Splat or subsurface ocean? The mysterious positioning of Pluto’s heart

Sarah Al-Ahmed: We're investigating the heart on Pluto this week on Planetary Radio.

I'm Sarah Al-Ahmed of The Planetary Society with more of the human adventure across our solar system and beyond. It's been almost a decade since NASA's New Horizons spacecraft flew by Pluto and the data still yields results. This week, we're exploring the origins of Sputnik Planitia, the western lobe of the heart-shaped impact feature on Pluto. It might be able to tell us whether or not the dwarf planet actually has a subsurface ocean.

Adeene Denton from the University of Arizona will join us to talk about the work she and her colleagues at the University of Bern in Switzerland have been doing to understand oblique impacts or as they call them, splats. Then Bruce Betts, our chief scientist joins me for a roundup of the most significant impacts in our solar system in What's Up? If you love Planetary Radio and want to stay informed about the latest space discoveries, make sure you hit that subscribe button on your favorite podcasting platform. By subscribing, you'll never miss an episode filled with new and awe-inspiring ways to know the cosmos and our place within it.

On July 14, 2015, NASA's New Horizons spacecraft flew by Pluto. It was the first time we'd visited that world and its moons and our first mission to explore a body in the Kuiper Belt. What we found there was astonishing. Pluto is a complex world with varied geology and five moons, the largest of which is Charon (KAIR-on) or Charon (SHAR-on), depending on how you like to pronounce it.

Pluto has glaciers, crevasses, thin clouds and a beautiful heart-shaped feature filled with nitrogen ice. Tombaugh Regio, the so-called heart on Pluto seemed to be the remnant of a large collision. But it was puzzling. What created this feature? Why was it located near the world's equator? And what could that tell us about its interior? The laws of physics tell us that we should expect a feature like this to migrate toward one of the world's poles. Its placement near the equator has caused a lot of people to speculate that perhaps, Pluto has a subsurface ocean, and that's what's stabilizing this world and allowing the heart to stay near the equator. But there are other explanations.

To explore the heart on Pluto and what it can tell us about this world, we're joined by Dr. Adeene Denton, a research scientist at the Lunar and Planetary Laboratory at the University of Arizona. She's a geologist and a planetary scientist who studies and models giant impacts in our solar system.

She and her colleagues at the University of Bern in Switzerland published a paper earlier this year in Nature Astronomy. It's called "Sputnik Planitia as an impactor remnant indicative of an ancient rocky mascon in an oceanless Pluto." As the title suggests, their team is investigating an alternative to the liquid ocean underneath the surface of Pluto. They've modeled whether or not an impact on Pluto at an extreme angle or as they call it, a splat, could be the explanation.

Hey, Adeene. Thanks for joining me.

Adeene Denton: Hi. Thanks for having me.

Sarah Al-Ahmed: In all of my time hosting Planetary Radio, I have not had a chance to talk in depth about Pluto yet, so I am so ready for this.

Adeene Denton: It's always good to talk about Pluto.

Sarah Al-Ahmed: Well, you're a geologist and a planetary scientist, but you focus primarily on giant impacts. How did you get into this line of work?

Adeene Denton: Well, the same way anyone gets into anything, you start out doing something else and then you find out that you can blow up planets as a job and then you start doing that instead.

Sarah Al-Ahmed: Virtually, of course. We don't have a Death Star over here.

Adeene Denton: Yeah, they don't let me have the Death Star. That would be bad. That's not ethical. No. So, I started out as a geologist in undergrad and then I got into planetary science through being an intern at the Lunar and Planetary Institute in Houston where I got to go into the crater at Meteor Crater, Arizona and study the structural geology of the crater and how it formed.

And that got me into impact cratering very early on and I thought it was the coolest thing ever because it's a geologic process that operates everywhere from Mercury to Pluto, right? It's kind of this great equalizer in terms of trying to understand planetary histories.

So, I ended up getting my PhD in Pluto, studying its largest impact basin which is Sputnik Planitia. And I've continued to explore how giant impacts can show us the signals of what's going on beneath the surfaces of icy bodies and ocean worlds across the solar system ever since.

Sarah Al-Ahmed: And just to define terms because I know a lot of people are familiar with this feature on Pluto, but a lot of people just call it the heart. So, what is Sputnik Planitia and what is Tombaugh Regio and how do these two things connect?

Adeene Denton: Well, it depends on who you talk to actually because not everybody's fantastic at differentiating between the different areas on Pluto that are all kind of intermingled. So, I study an impact basin which is technically called the Sputnik Basin because Sputnik Planitia is technically just the bright deposit of Pluto's heart that you can observe in the images. It's the really bright, high-albedo feature and that's the nitrogen ice, that's the interior of the basin. That's called Sputnik Planitia.

But that lies within a much larger basin which is called Sputnik Planum or the Sputnik Basin, depending on who you talk to. But a lot of people love to conflate the two. So, if you go watch a talk of mine, I've probably called the basin Sputnik Planitia and that's not technically right. But it's Pluto, you know it's fine, it's fine.

And then, Tombaugh Regio is the much broader area that encompasses not just Sputnik Planitia which would be the left side of the heart, but also the much more diffused terrains to the right of it that form the eastern lobe of the heart. So, Tombaugh Regio is the biggest. And then if we zoom in to Sputnik Planum and the Sputnik Basin. And then we can zoom in again to just Sputnik Planitia which is the bright deposits that are filling that basin.

