Podcaster: Paul M. Sutter
Title: AaS! 245: What Made the Moon?
Organization: INFN Trieste and OSU CCAPP
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Description:
Why is the moon so special? Why do we think a giant impact created it? Why didn’t the same thing happen to other planets? I discuss these questions and more in today’s Ask a Spaceman!
Bio: Paul Sutter received his Ph.D. in Physics from the University of Illinois at Urbana-Champaign as a Department of Energy Computational Science Graduate Fellow. He then spent three years as a Postdoctoral Fellow in Next-Generation Cosmic Probes at the Paris Institute of Astrophysics, and is currently an INFN Fellow in Theoretical Physics in Trieste, Italy, and a Visiting Scholar at the Ohio State University’s Center for Cosmology and Astro-Particle Physics. He is inexplicably drawn to positions with very long titles.
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Transcript:
Look around at the rest of the solar system, and I dare you to find something as weird as the moon. First off, it’s big. It’s huge.
I mean, it’s small compared to the Earth, it’s just around 1% of our mass, but as solar system objects go, it’s on the big side. Heck, Mercury is only a few times more massive than the moon, and it gets to be its own planet. And yes, there are other large moons out there, but those are all around the giant planets like Jupiter and Saturn.
The moon is big relative to the Earth. There are larger moons like Ganymede and Europa, but those are tiny, miniscule, nothing at all compared to their parent planets. The moon compared to us is huge.
There is no other system like this in the solar system except for Pluto and Charon, and we all know what happened to Pluto. And just look at the inner solar system, our nearest comparables. Mercury, no moons.
Venus, no moons. Mars has two moons, but come on, they’re really just captured asteroids and they’re not likely to stay in orbit for long, astronomically speaking, so those don’t really count. Of all the rocky planets, we’re the only one with a substantial moon.
But that’s not the only weird thing about the moon. The moon orbits us really quickly, especially given its size. In fact, if you add together the Earth’s own spin, add in the spin of the moon, and add in the orbit of the moon, you get the total angular momentum of the Earth-moon system.
And when you take that total, we blow the other rocky planets out of the water. How did we get so much spin? It gets weirder.
The moon is creepy. Yes, that’s an actual science term, but it’s an acronym. K-R-E-E-P.
K is the symbol for potassium, R-E-E stands for rare earth elements, and P is for phosphorus. Creep. Yeah, that’s a real stretch even by my standards, which are admittedly pretty low, but the idea is this.
These elements don’t usually like to hang out together. Rare earth elements, potassium, and phosphorus don’t usually like to hang out in the same rock. But lunar samples show a lot of creepiness.
You dig up a random scoop of moon dirt, or you pick up a random moon rock, there are a lot, relatively speaking, of, there’s a lot of potassium and phosphorus and rare earth elements all mixed together. And the only way to get those elements mixed together is to moltenize a significant fraction of the moon. And yes, moltenize is now a new word.
You have to turn it into a liquid. You have to turn rock into liquid. You have to moltenize it.
And that takes a lot of energy, a lot more energy than is typically involved in the usual planet formation process, which is violent and has a lot of energy, but not that much energy. Not enough to moltenize an entire world like the moon. And if the moon did form on its own, it’s very odd.
Like it has a very small iron core, much smaller than the other rocky bodies of the inner solar system, which suggests that it didn’t form through the usual processes like everybody else around here. And, and this is the real kicker, the Apollo missions were a goldmine in terms of our understanding of the moon. That’s because they brought back whole literal bags of rocks for us to study far more than we could ever hope to get with even fleets of robotic rovers.
So those missions gave us an exquisite view into what the heck the moon is made out of. And as much as I would like to say right now that the moon is made out of cheese, I instead have to say that it’s made out of us. Earth stuff.
That’s right. Lunar samples show very similar amounts of certain stable isotopes as the earth’s crust. If you dig up a random part of the earth’s crust, you just go in your backyard and dig it up, look for various stable isotopes of various radioactive compounds.
But you know, the, the kinds that aren’t radioactive, we should do a whole episode on radioactivity and isotopes. Just ask. And then you go scoop up your backyard on the moon.
They’re going to have the same ratios. What’s going on? A giant impact.
