Astronomy Cast

Ep. 565: When Worlds Collide

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So much of our Solar System has been shaped by enormous collisions early on in our history. Seriously, the nature of every planet in the Solar System has some evidence of massive impacts during some point in its history.

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Fraser Cain [00:01:07] 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. I’m Fraser Cain, publisher of the Universe Today. With me as always is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and Director of CosmoQuest. Hey Pamela, how you doing? 

Pamela Gay [00:01:25] I’m doing well. How are you doing, Fraser? 

Fraser Cain [00:01:29] The usual, just getting by day after day, the overwhelming sense of anxiety and existential grief is fading. And now I’m busy. Just busy. I’m like really busy. I got a lot of stuff going on now. 

Pamela Gay [00:01:47] Are you finding that the extroverts in your life are creating video conferences? 

Speaker 1 [00:01:53] No. 

Pamela Gay [00:01:56] I’m having video conferences with people who I have never video conferenced before. 

Unidentified [00:02:02] Right. 

Pamela Gay [00:02:02] And it’s just, okay, this is what we do. 

Speaker 1 [00:02:05] Are they spiraling? 

Fraser Cain [00:02:06] Yes. 

Speaker 1 [00:02:08] Yeah. 

Fraser Cain [00:02:08] I think, again, as the extrovert of this pair, I definitely am getting that process out of me because I do Astronomy Cast, I do the weekly space hangout, I do all of the other interviews and shows that I do, plus I’m hanging out with my son and I’m hanging out with my wife and I went for a walk with a friend of mine the other day. So I’m sort of feeling all that in, but I happen to live in a fairly wide open space. So we can get out a little more safely than I think a lot of people out there. So I can just imagine friends hunker down in New York City making these calls. 

Unidentified [00:02:49] Yeah. Yeah. 

Fraser Cain [00:02:50] We have a yard. 

Pamela Gay [00:02:52] You have a yard. 

Speaker 1 [00:02:54] Yeah. 

Fraser Cain [00:02:54] I have a yard and then I have an unlimited amount of forest behind that yard, right, that I won’t run into another single human being. So you know, just like tough it up. This is the service that you’re doing for your friends who are cooped up and are just dying for human contact. 

Pamela Gay [00:03:12] But extroverts out there, remember how you feel right now so that when all of this is over and us introverts are like, this is hard work extroverting. Remember how hard work it is for you to introvert. Just remember this. But I will say I am so grateful to live in the middle of nowhere because I do have that yard, that ability, and my heart reaches out to all of you who are in your studio apartments, who are living in situations that are less than ideal. We’re here for you and look for the weekly space hangout. Look for the CosmoQuest discord. We have a place for you to come and at least escape into a room on the internet only with people you like. 

Speaker 1 [00:04:02] Exactly. All right. 

Fraser Cain [00:04:04] So much of our solar system has been shaped by enormous collisions early on in our history. Seriously, the nature of every planet in the solar system has some evidence of massive impacts during some point in its history. When worlds collide, Pamela, tell us about. Well, I’d be interested to know why we see these these impacts all over the place before we start to talk about some specific events. 

Pamela Gay [00:04:33] Well, we’re at this really cool point in science where our understanding of how solar systems form is in the process of radically changing. Back when I was a graduate student, I remember learning that planets form in the solar nebula and sometimes they migrate to and fro. But in general, what you form is what you have. But but then, of course, there was the earth that got hit by something the size of Mars, and that created our moon. 

Fraser Cain [00:05:07] But other than that, but apart from that, right, everything was everything was standard. It all just formed in place. Exactly. Exactly. The earth was different. Yeah. Yeah. 

Pamela Gay [00:05:19] But but then we started to evolve this idea to include, well, there there was this great heavy bombardment and the the NIST model came up to explain how our solar system at one point had a resonance between Saturn and Jupiter that was responsible for flinging everything. Either and Yon, Uranus and Neptune went out to the outer solar system, switched places and the inner solar system. 

