Why Is the Moon So Big?

Hey, please take a second and leave us a review on Apple Podcasts, Spotify, or wherever you listen to the podcast. Thanks a lot. Hey, Welcome to Science Stuff, the production of iHeartRadio. I'm poor, Hey Champ, and today we are going to the Moon. You might think you know the moon, but actually it is full of scientific mysteries. We're going to be talking about where the moon came from, what it's made out of, and what it can tell us

Okay, welcome.

The last time we talked it was New Year's and we talked about how the Earth goes around the Sun. And now we're getting close to the lunar New Year, so I thought it would be interesting to talk about the moon.

Okay, that's actually my specialties. That's great to talk about something that I know.

Are you saying you're a lunatic?

Yes, yeah, we're proud to call ourselves lunatics.

Well, in what ways is our moon unusual in the Solar System?

Okay? Well, the weirdest thing about our moon is how big it is compared to Earth. There are a couple of moons out there, some of Saturn's moons and a couple of Jupiter's moons that are about the size of our moon or even a little bigger. But those planets are huge, right, you could fit I think it's thirteen hundred earths inside of Jupiter, right, Wow, huh. And so their biggest moon is a very small moon compared to the size of its body, meaning.

Like if Jupiter had a moon that's proportionately the same size as our moon, it would be like the size of Saturn or something.

Yeah, it would be like the size of Neptune at least. So yeah, And the only other planet that, if you want to call it a planet that has a similar is Pluto.

Right.

Oh, no, no, we don't talk about Pluto in this podcast.

You don't talk about about plu.

We don't know.

Pluto has a moon that's about a tenth of its size or something like that.

But plut itself is very small, smaller than us, right, smaller than our moon kind of.

Yeah, Pluto is around it could fit inside the continental US. So it's a pretty small body.

Okay, Oh, that's a very polite way to say it. It's a small body. Yes, yes, small body.

So that's the weirdest thing about our moon is how big it is compared to us.

So that's the first mystery about the moon. Why is it so big? None of the other planets have a moon this big.

Why is that?

Well, as it turns out, Charles Darwin's Sun George Darwin had a theory about that.

So that's actually a funny part of the history is our moon is relatively large. It's about the size of the Pacific Ocean basin, and so Charles Darwin's son, I believe George Darwin first theorized that the Moon might have been created from the Earth by being spit out of the Pacific Ocean basin, and the Pacific Ocean was the

Yeah, maybe that's came It was like a giant pimple that just popped off the Earth. That was the theory, yes, yes, or the Darwin giant pimple theory. Yeah, George Darwin didn't quite get thinks is right as this famous dad Charles Darwin, or did he?

Now we kind of think it's crazy, but really how we think the moon formed now was not all that different. It's just that that spitting out happened in a big event when something the size of Mars or so collided into the early Earth and caused that event where material got spit out and formed the moon.

That's a wild theory, Like we have a moon that's like gray white, but we think it happened because some giant mini planet crashed into the original Earth.

Yeah, so that's the idea is the original early Solar system was full of things that were flying all around, and some of them collided into each other, and the debris of that collision formed something kind of like a ring of Saturn around the Earth. Right, it was a ring, and then that coalesced together and became the Moon.

So like our Earth is a mix of those two original planets, and the Moon is a mix of those two original planets.

Exactly.

Yes, scientists think the way the moon form was that a planet the size of Mars crashed into a pre Earth planet about four and a half billion years ago, and then the debris from that crash clumped together and formed the Moon. But here's a big question, how do we know that? What makes us think that's how the Moon formed. Well, it turns out this idea started when we first walked on the Moon.

And this explains basically what we found when the Apollo samples were returned in the nineteen sixties and seventies. Huh, was that a lot of the isotopes of atoms were incredibly similar between the Earth and the Moon.

What do you mean isotopes?

So normally an atom will have the same number of protons and neutrons, and so an isotope is one that has different numbers of neutrons.

Okay than the normal one I see. And so it's where for the isotopes on Earth to match the ones on the Moon.

Yeah, So that turns out to be one of the key identifiers of where something formed in the early Solar System is it's the isotopes of its different atoms, and particularly good one is oxygen.

It turns out you can tell exactly where in the Solar System a piece of rock comes from by its isotopes. Like a piece of rock has more of one kind of oxygen or carbon, you can tell it came from close to the Sun, or near Jupiter, or way out at the edges of the Solar system.

