An essential ingredient for life in the oceans of Enceladus

Sarah Al-Ahmed: The prospects for habitability on Enceladus keep getting better, 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. Phosphorus, a key ingredient for life as we know it, has been discovered on Saturn's moon Enceladus. This is the first time that phosphorus has been detected in an ocean off of Earth. Chris Glein, a lead scientist at the Southwest Research Institute, joins us this week to talk about the discovery and its implications for the search for life. Then Bruce Betts will pop in to share what's up in the night sky this week. And now for some space news, NASA has announced a new project to investigate volcanic terrain on the moon. Artemis' cadence of robotic lunar missions will include a new scientific payload called DIMPLE, which stands for Dating an Irregular Mare Patch with a Lunar Explorer. It's going to study the Ina Irregular Mare patch, which is an area of hilly terrain created by volcanic activity on the near side of the moon. And China's plans for a moon landing are coming together. The China Manned Space Agency recently announced a more formalized set of plans to land a pair of astronauts on the surface of the moon before the end of the decade. The mission would involve separately launching a crude spacecraft and the lander segments which would rendezvous and dock and lunar orbit before landing on the lunar surface. The agency is also calling for proposals for science payloads to travel with lunar lander. And in Uh-Oh News, the US Senate has warned that if NASA's Mars sample return campaign can't stay on budget, it may be canceled altogether. The Senate committee responsible for funding NASA said that if the agency can't come up with a plan in six months that will contain the Mars sample return project to a lifetime cost of $5.3 billion, its remaining funding could be carved up and allocated to other programs, primarily Artemis. Our Planetary Society chief of Space policy, Casey Dreier, explains the situation in more depth in a recent article. You can read his article, see a beautiful new picture of Saturn from JWST, and learn more about these stories in our July 21st edition of our weekly newsletter, the Downlink. Read it or subscribe to have it sent to your inbox for free every Friday at planetary.org/downlink. Saturn's moon Enceladus has a subsurface ocean underneath its icy crest. There are many water worlds in our solar system, but studying their contents is usually very difficult. We don't yet have the technology to land on an ice world like Europa or Triton and tunnel through the protective ice to the hidden waters below. That's why Enceladus is such a treasure trove. This moon of Saturn sprays jets of water from cracks in its ice that we can sample and analyze from space. NASA's Cassini spacecraft orbited Saturn from 2004 to 2017 before it dove into the planet's cloud tops and ended its mission. Cassini taught us so much about Saturn. We learned more about its rings and moons than we ever could have dreamed. But Cassini also flew through and studied the material spewing out of Enceladus. What it found there has been amazing. In recent years, we've learned of the detection of organic compounds in the water and evidence of hydrothermal vents under the ocean. But we can now add one more exciting puzzle piece to the mix, the detection of phosphorus. This is the first time that phosphorus has been detected in an ocean off of Earth, which has profound implications for the potential habitability of Enceladus and other ocean worlds. Our guest this week is Dr. Chris Glein. He's a lead scientist at the Southwest Research Institute in San Antonio, Texas. He's a geochemist who uses his expertise to answer some of the biggest questions humanity has ever faced. How did life develop on Earth? And are we alone in the universe? Chris decodes stories told by molecules and isotopes. He uses thermodynamic modeling, hydrothermal experiments, and balanced chemical reactions to paint a picture of conditions and processes on distant worlds like Saturn's moon Enceladus. He and his team's new papers called Detection of Phosphates originating from Enceladus' ocean. It was published on June 14th, 2023 in the Journal Nature. Thanks for joining me, Chris.

Chris Glein: Hi, Sarah. Glad to be on.

Sarah Al-Ahmed: It's really funny because when I first read this headline, I was sitting on the couch scrolling through articles on my phone and my partner Dan was sitting next to me and I must have made some kind of strange noise because he turned to me and asked what I was so excited about, and I fully looked him in the face and said, "They found phosphorus on Enceladus." And he got this look on his face like, "I'm happy you're happy, but what?"

Chris Glein: Yeah, I like to say that we found P in Enceladus' Ocean.

Sarah Al-Ahmed: Oh man. But I mean, this is a huge discovery and I'm hoping that we can explain why this is such a big moment, not just for understanding Enceladus and ocean worlds, but for the search for life, because this is quite a headline.

Chris Glein: Right. We're digging a lot deeper than just looking for water now.

Sarah Al-Ahmed: But before we get into all those details, how did you find yourself studying Enceladus?

Chris Glein: I started in undergrad. I got really interested in astrobiology. It was the new hot topic in the late aughts. And so I was reading all these popular books about astrobiology and if there could be life elsewhere in the solar system or universe. And I initially actually started studying Mars, so that was kind of like around the time when people were excited about the two rovers on Mars, Spirit and Opportunity. So I was really excited about that. I started grad school in 2006 and I initially started working on Mars stuff related to water on Mars and what that might mean. But then later that year, some papers came out reporting that a plume was discovered coming out of Saturn's moon Enceladus, and I thought that that was just the coolest thing ever. So I quickly shifted gears because I thought that this is what I really want to study, and it seemed like it was a topic that people hadn't really dove into to the same degree that Mars had been studied. So I thought it was a great opportunity.

Sarah Al-Ahmed: Yeah, it's a hard call, which one of the worlds in our solar system is my favorite. Sometimes I waffle between Enceladus and Io because who doesn't love volcanoes, right? But if I had to bet on any one world in our solar system having life that we could find now, it's probably Enceladus.

Chris Glein: It could be. I don't really like taking bets, competing planets or moons against each other. Enceladus looks like it's a pretty comfortable place to be if you're a microbe. Mars also looks like it has a lot of the ingredients for life, and we're still learning about other bodies like Europa and Titans. I wouldn't count them out either.

Sarah Al-Ahmed: Oh, for sure. But the plumes on Enceladus are the key here, I think. Europa is a great candidate, but there is some indication there might be plumes. But you look at the Cassini images of Enceladus and you can clearly see this water spraying from a subsurface ocean into space. It's absolutely startling.

Chris Glein: Yeah, it's really breathtaking. And Enceladus is doing us a great favor because a lot of the more interesting environments for life, what we could think of as habitable environments we think are in the subsurface, like the subsurface of Enceladus or the subsurface of Mars. And usually, it's a lot of work to get into the subsurface. You have to try to figure out how to design a mission that will drill down to get to these kinds of environments. And Enceladus, it turns out just by it's geophysical situation, is given these free samples into space. So it makes our job as scientists a lot easier and also what we ask the engineers a lot easier.