Sarah Al-Ahmed: Hopefully, everyone at home has their pictures from New Horizons so they can follow along because it's such a beautiful and diverse region. And that's what's so intense about this for me because so many of us were looking forward to those New Horizons images of Pluto. That was almost a decade ago in 2015. But when we actually got those images back, I was kind of slack-jawed. I really didn't expect it to be such a complicated world all the way out there. What was it like for you experiencing those images for the first time?

Adeene Denton: I don't think anybody expected it to be as complicated as it was. That was actually the summer that I was an intern at the Lunar and Planetary Institute when we were getting those pictures back. And one of the premier New Horizons team members happened to work there. And so, we got to see all these images as they were coming back and I was captivated by it. I just had no idea that something so far away could be so geologically active.

I think those pictures are some of the most beautiful images ever taken. But I look forward to future spacecraft missions taking even more incredible images down the line.

Sarah Al-Ahmed: Oh, I hope so. I hope in our lifetime, we see another mission to Pluto. I mean, I take one to Uranus and Neptune first probably because we haven't been back there since before I was literally born. But I mean, come on, what an interesting world and all of its moons and how they interact with each other. So much more complex than I ever imagined.

And today, we're talking about the complexity of the giant impact on Pluto. But this isn't the first time that your team at the University of Arizona has collaborated with the University of Bern on a similar subject. How did your two teams end up working together?

Adeene Denton: This project came out of a longstanding collaboration between ... So, I was a postdoc at the University of Arizona at the time that this study was published. And my advisor, Dr. Erik Asphaug, has had a longstanding collaboration with Dr. Martin Jutzi who is at the University of Bern.

And together, the three of us and Martin Jutzi's PhD student, Harry, who's the lead author on the paper, came together to work on this idea. And it did indeed come out of work that Martin and Erik have been doing since 2011. They've had this incredible longstanding collaboration to essentially take a kind of strange idea about planetary scale impacts and see what happens in real life.

So, the 2011 paper that you're probably talking about is the lunar fireside splat paper probably.

Sarah Al-Ahmed: Yup.

Adeene Denton: Yeah. That's where the term splat as an impact term originated because the idea behind that paper was that our moon, the earth's moon has this incredible nearside-farside dichotomy where, well, most of us wouldn't know it, the farside of the moon looks completely different from the nearside in many ways.

And they suggested, "Okay. There's a lot of other ideas where the moon could form this weird hemispheric asymmetry through internal processes. But what if it was an external process? What if you had another moon, a companion moon hit the moon and splats onto the moon so that the moon accretes its companion moon and then bam, you form an asymmetric moon?" It's so easy.

Sarah Al-Ahmed: That makes a lot of sense. And honestly, I think the first time I saw the pictures of the farside of the moon, I was just as puzzled as I am today and I don't know what we're going to need to do to actually solve this problem, the formation of the moon and this weirdness between the farside and the nearside.

But what a place to start. And then, we extend that idea of this splat to Pluto. And clearly, this heart on Pluto is some kind of result of an impact. What is it about the heart that you'd say are its most puzzling characteristics?

Adeene Denton: Many things. So, I'm just going to say Sputnik Planitia throughout this talk but, listeners, just refer yourself to the explanation that I have given and continue yourself with knowing that I'm referring to the impact basin.

And the reason we think it's an impact basin is because New Horizons data suggests that it's a large hole in the ground, that the bright white that's filling Pluto's heart is nitrogenized that is filling the interior of this larger basin. And the reason it's probably an impact crater is because it's really hard to make a really big hole in any other way. So, ipso facto, something hit Pluto.

The other possibility is, of course, that if you load the surface with enough nitrogenize, you could basically cause Pluto's ice shell to bow down underneath the load, kind of like a bend and form a base in that way, similar to how the ice sheets on Antarctica are artificially depressing the topography.

But based on how thick we think that nitrogenize actually is, it's not thick enough to do that. So, probably a giant impact. And one of the things that's most unusual about it is we don't really know how old it is because we don't know how old the surface of Pluto even is, right? The reason we know how old the moon is, is because we sent people to the moon and they picked up some rocks and they brought them back and then we dated them in the lab and can continue to learn from them today.

It's not so easy on Pluto because we don't have any kind of benchmark for how old the surface is. It's all relative. What we do know is that Sputnik Planitia, this impact basin is the second oldest feature on the surface of Pluto, which means this impact probably happened really, really early on. And then since then, an untold number of things must have happened to it over potentially billions of years of time to cause it to go from whatever it looks like when it formed to what we're seeing today.

So, the presence of this basin indicates the passage of geologic time, but there's so much that we're missing.

Sarah Al-Ahmed: That just leads me to a question that might be totally off-topic, but if this is the second oldest feature, what is the actual oldest feature on Pluto?

Adeene Denton: That's a feature that nobody talks about because we don't have very good pictures of it. And that's because the New Horizons mission specifically targeted Sputnik Planitia as the area they wanted to image most, so we have really good pictures of that. But when it comes to some of the other areas on Pluto's surface, it looks more like the kind of picture that you get when you accidentally unlock your phone and it's in your pocket. A lot of those for the rest of Pluto, unfortunately.

But from what we can tell, Pluto appears to have this large north-south ridge-trough system that has been observed in the, shall we call it, less good portions of the image data. And that is the oldest feature. Everything else on Pluto's surface appears to overprint it, including Sputnik Planitia, which basically smacks right into that ridge-trough system to overprint it and form this large basin.

So, this ridge-trough feature forms very early on in Pluto's history. And then, bam, Sputnik Planitia forms sometime after that. And then the rest of Pluto's geologic history that we can observe accumulates over time.

