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Scientists In The Dark Over Birth Of The Moon


This is SCIENCE FRIDAY, I'm Flora Lichtman, filling in for Ira Flatow today. The moon, it's our nearest neighbor, but we don't know much about where our companion came from. In the 1800s, Charles Darwin's son, Sir George Darwin, proposed that maybe the moon just popped off from the Earth when the Earth was spinning much faster than it is today.

Others thought the moon might be a sort of primitive asteroid, which the Earth captured from space. But the most popular theory today dates back to the mid-1970s, when scientists suggested the moon was born in a huge smash-up between the early Earth and another planet they called Theia. Greek mythology buffs probably caught that: Theia is the mother of the moon goddess Selene, get it?

This week in the journal Science, two different studies tweak that smash-up idea in totally opposite ways. They can't both be right, so are we moving towards any sort of consensus on this, or might the moon mystery be lost to the cosmos? Give us a call, our number is 1-800-989-8255, that's 1-800-989-TALK. And if you're on scifri, you can tweet us your question by the @ sign, followed by scifri.

Let me introduce my guest. Erik Asphaug is a professor of Earth and planetary sciences at the University of California, Santa Cruz. He joins me by phone today. Welcome back to the program, Dr. Asphaug.

ERIK ASPHAUG: Oh thanks a lot, Flora.

LICHTMAN: So let's start with conventional wisdom. Take me through what people generally - how people generally think the moon was born.

ASPHAUG: Well, in scientific circles, the general idea that everyone's settled on - I'm at a science meeting right now - and if you ask anybody how did the moon form, they'd say it formed in a giant impact. And then when you press them a little further on details, it gets fuzzy, but basically the notion of planet formation has evolved to this idea that we had maybe 30 or 40 or 100 smaller planets, maybe the size of Mars, clobbering into each other as it orbited the sun very early on, in the first 20, 30, 40 million years of solar system history. And through this game of sticking to each other kind of like raindrops, you know, form in a cloud by tiny droplets accreting, the planets grew.

And so the last of this accretion events formed the moon, because accretion, when you have giant planets the size of Mars and Earth hitting each other, it's a pretty messy phenomenon.

LICHTMAN: But based on news this week, it sounds like that may not be the best theory.

ASPHAUG: Well, you know, the devil gets down to the details, and when you push these models forward, they start to come apart, and that's kind of the fun thing about science is you're sort of grasping at things, and they - sometimes you get them and hold them in your hand for a while, and they slip away like a fish.

Here we had this notion - and the reason this is so exciting right now is that starting in, you know, the mid-1970s, these sort of back-of-the-envelope calculations said hey, you know, we think planets formed this way, by collisions. We think we can explain the spin of the Earth, which is actually pretty fast if you add in the spin of the moon to the spin of the Earth, you know, that moon's orbiting the Earth, and if you think of it as one single planet, and if you brought all that mass together into one place, it would be equivalent to the Earth spinning with a period of only five hours, so much faster than it is today.

So that was kind of a puzzle. Why do we have this sort of high density of angular momentum in the Earth-moon system, and the other planets are spinning slower? And so it came up with this idea that hey, Mars hit the Earth, gave it its spin, not Mars itself but a planet like Mars...

LICHTMAN: Is that the Theia, or is that something different?

ASPHAUG: That's Theia. That's the Mars-sized planet that people have been invoking all these years, for about the last 40 years now. And that led to detailed models and dozens and dozens of papers exploring this idea of fleshing out various scenarios. And in the last 10 years, we really thought we were approaching the endgame, where we could make the Earth, we could give it its spin, we could, you know, make a moon...

LICHTMAN: And you had it all wrapped up?

ASPHAUG: We had it all wrapped up, and not only could we make a moon, but we could make a moon that has pretty odd composition. I mean, the moon has a composition that's really odd. It's like - it is like George Darwin said, it's like you pulled a piece of the mantle out of the Earth and popped it into the sky, but...

LICHTMAN: So it's made of mostly Earth stuff?

ASPHAUG: It's made of stuff that looks a lot like Earth stuff. The - what's really curious in these last couple of years - you can blame a lot of this problem on the busy work of geochemists - who found that the moon's not only like Earth rock, but it probably is Earth rock.

And this is really the detail that's got everybody in a bind now because the models, when they made the moon, and you see these simulations, and there's the moon orbiting the Earth, and it doesn't have an iron core, it doesn't have a lot of water, so it explains its pretty strange composition, the problem is it's an alien planet, it's a piece of Theia, it's not a piece of Earth.

LICHTMAN: Yet it has all of this - Earthy - Earth's components.

ASPHAUG: Yeah, when you look at the composition of the moon rocks in much greater precision, the isotopic composition of oxygen, silicon, titanium, you know, six or seven different isotopes now, match up to Earth. And these are like tracer dyes that sort of color the rock and tell you that it's kindred rock.

And so the moon really is a piece of the Earth, we think, and so models - or at least the standard model of moon formation that everybody was kind of heading towards in the last 10 years - that no longer seems to be on the table.

LICHTMAN: So, walk us through these two papers that came out this week and how they change our perception of the standard model.