Sarah Al-Ahmed: I mean, we're not even sure how thick the ice crust is on some of these moons, so we'd have to get really creative to try to tunnel down in there. Maybe one day. I know people are working on it.

Chris Glein: Maybe one day. You're absolutely right.

Sarah Al-Ahmed: But for people who aren't really familiar with the chemistry of life, why is finding phosphorus on Enceladus so important?

Chris Glein: Phosphorus is important because from studying life on Earth, we've come to identify that there's a big six elements of life, and these are carbon, hydrogen, oxygen, nitrogen, sulfur, and phosphorus. And it turns out that phosphorus is usually the rarest of the bunch. So if you go to places like the ocean on the Earth or your local lake or a river or someplace, oftentimes you'll find these other elements like carbon is readily available in the air, oxygen and hydrogen or in water, but phosphorus usually is not so easy to find. It's a more of a trace element. And it turns out that that's one of the reasons why when you add fertilizer to a plant, it'll start growing so much because it's limited in phosphorus. And if you provide that, it provides a spark for life to really take advantage of that resource. Finding phosphorus is a big deal because it's not usually very common on planetary bodies.

Sarah Al-Ahmed: Yeah. And I was joking around with one of my coworkers earlier today too about that classic thing that people in the United States learn in high school when they're going to their biology classes, they always hear mitochondria is the powerhouse of the cell, right? And the reason why mitochondria is the powerhouse of the cells because it creates ATP and the P stands for phosphate, which is literally what you guys discovered. It's key to life.

Chris Glein: Yeah. And as far as we know, it's indispensable because it serves this very important role of being able to shuttle around energy between different systems of the cell. And as far as we know, and chemists have argued, if there could be other substitutes for phosphorus. It doesn't appear that there's another element or type of molecule that can have that kind of role of being able to shuttle energy back and forth in a liquid water environment.

Sarah Al-Ahmed: This is one of those moments where we know phosphorus is on Earth, it's key to life. But you guys didn't just find phosphorus in the oceans of Enceladus. You found that the abundance of phosphorus in the ocean is what? 100 times more than there is on Earth?

Chris Glein: That's right. That was absolutely shocking when we came to that realization that it's not only is it not rare, but it's actually quite abundant in the ocean water that's come spraying out into the plume. And that actually took a little bit of a detective work to figure out what was going on to make phosphorus so abundant in that ocean water.

Sarah Al-Ahmed: I should probably ask you this question upfront because for many years I ran social media at The Planetary Society, and whenever I brought up Enceladus as a moon or any ocean world, the first question that everybody asked is, how is that even possible that it has a subsurface ocean? It's so distant from the sun, it's covered in ice. It's not even in the habitable zone.

Chris Glein: Right. This is one of the breakthroughs in planetary science over the past 20 or so years. We knew back in the early days of space exploration that these moons of the outer planets had a lot of water in the form of ice. It just turns out if you form a planet or a moon in the outer solar system, conditions are cold enough that ice can form and ice is abundant because oxygen is one of the most abundant elements in the solar system. So having the water part is no problem, it's just usually it's in the form of ice. But what we've discovered in the past 20 years or so is that there's this process known as tidal heating, and this is discovered earlier on in the Voyager flybys of Jupiter's moon Io that can impart a huge amount of energy into the interiors of moons that orbit giant planets. And so IO has these fantastic volcanoes that are erupting nonstop because all that energy is going into the interior. There was speculation around that time that maybe a similar process could happen in Jupiter's moon Europa, and we're still very interested in that. Now, if we flash forward a couple decades, we arrive in Saturn. And what we've learned throughout the course of this Cassini mission that we had an orbit around Saturn is that tidal heating can operate also in Saturn's moons. And it turns out Enceladus has a proper orbital configuration that it's in a sweet spot of receiving lots of tidal heating from Saturn.

Sarah Al-Ahmed: It's amazing that this is even possible. We'll get into the details later. It's actually quite awesome that you can have oceans underneath the surface far beyond where we are in our solar system because its position in space actually leads to this result of having more phosphorus in the oceans.

Chris Glein: I like to say that the outer solar system is wet. We're learning that there's probably more water out there beyond Jupiter than there is in the oceans on Earth.

Sarah Al-Ahmed: It just goes against everything that I assumed when I first began learning about moons and worlds in general. I understand why people get so confused and why they want to know how these oceans can even exist because it's so cold out there, and yet there's just so much we're learning about the opportunity for places that could be habitable or just ocean worlds in general. I mean, even Pluto might have an undersurface ocean. But your research and almost everything we know about Saturn is made possible by NASA's Cassini spacecraft. One of the things that was made abundantly clear by Cassini is that Enceladus has these plumes. We got these amazing images of it. Cassini even flew through the plumes. So which plumes on Enceladus did Cassini fly through? Where were they located?

Chris Glein: So Enceladus has it's erupting stuff from its south pole. That seems to be the area where all the activity is concentrated. What we found from Cassini, this is looking at the surface using images and using different spectrometers from remote sensing and space, we found out that there are these huge cracks on the surface of Enceladus around the south pole, and we call them the tiger stripes because they can look like the stripes on a tiger. These cracks actually tap into the subsurface ocean on Enceladus. So they actually serve as conduits where liquid water can come up through these cracks, and then it erupts in what we call jets. So there are these jets all along these tiger stripe fractures at the south pole. Over 100 of these jets have been mapped out. What happens is these jets then erupt into space. And then when they get up to about 10 to 20 kilometers in altitude, then they kind of coalesce and merge together. And that forms a larger plume structure. So you could say that there's over 100 jets, these little features that shoot up out of the cracks, and then there's one huge plume that goes into space.

Sarah Al-Ahmed: How much water is this thing putting out?

Chris Glein: It puts out about 200 to 300 kilograms per second of water. And that would be enough to fill up an Olympic sized swimming pool very quickly. So it's spewing out a lot of water. It's interesting because if this process were going on for a large portion of solar system history, it might have removed a huge chunk of Enceladus' original water budget, which is interesting to ponder.