Fraser Cain [00:05:53] We just got beat up. 

Pamela Gay [00:05:55] We got beat up by the giant planets. And so that was the first major evolution of how did we get to where we are? And then suddenly, in the past year and a half, there have been a series of press releases that are basically saying, OK, now that our computers are getting better and we’re thinking about this a little bit differently, Jupiter’s core is really too fluffy. And that was the actual word. I kid you not, they used Jupiter’s core was too fluffy. How do we explain this? 

Fraser Cain [00:06:31] OK, we’ll see. I mean, how do we even know how fluffy Jupiter’s core is supposed to be? And then how do we measure how fluffy it is today? 

Pamela Gay [00:06:43] Well, so computer models can tell us how fluffy it should be. And because we had Cassini at Saturn, these things should scale politely. So what we see at Saturn, we should see at Jupiter just scaled differently. And we didn’t. Saturn and our models happily matched one another in terms of having this nice, dense core that could have then attracted all of the gas particles around that dense core, building up this gassy world. It all was consistent and made sense. We can figure out the density profile of an object by looking at its moment of inertia as it rotates. A moment of inertia is a fancy pants way of saying how much of its core is located in its center versus in its outskirts. And we can see how the moment of inertia affects rotation with the standard example is ice skaters. They put their arms out, they slow down, they bring their arms in, and you’re no longer sure how they’re standing. Well, you can do the exact same experiment rolling cans down an incline. Take a can that just has the weight around the outskirts of the can. Take a can that is equal density throughout the entire can. Take a can that you have suspended a core of material in the center of the can. Send them all three rolling down that incline, and they’ll actually go down the incline at completely different rates because of the moment of inertia. They have to roll. They can’t slide because sliding is just gravity, and that’s a different result. But as long as they’re rolling, you can see their difference in behavior based on the profile of the content inside them by how they roll. Well, with planets, we look at how spacecraft go around them and how they go around themselves. And by looking at the detailed nuances of rotation, these flybys that we get allow us to get at the moments of inertia of these worlds. And when you happen to have a spacecraft that hangs out for a long period of time, you can measure in detail the moment of inertia of a world. 

Fraser Cain [00:09:06] Right. So, I mean, we’ve always talked about this, that figuring out the mass and density of some object is very difficult unless something is orbiting it. And once you have something orbiting it, for example, a spacecraft that you’ve sent to it, then it has all kinds of really good sensitive instruments on board, unlike ganymede, boring ganymede, which is really slacking off. You’ve got a way to really measure the exact behavior of the planet itself that your spacecraft is orbiting. Absolutely stunning. So then based on these measurements and the fluffiness of the core, what do they think happened to Jupiter? 

Pamela Gay [00:09:49] Well, there’s this really cool model that was put together where what they looked at was if you have a still forming but fairly well -formed Jupiter and then slam into it violently with a many Earth -mass object, what does that do? And effectively what it does is it inserts a whole bunch of energy and that heat fluffs up that core that had just condensed down, but the heat makes it more energetic and allows it to poof back up. And so by slamming something into it, adding energy to the system, it’s a way of just increasing the volume the core takes up, lowering its density and, well, fluffy. 

Fraser Cain [00:10:39] And is this a temporary thing or would it… It’s lasted this way for billions of years. So what kind of an object must it have taken? 

Speaker 1 [00:10:53] Like another object. 

Fraser Cain [00:10:55] Like something Saturn -sized, something Earth -sized, something Neptune -sized, like big though, right? 

Speaker 1 [00:11:01] I mean, Jupiter is big. 

Pamela Gay [00:11:02] It would have been between Earth and Saturn in size. 

Fraser Cain [00:11:06] Wow. 

Pamela Gay [00:11:06] And this is the thing that we’re now starting to realize is there are a bunch of these multi -Earth -mass things in the outer solar system careening around that weren’t given the chance to stay planets. Right. 

