What Can Cave Life Tell Us About Alien Ecosystems?

If instruments do someday detect evidence of life beyond Earth, whether it’s in this solar system or in the farther reaches of space, astrobiologists want to be ready. One of the best ways to learn how alien life might function can be to study the organisms called extremophiles, which live in incredibly challenging environments on or in the Earth. In this episode, Penelope Boston, a microbiologist who has worked for many years with NASA, speaks with Janna Levin about the bizarre life found in habitats such as caves, how it would be possible to detect life beyond our solar system and what it would mean for humanity if we do.

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Transcript

[Theme plays]

JANNA LEVIN: Life on Earth is amazingly resilient. Organisms known as extremophiles survive in caves stretching thousands of feet into the Earth’s crust, in deserts that go decades without rain, under high pressures kilometers deep in the sea, in toxic waste dumps that are entirely unnatural and inconceivably inhospitable.

These tiny extremophilic organisms can live, persist, even thrive in environments we would consider dire. If life lingers on the peripheries of our world, maybe there’s a chance that life exists elsewhere in the universe.

I’m Janna Levin, and this is “The Joy of Why,” a podcast from Quanta Magazine, where I take turns at the mic with my co-host, Steve Strogatz, exploring the biggest questions in math and science today.

In this episode, we talk to astrobiologist and speleologist — that is to say, cave explorer — Penelope Boston. Penny to her friends. We ask her what can extremophiles teach us about the fascinating breadth and immense diversity of life on Earth? And how can this knowledge help us recognize life beyond our planet?

[Theme fades out]

Penny studies the microbiology of cave environments to understand the potential for life in the subsurface of other worlds. She was a professor and co-founder of the Cave and Karst Studies Program at New Mexico Tech, served as associate director of the National Cave and Karst Research Institute, and was the former director of the NASA Astrobiology Institute. She is known for exploring some of the toughest caves in the world.

Penny, it’s so great to speak with you. Thanks for joining us.

PENELOPE BOSTON: Hi, Janna. It’s really a lovely opportunity to get to chat with you about some of my favorite weird organisms and how they fit into astrobiology.

LEVIN: Yeah, I was very excited about this topic. I remember the first time that I learned that there were microorganisms in deep hydrothermal vents essentially metabolizing hydrogen. So we often think of life forms as metabolizing organic material. And so some of these extremophiles are not metabolizing organic material. It’s like they’re gleaning electricity from rocks and other minerals. And I’m wondering, how surprising was this discovery to, say, experts who were considering just discovering other life forms on Earth?

BOSTON: It was really eye-opening, you know, the early work on the hydrothermal vents in the 1970s was very early in my career. I was a student and that was the first inkling that we had that maybe organisms could make a living in a way that was not dependent directly on sunlight driving photosynthesis. So that was very formative, really, for the science that came after.

LEVIN: I like this concept that they’re making a living.

[Both laugh]

BOSTON: Yeah. We all have to do that.

LEVIN: We all have to do it. It’s all about resources, right?

BOSTON: It is. It totally is.

LEVIN: So, do you have a favorite extremophile? And, regardless, can you give us a description of the kind of range or examples of extremophiles that we might not be aware of?

BOSTON: Oh my gosh, do I have favorite extremophile? No, really, all of them. They’re all my babies.

I love the organisms that seem to be involved in the transformation of some minerals into ores that we actually mine. So even copper ores, to me, seem to be a product of microbial manipulation. There are organisms that are happy being frozen in permafrost for tens of thousands of years and are still viable. There are the organisms we studied in the very hot caves in Chihuahua, the Naica system, where we were able to extract organisms from being trapped in these very gigantic crystals there. I mean, I think that organisms living and surviving for very long geological periods of time is something that is a new forefront for us in geomicrobiology and astrobiology to look for.

LEVIN: Wow, amazing.

And then I guess I’m wondering about the sorts of conditions in which we find extremophiles, especially those that are not extracting organic material.