Sarah Al-Ahmed: So, that means that the mystery of that ridge-trough system isn't something that we can solve with this splat, unfortunately.

Adeene Denton: No, but the splat can solve a couple other things. The other thing that is relatively poorly understood about Pluto's heart is its location. It's rather unusually located. And what I mean is in tactical terms, the center of Sputnik Planitia is located very, very close to the Pluto-Charon tidal axis, which in layman's terms just means it's really close to Pluto's equator, basically. It's right on the center of Pluto. And that's really unusual for a large impact basin.

And the reason for that is a little complicated. Basically, when you form a large impact basin, what you're doing is making a giant hole in the ground, right? So, you're literally removing material to create a hole which means on a large scale, you're creating a large mass negative. There's mass missing there. And planets don't like it when there's a large amount of mass missing in one area. And they will tend to rotate themselves to position a large mass negative like an impact basin at one of the poles.

This is why we think, for example, the moon's South Pole-Aitken Basin, one of the largest impact basins. The solar system is also at the pole. Similarly, the asteroid Vesta has one of the largest impact basins in the solar system relative to the size of the body it's on. That's Rheasilvia. And that is also at the South Pole.

But the converse of that is also true. If a planet accumulates a bunch of mass in one spot, it also doesn't like that and will tend to rotate itself to put that mass at the equator. And the best example for this is Tharsis on Mars, the massive volcanic province that's all concentrated on one side of Mars. And Mars has sufficiently rotated itself to put that extra mass close to the equator.

So then, it raises the question, okay, we have these mass negatives and mass positives at other locations on the solar system. And then for Pluto, here's this what we think is a large impact basin that's at the equator. So, that means somehow, it's a mass positive and not a mass negative, which there's a couple ways to do that. We've seen it on the moon with impact craters though a lot smaller than the one on Pluto.

And it's possible that the nitrogen ice that's filling the basin today is doing some of that work because nitrogen ice is more dense than water ice which composes most of Pluto's ice shell. So, you are putting a more dense load with that nitrogen ice that's filling the basin.

But the math suggests that you would need about 27 kilometers of nitrogen ice filling up Sputnik Planitia to really force it to rotate to the equator. And we really think there's somewhere between three to 10. So, it's probably not enough to do it which means that there has to be some sort of subsurface component beneath the surface of Pluto that is giving the Sputnik Planitia impact basin extra mass that forced it to rotate to its current location. And we don't know what that extra mass is.

One of the ideas that's had a lot of uptake because it's really fun is that Pluto has a subsurface ocean. So, it has the water ice, ice shell that we can observe on the surface today. And then underneath that, it has a liquid water ocean that is underneath the surface of Sputnik Planitia, adding that extra mass because water is more dense than ice. And that's really fun because then that implies that Pluto had an ocean in the past and potentially has one today, which then implies, "Ooh, is Pluto, habitable?" All sorts of cool stuff like that.

But that's not necessarily the only option. And that's where this paper comes in. What we're trying to do here is open the scope of what could be possible at Pluto. And the reasons for doing it this way are multifold and very complex, but the first being that, yeah, as you said, it's pretty difficult to get an ocean on Pluto, not impossible, just kind of hard because Pluto is small and it's far away and it's not orbiting a giant planet. So, it's not getting the kind of tidal heating that say Europa and Enceladus are getting. It's just out there by itself.

Well, it's got Charon. But even though Charon is a relatively large mass satellite, it's not the same thing as orbiting Saturn. So, the way to get Pluto to have an ocean is to have it form really, really early on in the solar system's history where massive heat-producing isotopes like aluminum-26 are still there ready to decay to give Pluto extra heat beyond the heat of its initial accretion and differentiation to then form an ocean early on. And then maybe you can keep it over time.

But that places a pretty firm time window on when Pluto formed. And it's not impossible, but people who simulate formation modeling of a solar system tend to form Pluto and the Kuiper Belt later than that. And if Pluto forms later than that, then it forms relatively cold and then it's a lot harder to get an ocean and have it stick around, and particularly have it be an ocean that's thick enough to actually affect the mass deficit at Sputnik Planitia.

So, that's the kind of framework that we were working with for this paper is, okay, say Pluto forms later, say it forms colder than we think which would be a huge bummer for having an ocean, but might potentially be slightly more realistic. Can you still get a mask on? Is there another way to do it? And the answer is yes. Unfortunately, I don't have a solar system time machine so I can't tell you if Pluto formed hot or cold.

I build Pluto on the computer. And to do that, I have to vastly simplify what Pluto is to what a computer can handle. And then I have to hit it with something. And what happens after that is the result of all the knowledge we have about what happens when two things collide at a massive scale.

And there's many things there that we don't fully understand. So, I'm attempting to approximate a process that nobody's ever seen. And because of that, I think our models do a pretty good job. But again, who's going to check me? You would have to watch two planets collide and then come back to tell me what happened. And then, I would fix my models and be very happy.

All this to say that I think that the work that we do as computer modelers is a little bit of science fiction, right? I'm giving you a possible world out there and I'm telling you what could have happened to that world. And then I'm saying Pluto, the real planet, the one that we can see might have happened a lot like the planet that I made up. But they're not the same. We're learning more about the process of what happens to something like Pluto, then determining exactly what happened to Pluto for real.

So, I like to think of it as really, really hard sci-fi.

Sarah Al-Ahmed: I did have a question about the timing of this impact because clearly, we don't know how long ago it happened, but we do have some clues as in Pluto and its largest moon Charon, the center of mass of the system isn't even inside Pluto. That's how connected these two objects are, how similar in mass they are, but they're tidally locked to one another.