Fraser Cain [00:11:24] Like how long, you know, if the solar system formed say four and a half billion years ago, how long until most of these objects were scooped up and merged together into other things? 

Pamela Gay [00:11:38] Well, it looks like our universe was pretty settled down by three and a half billion years ago. So there would have been the early days of ducking cover but without Saturn and Jupiter purposefully thinking things around with their resonant orbits. And Jupiter probably got fluffed up before the great heavy bombardment. So this was while things were still forming, it just happened to careen into something else. 

Speaker 1 [00:12:10] Right. 

Pamela Gay [00:12:11] And thus it got fluffed up in its early days and as it settled down, it kept that overly large core. 

Fraser Cain [00:12:21] So we know about the Earth and that the form, you know, we’ve talked about this on many episodes so we’ll just take this that everybody knows that the Earth was smashed by a Mars sized object early on in its history and created the Moon. So we know about the Earth, we know about this new evidence of Jupiter. What else do we see as evidence in the solar system for these kinds of gigantic collisions? 

Pamela Gay [00:12:45] Well, another case is Uranus. And we’ve always known that something weird probably happened to Uranus because where most objects in our solar system orbit such that if you took your right hand and curved it around the world, your thumb points up through its north pole and your fingers are pointing in the same direction the world is rotating. And this is consistent, north is up, following the sun, everything is going around and around the same direction is pretty consistent. And the variations in tilt we see are generally minor. Jupiter, which is the largest object, the hardest to tilt, has the smallest tilt. Our own Earth is 24 -ish degrees, Mars I believe is 29 -ish degrees. But Uranus is 98 degrees tilted over from what you would expect compared to the sun. Its north pole got flipped below the disk of the planets. 

Fraser Cain [00:13:56] I mean, those of you recalling your trigonometry recall that there is 90 degrees in an angle. And so for it to be, you know, you would expect that you would measure the angle that it’s tilted over somewhere between zero and 90 degrees. And yet astronomers measure it at 98 degrees, meaning it is completely that, you know, we measure it from that north pole and that north pole is flipped right over. So what happened to Uranus? 

Pamela Gay [00:14:34] Well, we’ve been saying for ages that it was probably either hit or torqued over. And the idea of torque is what’s happening that gets our moon in a tidal lock with the planet Earth. It’s where you have something that is tugging on the world in such a way that there’s essentially a lever arm flipping the world sideways. With the moon, its center of mass is not its geometric center. It is actually higher density on the side closest to the Earth. So every time it tries to rotate away from us, our Earth grabs onto that over density of mass and pulls it back. And this was what allowed us to essentially torque it into place the way it is right now. Now, trying to figure out what the lever arm is with an ice giant like Uranus is deeply confusing. So when people didn’t think collisions were common, there was a whole lot of maybe it got hit, don’t really know. 

Fraser Cain [00:15:44] Right. Well, I mean, you have that whole situation where in, as you mentioned, the Nice model, where as the planet shifted around, Uranus and Neptune went way out and in fact, they switched places. 

Pamela Gay [00:15:58] Yeah, we think. 

Fraser Cain [00:15:59] And so you can imagine switching, two planets switching places at some point would bring them close enough to cause some kind of interaction. So that’s still on the table. 

Pamela Gay [00:16:13] But recently folks have been trying to figure out where moons come from. And this is not as trivial as we thought. Early on, the idea of where do moons come from, it came down to, well, they get stolen. Mars stole Phoebus, Phoebus and Deimos. Of course it did. And they form in situ. Look at Jupiter and the Callisto, Ganymede, Io, Europa, these worlds formed in essentially a mini solar nebula. And we use all that. We can actually start to see the eddies in solar systems forming around alien stars where it looks like this idea plays up. And so we basically had two ideas. They are stolen or they form in place. But then of course there’s Earth’s moon and that’s an exception. And how is it our world is always the exception? 

Speaker 1 [00:17:12] Right. Yeah. Yeah. Right. 

