Discover Science Podcast: Sarah Hörst on life as we do not know it

Atmospheric chemist and a leading researcher of Saturn's largest moon, Titan, talks with physics and astronomy alum Donna dePolo and astrobiology professor Carlos Marsical

Discover Science compilation image of hosts and Sarah Hörst

Counterclockwise from top-left: Sarah Hörst, Carlos Mariscal, Donna dePolo

Discover Science Podcast: Sarah Hörst on life as we do not know it

Atmospheric chemist and a leading researcher of Saturn's largest moon, Titan, talks with physics and astronomy alum Donna dePolo and astrobiology professor Carlos Marsical

Counterclockwise from top-left: Sarah Hörst, Carlos Mariscal, Donna dePolo

Discover Science compilation image of hosts and Sarah Hörst

Counterclockwise from top-left: Sarah Hörst, Carlos Mariscal, Donna dePolo

Discover Science podcast identifier
The Discover Science podcast is available on Apple Podcasts, Spotify and other major platforms.

Titan is Saturn's largest moon and is the subject of distinguished atmospheric chemist Sarah Hörst's research. The distant moon is considered to be one of the most Earth-like worlds in the solar system, and its potential to host life is the topic of conversation in this episode of Discover Science.

Hörst is an associate professor at Johns Hopkins University in the Department of Earth and Planetary Sciences and an adjunct astronomer at the Space Telescope Science Institute. Hörst is also part of the team leading NASA’s Dragonfly mission – a rotorcraft-lander expedition to Titan's surface.

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Discover Science: Sarah Hörst on life as we do not know it

Planetary scientist Sarah Hörst speaks with alumna Donna dePolo and Associate Professor Carlos Mariscal about Saturn's largest moon, Titan.

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Hörst speaks with alumna Donna dePolo and Associate Professor Carlos Mariscal.

DePolo graduated in 2021 with degrees in astronomy and physics and was also a successful Wolf Pack athlete as a member of the swim team. As an undergraduate, dePolo published research titled "The flickering radio jet from the quiescent black hole X-ray binary A0620-00" with Assistant Professor of Physics Richard Plotkin.

Mariscal is an associate professor of philosophy interested in understanding the origin of life, the nature of extreme organisms, and what we can know about life in the universe. Mariscal is faculty in the Ecology, Evolution and Conservation Biology graduate program where he works in areas related to the evolution, origin, and distribution of life in the universe, a field known as astrobiology.

The Discover Science podcast is recorded in the Reynolds School of Journalism Radio Studio. Past episodes have been produced in partnership with the Reynolds School's Hitchcock Project for Visualizing Science.

Three podcast participants in the Radio studio

Conversation highlights: quotes from Hörst

"Well, first of all, all chemistry is interesting. So that's all I’ve got to say about that."

"Is it possible that life exists that does not use liquid water as a solvent or transport medium? Is there life that is not based on carbon the way life on Earth is? Titan helps us answer that question because on the surface we have huge lakes and seas made out of methane and ethane. If life could figure out how to use methane and ethane, we have one place here in the solar system where we have a non-water liquid sitting there that life could potentially take advantage of."

"I would be slightly disappointed if [life on Titan] was water-based, to be completely honest. I mean, you have this thing sitting right here where you can be really, really weird. And instead, Titan's like, No, I'm cool with the water. [...] I want there to be weird little Titan fish that are living in the methane lakes because that would be really cool."

"I really expect when we get to the surface with Dragonfly that we are going to find some substantially weird chemistry. And it's going to be very frustrating because I predict we're going to have to spend a lot of time trying to prove that the weird chemistry is not life."

"You hear a lot of, 'we need to go to Mars or wherever to protect humans.' Let me tell you, as a planetary scientist, Earth is really our only good option. All of the other planets really desperately want to kill you in a whole bunch of different ways. This is the only one that is even remotely hospitable to us. And so it's incumbent on us to actually keep it hospitable and habitable because the other options are just not a good idea."

About the Discover Science lecture series

The Discover Science podcast is an offshoot of the public lecture series by the same name. The Discover Science Lecture Series was founded by the College of Science in 2010, with the goal of bringing the country's top scientists to the University to share their knowledge, research and wisdom with the community.

"Science encompasses a wonderfully diverse collection of explorations into the unknown. We invite science lovers and the science-curious to join us and experience the extent of the science universe as the best scientists on the planet visit the University of Nevada, Reno for our Discover Science Lecture Series," Jeff Thompson, executive vice president and provost of the University and founder of the Discover Science Lecture Series, said.

Past speakers in the series include astrophysicists Michio Kaku and Neil deGrasse Tyson; Robert Ballard, who discovered the wreck of the Titanic; and Bill Nye the Science Guy.


Sarah Hörst:

This is going to going to go well. I can already tell.

Donna dePolo:

So close your eyes if you can, and imagine a landscape thick with haze drifting over vast rolling hills of sand, a river of liquid methane cuts through surrounding mountains made of ice and flows in and out of lakes and into a vast sea. Saturn and its rings are obscured behind an orange-brown haze. The sun is a nondescript glow in the twilight sky. 