Sarah Al-Ahmed: Yeah, I was actually thinking about this and having a conversation on a previous show. We were talking about Saturn's rings and what might have formed them and how some of the other moons in the system might have been perturbed by whatever situation created that ring, and just thinking about how much water that Enceladus is putting out and whether or not it's shrinking itself or tapping itself out of water over time is a really interesting idea.

Chris Glein: Yeah. This whole topic is actually a hot subject and controversial in planetary science where I study, at least right now, the question of how the rings of Saturn formed when they formed and how that relates to the formation of some of these moons, including Enceladus. People have vigorous debates about this, whether the moons and rings could be young or if they could be old. And we're still working on this. It's very exciting to see progress being made.

Sarah Al-Ahmed: But at least we know that Saturn's E ring has a cause and that E ring is caused by the water coming out of Enceladus.

Chris Glein: Yeah. So Enceladus is a small moon, but it has a huge impact in the Saturn system. It's constantly spewing out water. And like you said, it forms Saturn's E ring and it redistributes some water throughout the system. It looks like even Titan, which is quite far from Enceladus, gets its oxygen supply to its atmosphere from pollution that originates from Enceladus.

Sarah Al-Ahmed: Wow, I did not know that.

Chris Glein: Yeah, there's a couple of unexpected oxygen buried molecules like CO and CO2 in Titan's atmosphere. People have constructed models and it's found that the water supply from Enceladus appears to be adequate to explain what we find on Titan in the atmosphere.

Sarah Al-Ahmed: That's amazing. Because Titan similarly is one of those moons that's just... I don't know, everything about it is so strange and awesome. And knowing that there's some kind of connection between those two moons makes it even more fascinating. So we made this discovery by allowing Cassini to go through and analyze these plumes. What instrument on board allowed it to test the composition?

Chris Glein: The main instrument that discovered phosphorus is known as a Cosmic Dust Analyzer. We call it CDA because we love acronyms with NASA. And what it is it's known as an in-situ instrument. So it samples particles that are around the spacecraft. And it's also a mass spectrometer. So this is an instrument that can sample material and then it can weigh the different molecules or other chemical constituents that are in that material. It's very valuable because if you can weigh a molecule, then that can help you understand what molecule is present. And so what this instrument did, it was on Cassini before Cassini ended. Cassini flew through the E ring of Saturn, like we discussed earlier. The E ring is supplied from Enceladus' ocean water that erupts in the plume. And why we flew through the E ring and not the plume directly is because the E ring had more material for us to sample. The plume fly-throughs are very fast. It takes about a couple seconds and you're through the plume and it's done. So by going through the E ring, we were able to acquire much better statistics, collect more data, and that allowed us to look for more trace chemicals like phosphorus for example.

Sarah Al-Ahmed: What other chemicals other than phosphorus make up these particles coming out of Enceladus?

Chris Glein: So the main chemical is water in the form of ice. The plume ice grains are mainly made of ice. And then one of the earlier discoveries from Cassini was finding sodium chloride in some of those plume particles. So that's normal table salt. And that was our first smoking or salty gun that there was a liquid water ocean that was feeding the plume. So that was really exciting when we first found that. And then fast-forward about a decade, and now we're detecting sodium phosphate salts in the E ring grains that originated from Enceladus' ocean. Sodium phosphate is one form of phosphorus. What's interesting about that to astrobiologists especially is sodium phosphates are way more soluble than some other forms of phosphorus. So the one that all of us probably know of is calcium phosphate. It's also known as appetite if you're a geochemist. And if you're just a normal person, it's what your bones are made out of, calcium phosphate. It's great to make bones out of calcium phosphate because it's strong and it's not very soluble. So your bones are not just going to dissolve in your blood. That'd be a bad day. Instead, it's pretty insoluble. But on Enceladus, it looks like the main mineral form that's available is sodium phosphates, which is great if you're thinking about habitability because it's much more available in the ocean water.

Sarah Al-Ahmed: Yeah. And I remember when we learned as well that there was evidence that there were hydrothermal vents in the ocean on Enceladus based on the composition of the E ring.

Chris Glein: Yeah, I was part of that. Well, I was part of half of that discovery. Initially, the Cassini Cosmic Dust Analyzer found the first hint of hydrothermal activity by finding silicon bearing grains that came from Enceladus. So we think that results from hot fluids that's mixing with colder ocean water. And then a couple years later, I was part of a team that discovered molecular hydrogen, that's H2, in the plume, and we think that that's produced by water interacting with rocks at high temperature to release hydrogen, which could be a food source for microbes.

Sarah Al-Ahmed: Yeah, I mean it's a big deal because there's a lot of evidence that suggests that life on Earth may have started around hydrothermal vents in our oceans. This is an interesting point to me because this gives us some kind of indication of the temperature of the water in Enceladus.

Chris Glein: We're still not too sure about what the temperature in the deep portions of Enceladus what that might be. From the findings of silicon or we call it silica, it's SiO2 and hydrogen gas, we're able to set a lower limit to the temperature. So we think it's greater than about 100 degrees Celsius or roughly the normal boiling point of water that most of us know about if you live near sea level. But we don't know exactly how hot those fluids might get. That's still an open question. And then why hydrothermal systems are so attractive to many scientists who think about the origin of life is they provide a fantastic energetic foundation for the types of reactions that many scientists think are needed to form the building blocks of life. It turns out hydrothermal systems are really great at generating what's known as chemical disequilibrium. So this is where molecules are formed and then mixed together. And then that combination is no longer happy to coexist. It's unstable from a geochemical standpoint. So if there's any kinds of catalyst available or if there's a microbe available in an inhabited system, then that can help to support the synthesis of more complex organic molecules.

Sarah Al-Ahmed: And we have detected organic compounds in these ice grains as well.

Chris Glein: Yeah, we've detected organic compounds in both the E ring and in the plume itself. It turns out we're just sort of getting hints of what those molecules might be. Cassini was not designed to do intensive characterization of organic molecules because organics can get quite complex. And so we got some hints that there are certain structures like certain features containing oxygen or nitrogen. We've also detected features associated with the benzene ring. This is a six member hexagon that's found in certain organic molecules. We've also detected that these molecules go up to quite high masses, above 100 mass units. So they appear to have quite a lot of complexity, which is very interesting.

Sarah Al-Ahmed: That is. We should be clear that the finding of organic compounds does not directly indicate life. We find organic things on Mars, comets, asteroids. They're all over the place, but we are creatures made out of organics. So this is still a really exciting finding even if it doesn't directly point to life.