That’s what’s going on. As early as 1898, a scientist by the name of George Darwin, not Charles Darwin, evolution guy, but this is one of his sons, proposed that the earth and moon had a common origin. It was known by that time that the moon is slowly receding away from us.
And he wondered if that one time in the distant past, the earth was spinning so rapidly that the kind of got spun off. Like when you spin those pottery wheels way too fast and stuff just goes flying. But it turns out that doesn’t work physically because if you spin fast enough to make a moon, you also basically destroy the earth, which is no bueno.
It would take all the way until the 1970s when astronomers started to take seriously another idea. Maybe the earth got whacked really, really hard. And the moon split off like a tooth flying away from a boxer’s face.
And you got to hand it to the astronomers who first proposed this and why I will contend to my dying day, the creativity of scientific exploration. Someone was just sitting around one day, maybe out for a walk and they thought, hey, maybe a giant proto planet hit the earth and made the moon. Wouldn’t that be cool?
Yeah, it would be cool. And it’s kind of right. It’s called the giant impact hypothesis.
I suppose all the comic book punching sounds like blam and kapow hypothesis sounded a little too corny. So we’re going to go with giant impact hypothesis. Here’s the deal.
Let’s rewind the clock about, I don’t know, four and a half billion years, just a hundred billion years or so after the solar system first started coalescing. You know, it started off as just a disc of gas and dust that slowly compressed and then bits of the gas started pinching off and forming these little nuggets that would smash into each other and then slowly collect together. And then once they got a little bit big, they started using their own gravity to pull down surrounding material.
And by about a hundred million years, you end up with a series of proto planets. These objects are not quite planets, not their full grown size yet. And there are a lot of them, a few dozen, maybe, maybe more, maybe hundreds, but definitely a few dozen objects that could one day become planets.
But as you may have noticed, there are not dozens of planet planets in the modern day solar system. So something had to happen to these proto planets. Some of them got scattered away, thrown off into interstellar space, and some of them smashed together.
There just isn’t room for all of them. There’s a solid chance. Yeah, we try to track this in computer simulations of growing proto planets to see what kind of orbits are stable, where material starts to collect, you know, following the physics of this very early story.
And there’s a solid chance that we shared an orbit with another proto planet. There was us here on the earth and then sitting at a gravitational stability point called a Lagrange point, did a whole episode on Lagrange points, sitting about 60 degrees away from us on same orbit that you can have another collection of material just happening there. But this was never going to last long.
This is an unstable situation. So we have our proto planet forming that will become the earth right here and then sharing our orbit either ahead of us or behind us in our orbit, you can have another clumping of material, another proto planet, but it is so unstable. All it takes is a little extra tug, you know, an extra pull from Jupiter.
Jupiter starts to grow in mass and now it exerts its gravitational influence across the solar system. Maybe the outer planets rearrange and shift themselves and that changes the gravitational stability of the inner system. Maybe another object strikes that other proto planet, setting it into motion.
But once it was set in motion, it was always going to hit the earth. There is no other path for it. I mean, unless you give it so much energy just goes flying out of the solar system.
But if you give it a little nudge, it’s going to wind up hitting us. And that other object has a name, we call it Theia. This is named from the Titan from Greek mythology, who is the mother of Selene, who is the goddess of the moon.
That’s fitting. I like that. Once Theia was set in motion, it was going to hit the earth.
We were going to meet now as impacts go, it didn’t have to be all that energetic. Our best guess based on detailed high resolution computer simulations that follow all the gory details of what happens when two planets collide, is that Theia struck the earth at an oblique angle with a speed of around 9.8 kilometers per second or around 20,000 miles per hour. Which, okay, to be fair, that’s very, very fast.
But as collisions go, it’s not that fast. We have asteroids hitting the earth upwards of 100,000 miles per hour, five times faster. Easy.
Solar system can do that, easy. This is considered a low energy collision, believe it or not. But once Theia struck the earth, all hell broke loose.
Theia slammed into us, even if it’s a glancing blow, even if it’s an oblique angle, even if it’s not all that fast, even though it’s super fast, but not all that fast of what it could be. It is still nasty when you have two proto-planets hitting each other. It’s not fun.