The landscape is alien in many ways, but feels strangely familiar. Glancing around at the Highlands, blanketed in water, ice, the glistening lakes and the organic sand dunes, you can't help but be reminded of our home planet Earth. Now imagine what might life look like on this extraterrestrial moonscape. Welcome to the Discover Science podcast, where we speak to leading researchers about the exciting discoveries of our time.

I'm Donna dePolo, a recent physics and astronomy alum of the University of Nevada, Reno, interested in planetary science and other astrophysical phenomena. I'm here with my friend and co-host Dr. Carlos Mariscal, associate professor of philosophy, interested in understanding the origin of life, the nature of extreme organisms, and what we can know about life in the universe.

Carlos Mariscal:

Thank you, Donna. The landscape we just asked you to imagine is that of Titan, Saturn's largest moon, and the subject of distinguished atmosphere chemist Dr. Sarah Hörst's research. Dr. Hörst is an associate professor at Johns Hopkins University in the Department of Earth and Planetary Sciences and an adjunct astronomer at the Space Telescope Science Institute. Dr. Hurst is a leading researcher of the complex organic chemistry occurring in the atmosphere of Titan.

With her research, she contemplates life as we do not know it. What could it be like on this lunar landscape? Welcome, Dr. Hörst. It's a pleasure to meet you.

Sarah Hörst:

It's nice to meet you. Thanks for having me. 

Donna dePolo:

All right. So before we jump into the big questions of life, I want to set a little bit of framework for where in the solar system we're looking at. Titan is the largest moon of Saturn and it's the second largest moon in our solar system. And it's unique in our solar system as it's the only moon to possess a dense atmosphere composed of 95% nitrogen and 5% methane.

Additionally, it is the only known object in our solar system beyond Earth with clear evidence of stable liquid on the surface. That brings us over to you, Dr. Hörst. Can you tell us a little bit about your research into Titan's atmosphere, like what makes it unique? What's complex about it?

Sarah Hörst:

Yeah, definitely. I think you hit on a couple of the things already, so Titan is really unique in the solar system and there are a couple of reasons for that, which you already mentioned. It has a dense atmosphere that makes it really weird. It's the only moon in our solar system that has an atmosphere really at all. And so immediately we're like, okay, that's weird. We should figure this place out. The other thing that makes it really unique, which you also mentioned, is the only other substantial atmosphere in our solar system whose primary gas is nitrogen besides Earth. And so just those two things alone. If nothing else was happening in Titan's atmosphere, it would still be incredibly interesting to us. But that's not the only part of the story.

That methane that you mentioned gets broken down by light from the sun, and that starts chemical processes that result in the formation of really complicated organic compounds in the atmosphere. And so that's what we're so interested in. What are those compounds? What happens to them when they're on the surface? Is there the possibility that life exists on the surface that has figured out how to take advantage of these compounds?

And even if there isn't even if there's no life on Titan today, never was, never will be sorry. A lot of these processes happen in the atmosphere of the early Earth. We don't have good records of the time when life originated on Earth, and so instead of building a time machine, what we do instead is build spacecraft and travel to Titan and study those conditions to try to help us understand the origin of life on our own planet.

Carlos Mariscal:

So it's analogous with respect to the atmosphere of early Earth. It's much smaller than Earth, much farther further away, much colder, much darker. So it's not going to be analogous in other respects. Is that the…

Sarah Hörst:

Yeah. So, the main analogy that we draw basically is that the current Earth's atmosphere is about 78% molecular nitrogen, and the rest basically is oxygen. But the oxygen is only present because there's life on earth. Before there was life on Earth, there was no oxygen in the atmosphere. And instead, we had a nitrogen-dominated atmosphere that probably had a fair amount of methane in addition to some other compounds.

And so, it's the chemical analogy that we're drawing. But you're right, the temperature of early Earth would have been much warmer. We're closer to the sun, so we have more sunlight coming. Gravity is different. All these things play a role on the chemistry, but it's kind of one of these situations where beggars can't be choosers. We have something that has at least some of the same conditions happening. 

And so, you know, we'd be really remiss to not study it. And as with all analogies, it's never going to be perfect. That's why it's an analogy and not the exact thing, but there's still a lot that we can learn from doing that and really help us understand more about our own planet.

Carlos Mariscal:

Thank you. 

Donna dePolo:

That's really awesome. In what ways does Titan's atmosphere … I know there's a pretty thick haze also surrounding the planet. My goodness. Pretty thick haze also surrounding the moon. How does this serve to protect the potential of early life-like phenomena from hazards. We’ve got different radiation going on, cosmic rays, we’ve got UV or maybe a little bit of UV. What is it, a buffer? What's going on there?

Sarah Hörst:

Yeah, that's a great question. First, though, because you just called Titan a planet and I promise you I'm going to do it a billion … I've already done it a billion times today. We tend to, as planetary scientists, if a planet, if a world does planet things, we tend to call it a planet, even if it's not a planet. Excellent. So this isn’t really a political statement on the whole Pluto thing or anything. It's just like I'm going to call Titan a planet about a billion, zillion times, so it's fine. So there's that.

But back to your question about, you know, the role that the haze layer might play. So one thing that's really interesting about particles in the atmosphere is they interact with light differently than gases do. And one of the potential consequences of that is that they can absorb light that otherwise would get through the atmosphere. And so, as an example, UV light does not really reach the surface of time at all because it gets absorbed much higher in the atmosphere, some by haze particles, some by some of the other molecules in the atmosphere. 

