The slow evolution of Europa

Sarah Al-Ahmed: Understanding the formation of Europa, 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. Jupiter's moon Europa is one of the most exciting locations in the Solar System in the search for life, but a crust of ice, guards the secrets of its potential subsurface ocean. This week, Kevin Trinh from Arizona State University joins us to discuss his research on Europa's formation history and the consequences for the moon's habitability. The Planetary Society's Jason Davis looks forward to the upcoming total solar eclipse in 2024. Then Bruce Betts, our chief scientist, joins me for What's Up and a commentary Random Space Fact. On April 8th, 2024, a total solar eclipse will pass over Mexico with the United States and Canada. There are many types of eclipses, but of all the various ways that the sun, moon and Earth can align a total solar eclipse is the most spectacular. Here's Jason Davis, The Planetary Society's senior editor to talk about our newest article, Total Solar Eclipse 2024: Why it's worth getting into the path of totality. Hi Jason.

Jason Davis: Hey, Sarah.

Sarah Al-Ahmed: So many years ago, it was 2017 and I went to go see my first total solar eclipse, and I tell you, I still think about it to this day. I still have dreams about it. It was that impactful of a moment. Did you get to go see that eclipse?

Jason Davis: No, I didn't. I have actually never been in the path of totality for a total split.

Sarah Al-Ahmed: We've got to get you there, especially this time.

Jason Davis: Yeah, yeah.

Sarah Al-Ahmed: So why should people try to get to the path of totality?

Jason Davis: There is a huge difference between just being able to experience a partial solar eclipse and actually being in the path of totality, and so if you're in a partial solar eclipse, you're going to see the moon kind of take a bite out of the sun, and that is a really cool thing to see. I've seen it a couple different times, but if you have the chance to see totality, that's where the actual deep part of the shadow hits that travels across the globe. If you have a chance to get into that path, you'll actually see something completely different, and that is where the sun is completely obscured or at least the main disk of the sun is completely obscured for a couple minutes, and the sky turns twilight and it's just, as you said, this is such a memorable event. People that see it just say that there's just nothing like it, that you have to do it.

Sarah Al-Ahmed: I would totally agree. You can try to learn what it's like, but until you're standing there with sunset and all directions and that the stars and planets come out in the middle of the day, you just can't fathom what it's like.

Jason Davis: Yeah, yeah. I'm definitely going to go see this one and it's fortuitous that this is the first of three articles we're going to do on this as I'm planning to go see it myself. I'm kind of gearing up to be an eclipse nerd for this one as well. It sounds amazing and I cannot wait to experience it.

Sarah Al-Ahmed: And I think it's really important to underscore that there's a very small path along which you can actually see the totality of the eclipse. If you're even a few miles off in the wrong direction, you're not going to get to experience the full glory of this event. But once you do get to that path of totality, there's a lot of really cool cosmic phenomenon that you'll experience while you're there.

Jason Davis: Besides the sky turning dark and like you mentioned, it looks kind of like sunset in all directions on the horizon. You'll get to see the corona of the sun. Hopefully the sky's clear for you, and that's kind of this with the gas where white structure that kind of blows out from the sun. Actually the sun's atmosphere, stars and planets come out. You'll be able to see at least some of the brightest objects in the sky for those few minutes. Another thing that people talk about is animals, depending on what animals and insects are around, and this really confuses animals that all of a sudden the sky gets dark in the middle of the day. There's been scientific studies that show animals engage in their nighttime rituals. They think it's time for bed essentially. Even nocturnal animals might do the opposite. They might think it's time to get up, just all these different things to pay attention to and temperatures is another thing. Drop in temperature of at least five degrees Celsius, which is 10 degrees Fahrenheit depending on the weather at your location. Just a lot of really neat things to see if you managed to get into the path totality.

Sarah Al-Ahmed: Yeah. In 2017, we actually measured the temperature as the eclipse went by and afterward the change in temperature was so dramatic it kicked up a wind that almost blew away my tent. So hammer your tents down during this event is what I'll say. But how many people actually live along this path of totality?

Jason Davis: So about 635 million people will be able to see some part of the eclipse, but only 43 million people live in this path of totality, and that's just 0.5% of the world's population. So a very small number of people will actually live there and get to experience it firsthand, and so if you have the means and you're able to travel to get into it, it's very much worth it if you could do it.

Sarah Al-Ahmed: And this is going to be the last chance to actually see a total solar eclipse in the contiguous United States for quite a while, right?

Jason Davis: Yeah. This is the last one until 2044 in the contiguous US, and that one will only kind of touch the very tippy top of the US like Montana and the Dakotas, but this one just cuts a swath right across the country. This is your best chance and your last chance to see it for a while, so totally recommend it.

Sarah Al-Ahmed: Well, I'm looking forward to you getting to experience your first total solar eclipse, Jason.

Jason Davis: Yeah, great. Looking forward to it.