Chris Glein: That's absolutely right. So organics do not equal life, but they look to be a necessary condition for life as we know it, but may be not sufficient. So that's why we're also looking for other ingredients that you would need in addition to organics like phosphorus as we've been talking about.

Sarah Al-Ahmed: Can we make assumptions about the chemical makeup of Enceladus ocean as a whole just from this data? You're looking predominantly at stuff that's coming out of one pole of this moon. And now it's in space and being changed by the environment out there. Can we actually make inferences about what's going on with the entire ocean?

Chris Glein: We could make inferences. We don't have a complete picture because the data are so limited. So we're able to try to connect the dots through modeling efforts to see what kinds of conditions would be self-consistent. And so I've made these kinds of models. Other people have also contributed to this kind of work. So we think we have a pretty good understanding now of what the basic characteristics of the chemistry that ocean look like where it's a moderately salty body of water, has a slightly alkaline pH, that means the pH is similar or maybe even a little bit higher than Earth's sea water. We have some indications of the total volume of liquid water down there. It's temperature, we think most of the ocean is near the freezing point of water. But then there's other questions that we're still struggling with, some of the other elements we haven't found like magnesium or calcium for example. Or even sulfur is still a little bit up in the air. So we're working on these issues, but we don't have a complete story yet. What's cool though, if you want to turn that around too, is it's also phenomenal to think that we even know anything about an extra terrestrial ocean because we cannot see this ocean. It's completely through the use of remote data like the gravitational tugs that Enceladus imparts or looking at the composition of the plume that helps provide a window into this unseen environment that is so interesting.

Sarah Al-Ahmed: Yeah. And it's not like Cassini spent its entire time just hanging out around this one moon. This is a side project on Cassini's wild adventure. I can't even imagine what we could do with, say, an Enceladus orbiter or something.

Chris Glein: Yeah. Cassini, none of its instruments were designed to try to address the habitability of an ocean on Enceladus. We didn't even know that an ocean existed when Cassini was being designed. So if we could design a future mission that would go after the last remaining questions of habitability and start to take some steps toward looking for biosignatures, so possible evidence of life on Enceladus, we could make much more progress on these questions of astrobiology.

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

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Sarah Al-Ahmed: Yeah, I know there's a proposed mission, the Enceladus Life Finder, but it's still a dream right now. We don't really have any other missions going to Saturn that are going to help us figure these things out anytime soon.

Chris Glein: Well, what's really encouraging is you may have heard the planetary science and astrobiology communities with the National Academy of Sciences, we put together what's known as a decadal survey last year and a flagship mission to Enceladus known as Enceladus Orbilander was recommended to be the second-highest priority flagship mission after a Uranus orbiter and probe. So the scientific community is starting to rally around and recognize that Enceladus is an important place to try to look for life elsewhere in the solar system.

Sarah Al-Ahmed: Yeah. And if anybody wants to learn more about the decadal survey, we are really into the details on this one. We have a whole article that I'll link to on the page for this episode, planetary.org/radio, so you can read more about it because some of these things that are being planned and are going to be priorities for the next 10 years are just so exciting. And I really want to go to Uranus and Neptune. The ice giants need some love as well, but oh my gosh, if we could learn more about the oceans on this world, I'd be so happy.

Chris Glein: Yeah, so it's going to be a great thing, especially for any younger listeners, this will be their kind of generation that will go after looking to see if there's any life or evidence of life in the solar system because we haven't had a mission that's been dedicated for looking for life since Viking landed on Mars. We've had missions that have been designed to look for ingredients that life would need like liquid water. Follow the water's been NASA's thing, but we're now starting to gear up for a new dedicated search for life on ocean worlds that will help to synthesize all the science that we've discovered over the past few decades. From studying life on the Earth and exploring a lot of these other ocean worlds, we'll be able to use all those insights to design much more robust tests of life in our next missions.

Sarah Al-Ahmed: Yeah. If I was still a kid, learning this kind of thing would've completely changed the trajectory of my career. I mean, I decided to go into astrophysics and learn more about planets, but I think Enceladus would've motivated me to try to make it my whole life's mission if I had known when I was younger.

Chris Glein: There's so many great things to study out there. I'm constantly mesmerized by what the James Webb Space Telescope is doing and figuring out the details of exoplanets, I think. It's very difficult to choose.

Sarah Al-Ahmed: Am I remembering correctly that I hear that you are going to be leading a team that's going to be using JWST T to try to learn more about Enceladus, right?

Chris Glein: Yes. Yes. So we had a paper just about a month ago where we gave a first look at Enceladus using JWST. And what was super exciting about that is JWSTS can see the plume. And so we had to send a spacecraft known as Cassini to discover the plume all the way to Saturn. So that's about a billion miles away. And it turns out now we have a telescope that's near the Earth that can actually see the plume erupting from the south pole of Enceladus. And so what we're planning to do in cycle two, so this is in the next year, is we're going to take a much longer look at Enceladus than we did with the initial look in cycle one to try to see if we can find any evidence of any organic molecules. So following up on Cassini's findings to see new information about organic molecules that might be on Enceladus, look for other kinds of building blocks for life. Like ammonia's a big one. We want to see how abundant ammonia is in that environment. One that's been really confusing from Cassini is hydrogen peroxide, H2O2. Hydrogen peroxide is known as an oxidant. So it's kind of like the other half of the yin and yang with hydrogen. Hydrogen is like one of the most reduced things, and H2O2 is one of the more oxidized things, and it turns out H2O2, hydrogen peroxide, would be a great source of oxidation for powering metabolism. And so there's been some models that have been published looking at this, and we're very interested if hydrogen peroxide could be readily available on Enceladus' surface and what that might mean for powering metabolism in the ocean.

Sarah Al-Ahmed: That's really interesting. My gosh. I'm wondering about the composition of this ocean. Can it tell us more about the interaction between the ocean itself and the core of Enceladus?