And I imagine that if Pluto had been wobbling about as it was trying to sort out this impact and potentially a subsurface ocean, that might change how long it took Charon to tidally lock or how that interaction plays out.

Adeene Denton: I should be having a paper come out in a couple months that will address the initial Pluto and Charon formation scenario, because we've also been, the group that published this paper has also been revisiting that. Some really exciting stuff, I'm very excited about it.

But yes, as you say, Pluto and Charon aren't actually a moon in its satellite so much as they are a binary, a large mass binary where because Charon is so large, it's half the size of Pluto and one-sixth of its mass. The center mass of the system lies in between the two, though obviously closer to Pluto than to Charon.

And your question is, could the formation of Sputnik Planitia have affected Charon's tidal locking? Probably not just because based on the simulations that we've been doing ... So, let me rewind because not everybody knows that we think that Pluto and Charon itself, the binary formed from a giant impact as well. So, I think this is so neat. So, feel free to cut all this.

Sarah Al-Ahmed: No, it is really, really cool. I mean, I don't know ... Personally, I find Pluto and Charon to be one of the most romantic situations in our solar system.

Adeene Denton: Yes. Oh, they're very good friends.

Sarah Al-Ahmed: They're just dancing their way in the dark. Really, though.

Adeene Denton: Well, you'll see. So, I wish my paper was out because we're going to publish a paper where we've basically established a new form of what happens when two large bodies collide to capture a moon called kiss and capture because Pluto and Charon are girlfriends. But that's a different discussion. Let me backtrack.

So, the reason that we think Pluto and Charon formed through giant impact in the first place, so this would be an even bigger impact to the impact that forms Sputnik Planitia, right? So, it's kind of wild that it happened twice, but it must have because that's what the geology suggests.

We think Pluto and Charon formed from a giant impact because we think the earth and the moon system formed through a giant impact where basically a Mars-size impactor hits the earth early on in its evolution and essentially gets caught. And then, the earth-moon system stabilizes over time.

And the reason we think this is because the earth and the moon are similar to Pluto and Charon in that the moon is unusually massive relative to the earth. If you look at Mars and its two tiny moons, Phobos and Deimos, they're little potatoes compared to Mars. And the moons of the giant planets are less than 1% of their mass, but Pluto and Charon and the earth and the moon are very, very different.

And there's a whole isotopic reasoning behind why the earth and the moon likely formed through giant impacts that I will not explain. But that is the foundation for why people then suggested, "Okay. It's so interesting that Pluto and Charon are very similar in a lot of ways." That's also probably a giant impact.

Now, I'm working my way around to answering your question. I'm coming back around. So, based on the most up-to-date models that we have, it is highly likely that Charon's tidal locking actually happens relatively quickly, very, very fast. So, Pluto and Charon collide. Or in this case, potentially, Charon collides with Pluto. And then there's a brief chaotic period that lasts on the order of hours after which Charon is then caught into orbit with Pluto and tidally locks almost immediately before it's even reached its current position.

So, Charon is about 16 Pluto radii away from Pluto right now, which sounds really far but that's actually pretty close in terms of its satellite and a planet. But Charon gets caught several Pluto radii away from its host Pluto, and then becomes tidally locked and then starts to migrate out that way.

So, I don't think that the formation of the Sputnik Planitia impact would have actually affected that process just because it happens really fast. They kind of snap into place. And again, we don't know how much time there was, as you say, between the initial Pluto-Charon impact, if that's what happened, and the Sputnik Planitia forming impact.

But what we know is that enough time passed to form that ridge-trough system first and then Sputnik Planitia happens. So, probably, Charon was far enough away from Pluto at the time that Sputnik Planitia forms that it was far enough away to go, "Wow, what's happening on Pluto?" And just keep being itself.

Sarah Al-Ahmed: I know that Pluto is this object of fascination, both because it's so far away and because of the pop culture and this whole argument, "Is Pluto a planet? Is it not a planet?" That's not the important thing here.

What's important is that this world is so complex, so geologically diverse despite being all the way out there. And it's one of the few Kuiper Belt objects that we have this kind of up-close data on. And all we have is that one flyby, really.

Adeene Denton: Yes.

Sarah Al-Ahmed: So, this object can tell us so much not just about our solar system and its formation, but also about other exoplanetary systems and the differentiation across these bodies and other systems.

Adeene Denton: Yeah. It offers us this huge window into places that we'll probably never get to see. Eight out of 10 of the largest Kuiper Belt objects are a body with a large mass satellite like the Pluto-Charon system and they probably evolved in similar ways. And the only way we can really know more about any of those is by trying to understand Pluto. There's so much to learn.

Sarah Al-Ahmed: But we're not supposing that Sputnik Planitia was created by a direct head-on collision here. What these simulations are suggesting is that this was more of a glancing blow, a splat situation. Why do we think that's the most likely scenario here?

Adeene Denton: Most likely is such a kind way to put it. It could have happened this way is what I will say because our models made it work. It certainly could have happened some other way. So, the first thing I will say is that a head-on collision where two bodies straight up just hit each other at a 90-degree angle is pretty unlikely. The average impact angle of an impact in the solar system is 45 degrees. Why? Basic geometry, unfortunately. If you hated eighth-grade geometry, me too, but I'm confronted with it almost every day.

So, if the average impact angle of the solar system is 45, that changes how the impact happens, right? Because now, your velocity vector isn't entirely vertical. You have a horizontal component and a vertical component. So, that changes how the crater gets excavated.