Fraser Cain [00:17:13] I mean, literally like the first rule of astronomy is you’re not special. Exactly. Exactly. Right. 

Pamela Gay [00:17:20] But we didn’t want cool stuff to be happening. We wanted the universe to be stayed and regimented and the universe doesn’t care what we wanted. It’s going to do whatever it’s going to do. And so a team of scientists working through trying to model the formation of moons couldn’t make sense of Uranus because here’s a world that has tenuous, wispy rings of material. It has far flung moons. The masses make no sense. If you look at it as it all formed in situ or was stolen. Now, if you add the third option, which is you take a many Earth mass icy planet. So take Pluto or Triton and make them multiple times the mass of the Earth, which is consistent with the object that we think is out in the Kuiper belt that Michael Brown is searching for. 

Fraser Cain [00:18:25] Yeah. So planet 10. Right. Planet nine. 

Speaker 1 [00:18:28] Right. Planet nine. 

Pamela Gay [00:18:28] However you choose to number it. Imagine it had a self -destructive sibling that three to four billion years ago, maybe four and a half billion years ago, rammed itself into the young Uranus, broke apart, shattered, spewed vaporized material in all directions. Well, the thing is, when our Earth got hit by Theia, that Mars sized object and everything splattered, rock cools much faster because it cools at a much higher temperature. So we ended up with two objects quickly. So the rock that we have here at Earth, it solidifies at a much higher temperature and this changes the rate at which things are able to solidify. Out in the outer solar system, you have everything getting vaporized, but then it solidifies at an even colder temperature and that changes the rate. And that extra time that was gained by the fact that you’re in a colder part of the solar system and this stuff cools too solid at a colder temperature. It all played out so that Uranus was able to suck in all of the vapor except for a little bit that was left in the outskirts where rings formed and a few big chunks that became moons. 

Fraser Cain [00:20:08] And possibly, and I know as well, that in fact it might have taken several of these torquing events to actually complete this process of throwing Uranus totally over on its side. That it might have been not just one but actually a bunch of these events, one after the other as Uranus scooped up all the material that was roughly in its orbit. 

Pamela Gay [00:20:34] And this is where what matters is a combination of where did the blow hit and what was the difference in velocities. So this is one of those really frustrating things where there’s multiple ways to get at how Uranus is tilted on its side. But the easiest way to account for the rings, the moons, and the tilt is one massive object coming in hitting it with a glancing blow, tilting it over, shattering, creating the rings, the moons, all of that at once. 

Fraser Cain [00:21:13] Now, you mentioned Uranus. We talk about something having 98 degrees but that is not the most tilted angle in the entire solar system. There’s another place that’s even weirder. 

Pamela Gay [00:21:25] Yeah. 

Speaker 1 [00:21:26] Or equally weirder. 

Fraser Cain [00:21:28] Anyway, let’s not make them fight. 

Pamela Gay [00:21:30] Venus was designed for the left -handed people in our solar system, and as a left -handed individual, I am pro -Venus. 

Fraser Cain [00:21:37] So as long as you measure Venus’s orbit using your left hand and not the right hand like you do for every other, pretty much every other body in the entire solar system, then Venus is doing fine. Exactly. 

Pamela Gay [00:21:48] Either that or it’s just doing thumbs down. So again, the way we do this is it’s supposed to be with your right hand, you wrap your fingers in the direction of rotation of the world. And when you do this with the Earth, you end up with a moderate tilt. It gives us the seasons. We’re happy. With Venus, you have to tilt your arm upside down. 

Fraser Cain [00:22:11] Thumbs down. 

Pamela Gay [00:22:12] Thumbs down. Thumbs down. Venus is giving us a thumbs down. And not only that, but its rotation rate is totally off. We expect certain rotation rates based on conservation of angular momentum of the whole darn solar system. And there are exceptions. Mercury is in a resonance with the Sun due to tidal forces. But everything else kind of more or less makes sense except for Venus where its days are longer than its year. 