Sarah Al-Ahmed: Jupiter's icy moon Europa has captivated scientists and the public for decades. We could say that it began in 1610 when the Italian astronomer Galileo Galilei pointed his homemade telescope at Jupiter. He discovered the now famous Galilean moons Io, Europa, Ganymede, and Callisto. Centuries later in 1989, NASA launched the Galileo spacecraft to investigate those moons and the planet that they orbit. The Galileo mission spent nearly eight years in the Jovian system exploring Jupiter and the worlds around it, but of those moons, Europa stood out. Data from the spacecraft suggested that this moon held hidden wonders beneath its icy crust, a potential subsurface ocean with more water than all of Earth's oceans combined. Scientists including our guest, Kevin Trinh, are still combing through the spacecraft data today. Kevin is a PhD candidate at Arizona State University's School of Earth and Space Exploration. As a first generation student, Kevin began his academic journey at Bowdoin College in Maine, USA. Now, he continues his graduate studies at ASU under the guidance of Dr. Joseph O'Rourke. NASA has recognized Kevin's determination and talent. He's part of their Future Investigators in NASA Earth and Space Science and Technology or FINESST program, which awards grants to promising graduate student research. Kevin's studies focus on Europa's internal differentiation, evolution, and potential habitability. He uses numerical models and fundamental theory to investigate the geophysics and geochemistry of icy moons. These things could impact the formation of these worlds subsurface oceans, but also their metallic cores and potentially seafloor volcanism. Right now, the European Space Agency's Jupiter Icy Moons Explorer, or JUICE mission is cruising through space on its way to Jupiter and it'll arrive in 2031. The spacecraft will investigate three of Jupiter's moons, not just Europa, but also the two giant moons, Ganymede and Callisto. Each of these moons with their unique geologies and potential subsurface oceans hold keys to understanding the conditions necessary for life in our Solar System. But for a real in-depth investigation of Europa, we're going to need to wait for NASA's Europa Clipper mission, which is set to launch in 2024 and arrive at Jupiter just a little bit before JUICE in 2030. Kevin's newest paper called Slow evolution of Europa's interior: metamorphic ocean origin, delayed metallic core formation and limited seafloor volcanism, was published in Science Advances on June 16th, 2023. He and his team are using Galileo's data to set the stage for future missions. Hi Kevin, welcome to Planetary Radio.

Kevin Trinh: Hi, Sarah, nice to meet you as well.

Sarah Al-Ahmed: So you're a grad student at ASU and also the lead author on this paper, and I got to say, grad students deserve way more credit for the amount of work they're putting in behind the scenes. I feel like it's really easy to just see the professors and the big name doctors on all the papers, but in my experience, it's the skill and the coding and the data processing of the grad students that contribute just as much to these projects as everybody else.

Kevin Trinh: Yeah, thank you. I definitely feel like a lot of hard work was put into it. It's nice to see that all of my time spent, I guess not just writing and doing code, but the act of putting together a story, presenting it, refining the idea has come into something that I can now read and it's now in my own zone zero, which is nice. I've also am very thankful for my mentors, Joe Rourke, my PhD advisor and Dr. Carver Pearson. Both are the co-authors of this paper. They really made sure that my hands were on the steering wheel, so I was learning a lot from putting together this paper and it's been a great experience.

Sarah Al-Ahmed: What initially drew you to Europa as a research topic because there's a lot of cool icy moons out there. Why Europa?

Kevin Trinh: Yeah, so I guess this also ties into why I chose ASU because one factor into me choosing a grad program was doing research related to icy moons, particularly Europa. So I tried to get a sense of what projects I might be working on going into a program. I actually first learned about Europa in fourth grade. Before that, in kindergarten, I had this space encyclopedia that I picked up at a bookstore. It talked more generally about space, like how sun was really hot, some planet were really big or really small compared to Earth and everything felt, I guess, otherworldly. But I learned in fourth grade about Europa, how there's strong evidence for an ocean underneath its surface, and NASA had the saying, "Follow the water." When it comes to the search for life, but I always felt like a lot of the conversation was on Mars and it didn't make sense to me as a kid why we weren't talking about Europa as much. So I always felt like Europa was a, I wouldn't say hidden jumps into pretty high profile moon now, but I always had this appreciation for Europa because of it. Now, there's a childhood meeting aside from me wanting to study it in grad school now.

Sarah Al-Ahmed: Yeah. I think I learned about Europa from, man, this is a throwback, The Magic School Bus, Solar System computer game. I think, is how I learned about Europa. These things really impact you when you're a kid and I love to see you following that life path to studying it and teaching us more about its evolution, because there's a lot that we don't know about that moon, and it's an incredible target to study primarily because of its subsurface ocean and the search for life.

Kevin Trinh: I feel very lucky to be doing what I want to as a kid. And there are so many interesting questions, not just Europa, but the icy moons' community in general.

Sarah Al-Ahmed: So what is the main proposition of your paper?

Kevin Trinh: Europa is famously known as an ocean world that has earth-like conditions at the seafloor in terms of temperature and pressure. The water is in contact with the rock, so it's natural and reasonable to suspect there might be the kind of chemistry between the rock and water that might be favorable for life as we know it. And a big part in assessing Europa's habitability is understanding how the moon formed and evolved over time. One of the main premises of our paper is that Europa is much smaller than the earth. You need about a hundred Europas to add up the earth's mass, so the physics and chemistry might not be very earth-like. We're biased as people live on this planet to think about things that are familiar to us, but for such a small moon, you might have a lot of changes in the moon's eternal structure over time, and that's going to impact things like the prospect for volcanoes at the seafloor that gives us heat and chemical energy into the ocean. Metal core formation that tells us where the metal is and also the formation of the ocean. That's going to have something to do with the composition of the ocean and just as we care about the makeup of the atmosphere that we breathe, fish or just aquatic things are going to care about what's in the water.

Sarah Al-Ahmed: What do we think the internal structure of Europa is like?

Kevin Trinh: There's most likely an icy shell with a salty ocean underneath. Beneath that, there's either one or two, I guess big layers that we are interested in. There's some kind of rocky component, we might call it the rocky mantle, and then depending on how hot Europa got in the past, it might also have a metal core at the center. Many studies assume that Europa has a metal core at the center, but what we did was put a question mark on that metal core showing that it is a outcome, but it's not a guaranteed one. Given what we understand about how planets and moons form.