And the reason why, among other reasons why we think that Sputnik Planitia formed from a more glancing blow is just look at it. It's not even circular. So, most impact craters are circular. And the cool thing about impact physics is when you hit something hard enough, you can change the impact angle somewhat and you'll still make a circle just because the impact is so dynamic.

But that changes when you get to impacts about 30 degrees. So, for those of you listening, I'm establishing the reference frame. So, a head-on collision where a body just drops straight down and hits would be 90 degrees. So, 30 degrees is quite oblique. We're glancing at quite a glancing angle and that's where things start to become elliptical. You can get elliptical craters. So, that's something that we see on everybody in the solar system.

There's a small fraction of craters that are elliptical and they're very neat. But Sputnik Planitia is huge and it's also elliptical. So, we think that a similar thing happened there where the impactor which was likely very large came in and at an angle grazed the surface of Pluto and proceeded to then excavate Sputnik Planitia from there.

Sarah Al-Ahmed: How large of an object are we talking here?

Adeene Denton: Pretty big. Say a moon of Saturn. Size of a moon of Saturn.

Sarah Al-Ahmed: Oh my gosh, that is quite large. So, we've got this object. It's coming in at this glancing blow. It's pretty large. Is it large enough to potentially be differentiated?

Adeene Denton: Yes. And that's the main thing. So, previous impact simulations that were done actually by me, so I'm kind of proving myself wrong which is always really fun and delightful to do. It's like when you're running a relay race and you take the baton from somebody else but you're high-fiving yourself.

Previous impact simulations assumed that the impactor that was hitting Pluto was around 400 kilometers in diameter. And that'd be around the size of Saturn's moon Mimas, which is already close to the threshold of differentiation but we were assuming, "Ah, it's just a really big snowball. Don't worry about it." But as the impact angle changes, and we assume that Sputnik Planitia formed through an oblique impact to make it the elliptical shape that it has, the impactor has to be bigger to provide the same amount of excavation force basically, right? Because now, like I said before, you don't just have the vertical component of the impact. You have vertical and horizontal. So, you're losing some of your momentum. So, the initial impactor needs to be bigger.

So now, we've got an impactor that's more than 700 kilometers hitting Pluto. That is differentiated. And by differentiated, what we mean is it has a rocky core and then a water ice, ice shell because in the outermost portions of the solar system like the Kuiper Belt, when we differentiate, there's not a lot of metal so you're not going to have rocky bodies with metal cores. You're going to have icy bodies with rocky cores. So, that's what we think would be hitting Pluto in this scenario.

Sarah Al-Ahmed: We'll be right back with the rest of my interview with Adeene Denton after the short break.

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Sarah Al-Ahmed: I think this is where it gets really cool because in that case, that means that there could literally be part of this core or the entirety of the core of this object perhaps just buried right under the surface of Pluto and that's why we're getting this totally wacky mascon. That's cool.

Adeene Denton: Yeah, it is really fun. Basically, what happens in these cases is this differentiated impactor hits Pluto. And because it's hitting it at such an oblique impact angle, the impactor starts to break up. And then the rocky portion of the impactor, the rocky core, remains largely intact and then gets embedded in the ice shell and then sinks down into it, leaving behind this rocky remnant inside Pluto of the impactor.

Sarah Al-Ahmed: But now, we've got all this ice burying it away from our eyes. How do we figure out what's underneath there and what it's composed of?

Adeene Denton: Well, Pluto orbiter is the answer to that question.

Sarah Al-Ahmed: Pluto orbiter.

Adeene Denton: I would love ... Yeah, if you think-

Sarah Al-Ahmed: Pluto sample return. Let's go.

Adeene Denton: Pluto sample return sounds great. Yeah. So basically, again, the concept behind a mascon is that which so like a positive mass anomaly, also called a mascon, the concept behind it is that the thing that's providing the extra mass if it's not on the surface is buried. And then right now, it's very difficult to say whether it was an ocean or whether it is indeed this rocky remnant that's buried in the ice shell. They can both provide that component that we need to reorient Pluto.

But the telltale signs of either one would be hidden in Pluto's gravity signature, for example, which is something that we don't know. It would also change the morphology of the Sputnik Planitia basin itself because if you bury a large rocky chunk underneath only part of the basin, and the reason it's only under part is because in an oblique impact, you kind of hit the surface and skid and then deposit materials. So, we know where the rocky component would be relative to the basin. And that would change the basin topography quite a bit.

But you've looked at pictures of Pluto. You know that the basin of Sputnik Planitia is filled with material so we can't even see the floor, right? We've got to go back and get better topography data, better gravity data. And the only way to do that is to orbit Pluto.

So, call your friends, advocate for a Pluto orbiter because that's the only way to really get to the bottom of this one.

Sarah Al-Ahmed: In order to build this understanding of Pluto, you had to do some computer modeling. It's a smoothed particle hydrodynamics simulation or SPHLATCH.

Adeene Denton: Yes, SPHLATCH.

Sarah Al-Ahmed: In simple terms, kind of what is a SPHLATCH simulation and why is it really good at giving us answers in this scenario?

Adeene Denton: Sure. So, we call our code SPHLATCH, no relation to the splats that we've been talking about, though during the course of this project, we started calling Sputnik Splatnik because that's what we're making. SPHLATCH stands for SPH LATCH. LATCH is the code. SPH stands for smoothed-particle hydrodynamics which is a class of codes. There are many different SPH codes out there. In fact, it's quite easy to build your own. And so, I encourage you to try to do that if that is something that is of interest to you. And you can look up, there are guides to how to build your own SPH code.