BOSTON: Yeah, you know, [they] may go incredibly far back in the history of life on our planet. We are human-centric for obvious reasons because we’re humans, right? So we’re used to eating food and that’s already fixed energy for us. And then we burn that with various metabolic processes; we breathe oxygen and so forth. But if you hark back three to four billion years ago when our planet was young, there was no free oxygen …

So the type of photosynthesis that drives most of our food chains on Earth now was not possible. It was not happening. So other molecules were being used besides water, hydrogen sulfide being one of them.

LEVIN: So this was before there were organisms that photosynthesized at all?

BOSTON: This was early on, when the form of photosynthesis that organisms were beginning to develop actually relied on non-oxygen, non-water. And so it was splitting other molecules, like hydrogen sulfide.

At the time, there were also lots of chemically reduced conditions rather than oxidized conditions. And the early organisms were making use of energy sources that were much more varied than we commonly think now.

But those organism types, those metabolic pathways, they’re still with us. They’ve just been pushed to the edges of our biosphere because of the dominance of the oxygen atmosphere that we now have.

LEVIN: This is fascinating. So, these were multicellular organisms or single-celled organisms?

BOSTON: No, single-celled organisms persisted for a very long time. Just recently, there’s been new clues as to when the first possible multicellular organisms arose. And it’s a little bit earlier than we thought.

LEVIN: Oh wow.

BOSTON: So that’s a major find, . But, for a very long time, single cells were the lifestyle that everybody was practicing. And it took quite a while to figure out how to even make, you know, strings of yourself, to be multicellular at that very low level.

LEVIN: Mm-hmm. And to make that energetically favorable.

BOSTON: Yes, yes, how can you compete? Because, you know, if you’re a tiny organism, you’ve got a very lean diet when you’re in an environment where you have to extract energy from minerals or abiotic gases. It’s much easier if you’re chomping down on organics. You get more bang for the buck if you eat, you know, a cheese sandwich than you do if you chew a bunch of rock.

LEVIN: I’ve always found that to be the case.

BOSTON: Yeah, absolutely.

LEVIN: So, if I’m understanding correctly, really we didn’t have the proper atmosphere that we have now, and we didn’t have oceans. So this is really early in the Earth’s history.

BOSTON: Well, we had oceans. We needed water. There’s a lot of thinking about when the oceans started to happen. That’s very unclear. So there’s the idea that the volatiles, including water, outgassed from the early Earth, which was made out of all these sort of chunks of stuff that came together to form Earth and the other planets. And that when all those pieces stuck together, at some point they reach enough mass that the gravitational effect is enough to start helping the planet differentiate into layers of material. As part of that process, you would get outgassing of these vaporous compounds, including water, and eventually you get oceans.

There is a competing theory — and both of them may be true — that you’re getting a large amount of the water from impacting objects after the Earth mostly came together. We don’t really have it nailed down. And I suspect it’s probably both, right?

LEVIN: Mm. Right.

BOSTON: It’s not a simple answer. So it took a long time to get the oceans together. And that was necessary to have liquid water because our type of life is based on liquid water.

LEVIN: Now, these very early extremophiles. Did they rely on water, even if they were metabolizing leaner systems like minerals, rocks, electricity?

BOSTON: They did because we’re made out of water.

LEVIN: Mm, so they needed it for their cellular structure, even if they didn’t need it energetically.

BOSTON: Yes, that’s right. We are all creatures that are basically bigger or smaller bags of water with stuff dissolved in it. With structures that help the chemistry happen. Those structures probably became more elaborate over time, but the basic notion of a bag of soup is really what we all are.

LEVIN: And then once they have this sort of soup, these salty soups, they’re able to conduct electricity. Is that kind of the major way that they were gleaning energy from things like hydrothermal vents, or just raw minerals?

