Are The Moon's Mountains Due to A Cosmic Collision?
IRA FLATOW, host: One theory of how our moon formed suggests that when the Earth was very young, something hit it with an impact so big that it broke off a large chunk that remained close by and became our moon. And looking at the craters on the moon, that should tell you that the lunar surface is no stranger to impacts.
But this week comes a new idea. What if, researchers say, the Earth once had two moons, two - one big, one small - and the smaller one splatted onto the far side of the moon and stuck there?
That idea, they say, could explain the highlands on the far side of the moon, and perhaps could explain differences in the composition of the lunar surface from one place to another.
Joining me now to talk about it is Erik Asphaug, one of the authors of the idea published as a paper in Nature. He's a professor of planetary science at the University of California Santa Cruz. He joins me from his office there. Welcome back to the program.
ERIK ASPHAUG: Hey, thank you, Ira.
FLATOW: This is a new idea?
ASPHAUG: Well, it is. It's sort of a stringing together of a couple of ideas into a new one.
FLATOW: Ah-ha. We had a second moon at some time?
ASPHAUG: Well, you described the hypothesis really well, that you had one moon, the one we know and love, and then you had a smaller moon, about a third the diameter in order to make the model work, and it went unstable.
If you go back to the giant impact theory of the origin of the moon, a big planet the size of Mars careens into the Earth, crashes, makes a huge, big mess, sets the stage for the rest of our planet's history, pretty much, and from that disk of material pops a moon.
And it's - you see it in models, and you see it in lots of analyses, but you get multiple moons at the start. So that part of it's not entirely new. What's new here is kind of the physics of what happens when this last moon, the last of the small moons, plays out.
FLATOW: So the original idea that there was this collision, with a large object, that's the same? That's not changing?
ASPHAUG: That's not changing.
FLATOW: But you're saying that the models show, hey, you know, these shouldn't be just one object, one splat leftover, one piece of debris. There should be a few of them, and they also agree with that model.
ASPHAUG: Yeah. And so what's novel here is the endgame. As you form the moon pretty quickly - and people haven't spent a lot of time really thinking about the fate of the debris. There have been some papers that sort of study how long these should stick around.
But then we started approaching it from the other end of the picture. We have been paying close attention to a lot of models, trying to explain the far side of the moon.
FLATOW: What needs to be explained there?
ASPHAUG: Well, in 1959, the Soviet Luna 3 mission was the first human spacecraft to take any pictures of the backside, and it looked totally different from the near side. If you look up in the sky, this evening or in the coming days, as the moon waxes, you see the familiar man in the moon or the bunny in the moon, and these are all caused by the huge lava flows that Galileo saw with his telescope, and he called the marea: the oceans, the seas of the moon.
Now, we know that there are volcanic planes, but the moon on the near side has been totally resurfaced. About half of it's been resurfaced by these lavas. And you go back on the backside, and nothing of the sort. It's hills, mountains, and if you measure the gravity of the moon using an orbiter, you find out that the crust on the far side of the moon is about twice as thick as the crust on the near side.
And none of the theories that were out there seemed quite adequate to explain that.
FLATOW: So if you smash another object into it, then you can get that extra crust. You can shape the landscape, things like that?
ASPHAUG: Yeah, you know, modelers, we like to play God.
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ASPHAUG: We like to say, you know, here we have a computer code. Let's see what happens if X hits Y. And, you know, you have to obey certain rules of physics. So if you have a colliding object hit the moon, you have to say, well, where did it come from?
Now, if you have the moon being clobbered by a cosmic bullet, say, somewhere else in the solar system, it's going to hit the moon so fast that, just like all the other holes you have in the moon, it's going to form a crater. And it'll actually make a thinning of the crust. It'll actually make a hole. And we see lots of holes in the moon.
And what we wanted for our theory was a collision that was quite slow. Now, slow by cosmic standards is still about 5,000 miles per hour, but we needed something that was so slow that it would have come from the Earth-moon system itself. And this is where we tried to connect these two theories together, one of a slow cosmic collision that the model shows slaps onto the moon like a pancake and makes this extra crust on the backside, and where this object could have come from in the Earth-moon system.
And so this is where we went back to the old theories that were looking at the stability of these moons, and there are a couple of key spots where a second moon could hide out for about 100 million years and be orbiting along the moon that we know and love. And these are called the Lagrange points, or the Trojan points.
FLATOW: Interesting. We have a tweet that came in from Sean(ph), who says: Have you considered a similar giant impact history for Mars, which also exhibits a high highland-lowland divide?
ASPHAUG: Well, this is a very good question. We looked at Mars, and a couple of other research groups looked at Mars as a collision. And this is an idea that goes back to the mid-1980s, where Don Wilhelms and Steve Squyres looked at, you know, how much mass was lost if it was a collision.