Sarah Al-Ahmed: Every single time, it's way more complicated than you think it might be.

Kevin Trinh: All planets and moons are complicated in their own ways. A lot of literature assumes Europa had a metallic core to start out with or maybe formed shortly after the moon itself forms, and that's more similar to Earth in the sense that you start out with a spread out cloud of dust and gas, all that matter comes compact into a sphere and that's going to generate some heat. And the more mass you have, the more heat you can get, and for Earth you can get hot enough during the accretion process. So the formation of this planet or the moon, so that you can melt metal and that allows for the dense stuff, the metal to migrate towards the planetary sensor. For Europa, you might retain little, if any heat from the accretion process, even if you were to assume that all of its gravitational energy converted into heat, you are still not guaranteed to form a metal core. So I think that's one big thing we need to keep in mind when studying small moons like Europa compared to a large rocky planet like Earth. The Earth is also very unique compared to other rocky planets because of the moon forming impact, that's going to generate a lot of heat that will also exchange some materials. So the Earth is also a unique case, but for moons of giant planets, so like Europa and other icy moons like Ganymede, Callisto, Io, they form in a disk of material around a big planet like Jupiter. So the dynamics may different, this is a detail that is really important when we're trying to think of how Europa performed and evolve over time.

Sarah Al-Ahmed: You pointed out Europa is very small, but it has a lot of water on this tiny moon. If it does in fact have a subsurface ocean, how much water are we talking here?

Kevin Trinh: There should be twice as much water as all its Earth's oceans combined. So even though Europa is slightly smaller than our moon, water is pretty common in the Solar System as we get farther from the sun. The hard part is that it often exists as ice, but in a lot of ice and moons, especially ones where you have a lot of VOC for radioactive heating or you have tidal heating too, that can melt some of the ice below the surface and it might maintain ocean depending on which moon you're talking about.

Sarah Al-Ahmed: If you're doing a scale comparison, it's like if the Earth is a basketball, then Europa is this size of a golf ball maybe with twice as much water as our entire planet? That's amazing.

Kevin Trinh: Yeah. It's weird because we live on the surface of the Earth and the surface is really just this, I hesitate to call it a small part of a planet because that's where a lot of the action happens, and that's a lot of what we see. But a lot of Earth's surface is just continental land masses, two thirds of it about is water, whereas a place like an icy moon, the entire surface is ice, and then there might be a global ocean underneath or a regional for some other moons, there's a lot of ice out there, and for some moons there's plenty of heat to go around to melt some of the ice and keep it melted for a long period of time.

Sarah Al-Ahmed: But even if it has an ocean, that doesn't necessarily mean that the situation is favorable for life, but in this situation, we've got a subsurface ocean that's touching this rocky mantle and that exchange of chemicals between the mantle and the ocean could help make it more favorable for life depending on what's going on down there.

Kevin Trinh: Yeah. So that's one of the big reasons why Europa stands out compared to other icy moons. I'd say, Europa, Enceladus are the two most promising ones for habitability. I think, the answer about which one's more habitable might depend on which scientists you ask, but yes, the water at Europa is likely in contact with the rocky seafloor. The same cannot be said for larger ice and moons like Ganymede and Callisto. So the neighbors of Europa, they're much bigger and Callisto is about half it's mass and water and ice. Ganymede is 30 to 50 weight percent, so these are very, very water rich moons. The problem is that there's so much pressure at the seafloor that the water compresses into high pressure ice, and that's going to limit the extent to which water and rock can interact with each other to release the kind of soleus that life might want.

Sarah Al-Ahmed: That's a great point. It's like if we're trying to rank places by their potential habitability, a moon that's smaller might actually be more favorable to these chemical exchanges that begin life, if it's got the right hydrothermal or volcanic conditions. Which spacecraft data have we used to kind of figure out Europa's internal structure or at least what we know about it so far?

Kevin Trinh: I'd say the Galileo spacecraft, which launched in the Knight's '90s is the main one. Understanding of Europa's interior structure mostly comes from gravity and magnetic data. So the gravity data allows us to get a sense of how a mass is distributed inside of Europa, and we don't think of specific rock compositions necessarily. We assume a number of layers, so water and ice, they have similar density, so we assume they're the same thing for the sake of gravity modeling purposes and then rock and then metal. The density differences between them are large enough where we can treat them as different layers, and then we try to find internal structures that are compatible with the kind of gravity that the spacecraft experiences. So as spacecraft flies by a body like Europa, it'll experience different amounts of tugs as it passes by the moon, and that's going to be related to how much mass is between the spacecraft and Europa. And then there's magnetic data. The Galileo magnetometer detected an induced magnetic field, so the Earth has our own magnetic field generated by the vigorous convection of liquid metal in our core. Europa doesn't have that core hosted dynamo. Instead, Europa exists inside Jupiter's magnetic field, but there's a perturbance in the magnetic field once we pass by Europa. That is best explained by a global conductive layer and a salty ocean is a very strong explanation for that. So magnetometer grab a data from the Galileo spacecraft, is I'd say our biggest contributor to what we think is inside of Europa, but there'll be Europa Clipper that will launch October 2024, at least that's the schedule, and it should arrive towards the end of the decade, so I'm very excited about that too.

Sarah Al-Ahmed: Having a dedicated mission to Europa is going to be great, but thankfully we also have the Jupiter icy moons' explorer from the European Space Agency that's on its way to Jupiter right now.