BOSTON: So, minerals laying around don’t do much. You have to chemically react them in some way. The way life eventually learned to do this was by being clever and forming proteins that fold up into complex shapes, and those proteins, they can then catalyze or start the process of different chemical transformations. We don’t know what the sequence really was.

And I suspect that, you know, when you have a laboratory the size of a planet, you’ve probably got more than one kind of experiment going on at a time. My suspicion is that all these different kinds of chemical, natural experiments were going on, becoming more and more complicated over time.

And the minute you get even molecules that are different from one another, they still need to grab stuff from their environment in order to make more molecules, even maybe before you have what we would call living organisms in the sense that we now understand that. Natural selection comes into the picture pretty early.

LEVIN: I’ve heard that there’s a Nobel Prize–winning physiologist, Albert Szent-Györgyi, who said once that life is nothing but an electron looking for a place to rest. Do you share this sentiment?

BOSTON: Well, not exactly. All right. I think that he’s representative of a time where we were trying to be as reductionist as possible, which is a very powerful science tool to get your teeth into a problem. But what we know about the real natural world is that it’s a complex system in the mathematical sense of that. And so, really, there’s no single level of complexity that holds the entire answer.

And so, that’s a piece of the puzzle, and it’s true that organisms live by shuffling electrons around. But they couldn’t do that if there weren’t all these other levels of complexity that we see in the mature system.

So we’ve come a long way in our thinking about how to really work with complex systems. It’s really hard. It’s easier to make a simple system, like in a physics experiment, where you can control all the conditions. But folks, life is not like that.

LEVIN: That’s what electrons want to do.

BOSTON: They do.

LEVIN: They want to go from a high-energy state to a low-energy state.

BOSTON: They do.

LEVIN: So the physicist in me imagines the Big Bang, high-energy, and we’re just trying to drop down ever since then.

BOSTON: Right, that’s exactly right. The way I think of life is, inside every organism, no matter how complex or relatively simple it is, it is a temporary place where the organism keeps entropy at bay.

LEVIN: Fascinating. Mm-hmm.

BOSTON: It exports waste, takes in energy, and makes a more ordered system on the inside of an organism at the expense of the external environment.

When that organism dies, of course, entropy wins in the end, as we know. But it’s a temporary suspension of entropy within the organism.

LEVIN: Absolutely. Now, do you think that all life as we know it might have evolved from extremophiles?

BOSTON: Oh yes. I think, if you had a time machine, and you were able to go back and directly interrogate that very early Earth environment, that you would find way more metabolic pathways than we even know now. So we have the relic extremophile complement that have survived in spite of the changing planetary conditions over time.

And people are doing amazing work trying to dig in to “bug guts,” as I call them, to look at the DNA and look at the history of how these metabolic pathways have evolved, not only the genomic work, but the proteomics and even other molecules that are complex carbohydrates and so forth, to try to figure out who’s related to who. But, you know, there have to be missing gaps, and so it’s hard to infer that. But just looking at the history of the planet itself, it had conditions that are extreme from our point of view in the modern world.

LEVIN: Mm hmm. And can you point to strands of human DNA that we can identify as, “aha, that was an ancient extremophile”?

BOSTON: Oh, yes, there are wonderful schemes that have been published that show how much DNA we personally, our species shares with bacteria.

LEVIN: Mm hmm.

BOSTON: I think of mitochondria as being such an amazing transformation. The great evolutionary biologist Lynn Margulis had a real hard time, you know, trying to persuade people in the ’60s —

LEVIN: Absolutely.

BOSTON: That symbiosis was the way that big cells came together. So that was another leap. But, yeah, if you go way back, everybody was chewing rocks and slurping gas, and probably early organisms clinging to, you know, minerals for some of their structure, even, that’s been suggested.

LEVIN: And as life became more abundant in what we, obviously, in a biased way, called a temperate environment — because it’s temperate for us—

BOSTON: Right.

