Revisiting the discovery of phosphorus on 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.

Andrew Lucas: And I'm Andrew Lucas, the audio editor for Planetary Radio. We hope you all had a very happy New Year's Day. So unfortunately, Sarah is out sick this week, but don't worry, she's going to be back next week with new exoplanet science from the James Webb Space Telescope. Today, however, we're looking back on one of the most significant recent headlines in planetary science, the detection of phosphorus in the oceans of Saturn's Moon Enceladus. Phosphorus, a key ingredient for life as we know it has been discovered on Saturn's moon Enceladus. This is the first time phosphorus has ever been detected in an ocean off of Earth. Chris Glein, a lead scientist at the Southwest Research Institute, joins us to talk about the discovery and its implications for the search for life off of Earth. Then Bruce Betts pops in for what's up. If you love Planetary Radio and you want to stay informed about the latest space discoveries, please make sure to hit that subscribe button on your favorite podcast platform. By subscribing, you'll never miss an episode filled with new and awe-inspiring ways to know the cosmos and our place within it.

Sarah Al-Ahmed: 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 de-codes 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 paper is called Detection of Phosphates Originating From Enceladus' Ocean. It was published on June 14th, 2023 in the Journal Nature. Thanks for joining me, Chris.

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

Sarah Al-Ahmed: 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've 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. He got this look on his face. I'm happy you're happy, but what? 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.

Christopher 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?

Christopher 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. And 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, and sometimes I waffle between Enceladus and Io because who doesn't love volcanoes? 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.

Christopher 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.

Christopher 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 its geophysical situation, is giving 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 are 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.

Christopher 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?

Christopher 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 river 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 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 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.

Christopher 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 a hundred times more than there is on Earth?

Christopher 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.

Christopher 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 was 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 in 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, and as 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.

Christopher 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?

Christopher Glein: So Enceladus has, its erupting stuff from South Pole. That seems to be the area where all the activity is concentrated. And 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 kind of look like the stripes on a tiger. These cracks actually tap into the subsurface ocean on Enceladus. 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 a hundred of these jets have been mapped out. And 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 a hundred 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?

Christopher Glein: It puts out about two 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. And 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've formed them and how some of the other moons in the system might've 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.

Christopher 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. And 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.

Christopher 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.

Christopher Glein: Yeah, there's a couple of unexpected oxygen buried molecules like CO and CO2 in Titan's atmosphere, and 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?

Christopher Glein: The main instrument that discovered phosphorus is known as the 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. And 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's on Cassini or it was on Cassini before Cassini ended and 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 are 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?

Christopher 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 ringing grains that originated from in Enceladus' ocean. Sodium phosphate is one form of phosphorus. And 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 apatite. If you're a geochemist and if you're just a normal person, it's what your bones are made out of calcium phosphate, and 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.

Christopher 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.

Christopher 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 a hundred 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 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 rings as well.

Christopher 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, and we've also detected that these molecules go up to quite high masses above a hundred 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.

Christopher 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?

Christopher 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, its 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 extraterrestrial 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 and on Cassini's wild adventure. I can't even imagine what we could do with say an Enceladus orbiter or something.

Christopher 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.

Christopher 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: 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.

Christopher 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 has 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.

Christopher Glein: There's so many great things to study out there, study. 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're going to be leading a team that's going to be using JWST to try to learn more about Enceladus, right?

Christopher Glein: Yes, yeah. 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 JWST 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. And one that's been really confusing from Cassini is hydrogen peroxide, H2O2 and 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 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?

Christopher 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 like 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've formed by this process known as a accretion from the agglomeration of many smaller bodies, which might be similar to a type of meteorite known as a carbonaceous chore. 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 what 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 the 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.

Christopher 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 Enceladus, 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?

Christopher 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.

Christopher Glein: Yeah, it'd be interesting. And when I say young, young could still be a hundred million years, and so even if a body like Enceladus or other moons around Saturn are only a hundred 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?