But basically, a smoothed-particle hydrodynamics code, henceforth referred to as SPH code, approximates something, and in this case, a planet to planets hitting each other as a series of particles. So, when I say we're hitting Pluto with something, what's actually getting hit is a version of Pluto that is composed of a lot of overlapping particles.

SPH codes were originally designed to study galaxies, so they studied galaxies colliding. So, it makes a lot of sense as to why you might assume that two galaxies colliding are composed of a bunch of particles that can then deform relatively easily as they move past each other.

What we've done is used that same approach and built planets with it. And to do that, we actually had to implement strength models basically so that Pluto doesn't just behave like a liquid when you hit it. It's not going to behave like a liquid because it's a solid planet and not a galaxy. So, we implement a strength model so that all of those little overlapping particles can hold together until they're overcome by the force of the impact.

Sarah Al-Ahmed: Every time I see a video of these kinds of models, it's always really fun. Do you have some cool videos that you've made to watch this play out?

Adeene Denton: Making the videos is the best part, I'm not going to lie to you. I don't have any good videos of this impact. We have videos. They were really funny and I don't have any of them. But basically, one of the funniest things that happens is during the splat, Pluto gets hit forms the impact. And then because it's happening at an angle, Pluto starts rapidly rotating for a little bit, kind of spins like a top for a while before it calms down. So, that's what I can tell you about this impact.

Sarah Al-Ahmed: You're doing this in 3D, right?

Adeene Denton: Yeah.

Sarah Al-Ahmed: Because I know a lot of the previous versions of this kind of research were done in two dimensions, which I feel like probably loses a lot of the complexity of the system.

Adeene Denton: Yeah. And I can say this to someone who was doing the 2D simulations as well. Wow. Why would they do that? It just depends on what you're trying to study.

The problem with doing things in three dimensions is usually, you have to do them at relatively low spatial resolution just because of computation time. What you can get out of a 3D simulation usually depends on how much computing power you have and how much time you have to sit around and wait.

So, in this case, we were able to strike the balance between looking at Pluto at a high enough resolution to see if we could reproduce Sputnik Planitia. And also doing that fast enough to get an answer while we're all still alive.

So, yes, it's the only way to really try to reproduce an elliptical crater as well, right? Because in 2D, you're limited to vertical impacts, which means the Sputnik Planitia that you're making is going to be circular, which while it's not. So, that was one of the main motivators behind using an SPH code is because we could do 3D and see what happens when you actually try to make an elliptical impact basin on Pluto.

Sarah Al-Ahmed: Well, if we think a good chunk of this is buried underneath all of the ice, that can't be the case for all of the material from this body. Some of it must have flown off around the system. Is there a possibility that some of those leftover bits are anywhere on the surface of Pluto or even Charon?

Adeene Denton: Yes. Not all of the impactor makes its way out to Pluto. Some debris is left in the system. And if you go to our paper and you happen to look at the figures, you'll see Pluto made up of all those little particles and you'll see a bunch of particles that are also just kind of floating out there in the system.

Most of those particles will probably re-impact the surface of Pluto, but a small fraction of them will probably be out in the system for a while. That's not something that codes like these are good at tracking. So, an impact code like SPH is designed to look at what happens during impact. And that means we're looking at what happens at a 10th of a second of time. So, we run out these codes to, in this case, six hours, 10 hours.

The lifetime of debris from an impact in a system like Pluto could be on the order of years, tens of years. And that's not something that these codes are designed to do. You can't have a code that's checking every 10th of a second what's going on run out for 10 years because you'd be dead. So, that's an astrophysics problem, not a geology problem.

So, yes, I agree. It's highly likely that an impact like this, some material ended up out of the system entirely. Some of it ended up on Charon probably. And a lot of it got scattered all around Pluto. But the only way to really go back and try to figure that out would be to, again, Pluto orbiter. And it will be a little bit difficult because most of the rocky core goes straight down preserved at the base of Pluto's ice shell. Why? Because rock is much stronger than ice.

So, most of what's getting scattered across the system is ice. And trying to reconstruct what happens when you have ice ejecta land on Charon which is also made of ice, that's tough. That's a challenging question. It's not impossible, it will just be difficult to figure it out.

And even though I can't tell you for sure, "Ah, stuff landed on Charon," definitely. Because we're pretty sure that we have rocks from the earth that landed on the moon from previous asteroid impacts and we have lunar meteorites that have landed on the earth, I think I can pretty comfortably say that that probably also happened in this scenario. I just can't tell you how much of it or where it is.

Sarah Al-Ahmed: Without actually going there and having an orbiter. I mean, even that impact on Vesta you were talking about earlier, still rains rocks down on earth, all this distance away all this time later.

Adeene Denton: Oh, yeah.

Sarah Al-Ahmed: I'm sure there's little bits of this impact just flying around out there in the Kuiper Belt peppering everything.

Adeene Denton: Oh, absolutely.

Sarah Al-Ahmed: Now, I want a Pluto orbiter. Thank you. I got to put that on my list of things I want to see.

Adeene Denton: That's so optimistic of you. But I agree, we can advocate for it. I do think it would be incredible to go back. But obviously, realistically, we need the Uranus orbiter probe first. But then, Pluto.

Sarah Al-Ahmed: Then Pluto. Honestly, if only we could have orbiters around every world in our solar system, imagine what we could learn if we could compare all of these. It is so intense what we could learn. But in the meantime, we're just kind of piecing together everything from what we have.