LEVIN: Did extremophiles then get pushed, in some sense, to the periphery or have they always just kind of lived where they’re happy? And it just took us a long time to realize they had endured?

BOSTON: Yeah. I think, Janna, it’s a bit of both. When you got the, what’s the so-called “oxygen revolution,” when oxygen, you know, accumulated as a byproduct of water-splitting photosynthesis, which took a long time because it had to oxidize the whole ocean first.

LEVIN: Wow, yeah. That’s a project.

BOSTON: Yeah, you know so it took a heck of a long time. So in some sense, a lot of those niches were displaced by organisms that could tolerate an oxygen environment. It’s not an either/or thing. So, you can see that, you know, some organisms are perfectly happy sniffing in the oxygen that we have at Earth’s surface right now.

But there are a lot that are so-called microaerophiles. And they tolerate oxygen, but at lower levels, varying levels. So I think that that’s putative evidence that organisms that were extremophiles that did not tolerate oxygen, they got pushed to the margins. But they also proliferated in niches where oxygen doesn’t penetrate.

LEVIN: And in your work, you explore these really difficult-to-explore caves. Why are caves of particular interest in your area of expertise? What is unique about subterranean microbiology?

BOSTON: Yeah, I mean, so I could just regale you for hours, but I will spare you. I’ll just hit on the high points. There are some really clear advantages. One is that you can consider a cave a semi-closed system and when you look at the genetics of the organisms that we find there, we find vast biodiversity. And we also find that most of the organisms are not the ones that we find on the surface, even on the surface right over a given cave.

So what that says to us is that however organisms get into a cave, they’re immediately subject to really different conditions. And that promotes an evolutionary pace that pushes them in the direction of becoming a very unique little biosphere, a miniature ecosystem, and that each cave has the potential for doing that. And so, I think of them as little baby planets that we can study.

LEVIN: And they’re different from cave to cave?

BOSTON: Yes. People have a really simplified idea of caves. So, you know, we all grew up as kids going into caves as summer holiday entertainments. And I always thought they were fascinating, but it wasn’t until I started to go into caves all around the world that I saw how diverse they are.

So there’s hot ones, and there’s cold ones, and there’s ones that contain ice. And there’s ones that are saturated with — to us — a poisonous environment, either high CO₂ or high hydrogen sulfide, or weird aldehydes or something. Some of them are wet and some of them are dry and some of them are in deserts and some of them are on high mountains, where you have to go down a kilometer through ice to get to the inside of the cave. So each one of those combinations is like a whole separate extreme environment.

LEVIN: They’re like wild terrariums.

BOSTON: Yes! That’s exactly it!

LEVIN: So there’s biodiversity, not only among the caves — they’re biodiverse — but also within the caves, there’s tremendous biodiversity?

BOSTON: Yes, there is. Some caves are extremely large, like Lechuguilla here in New Mexico, where I am located. It’s one of the larger caves in the world and it has tremendous diversity in terms of the mineralogy that’s in there. And also because there are these evolutionary pressures and because organisms in a cave have limited ways of getting around, it makes these little evolutionary mini pockets as I think of them. So if we look at diversity in one patch and then we go a few hundred meters away, we can find a very different complement of microorganisms. So they’re chopped-up environments.

LEVIN: And they all source energy in different ways. Would you call all of these organisms in these caves extremophilic?

BOSTON: It depends. So if you’re an organism in a cave, your biggest challenge is making a living with energy because there’s a limited amount of organic material that comes into you.

Some caves are richer than others. You know, if you’ve got a cave with a river flowing through it that sources from the surface, you’ve got quite a reasonable amount of organic material coming in.

But if you’re a cave in Saudi Arabia, you have whatever ancient carbon might be trapped in the rocks, and the bioproductivity of your friends who are munching down the minerals and making biomass. So life as an organism that needs organic material becomes harder.

