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How Cosmic Collisions Have Shaped Our Solar System


You're listening to SCIENCE FRIDAY. I'm Ira Flatow. And for the rest of the hour we're going to talk about collisions, space collisions, space impacts, with Erik Asphaug, who's Ronald Greeley chair of planetary geology, School of Earth and Space Exploration at Arizona State University. Welcome to SCIENCE FRIDAY.

ERIK ASPHAUG: Thanks very much, Ira.

FLATOW: You must be very busy since this last collision in Russia of this asteroid.

ASPHAUG: Well, there's actually a meeting next week to talk about it. It's called Planetary Defense, and to the extent that we can do anything about these things. And it's sort of like - this is sort of like a mosquito compared to the big wasps that are out there.

FLATOW: And there are big wasps out there with our names on them?

ASPHAUG: Well, none that we know of yet. I wouldn't be surprised if somebody at some point announced a - an actual collision that would happen in the next hundred or 200 years. The problem is chaos, and things are never quite understood as precisely as we hope. So predicting where is something is going to be 100 years from now, it's always a probability exercise.

FLATOW: Mm-hmm. There's a possible collision on Mars, I understand. Is that correct?

ASPHAUG: That's right. It's about a one in 600 chance that a comet is going to hit Mars, and, you know, we'll - for now, it's just the waiting game.

FLATOW: And there obviously have been lots of collisions on Mars before, right?

ASPHAUG: Yeah. You look on any map of Mars, you see it's all full of craters. These were all a comet or a meteor or asteroid that hit the planet.

FLATOW: Mm-hmm. How do we know much about the really big impacts that have taken place on our solar system?

ASPHAUG: Well, one - you have two ways. Look at the witness plate, the craters. Some of these are gargantuan, like - well, the K-T impact as we call it, the one that extinguished the dinosaurs, that left a crater about 180 kilometers, about 100 miles across in Southern Mexico, present-day Southern Mexico. And you look at the moon and it's full of craters, and it really took a revolution of our thinking to realize that the moon is actually bombarded; it was Hooke.

Back in the late 1500s he postulated the moon might be hit by impactors, but then he said that's such a ridiculous idea because where would these things come from? [POST-BROADCAST CORRECTION: Astronomer Robert Hooke did his work on lunar craters in the late 1600s, not 1500s.]


FLATOW: So people really didn't know where these things came from.

ASPHAUG: Until maybe the middle of the century, I don't think people were willing to accept that the solar system is as messy as we now know it is.

FLATOW: Yeah. Well, our solar system started as a - with impacts, right? I mean that's natural for our solar system.

ASPHAUG: Yeah. That's the standard model now, is that it grew out of nuggets of gravel and dust and these grew into bigger boulders and agglomerations. And all these collisions started out very gentle, you know, kind of like you're making a snowball. You're patting it together, and then they became more and more violent and energetic as time went on.

FLATOW: In fact, our moon was formed from a giant collision. Isn't that what the thinking is then?

ASPHAUG: That's what we think. The...

FLATOW: How did that happen?

ASPHAUG: Well, the details have unraveled. I think - if you'd asked me 10 years ago, I would have been more confident in telling you what happened. And now I don't know what happened, other than I would bet my career on the fact that there was some kind of a giant impact that formed the moon. The details indicate that it was a big mess. The standard model is that a planet the size of Mars collided with the proto-Earth when the Earth was almost formed, and this merger - it's really - you can think of it as a collision, but the gravitational attraction is so great that even though they're colliding at about 15 kilometers per second, they're still bound together. And they spin around and they fling off bits and that formed the moon.

FLATOW: But you said that now you're not so sure.

ASPHAUG: Yeah, yeah.

FLATOW: What has happened in those years...


FLATOW: ...that throws that, maybe, up in the air.

ASPHAUG: Well, you know, the devil is in the details. And if people would just stop doing geochemistry, we'd probably be done...

FLATOW: Oh darn.



FLATOW: New information.

ASPHAUG: New information. And so the geochemists, they've been looking at these Apollo samples. We have pieces of them in that the Apollo astronauts brought back in one of the - a couple of Russian probes - autonomous robots brought back, and we have some meteorites from the moon. And these rocks - if you look at the chemistry, they are actually identical to the Earth in what we call their isotopic composition, the ratio of different atoms, different species of atoms, so the ratio of oxygen isotopes - oxygen comes in three flavors. And all these different isotopic systems are the same for the moon as for the Earth, which tells you that the moon came out of the same reservoir as the Earth.

OK. That's fine. But when you look at the model of an alien planet, a Mars-like planet hitting the Earth and spinning off bits, the moon should, according to these models, be made out of the alien planet.

FLATOW: Green cheese.

ASPHAUG: Yes. It should be green cheese. And instead, it's Camembert, it's whatever, you know, the Earth is made of.

FLATOW: So it doesn't fit the model.

ASPHAUG: It doesn't fit the model. And so now people are spinning off new models like...

FLATOW: So what ideas are they thinking that might have happened?