The proto-earth was a little bit smaller than modern-day earth. Theia was probably around the mass of Mars. When those two kinds of objects hit each other, there’s a lot of energy.
Material gets ejected. The crust is vaporized from the heat of the impact. The energy is released.
The core of Theia, the high density iron nickel core of Theia survives, but it becomes embedded, lodged inside of the earth, where it makes its way to meet our core. Our two cores mix together. The mantle of Theia largely survives, but again, mixes with ours, so now we get a super mantle.
Our crust gets blown to smithereens, ejected into space. The crust of a planet is always the lightest material, the least dense material, and the easiest to destroy material, so that’s what gets destroyed. Our crust and Theia’s crust go into orbit around the earth.
What happens next is a little hard to follow. The physics is kind of complicated, but our best, most detailed computer simulations that try to follow the physics of what happens in a collision like this, and it also all depends a lot on exactly how the impact unfolded, what Theia was made of, how big its core was, how big its mantle was, how much crustal material it had, et cetera, et cetera, et cetera, but here’s the general story, the general picture is that some stuff went flying away, never to return, gone, we lost it. Too much energy, blew up. Some of that crustal material that got flown into space, rained back down onto the surface of the earth and gave us back our crust, but then a big chunk remained in orbit.
Eventually, that material would coalesce, because it’s in orbit around the earth, it starts bumping up against each other, they start sharing orbit, they start all these little micro collisions, just like the collision where Theia and earth shared an orbit, but ended up merging together. You have all these bits and pieces in earth orbit, they coalesce. Some simulations suggest it took only a few hours, some say a few centuries, or maybe a few thousand years, but eventually the deed is done.
The remaining material becomes its own solid object, the moon. Some models suggest that a second moon, just a few hundred kilometers across, formed out past the far side, we get material at the orbit of the moon, and then out past the far side at another Lagrange point, you get another collection of material, a few hundred kilometers across, but through tidal interactions, that gets brought in and it approaches the moon and pancakes itself. That’s the word the researchers who did this study use, it’s not mine.
Pancakes itself on the far side, which might explain why the far side of the moon is lumpier and weirder than the near side, that’s a possibility. Another possibility is that this wasn’t a low energy glancing blow at all, you know, relatively speaking, that instead the proto-earth was spinning really quickly, and then we really got nailed. Not a polite tap, but a full-on wrestling body slam by Theia.
This impact delivered more than enough energy to vaporize, not just the cross, not just the mantle, but everybody and everything, turning both of us into this giant donut-shaped ring of plasma known as a synestia. This is taken from syn, a prefix meaning together, and Hestia, the Greek goddess of the hearth, since we seem to have a Greek goddess vibe going on here. So if there’s enough energy and things are spinning rapidly enough already, when we get hit we just get vaporized and turn into a donut of material.
Eventually that donut cools off, it only lasts a few decades maybe. The inner portion collapses, the heaviest material goes to the center, forming the core of the earth, surrounded by the mantle of the earth, and then some of the crustal material gets distributed between the crust of the earth and then the leftovers make the moon. But no matter what, whether there are second moons, whether it’s a glancing blow, whether it’s head-on, whether it’s really kind of low energy, whether it’s kind of high energy, the broad picture emerges, and this is why the Theia giant impact hypothesis, one of the reasons it’s so compelling, one is that it fits a lot of the evidence, two is that it’s very general.
We like it in physics when things are generic. When you can tweak the model and play around, like what if Theia’s over here, what if it’s made like this, what if it hits us like this, and you get the same general picture. Yes, the details are different.
In one version you get hot donuts, in the other you get second moons, in the other you just get normal moons, but the general picture is the same, which is that when you look at the moon you’re looking at pieces of crustal material. Some of it is ours, that was on the proto-Earth already and got blasted into space, and some of it is Theia’s, and that what we have here on the surface of the Earth when we walk around and dig into our backyard, we dig into the crust of the Earth, we are digging into a mixture of the proto-Earth’s crust and Theia’s crust, and that when we dig into the moon we’re doing the exact same thing, we’re digging into some of proto-Earth’s crust and some of Theia’s crust. The general picture is that no matter what, this impact involved the release of, let me check my notes here, yeah, yeah, a lot of energy, more than enough to moltenize, so there’s that word again, join Patreon to support the creation of new science-sounding words like moltenize, that’s patreon.com slash p-m-sutter, p-m-s-u-t-t-e-r, that is how you can keep supporting the show and I truly do appreciate it. This is more than enough energy to moltenize the moon, which explains its creepiness, not creepy weirdo creepiness, but creepiness as in potassium, phosphorus, and rare earth elements. This also explains why the moon doesn’t have a big core, because it was just made out of crusty surface material.