So we think that early Earth may have had something similar. And the reason why that's important is that before life really dramatically change the composition of Earth's atmosphere, there wasn't any oxygen, which meant there wasn't any ozone because the ozone layer forms from sunlight impacting or radiating the oxygen in the atmosphere. And so there wasn't anything in early Earth's atmosphere to protect nascent life on the surface unless there was a haze layer.

And there are a lot of reasons why we think that early Earth did have a haze layer. And if so, it would have played the role that the ozone layer plays. Now, it would have been able to absorb a lot of those dangerous photons and prevent them from getting to the surface. And the reason why that's important is the same reason that the ozone layer is important to us today. Right? Those energetic, you know, the energetic light from the sun is very dangerous to us. It's why we have skin cancer, right? Those energetic photons can actually destroy our DNA, the molecules that are really important for life. And so, you can think, especially when we had brand new, just trying to figure out how life goes creatures on the surface, they haven't necessarily figured out self-repair mechanisms yet, all of those kinds of things. If they get hit by a lot of that UV light, that might be lights out for them. And so that's one thing that we think could be really important, both for the potential for life on Titan, but also early Earth.

And then just thinking about what are the conditions that are required for life. Is an atmosphere really important? Is it one of those like, you know, it's like the walk-in closet in the house, like you'd love to have it, but, you know, maybe you're not going to be able to buy that house, and so you're just going to have to get along without one.

Carlos Mariscal:

So I realized we are triangulating here from a physics and astronomy perspective, with which we've been asking you questions about now and now we're going to switch to a more philosophical and biological perspective. So we're coming at it from both sides.

Sarah Hörst:

Lovely. I didn’t know what I was getting into when I signed up for this.

Carlos Mariscal:

But it's great. Hopefully, this will be nice. Interesting questions for an interesting I'm excited.

Sarah Hörst:

Let's go.

Carlos Mariscal:

I spent some time a couple of years ago at the Earth Life Sciences Institute in Tokyo, and one of the things that people were advocating there was this slow chemistry. I also heard people talking about messy chemistry, but slow chemistry. And it was this observation that most of the research that we do, and it's not just in chemistry, it's across academia.

Most of the research that we do are the kinds of projects that can be done within a graduate career or within a postdoc or within the tenure track. So we're looking at 4 to 6 years, more or less, and that leaves entire processes that would be longer and perhaps equally interesting as things that we never study – think processes that could take it ten, 100 a million years. 

So, here's a very hypothetical question for you. Let's say a time where no object and you could spend an infinite amount of time or as long as you want to study Titan, what would you want to study there?

Sarah Hörst:

Yeah, it's funny that you ask this question because it's like a joke that I make a lot of times when I have kind of academic seminars when people ask some questions. So in my lab at Hopkins, we run experiments where we simulate the chemistry that happens in Titan's atmosphere to try to understand it better and figure out what molecules are made, you know, would that be a tasty treat for and is it life like you know, all those things.

Um, one of the problems is that the chemistry that happens in the top of Titan's atmosphere happens on a timescale scale of many hundreds of years. Um, and so the joke that I wasn't, which is exactly what you brought up, the framing this question is that of course we have to find a way to speed that up because none of my grad students want to be in graduate, right?

So, we do things to make the processes happen faster in the lab that are artificial and we don't necessarily know what impact doing that has on the answers that we get. And so if I could, I would much prefer to run my experiment for many hundreds of years than for like a week. But of course, as you mentioned, like my grad students, I want to do it. Funding agencies don't want to do it. And honestly, I plan to retire at some point. And so I'm not particularly excited about running 100 years, but there are some I think it's incredible. Like there are some experiments that have been very, very like long-lived for people who are trying to measure the viscosity of the pitch, which is like, you know, it's had six results or seven results. 

And so the entire history that this experiment has been running and I mean, I appreciate the commitment to it cause I'm not sure that I have that much strength of character to be able to like, decide to do something like that. But I think it's a really interesting problem and I'm not surprised that it came up in that context when you were at LC, but it definitely, I think especially in a lot of origin of life research.

And actually, it's funny because it came up when I was talking to some high school students earlier today. We try to do things very quickly in the lab and then if the result we want in terms of origin of life, if stuff doesn't happen, then it's just kind of like, oh, it can't happen that way.

But what if we left that thing sitting there for like 500 years? You know what I mean? Like, we didn't, we left it sitting there for two weeks and a creature didn't crawl out of it. So now people want to eliminate it as an option. And it's like, I don't think the origin of life took the timescales that any of our laboratory experiments are operated on.

And so it's very, very possible that we're missing out on what would be a relatively straightforward answer if we ran the experiments on the right timescales. But at some point, you just have to accept that this is how the cookie crumbles. You know, do the best you can with what you have. And at the end of the day, and we say this all the time, we were kind of pitching Titan as like, you know, the cool place that we should go to. The universe has been running this experiment for four and a half billion years on Titan.

So fine, let's go collect the answer. Like we don't need to run the experiment ourselves. It's already been run. Someone else did all the work. There were no grad students harmed. So let's. Let's go figure it out.