ASPHAUG: Oh, there's four of them. One is that Earth was spinning so fast that it only took a tiny little pinprick, kind of, of an impact to fling off pieces that would form the moon, and those would come mostly from the Earth. And that actually goes back to Darwin's idea, the son of Charles Darwin, George Darwin. He postulated that the Earth was spinning with a day about two hours long. And if you spin the Earth that fast, it's just going to shed mass off its equator. And that goes back to the middle of the 1800s.

One is called a hit-and-run collision where the Mars-sized planet, instead of merging with the Earth, it comes by a bit faster and it bounces off the Earth. And so the moon gets made out of the mess that it left behind, but little of it gets incorporated into the Earth. And the question that I have, personally, is, you know, are we ever going to know? Is it one of those solvable problems or is it one of those that we have to just wonder about?

FLATOW: Any other ideas? You said there were four of them. I think I heard three.

ASPHAUG: Oh. The other one is that the moon got coated in Earth stuff after the moon formed.

FLATOW: Earth stuff.

ASPHAUG: Earth stuff.

FLATOW: That's a Carl Sagan-ish sort of thing. Earth stuff.

ASPHAUG: Billions and billions of pieces of Earth stuff.


ASPHAUG: And the moon forms, and then you have this inner disk that's interacting intimately with the Earth, and it becomes Earth-flavored. And then these migrate outwards and plaster themselves onto the moon. And so that idea would say that the moon has maybe - the outer hundred kilometers of the moon is really Earth and the inner is the alien planet.



FLATOW: If you want to ask a question, this microphone is open in the audience. You can talk about collision. What I find always very extraordinary to talk about is that - let's talk about, for example, Mars - that you can find pieces of Mars lying around on Earth, that have been traveling around for thousands of years, they got ejected off the planet, right?

ASPHAUG: Oh, yeah.

FLATOW: How does that happen?

ASPHAUG: Oh, it's amazing. I mean, well, I was thinking when I was listening to the discussion about the desert and the microbes on the desert. It doesn't take a lot of imagination when you look at the moon and you see Tycho, which is - if you look with binoculars at the moon, you'll see this one crater on kind of the bottom left part of the moon as you're looking through your binocs. And it has this bright ray network, and these go all the way around the moon. And many of these would've left the moon. And so it doesn't take a lot of imagination to realize that ejecta leaves planets behind.

FLATOW: Right.

ASPHAUG: And so one of the most creative and interesting ideas, and I think one of the most profound ideas in modern planetary science, is the notion that planets could swap microbes, not just rocks, back and forth, and especially these hard, hardened little desert buggers. If they did evolve on Mars, you could imagine a collision into Mars transporting them to Earth. And so one of the ideas that's out there that I find fascinating, is that we are all Martians, because life would've probably...

FLATOW: Don't look at me when you...


ASPHAUG: You know, Mars would've been habitable probably before the Earth was.

FLATOW: So if there is a collision going to be with Mars - you say there's a finite probability to this - can you watch for stuff being ejected in real time? Can you see stuff that might be ejected off of Mars in a way?

ASPHAUG: Maybe from the Earth. I mean, Mars is far. We have orbiters. The Europeans have their Mars orbiters. We have a Mars orbiter. We have a couple of Mars rovers - three Mars rovers on the surface. And so these will all be looking for signs of what occurs. What's more likely to happen, I mean, comets are messy things of themselves. They come from this primordial phase of solar system history where things haven't quite accreted yet. So they're tiny bodies. They would've liked to have grown up into Saturn or something, but they didn't. They're relics, and they're leaking material. As they come into the sun, they become active and they become like a firework. And if any of you have seen a big comet, it has a big tail. And so it's more likely the nucleus of the comet itself hitting Mars, is that Mars gets hit by a firestorm of little meteors.

FLATOW: Is that how you get the rings in Saturn? Where did those - all the debris in those rings come from?

ASPHAUG: Well, nobody knows, other than that the rings of Saturn are almost entirely composed of water ice, which is kind of odd because Saturn has its own flavor of composition. Titan, the biggest moon of Saturn, is about half rock. And so how do you come up with stuff that's mostly made out of ice? And so we theorists like to dream up scenarios. And one of the scenarios you can think of is, well, you look at the moon. The moon's mostly made out of silicate rock. It has almost no core, and that's why we think it formed in a giant impact. It got slung out of the leftover bits of this collision. The iron all goes into the core of the Earth.

So imagine a collision around Saturn where all the rock goes into the core of Titan. So one idea for the forming of all these icy bodies around Saturn is a similar sort of a giant impact that slings off bits of the mantle, which is the differentiated icy stuff. These are all theories, and I think, you know, you really need samples to compare them all.

FLATOW: You do the easy work.

ASPHAUG: Oh, yeah.

FLATOW: You come up with the theory, and then somebody has to actually go out and get a piece of it and bring it back.

ASPHAUG: I have a whiteboard and a window. That's my...

FLATOW: That's it.