Adeene Denton: Yup. It's so hard. They ask us to choose and you just ... It's so sad. But there's still a lot we can learn from Pluto and from the New Horizons data that we have, so we're certainly not done trying to learn about Pluto.

Sarah Al-Ahmed: Beyond Pluto, are there any other bodies in the solar system that we can look to, to use this kind of understanding of the splat that we've seen with Pluto and potentially on the far side of the moon to look at other bodies?

Adeene Denton: Yes. So, like I mentioned before, eight out of 10 of the largest Kuiper Belt objects are binary systems with one large dwarf planet-sized body and a satellite. And some of these are more complex than others.

Your listeners might be more familiar with say, Eris, which was the next dwarf planet discovered after Pluto by Mike Brown, which eventually caused Pluto to no longer be a planet. But there's also the Haumea system which is incredibly complicated.

Haumea is not a binary, it's not technically a trinary either. It has two satellites, Hi'iaka and Namaka. And it also has a ring. And it also has a collisional family. Haumea has everything. It's so complicated. And it's also spinning with a period of four hours, which means it's spinning so fast that the entire body is elongated so that it looks a lot more like a loaf of French bread than a planet.

Sarah Al-Ahmed: Little like 'Omuamua.

Adeene Denton: It's giving 'Omuamua, a little bit. I'd love to see Haumea shape models because they just look like French bread to me. But the Haumea system is so interesting because it has all of these things and people have been trying to figure out, how do you have all of that? Does it happen all at once? Could one giant impact have caused Haumea to spin off enough debris to create this whole collisional family, a ring and two moons, and start spinning fast enough to look like French bread? Could it have been one impact? Did it need more?

This is a system that we really don't understand. And so, that's kind of the next big step is trying to tackle some of these more complicated systems in the Kuiper Belt.

Sarah Al-Ahmed: Thanks for joining me. I feel like so often, I'm told that I have this high enthusiasm for space. Then I'm constantly just having a great time and laughing about it and I feel like I found a kindred spirit in you. You definitely have a joy to the way that you're doing this research and I absolutely love it.

Adeene Denton: We're just having a good time. I blow up Pluto and I say, "Oh, what happened this time, Pluto?" And then I do it again. It's good fun.

Sarah Al-Ahmed: That sounds like a lot of fun. And now, I got to go figure out how to make my own SPHLATCH.

Adeene Denton: Oh, yeah. I encourage you to try.

Sarah Al-Ahmed: Well, thanks so much for joining me, Adeene.

Adeene Denton: Yeah, thanks for having me.

Sarah Al-Ahmed: Since we're talking about Pluto, I want to send a thank you to all the space advocates worldwide who helped make the New Horizons mission to Pluto possible. This is one of those missions that very nearly did not happen. From 1990 to 2000, NASA considered and rejected four missions to Pluto. Our members, school kids, the National Academy of Sciences and planetary scientists all over the United States wrote the US Congress to support this mission.

And finally, in 2001, New Horizons was approved for its mission to the Kuiper Belt. We'll have to get on that Pluto orbiter idea at some point after we've checked off all of the other big priorities on the Planetary Science Decadal Survey. And I'm already brainstorming ways to celebrate next year's 10th anniversary of the flyby of Pluto. If you have any ideas or requests for guests, let us know.

Now, it's time for What's Up? with Dr. Bruce Betts, our chief scientist.

Hey, Bruce.

Bruce Betts: Sarah.

Sarah Al-Ahmed: Are you getting in that Halloween spirit? I hear the little zombie-ish coming out in you.

Bruce Betts: [inaudible 00:49:05].

Sarah Al-Ahmed: But I spoke with Adeene Denton from the University of Arizona about Pluto. It's been a long time since we talked about Pluto on the show. And about their team's idea that perhaps there's other ways to explain what happened with Pluto and this giant impact that's near the equator without having to suppose a subsurface ocean. There's a lot of complexity there and we've only flown by Pluto once, so trying to disentangle all these facts and try to figure out what actually happened there is kind of super complicated.

Bruce Betts: It's true, but people will have trouble proving it wrong for a long time.

Sarah Al-Ahmed: That is a benefit, right? Until we get Pluto orbiter.

Bruce Betts: Yeah. I mean, wow. Pluto is just, like so many places we've gone in the solar system, it was just a shocking surprise how complicated and potentially active in geologic sense. But at the very least, just ridiculously complicated geology which is not what most people at least expected from a body that's sitting that far out. You'd think it'd just be a frozen simplistic mess so it was the wrong choice as opposed like Triton which is a frozen weirdo mess.

But also, same thing. Way out there, very surprising. Had geysers. You've got glaciers moving around on Pluto. You got the cool different ices. It's all sorts of excitement that wasn't expected.

Sarah Al-Ahmed: Yeah. I know that everyone is super into that actual image of Pluto where you can see all the details, but I think that the images of Charon or Charon, the moon, where you can see that it's all red from those kind of tholins that have been blown off of Pluto, that's insane.

Bruce Betts: Yeah.

Sarah Al-Ahmed: And then, that kind of really angled shot that we got where you can see almost the really thin clouds almost on Pluto. It's so insane what's going on there and so much more complex than I ever thought. But how was I supposed to expect anything? We've only flown by Pluto once.

Bruce Betts: Well, that's the point. It's also yet another reminder of why we explore. We don't know what we'll find. And we keep being reminded of that over and over again. And then we find, "Oh, that's neat." And then we ask questions they're addressing which is, how do you get something like that and how do you keep it stable when you're on average 40 AU away from the sun, 40 sun-earth distances?