So we think the whole Solar System essentially was a big cloud of gas and dust that was leftover from some other star that exploded, right, huh, And that cloud kind of collust around the early Sun, and then when that Sun turned on, you know, very early in the Solar system four point six billion years ago, it heated that cloud and basically boiled off different isotope ratios as you went different distance from the Sun. And then when the planets formed, out of those different isotopic ratios as

Interesting, it's like the rocks have a tag that says where in the Solar System formed.

Yeah, so when we see a meteorite that we have identified has come from Mars or a meteorite from the Asteroid belt, these will have different oxygen isotope ratios than the Earth. But the Moon has almost the same as the Earth, like too incredible precision, such that the only way that we can explain it was that they essentially

I see the match is too good. It's just too identical to the Earth.

Yeah, Like these are identical twins, you know, they had to be born from the same mother.

I see the DNA says their siblings, same mother, same father.

Okay, And so there were lots of theory going around, and as far as I understand it, there was some international meeting about this in the early nineteen eighties and there was a lot of debate, and then out came the giant impact as the winner, and it's kind of been the standard since then.

Interesting. So it's not like we have some smoking gun that says that this is what happened, or that there was a camera or somebody saw this happened. It's like, well, we can't think of anything else, and this is the only thing that we have left.

Yeah, and everything that we find out knew about the Moon seems to fit with the theory of the giant impact. Like what like one of the things we found out was from seismometers on the Moon. We've been able to figure out that the Moon has a fairly small iron core, right, and the Earth actually has a really big iron core for its size. And so where did this extra iron on the Earth come from and the lack of iron

Oh.

I see, what do you think the junior Darwin would feel now that you've disproven his theory and not lived up to the greatness of the Darwin name.

I don't know. I think scientists by nature like to know the truth or the facts about the matter, and if an idea that we clung to turns out to be wrong, we should kind of give it up.

Yeah.

So I think that was a strong philosophy of science then and it still is today.

Nice all right, when we come back, we're going to tackle the second big mystery about the moon, which is why is the Moon always facing us the same way? In case you haven't noticed, every time you look at the Moon, no matter where you are on the planet or what time of the month or year, it is, the features on the Moon always look the same, even though it's spinning around the Earth. How can it be that it's training at just the right rate for it to always look the same to us?

We'll answer that.

Question and more after the break.

We'll be right back. Hey, we'll come back.

We're answering questions about the moon, and so far we've talked about where the moon came from four and a half billion years ago and how scientists figured out what happened. Now we're going to answer another big question about the moon, which is why is the Moon always facing us the

Okay, So the moon is what we call tidally locked which essentially means one side is always facing us.

Right. Is that another characteristic of science.

If you don't know, we give it a fancy name and basically says this is all we know about it. Yeah. So a lot of moons in the Solar System are also tidally.

Locked, not just our moon.

Like Jupiter's moons gets large. Four moons are all tidally locked. So one face of Io is always facing Jupiter, one face of Europa is always facing Jupiter.

Whow, we're not alone. Our moon is not the only one. The moon is what scientists call tidally locked, which basically means it's always facing us the same way. But there is a reason why this happens, and it has to do with how gooey the Earth and the Moon are.

This happens essentially because of how gooey an object is.

Uh huh.

So if something's spinning and it's gooey inside, it can dissipate energy.

Okay, meaning like if you were spinning a water balloon.

Yeah. Or a good analogy is if you're spinning a hard boiled egg versus a normal egg. Right, the hard boiled egg will spin and spin and spin, right, but the non heart boiled egg will slow itself down pretty quickly because that ah, yeah, that goo inside is kind of churning around and dissipating energy of that spin.

I see, if I spin a raw egg in space, it'll eventually slow down because the liquid inside is losing energy.

Yeah, And so something akin to this happens inside of a body like the Moon that initially early on at least had a large liquid inside of it, right.

Oh. It initially the Moon was gooey, oh yeah, because it was formed from the giant impact right, which was really hot and made a lot of things melt. What doctor Ziegler is saying is that when the Moon formed, it was probably spinning really fast, but the inside of it was gooey. It was melted rock or lava. It was still hot from one the huge collision that formed the Moon happened, and that melted inside made the Moon stop spinning.

It's sort of like.

If you take a regular plastic water bottle and you try to toss it up into the air so that it spins, it'll eventually stop spinning in mid air, and that's because the water inside.

