Unexplainable - The Hitchhiking Microbe’s Guide to the Galaxy

Episode Date: April 15, 2026

Can microbes travel through space on meteorites? It’s an idea called “lithopanspermia,” and to work out if it’s even feasible, some researchers decided to shoot microbes — with a gun. Guests: K.T. Ramesh, professor of science and engineering at Johns Hopkins University; Lily Zhao, mechanical engineer at Johns Hopkins University For show transcripts, go to ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠vox.com/unxtranscripts⁠ ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠For more, go to ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠vox.com/unexplainable⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠ And please email us! ⁠⁠⁠⁠⁠[e-mail]⁠⁠⁠⁠⁠ We read every email. Support Unexplainable (and get ad-free episodes) by becoming a Vox Member today: ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠vox.com/members⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠ Thank you! Learn more about your ad choices. Visit podcastchoices.com/adchoices

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This episode is brought to you by Defender. With a towing capacity of 3,500 kilograms and a weighting depth of 900 millimeters, the Defender 110 pushes what's possible. Learn more at landrover.ca. I think if you're interested in life, you're always interested in basic questions about life, where it comes from and what it can do. KT. Ramesh studies a lot of things. I'm a professor of mechanical engineering and material science and earth and planetary sciences.

At any given time, I'm not quite sure what I do, but one of those three. But in the last few years, he's also been exploring questions of life. And specifically, this kind of old idea that has a name that's somehow both technical and salacious at the same time, this thing called lithopan spermia. So a good way to think about it is to just break that word up into three parts, right? So little stone. So this is basically about rocks, pan, everything, spermia seeds. So the general idea is the idea of being able to seed life throughout the universe

or throughout the solar system through rocks, through basically life being carried in rocks that move around inside the solar system. A version of this idea goes back as far as the 5th century BC, when a Greek philosopher suggested that maybe life came to Earth cosmic seeds. Later in the 1800s, Lord Calvin proposed that life might have come here on a meteorite. Others put their own spin on this concept. And while people in more recent times have investigated the ways that life could have developed right here on Earth, no, extraterrestrial sperm required, the lithopanspermia idea has lingered on. It has come up as people talk about space exploration,

for example. People have asked, can life travel between planets or moons on rocks? because they want to figure out where life might be in our solar system. We have some policies that all our countries that are involved in the space program have agreed to. One of those rules is that we'll be really careful about potentially contaminating another planet. So far, we do not have definitive proof of life anywhere but Earth. But if we think there might be life on another planet or moon or asteroid, we would want to be extra careful to make sure we didn't contaminate that life with our own microbes. You don't want to take earth life and take it to Mars and let it loose and now it's all over Mars, right?

We also want to be really careful we don't bring something back to Earth from some other part of space. We're clean, let us in. What happened to King? Nothing is attached itself to him. We have to get them to the infirmary right away. We don't want to accidentally reenact the movie alien and contaminate ourselves. What kind of thing? I need a clear definition. An organism. Open a hat.

So the way we do this is we define some bodies in the solar system as being restricted bodies, meaning these bodies you have to be really careful about because they may have life. Right? Mars is one of those bodies. So if you go to Mars, you want to be really careful about sterilization. If you bring anything back from Mars, you want to be really careful about how you bring it back. Wait a minute. If we let it in, the ship could be infected. You know the quarantine procedures. 24 hours for decontamination.

So restricting a place like Mars seems pretty obvious, but people have questions about other spots. Like, let's say there was life on Mars, right? And then an asteroid slammed into it at some point and sent little chunks of Mars debris flying through space. The question is, could life have traveled, lithopanspermia style, on those little bits of debris over to a Mars moon, for example? and should we, therefore, be cautious about visiting Mars moons. When KT first encountered this question, he was skeptical. Why would you ever ask this question whether the answer is obviously no? That was my view of it.

But after several years and some more research, he thinks the answer is actually a little less obvious. This is unexplainable. I'm Bird Pinkerton. And today on the show, we're talking about lithopan sphermia. but also we're talking about how researchers try to push life to its limits. Try and figure out what the limits of life might be. KT first got interested in the Lithopanspermia hypothesis when the National Academy of Sciences approached him to work on a project about it.