Carlos Mariscal:

That's brilliant. And I really should leave that there. But I'm very curious, you know, you sometimes should exit on the right things, but. But I'm interested. You said you could speed up some reactions. I assume that, you know, you don't have the dial for time, but you know that some things can catalyze reactions, make things go faster. That also has got to be the things that you focus on.

Right? Like you would maybe make methane produced faster and it would have some effect that you might be able to quantify and might not.

Sarah Hörst:

Yeah. So, the main thing that we change, there are two things that we change that make what we're doing happen faster than what happens on Titan. The one thing we change is pressure. So we run our experiments at a higher pressure than where this chemistry happens on Titan. And the reason that speeds up the experiment is that when there's higher pressure, the gases collide with each other more often.

You can imagine if you are at a party and you have been instructed to go find someone who has a great cocktail recipe, right? You're going to like go around the room talking to people, trying to find this person who has this great recipe right? If the room is, you know, ten square miles and there are five people in it, it's going to take you a long time to find the cocktail recipe person because you're just spending a bunch of time trying to meet the person, right?

If the room for the party instead was, you know, like 50 square foot room, it's going to be much faster to find your cocktail recipe person because you just interact with everybody faster. So this is the same thing. We've increased the pressure, our molecules are meeting each other faster. And so that speeds up the chemistry. Unfortunately, if you go to high enough pressure, it actually changes the chemistry that's possible.

So you've not only sped it up, but you've changed it. We have to be really careful about kind of trying to toe the line between making it happen faster or making it happen differently. That's one thing. The other thing that we change is the amount of energy that we put into these experiments, the way in which we simulate the sun, is at a much higher energy density than what is actually happening on Titan.

And that also just speeds the chemistry up because the molecules aren't sitting there waiting for a photon to help them go off on their new chemistry adventures. They don't have to wait as long again. There's the possibility that we're only speeding it up. Sure, but there's also the possibility that we're changing it. And it's not super straightforward to actually mess around with both of those variables and see whether or not we're changing things. Because in the lab, other things change too when you change them. So sure. But that's how we speed it up. And it you know, it just is what it is.

Carlos Mariscal:

I mean, but it's interesting either way, right? Like it might turn out not to be an analog for Titan, but it would be an analog for a hypothetical situation.

Sarah Hörst:

Oh, yeah, absolutely. I mean, it's not that we're not we are doing these experiments. We learn things, we may not learn the thing that we are trying to learn, but we will learn a thing that will help us understand. At the end of the day, you know, how all these processes are working and in atmospheres.

Carlos Mariscal:

One of the reasons people are interested in Titan is because of its potential for life. And so a lot of astrobiologists are interested in the question about drawing the line between interesting life-like phenomenon and uninteresting abiotic chemistry. I kind of want to flip this question on its head and ask you what would be the most interesting thing you think we could find on Titan that you would still not wish to call life and on the other hand, let's say we did discover life on Titan.

What would be the least interesting thing we could discover about it? So in other words, is there interesting chemistry and boring biology?

Sarah Hörst:

Yes. Cool. Okay. Well, first of all, all chemistry is interesting. So that's all I’ve got to say about that. I might come back to that in a second.

Carlos Mariscal:


Sarah Hörst:

Taking a step back, and thinking about your question, Titan is interesting for astrobiology reasons, for a couple of reasons. And I think you can kind of boil it down to two things. So we have two really big-picture questions about life in the universe. And this is not new information from my brain because I'm brilliant.

I have stored a lot of this thought process from Jonathan Lunine, who's a very famous astrobiologist. But there are two big questions, and one of them is, is life ubiquitous? Which is to say that it's probably relatively easy for it to originate. If you give it all the red stuff, it goes off and has a party and you have life.

There's no, you know, low probability event that causes it. It's just like the stuff here in life. And that would mean that it's everywhere the ingredients exist. Great. If it's easy, if it's ubiquitous, then anywhere that has liquid water and energy source and organic material has life. If that is the case, then we should be able to find life in all of the subsurface oceans in the solar system. 

So, Europa should have life and Enceladus should have life, Triton should have life. Pluto might have a subsurface ocean, it should have life. Maybe did I mention Enceladus? I don't remember. That's one question. Titan helps us answer that question because Titan also has a subsurface ocean. So that's one thing. The other big picture question we have about life in the universe, assuming that it exists elsewhere than Earth, although actually this will still answer the question, but is life diverse?

Which is to say, are there lots of different chemistries? Right. Is it possible that life exists that does not use liquid water as a solvent or transport medium? Is there life that is not based on carbon the way life on earth is? Titan also helps us answer that question because on the surface we have huge lakes and seas made out of methane and ethane. If life could figure out how to use methane and ethane, we have one place here in the solar system where we have a non-liquid-water liquid sitting there that life could potentially take advantage of. I think those are the two, you know, kind of big-picture things that we're interested in. Both of them can be answered by looking for life on Titan.

None of that is related to the question that you asked me, but I'm getting there I promise. If we found life on Titan like that would be – I desperately want to curse right now – It should be the wrong answer, but that would be really cool. Insert whatever curse word came to mind when I said that just now.