Of it slashes around.

Well, the same thing happened to the Moon, and actually it's also happening to the Earth.

And the Moon is doing this today to the Earth. Actually, and so the Earth is actually slowing down. Right, Our day is twenty four hours now. It used to be twenty two hours, and earlier on in the Earth's history it was as short as four hours. So we think when the Earth and Moon formed that the Earth day was about four hours long.

Really, yeah, we were spinning that fast.

Yeah, and then the Moon has caused us to slow down that we're now at twenty four hours a day. What maybe in a billion years will be at twenty five hours a day or something like that. Right.

Oh, so the reason the day feels this long is because.

Of the moon.

Yeah, it's the moon's fault.

You had a long day, Blame the moon. Howl of the moon.

Yeah, but if your day was too short, if it went by too fast, then think the moon.

Right, Oh yeah, that's right, that's right.

So the same way that the Earth day is slowly getting longer because the Earth is slowing its spin, the Moon went through something similar. It's just because the Moon is a lot smaller, it happened a lot faster. Basically.

Okay, maybe take me back, So we had this big crash, A lot of stuff went out there into space eventually formed the moon. The moon had a squishy inside, meaning like it was lava inside.

Right basically, right, Yeah, basically it's magma if it's not erupted.

Not the whole other episode. It was magma or lava melted rock. It was melted rock, and it was spinning super fast.

Yeah.

So we had a fast spinning Earth and a fast spinning Moon, and basically them tugging on each other slowed each other down. Okay, So if I was a live bike there and on Earth, I would die because I couldn't breathe and standing on lava. But if I could somehow be there, my day would be four hours long, and I would see the moon like basically spinning in place. The Moon would look different every time I looked at it. Yeah, you would see the far side of the moon back then.

Oh okay, And over time we slowed down and the moon slowed down to the point where now it's always looking at us the same way. Yes, So is it just luck that we just happened to be alive at a time when it's spinning. It just the right time for it to be always looking at it, or is it stuck there?

I wouldn't say it's that lucky, because we think it's stuck very quickly, like in potentially a few thousand years to less than a million years, Like the moon slowed itself down very quickly. So the moon'spent tidally locked for most of Earth's history, really, and then it got stuck there. Yeah, and then it's been stuck there. And it's a little arbitrary which side ended up stuck facing the Earth. And that's still a debate of why the near side of the Moon is the near side.

Okay, you might be wondering, if the Moon went from spinning really fast to slowing down, why did it stop. It's still going around to Earth, so why didn't the Moon suddenly start spinning in place the other way? Well, according to doctor Siegler, it all has to do with the shape of the moon.

The Moon isn't a perfect sphere. It's kind of egg shaped itself or what rugby ball shaped or whatever you want to call it. Yeah, just slightly.

Yes, the Moon is not perfectly round. You might have heard that the Earth is not perfectly round either. It's a little wider around the middle or the equator. That's because the Earth is spinning, and so it bulges out, but.

The Moon isn't round.

In a different way and for a different reason, the Moon doesn't bulge out, it's actually stretched out. It looks slightly closer to a rugby ball or an American football ball with one of the pointy ends pointing at the Earth. And the reason it's that shape isn't because it's spinning, but because the Earth is pulling on it. As the Moon goes in orbit around the Earth, the Earth's gravity pulls on the Moon to keep it in that orbit,

Okay, Yeah, that would be a good description of it.

Yeah, okay, And so that's what causes the Moon to look sort of like a football shape pointing at us.

Yeah, and at some point in the Moon's history it became solid enough, it cooled off enough that it froze in a football shape at that time.

Okay, so here's what scientists think happened. There was a pre Earth planet somewhere close to our current orbit. He got smashed by another planet about the size of Mars. Some of the debris stayed in place and became the Earth, and some of the debris floated out in space and became the Moon. This new Moon was an orbit around the new Earth, but it was gooey and melted on

At the Earth.

Wait, wait, wait, wait, I think what you're saying is that just because of the particular history of when the Moon cooled is the reason we're always seeing the same side. Because if it had stay squisher longer. Maybe the moon would be rounder now, less football.

Shape, maybe, but I think it would still become tidally locked eventually.

Yeah. Okay, that's good.

That's a good thought. I have to think about that one more than live on the air.

Am I gonna get my name in a paper?

Now? The cham.