Kevin Trinh: Yeah. JUICE is going to be really exciting. And so from my understanding, JUICE is going to be focused on Ganymede, but it'll also have flybys of Europa and Callisto, and I actually think that's advantageous. There's a lot of synergies when we have more data on not just Europa, but its neighbors as well, because these moons exist as part of a system and they do interact with each other in the sense that Io, Europa and Ganymede, they're an orbital resonance for each other. So every two times that Io orbits, Europa orbits once, and every two times, Europa orbits, Ganymede orbits once. That affects how much tidal heat that each body gets with Io getting a lot, Europa getting a lot, but not as much. Again, we're getting a very small amount, understanding why these moons are different despite forming all around Jupiter is going to tell us a lot about Europa and how the other moons formed as well, so ideally the physics and chemistry we use to describe how Europa forms should be consistent with the other moons as well.

Sarah Al-Ahmed: Your paper supposes that Europa underwent potentially a kind of slow evolution. What are the other potential formation scenarios that we're talking about here?

Kevin Trinh: Our idea of Europa evolving slowly is pointing out that a small moon like Europa could have formed as a cold mixture of ice rock and metal or cold mud ball, put it that way. And over time as we have heating from radioactive isotopes and tidal heating, we'll eventually melt stuff and slowly convert into a layer structure. The alternative is to assume that Europa was layered to begin with, and that's a pretty common assumption in the literature, but it's a hard one to support given that if we assume all of Europa's accretional energy got converted into heat, then we still might not have enough of a temperature increase to have that layered start. So I find it hard to argue for what's typically assumed, which is Europa started out layered. Instead, we have to overcome these hurdles. So while there's a lot that we don't know about Europa, we do have a good idea of how much, what the mass and radius of Europa is, and that's going to put some constraints on Europa's formation conditions.

Sarah Al-Ahmed: What is the formation timeline that you think is most likely given the data that you've analyzed?

Kevin Trinh: Europa most likely forms, I'd say between three to 5 million years after calcium aluminum inclusions or CAIs. Those are the first solids who have condensed in the Solar System, so they provide I guess a time reference point for us, but it's also a physically significant one because the earlier we form in the Solar System, the more aluminum-26 we have. That's a short-lived radioactive isotope and that contributes a lot of heating and it's very sensitive to our uncertainty in the formation time of Europa.

Sarah Al-Ahmed: When I was first studying Planetary Science, I was honestly shocked to find out how much of a world like Earth's internal temperatures because of this radioactive material. I always thought that it had to be mostly the heat from formation, but the radioactivity contributes a large amount of the heat inside of these worlds.

Kevin Trinh: Yeah. The amount of heat that each planetary body gets, it definitely varies. There's a different balance. So Earth is like you mentioned, primarily driven by radioactive isotopes, but the accretional heat is also significant because there was enough to form metal core during the formation of Earth itself. I would like to say radiogenic heating is very important for a lot of bodies, but there are cases where a moon might be small enough and not very rich in rock compared to ice, so that you don't get that much radioactive heating compared to other heat sources like tidal heating. I think Enceladus is the best example of that, a very ice switch moon with a small enough size so that the heat that you generate from the interior can conduct away pretty fast. But it's always a balance, a different balance of radioactive isotopes and tides from whatever moon or planet you're looking at.

Sarah Al-Ahmed: This paper proposes that Europa's oceans may have this metamorphic origin, and I'm sure a lot of listeners are throwing back to their early science classes about rocks, but what would it mean if Europa's ocean had this metamorphic origin?

Kevin Trinh: To put things in context, when I use the word metamorphic, I mean that the ocean itself formed as a result from warming up the rocks. The alternative is that we melt ice directly, and since water is much less dense than rock and metal, the water should migrate to the surface and that can form the ocean. Now, if you form the ocean metamorphically, we're taking the oxygen, hydrogen that's directly bonded to the hydrogen minerals inside of Europa's rocky interior, and at high temperatures the hydrogen and oxygen will be released from the rock, and that can be combined into a fluid, probably a super critical fluid depending on the pressures where the VOC is dehydrating. But this fluid is really hot, it's really reactive and it's low viscosity, less than water. It's going to want to shoot up to the surface. I haven't done modeling myself, at least not in depth modeling on the dynamics and timescales of how that fluid migrates from the interior to the surface, but the ocean formation process for a metamorphic origin is going to have high temperature and pressure conditions. So that's going to govern the rate at which chemical reactions proceed, and that's going to be a very different physical scenario compared to form the ocean by melting ice and having that water percolates the surface.

Sarah Al-Ahmed: If that was the case, would that mean that the materials on the seafloor were also more hydrated?

Kevin Trinh: There are different ways and different times when you can hydrate rock. So one case is at the beginning, when you accrete the material that eventually formed Europa, maybe the ice melted and hydrated the VOCs then and there, or maybe the rocks accreted directly as hydrated material. And the seafloor might remain hydrated over four and a half billion years, Europa's entire lifespan, and that would be due to the ocean keeping the seafloor at low temperatures. If that's the case, we shouldn't expect too much rock water reactions to be happening, but it's also possible that Europa got warm enough to dehydrate most of the seafloor as well and eventually to volcanism or for complete dehydration, but then you have hydration from the top down. Well, after Europa formed and that can release some heat and chemical energy. So there are a lot of possibilities regarding rock water reactions, but yes, the formation of a metamorphic ocean has some implications for when rock water reactions can happen and how extensive they may be.

Sarah Al-Ahmed: Given that it might be going through this long evolution over time, and there's all these different factors, like its radioactive content and its interactions with Jupiter. Do we even know if it's at the phase where it's cooling down or if it's still heating up over time?