But I would be slightly disappointed if it was water based to be completely honest. I mean, you have this thing sitting right here where you can be like really, really weird. And instead, Titan’s like, no, I'm cool with the water. That's fine. I think that's relatively unlikely for a number of reasons. Titan's subsurface ocean is pretty deep, and so of all of the ocean places I just mentioned, Titan is probably one of the less likely subsurface liquid water oceans to harbor life. 

That would be a bummer. I want there to be weird little Titan fish that are living in the methane lakes because that would be really cool. I mean, first of all, that would tell us immediately… I mean, that would be groundbreaking. That would be a pain because it would tell us that we can look for life in more places than we have been looking because we have mostly focused on places where there can be liquid water.

I would be a little bummed out if Titan didn't serve us up like the weirdest option that it could. But again, I would I'm not going to like say no. Like if there's fish in the subsurface ocean, like, that's fine. Like, awesome. Thank you. In terms of like weird chemistry, I mean, I think we already know that Titan's chemistry is really weird.

The chemistry in the atmosphere is so much more complex than we believed it could be. We still don't understand exactly why it works the way that it does. I really expect when we get to the surface with Dragonfly that we are going to find some like substantially weird chemistry. And it's going to be very frustrating because I mean, I predict right now we're going to have to spend a lot, a lot of time trying to prove that the weird chemistry is not life.

And maybe it will turn out that the weird chemistry is life. But I am very nearly positive that we are going to find some really seriously weird chemistry and then have to spend a lot of time trying to figure it out. But that would be exciting. I would be a little bit disappointed if Titan was like, Nope, you guys figure this out years ago.

I mean, the good news for history, planetary exploration already tells us that that is not what's going to happen. Like places are always so much weirder than we imagined. They could be. And that's one of the fun things about going somewhere again, right?

Carlos Mariscal:

I mean, the Viking missions and Mars detected lots of things. I mean, famously, Gil Levin still … actually he passed away recently. Yeah. He, until his death, he still argued that he had discovered life on Mars.

Sarah Hörst:

Yeah, he was very sure that Viking showed evidence of life on Mars. He was probably the only one like it.

Carlos Mariscal:

It was his experiment, I mean.

Sarah Hörst:

I mean, he knew it better than anybody else. And so on the one hand, you're kind of like, okay. And on the other hand, you're like, wow.

Carlos Mariscal :

But so, so life detection. I mean, we had the problem with the Viking landers in the seventies where we had four separate, depending on how you measure it, experiments to detect life. And it seemed fairly difficult to try to like thread the needle so that all of them were successful in detecting life. So people decided it was inconclusive, which meant no detection.

And so I guess the first part of the question is, are we sure we could even detect life as we know it on Titan? But then the second part of the question is directly related to Dragonfly, which you just mentioned. So I know you're a part of it. I know. And I guess I just want to give you space and time to tell us about it, what your role is, what you're interested in, and some more about that.

Sarah Hörst:

Yeah, that's a that's sort of … I’ve never thought about that question before. My brain hurts a little bit right now. [laugher]

Carlos Mariscal

I'm sorry.

Sarah Hörst:

So first of all, I mean, I think it's one thing that's interesting, you know, talking about all this life stuff is that in terms of solar system missions, the only mission that has ever flown, that had life detection as one of its goals was Viking.

After that, everyone got kind of freaked out and just stopped proposing to do those kinds of experiments, in part because I think that people take Viking as a cautionary tale. And I really appreciate the perspective of Kevin Hand on this issue. He's at JPL. You know, a lot of times people think of the Viking experiments as failures because they didn't detect life.

And one of the things Kevin always says is, you know, the Viking experiments would have been failures if they didn't detect life. And then the next mission we sent to Mars found life. And that's not been the case. And we haven't sent life detection missions since Viking to Mars, but we have sent a lot of missions to Mars and none of them have found, you know, little Martian grasshoppers hopping around or creepy crawly whatevers.

There're no elephants stomping around. You know, we haven't found those kinds of things. Now, that doesn't mean that Mars doesn't have life, because, again, we have not sent things that could detect microbial life necessarily. But they tried to do something really hard with Viking and it's not clear to me that it actually failed. It may turn out that it gave us absolutely the correct answer. It just wasn't the answer that people wanted.

Yeah, right. In terms of trying to find life with Dragonfly on Titan, Dragonfly is not a life detection mission. I'm contractually obligated to say that as often as possible. Only mostly kidding. So it's not a life detection mission, and that is not one of our goals. We are going there to characterize the surface and near-surface environments.

We're going to look at the composition of the surface. We're going to look at the composition of the subsurface. We're trying to understand the geological processes, the way in which the atmosphere interacts with the surface, and things of that nature. If there is life on Titan that is doing life things enough that it has modified the chemistry substantially enough that it is detectable with the kinds of instruments we can fly on spacecraft, we will detect it, and we might sit and stare at it and go, What's that?

And not actually figure out that it's life, but we should be able to detect large-scale changes like that. So, for instance, if you somehow flew an instrument to Earth that could detect Earth's atmospheric composition, it had no way to measure anything else, which would be a silly thing to do if you were sending a spacecraft to Earth.