It explains why we share common isotopes, because both the Earth’s crust and the moon are made out of the same mixture of proto-Earth and Theia. It also explains the angular momentum, because we took a big punch to the face and we went spinning. And it explains why the moon is so big, it’s basically the leftovers of a proto-planet.
You know, as wild ideas go, it seems to be a pretty good fit. But like all hypotheses, it’s not perfect. If you have enough energy to moltenize the moon, you also have enough energy to moltenize the Earth.
And there’s no evidence that Earth was ever covered in a globe-spanning magma ocean. There’s evidence for some magma oceans on the globe-spanning surface, but not a globe-spanning one. So that needs explaining.
How do you get just the right amount of energy to completely moltenize the moon, make it liquid, but preserve big chunks of the Earth, and not make that liquid? That’s tough. Also, the moon does have some volatile elements.
In astronomy, this means elements that are easy to lose, like water. It has a lot of water trapped in its rocks, and a giant impact, energy event should have gotten rid of all the water. It should have vaporized.
The water should be gone instead of being trapped in the rocks. And there are some issues, like the details of the abundances of the moon’s various elements. Some of them match the Earth.
Most of them fit with the giant impact hypothesis, but some don’t. You know, some of it may come down to the details of what the Earth was made of, how it impacted the Earth. If some material got blasted away, not to join the Earth-Moon evolution dance, but instead just was left alone.
And if you change, say, the density or the properties of the Earth, or you change the, you know, the impact angle, you can maybe address some of these issues. But some issues still stand out. I mean, nothing’s perfect.
That’s okay. As usual in science and in life, we can only move on. The giant impact hypothesis does have some shortcomings.
It isn’t able to explain all of the available evidence. If you haven’t been paying attention by now, I’ll say it again. There is, like, no scientific hypothesis that is able to meet all of the evidence.
Every single hypothesis that we generate in science, every single one has shortcomings, known shortcomings and unknown shortcomings. We know quantum mechanics comes up short. We know general relativity comes up short.
We know Big Bang cosmology, Lambda CDM comes up short. We know the dark matter hypothesis comes up short. We know evolutionary theory isn’t able to explain all of our observations.
Germ theory isn’t able to explain everything. There are always mysteries. But the mysteries surrounding a successful theory don’t add up to enough to dethrone the theory.
So, yes, there are shortcomings with the giant impact hypothesis. It does not explain all of the available evidence. It explains the vast majority of the available evidence.
And it doesn’t dethrone the giant impact hypothesis. The shortcomings, like, how do you not moltenize the entire Earth? How do you get exactly the right elements?
How do you still have some volatile elements? How do you have all that? It doesn’t dethrone giant impact for two reasons.
One, these are tiny little details that might be explained with a more refined understanding. Like, if we keep pushing with our simulations and our understanding and our possibilities, we may be able to resolve this. It’s, like, not out of the question that we can use the giant impact hypothesis to explain all of these data points.
And two, no one has come up with a compelling alternative. You want to say, oh, we just captured the moon. Well, that opens up even more worms than the giant impact hypothesis does, because then you have to explain why the moon is so weird if it just formed on its own, because it doesn’t look like it formed on its own.
No one has a compelling alternative. And we’re able to use this theory to meet the vast majority of observations. And it seems like we have a pretty good shot of explaining all of the observations if you just give us a little more time and research funding.
That’s why it maintains its status as the leading theory of the moon’s formation. We can only move on. It’s the imperfections that push us forward, which is great perspective, not just for science, but for life if you needed a little dose of astrotherapy there.
But before I go, I want to address something else. All this talk about Theia and giant impacts and moltenization opens up another question. We started this episode with the moon being special, which it is.