Most caves have at least some so-called heterotrophs, which means that you have to have organic carbon. And so it’s a mix. And the more harsh the conditions are in the cave with respect to nutrients of the organic sort, the more it’s pushed in the direction of organisms that are using non-organic materials for energy.

LEVIN: Hm. Now, what drew your attention to cave microbiology?

BOSTON: Well, Mars. I had been working, you know, on what we used to call exobiology, now it’s usually referred to as astrobiology, ever since my earliest days. In fact, I fell in love with the idea when I was a little kid.

In the early 1990s, with a couple of colleagues, we were sort of digesting what had been learned about Mars so far and the fact that the surface is, you know — what can I call it? A blasted, cold, dry hellhole.

And so the potential for living organisms was not promising, probably off the table. But we began to think about the subsurface, because here on Earth people were beginning to look at the subsurface for microorganisms more seriously.

You know, when I was a student in the ’70s, basically the story was, well, you go down about a meter or so into the soil and basically the microorganisms die out, you know, and there’s nobody home, because we weren’t thinking about the deep subsurface. We weren’t thinking about caves. We weren’t thinking about rock fractures. So it was a revolution in thought that was going on at the time.

And so we were looking for ways to get into Earth’s subsurface. But drilling is very expensive, and we were all young investigators, so we thought, “Oh, wow, caves!” And then we saw a National Geographic special on TV that talked about Lechuguilla Cave right here in New Mexico. And, boy, that clicked, you know, that was it.

LEVIN: So you went running.

BOSTON: We went running, and we knew nothing, and we weren’t prepared, and it was a horrible trip. We all got injured, or an infection, or something.

LEVIN: You brought some back with you.

BOSTON: Yes, yes.

LEVIN: So do you still believe that [the] subterranean Martian surface holds promise for life?

BOSTON: I do. But I think it’s going to be very deep. Deeper than our current near-term or even mid-term capabilities to get in there to find it.

LEVIN: Are there other moons, planets, promising exoplanets beyond our solar system where you think that there’s a good likelihood to search for these kinds of extremophiles?

BOSTON: Oh, yes. In the astrobiology community, both at NASA and other space agencies around the world, and academia and national labs, we’re all thinking about the icy moons that appear to have an interior fluid. And we call them ocean worlds. I mean, as a cave person, I think of them as ice-covered, fluid-containing, planet-sized caves because they’re closed to some extent to materials and energy exchange with the outside.

But whatever you call them, the fact that there are these intriguing environments where we know, for example, Enceladus — a tiny moon, a beautiful glowing white object around Saturn — has fractures that are poofing out materials. And the wonderful Cassini mission detected that there were organics in those. I mean, that’s clearly a high priority.

Europa, the first moon to have been identified as having a fluid interior, back from the Voyager missions around those bodies, gave rise to that understanding. So, you know, those are high targets for us in astrobiology.

How to actually study them? Boy, that’s a technological feat still to be cracked.

LEVIN: I can imagine. And that’s just in our solar system. And now we know that there might be more planets in our Milky Way galaxy than there are stars, which is to say hundreds of billions of planets.

BOSTON: Yeah. I mean, it’s so exciting. You know, it’s so exciting, Janna, because, my lifetime has spanned so many amazing scientific revelations, but nothing more exciting than the fact that we now have confirmed planets around other stars. And that was a hypothesis when I was in high school.

You know, and seeing this develop, our galaxy is opening up to us. And we have new fabulous tools like James Webb Space Telescope and others coming down the pike in the future. So, I want to live 500 years or more so that I can see how it all turns out.

LEVIN: So, you must believe that life will be recognizable to us. How will we recognize it if it’s so on the border of what’s organic and what’s inorganic in its processes? What tools could we possibly use to assess this?

BOSTON: Yeah, that’s a fabulous question. We wrestle with this. I mean, this is a great deal of the ongoing work of astrobiology is to figure that out.

So we have wonderful knowledge now about Earth’s life processes. The particulars of those are fascinating and enriching and wonderful for us. But our task in astrobiology is to take what we know that we can reduce to as general a principle as we can.