But, you know, you would find out that the atmospheric composition is really weird on Earth, and if you were clever, you could eventually figure out that it's because there's life. You have to rule out a lot of other things, first. That said, I haven't really thought through what would happen if there was life that was relatively similar to Earth on Titan. What would that look like in terms of detectability? I will say it's wildly unlikely. So although we can make a lot of the molecules that life on earth is based on in Titan's atmosphere, things like amino acids, which are the building blocks of proteins, nuclear basis, which are the building blocks of DNA and RNA, those molecules are really unlikely to be used on life, at least on the surface, because the temperature so cold and so molecules really behave differently at those temperatures.

So things like membranes would need to be made out of different material to still be flexible at 94 Kelvin and stuff like that. Um, so, I don't know, I'm trying to think about the life detection thing too hard with Dragonfly. We're going to get data that are going to be really hard to understand and life will be the last resort explanation unless we have little creepy, creepy crawlies running in front of our cameras, which would be lovely. If life could be like nice to us and just like do something like that, that would be really helpful. But it's a little bit, a little bit challenging.

In terms of Dragonfly. So Dragonfly is amazing. Dragonfly is a dual quadcopter, so it's basically like a relocated blue lander. We’ll behave in some ways, a lot like the Mars rovers that everyone is very familiar with. But instead of driving between the places that we study in detail, we fly. And that's just because it's much easier to fly on Titan than it is to drive. And so we'll be going to Titan and we're supposed to launch in 2027. 

If we do that, that gets us to Titan around 2034. So patience is a virtue and we will be primarily focusing on characterizing the composition of the surface, in particular trying to understand the composition of the really complicated organics I mentioned before, and also understand how they've been modified by processes that happen on the surface. But we'll also be studying the geology a lot, using images from cameras. 

We have a seismometer, so we'll look for Titan quakes and try to understand the subsurface properties based on the way that the surface moves as can be recorded with a seismometer. And so those are the kinds of things we're going to be doing.

I am a member of the science team and what that basically means is that I was one of, you know, a couple handfuls of people who wrote the Science Justification when we originally wrote the proposal to NASA for the mission, and then, you know, helped kind of shepherd the science part along as we were in competition. 

And then once we got selected, the role of the science team is really just to sit in total your thumbs while the engineers build the spacecraft and occasionally just ask questions that are probably a little bit ridiculous just to make sure everybody is on the same page. And then we go to the launch and cross our fingers and close our eyes and scream and do whatever else you do.

And like 20 years of your life is put on a giant rocket and lit, and then we will spend a bunch of time again, kind of just twiddling our thumbs while we wait for it to get to Titan. I joke about that. I mean, there's a lot of work to be done with calibration and making sure that our models are right. 

We want to be at the point that when we land on Titan, we … I would say hit the ground running, but that's a really weird thing to say for a quadcopter. Hit the ground flying? Do you want to hit the ground? But we're you know, that we're ready. We don't want to get to Titan and be like, oh, if I had just run that model for a year, then I would know the answer to this question.

So, the science team, we spend a lot of time thinking about hypotheticals like, okay, so if this happened, what would that look like in the data? What would this look like? Do we need to run more experiments? Do we need to calibrate this thing more? But then once we get there, then we're kind of the stewards of the data, not just for the team, but for the for the whole well, I mean, for all of humankind, actually.

And so, you know, it's kind of our job. Yeah. To figure out some of the first science results and you know, we're the ones that have really put in a lot of work to think about these instruments and what they can do, but also to make sure that the data that are generated are, you know, as clean as possible. 

They've been well-calibrated. They you know, they have had integrity checks and all of these things so that when they do become publicly available, the rest of the players, the planetary science community, and the interested public has the best possible data that they can go off and figure out what you know, what kinds of interesting things they can find from the data, too.

So that's kind of one of our other jobs is to be the stewards of the data that we collect and really make sure that for for all of human history subsequent, that these data are as good as possible.

Donna dePolo:


Carlos Mariscal

That's really cool.

Donna dePolo:

I've got to follow up on that really fast. So it's kind of in the context of Cassini and Galileo, but like since Titan does have this possibility, this potential for life, what are some of the safety measures that Dragonfly has to take then? 

Sarah Hörst:


Donna dePolo:

Entering the atmosphere.

Sarah Hörst:

Every NASA's mission. Well, actually, every mission. But I'm going to just focus on NASA's for the moment, has to work with NASA's Office of Planetary Protection. So everybody in the solar system has a planetary protection classification and that determines what kind of precautions have to be taken for the spacecraft. So, for example, if you're sending a spacecraft to the sun, you're fine. Like, we don't think that life exists on the sun. We don't think that earth life could exist on the sun. So it's fine. The missions that tend to have the most stringent planetary protection are missions that go somewhere that we think could have life or could harbor earth life, but then bring things back to Earth. So that's what we have to be the most careful about because we have two directions going. We have to worry about contaminating that place, but we also have to worry about … I feel like if anybody’s seen Andromeda Strain … we don't go down that road. And so that's just to say that every mission has to work with the Planetary Protection Office and come up with a plan. And so it's an iterative process. It's something that is an ongoing conversation between the mission and Planetary Protection throughout the process.

But it will involve a lot of things like making sure the instruments are very clean and there are different ways that you can do that. A lot of times you can hit the instrument very hot to kill anything that might be on it. Or you can do like has like superheated hydrogen peroxide valves and all these awful things that sound exactly as awful as they are.