Christopher 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 that I alluded to earlier where you take a meteorite to represent Enceladus' core, in this case a certain type of carbonaceous chondrite 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, 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 past that you can get more phosphorus absorbed up into the water because of it.

Christopher 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. And 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, 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 moons of 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.

Christopher 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.

Christopher 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?

Christopher Glein: Yes, absolutely. So what's really intriguing about the comparison is if you look in seawater phosphorus is a hundred or a thousand times less abundant in terms of concentration, but there are certain environments on Earth that might be like little Enceladuses, and these are known as soda lakes. So one famous example is probably close to you, is known as Mona Lake. It is a salty lake that's chockfull of baking soda, and another kind of soda is known as washing soda. It's a higher pH version of baking soda that's abundant in Mona Lake. And there are these other soda lakes in Washington state and British Columbia that are even higher in phosphorus. And it turns out those environments have the baking soda being very abundant, and it turns out phosphorus is also abundant. And 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 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?

Christopher 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 Whitlockite 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.

Christopher Glein: Absolutely. So we're really diving into the nitty-gritty of the geochemistry now. 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 sea shells are made out of. And the common theme between those two minerals is calcium, and 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 phosphorus 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. And when you have lots of carbonate present in the soda ocean, the carbonate wins, 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.

Christopher Glein: And 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 Planetary Society is 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.

Christopher Glein: Yeah. We're going to be continuing to advocate for these kinds of missions. A 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 JWSD, 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.

Christopher 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, and 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 of a larger field of oceanography that we don't yet understand.

Sarah Al-Ahmed: Wow, just 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.

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

Sarah Al-Ahmed: 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?

Christopher 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, and 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.

Christopher Glein: Yeah, it was a global effort, and 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.

Christopher 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.

Christopher 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.

Christopher 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.

Christopher 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: Hey, Sarah. How are you doing today?

Sarah Al-Ahmed: Doing good. I'm wondering, did you become interested in space as a kid, or were you one of the people that found that passion later in life?

Bruce Betts: Oh, I found it very early. So I was interested from a few years of age. A couple of significant things, one was watching from a distance, but the final launch of Apollo to the moon, Apollo 17 from a hotel in Florida, and that was profound. And then I had a second, third cousin who worked at JPL, and it was the days before the internet, so he would send me packages of the press release photos of Viking and Voyager, and that's what got me really fired up. Pretty pictures. Still love them to this day. And so those were kind of two of the pivotal things. And then just books that taught me more about it and time travel to the future and using the internet and then traveling back, the usj.

Sarah Al-Ahmed: The usj. One of our members, Gene Lewin, sent in a poem about the Viking missions that I thought was really beautiful.

Bruce Betts: I would like to hear it.

Sarah Al-Ahmed: Yeah, and it's cool because another person actually wrote me today saying that they wished more people knew about the Viking landers and their tests and search for life. So it was perfect. But here is Gene Lewin's poem about the Viking landers. "Off to Martian surfaces a Titan centaur [inaudible 00:53:56] the way with orbiters and landers to last for 90 days. Twin galactic long chips, Vikings one and two, searching for the signs of life as the mission so ensued. Alas, nothing definitive though if I were to decide when you see Vikings come ashore, it would be wise to hide. Instead of staying there on Mars, the Martians had a plan. They pulled up stakes and came to Earth and mingled among man. The landers when they both touched down, used a Dacron polyester shoot. The Martians may have upcycled it, explaining those folks in leisure suits. You see, it was the seventies, and with fashion ebbs and flows, Martians were hiding in plain sight in those casual pastel clothes."

Bruce Betts: Nice.

Sarah Al-Ahmed: But nah, man, I am looking forward to a day when we can go back to Mars thoughtfully, carefully and do these experiments again, because what happened with those Viking landers, if you're out there listening and you don't know, their experiments for life were very fascinating, very, what is the word? They couldn't determine whether or not there was actually current life on Mars, extant life at the moment. But they did provide some really big mysteries that I want to go back and learn more about.

