IRA FLATOW: This is Science Friday. I’m Ira Flatow. About 25 years ago, a little piece of Mars caused a big commotion. A potato-sized meteorite from Mars known as Allan Hills 84001 was picked off the ice in Antarctica in 1984.

Lots of Martian meteorites are found on Earth. What made this one different was that in 1996 the late astrobiologist David McKay decided this 4-billion-year-old piece of Mars showed evidence that the Red Planet had once contained life. Mineral traces and structures on the rock, McKay thought, could only have been created biologically.

You can still read the press release for when that research was published, Meteorite Yields Evidence of Primitive Life on Mars. It’s up there in our website, ScienceFriday.com/MarsRock. We talked about the Allan Hills meteorite in 1997 after a flurry of research began to challenge McKay’s findings. Here’s planetary geologist Ed Scott.

ED SCOTT: We’ve been focusing on the carbonate minerals that were alleged to contain the signs of life. Our group is convinced that the carbonate minerals must have formed at high temperatures. They didn’t form over long periods of time from water which was oozing through the fractures in the rock. We think they’ve carbonate-formed at very high temperatures, well over 1,200 degrees Celsius. We think they formed very rapidly in an impact.

IRA FLATOW: Further research in the decades since has ruled out many of McKay’s conclusions. But the meteorite is still an important part of research into ancient Mars and conditions for possibly living there. Producer Christie Taylor recently revisited the storied past of the Allan Hills meteorite for the SciFri Book Club. Welcome back, Christie.

CHRISTIE TAYLOR: Hey there, Ira.

IRA FLATOW: Nice to have you back. And, of course, we are reading The Sirens of Mars by planetary scientist Sarah Stewart Johnson this month, all about the search for life on Mars, which includes the bits of Mars that have landed on this planet. I take it Sarah talks about this meteorite in the book, right?

CHRISTIE TAYLOR: She does. It gets an entire chapter, which feels very fitting, given its importance in our understanding of Mars.

IRA FLATOW: Sounds great. Take it away, Christie.

CHRISTIE TAYLOR: Sure. So I talked to astrobiologist Andrew Steele. He’s been studying meteorites like Allan Hills for decades. He actually worked with David McKay, starting shortly after that first very controversial publication. And he also works on both the Curiosity and Perseverance rover missions. So I started by asking him to just describe this piece of rock from Mars. And, as you said, it is indeed potato-sized.

ANDREW STEELE: It looks like any other rock. It’s very fine-grained inside. It means the crystals are really small. And it has, in some places, this kind of reddish tinge. And that reddish tinge is these small rosettes of carbonate. On Earth, such things can be made by life. And that really drew the eye in, of Dave McKay and his team, into really looking at those.

So it really looks quite nondescript. It’s quite heavy. It’s blackened on the outside because of its journey through the atmosphere as it melts. But apart from that, it’s really quite a nondescript rock. It’s small and yet so important in our understanding of the universe now.

CHRISTIE TAYLOR: Why did David MacKay think that this meteorite contained evidence of life?

ANDREW STEELE: At the time Dave MacKay began the studies on Allen Hills 84001, we had a range of measurements on early Earth samples, for instance, that indicated that life was, on this planet, around about 3.6 billion. And the initial dating of this Martian meteorite, Allan Hills 84001, showed it was an equivalent age of when life had started on Earth, and it was the equivalent age of that rock on Mars.

And so, their initial analysis of the rock showed the presence of organic molecules in polycyclic aromatic hydrocarbons. It also showed a kind of disequilibrium texture that life tends to like to do. It kind of makes minerals in strange places. In these carbonate globules also, there was these small magnetite grains that certain species of bacteria like to use on Earth, magnetotactic bacteria. And, all in all, there were four to five lines of evidence, all pointing towards a possible biogenic explanation.

CHRISTIE TAYLOR: One of the lines of evidence that you just described was just, he thought he saw something that might be fossilized bacteria. What did that look like?

ANDREW STEELE: As you go into the microbial world, the shapes that the microbial world adopt, at the simplest level, are pretty uniform. They’re either small circles, small spheres, small rod shapes. And on the rock themselves, they found evidence of these small shapes around about the size range, a bit smaller actually, than Earth-bound bacteria.

And at the time, again, in ancient rocks on Earth, it was the search for fossils, which was very much part of the debate at that time. And they used a technology, at that time which was cutting edge, to look at Allan Hills 84001 in a way that many of the rocks hadn’t been looked at like that before. And so they find these fabrics and features that really pointed them towards it being evidence of fossil life.