And he realized that in order for a cell to make it from one place to another place on a meteorite, it would have to overcome a truly wild series of obstacles, a kind of lithopanspermia gauntlet. So let's say there was some life out there, maybe on Mars. First, again, something like an asteroid would have to come and smash into the planet. And then lots of little bits of Mars would go whizzing off into space, giving us meteorites,

right? Hopefully with some life on them. But obviously, when one thing smashes into another thing, you can get very high pressures. And KT told me that, The bits of rock that get flung up and out of space are not usually right underneath the incoming objects. They're usually kind of on the edges. But they're still probably experiencing a lot of pressure, and it's hitting all of a sudden, like an anvil dropping from the sky and then bouncing away.

So it's actually a shock. And one of the things we worry about is not just what the pressure is, but how fast it's put on. Because if I put the pressure on very slowly, maybe you had to be. adapt to it. And in fact, we know there are some bacteria that live way below the surface of the Earth. So there are bacteria that can adapt to high pressures. Maybe not this high, but high pressures. But if you do it as a shock, it happens really fast. But let's say, just for the sake of argument, that some microbes do survive this shock of pressure, this takeoff period. Now, they're on a meteorite hurtling through space. And for the lithopanspermia

a hypothesis to work, they also have to survive space travel. They get really cold and they get really dry. They get bombarded by radiation. And we know that you make things really cold, you can keep them from growing, you make them really dry, you can kill them, you put radiation on, you can kill them. The meteorite might also have to travel through an atmosphere or move at really fast speeds. So the microbes are also probably getting hot for a while. They probably experience a bonk when the meteorite hits.

a surface somewhere. So all in all, the likelihood of surviving the sort of panspermia gauntlet seems low. And yet, as KT was digging into the literature, he was also learning that life is pretty hardy. Like for decades now, researchers have been turning up examples of life here on Earth that can withstand pretty intense conditions. Organisms that can survive desiccation, organisms that can survive extreme cold, that can survive radiation. We call them extremophiles. They like extreme conditions.

Sometimes what makes them good at surviving one thing can also help them survive other things. So after he finished his reading, KT thought that maybe, just maybe, there was at least a chance that some microbes could survive the journey through space, despite all the radiation and the cold and the dryness. But he still wasn't sure that any microbes would even make it. to that part of the journey. Because he still wasn't sure if any microbes could even survive those initial pressures from the formation of the meteorite. That question, we did not have a good answer to.

Researchers had done a bunch of different experiments to try and work out if microbes could withstand huge pressures, and he respected the researchers who'd done these experiments a lot, but their results didn't give him a clear picture. So that's how I got into this, is I felt like the data wasn't really there to justify by us saying one way or the other. And I figured that maybe I could do this a little more cleanly. So I took on this project of saying, maybe I can develop an experiment that would let me take bacteria

and subjected to these really high pressures for very short times, just like a shock, and then measure how much of it survives. So I wrote a proposal to NASA saying, okay, let me go see if I can do that. Which is how he wound up in his lab, shooting bacteria with a gun. More on that. After the break. It's all about you. And when you fly with Virgin Atlantic in their upper class cabin,

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Before the break, I told you that KT. Ramesh shot bacteria with a gun for an experiment. If I'm being strictly accurate, though, he actually had one of his grad students do this, a mechanical engineer named Lily Zow. He's like, what if we try to shoot some bacteria for alien research? And I was like, why not? Sure. I'll shoot some bacteria with you. KT and Lily were not the first people ever to shoot stuff at bacteria.

But KT had a sort of new study design for this experiment that was a little bit different from the experiments that had come before. This technique that he thought would give him the clear answers he was looking for. The idea was to take some bacteria and then put them as a kind of layer between two steel plates. Like a little sandwich with the little cells. the middle. And then you're going to take this sandwich of metal plates and put it inside a target chamber. So I have in my lab a giant gun, and inside that gun I can launch projectiles at high velocity. So my sandwich containing my cells is sitting inside the target chamber. I then close the target chamber, evacuate it. So I take all the atmosphere out.

and then I launched my projectile. My projectile is carrying another metal plate. So I've got one metal plate hitting a sandwich of metal plates and the sandwich of metal plates contains the bacteria. So I'm sorry. Yeah. You have the sandwich of metal plates and you are firing a massive gun at the plates.

And the gun doesn't shoot like a bullet. It shoots another metal plate essentially. and that slams into the sandwich. Right. And what that does is sends waves into the metal plates. And those waves are like shock waves. And they generate the really high pressures inside the cell. And that's how we generate the high pressures for the experiment.