Bruce Betts: Okay. One instrument and a small set of people claim there was evidence for life, but there was plenty of evidence that it was a non-biological release.

Sarah Al-Ahmed: Totally.

Bruce Betts: And the others are pretty negative. But yeah, it was a weird way to start Mars landed exploration to focus entirely on, we'll send a mission that looks for life and looks for life and looks for life, which now we've got a much more methodical way of surveying the planet, figuring out good places to look for evidence of past life, which is far more likely, particularly on the surface because we've learned the surfaces pretty nasty, not compared to my friend Venus, I'll talk about. Or called Venus my friend before, I'm sorry, Mars. Anyway, yeah, there are, it'll be nice and it'll be great if and when we get the samples back from the surface of Mars, which have been taken and are being taken by Perseverance, because that'll allow us to go crazy in Earth laboratories with much more advanced techniques.

Sarah Al-Ahmed: Right? You think we're going to learn stuff from the moon samples and the Osiris Rex rocks and all those things, but oh my gosh, the things we could learn from those Martian samples, once we get them, hopefully we get them. And if anybody wants to support the Mars sample return mission, we have a petition going on on our action center. So I'm going to link for that on this episode of Planetary Radio.

Bruce Betts: Nice.

Sarah Al-Ahmed: That way, if you want to kick in your vote behind actually bringing those samples back from Mars, we'll make sure it gets to the right people who can hear that.

Bruce Betts: How about we travel to Venus and I'll give you a [inaudible 00:56:52].

Sarah Al-Ahmed: All right, let's go to Venus.

Bruce Betts: All right, Venus. Probably know, sulfur dioxide clouds. That's what makes it so we can't see the surface. Pretty exotic, pretty wild, pretty nasty. But people often picture, I think when they hear sulfur dioxide, acid rain that is coming down on the surface, but it actually, the clouds are up at tens of kilometers and then it rains. And sulfuric acid evaporates around 300 degrees Celsius, which means you easily hit places where it evaporates and then goes back to the clouds and recondenses. So there is a cycle, but it never reaches the surface. So the surface is quite enjoyably pleasant, I mean, except for the crushing pressures and melting temperatures. But other than that, it's good. No acid rain. Yay.

Sarah Al-Ahmed: Yay. See, that actually explains a lot because I remember the first time I was looking through the Soviet Venera images of the surface of Venus, I remember thinking, but where are the pools of sulfuric acid? Because at the time I was younger, I didn't know that the rain didn't reach the surface, and I kept thinking like, why aren't there entire lakes of this stuff? That sounds horrifying. But there's-

Bruce Betts: Oh, there are, but they're in my backyard.

Sarah Al-Ahmed: Keep the dogs inside.

Bruce Betts: All right, everybody go out there, look up the night sky and think about fruit. Thank you and goodnight.

Andrew Lucas: We've reached the end of this week's episode of Planetary Radio, but we'll be back next week with new results from JWST about the exoplanet K2-18 b. You can help others discover the passion, beauty, and joy of space, science and exploration by leaving a review and a rating on platforms like Apple Podcasts. Your feedback not only brightens our day, but it also helps other curious minds find their place in space through Planetary Radio. You can also send us your space thoughts, questions, and poetry at our email at Planetary Radio at Planetary.org. Or if you're a Planetary Society member, leave a comment in the Planetary Radio space in our member community app. Planetary Radio is produced by The Planetary Society in Pasadena, California, and is made possible by our dedicated members around the world. You can join us and help many more amazing space mission launches to success at planetary.org/join. Mark Hilverda and Rae Paoletta are our associate producers. Me, Andrew Lucas is the audio editor. Josh Doyle composed our theme, which is arranged and performed by Pieter Schlosser, and I'm excited I get to say this. Until next week, ad astra.