Now, Mars' northern hemisphere is thinner than the southern hemisphere of Mars, in the very same way that the moon is. In this case, it's a, we think, a very high-velocity impact that actually makes a crater.
According to this model, the northern plains of Mars, the lowlands, are a big, huge, planet-scaled crater cavity. In the case of the moon, what's curious about that collision is a much slower event doesn't make a crater, and it just splats stuff onto it. It's very much like the physics of a mud clod hitting a wall. Anybody at home can reproduce this moon-moon collision with a couple of snowballs.
FLATOW: Eric, stay with us, okay. We're going to take a break and come back and talk more about this theory about how the moon was formed from two moons: one little one that splatted into the bigger one and created what we have today. 1-800-989-8255. Tweet us @scifri. Stay with us. We'll be right back after this break.
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FLATOW: I'm Ira Flatow. This is SCIENCE FRIDAY, from NPR.
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FLATOW: You're listening to SCIENCE FRIDAY. I'm Ira Flatow. We're talking with Erik Asphaug, one of the authors of an idea published as a paper in the journal Nature. He's a professor of planetary science at the University of California Santa Cruz, talking about the possibility, the idea that there were not - we once had two moons, a big one and a small one, and the little one smashed splat into the big one on the far side of the moon and created the kind of geology they have over there now.
Is there any way, Eric, to test that theory out?
ASPHAUG: Well, you know, the easiest way, the most expensive way is always the easiest way, and that's to have another major moon mission that goes and grabs rocks from the far side.
FLATOW: Well, we've been looking for a reason to go back to the moon, haven't we?
ASPHAUG: Yeah, you know, and I think one of the things about this story, you know, it got a lot of attention, and I wasn't expecting this level of attention, and part of it is just everybody loves the moon. You see it every afternoon. You see it this evening shortly after sunset. It'll be really pretty there up in the sky. And the science questions are quite compelling.
One of the things about the moon, if you go and grab a rock from the moon, you bring it back to Earth, chances are it's about four billion years old. This is older than almost any of the Earth rocks you're going to find.
So one of the reasons why we want to understand the moon and figure out how it got to be and how it's got to have this asymmetry and what kind of a collision environment was going on and what kind of an orbital environment was going on at that time is because that's the hidden record.
We're not going to find it here on Earth. It's all been subducted back into the mantle. We don't have any rocks from that epic. So it's kind of a museum up there that we can go to.
FLATOW: Let's go to the phones. Randy(ph) in Elkhart, Indiana. Hi, Randy.
FLATOW: Hi there.
RANDY: I recently heard that Earth has a Trojan satellite also. I wonder when that puppy is going to hit us.
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ASPHAUG: Well, the funny thing is it's actually a pretty safe object because, yeah, people have been searching for Earth's Trojans for quite a while, and so it was a big news event that we found this - you know, just to explain to the listeners, a Trojan is a satellite that orbits the sun at the same orbit that the Earth orbits the sun.
And the Earth's gravity kind of keeps it put, about 60 degrees ahead of the Earth. And so this thing's corralled by Earth's and the sun's gravity operating together. So this is actually quite exciting from a - you know, if you believe in a future that looks like "Star Trek," where you're going to be building these big spacecraft out of something, you're going to build them out of asteroids. And so that might be a place where we go and find stuff and maybe are able to build stuff.
FLATOW: Let's go to Lou(ph) in Denver. Hi, Lou.
LOU: Hello. I have a question about our good old moon, and that is: Why do we always see the same face? Is the moon pendulous, so gravity pulls that one face toward us?
ASPHAUG: You know, if you look at almost all the big satellites in the solar system, they all do the same thing. They have one face facing their planet. And in some cases, like Pluto and its moon, Pluto always has the same face facing its moon. And so they're doubly locked together, kind of like a bolero spinning through space.
FLATOW: So this impact may not have set that up, that synchronicity with the Earth?
ASPHAUG: This impact probably messed up synchronicity that already existed. This kind of locking-in happens pretty quick. It's kind of like an unstable bicycle tire, you know, it'll always end up with one side on the bottom. You've got to balance your wheels if that's not going to happen.
And so anything that's a little bit unbalanced is going to get locked. This impact probably knocked it a-kilter, and then it came back to the way it was, and so it's one of those dynamical things where the big planet likes to lock its moon into place.
FLATOW: Is there any evidence that parts of that moon that originally - you know, the big splat that hit the Earth, that pieces may have fallen back on the Earth, created craters or things we haven't discovered because they might be in oceans or something like that?
ASPHAUG: You know, that certain happened, that the debris, you know, created a big debris field, and a lot of this re-impact hit the Earth. Some of it goes into orbit around the sun, and it's going to hit the Earth a lot later.