Things like, an organism has to get energy. It has to do something with waste. It has to be able to store information, so it can make more of itself. How can we take what people are beginning to call “agnostic biosignatures”? Signatures of life that don’t depend on the precise nature of the chemical interactions, but really have a very sound base in physics and in what the resources would be for energy and materials in a given planetary habitat type, even though that may be radically different from anything we have on Earth, even in extreme environments here.

We’re trying to pull out what are the unifying concepts, and that we can use to allow us to recognize lifelike processes — even if it’s made out of silicon, who knows? We start with what we know, we start with Earth life, but we have to keep our imaginations really flexible.

LEVIN: So instead of bags of soup with water as a base, we could imagine discovering bags of soup with silicon as a base.

BOSTON: Maybe. Bags of liquid ammonia–based soup at very low temperatures. Yeah.

LEVIN: Fascinating.

BOSTON: I mean, there are some fundamentals, and we don’t know the degree to which we have those. It could be that, you know, certain behavioral patterns and some of the work that I’ve done with my colleagues look at even physical arrangements of organisms that we see in many, many different environments at many different scales. So it may even be that there are some kind of resource-seeking behaviors that cause the construction of certain recognizable structures. We don’t know. We’re just sticking our toes into the, you know, the ocean of complexity of that, but it’s a fascinating challenge, though, you know?

LEVIN: Absolutely. Now, what do you think when people hypothesize that there could be life in the Sun, in the sort of photosphere of the Sun? Do you think that’s absurd because of the lack of, sort of, cellular materials?

BOSTON: No, I don’t think it’s absurd. I think it depends on what you’re calling life. So, you know, we have this conundrum in biology and astrobiology, that has beset biology for the last hundred-plus years. And, you know, what is life? What are the characteristics that make something alive?

We started with a laundry list of characteristics that are common for Earth life. But I think that I call them lifelike processes. And, for me, processing of something is necessary, and controlling the environment at some level is necessary. Is that a lifelike process? I don’t know.

BOSTON: But I would be willing to consider it on a spectrum of what I call “life-iness.” There are high life-iness processes and low life-iness processes. Natural crystal precipitation, where you get the geometry coming together, that’s low life-iness. But that doesn’t mean that it hasn’t interacted with life. It has.

Microorganisms affect how a crystal in its environment will grow because it shifts the thermodynamics of the system. It shifts the chemistry. So, you know, life in the Sun? I don’t know. There have been several good science fiction stories written about that.

LEVIN: Right. So, you talk about low life-iness and high life-iness and I have intuitively a sense of what you mean by that. But is there a way in which you really understand how there became a transition from not-life to low life-iness and then to high life-iness?

BOSTON: I think the crux of the matter is really the ability to store information. Our type of life does it with DNA now.

LEVIN: Mm hmm.

BOSTON: There are theories about it first being encoded in RNA, which can function both as an information-storage and as a functional molecule, like proteins now do. That’s one theory.

There’s another theory that the structure was provided. The information storage was molecules sticking to certain kinds of electrically complicated, structurally complicated clays. But whatever it was, the degree of life-iness to me hangs on the ability to store information, and transmit that information, and to perpetuate those information patterns.

LEVIN: Do you think that there’s a likeliness, not just that there’s extremophiles out there in the universe in this rich exoplanetary terrain, but that it will have also made the crossover to complex life?

BOSTON: Oh, yes. I think it’s a numbers game, Janna, you know? What you pointed out before, the fact that we probably have more planets than stars, even in our own galaxy, not to mention all the other bajillion galaxies out there. It’s out there.

The question is, how do we find it? It becomes very challenging, even imagining our future advanced telescopic tools, to perceive the signatures of life at a planetary scale from the vast distances that we are, even to the closest exoplanets. So, it’s a different kind of challenging… a challenge that requires a whole planetary signal like we have here on Earth.

