Just take a look at the surface of the Moon and you can see it experienced a savage beating in the past. Turns out, the whole Solar System is a cosmic shooting gallery, with stuff crashing into other stuff. It sure sounds violent, but then, we wouldn’t be here without it.
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Female Speaker: This episode of Astronomy Cast is brought to you by Swinburne Astronomy Online, the world’s longest running online astronomy degree program. Visit astronomy.swin.edu.au for more information.
Fraser: Astronomy Cast episode 333: When worlds collide. Welcome to Astronomy Cast, our weekly facts based journey through the cosmos where we help you understand not only what we know, but how we know what we know. My name is Fraser Cain. I’m the publisher of Universe Today and with me is Dr. Pamela Gay, a professor at Southern Illinois University Edwardsville and the director of CosmoQuest. Hey Pamela, how’re you doing?
Pamela: I’m doing well. How are you doing?
Fraser: I’m doing great. Cold though.
Fraser: Yeah, it was like –
Pamela: Okay, define cold.
Fraser: Minus 12 Celsius. My pipes froze. My toilets don’t work. It’s cold for the west coast of Canada.
Pamela: Okay, we’re running about the same temperature here so…
Fraser: Now before we get on to the show today, there’s a little bit of promotion going on for 365 Days of Astronomy. Do you wanna mention that?
Pamela: Yes. So, our 365 Days of Astronomy podcast. A winner of the –
Fraser: Parsec winner!
Pamela: Yes. Unlike Astronomy Cast.
Fraser: Unlike Astronomy Cast. Yeah.
Pamela: It’s a community paid for podcast that relies on donations from people like you. But we just haven’t been getting the donations we need this year. It’s like people forgot that donating made it go. So our number of listeners is going up and up and up and our donations are going down and down and down. I love my staff. I’d like to pay Aviva and Richard continually, forever, and always so that we can keep sending you science fiction stories, science stories, interviews, all of the wonderful, diverse content that we have to offer. If you can help support the show, that would be absolutely amazing.
There’s a donate button on the right hand side of 365daysofastronomy.org, which will redirect you to their CosmoQuest page. Help. Another way you can support that show – and this show, actually – is to buy our apps. We have podcast apps that are in the iTunes and Android stores. Just look up “365 Days of Astronomy” or “Astronomy Cast.” We also have CosmoQuest apps. Only for Android there. But you can support all of us by buying our apps and getting something you can learn from. Fraser has his Universe Today Phases of the Moon app and sometimes your purchase of educational material helps us educate the world.
Fraser: Awesome. Also contribute shows. We’re always looking for more people to participate. I did a bit of a rant in the Weekly Space Hangout this week which is that we have really organized this whole community to be as inclusive as we possibly can. There are so many ways that you can get involved and connect with us and connect with other space fans and create content and get that content out to a wide audience. We want to help you. So just reach out and let us know.
Pamela: And to use the NPR line – National Public Radio, here in the US – don’t just rely on somebody else to donate. Because that seems to be what’s happening – is people are like, “Ah, I donated last year. This year is somebody else’s turn.” Well, the problem is when everyone assumes this year is somebody else’s turn; no one ends up donating this year. So please consider helping if you can.
Fraser: Alright, let’s get rolling.
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Fraser: Just take a look at the surface of the Moon and you can see it experienced a savage beating in the past. Turns out the whole solar system is a cosmic shooting gallery with stuff crashing into other stuff. It sure sounds violent, but then we wouldn’t be here without it. So this is this idea that really the entire history – the evolution of the planet Earth comes from stuff smashing into each other.
Fraser: It’s very counterintuitive that really we wouldn’t be here if there wasn’t apocalypse after apocalypse for poor planet Earth.
Pamela: Well, hey, it’s what erased the dinosaurs and allowed us to evolve into being the master beings at the top of the food chain.
Fraser: Move over, dinosaurs.
Pamela: We don’t need you, T-Rex. Go away with your tiny, little arms.
Fraser: Let’s go back to the beginning and we’ll start with the Solar Nebula concept and then push that a little forward until we get to the point that appreciable objects are smashing into other objects. How did this happen?