The cham procrastinating hot football theory of moon tidal walking. Yes, rolls off the tongue exactly.

Okay.

There are a couple of interesting things about this scenario. The first is that the arrangement of having something football shape pointing at you in orbit is stable, meaning that if the football, for some reason tilton starts to point in a different direction, the physics of the orbit will

Yeah, if they were like a giant impact or someone crashed a really really big rocket into the Moon, that they were enough to unlock it, you could have imagined that it would lock again in that other way.

Like I'm tired of seeing this side of the moon, Like, I'm done with it, Let's see the other side. Somebody's give it a little push and it'll lock back to the other side exactly. Yeah.

Okay.

You might be wondering, why would I want to look at the other side of the moon? Is it really that different? Well, it turns out that it is different, and scientists don't really know why.

But there are all these weird things we've learned about the moon. Now, is that the crust on the far side of the moon is a lot thicker than the crust on the near side of the moon. Whoa, and like the near side of the moon has all the lava flows right far side has none of that? What? And so why did the near side of the moon and the far side of the moon end up so different? And is that because of the tidal locking or is it a cause of why we tidally locked with this face?

Did it tidally lock because one side is different than the other? Or is one side different than the other because it's going around the Earth always facing the same way.

Yeah, And there are different ideas about what caused that difference. That the Moon might have had a giant impact of its own that created the difference originally, or that just imagine you tidally locked when you're still really close to the Earth and the Earth is still a molten ball of lava, but it really heats one side more than

In other words, we sort of know why the moon is always facing us the same way, but we don't know why it's this side of the Moon that's facing us. It could have been random, or there could be more things, but the moon and its history that we don't know about. Okay, when we come back, we're going to tackle the last mystery of the Moon, and it's a big one. It's a secret that could determine the future of space travel and it could help us understand the reason why there's

We'll be right back.

Hey, welcome back. We're not going to tackle the big open questions about the Moon. It turns out there's a lot we don't know about the Moon, including a question that might have huge implications for the future of space travel and what we know about how life started on Earth. Here's doctor Matt Ziegler, a planetary scientist involved in the New Artemis mission stake astronauts back to the Moon. Okay, Matt,

Oh? Yeah, that depends on which subdiscipline of lunar science you're in.

I think, really, what flavor of lunatic are you? So?

One thing we don't know about is what's on the interior of the Moon.

We don't know.

Yeah, we have lots of pictures of the surface of the Moon. We know that sometimes stuff came up from the surface of the moon, But we want to figure out what's going on inside of the moon and how.

Wait wait before in the episode, you told me that we know how much iron is inside the moon.

Yeah, we know kind of basic things that like, the moon has these different layers and they're about this big. And we know that because in the nineteen seventies we had seismometers around the moon that we're recording moonquakes.

Uh huh.

And so from listening to a moonquake, you can tell kind of how thick the different layers are, what the density of the interior is, but you don't really know the composition entirely. You're kind of guessing based on density. And so one of the ways you can figure out the composition is by measuring how much heat is coming out.

Yes, it turns out we don't know exactly what the inside of the Moon is made out of. So one project doctor Siger is working on is to take the moon's temperature. There are radioactive elements inside the Moon that are giving off heat, and so by burying thermometers all over the moon, you could reconstruct what all those radioactive elements are.

And just this last year about a year ago now, we had the first geothermal heat flow probe since then land on the Moon with one of these missions that's kind of leading up to Artemis.

Basically, we put a thermometer on the Moon.

Yeah, we drilled the thermometer about a meter down below the nerve. And if we do this in enough locations around the Moon, we can start to put together this picture of how the heat producing elements are different around the whole Moon.

Okay, And so the mystery is what exactly's in there? What could be inside the moon?

Yeah? Is the material in the Moon the same or different than the material deep inside of Earth? Right? And that gets back to this idea did the Moon form from the Earth?

Right?

We know that these oxygen isotopes and sauch are telling us that the Moon and the Earth seemed to form together, but the composition of the deep interior is going to be another data point telling us did they form from the same spot?

I see, what if you find that the inside of the Moon is made out of cheese.

Well then then we better go there and mind the cheese. It's hard milking cows. And I don't know, I think it still might be cheaper to get cheese from cows on the Earth.

But I don't know, but it's moon cheese. You can charge your premium, Yes, exactly.

Okay. The last mystery.