Fraser Cain [00:22:56] Yeah, I’ve heard this said that you could walk on Venus and you could out walk the Sun. This is so if you so if Venus wasn’t horrible enough for you already, you could always just have the Sun beating down on you and you just walk at a regular walking pace just around and around the planet and you would be faster. You could outrun the night or you could do the opposite and never see the Sun if you wanted to. Both sounds so weird. So what happened to Venus then? What could have caused this situation? 

Pamela Gay [00:23:30] And a few years ago, 

Fraser Cain [00:23:31] I would have said, well, 

Pamela Gay [00:23:32] there’s models that think it could have been brought about by a combination of torques from the planet Earth and its orbit around the Sun that maybe it was collided with. And because we don’t have enough data, I still can’t tell you except that if Venus follows the pack, it got hit as well. And the question is when and by what? And Venus is one of these worlds that the more we look at it and the better our computers get, the more variables we’re allowed to include in our models. We’re starting to realize that quite possibly just within the last millions of years, Venus’s climate might have been just a slightly warmer version of Earth like everyone thought it was until Mariner 2 got there and crushed all of our dreams. 

Fraser Cain [00:24:29] Although people really should have been looking at that, taking that thumbs down as a hint. Yeah, it’s true. 

Speaker 4 [00:24:35] It’s true. 

Pamela Gay [00:24:35] Yeah. 

Speaker 1 [00:24:35] How to, you know, 

Fraser Cain [00:24:36] is there life on Venus? Thumbs down. Thumbs 

Speaker 1 [00:24:39] down. Yeah. 

Pamela Gay [00:24:41] But we don’t know. Something traumatic and multiple epochs in time has happened to Venus. 

Fraser Cain [00:24:51] So again, possibly, I know astronomers have absolutely worked out models where something big smashed into Venus, rolled it over. But I mean, if all that happened to us was we got knocked over 23 and a half degrees and we got a moon, just imagine what it must have done, what must have happened to Venus. 

Pamela Gay [00:25:16] And the models that I’m desperately hoping someone will do and publish, because if you get boring results or you get null results, you don’t always publish. And this makes me sad. The model I would like to see tested and published if it’s a null result place is a model that looks at, assume as it appears to have been the case that Venus was still a perfectly average, not death world until a couple of million, a few million years ago, then slam something into it that would trigger the atmospheric changes that we have seen and see if it’s possible to do this in a way that also flips the world. I don’t think you can flip the world without destroying the world. So I’m pretty sure that this model will go, no, you can’t do this. I’m pretty sure this had to have been an ancient event that flipped it upside down. But I still just want to know. 

Fraser Cain [00:26:24] I want to know. And we’ve done several episodes on several articles on Universe Today as people release models showing here’s how it might have happened. And this might have explained why it also doesn’t have a moon. Like it gobbled up its moon and then that flipped it over. So there are a bunch of interesting ideas. And then, I mean, we see evidence of really catastrophic events happening on Mars. We see really strange features on Mercury, which are evidence of some ancient collision. We’ve got to go back to Neptune and Uranus to try and trace them out as well. But there’s one last world that NASA is actually planning to visit, which is asteroid Psyche, which is the leftover core of a planetoid, which is pretty cool. 

Pamela Gay [00:27:14] And this is one of those things where you have to wonder how many other might have been worlds have we just not found the cores yet. 

Speaker 1 [00:27:26] Right. 

Fraser Cain [00:27:27] And like, is that what our, you know, our metal meteorites are is they are the cores of, of dead planets. Yeah. 

Speaker 1 [00:27:36] Yes. 

Pamela Gay [00:27:36] And, and just one, one of the things that has been getting me is the Atacama Large Millimeter Array, ALMA, is out there looking at solar systems forming around alien stars. And the images that we’re getting back are a hundred or hundreds of astronomical units in size. Our solar system tapers out at about 55 AU and becomes just plain boring and we haven’t seen the Oort cloud yet. But these solar systems that we’re seeing around other stars are so much bigger, which implies that things migrate inwards. And then if the NIST model is correct, gets flung back out to a different point. Solar systems are yo -yoing. 