ASPHAUG: Well, the standard model was based on the assumption that the spin of the Earth that you've got now, of the combined Earth system with the moon, is the spin that you had to begin with. It was based on the premise that you can't really lose spin. You could transfer spin from the Earth to the moon or from the moon to the Earth, and that's what's happening right now.

The Earth is slowing down ever so gradually while the moon is orbiting into higher and higher orbits every - you know, several centimeters per year. And so the idea that you had to start off with an end state after this collision that had the same spin that you had today,, gave us a powerful constraint on the problem.

You couldn't just crash anything into anything. You had to pick things that had the same angular momentum that we have today. This paper by Cuk and Stewart is - that came out in Science today is fascinating because they show that you could actually start with a very fast-spinning Earth, twice as fast as we thought possible just a few years ago.

LICHTMAN: I thought that our spin was slowing down.

ASPHAUG: We are slowing down. We're slowing down but only because we're transferring it to the moon, and so the - if you count both of these bodies today, that spin should be remained constant throughout the course of solar system history.

And what they're suggesting is that you can lose spin from the Earth-moon system and transfer it to the sun-Earth system. And it's a very complex calculation. I mean, it's not just trivial that it took 30, 40 years to figure this out. But once that is on the table, and you say hey, I can start with an Earth that's spinning with a period of two hours, now you have an Earth, that if you were to look at this - and you can download the simulations from Science magazine - and you can see that - or from Harvard - and you can see that this Earth looks kind of like a muffin.

It's spinning so fast, it's got kind of a two-to-one axis ratio. And so their solution is hey, I can make the moon out of Earth's mantle, I'll just sort of hit it with a tiny projectile - you know, not tiny but much smaller than we thought previously - so Theia becomes smaller.

And you hit it, and off of the equator of this rapidly spinning muffin comes this stuff, and it's mostly Earth's mantle.

LICHTMAN: Little muffin that becomes the moon.

ASPHAUG: Yeah, a little moon muffin. And so you say hey, I solved the problem. I've got the moon. It's made of Earth's rock. And it's the right mass. And, you know, that's all fine and good, but, you know, removing a constraint from scientific problems is a double-edged sword because now anybody can play this game.

And so there's probably three, maybe four new ideas on the table based on, you know, different kinds of collisions that no longer have to conserve spin.

LICHTMAN: Do you expect to see the whole field of moon research just totally blown up by this constraint that's been removed?

ASPHAUG: It's kind of like the impact simulations themselves. There's this huge global mess of stuff. There's papers coming out, pretty rapidly. You can hardly keep track of progress in the field. I can just sort of state maybe three of the main ones, like what if Theia hit the Earth and didn't accrete with the Earth but bounced off and left the Earth as a big spinning mess.

What if you had two equal-sized semi-Earths accreting, and then you make the moon that way? And that's a paper recently coming out by Robin Canup. And what if you had - this started with an already-spinning muffin and then just hit it with a small thing?

And the problem is all of these can give you disks around the Earth that - from which the moon can form. And the hidden secret behind all this - I think what a lot of listeners probably don't appreciate or don't know, and a lot of scientists probably don't know either - is nobody's actually made the moon in any of these simulations.

What people have made is a disk of material orbiting the Earth from which the moon would eventually be made, we think.

LICHTMAN: Now, it's true that the moon's moving away from us, right?

ASPHAUG: Yeah, it's moving at a fairly fast clip, kind of the same rate that continents creep around on the Earth.

LICHTMAN: I mean, does that mean that someday we're going to lose our moon?

ASPHAUG: We'll never lose it because it's being basically torqued out into Earth by the spin of the Earth. The moon raises tides on the Earth, as everybody knows, and those tides kind of get slung around by the Earth a little bit ahead of the moon and its orbit, because the moon orbits the Earth once a moon, the Earth spins every day, and so this tidal bulge keeps pulling on the moon and kind of ratcheting around outwards and outwards into outward spirals.

And that drains the spin of the Earth. What's going to happen billions of years from now is Earth's going to slow down until it equals the spin rate of the moon's orbit, and then you'll have a double planet locked and frozen together until the end of time.

LICHTMAN: That's nice, but does that mean that our days are actually getting longer? That drives, sort of, my feeling.

ASPHAUG: They are, yes, they're getting longer. Yeah, the Earth's spinning down. And this actually goes back to George Darwin's original idea that the Earth could have been spinning really fast because being the son of Charles Darwin, he was familiar with the fossil record, and there were these puzzles when you looked at certain fossils that should have little layers accreted on the - you know, I think it was corals every day.

You know, go deep back into the fossil record, and you start to see 400 layers accreted per year instead of 365. And so there was this evidence that there were more days per year, which indicates that the days were shorter. And so our days are lengthening, the months are lengthening. There's nothing you can do about it.


LICHTMAN: I think that's a great place to call it. Thank you, Dr. Asphaug, for coming and joining us on the show.

ASPHAUG: Oh, you're most welcome, thank you.

LICHTMAN: Dr Erik Asphaug is a professor Earth and planetary sciences at the University of California, Santa Cruz. And up next, the science of polling. We've got the numerati here. You'd better stay tuned. Nate Silver and Sam Wang are here to demystify this confusing polling landscape. Don't go away.


LICHTMAN: This is SCIENCE FRIDAY from NPR. Transcript provided by NPR, Copyright NPR.

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