Chris Glein: Yes, absolutely. So the water itself is just water, H2O. But what's great is finding all the contaminants in the water, if you will. So the things like sodium chloride. Another one that's been found is sodium bicarbonate, so this is baking soda. So this tells us there's a lot of sodium in that system. So for a geochemist, this is telling me that the sodium has to come from the rock because minerals and rocks contain sodium. So many of us are familiar with different kinds of minerals and rocks. One kind of mineral is known as feldspar, which is really common in granite. It gives granite kind of the whitish appearance in some of the mineral grains. And so that's a source of sodium, so by looking at some of these different metals like sodium. Chlorine is not a metal but it's another impurity that can tell us something about how extensive the water rock interaction has been on Enceladus. We can actually construct models by trying to connect potential building blocks, so things like comets and meteorites. We think icy bodies like Enceladus might have formed by this process known as accretion from the agglomeration of many smaller bodies, which might be similar to a type of meteorite known as a carbonaceous chondrite. This is a really primitive type of rock from the early solar system. And then comets are also thought to be very, very primitive. From the early days of the solar system, they've been left in deep freeze. And so we can try to piece together what we think were the starting materials, the meteorites and comets, and then at we see in the ocean today and then using models we can try to simulate, "Okay, if you start from different meteorite compositions and if you react them with this much water at these kinds of temperatures or this other type of fluid, can you reproduce the composition of the ocean as we infer it today?" And it turns out that we can in many cases, and that can give us deeper insight into how extensive the water-rock interaction has been. What we find is that it's been very, very extensive. We think probably the entire rocky core at the center of Enceladus has been permeated by hydrothermal fluids.

Sarah Al-Ahmed: As I was reading your paper, I realized it was making claims about the density of the core of this moon and how it's been altered over time by its interactions with the water. And the fact that we can even begin to fathom what that means based off of measurements of dust grains and space is just ridiculous. That's so cool.

Chris Glein: Yeah, I got to give a little bit of love to my geophysicist friends. So they're the ones who are able to derive the density. By looking at Enceladus' gravity field, we can understand, well, how is mass distributed in the interior? And it turns out those data show that you have a mass anomaly in the very center of Ellis, which is what we call the rocky core. So we're able to then constrain the density of those rocks. And it turns out the density is actually kind of low. It's more similar to clay-type rocks like mud and less similar to some kind of hard rock. If you went to Hawaii, I just got back from a vacation to a big island in Hawaii last week, and you look at those basalt fields, those are hard rocks. Those are dense rocks, relatively pristine rocks. It looks like the density of the rock inside Enceladus is low. It's more like mud. So that pristine rock has been altered by reactions with liquid water and it's now this muddy type of clay rock.

Sarah Al-Ahmed: Could that in any way help us try to figure out the age of this or how long the ocean's been interacting with the core?

Chris Glein: Yeah, people are starting to make models. Whenever you get great data like this, people start trying to make models. It turns out that this might lead to a paradox because most people assume that Enceladus could be very old because we think the solar system is old. If you look at the rates of minerals reacting with water, those kinds of reactions are generally fast. And so this might be one argument that Enceladus could be quite young. And this could get back to our initial discussion about Saturn's rings and if something very terrible happened in the Saturn system around the time of the dinosaurs.

Sarah Al-Ahmed: Yeah. That would explain a lot of things for me actually, but also open up whole new realms of questions. Gosh, we need to send 50 more spacecraft to the system.

Chris Glein: Yeah, it'd be interesting. And when I say young, young could still be 100 million years. And so even if a body like Enceladus or other moons around Saturn are only 100 million years old, that's still much longer than we can do a laboratory experiment, reacting water with rocks and organic compounds. So a lot of scientists, including myself, are really interested in what a natural geological experiment can accomplish over millions of years when organic molecules are interacting in an environment where energy is constantly supplied. We don't really have a good sense of what can happen over these kinds of timescales. So we're really interested in finding out.

Sarah Al-Ahmed: I do have a question about the experiments we do on Earth in order to understand what's going on with Enceladus. Because in order to just figure out what the chemical composition is, we have to do all kinds of interesting research with different chemicals on Earth and how they interact with each other. And so who are your partners here on Earth in order to do this kind of science?

Chris Glein: I've been very fortunate to have great partners. I've mentioned before, Frank Postberg is the lead scientist in Germany, and he spearheaded the effort to analyze the Cassini CDA data in great detail to help identify phosphorus. And then on the other side of the world was my colleague Yasu Sekine, leading a Japanese team, and they performed experiments to help clarify the geochemistry of phosphorus where when you have water reacting with rock under Enceladus-like conditions, how would phosphorus behave? We didn't have a completely clear understanding of that, but what they did is they did the kinds of experiments I alluded to earlier where you take a meteorite to represent Enceladus core, in this case a certain type of carbons contrite, and reacted that at high temperatures with liquid water under Enceladus-like conditions. And we found that under the ocean conditions of Enceladus, and these conditions really are where you have all that baking soda in the water, the ocean's loaded up with baking soda, and that changes the way the water and rock interact. Having all that baking soda, it turns out it supercharges the solubility of phosphate minerals. It makes them much more soluble than we normally think of, and that's how you get all this sodium phosphate leached out of the rock into the ocean water.

Sarah Al-Ahmed: As I was going through your paper, there was also a whole subsection on CO2 and the freezing line in space, this primordial freezing line where you can get these dry ices, and pass that, you can get more phosphorus absorbed up into the water because of it.

Chris Glein: Right. That's the larger implication of where Enceladus is doing us a huge favor by spewing its guts into space and we're able to learn about it, but we're also able to learn about more general concepts that we think apply to other icy worlds where we may not have as ready access to the ocean water. So we can learn maybe about the kinds of geochemistry that Europa can support or Titan, or even Pluto like you said earlier. What we've learned from this study of Enceladus is having that bicarbonate, the sodium bicarbonate, the baking soda, baking soda is formed from CO2 reacting with rocks, and so CO2 appears to be a very important ingredient to making phosphorus available. And so if we think about the solar system in a bigger picture, we can look at the distribution of ices in the solar system. And it turns out that as you move further away from the sun, more volatile ices become stable and they can be incorporated into the building blocks of planets and moons. So if you start moving away from the sun, eventually you don't just have rock, but you have rock plus water ice. If you move further away from the sun still, then CO2 ice can be a major constituent in the building blocks of these bodies. What's very interesting is if CO2 ice is abundant and it can react with liquid water and rocks inside some of these ocean worlds, then we think the same kind of geochemistry should happen on these other kinds of bodies. So possibly this type of chemistry that makes phosphorus so soluble in Enceladus could also occur inside Europa's ocean or inside Titan's ocean, or if the moon's a Uranus. So we're thinking about sending a mission to Uranus in the next 20 years. If those moons have oceans, maybe this kind of chemistry can also operate in their oceans to make phosphorus readily available in the ocean water there, which should be extremely exciting.