I have this lifelong bet with my friend, Seth Shostack, who is one of the senior scientists at the SETI Institute looking for intelligent life. And I told him I think that we may find signs of life on an exoplanet before we actually crack the case here in our own solar system, because we have so many exoplanets and here, in order to detect life, it’s going to be cryptic. You know, it’s going to be subsurface or it’s going to be very relic on the other bodies in our solar system.

So, you know, we’ll see.

LEVIN: When we explore the solar system, also, we can send probes there and dig up samples. But does that introduce the possibility that we’ve contaminated other planets? I mean, is it possible we’ve seeded Mars with terrestrial extremophiles that will survive?

BOSTON: Well, undoubtedly, Earth organisms have been transported on earlier spacecraft. The reasoning for why that would not have materially contaminated Mars, for example, is that Mars is such a harsh environment on the surface. But as we go forward, we have to worry about what you’re talking about, which is, you know — we call it in the business “planetary protection.”

Planetary protection means keeping Earth safe from any organisms that we may return to Earth to study, and keeping a planetary body safe from contaminating organisms that we may transfer. We have to worry about this extremely when we begin to drill down to icy aquifer layers in Mars.

So, a lot of work has been done on that. There’s also an international organization that deals with all things space on an international level called COSPAR, and COSPAR has a special section worrying about planetary protection. I’ve done a lot of work on those issues myself, so it’s something we take tremendously seriously.

LEVIN: So what did you think about this Moon mission? I believe it was an Israeli mission, that sent tardigrades — tiny extremotolerant organisms also known as water bears — to the Moon, and then crashed, possibly ejecting the tardigrades. Is that a concern for you or do you have this sort of same sense, well, it’s, very unlikely they’d survive on that harsh surface, but we got to do better going forward?

BOSTON: It’s very unlikely that they would survive on the Moon in any significant duration. This is not a NASA opinion, but my personal opinion is that you should never take the life of any organisms, no matter how small, for a stunt. And to me, this was not a legitimate scientific enterprise.

What was the point of this? We know that tardigrades are not going to survive very well on the Moon, not unless you created some kind of habitat for them. So I think it was a waste of a bunch of good tardigrades, and I am a huge tardigrade fan.

LEVIN: They are adorable.

BOSTON: Yeah, they are completely adorable. I have stuffed ones right here in my office. I hope that future tardigrade experiments on the Moon are done in a proper way, where there’s some point to seeing how they respond to the lower gravity, some aspects of the radiation environment.

We’re about to fly yeast and plant experiments on the Moon, these are all legitimate. But just sending them there for the heck of it, not a fan.

LEVIN: Mm hmm. Now, there’s an even more extreme view about life in the solar system — and one that suggests that maybe extremophiles hitchhiked on an asteroid to get here in the first place, the idea of panspermia, that maybe we are aliens already. We are evidence of extraterrestrial life. What do you think of that idea?

BOSTON: I think it’s possible. When I first started contemplating it many years ago, we didn’t have a mechanism. And a lot of the ways that it was suggested that panspermia might happen were pretty goofy.

But the big breakthrough came really when we discovered that one class of meteorite appeared to have been blasted off Mars and some small portion of that material made it to Earth. These are the so-called SNC, or SNC meteorites. [Editor’s note: SNC refers to the three sub-classifications for these meteorites: shergottites, nakhlites and chassignites.]

And that was a revelation because what that caused people to do, like orbital dynamicists and so forth, was to actually work the issues that you would need to figure out whether or not that was physically possible. Could a big impacting asteroid blasting into Mars, for which we know there have been many large impacts, would that debris have made its way to Earth? And the answer is yes, it was very plausible.

And now, of course, we have this relatively big collection of SNC meteorites that have been gathered on Earth. So, we have, you know, DoorDash delivery of little bits of Mars that have been brought to Earth for us to look at.