Why do you have, like, you just have a giant gun because you're firing things at things all the time? Right. So my lab is mostly about impact. I'm interested in impact problems. I do work for a bunch of places, all of whom I interest in these impact conditions. Do you ever use the massive gun for like silly things? I will never admit to that. Lily told me that it takes a long time, like almost an entire day, to set up this gun.

So the potential for silly things is somewhat limited. And also they had their hands full with this experiment. So they had their gun, but they also needed to put some actual bacteria in their steel sandwich. And several of the previous experiments on pressure and microbes had looked at E. coli, which is like the bacterial equivalent of a lab rat, kind of. But E. coli is not an extremophile. It's not able to withstand super intense conditions, and it's comparatively kind of puny. I didn't say it. I didn't say it, Eucaly. Don't get me sick. Don't come after me. One of their collaborators, Jocelyn de Ruggiero, she'd suggested that they work with a more hardcore, bacterium. One so tough that its nickname is Conan the bacterium. But you can call it Dynococcus

radiodurans. And the name should tell you something, Radio Durants. To me, that says radio, so radiation. And Durand says endurance is how I remember it. So this thing can handle a lot of radiation. Dynolkakis Radio Durans is also good at being cold. It's good at being dry. Basically, it's good at a lot of the things you would want a bacterium to be good at if you were going to send it off into space and hope it would survive. So the team started to work with it. And the first tests they ran were relatively simple. They put some of these bacteria in the steel sandwich.

They put the steel sandwich in the chamber with the gun. And then they left it there for a bit. Because even without the shock of pressure, it's just kind of stressful for a microbe to be living in a steel sandwich without an atmosphere around it. So they wanted to see how much stress in this experiment was coming just for, from that versus from the shock itself. But once I had that data... Then we shorted at the lowest velocity we could get,

which ends up giving us a pressure of around 1.4 GPA in this case. 1.4 GPA, or 1.4 gigapascals, is somewhere between 12 and 13 times the pressure at the deepest spot in the Earth's ocean, so the bottom of the Mariana trench. So imagine that you are a macrob minding your own business, and then out of nowhere you are first slammed with many Mariana trenches worth of pressure, and then sort of just as suddenly, all that pressure disappears again.

If this happened to you or me, we would be very dead. And that's kind of what Lily and KT assumed would happen to most of these cells, too. After the gun firing, Lily did some microbiology work to figure out sort of what percentage of the cells had survived. What I was expecting to hear was that there was nothing left. But instead, we saw that it was close to the control, like, 95% to 97% survival compared to the control configuration that we had. So basically, it seemed like close to all the microbes had survived. And I said, well, there's something wrong here. You know, it doesn't seem like this is right, because we were expecting 10 to the minus 6.

So one in a million, and this is now like one. So there's probably something not right in the experiment, and she said, yeah, I think there must be something going on. Many things can go wrong along the way that could give you an incorrect result. So we wanted to make sure that the survival that we were seeing was absolutely correct. So she started all over again and built the whole thing and we did it again. And again, they got similar results.

Only a small percentage of the bacteria seemed to be dying. And when Lily did more analysis, she found that the shocked bacteria were stressed out, but only about as stressed out as the control bacteria, the ones that had just gotten to

hang out in the steel sandwich with no pressure involved. So it seemed like the pressure had not faze them much at all. To me, this was really amazing, right? So the good thing about science is you have data. You repeat it. Did you get the same kinds of data?

All right, now, maybe there's something here. And once you were sure, what was your reaction? I mean, that was like, okay, let's go higher. So they went higher. They up the pressure to 1.9 gigapascals, which is roughly 17 times the pressure at the Mariana trench. And when they checked the results, the bacteria still seemed relatively okay. So they went even higher than that to 2.4 gigapascals.

For context, if you apply around 2.2 gigapascals of pressure to room temperature water, it changes into a kind of ice. In fact, at this pressure, their equipment was starting to fail. So the screws of the metal plates were shearing open at 2.4 gigapascals. But the microbes? At 2.4 gigapascals, the survival that I found was around 60%, so still pretty high. They did find that the bacteria that it endured this higher pressure also seemed to be stressed out in a new way. So the researchers are careful about making grand conclusions from sort of limited data here.