FLATOW: That's an inexpensive office. Yes.

UNIDENTIFIED CHILD: When will the next asteroid hit Earth?

ASPHAUG: Well, you know, tiny asteroids are hitting Earth every night, asteroids the size of your fingernail. I don't know if you'd call those asteroids or not. But the next big asteroid, we don't know. The, you know, there's about five or six of them out there that have some small probability of hitting Earth sometime in the future. One of them you might have heard of. It's called Apophis, and I think it was - terrible choice to give it this name after the Egyptian god of gloom and darkness.


ASPHAUG: But it's - it has sort of, you know, one in several thousand chance of hitting us. The problem is, it's kind of like throwing darts. You never really know if you're going to get the bull's eye or not. And so we can't say for sure when that's going to happen. Statistically, you don't have anything to worry about. But the chances are a huge asteroid could hit us during our taping session right now.


FLATOW: Yeah, you just want the ratings to go up.


FLATOW: Thank you very much. Well, let's talk about Mercury a little bit, because we - there is actually a probe that's going around Mercury.


FLATOW: And we're learning different things about it that we never thought, right? I mean, new ideas about how Mercury formed?

ASPHAUG: Yeah. I mean, Mercury, the first - the Mariner mission that flew by Mercury back in the, I guess it was late '60s, early '70s, they imaged Mercury and it looks just like the moon.


ASPHAUG: And the idea was like...

FLATOW: Ooh, dead rock out there.

ASPHAUG: Yeah, just dead rock, cratered - heavily-cratered. And now, going back with modern instruments where you have lots of different detectors that can look for composition, they find that it's fairly volatile-rich. I mean, it has a lot of things like sodium and things that would evaporate easily: sodium is one of them, sulfur is another. And so the idea that - and the moon has none of these things. And again, getting to the idea of how do we know there was a giant impact, well, the moon's quite dry. And the moon seems to have, you know, you could explain it as having lost a lot of its volatiles in this collision. Mercury people would like to explain its anomaly by a collision. The anomaly of Mercury is - you look at Venus, you look at Mars, you look at the Earth, they're all about 30 percent iron by mass in their cores. Mercury is about 60 percent iron by mass. And so it's lost all its rock.

FLATOW: Doesn't belong there. So.

ASPHAUG: It doesn't belong.

FLATOW: So it came some other way? Some other place...

ASPHAUG: You know, and people try to put all these things together. If any of you listeners are interested in pursuing this field, you know, you start to come at it from many approaches: chemistry, math, physics, and you can say well, I got an idea. Mercury accreted differently than the rest of the planets. It's closer to the sun. Rock was somewhat harder to grab a hold of closer to the sun.

FLATOW: Let me just remind everybody that this is SCIENCE FRIDAY from NPR. And so it's a different animal than we thought it was.

ASPHAUG: Different beast.

FLATOW: Different beast. I mean to go to the audience just after you said beast, but the next person at the mic.


UNIDENTIFIED MAN: Thank you for that compliment.


UNIDENTIFIED MAN: With the near miss and the impact in Siberia, there's been in a lot of discussion in let's say public awareness of tracking asteroids and possible collisions. And I know there are some very large asteroids that are tracked from optical telescopes and some discussion about the fact that the one that impacted - or nearly impacted - in Siberia couldn't have been tracked because it came from the sun side. And then some discussion about a possible tracking mechanism through a space telescope looking for, I believe, IR images. Can you talk a little bit about some of that possible tracking technology and the likelihood, and perhaps estimation of what a system like that might cost to put up in space, to give us some element of pre-warning, perhaps?

ASPHAUG: That's a great question. I mean, I can answer the - cost is kind of well-understood. It's about a half a billion dollars for a dedicated survey mission in space, of the kind that they envision for putting it closer to the sun than the Earth is, to try to see the ones that are one the sunward side. In fact, the B612 foundation, it's a private - it's a non-profit organization. They're trying to do this through private funds because federal government procurement processes are very slow, and this is something that's on people's mind. And there's a lot of money out there, and people - and a half a billion dollars sounds like a lot, but it's actually small compared to a lot of government endeavors.

The main thing is you're trying to retire risk. Right now, if we don't know what's out there, I sort of jokingly said something could hit us during our taping session, and it's true. And in about 20 years, that'll probably no longer be true. We'll have closed the gap on all the big ones and - to maybe the 99 percent level. There's always going to be something out there. Asteroids and comets are dark, they're small, they're hard to see. You mentioned IR - infrared. I mean, that's something where they start to stand out when you look at them in different wavelengths.

FLATOW: All right. There's the message for today, dark. Hard to see they're out there. Be afraid. Be very afraid.


FLATOW: Thank you very much, Dr. Asphaug.

ASPHAUG: Thank you very much, Ira.

FLATOW: Erik Asphaug is the Ronald Greeley chair of Planet Geology at the School of Earth and Space at ASU, just up through out here at Tempe, Arizona. Transcript provided by NPR, Copyright NPR.

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