Pamela: Well, the “how” has to do mostly with – well, you take two things and set them loose in a solar system and there’s a fairly good probability that, given the fullness of time, they will hit each other. It just happens that in our own solar system, for instance, the orbit that the Earth started out on and the orbit that another object started out on – an object roughly the size of Mars – intersected about four to four and a half billion years ago. And when those two orbits happened to intersect at the same place at the same time, we formed our Moon. We lost some of our crustal materials. We gained a bunch of heavier materials. We ended up with a uniquely composed Earth and Moon.
Fraser: But I mean these kinds of collisions have been happening probably before that. I mean, that was one gigantic collision but to make the very existence of the planet itself was caused by collision after collision after collision – objects smashing.
Pamela: Yeah, so that’s the collisional creation model. So there’s collisions that occur both elastically, inelastically. What that means is you have collisions where two things essentially stick together. So if you think about it, every time we have a small rock come along and get sucked gravitationally toward the Earth and colliding with our planet, our planet gets bigger by that amount of stuff. So that’s collisional creation. Well, in the early solar system, we were basically a giant dust bucket of a solar system and larger particles of dust gravitationally pulled in and happened to collide with other things and they stuck together via chemistry, via the electrostatic force and eventually, via gravity.
Fraser: Sorry, just to interrupt you – that electrostatic force – that’s one of the amazing parts for me – is to get that whole process started before you had gravity, you had –
Pamela: You had chemistry.
Fraser: Chemistry and electricity. Right? Negative and positively charged stuff sticking together.
Pamela: Chemistry was definitely one of the larger ways that, once things got going, things stuck together. But, yeah, it’s all part of the total story of – things are held together not just with gravity but also through molecular bonds, through ionic bonds. So ionic, covalent, all of these things come into play in creating worlds. What has the bigger part depends on are you looking at a tiny potato or are you looking at a giant sphere? It’s those spheres that are definitely gravitationally dominated. In fact, it’s gravity that is causing everything to crush down to its equal potential surface and being nice and spherical.
Fraser: Right. And so you’ve got this situation. You’ve got these tiny little pieces of dust. They’re pulling together. They’re collecting into larger and larger gravel type things. Those are collecting together into rocks – piles of gravel. And eventually it’s sort of clumping the whole solar system together into all of these blobs of matter.
Pamela: So, yes. We have this constant increasing of blobs. If you’ve ever watched snow on a day where you have the nice drifting snowflakes, the snowflakes will collide together until you get something that isn’t so much a snowflake as a giant blob of snowy substance falling from the sky. That’s a collisional creation process. It’s just a less violent one than we think of when we think of what happened in the early solar system.
Fraser: So I can imagine these sort of larger and larger – they’re asteroid size and they’re becoming Moon size and they’re smashing into each other, right? And at a certain point – like, when did we get the planets. Pre-Moon creation, when did we get the planets that we’re seeing today?
Pamela: We started to get them about five billion years ago. One thing to remember is when you do that “prisshh” crunching, crashing thing in your head, you’re forgetting that everything started out molten. So initially, you actually had two molten blobs of mud, essentially, except hot instead of wet. Well, it was wet too in some cases. So you have liquidy things colliding into one another so it’s more like “sploof.”
Fraser: That’s why I said “blobs.”
Pamela: Yes, but then you made crashing noises.
Fraser: Oh, “perkwarrr.” Yeah, alright. I’ll work on my sound effects.
Pamela: Yeah, I don’t know how to make a good splashy noise either. But you had these molten, splashy objects merging together into larger and larger bodies starting to cool off. Planets really evolved quickly. Some models show that it only took about 300,000 years to start getting things like Jupiter. So it was a very rapid process. But then we had about 4 to 4.5 billion years ago, we had an object the size of Mars collide with the original Earth. In that process, their heavy cores merge –
Fraser: Okay, now, hold on. You’re glossing over a very fascinating piece of information. Like, “Mars, I think, crashed into Earth. We got a moon.” No, no, no. No, no, no. Back it up. Back it up. So what was going on? Like there was the Earth and there was like a Mars-sized or two Mars-sized objects? What would it have looked like?
Pamela: Well, there was Mars clearly. So this is the thing. In the process of running our solar system backwards, there’s no definitive answer. We know that early in the solar system’s history, things were in a different place. At one point, we had Jupiter and Saturn in a resonance such that for every one time Saturn went around, Jupiter went around twice. And when that happened, all hell broke loose. You had gravitational resonances that were flinging things in and out all around. Heavy bombardments ended up occurring. So you had the outer solar system evolving through these different resonances.