Kevin Trinh: So my models allow for both to be possible. What really will determine whether Europa is heating up or cooling down over time, is where the tidal heating is distributed. The tidal heating could be mostly concentrated in the ice shell or there might be a substantial fraction of it, also in the rocky interior. The distribution of tidal heating has to do with the radiology of Europa's interior and that's not well understood. So our certainty is really going from all tidal heating into the ice shell to a lot of it and being in the rocky interior as well. If there's a lot of tidal heating in the rocky interior, then Europa could be warming up at present day, and you can also have seafloor volcanoes if there's enough tidal heating there. But if the tidal heating is concentrated in the ice and there's not enough in the rocky interior, then we might not have volcanism. I'm actually a bit pessimistic about volcanism. That's the most important variable in my opinion, when it comes to, what my models say about Europa both cooling down and warming up as possible, but we really need to know about tidal heating.

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

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Sarah Al-Ahmed: There's a lot of factors that go into the internal heat of this world, many of which we don't fully understand, but how would that level of internal heating impact the potential seafloor volcanism?

Kevin Trinh: So the amount of heating that the rocky interior experiences will determine how much rocks can melt and where it melts. So the closer that we have magma to the seafloor, the easier it is to form a volcano because not only do we need to have magma to spew out, we need that magma to reach the seafloor. If the magma is really deep in the interior, then it's harder for volcanoes to form because now that magma has along a road trip, it needs to survive through. We need to understand how much magma gets produced, but also just the dynamics of how magma can transport and in general, the more heat is going to be better for the prospects of seafloor volcanoes.

Sarah Al-Ahmed: And the better prospects we have for seafloor volcanoes, the more likely it is that potentially it could be habitable, right?

Kevin Trinh: Yeah. So water is not the only thing that life as we know it needs. We need some kind of, I guess this eco lube oil, so heat and chemical energy that's going to be good for it. We like gradients, so whether that's a redox gradient, a thermal gradient, it allows things to move and volcanoes are a good way of contributing to that.

Sarah Al-Ahmed: At least with Enceladus, we had the benefit of having these jets and this giant plume that's literally just launching water into space. We know that there's probably hydrothermal vents there, because we could test that water and see that there were silicates and all kinds of interesting things in there, but Europa poses a really intense challenge. I'm hoping that the data that shows there might be plumes there as well may be true because that might make this a little easier for us.

Kevin Trinh: Yeah. There have been some papers on Europa that argue for plume, but those papers are also very controversial as well. Having a plume will be nice if we can measure a plume, get a plume sample at Europa, that's going to be very exciting, not just for the people who work on compositions of Europa, but just the icy moons' community in general. But I'm glad you brought up Enceladus, because Enceladus is active pluming at the South Pole is part of what makes that moon very interesting as well, and also easy to study in terms of getting a sense of what the ocean composition is like. So the planetary decadal survey, this is something that experts in the Planetary Science community gather around every 10 years to put together a document to provide recommendations for NASA and the government on what to do based on the state of our field, what our priority questions are, and an Orbilander mission to install. This is the second-highest priority mission, and I think that's going to be very exciting. It will be a while, I will hopefully be a happy old professor by then, but there's going to be some interesting connections we make and I'm curious to see how things progress from now to Europa Clipper, as well as to the Enceladus Orbilander. So there's going to be a lot of exciting icy moon stuff in the future.

Sarah Al-Ahmed: We've been talking a lot about that decadal survey recently because there's so many amazing missions that it lays the groundwork for. So if anybody wants to learn more about that, I'm going to link to our article that's a breakdown of the decadal survey on this episode, so if you want to read that, you can go to planetary.org/radio and learn all about it. Something I want to come back around to, because this is so fascinating to me, is the idea that Europa might not actually have a fully formed core. It might not have fully differentiated because of the slow formation. How could that possibly be and what would the consequences be if that was true?

Kevin Trinh: One of the big premises behind a study again is that a small moon like Europa probably formed with little heat, probably formed cold, and now it has a longer ways to run before it can get to the temperatures where we can melt metal and kick off metallic core formation. It's a really interesting topic I think because one, it redistributes the iron inside of Europa and iron's really important for redox chemistry. It can play a big role in how energetic reactions support life in the ocean. If there is a metal core, that means the rocky interior or the rocky mantle will be more depleted in that iron, otherwise, if there is no metal core, there's going to be a lot more mixed in with the rocky interior. But metallic core formation also can act as a heat source, so just as the accretion process can convert gravitational energy into heat, the migration of dense stuff like metal to the planetary center will also convert gravitational energy into heat. That's something that we don't think about as much for the Earth, since Earth's foreignness metal core during the accretion process, but this could be a late heat pulse for Europa if it happened.

Sarah Al-Ahmed: This is such a complex question to try to figure out. You created the code for this analysis. What did you have to take into account when you're trying to build a model for the formation of a moon?

Kevin Trinh: I mostly assume different starting points, and then what I do is solve the heat diffusion equation. So this is a very famous equation in physics. There are different features you can add to the equation, which I did, which included things like tidal heating, silicate dehydration that actually consumes heat, so different things contribute heat, others consume it. My first year of grad school was actually spent entirely on just writing code. Luckily, I've had the postdoc work with Carver. He wrote a similar code in FORTRAN for Kuiper belt objects. So I use that as my initial inspiration for just radioactive heating and a body and how the heat can migrate out towards the surface. But I wrote all my code in MATLAB my first year and to include things like the dehydration of silicates and maybe other things in the future, like the formation of metal core, if I get to that, that's going to be very computationally expensive, so I needed to move to a different language. So I switched to C++, very fun, and not to unemploy my research, but the heat, the fusion equation, one of the aether models, that's something that has been done by other people. What makes a model unique are the assumptions that go into the modeling and the processes that include, so in my case, I assume that Europa formed code, it could form as a mixture of rock, metal and ice. Maybe all of the ice is already embedded into the rock as a hydrated silicate. And then I include things like tidal heating, the dehydration of silicates, and as silicates dehydrate, the rock interior actually shrinks because we're releasing less dense stuff and now we're left with denser dehydrated VOC. So a little more knobs, but I think what really makes the code valuable, it's not that I spent so much time trying to invite it, that's a big part of my learning process, but it's how I use it as well.

