compared to the planet, and so in order to make that size the Moon work, you need a huge impact, right, You need a ridiculously big effact. You can't just be hit by laroc that knocks off a bunch of stuff. You need a giant impact, like a planet killing impact. Yeah. And the current models say that the kind of thing that hit the Earth a long long time ago to make the Moon might have been something the size of a planet. It might have been a proto planet, might have been something the size of Mars. Right, So we're talking about like two planets colliding, right, This must have been amazing to watch, right, right or not if you're living in these planets, that's right. Well, we don't think there was anything alive on Earth when it happened. The best estimates say that this happened about um zero point one billion years or a hundred million years after the Earth was formed, So the Earth was still pretty hot and a nasty when this kind of stuff happened. We don't think there was any atmosphere of life yet collided with another planet about its size. That's incredible, right, two giant balls of rock just yeah, exactly, and um and that must have been pretty kindaclasmic, right, A lot of stuff must have gotten thrown off into space, and some of that stuff coalesced into rings. So the Earth probably had rings for a while, no way like Saturn. Yeah, I know that has been pretty awesome, right, I kind of wish the Earth had rings, Like what would that look like at night or during the day, you know, to see rings up in the sky. That would be pretty incredible. So then the two planets collected is the idea to planets collided, they form a new Earth with some stuff out there in rings that eventually became the Moon. That's right. And this collision was huge, right, It was like a hundred million times the energy of the asteroid that hit the Earth and probably killed the dinosaurs. So it's a ginormous explosion. It's nothing that anybody could ever survive. Yeah, I've seen I've seen the videos of the stimulations. Like it's basically the Earth just gets pulverized and then just kind of coagulates into this new thing. But it's like it's basically obliterated. Yeah, and here you'll see the strength of my fruit based analogy, which is imagined you take two watermelons and you throw them together. What happens, right, that's pretty much what's happened. You know, it's complete destruction. The watermelons do not survive that kind of impact. So then how do rings become moons? And why? Like, why does Saturn still have rings and nut moons? Yeah, that's a really awesome question because gravity, right, you would think if stuff is floating out there in space, then eventually gravity would coalesce, it would pull it all together into a moon. And that does happen if you're far enough away from the planet. If you're too close to the planet, then the strength of gravity tugging on you from one side and the strength of gravity tugging you from the other side are too different. Because remember the force of gravity depends on your distance from an object, and so one side of the Moon is closer to the Earth than the other side of the Moon, So the Earth pulls on one side of the Moon more than the other side, So it's literally pulling the Moon apart. But the Moon is far enough away from the Earth that the Earth is not strong enough to shred the Moon. But if the Moon was a lot closer right then it then it would be pulled apart. By these gravitational they're called tidal forces. By the tidal forces, so there's a region around every planet where you just cannot be a moon because if you are, you're gonna get shredded, right, So you just kind of get ripped apart. Yeah, it's like a blender. There is like a blender. Yeah, and we've seen this happen if you remember the comment that hit Jupiter um in the nineties. It passed really close by Jupiter before it hit and Jupiter pulled it apart into twenty six pieces because of his tidal forces. And so the larger the planet, the stronger the force of gravity, and the more likely this is to happen. Oh I see. So if Saturn had bigger rings or rings that were further apart, then the tidal forces would be less and then the little bits of it would have time when and kind of space to clump together. Yeah, but that's a whole other fascinating mystery, like how long has Saturn hand rings? How long will it continue to have rings? Right? We think that those rings are pretty stable because they have been there for like a hundred million years and um, and because Saturn has really strong title forces. So anything that coalesces, Saturn will tear up again, but we don't really know for sure. And Saturn does have some moons, and we don't know like are those moons in the process of being trashed, Like if you fast forwarded a billion years with Saturn look totally different or is it looked this way for a long time. It's amazing to me how dynamic the Solar system is. You know, like if you look at a picture of the Solar System from two billion years ago, you might not recognize it. You could have a different number of planets, and all the planets could have different number of moons, or the planets could even be in a different order. It's crazy stuff that's happened in our Solar system. Or even two billion years from now, it might look totally different. Yeah, well, in two billion years from now, I hope we've built massive interstellar structures so we're recognizable from space. Um. But even without that, Yeah, the planets could reorganize or realign, or things could drift this way or the other way, or something could come from another Solar system and knock knock into something and change everything. Yeah, you know, people might imagine the Solar system is really static because it's old, but we've only seen the recent history of it. And one of the best ways to figure out what is the history, what is the whole story here, what is the drama that's taking place out there in space, is to ask these questions, you know, like how did the moon form? Yeah, so that's