Uranus and Neptune probably started in very different locations than where they are now. We have all of the weirdnesses in the outer solar system from – well, Uranus is spinning on its side, basically.
Fraser: Yeah, Venus is spinning backwards.
Pamela: Upside down.
Fraser: Yeah, upside down. Right. So I envision it like cars speeding on the highway, right? You got these lanes and lanes and lanes of traffic. In the early solar system, it was 13 lanes and everybody was changing lanes and not checking their blind spots. Right?
Pamela: Well, and doing it during a snowstorm so that when they collided, they just kind of spun all over the place. It’s been snowing a lot. That’s the metaphor I’m going to go with today.
Fraser: Yeah, yeah. So would it have been like a Mars-sized object had gotten pushed into Earth’s orbit or were they sort of interacting with resonance and then they just kind of crunched into each other?
Pamela: We don’t know. We don’t know. It was the early – how that sort of thing happened, there’s multiple models and we just can’t get there from here. There’s so many things that have happened in the interim. Roughly three to three and a half billion years ago is when we were looking at all sorts of bombardments going through the inner solar system. 3.8 billion years ago. Then, of course, we have to worry about things that erased the history.
We still had volcanism in the past on Mars. We still had – well, it wasn’t so much volcanism on the Moon the way we normally think of volcanoes, but there was certainly lava flows in the Moon getting formed four billion years ago. It took time to cool off and settle into its current shape. So it’s really hard to decipher what happened in our universe that far back.
Fraser: Right. So, at some point, Earth was driving along in its lane, perfectly happy and someone in a Mars car smashed into it. What happened?
Pamela: I think –
Fraser: Not actual Mars, a Mars sized –
Pamela: No, it was a Mars sized –
Fraser: Another Mars.
Pamela: Different object. And I think the way to think of it is there was a missing yield sign. So no one yielded the way.
Fraser: Just merged. Yeah.
Pamela: They just merged.
Fraser: Yep. Okay.
Pamela: Yeah, with no getting into the left-hand lane. So, yeah, we had these two objects going along, minding their own businesses and then trying to occupy the exact same space. When you have massive collisions like this take place, you have all of the kinetic energy of the collision getting released as heat. That re-melted. It caused a giant splash of the lightweight material, a merger of the cores, and essentially, this is where the Moon came from. For a while, the Earth probably had a ring of material. The ring of material coalesced into the Moon. Some of it fell back down to Earth. It was a violent event but it wasn’t unique. That’s the thing to stress.
Fraser: Yeah. Well, that’s the thing is this is just one that we know of because we’ve got a piece of evidence. A moon.
Pamela: And so far, we’ve restricted our conversation to this solar system. But we now know our solar system is one of countless solar systems that are out there. There’s actually this really neat discovery all the way back in 2008 of a solar system that was about the age of our solar system. So it wasn’t young. It had been hanging out for a long time, fairly well evolved, and it looks like two planets – one the size of Earth, one the size of Venus – collided and in the process of colliding, just created a giant dust belt. There was nothing left.
Fraser: Like Star Wars. Right?
Pamela: No Death Star.
Fraser: Alderaan. Well, no, the Death Star – but why have a Death Star when you can just crash a Venus into an Earth? So we see the results of this impact here with the Earth and with the Moon and the distance of the Moon and what the Moon is formed out of. One piece of research – I don’t know if you’ve heard – that maybe the idea was that one of the explanations for the weirdness on the far side of the Moon is that there was a second moon that sort of slowly impacted onto the backside of the Moon.
Pamela: Right. So that particular model has a giant splash occurring. Essentially a ring forming, then a large body and a smaller body both forming and over time, they caught up with one another and the smaller body whacked into the backside of the larger body. And that became the near side and far side of the Moon. It explains why the high density core of the Moon isn’t actually in the center of the three-dimensional Moon.
Fraser: So more weird evidence is Venus. As we mentioned, it’s upside down.