Sarah Al-Ahmed: I really wish that before I had gone in and studied astrophysics, that someone had warned me what portion of my job was going to be coding because I had no idea going into it. And then you just kind of have to learn. If anybody out there is preparing themselves for a life of trying to get into planetary or astrophysics, you're going to need to learn how to code. That's a big part of it.

Kevin Trinh: Yeah. Coding is just such a useful tool. I was lucky to, I guess halfway through undergrad discover that I really like coding. It almost feels therapeutic for me, at least if I'm not spending months debugging the same problem, which has happened for me. But it's nice, it's like puzzles, just arranging numbers, doing operations and seeing what comes out.

Sarah Al-Ahmed: It's maybe a little melancholy for me that you spent all this time trying to learn more about Europa, because of its wonderful oceans and its potential for habitability only to come around to your research and find that maybe it doesn't have as much volcanism or maybe the core didn't fully formed. Maybe it's not as great for life as we want it to be.

Kevin Trinh: At first it was, because as a kid and going into grad school, I've always been very excited about the possibility of life, and I still am. But over time, I think I've really grown to appreciate the diversity of planetary bodies, even if Europa doesn't have life, I'm still going to think of it as a fascinating moon, and I like to think about why do a lot of moons form around one planet, but look so different from each other. I think the Galilean moons of Jupiter are a great example of that. I mean, Europa was not the only candidate for life. So we can look to and sell this or your other favorite astrobiology target, regardless of whether or not your past life. I think there's a lot of interesting things to learn about it.

Sarah Al-Ahmed: I do have a question about how this relates to the other moons in the system. If the ocean on Europa does have this metamorphic origin, would that tell us anything about the other Galilean moons in particular and what's going on with them internally?

Kevin Trinh: Yeah. I think that gets really interesting actually, when we try to bring up the idea of a metamorphic ocean origin to the other moons. So the four Galilean moons of Jupiter are Io, Europa, Ganymede and Callisto moving in order of distance away from Jupiter. So one big difference about Ganymede and Callisto compared to Europa is that Ganymede and Callisto are much more water rich. We'll need to accrete a lot more ice, even if we have some contributions from silicate dehydration, probably because we don't know of any silicates that are water rich enough to produce the ocean and ice shell of a Ganymede or Callisto if those silicates formed the initial Ganymede or Callisto. So silicate dehydration at least can contribute partly to the composition of Ganymede and Callisto as ocean ice shells, but not completely. I'm more interested in Io actually. So Io is famously known as a very volcanic world. It doesn't have an ice shell today, but I'm very curious whether Io was an ocean world in the past and somehow that water got removed over time. So that's another big question in at least the Galilean moons community. Does the gradient of, I guess, densities or ice rock ratios that we see in the Galilean moons today, did that exist when the moons form or did that revise as a consequence of the moons evolving? So I think it's very interesting to think whether Io had an ocean, maybe it dehydrated silicates as well, like Europa, but that ocean got removed over time. There's actually another paper that I'm a co-author of, but Carver Pearson, co-author of the paper that I published recently. I think it was within this past month, he published a paper on early Jupiter being very luminous, and this is in Planetary Science Journal, and if Io formed its ocean from silicate dehydration, depending on when that ocean formed, it could have been removed by Jupiter's high luminosity when it was young, whereas Europa formed this ocean late enough to retain some of that water and have the ocean Io it has today. So that's another very recent paper that came out after mine.

Sarah Al-Ahmed: I literally hadn't even considered that Io could have had an ocean at some point. Wow.

Kevin Trinh: Yeah, it's a very niche topic. I don't know how many people think of Io as a potential ocean world, but I think that probably gets overshadowed by Io's volcanism today. That's typically what people think about when people think of Io. But all these moons, Io, Europa, Ganymede and Callisto, they all formed around Jupiter, but somehow they look different. And when we're trying to investigate why these moons look different, it's also worth asking whether these moons were the same at some point.

Sarah Al-Ahmed: Cannot wait until we have the Jupiter icy moons explorer there and this Europa Clipper mission to really help us figure this out. Because if Io actually was a water world at some point, that would blow my mind.

Kevin Trinh: A lot can happen between the formation of a moon and today, so I'm very excited for the synergy that's going to happen with Europa Clipper and JUICE from ESA. A lot of exciting stuff to look forward to.

Sarah Al-Ahmed: But what do you have to look forward to right now? What's your next step in your research?

Kevin Trinh: Yeah. So right now I'm doing more modeling on Europa, also from the end of accretion going all the way to the temperatures where you could start melting metal. I'm trying to figure out what the composition of Europa's metal might be because that's going to tell us the temperatures required to melt that metal, and that has implications for the formation of a metal core, but also dynamo activity, so generating a magnetic field. Right now, I like to think there are three kinds of moons today. There are moons with an active dynamo today, and that only includes Ganymede. Moons have strong evidence for a past dynamo, our moon, and then there were all other moons. No one really knows if Europa had a dynamo in the past, but I'm curious whether it did, and in order to understand what a past dynamo might be like, we need to have a sense of the composition of Europa's metal. When could a metallic core have formed and what state would it be in? So still a lot of unknowns, but I'm still using computer models to try to figure out how did the metal change in composition over time?