It’s weird. Then we came up with a reason to explain its remarkable properties. Oh, it’s the product of a giant impact between the proto-Earth and Theia.
Isn’t that cool? Well, this leads to a question. Why didn’t the same reason giant impact happen anywhere else?
The Earth has a moon when the other rocky planets do not have substantial moons. And we said, well, we got whacked. We got hit.
Theia hit us. We got a moon. Well, why didn’t other Theias hit proto-Venus and proto-Mercury and proto-Mars to give them moons?
We haven’t really addressed the elephant in the room here, which is why is the moon so special? Why did it happen to us and not anybody else? Well, maybe it did.
Venus has this crazy retrograde motion. It orbits in the opposite direction as it spins. Maybe there was a giant impact there that turned it around.
Uranus all the way out in the outer solar system. Man, that planet’s sideways. Maybe it got hit, knocked over.
Maybe giant impacts happen to all the planets in this proto-planet phase when a solar system is only 100 million years old. And maybe it produces different things. Maybe it produces a giant crater over here.
Maybe it forces you into a retrograde spin over there. Maybe it gives you a big moon. I don’t know.
There, in many ways, the moon is always going to remain weird, and in some ways, unexplainably weird, and creepy, and at least in our hearts, moltenized. Thank you to Jack K., Walt, Susan T., and Stephen L. for the questions that led to today’s episode.
Keep those questions coming. That’s askaspaceman at gmail.com or go to the website askaspaceman.com. You’ll find a box there to send questions to me.
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They really do keep this show going. I can’t thank you enough. That’s patreon.com slash p-m-s-u-t-t-a-r. I’d like to thank my top contributors this month. They are Justin G., Chris L., Alberto M., Duncan M., Corey D., Michael P., Nyla, Sam R., John S., Joshua Scott M., Rob H., Scott M., Louis M., John W., Alexis, Gilbert M., Rob W., Jessica M., Jules R., Jim L., David S., Scott R., Heather, Mike S., Pete H., Steve S., Watt Wattberg, Lisa R., Koozie, Kevin B., Michael B., Eileen G., Toho Warrior, Stephen W., and Brian O. Thank you so much for all of your contributions.
They really do mean the world. I can’t thank you enough, and I’ll never stop. And I’ll see you next time for more Complete Knowledge of Time and Space.
You are listening to the 365 Days of Astronomy podcast.
[Speaker 2]
The 365 Days of Astronomy podcast is produced by the Planetary Science Institute. Audio post production is by me, Richard Drumm. Project management is by Aviva Yamani, and hosting is donated by LibSyn.com.
This content is released under a Creative Commons attribution, non-commercial 4.0 international license. Please share what you love, but don’t sell what’s free. This show is made possible thanks to the generous donations of people like you.
Please consider supporting our show on Patreon.com forward slash CosmoQuestX and get access to bonus content. Without your passion and contribution, we won’t be able to share the stories and inspire the worlds. We invite you to join our community of storytellers and share your voice with the listeners worldwide.
As we wrap up today’s episode, we’re looking forward to unraveling more stories from the universe. With every new discovery from ground-based and space-based observatories, and each milestone in space exploration, we come closer to understanding the cosmos and our place within it. Until next time, let the stars guide your curiosity.
End of podcast:
365 Days of Astronomy
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The 365 Days of Astronomy Podcast is produced by Planetary Science Institute. Audio post production by me, Richard Drumm, project management by Avivah Yamani, and hosting donated by libsyn.com. This content is released under a creative commons Attribution-NonCommercial 4.0 International license. Please share what you love but don’t sell what’s free.
This show is made possible thanks to the generous donations of people like you! Please consider supporting our show on Patreon.com/CosmoQuestX and get access to bonus content. Without your passion and contribution, we won’t be able to share the stories and inspire the worlds. We invite you to join our community of storytellers and share your voice with listeners worldwide.
As we wrap up today’s episode, we are looking forward to unravel more stories from the Universe. With every new discovery from ground-based and space-based observatories, and each milestone in space exploration, we come closer to understanding the cosmos and our place within it.
Until next time let the stars guide your curiosity!