And then one of the things that happen in between these steps is that there are people who go in and they swab the spacecraft and they culture it and they see like, are there creatures on this spacecraft walls being built?

And again, you know, that is obviously an iterative process, too, because humans interact with the spacecraft so much in the process of it being built, that because you have to be really careful to make sure that you know, Oh, this thing got messed up and we noticed it in a test and now we have to go back in. Okay, if we go back in and do this, do we have to go back through this process again? And so that's, you know, part of all these ongoing conversations, but it's something that the community takes really seriously. And we're really lucky that we have an office at NASA that has all the necessary expertise to really like just tell us what to do we'll do it. Like, I'm not a biologist. I don't know how to solve things on the spacecraft. Just tell me I will go do it. Like, I mean, I won't personally, but somehow. And so it is something that we really have to worry about.

Carlos Mariscal:

And it's a hard thing that keeps changing too, right? We used to keep things 120 degrees centigrade and then we discovered Strange 121, which could survive at this.

Sarah Hörst:

Yeah. I mean, I was, I was when I was talking to some of our high school students earlier this morning, I mentioned to them, you know, Mars is contaminated. What we knew about microbes when we sent Viking to Mars was basically nothing compared to what we know now and the kind of minimal protections that were put in place for Viking for sure did not kill all the microbes that were on Viking.

I always kind of want to send another mission to the Viking landing site and just be like, so what did you guys do? Was the surface of Mars inhospitable enough that eventually you surrendered? Or like, did you figure out, like, how to survive here and you just have this, like, thriving colony that's just hanging out on Mars, doing its thing?

And it's, you know, it's kind of interesting philosophical question, if that turned out to be the case, what should we do? Do we go like drop a bubble on it and like, well, you can have like this? Do we, you know, is the ethical thing to do to kill all of it? You know, what do you … I don't know what the right answer is there, but you're right.

I mean, it's definitely a moving target, as we understand more of what we call extremophiles, you know, creatures that can live in extreme conditions. It pushes the envelope further and further in terms of what we need to do to spacecraft to make sure that we're not carrying those things with us. 

Carlos Mariscal:

Yeah, it's interesting. I mean, there are a lot of people that are actually wrestling with that particular question, right? I'm thinking about Jim Schwartz and Eric Pers and others. But it's interesting to think that it might be possible to evolve like an earth creature that was on Viking to survive on the surface of Mars. But you wouldn't be able to imagine the same sort of thing on Titan, presumably.

Sarah Hörst:

I mean, one would hope not. It is the case that because there is water on the surface of Titan, although it's solid that we do have to be careful. So Cassini, Cassini's end of mission was to crash into Saturn. Really cool. Part of that was for scientific reasons which we love our science reasons, but part of it was because the Cassini carried a radioisotope power as its power source.

It, you know, landed crash-landed on an icy moon and there long enough it would heat itself a nice little pool of liquid water. And then the creatures that, you know, were potentially on Cassini might have a little party in their pool of water. And so we do have to be careful because of the fact that there is water even though it's very, very frozen. And so we have to make sure we don't get ourselves in a situation where we've created a of liquid. But it is definitely something that we have to be careful about, even though it's definitely less likely that I think, you know, Earth creatures would survive at 94 Kelvin than Martian surface temperatures. 

But I mean, you never know. Earth life – some of these things are tough cookies.

Donna dePolo:

The water bears at Titan …

Sarah Hörst:

Right, Tardigrades.

Donna dePolo:

So beyond Titan what's exciting an up and coming planetary research and futuristic missions? I know we've talked about Dragonfly a lot.

Sarah Hörst:

Yeah, that's a great question. So I think there are a couple of things that folks are really excited about right now. So, you know, we have this big new space telescope, JWST, that is pretty exciting. And I think especially for think about extrasolar planets where we're really just starting to move into a period of time in which we can really start to think about extrasolar planets in the same way that we think about planets in the solar system. And so exoplanets are kind of moving from what has traditionally been the realm of astronomy more into the realm of planetary science. And so I think that's really going to be really interesting. There are a lot of flavors of planets that exist that we do not have in our solar system. And so it will be fun to figure out what those weird planets are doing.

So that's one thing.

I think one thing that I'm kind of personally excited about because I'm involved in it as well. But I think the community is just really excited. NASA hasn't sent a mission to Venus in a long time, in many of our lifetimes, in fact. And we have two that have been selected now that should be launching in the late 2020s and early 2030s. One is called Veritas, which will be doing remote sensing from orbit. The other one is called Da Vinci, which is an atmospheric probe. That's the one that I'm involved in. It's going to be really fun to get the public excited about Venus again, because Venus is really interesting and just hasn't gotten the attention that it really deserves for a long time.

So I think that is one of the other things that I'm really excited about. And we also have this mission to Europa that should be launching not next year, 2024. I don't know. It's sometime soon. It's getting assembled, so that means like sometime soon it goes to the Cape and they send it into space. But that'll be really fun because Europa's awesome and so that'll be really exciting.

And we've got asteroid missions coming up. And so we have a lot of, you know, very different places that we're going in the solar system that I think will be really exciting. So, you know, stay tuned for all of those things in a decade or so.