one possibility, is that something smashed into the Earth through all this stuff out there that became a moon because it was far enough away from the Earth. But there's some that's not quite right, right, Like that doesn't quite fit what we see or know about the moon. Yeah, it's interesting because we can't quite make that story explain everything we see. And so one of the things we see, for example, is that we've been to the moon and we've looked at rocks from the moon. You've been to the moon. I mean we collectively as humanity. I like to take credit for humanities of humanities collective. The Royal week there. Okay, we have won a bunch of NBA championships, by which I mean me and Lebron James. Yeah, we are an acclaimed internet cartoonist. But which I mean me and you. We are Tom Cruise. You know, we are sexy in our fifties exactly. Humanity, not me personally, has been to the Moon and brought back rocks, and we've looked at those rocks, and those rocks look really similar to rocks on Earth. And you should know that rocks on every planet look different because they're formed under different circumstances from different bits and different temperatures and different ages. So you can sort of tell where a rock came from, Like really that different because they it all came from the debris of this Solar system, right, wouldn't it all be sort of the same. But like Mars is different from Earth, and we can we we found rocks on Earth that we can tell came from Mars because we know they're different. They have different structures, formed in different temperatures at different times and this you know, different totally different kind of tectonic activity on different planets, like Mars doesn't have any at all, you know, and letters of the bottoms that made in Mars Mars first, Yeah, exactly, make Mars great again. They don't like, they don't want to import our rocks anymore. The high tariffs on Earth rocks anyway, So they can look at these rocks from the Moon and they say, they looked just like rocks from Earth. Right. So that's really interesting because suggests that the stuff that the Moon is made out of is really similar to the stuff that the Earth is made out of. But if a really really big planet came and smashed into the Earth, that's not what you would really expect. You would expect that planet to have mostly survived or that or to bounce off, or for this stuff in the Moon to be made from that planet that came. Right, You started out with two objects, you have a collision, you end up with two objects. You expect sort of a you know, a connection between the incoming and the outgoing too, but instead both objects that survives seem to be like the Earth stuff, which is a bit weird. But we couldn't, um, couldn't this new planet have also mixed in with our old Earth. And so that's why it's the same, you know what I mean? Like maybe the Moon and Earth is a mix of these two pre crash planets. They have these simulations that are totally crazy where essentially the other planet gets like subsumed into the Earth like including it's it's like core, right, So there's some other planets internal bits, right, the iron and the nickel that makes up the inside of a planet is inside the Earth. Now, like if you cut the Earth open, you would find evidence for like, you know, a second planet in there. And then what made the Moon was like the Earth's crust just got ejected. It's like ice cream, like a swirly ice cream doesn't completely just mixed together, and that you would expect chunks. Yeah, I'm glad you're with me on the food out analogies. Now, trying to move us here towards dessert. We had our fruit as the meal is wrapping up. In some places fruit is considered dessert. I think we should do something a little bit more meaty. Um. Yeah. So that's one mystery is like why does the stuff on the Moon look just like the stuff in the Earth. And they can kind of make it work, right, Like they can make it work that most of the stuff from the Earth's crust turned into the Moon and the other planet just sort of got swallowed by the Earth. But it's tricky. It's not an easy thing to make work. And if that happened, then you'd expect crazy stuff to have happened on Earth, like you know, huge oceans of molten talk of magma like magma oceans which is just a phrase I love saying and hearing, magma oceans um. But we don't see any evidence of that on Earth. Like people who look at the history of rocks on Earth don't see evidence of these of these magma ocetions that you would expected from a huge such a huge collision. It doesn't look like we were hit by a big rock. Yeah, we don't see evidence for that on Earth. And you know, when you put together this kind of story, you want to check it multiple ways and does this make sense? And let's see it in this other way? And we can't find any confirmation for that currently, and the simulations are are pretty hard to get right. Before we keep going, let's take a short break. Man, that's wild. So we really don't know where the moon came from. Kind of Yeah. There are some other ideas, you know, crazy ideas about how two other planets may have collided and basically totally annihilated each other into some other sort of cosmic doughnut, which then spun around and formed the Earth in the moon. I said, that's right, we're doing double dessert here. One of the crazy ideas is that the Earth essentially was obliterated in this collision, and everything just became a big cloud, really fast spinning cloud, which ended up shaping shaped into this crazy donut shape. It's spun out a blob which became the Moon. Um. That theory has some trouble because it turns out the Moon doesn't really have like a solid iron core like you would expect, like, you know, the Earth has a as a release heavy metallic core like most rocky planets, but the Moon is pretty light. It's mostly fluffy, right. It has a little bit of a core, but not very much. And so that's better explained by saying that the Moon mostly came from Earth crust stuff rather than from like an