Speaker 1 [00:28:22] Yeah. 

Pamela Gay [00:28:24] And what all could have formed spread out across a hundred astronomical units. That’s just amazing to imagine. 

Fraser Cain [00:28:33] Yeah. And that’s one of the things that’s quite exciting is we do see planets colliding in other star systems. We see just all of this, just incredible diversity across, you know, the Milky Way so far with all the instruments. Newly forming planets are actually a lot easier to find. 

Speaker 4 [00:28:52] Yes. 

Fraser Cain [00:28:52] Newly forming star systems in infrared observatories than fully mature planets. So actually one of the greatest amounts of innovation in this field right now is happening in these newly forming planetary systems. So if there’s any answers that we’re going to get more quickly than others, it’s probably going to be what happens, what kind of mayhem goes on in these planetary systems early on in their history. Absolutely fascinating. Thanks, Pamela. Do you have any names for us this week? I do. 

Pamela Gay [00:29:22] As always, we are so very grateful to all of you that really allow us to keep doing the show week after week. Now it’s starting to be decade after decade. 

Fraser Cain [00:29:34] Decade after decade. 

Speaker 1 [00:29:36] Millennia after millennia. 

Fraser Cain [00:29:39] What was that? I said millennia after millennia. 

Pamela Gay [00:29:43] I’m not sure I’m as into the robot bodies as you are. 

Speaker 1 [00:29:46] All right. 

Pamela Gay [00:29:48] But we couldn’t do this without Suzy because what you see us doing here on the show, this is the fun part. We’re just like talking about what we love. Suzy has to edit. You let us pay Suzy to do the thankless job of making sure that this goes from us talking to you actually having a show. So thank you. And these are the people we’d like to thank this month. Brian Nelson, Kristen Brooks, Eric Frerringer, Martin Dawson, Cassie Pienfilenko, Dwayne Isaac, Frode Tenebaugh, Shannon Humber, Justin Proctor, Thomas Tubman, David Gates, Rachel Fry, Eron Zegrev,  Fredrik Sjorga, 

Pamela Gay [00:30:41] Claudia Mastromenet, please you can put in pronunciations. 

Fraser Cain [00:30:44] I apologize. I’m so sorry. Nuderdude. 

Pamela Gay [00:30:47] I know how to say your name. Paul Hayden, Omar Del Riviero, Brent Crenop, Tim Garrish, and Arthur Latz -Hall. Thank you. Thank you. I thank you more than I can pronounce your names and I apologize. 

Fraser Cain [00:31:03] Thank you. Thank you everybody and we will see you all next week. 

Speaker 4 [00:31:08] Thank you for listening to Astronomycast, a non -profit resource provided by the Planetary Science Institute, Fraser Cain, and Dr. Pamela Gay. You can find show notes and transcripts for every episode at Astronomycast. You can email us at info at Astronomycast .com, tweet us at Astronomycast, like us on Facebook, and watch us on YouTube. We record our show live on YouTube every Friday at 3 p .m. Eastern, 12 p .m. Pacific, or 1900 UTC. Our intro music was provided by David Joseph Wesley, the outro music is by Travis Searle, and the show was edited by Susie Murph. 

Speaker 1 [00:31:56] Arizona, meet Marvin. Timeless quality for your home. Marvin windows and doors are crafted with precision, designed to inspire and built to last. Whether you’re looking to enhance natural light, improve energy efficiency, or create stunning indoor -outdoor living spaces, Marvin offers innovative solutions tailored to your vision. Explore Marvin Design Gallery by Mountain West Windows and Doors in Scottsdale, Gilbert, Flagstaff, and Tucson today. Discover the difference at www .mwwd .com. 

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