Sarah Al-Ahmed: Well, when we have all this new modeling and someday with the added data from all these other spacecraft, when you are a venerated professor somewhere, come back and tell us what you found out about the core and it's dynamo. I'd love to hear it.

Kevin Trinh: Really looking forward to that day.

Sarah Al-Ahmed: Well, thanks for joining me, Kevin.

Kevin Trinh: Hey, thank you for inviting me.

Sarah Al-Ahmed: One thing is certain, Europa with its potential slow evolutionary processes remains one of the most intriguing celestial bodies in our Solar System. Even if it doesn't turn out to be the hospitable haven for life that many of us hope, it's unique beauty and the mysteries it holds, make it an invaluable subject for study. As the next chapter of exploration unfolds with the upcoming missions to Jupiter's moons, we're on the cusp of understanding more about Europa than we ever have been before. But for now, as always, the universe reminds us to be patient to wonder and to keep looking up. Now, let's check in with Bruce Betts, the chief scientist of The, Planetary Society for What's Up. Hey, Bruce.

Bruce Betts: Hey, Sarah.

Sarah Al-Ahmed: Nice to hear from you.

Bruce Betts: It's nice to be heard.

Sarah Al-Ahmed: Now we just went through a major storm here in California, so I'm glad to hear that you and everyone is safe.

Bruce Betts: Yeah. No, I'm talking to you too in my roof. No, I shouldn't make jokes about flooding. I'm sorry. Yes, the rare tropical storm in Southern California. Super rare. Fortunately we did okay, but it was certainly dependent on your exact locale and what happened. So best wishes to those who had serious problems with it. Rain, it's confusing. We're not ready for it.

Sarah Al-Ahmed: At least over here on this coast, but it did make for some really interesting imagery from space. I was tracking it with the GOES-West satellite the whole time.

Bruce Betts: Cool. I was really excited about targeting LightSail too, to take a picture of it, but then I remembered we burned up.

Sarah Al-Ahmed: RIP LightSail.

Bruce Betts: We did get some nice picture. I did them targeted, so we got some not as nice as GOES because that's what they do. But for us, we got nice pictures of some typhoons and hurricanes. They're impressive. They're really big. Did you know that?

Sarah Al-Ahmed: Yeah. But it is one of those things where you don't really understand how big until you're looking at it in the context of whole cities and continents. When you get the satellite imagery or you get something like LightSail from space, it really puts it in context. There have been some really exciting stories about Saturn and the big storms that happened there and how long they happened. So I'm hoping I can get someone on the show to talk about that. So cool.

Bruce Betts: I would be interested because it intrigues me how they are. I've started looking into it, but how they're so convinced from current data that they go back hundreds of years, but it's intriguing no matter what. So yeah, do that. Do that Sarah, talk to someone.

Sarah Al-Ahmed: I appreciate too. That's a story about storms I can report on without feeling sad for this Saturnians that aren't actually grappling with the hurricanes.

Bruce Betts: Yeah, they deserve it. No, I'm kidding. There are no Saturnians that we know of.

Sarah Al-Ahmed: That we know of. Dun, dun, dun. Well, we've got some really cool messages from people this week, and I was really happy because I love reading people's poetry on the show, and there are a few people that write in consistently to send us their awesome poems, but we've got a new one from someone I haven't heard from before.

Bruce Betts: Oh, cool.

Sarah Al-Ahmed: Bill Tight from Alamosa, Colorado wrote us a poem about Halley's Comet. It's called Osmosis, "On 28 July 2061. My children and their children and their children's children, and one more. And I will watch Halley's Comet pass overhead almost as bright as the binary star system Alpha Canis Majoris and the Waxing Gibbous alpine sky. A cloudless valley will focus eternity and the spaces will be porous. Terra Nova will sit on my lap and tug my ear. The Earth will hold its breath. I will be 103." 103. I don't know how old I'll be. I'll be in my '80s when it passes overhead, I think.

Bruce Betts: I thought you already were in your '80s. Let's just be your vast knowledge and maturity.

Sarah Al-Ahmed: But did you get a chance to see Halley's Comet when it came overhead? I bet it was beautiful.

Bruce Betts: Oh, it was very disappointing.

Sarah Al-Ahmed: Really?

Bruce Betts: Yes, it was. Yeah, no, it depended on where you went, but it was not a good app. 1986 was not a good apparition of Halley's Comet and people were, the general populace and the media have got people overly excited, which was kind of a lesson for me because the 1910 apparition was really, really good and there all sorts of wonderful stories, and my grandfather saw the tail spreading across the sky. And it was okay if you went to a dark site, you could see it, but it happened to not pass that close to earth. I have not actually done my homework to figure out 2061, would how it will look. No, it was neat. But other comets have been better in the recent decades in terms of visibility, but not in terms of historical significance.

Sarah Al-Ahmed: I've seen some really wonderful artwork of not the most recent pass by, but previous ones. It must have completely stunned people because it spun off whole new realms of science.

Bruce Betts: Yeah. And you can go way back with it. You can go back to the Bayeux Tapestry, of course, from 1066 AD documenting the Norman Conquest, and there's Halley's Comet. I mean, they didn't know that, anyway, so anyway, what else you got, Sarah?