Sarah Al-Ahmed: It might be very difficult to find worlds out there with liquid water just sitting around on the surface, but if we can have all of these ocean moons with these oceans just full of phosphorus and all these other organic compounds, it's quite possible that there's just life under the ice of a bunch of rocks out there.

Chris Glein: Yeah, it's really tantalizing to imagine that. And it's also intriguing if there's not life out there, what that would mean. It'd be a hugely important data point for scientists if we find all this attractive real estate where microbes would just have a heyday out there in the oceans of the outer solar system. But if we didn't find them, it would be profound. It would tell us something very important about the conditions on the early Earth or early Mars that might have helped to facilitate the origin of life. Or if we find life everywhere, then that might tell us that life is a very natural and probable outcome when you have water, rocks, and organic compounds interacting and evolving over geological timescales.

Sarah Al-Ahmed: Yeah, we're going to have to do more research, but every indication so far suggests that there's probably life out there somewhere. I just hope we can find it in my lifetime because I'm impatient.

Chris Glein: That's what a lot of us think too. But the data will show us. I think right now the data look very intriguing for habitability, and so it's worthwhile for us to go have a look.

Sarah Al-Ahmed: Does the situation with the baking soda in the water explain why there's such a high concentration of phosphorus in the oceans of Enceladus versus Earth?

Chris Glein: Yes, absolutely. So what's really intriguing about the comparison is if you look in seawater, phosphorus is 100 or 1,000 times less abundant in terms of concentration. But there are certain environments on Earth that might be like little Enceladus, and these are known as soda lakes. So one famous example is probably close to you, is known as Mona Lake. It's a salty lake that's chockfull of baking soda. And another kind of soda is known as Washington soda. It's a higher pH version of baking soda that's abundant in Mona Lake. There are these other soda lakes in Washington state and British Columbia that are even higher in phosphorus. It turns out those environments have the baking soda being very abundant, and it turns out phosphorus is also abundant. People at the University of Washington have made models looking at this kind of chemistry. They did it for early Earth and they first clued in the Planetary Science community that these are the kinds of environments that support abundant phosphorus, which got us thinking about, "Well, if Enceladus has a ocean that's rich in baking soda, might this concept also apply? And would this be very favorable for life?" And it looks like the answer is yes.

Sarah Al-Ahmed: Have you been to go visit some of these lakes just to see them for yourself?

Chris Glein: Oh yeah. I've been to these lakes in the more distant past. More recently, I haven't been there because travel's been at a standstill up until earlier this year. So I'm looking forward to going back soon though.

Sarah Al-Ahmed: I want to take a little fork because there was a whole topic in this paper that I knew absolutely nothing about, which I love. But your team's research suggests that there are systems similar to Earth's, and forgive me if I pronounce any of this wrong, hydroxyapatite calcite and this [inaudible 00:46:46] calcite buffer. I know nothing about this, but the papers seem to suggest that these kinds of buffer systems are really important to the balance of chemicals in our ocean and potentially life on Earth.

Chris Glein: Absolutely. So we're really diving into the nitty-gritty of the geochemistry now.

Sarah Al-Ahmed: Yeah.

Chris Glein: Basically how you can think about it is, so hydroxyapatite is a form of calcium phosphate. And then another mineral that you mentioned is calcite, and that's calcium carbonate. So apatite is what we find in our bones. And calcium carbonate or calcite is what a lot of seashells are made out of. The common theme between those two minerals is calcium. Calcium is really the key to understanding the availability of phosphorus. It turns out that in Earth's seawater and in many freshwater environments on Earth, the calcium takes over the phosphorus. It acts as a sink of phosphorus, and phosphorus is not soluble because calcium is holding onto the phosphorous so tightly in the form of calcium phosphate minerals. What Enceladus ocean chemistry does is when you have all that baking soda and all that washing soda, that provides what's known as a carbonate ion. And what the carbonate ion does is it bonds to the calcium. So it basically shoves off the phosphate from calcium, and then it replaces the phosphate with calcium carbonate. So it's kind of a competition for calcium between carbonate and phosphate. When you have lots of carbonate present in the soda ocean, the carbonate winds, it takes over calcium. And then phosphate is left to walk away and it's made available in the ocean water.

Sarah Al-Ahmed: I see. Thank you for getting into that because as I was reading through, anytime I stumble across something that I have heard nothing about, I love those moments because the deeper you dive into the science, there's so much to learn and so much that even experts or people like me that try to spend all of our time reading about it. You can't learn everything, you know?

Chris Glein: It really comes down to the data from Cassini though, because all this stuff in hindsight can make beautiful sense, but a lot of it's really difficult to just come up with if you're sitting in a room pondering what's happening on Enceladus. So we need to explore places because exploration, I know The Planetary Society's big on exploration, exploring these other worlds gives us these insights that we wouldn't normally come to. And that can be very powerful

Sarah Al-Ahmed: For both Earth and understanding other worlds. And the fact that Enceladus is a priority in the decadal survey means that we're going to be spending a lot of our time advocating for these kinds of missions. So if anybody out there isn't a Planetary Society member, you can help support this kind of research by becoming a member because this is literally our jam.

Chris Glein: Yeah, we're we're going to be continuing to advocate for these kinds of missions. The decadal survey was clear that this is going to be a priority in the coming decade or two decades, sending missions to the outer solar system like Enceladus to look for evidence of life.

Sarah Al-Ahmed: And then we can apply that information to what we know on Earth and then our exploration of exoplanets with things like JWST, because we're about to get a lot of information about at least the atmospheric composition of worlds far from our own, and we're going to have to figure out how to interpret that.

Chris Glein: Right. That's where everything's kind of interconnected. Earth is an ocean world. We've been talking about Enceladus, which is an ocean world. And then many of these exoplanets we'll find are probably also going to be ocean worlds. We may not get the same kinds of data from these exoplanets. We certainly won't be able to sample ocean water. I believe we won't be able to sample ocean water on an exoplanet anytime soon. But maybe we can take some of what we've learned from Enceladus and the Earth, and then we can try to construct new models under exoplanet conditions to get a more universal concept of how oceans evolve. So maybe oceanography on Earth and oceanography on Enceladus are like two branches, maybe larger field of oceanography that we don't yet understand.