Carlos Mariscal:

So it's interesting. Okay, we're on missions. You talked a little bit about exoplanets and I guess I wanted to ask to what extent you take some of the planets that we have here, in particular, Titan, to be good analogs for exoplanets. I mean, one of the things that you said was that there's we've discovered some hot Jupiter or some things that we don't have here.

So, the question is, is Titan a particularly good candidate as an analog for exoplanets?

Sarah Hörst:

I mean, there would definitely be Titan-flavored things. One of the challenges at least for now, is, you know, being able to study an object like Titan around another star is not really a thing yet. Because it's so small, because it's so cold, it doesn't give off a lot of energy that we can detect from a telescope. We pull it off in our solar system because we're close by. And so we've been able to study Titan with telescopes since the early 1900s. But you know, for that type of object around another star, it would be really hard. We're definitely going that way. And it'll be I mean, it's going to be interesting for some of the colder planets, especially around some of the less energetic stars, I think you would expect to see some objects where I think Titan might be a pretty decent analog. And so it is going to be interesting to see how all of these things play out. I mean, this, you know, having this very cool object that has this big puffy atmosphere is weird in our solar system, but there's not necessarily a reason why it's inherently weird. And so I think that's something that, you know, we're going to learn more about as time goes on.

Carlos Mariscal:

So, Dr. Hörst, it looks like we're really coming to the end of our time. Is there anything that we should have asked you but didn't, or just anything that you'd like to share with us?

Sarah Hörst:

Should have asked me, but didn't ... Well, that's such a hard question. [laughter]

I feel like I've been dropping some, like, fun facts on people today that I'm realizing a lot of people don't know. So I’ll just share a couple of fun things. One of my favorite fun exoplanet facts that I feel like a lot of people should know, and it's not a super recent discovery at this point, but I don't think it's really made itself into the public consciousness is that we know from this mission called Kepler that at least in our galaxy, in the Milky Way galaxy, there are more planets than stars.

So if you go to a site at night and you look up at a star, there is a very, very, very good chance that it has a planet and a pretty decent chance that it has more than one planet. So I think that's pretty cool. Now, when you look at the sky, you can think, Oh, that's not just a star, that's a star and planets.

I think that's awesome. And the other fun fact that I like to share that is Dragonfly related is that part of the reason why we're flying is because Dragonfly is probably the easiest place to fly in the solar system. Its gravity is about one seventh out of earth, but the atmosphere is about four times denser. And so you have a lot of atmospheric density to support you and you're not feeding as much gravity. If you were on Titan – and if you find yourself on Titan, holler, because I’m going to have some questions – so if you had some kind of rudimentary wings that you could attach to your arms somehow, you could flap your wings and fly under your own power on Titan, because it's that easy to fly. And so that's one of the reasons why we are flying instead of driving, because it seems kind of silly to not use this huge thing that you have to your advantage.

So those are two fun facts. You know, go home, share them with your friends. I don't know. Might come up in trivia someday.

Donna dePolo:

Those are some facts, and I just realized your dress was covered in Dragonflies and it's a very lovely. And that just clicked for me.

Sarah Hörst:

I'm always on brand and I'm very much like, it's subtle enough that I feel like it takes people a little while to like, Oh, that's what's happening.

Carlos Mariscal:

It's funny you were talking about flapping and Titan. Donna and I, when we were prepping were like, Oh, so Titan has nitrogen. Well, Earth has nitrogen. So, if you take off your helmet, you might breathe in until you suffocate. And then we realized how cold it was. So, you probably like freeze to death, but you'd be flapping your wings and flying, while being frozen and breathing and slowly suffocating to.

Sarah Hörst:

It’s so funny that you mentioned that because we had this – and this is like this is a little morbid and I’m sorry to the people who are going to find is a little bit morbid – but we got into this amazing conversation on Twitter one time about what would kill you first on Titan, because you have the cool temperature, which – not great. You have a lack of oxygen – also not great. But you also have a bunch of carcinogens in the atmosphere. So there's like carbon monoxide, there's hydrogen cyanide, there's like a bunch of molecules that are very toxic to humans. And so we were like, okay, well, but if your lungs freeze over first, like, things can’t be suffocated, or is that considered suffocation? And like, if it happens slowly enough, does the carbon offset it? None of us involved in this conversation had any expertise to know the answer to this.

But then we started thinking about all the ways that you would die on different planets. And suffice it to say that you and this might be a good note to end on, but, you know, you hear a lot of, you know, we need to go to Mars or wherever to protect humans and all of these things. And let me tell you, as a planetary scientist, Earth is really our only good option. All of the other planets are like really desperately want to kill you a whole bunch of different ways. This is the only one that is even remotely hospitable to us. And so, you know, it's incumbent on us to actually keep it hospitable and habitable because the other options are just it's not a good idea.

Donna dePolo:

They leave a little bit to be desired.

Sarah Hörst:

A lot a bit, as it turns out.

Donna dePolo:

Well, that's an excellent note to leave off on. Thank you so much for joining us here today, Dr. Hurst, this was a really fascinating conversation. Thank you. This was really good time. So signing off, I'm Donna dePolo.

Carlos Mariscal:

And I'm still Carlos Mariscal. Thank you for joining us today and keep discovering science.



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