The Earth's record, though, is so young. The average age of the Earth's surface is only a couple of hundred million years. Oceans are quite young, and the Earth is mostly ocean. We live on this incredibly active planet, and so seeing craters from that long ago, you're not going to see them on Earth even if they formed.
FLATOW: Andre(ph) in Jenison, Michigan. Is it Jenison?
FLATOW: Hi there, go ahead.
ANDRE: Why don't you think that the moon co-accreted alongside the Earth when they were both forming?
ASPHAUG: That was the original idea going back to these really, really smart guys like Laplace and Immanuel Kant. And they thought about, you know, how did this planet formation occur. Well, you have a disk that collapses. It spins out into like a pizza, you know, spins out as a flat thing.
And then planets can coalesce, and the moon would coalesce around the Earth. It's a great idea, and the problem is that the moon is so unlike the Earth compositionally. It doesn't have a core, for example, a very small amount of iron in its core.
And so why wouldn't it be made of the same stuff as the Earth if it formed along with the Earth? So that's the main argument against co-accretion.
FLATOW: Here's a tweet that came in from George Burger(ph), says: So what kind of orbit would the second moon have had to be in - to put it on the far side?
ASPHAUG: Well, the far side, you know, it could have hit anywhere on the moon, and then this moon, the tidal tugging by the Earth would lock it into place the way we see today.
But the - it does take a special kind of an orbit for this theory to work out. It's a co-orbital with the moon. So it's like this Trojan asteroid. This is a Trojan moon. And it's very stable for the first couple of hundred million years.
But I don't know if everybody knows the moon's actually receding from the Earth by about two inches per year. And early on, it was receding from the Earth much faster even than that. And so what was stable early on became unstable, and that led to the collision that we're theorizing was this - you know, it was sort of anchored in this one spot, and then eventually it got loose and got into trouble.
FLATOW: How close was the closest point? You say it was receding, our moon. How close was it at one point?
ASPHAUG: Well, if you go and look at the first thinking about how the moon formed that addressed this idea of - you know, we knew that the density of the moon was much less than the density of Earth back in the 1860s. And so we already knew that the moon had to form in some magical manner that we hadn't understood.
And so George Darwin, the son of Charles Darwin, he was a geophysicist who knew that the moon was receding from the Earth. The evidence is in fossils. You can see that the Earth's day used to be faster. The month used to be longer. You see this recorded in clamshells and corals that record the day and record the month.
And so he pieced together the fact that the moon's coming towards the Earth, you know, or is leaving the Earth. And if you go backwards in time, he calculated how long it would have been when the moon was part of the Earth.
And so he came up with what is known as the fission hypothesis, and he proposed that the moon popped out of the Pacific Ocean when the Earth was rapidly rotating. And so you can do that kind of argument, and that's what makes moon theory so fun, is you have a couple of pretty live hypotheses out there, and all of them have pretty significant implications for how it all started.
FLATOW: If the moon was so close, would it have affected the Earth's surface, maybe the water? Was there water at that time? Or would the seas have been - you know, we have tides now. Wouldn't they have been incredible at that period?
ASPHAUG: Yeah, the tides go with the square of the distance. And so as you get closer and closer in, the tides get amplified quite a bit. When you talk about these - this early epic, when the moon was really close, probably Earth had oceans, as of maybe 4.4, 4.3 billion years ago. We have maybe some evidence of the first oceans on the surface of the Earth.
If you go farther back in time, around the time the moon had just formed, and you think about the planet, well, Earth was, you know, probably this hot lava ball with a steam atmosphere and a lot of hydrogen in its atmosphere, and it was really a messy, nasty place.
So the tides that would be raised would be powerful and significant, but it wasn't really until about maybe 4.1 billion years ago when the solar system's activities had started tapering down a lot, and things had settled down, that somehow - and this is the big mystery - you know, we started the process of the first life on Earth.
FLATOW: Interesting, fascinating. Well, I think we'll just have to go back to the moon.
ASPHAUG: I agree.
FLATOW: And get some samples from the far side.
ASPHAUG: You can send me if you want.
FLATOW: Okay. I'm putting you at the top of the list. When they come and ask me, I'll put you right up there.
ASPHAUG: All right.
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FLATOW: Thank you, Eric. Fascinating, fascinating idea that we once had two moons, and one little piece broke off and hit into the backside of the moon and smushed together like clay and stuff. (Unintelligible) thanks a lot, Eric.
FLATOW: Erik Asphaug is a professor of planetary science at the University of California Santa Cruz and one of the authors of an article in the journal Nature, suggesting that the, as I say, the Earth once had two moons.
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