Sarah Al-Ahmed: Wow. Astro oceanography sounds like a topic that I would love to get into, and I hope someday people can major in it, get their PhD.

Chris Glein: That would be great. I think we're headed there though.

Sarah Al-Ahmed: Mm-hmm. But it begins with things like this research, with understanding more about our local worlds and how we can apply it to everything else. So thank you to you and your team for doing this research. Who else on your team was participating in this?

Chris Glein: It's been a pleasure working with everyone. Frank Postberg was on the team. I mentioned him earlier. He had a phenomenal group in Germany of many postdocs and graduate students who did a lot of the painstaking labor of performing laboratory experiments and looking through the data very carefully. The Japanese team also had postdocs and students. That team was led by Yasu Sekine. And then I was the main American lead at the time this research was performed. It was an interesting process because we're trying to coordinate a research project between Japan, Germany, and Texas where I'm from. So you can imagine trying to organize a phone call. Somebody's having to be calling in either at midnight or very early in the morning. I think we just kind of turns to decide how we would coordinate this.

Sarah Al-Ahmed: That's the beauty of space exploration. It at least feels to me, and maybe just because I'm steeped in it, but it feels like it's one of those fields where just international collaboration is so necessary and so much of a part of this research.

Chris Glein: Yeah. It was a global effort. Everyone had a unique role. It wasn't like we could just find somebody else who could do the same thing. Everyone was very specialized and they were the exact right person to help us make this discovery.

Sarah Al-Ahmed: Well, at least we're only coordinating across time zones on a single planet and not having to say coordinate with someone living on Mars, that might get a little wacky.

Chris Glein: I'd be okay with that. That would be cool.

Sarah Al-Ahmed: Just get the whole team on Mars time. We've already figured it out.

Chris Glein: Eventually, if we continue to be successful, we'll be doing that and possibly elsewhere further into the solar system.

Sarah Al-Ahmed: A whole research station on Enceladus with Saturn in the sky above. That sounds like a dream.

Chris Glein: I think the forecast might be... It would be interesting at first, but then it might get kind of mundane because the forecast would always be that it's snowing at the South Pole.

Sarah Al-Ahmed: Yep. Well, thanks for coming on to explain all of this, and I am so excited to learn more. I hope that JWST can really help us analyze this. But considering that it's already teaching us about worlds that are hundreds of light years away, I think there's a good bet.

Chris Glein: Thanks, Sarah. Stay tuned, everyone.

Sarah Al-Ahmed: Even the smallest detection of life on another world would completely change our understanding of life in the universe. We can't say for sure what's happening in the oceans under Enceladus' icy crust yet, but each new puzzle piece adds to the picture. Now, let's check in with Bruce Betts, the chief scientist of The Planetary Society for What's up? Hey, Bruce.

Bruce Betts: Space. Hey, Sarah.

Sarah Al-Ahmed: Have you been dancing with your shadow and letting your shadow lead?

Bruce Betts: Yes. Although I do like to lead occasionally just to stay in practice.

Sarah Al-Ahmed: We're going to explain that reference in a second. People have heard it at the last couple What's Ups? But before we get to that, what's up in the night sky this week, Bruce?

Bruce Betts: This planet's running away low in the west. You got Venus super bright, but getting lower and lower soon after sunset. It will sink below the horizon very shortly. Mars in a few weeks, it's up above, but much, much dimmer looking reddish. But the good news is we've got Saturn and Jupiter coming into the evening sky over in the east. Saturn rising in the late evening, and Jupiter rising a couple hours later. Both of them are very high up in the sky in the predawn. And a little look ahead, Perseids meteor shower coming up, coming up. I get psyched. And that should be a good year. That'll be not until August 12th, 13th. And that'll be the peak, but we're already getting some Perseids, so. Good meteors and not a moon interference, it'll be good stuff. We'll get back to you. Onto this week in space history, it was a big, big day in automotive history, the first driving by humans on the moon of a lunar vehicle, the lunar roving vehicle, Lunar rover, was driven on the moon for the first time in 1971 by the Apollo 15 crew.

Sarah Al-Ahmed: I wish I could drive a little rover around on the moon. That just sounds like a good time. But I'm sure there are difficulties that I am not thinking of.

Bruce Betts: That's hard to believe. Space? Exploration? Difficulty? Nah.

Sarah Al-Ahmed: Nah. Easy-peasy.

Bruce Betts: All right, onto random space fact.

Sarah Al-Ahmed: Why so sad?

Bruce Betts: I don't know. Just looking for something different. I'm not sad. Those are tears of joy because the Great Red Spot of Jupiter has a height or a depth, depending on how you look at it, of anywhere from 200 to 500 kilometers. That's a lot more than, say, your run of the mill hurricane, the typhoon, maybe a couple kilometers. Yeah, 300 to 500 kilometers deep as Juno has found out as it orbits and studies Jupiter. So pretty cool base. Still horizontally bigger than Earth, but a few hundred kilometers in depth. More height, depending on how you look at it.

Sarah Al-Ahmed: And that storm is terrifying. I can't even imagine how many even more giant storms there are out there in the universe. Yikes. But okay, so this is a big moment. We announced a few weeks ago that we're moving our Space Trivia contest out of Planetary Radio and into our Planetary Society member community. So what was our last question, Bruce?

Bruce Betts: Well, for better or for worse, it was I quoted the wonderful heavy metal band warrant in my PhD thesis saying, "Dancing with my shadow and letting my shadow lead." What shadow was I referring to in that reference? How did we do?

Sarah Al-Ahmed: That was a hard question, but Planetary Radio fans were undeterred. They took to Google, they downloaded your paper, they read it. And in some cases, they even cited the chapter and page number of the answer. Just in case you're curious, it's chapter 6, page 115, but the answer is the shadow of the moon Phobos. What was your thesis about, Bruce?