But they think what's happening is that the bacteria is now spending more time doing damage control. So it's seen high stress and it's reacting to that stress and managing itself in various ways. Eventually, though, the surviving microbes seem to be able to be. able to pull themselves together again and be right back in that kind of stressed out because of the suboptimal steel sandwich state. To Lily and to KT, this was exciting. It's not the first time people have ever reported bacteria being resistant to pressure, even high pressure. But again, KT wanted to do this study because he wanted clarity. And the data here were pretty clear. These bacteria had a high survival rate even when they were subjected to high pressure.

I have been wonderstruck by how much these things can take. It's just amazing. What does all this mean for the lithopanspermia hypothesis? This question about microbes traveling through space. My impression from talking to KT and to others is that this is a useful data point. Remember, this whole experiment tells us a bit about the takeoff part of the lithopanspermia gauntlet. Could bacteria survive the immense pressures that occur as a meteorite is formed? There's still more follow-up work to be done. These pressures are high to us, but still relatively low if you're talking about meteorite formation. But if KT once thought that it was totally obvious that no life could survive the lithopanspamia gauntlet, he now thinks that if you

start with enough cells to begin with, under some conditions, The possibility that some tiny fraction of cells might make it is not zero. You go from saying this is improbable to, well, it's possible. It's still a low probability, but it is possible. Beyond questions of lithopanspermia, though, I think it's just as interesting to look at the ways that this research contributes to our understanding what life right here on Earth can do. Like, we've known that microbes,

can live at high pressures deep in ocean mud, but those pressures are an order of magnitude less than these pressures, and they're consistent, not sudden. Here, it's a little like KT and Lily are telling us that Looney Tunes is a documentary, that you really can smash something with an anvil, and in the next scene, it'll be zooming around,

ready to catch a roadrunner. It's something that, understandably, both KT and Lily have further questions about. Lily's doing research to see essentially if she can breed like a super pressure-resistant bacterium. So if she slams bacteria with a bunch of pressure and then takes the survivors and lets them grow, will she get bacteria that are even more pressure-resistant? And then KT, meanwhile, wants to figure out what makes these bacteria so pressure-resistant. He wants to know if other extremophiles can also survive high pressures or if fungi can.

He wants to know if there's something special about Dynococcus radiodurans that helps them with pressure, like their cell walls. That's my current hypothesis. But so far, every time I've tried something, I've been wrong. So we'll see what happens. Ultimately, we may never know whether or not life has traveled around the solar system. But there is still plenty of questions about life right here on Earth for us to chase after. And maybe if we fire our Acme science gun at a number. off organisms, we might even catch some answers.

You want to hear more about microbes in space and other ideas about panspermia besides life traveling around on meteorites. We have another episode all about the poop we left on the moon that you might really enjoy. It's called moon poop, and we'll link to it in the transcript. Also, in my conversation with KT, we were mostly focused on the idea of microbes traveling between something like Mars and a Mars moon. But for those of you who are Earth curious, we also have a third.

three-part series called Origins, all about how life might have developed right here on Earth. We will link to that as well. This episode was produced by me, Bird Pinkerton. It was edited by the wonderful Joanna Solitaraf. Christian Ayala did the mixing and the sound design. Melissa Hirsch, check the facts. Noam Hasenfeld does our music. Jorge Just, Meredith Hadnatt, Sally Helm, and Amy Padula,

are the fact that octopuses have donut-shaped brains. Thanks always to Brian Resum. for co-creating the show with me and Noam. And a big thanks to Lily Zow, who has now defended her Ph.D., and to K.T. Remesh. Thanks also to their collaborator at Johns Hopkins, Jocelyn de Ruggiero, who helped me better understand biology here. And thanks to Paul Hazel at UNSW, Canberra, Australia, Gareth Appleby Thomas at Cranfield University, and Peter Dorn at Louisiana State University, for helping me understand different aspects of this paper and of Lithuania. pansepermia. If you have thoughts about life in space or if you have subjects you think we

should explore, please email us. We are at Unexplanable at Vox.com. I have been endlessly fascinated with microbes lately, so please send me weird microbiology stuff. I love reading through it. If you would like to support this show and the journalism that Vox does, we would love it very much if you would become a member. It's a very easy thing to do. Just go to Vox.com slash members and you will get access to all of Vox's journalism, but you also know that you're supporting all of Vox's journalism, including this show. And for those of you who have emailed us to let us know

that you've signed up because of Unexplainable, thank you. I also want to thank the people who have left us a nice review and the people who have told people in their life about the show. All of you are, frankly, the best. Unexplainable is part of the Vox Media Podcast Network and we will be back very soon with another episode about everything that we do not yet know.