Sarah Al-Ahmed: Well, I did have this one other comment that someone sent us in our member community. Daniel Wright sent us this message and this just made me so happy. He said, "First episode of Planetary Radio that I've listened to and it convinced me to become a member. Thank you for continuing to put out these excellent discussions with an extensive backlog I can now go and enjoy." That's the kind of message I love hearing.

Bruce Betts: Yeah, that's good stuff. And yeah, there's over a thousand episodes you can dig in and have all sorts of fun.

Sarah Al-Ahmed: And you've been here for the entire history of those episodes.

Bruce Betts: I know. Crazy, wacky me.

Sarah Al-Ahmed: How do you wrap your brain around the fact that a thousand plus episodes, you and Mat are legendary?

Bruce Betts: I don't know about that, maybe Mat. It is very strange, when he, because it's one of those things that just, I do it weekly. It's fun. We share information and usually I'm not thinking about, "Wow, this is my 1105th show, and that's pretty amazing." And I also should never use that voice ever again, but it's pretty wild. I remember every one of them like they were yesterday.

Sarah Al-Ahmed: Really?

Bruce Betts: No, not at all. Not even a little bit.

Sarah Al-Ahmed: Well, let's go into it, Bruce. What is our Random Space Fact this week?

Bruce Betts: Well, kind of funny. As I mentioned to you, we independently came in with commentary things, and so I want to talk comet tails, which they're really long. They make those hurricane dimensions seem really like nothing. And so the longest tails that have been detected, you had in 1996, the nice apparition comet of Comet Hyakutake, it was detected by chance. The tail was detected by the Ulysses spacecraft when it passed through, and it was about 500 million kilometers. And so that's over three times the Earth's sun distance from the comet when they detected it with their particles and fields instruments. And then you had the detection about twice as far out of the Cassini hanging out around Saturn, and they checked the data years later and they found an enhancement in proton flux from Comet Ikeya-Zhang. That's 153P for those playing the periodic numbered comets game. And it was over a billion kilometers from the comet and they could detect protons. So it's not clear that'd be very visible at that distance, but the comet tails are amazing how much they spread out over planetary distances.

Sarah Al-Ahmed: Yeah. A friend of mine lovingly says that comets are like little kids because they make a mess everywhere they go.

Bruce Betts: They are. And there's the common analogy of them, they're like cats. You can't predict what they're going to look like beforehand and they've got tails. But what's interesting is, as opposed to dog tails or cat tails, and I've used this, mentioned it before, but sometimes people don't realize that the tail is always pointing away from the sun. The two tails roughly away from the sun. And so when it's headed towards the sun, it gets animated, right is the tails behind it. But when it's going away from the sun, the and tails are in front of it, which I tried to get my dogs to do that trick and they would've none of it. None of it whatsoever.

Sarah Al-Ahmed: That is pretty strange. Thinking about the comet, kind of like going away from the sun, going into its own tail as it's traveling.

Bruce Betts: Yeah. Hanging out in vacuum of space with solar effects. It's weird.

Sarah Al-Ahmed: This week we had a really cool opportunity to talk about Europa, and I wanted to bring this up to you because I feel like in my heart, Europa was the first place that I really dreamed that life would be off of Earth when I heard about it. But this result that I talked about with Kevin Trinh basically suggests that if Europa formed over really long timelines, it might be less of an awesome place for life than we hope it would be. Just because it would impact the level of seafloor volcanism or whether or not it has a core, all kinds of stuff that makes me a little happy, sad. Happy that we know it. Sad that it might mean that Europa isn't the hotspot for space shrimps that I hope it is.

Bruce Betts: Space shrimp, sea monkeys. They can survive anywhere.

Sarah Al-Ahmed: Oh, no. We should send sea monkeys to Europa.

Bruce Betts: Oh, the planetary protection, people would really, they would love that.

Sarah Al-Ahmed: We would never do that. We're just kidding.

Bruce Betts: They have been sent into space many time. Sea monkeys, which are actually a brine shrimp, but were sold under the name sea monkeys for years in the back of magazines. It was weird. The Europa is what it is, feel free to picture your shrimp and whales. Well, maybe not whales because that whole breathing thing, but fish, aliens however you want. And they're probably not there, but maybe even if there's life there, it's probably microbial, which is fun, but maybe not the shrimp. I don't know. The best brilliant thing is we learn more, but we don't know, and when you stick your liquid water ocean under tens of kilometers of ice, it makes it tough to figure out what all is swimming around down there or not swimming around down there.

Sarah Al-Ahmed: We're just going to have to keep trying, and when they eventually get it, Planetary Society members will celebrate whether or not we're there to see it.

Bruce Betts: It's true. And Planetary Society members have been key players in keeping Europa missions, including Europa Clipper going out and exploring it. So it's a neat place whether there's life or not. It's got a lot of weird fascinating stuff going on.

Sarah Al-Ahmed: And hey, there's always Enceladus.

Bruce Betts: We'll always have Enceladus. All right. All right, everybody go out there, look up the night sky and think about happy little thoughts of swimming fish in Europa. Thank you. Goodnight.

Sarah Al-Ahmed: We've reached the end of this week's episode of Planetary Radio, but we'll be back next week with the team from University of Washington, whose new planetary defense algorithm just detected its first potentially hazardous asteroid. Planetary Radio is produced by The Planetary Society in Pasadena, California and is made possible by our icy world loving members. You can join us as we advocate for the search for life and missions like Europa Clipper at planetary.org/join. Mark Hilverda and Rae Paoletta are our associate producers. Andrew Lucas is our audio editor. Special thanks to Mat Kaplan for helping us edit this week's show. Josh Doyle composed our theme, which is arranged and performed by Pieter Schlosser. And until next week, ad astra.