Now, they’ve been through a lot. It’s not like going to Mars and digging up a bit and going, “Oh yeah, well, we can tell everything,” because they’ve been through a lot of trauma. But nevertheless, that was a revolutionary finding.

LEVIN: Is there any reason to suppose life would do better emerging on Mars than it would emerging on Earth?

BOSTON: Maybe. It really depends on what school of thought you are in terms of the temperature regime and the conditions on early Mars. Early Mars clearly was much more similar to Earth in its early history. Now it has lost its atmosphere, its ability to retain heat and modulate the temperature and retain water vapor in the atmosphere and liquid bodies, but we see the evidence of that in the past. So we don’t know.

If we are ever lucky enough to actually find organisms that are either well preserved enough to investigate or perhaps even still living in the deep Martian subsurface, we can look at how they’re put together. Are we related? Are we not related?

LEVIN: Now let’s say we make this discovery, and we find [an] alien life form — however small, however different. What do you think it’s going to mean to us as a society, you know, a species on a planet brimming with life, if we were to make that discovery?

BOSTON: Boy. You know, I think about that all the time. No surprise, I don’t know.

I don’t know. Those of us who are involved in science, who are members of the public, who are interested in science, I think it will be literally earthshaking.

But there are many people on Earth who are not interested in that. They’re not engaged in that. That’s not part of their daily life.

How will they respond? Will they just yawn and go, “We don’t care”? Will they think of it as something evil? I really can’t predict.

LEVIN: Mm hmm. And they might just reject the reality of the discovery altogether. I mean, we still have people who are proponents of a flat Earth or think the universe is 6,000 years old. So people can be resistant.

BOSTON: Science cannot compete with a deeply held other mythology or bias.

LEVIN: Yeah. Now a question we like to ask our guests here on “The Joy of Why” is what about your work and your exploration brings you joy?

BOSTON: Oh gosh, everything. I love beauty and I find beauty everywhere in the natural world.

I love just being in nature. I love going to places that are so different that it shifts my thinking.

As a kid reading science fiction, that’s what I wanted to do, was go explore other planets. And I feel that I’ve achieved that on Earth because caves are so different from the surface. And so, really, I’m a cheap date, you know? It doesn’t take much to make me happy. Seriously.

LEVIN: Just alien life forms or subterranean exploration. Forget the dinner and a movie.

We’ve been chatting with astrobiologist and cave specialist Penny Boston. Penny, thank you so much for this fascinating conversation.

BOSTON: Oh, my pleasure. Lovely to talk to you, Janna.

LEVIN: Lovely to talk to you.

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LEVIN: Thanks for listening. If you’re enjoying “The Joy of Why” and you’re not already subscribed, hit the subscribe or follow button where you’re listening. You can also leave a review for the show. It helps people find this podcast.

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“The Joy of Why” is a podcast from Quanta Magazine, an editorially independent publication supported by the Simons Foundation. Funding decisions by the Simons Foundation have no influence on the selection of topics, guests or other editorial decisions in this podcast or in Quanta Magazine.

“The Joy of Why” is produced by PRX Productions. The production team is Caitlin Faulds, Livia Brock, Genevieve Sponsler and Merritt Jacob. The executive producer of PRX Productions is Jocelyn Gonzales. Morgan Church and Edwin Ochoa provided additional assistance.

From Quanta Magazine, John Rennie and Thomas Lin provided editorial guidance, with support from Matt Carlstrom, Samuel Velasco, Arleen Santana and Meghan Willcoxon. Samir Patel is Quanta’s editor in chief.

Our theme music is from APM Music. Julian Lin came up with the podcast name. The episode art is by Peter Greenwood and our logo is by Jaki King and Kristina Armitage. Special thanks to the Columbia Journalism School and Bert Odom-Reed at the Cornell Broadcast Studios.

I’m your host, Janna Levin. If you have any questions or comments for us, please email us at [email protected]. Thanks for listening.

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