Sarah Al-Ahmed: It gets really complex when you get that strange angle in there. And it's so silly that this is what my brain goes to. But at Eclipse-O-Rama and at a few other events that I've been to, we do this thing where you do crater simulator essentially, where you throw objects into a bunch of corn starch with layers so you can see what it looks like.

And Adeene pointed this out in the conversation that most of them are pretty round, no matter what kind of angle you come in at. But if you come in at a really steep angle, you can get those really weird-looking craters. Have you done this? I'm sure you've done this before.

Bruce Betts: I believe I wrote the text that was used in the experiment. Not necessarily at Eclipse-O-Rama, not necessarily means I've done it but, yes, I have done it. I've done it in various ways. And it is neat and I think it's even more true although I could be wrong, but it's certainly true with hypervelocity impacts which is not at least what my arm throws, unfortunately. You get that where most angles will create a circular crater which is why most craters you look at on the moon or anywhere else are circular. You really have to work your obliques to get the non-circle.

Sarah Al-Ahmed: Other than Pluto, what are some of the coolest impacts or impact craters that we've seen in our solar system?

Bruce Betts: Well, if you had to find this coolest, Meteor Crater, because we can go there.

Sarah Al-Ahmed: I still haven't been.

Bruce Betts: Oh yeah, it's really lame. It's totally not a problem that you haven't been there. No, it's just neat, especially when you're planetary fanatic and impact cratering has been the dominant geological process in most of solar system history and still is for most places to actually see, even though it's just a wee little one kilometer crater, to see it and know that a rock came from space and made that thing, it ties it all together. But again, you haven't missed anything, I'm sure. I mean, I am sure. No. Okay. You did. You should go there someday.

Sarah Al-Ahmed: It's on my space life goals list.

Bruce Betts: Okay. Good, good, good. But if you go up to the big old hand craters, I mean, let's take the moon. It's a nice cratered body. It's fun and exciting that all those giant hundreds of kilometer wide basins got filled in by dark lava. So, we get those neat circular features. Basaltic lava filling in amongst the lighter materials formed the islands.

We got the South Pole-Aitken Basin which is one of the largest craters in the solar system. But it's so old that it wasn't recognized as early and isn't as well-defined, but is this huge thing that now it has a lot of interest and people wanting to go to the South Pole and ski now and look for water ice.

Mercury, Caloris Basin, very spiffy keen, also one of the largest in the solar system. Interesting little historical tidbit that when Mariner 10 flew by Mercury in the early '70s, it flew by and only saw one side of Mercury due to the orbital geometry flying by. Caloris Basin was right on the edge of that coverage. So, you had to wait 40 years for spacecraft that went to Mercury.

Sarah Al-Ahmed: Messenger?

Bruce Betts: Yeah.

Sarah Al-Ahmed: Yeah.

Bruce Betts: You win. You could quiz answer. Messenger to see the other half of Caloris Basin. I don't know. That's just your little random trivia. Craters on Mars have almost unique, depending on possibly unique morphology to a lot of them which is a fluidized ejecta blanket. So, it came out and it didn't just throw out the ejecta and the land on the surface, it flew, came out and hit and flowed. That's true not for all craters on Mars, but a certain size range and certain latitudes.

Basically, the concept is that it vaporized ice in the subsurface or otherwise created a temporary liquid slurry-type muddy thing that flowed out. So, you see all these flow features, but there are other places you don't see them.

So, I published a paper about thermally distinct ejecta blankets way back when. But the silly part of it is I really want an acronym for them because you're just referring to it over and over. It ended up being EDITHs. No one uses it.

Sarah Al-Ahmed: EDITHs.

Bruce Betts: Ejecta distinct in the thermal infrared. But for a long time, they were TIRDs, "thermal infrared". I have real opinions about acronyms. But anyway, I've strayed far from the subject.

Sarah Al-Ahmed: What's our random space fact this week?

Speaker 5: Random Space Fact!

Sarah Al-Ahmed: Trick or treat?

Bruce Betts: A little of both. A little of both. So, interesting. Edmund Halley, that guy, the guy with the comet named after him, he suggested that the Northern Lights, what would be called the Aurora Borealis were formed and occurring having something to do with earth's magnetic field.

And 100 years or so later in the early 1800s, Sir Edward Sabine established the first magnetic observatory at the University of Toronto or what's now the University of Toronto, and put up various things and figured out that indeed, earth's magnetic field had something to do with this. And he also figured out that their worldwide magnetic disturbances can occur and that they're related to the number and strength of sunspots.

So, thank you, Sir Edward Sabine and Edmund Halley for just thinking big. Thank you.

Sarah Al-Ahmed: I imagine that's very similar to solar eclipses and those kinds of things back in the day. Suddenly, we're at solar maximum. You get these crazy lights in the sky, something you haven't seen before because you live at a more equatorial latitude and suddenly, that's got to be so scary. I mean, beautiful but terrifying if you don't know what's going on. So, I'm glad that research finally got done.

Bruce Betts: I think the people living in those latitudes had probably figured out they weren't getting hurt by them, but I could be wrong. But yeah. Weird answer though. I mean, hey, there are lights in the sky, it has to do with the sun.

Sarah Al-Ahmed: At night?

Bruce Betts: All right, everybody, go out there and look up the night sky and think about good humor, Eskimo pies, bicycles, missile pops. Thank you and good night.

Sarah Al-Ahmed: We've reached the end of this week's episode of Planetary Radio, but we'll be back next week to learn more about Spaceport Nova Scotia, the first commercial spaceport in Canada.

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