Pamela: Yes. And Venus is actually one that is more frustrating to understand because there’s two different ways of looking at it. You can either look at it as the result of an impact flipping it over, potentially. But there’s nothing, looking at its surface, to prove that. But the thing has been resurfaced recently by volcanism, we believe. Then the other way you can look at it is some sort of a resonance interaction over time – potentially even with the Earth – could have torqued it and torqued it and torqued it until it flipped over. So you can do theoretical models either of collisions or of this gentle nudging over time and in both cases, manage to flip it on its head.
Fraser: Right. And you can end up with a situation where it too could have briefly gotten smashed by something, had a cloud of material, had a moon but if the moon was below the Roche limit –
Pamela: It would have gone back down –
Fraser: It would have gone back –
Pamela: Which is actually what we’re seeing happen with Mars. Mars is eventually gonna get impacted by its lowest moon.
Fraser: So more evidence. Let’s talk about Mars. Captured asteroids?
Pamela: Captured asteroids.
Fraser: Yeah. Okay. Not similar formation as to what happened with the Earth?
Pamela: No, in fact, this is something that we see repeated over and over throughout the solar system is many of the worlds have captured asteroids orbiting around them. I actually am kind of amazed that we don’t have a captured asteroid floating around the planet Earth. Our solar system is filled with rocks. Gravity reaches out forever and when things pass one another with the correct difference in velocities, instead of an asteroid flying by, it ends up getting captured into an orbit. We also see this happening with Kuiper belt objects, where Neptune’s Titan appears to be a captured Kuiper belt object. Triton, not Titan.
Fraser: Triton. Right.
Pamela: Neptune’s Triton appears to be a captured Kuiper belt object. Our solar system is filled with rocks and ice that got captured from other places.
Fraser: Okay. Well, explain Uranus.
Pamela: That one – it was either something where it got seriously torqued during this – there was probably a period of time, in the past, where we had all four of the giant planets – the ice giants and the gas giants – in basically a tumbling blob of inter-orbiting objects. It could have, during those interactions, gotten flipped over through some sort of a torquing mechanism or it could have happened via collision again.
Fraser: So there would have been like another Neptune or something that smashed into it?
Pamela: It wouldn’t have necessarily had to have been another Neptune. It could have been a smaller, more dense object. Lots of different things. It’s all a matter of the rate at which two things collide together and their masses. So you can either have slow and big or fast and small.
Fraser: Now, there were some periods in the history of the solar system where the mayhem was cranked up to another level and the sort of famous one of this is the Late Heavy Bombardment. So what was going on there?
Pamela: Its name really says it all. It was a period where we had all sorts of rocks from throughout the solar system – so minor planets, small bodies – getting flung in all directions and as a result of this – and this was related to Jupiter and Saturn being in resonance with one another. This is that Jupiter going around twice for Saturn going around once period. We believe that may have been one of the driving actions for it. But we’re still trying to figure out all of those things.
Fraser: What would it have been like to be living on the Earth during that period? Apart from suicidal. Apart from it being a death sentence, what would it have been like?
Pamela: So first of all, the Earth hadn’t fully cooled yet. So you’re dealing with a very different planet. Our oceans –
Fraser: Okay, I’m on my lava boat. Hook me up.
Pamela: Our planet was fairly free of volatiles at that point, so not a lot of water, not a lot of carbon dioxide, all those sorts of things. They were starting to get delivered probably via comets. So the heavy bombardment was a good thing from that perspective. But instead of getting a giant thing hitting us every couple million years, it was happening probably every few thousand years.
Fraser: Wow. And you can see that in the story in the surface of the Moon. All of these craters that were formed within a few hundred million years of each other.
Pamela: And out to about a billion year period. This is when we saw some of the biggest craters on the Moon forming, the giant basins that you look at. It was a time when there was also just more debris around. At the end of the Late Heavy Bombardment, there just wasn’t as much stuff to do the colliding.
Fraser: So we’re now – things have calmed down. Things have settled down with our solar system. Not as much stuff flying around, but there are still some collisions happening all the time and even in our future.
Pamela: And this is where the topic of our next episode comes from – the Chelyabinsk impact in Russia. The solar system’s still filled with rocks of varying sizes from a few centimeters to kilometers to tens and hundreds of kilometers across that are on orbits that aren’t entirely stable, which means they will evolve over time, and on Earth-crossing orbits.