Bruce Betts: I don't know. No one wanted to read them. They all thought, "Oh, sure, it's fine." No, it was using a little known, still little known dataset, the Termoskan dataset from the Soviet Phobos '88 spacecraft. Remember, this was during a period when the US had nothing going to Mars for between Viking and eventually Mars Observer, yeah. So anyway, I ended up doing my thesis with this data set that things kept changing because one spacecraft failed, then the other spacecraft failed in orbit. But what they did get was Termoskan had a thermal infrared imaging channel and a visible channel. And so I studied many different things from channels and valleys and thermal modeling of the surface. But one thing I looked at was the shadow of Phobos where they had it visible and in infrared to estimate the thermal, inertia. In other words, the resistance, the heating and cooling of just the upper millimeter or so on the flanks of catharsis volcano, because that eclipse will only last like 20 seconds. And so it is amazing that the surface in those areas cools down like four or five Celsius or Kelvin's in that time because it's really light and fluffy is what it said. A lot of dust fallout. And then we use the daily changes in temperature to estimate the thermal inertia of the upper centimeters/meter. And you can use seasonal changes, but this was a weird, unique opportunity at that time to use shadow of Phobos. Anyway, I could go on and on, but people, can I refer you to chapter 6? Page what?

Sarah Al-Ahmed: 115.

Bruce Betts: Of my PhD thesis. Or you can look up the paper in the Journal of Geophysical Research by myself and my colleagues, Bruce Murray, my PhD advisor and co-founder of The Planetary Society and Tomas Svitek. So it was a good time.

Sarah Al-Ahmed: That is a good time. And our winner this week. Oh, man. Big moment. Drum roll please. Greg Sakora from Shelby Township in Michigan, USA. Greg, big cheers. Thank you so much for participating. I just put together your prize. So we've got an awesome grab bag of space prizes and I hope you cherish it. It's funny because I said in a previous show that this was going to take people in a deep dive down Google. One of our regular listeners, Pablo Kumesha from Minsk Belarus wrote in to say, "My sweet summer child, what do you know about Googling?" Excellent Game of Thrones reference. He said, "I still remember one Space Trivia question about penguins as part of some Planetary Society project logo." He said the search took him hours and in the end he found no answer, and that feeling of futility still haunts his nightmares.

Bruce Betts: Wow. I'm so proud to have had such a profound and disturbing effect. No, well, I'm sorry. Anyway, I would like to, before we move on, to thank everyone who over the last 20 plus years has entertained me with their answers to Planetary Radio trivia. And I hope you join our member community and keep playing with the Space Trivia.

Sarah Al-Ahmed: And I think I speak for all of us, Bruce, when I say thank you. You have been the rockstar behind this competition for 20 years, and that's over a thousand trivia questions. And you didn't repeat yourself. That is a prodigious task.

Bruce Betts: Not as far as I know. Maybe once or twice I screwed up, but I really tried not to, which was why we ended up with questions about my thesis and penguins on stickers.

Sarah Al-Ahmed: So many wonderful comments that people sent in on this. I usually don't read every single comment that pertains to the trivia question, but some of these were just so good. Elijah Marshall from Leeton, Australia wrote in to say, "Dang, Bruce. Sneaky way to get the views up on your PhD thesis."

Bruce Betts: Totally. They were so low, I cannot imagine why.

Sarah Al-Ahmed: And David Floyd from Athens, Georgia, USA said, "I have no idea on this one. And Google didn't help me out with finding warrant lyrics."

Bruce Betts: What?

Sarah Al-Ahmed: And another listener from Australia wrote, Ian Gilroy from Maroubra, New South Wales said, "The answer was Phobos, the Martian moon. Unless Bruce was referring to Phobos, the Greek God of fear and panic. Is there something Bruce isn't telling us?"

Bruce Betts: Wow. I can neither confirm nor deny that.

Sarah Al-Ahmed: Yeah. But as is tradition, people sent us wonderful poetry about your Space Trivia question. They have to read both of these because you deserve to hear the beautiful poetry that people sent in. David Fairchild, our poet laureate from Shawnee, Kansas, USA wrote in, "His thermal and visible studies would make him a PhD czar. And Bruce Harolds Betts with little regrets was seeing a shadow on Mars. The shadow of Phobos was moving. The Termoskan data would show penumbral eclipses resulting with each 20 seconds or so."

Bruce Betts: Wow.

Sarah Al-Ahmed: It's clever.

Bruce Betts: That's very impressive.

Sarah Al-Ahmed: Right? They even brought in the Termoskan data? Like, woof.

Bruce Betts: Yeah. Nice.

Sarah Al-Ahmed: And this one's a little longer, but I do want to read this because I thought it was really beautiful. Gene Lewin, another one of our favorite Planetary Radio poets wrote, "The colors of the spectrum reveal secrets in their hue, each frequency and elements and atmospheric clue as we gaze into the universe with increasing acuity, attending telescopic vision through improved technology. But within our solar system, our planet still can mystify. We rely on their albedo to illuminate and clarify. Light can show us features, but shadow is needed too. Shadows give us contrast, helping determine what is true. A student then of Caltech didn't need to use just light, for the shadow of the moon Phobos provided him with new insight. So do not fear the darkness. Answers do lie there. Variations in the surface in his thesis he did share. And truly this did warrant his awarded PhD, the chief scientist and program manager of The Planetary Society."

Bruce Betts: Wow, that was impressive.

Sarah Al-Ahmed: Right?

Bruce Betts: It even worked in warrant.

Sarah Al-Ahmed: But I just want to thank everyone so much for participating in the Space Trivia contest on this show for all of these years. It's been 20 years and everyone clearly loves this contest. And I'm so looking forward to the next iteration of this. I feel like we've had some conversations about it. And as we move this Space Trivia contest into our member community, I'm hoping it's going to allow so many more people to participate. And already I've collected some really cool swag to give away for this, including a piece of Star Trek stuff signed by one of the cast members. So keeping it just for such an occasion.

Bruce Betts: Now, what's the status of my suggestion that all the questions have to do with my thesis?

Sarah Al-Ahmed: We'll take that into consideration. But please, everyone, if you want to send more poetry and love to Bruce, you can either message him on our Planetary Society member community or email it to us at [email protected]. I will send all of it to Bruce because we need to shower him in the love he deserves.

Bruce Betts: Aw, I told you they were tears of happiness. All right, everybody, go out there, look up the night sky and think about how great you are. 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 to talk about a mysterious hotspot on the far side of the moon. Planetary Radio is produced by The Planetary Society in Pasadena, California and is made possible by our ocean world obsessed members. You can join us as we continue to support the search for life off of Earth at planetary.org/join. Mark Hilverda and Rae Paoletta are our associate producers. Andrew Lucas is our audio editor. Josh Doyle composed our theme, which is arranged and performed by Pieter Schloser. And until next week, ad astra.