entire blob of planetary stuff, where you would get like the same same amount of serving of the core as you would have the crust. Um. So all these theories have some problems. Well, what are some other crazy ideas? Yeah, Well, a theory that exists for a long time was just that the Moon was captured, you know, like maybe the Moon came from somewhere else or used to be its own planet, or was wandering around just sort of got sucked into the Earth's orbit. And this theory has a couple of problems. One is that that's really hard to do. Like you do simulations and you have another planet approach Earth, as we were saying before, usually they you know, spin off each other and then one gets flung out into space or they end up colliding right to get it to come into stable or with that last for billions of years. That's really really hard to do. That's a one in a billion chance. So that might be possible, but it's hard to do. And the other problem is it doesn't explain why the Earth and the Moon looks so similar, right, they have like these really similar rocks on them. The Moon came from somewhere else. I was captured. It shouldn't have the same basically rock d n a that we have. Okay, but I heard another idea is that maybe the Moon fell off to Earth like it was part of us, but then it was like I'll see you later. Yeah. I think this is a really popular, super ancient idea, Like if you look in the historical documents, people speculate about this, you know, a thousand years ago before really anybody knew physics, and somebody even wrote that the Pacific Ocean was like the scar of the moon leaving the Earth. You know that a huge chunk of land to just like floated off into into space because the Earth of spinning, um, and it got turned into the moon. The moon is Atlantis? Is Atlantis? Oh my gosh. I love when you can connect to mysteries at once. Um. But there's basically no data to support that at all. I mean, it's just I mean, we know the Pacific Ocean is not formed by somebody taking a scoop out of the Earth, um. And so it was just like random speculation. But it was a popular idea for you know, a long time among the medieval and ancient folks. So what's kind of the best current thinking about where the moon came from? I think, um, if you ask most scientists, and I haven't asked most scientists, but I've asked a few. He was asked science scientists. I've asked a scientist who's an expert in planetary science, and I've asked myself. I'm a scientist. Um, So yes, I've asked scientists about this. That's the deep research we do for this show, folks. UM. And the prevailing theory is the giant impact, right, it has some problems. There are things we don't understand about it. But that's sort of the progress of science. Right, we say, here's a bunch of ideas. This one doesn't quite work, but it mostly works. The elements of it that explain things we see. There's just stuff to figure out. And so that's that's the prevailing ideas, the giant impact. It's the best idea we have. That's right, that it's that or the Big bang? Man. Yeah, and I like this process of science. You know, we have one idea, we refine it, We refined it, and then we see doesn't fit the data better than it used to. Is it sort of coming together? It's like solving a murder mystery, right, You look for clues, you come up with the theory. Something doesn't quite work, makes you change your theory. Eventually you have a story that explains everything you see and that fits in with what other people say and that makes sense. You get more data, and that's what we're looking for, and we have a story. They definitely wasn't murder. A huge planet died um in the making of our moon, but we don't quite know how it happened. And it might be that, you know, it's just sort of weird, and it's it seems unlikely that this sort of configuration would happen, and maybe it was unlikely, or maybe there's some part of the story we haven't understood, like there was two cataclysms, or you know, two planets hit the Earth or something like that makes you wonder, what if that other planet or asteroid hadn't hit us, you know, if it just missed by a few degrees, we wouldn't have a moon and we would have a super different planet Earth, right, Yeah, and things could be very different. You know. The reason we have tides is because we have the Moon, and the moon stabilizes the Earth's orbit, and we think that the collision that caused the moon might have also caused the tilt of the Earth, which means if we didn't have a moon, we might not have tides, we might not have seasons, and that's a pretty big change in what life on Earth is like, right, And a lot of people think the tides were critical to life because as the sea comes in and out, it's sort of like a lot of slashing around, which is what you need to mix up those basic organic chemicals into something that might turn into life and so yeah, we might have the Moon to thank for the fact that we're even here to ask about it. It just that a planet had to die. That's right to get to But in the interest of all things good, so that we could be here eating watermelons, somebody had to sacrifice a big planet. So yeah, that's another one of these crazy mysteries that are just staring at us in the face every single night. And when you think about the Solar System next time, remember it's not static, it's dynamic. There is stuff happening, and there is a story. It's playing out really slowly right geological cosmic time scales. But if you took a time lapse video of the Solar system, it would seem like a crazy dance party. Yeah, so next time you are mooning the moon, pull up your pants and show some respect, because that we owe the Moon a big thank you. All right, thanks for listening to everyone, See you next time. If you still have a question after listening to all these explanations, please drop us a line. We'd love to hear from you. You can find us at Facebook, Twitter, and Instagram at Daniel and Jorge that's one word, or email us at Feedback at Daniel and Jorge dot com