These are the Apollo class of asteroids. Things that have a greatest distance from the Sun that is quite often out in the asteroid belt, but a distance closest to the Sun that’s closer than the Earth is to the Sun. That means they cross our orbit twice a year. If we happen to be in the same place at the same time, which is what happened with the Chelyabinsk asteroid – which became a meteor, which hit – then you end up with collisions.
Fraser: How common are these collisions gonna be? How rare – and we’ll get on to the Chelyabinsk in the next episode, but what can we expect over the next few thousand years?
Pamela: Well, this is something that we’re still trying to figure out. This is where space projects, like the Large Synoptic Survey Telescope that’s getting built down in Chile, are so important because we don’t actually have a complete census of all the asteroids. So I can tell you as near as we know. Things at the Tunguska side are an every few hundred year event. Things like extinction level events are probably every couple hundred million year event. But we don’t know that.
Now, one of the nice things that we do know is recent models to look at the dynamics of the inner and the outer solar system, find that for at least the next five billion years, the orbits of all of the planets, when you run numerical models, taking into account all the asteroids we do know and different projections for what we don’t know – when you run all those numerical models, there is essentially zero probability of worlds colliding for five billion years.
And, you know, in five billion years, the Sun bloats up, we all get fricasseed anyways. It’s not gonna matter what happens in five billion years. So the planets, on their orbits today, are nice and stable. In our solar system, we’re not gonna be witnessing any massive collisions any time soon.
Fraser: Well, you already jumped to my next question which is to run things far, far forward. So in five billion years, we’re not gonna see the orbits move around at all. But when the Sun does bloat up as a red giant and the solar wind kicks up, we’re gonna see some shifting a bit?
Pamela: We’ll see everything migrating. What happens is, as the Sun loses mass, objects’ orbits – the thing that they’re orbiting is no longer the same size so their orbits end up expanding out. So the Earth, Mercury – Mercury just gets eaten. Earth is going to migrate out. Mars is going to migrate out. Everything’s going to migrate. It’s going to be an interesting time that we shan’t witness. But –
Fraser: Speak for yourself. I’ll be on my third robot body by then.
Pamela: As far as we know, the Earth will stay outside of this expanding red giant star.
Fraser: Then, though, it’s gonna turn into a white dwarf.
Pamela: Right. And –
Fraser: Will the planets remain in their same orbits or will they shift inward again?
Pamela: No. So their orbit doesn’t depend on the diameter of the thing they’re orbiting. It depends on the mass of the thing that’s being orbited. So you can take the Sun and squish it down until it’s a few centimeters across and has the density of black hole and acts like a black hole and our orbit won’t change at all. You can expand it out and the only reason the orbits change is because during the expansion, it loses mass. Once it’s done losing mass and collapses back down to a white dwarf star, it still has the same mass after the collapse. Therefore the orbits don’t change.
Fraser: Right. Unless you were really close and some will get caught in the gravitational frame dragging or something like that.
Pamela: Right. Now, to be clear, the entire envelope of the star gets lost as the sucker becomes a white dwarf. So what’s left behind is the core of the star. But it’s the losing of the mass, not the diameter of the star that causes orbits to change.
Fraser: If you then ran those orbits for billions, trillions, quadrillions of years into the future –
Pamela: You can’t.
Fraser: As the universe expands –
Pamela: Well, the universe’s expansion has nothing to do with planetary orbits.
Fraser: No, I know. But I’m just saying the universe is gonna be around for quadrillions of years after the Sun dies. So will they eventually crunch into each other and form one big planet? Orbiting the white dwarf?
Pamela: We don’t know. That’s the thing is numerical models are unstable at that point because we don’t know all the different factors that will occur. We don’t know. Are other stars going to pass too close to our solar system? We don’t know all of these different things. It’s gonna depend on what hits what, how much mass the planets absorb via the mass loss of the Sun, all those different effects. Solar heating will have an effect on the orbits of smaller objects. There’s lots and lots of variables and so the models aren’t stable more than a couple billion years. They’re really not stable for more than a few thousand years for the small objects, but for the bigger stuff you can go a few billion years.
Fraser: Right. But not the few quadrillion that I’m trying to go out to.
Pamela: No, can’t go there.
Fraser: Alright. Cool. Alright, well, thank you very much, Pamela.
Pamela: My pleasure.