Doctor Seeger brought up about the Moon is a pretty interesting one. It's tied to the new Artemis missions that you might have seen in the news, where we're bringing astronauts back to the Moon and what they find could revolutionize the future of space travel.

I would say. Another big mystery and one of the big motivators of the Artemis program coming up to the Moon and the whole international exploration of the Moon is is their ice at the poles of the Moon.

Oh, we don't know.

No, So we have craters, and because the Moon has a very low tilt, a crater near the pole will literally be a place where the sun don't shine.

Right, I see, it's the moon hole. The moon hill is where the sun don't shine. Yes, yes, it turns out there might be places on the Moon with water on it. At the north and south poles of the Moon, there are craters so deep that they never get hit by light from the sun, and because they never feel the heat of the sun, they are super cold, which means they could be full of ice.

And so these places on the surface of the Moon, because get no direct sunlight, actually can be colder than like the surface of Pluto. Whoa, And so any water molecules that land there, you know, maybe a comet hit the Moon, or maybe an asteroid that had like water in its rock materials hit the Moon. Ah, that water should stick at the poles.

That's right, there could be huge lakes of ice inside of craters in the North and South poles of the Moon. You might have heard on a previous episode of this podcast or somewhere else that scientists think most of the water that we have here on Earth actually came from asteroids or comets that flew here from the far edges of the Solar System. In other words, most of the water that you drink that makes up your body could be important. Well, the same thing probably happened on the Moon.

And if there's water.

There, it could have a big impact onto things. The first is space traveling, and we're hoping that that ice is there, both because we want, you know, astronauts to drink martinis on the Moon and could generally stay alive and hydrated.

Yes, and hydrated, and also that if you saw the Space Shuttle ever launch, it had a big cloud coming out. What that was was mostly steam. Right, So water is rocket fuel.

You could make rocket fuel out of water. Yeah, that's right. You can turn water into rocket fuel. It's a process called electrolysis where you separate the H two O into H and oh, and once you have oxygen gas and hydrogen gas, it can burn those to push rockets further out into space.

So basically, if we can find enough ice on the Moon, the Moon also becomes a fueling station for our exploration of the universe. Right, that's wow. That's another exciting thing about it.

And the other exciting thing about finding water on the Moon is that if there is water there in deep craters, it would be sort of an untouched record how water came to our solar system neighborhood billions of years ago.

It's kind of like flypaper and it essentially should record the delivery of this water to the Moon.

It might tell us like, yes, we did get water from asteroids or comets.

Yeah, and tell us you know where the sources of our water came from. Yeah, And this is really important for understanding the big pictures of like, well, could there be life somewhere else out there? Right, because now we're finding all these other solar systems. And did the same thing that caused water to somehow come from outside the outer Solar System to Earth? Is that a common process that happens in every solar system or is it a weird thing that only happened in our Solar system?

Yeah?

That you would have a planet that's warm enough to live on because we're close but also wet.

We want to know more about why Earth has water because water is the key ingredient in life.

Yes, and the Moon holds a record of water that the Earth does not.

You're making the Moon seem like a really good tourist stop, Like it might have fuel for you to go to other parts of the universe. It might have some good history so you can read while you're there. Yeah, and it might have some cheese inside also for your snacks. Exactly.

Yeah, So in a thousand years, the Moon might be the wall drugs. You'll see signs for a thousand miles before you get to the name.

It might be the BUCkies of space.

Yes, the moon is, but we've only got one.

Well, the buggies are really hard to find too, aren't they.

Yeah?

Yeah, all right, last question, Matt. When you step out in a warm Hawaiian night you see the full moon, what are you thinking that?

Well, just last night, I think we're out and I saw the moon with my kids, and I said, you know, in a couple of weeks, there's going to be people behind that thing, right, And that thought alone is pretty neat and exciting. Yeah, that there will be people there, and maybe you know, in a few years it will be all of us, right, a lot of us going

And it is a part of the Earth, right.

Yeah, it appears to be part of the Earth and part of our formation of the Earth. And we're part of the Earth, right We're made of the material from the Earth too, So the moon is.

Our brother, all right.

So to recap scientists think our moon came from the random collision of two small planets. The moon is always facing us the same way because it's shaped a little like an American football. We're not one hundred percent sure what's inside of it. It could still be cheese. And the moon might in the future serve as a gas station for our exploration of the cosmos. So if all of that doesn't turn you into a lunatic, I don't know what will. Thanks for joining us. See you next time