CHRISTIE TAYLOR: And then, of course, others disagreed. What was their reasoning, and what did their research show to sort of counter that postulation?

ANDREW STEELE: Well, it was several things. One was it could be an artifact of sample preparation for these kinds of analyses, which is where my initial work came in, showing that it potentially wasn’t. There was also that these features are on the small side of what we know about life on Earth.

And so, that kind of jumpstarted an effort amongst the scientific community to delineate what the smallest form of life could be on this planet, at the time, ultramicrobacteria or nanobacterial features that were being seen and were highly contentious then and still, to some extent, are contentious today. So the lower-size limit for life, how small could an organism be? How could you pack all this cellular material into something so small?

CHRISTIE TAYLOR: You’ve said that we couldn’t even find Earth life that was contaminating this meteorite until you looked at it and imaged it. Is it really that hard to find bacteria on a piece of rock?

ANDREW STEELE: Well, the thing is, I think, the scale of the problem is really interesting, yes. We did find contamination. Dave McKay actually brought it to me to classify, because he had found it and said, this is unusual. What is it? And so we classified it as a bacteria from Antarctica. And at the time, the extent of life on Earth wasn’t really known as it is now. And certainly, the extent of microbial life in Antarctica wasn’t really appreciated as it is now.

CHRISTIE TAYLOR: We thought Antarctica was sterile.

ANDREW STEELE: Pretty much, pretty much. This analysis showed that this was probably a type of bacteria called an actinomycete and was growing on the rock. And at the time, a lot of techniques said there is no life in this rock. But there was. There was Earth life. And that struck me as being, we can’t see the wood for the trees.

But if you think about trying to find a single microbe or a small group of microbes on a rock, you have to understand the scale of that. So you can literally fit a hundred bacteria on the pointy bit of a pin. And so finding those within a rock is not easy.

And the amount of carbon, for instance, in a single bacteria is 0.1 with 13 naughts, right? It’s vanishingly small, 10 to the minus 13 of a gram of carbon. So detecting microbial cells at that level is not easy.

CHRISTIE TAYLOR: Just a quick reminder that I’m Christie Taylor, and this is Science Friday from WNYC Studios. Talking to Dr. Andrew Steele about meteorites from Mars and how we can study them.

What is the biology of microscopic life tell us about how to look for it in samples of rock?

ANDREW STEELE: That’s a really interesting question. Life has an ability to be able to kind of concentrate the ingredients it needs, either to make itself or to eat or what it excretes. And what we found in meteorites left over from the building of our solar system, we see that a lot of organic chemistry goes on in those rocks. Thousands and thousands of compounds are made. But life doesn’t choose all of them. It only chooses a subset of those.

So if you look at that, what you need to do is look for concentrations of these letters above and beyond what organic chemistry would do in these rocks. And I think the biggest lesson– for me, certainly, and others in the community– was that the scale of that search is one where you have to be ridiculously sensitive with the instruments that you’re making the measurements with, and ridiculously careful that, during all of that process and during the time that meteorite has been on Earth, that Earth life doesn’t interfere with those measurements.

CHRISTIE TAYLOR: Well, I have a question sort of on that vein. Someone in Antarctica finds a new meteorite. They send it to you. What kinds of instruments do you then submit your samples to? How are we actually probing what’s in there?

ANDREW STEELE: Well, I think it’s like with any sample. If you’re an Earth geologist looking at Earth life, you would first go to the place where you picked it up so you would know the context of the sample. I’m picking it up from this rock face, and this rock face looks like a sedimentary rock or an igneous rock.

And the first thing you do is, obviously, you photograph it. You just look at it, describe it, its context, what its mineralogy is, what kind of rock is it. And then you can do a couple of different things. And I tend to do both.

One is to look at fresh fracture surfaces, where you just chip little bits off and put them under a light microscope at first. So you start at low magnification and slowly increase the magnification that you’re looking at the rocks, so that you can look with increasing resolution and increasing detail at the mineralogy of the rock. Or you can slice it into really thin slices and look through the rock.

And then at that point in time, I use a spectroscopy technique called Raman spectroscopy. And what that allows me to do is look within the rock.

IRA FLATOW: We need to take a break. We’ll be back with more on Mars, meteorites, and the hunt for signs of life in a moment. Stay with us.

This is Science Friday. I’m Ira Flatow. In case you’re just joining us, SciFri’s Christie Taylor is talking to astrobiologist Andrew Steele about looking for signs of life in meteorites and ancient rocks.

CHRISTIE TAYLOR: All right, so we’re walking through looking at a new meteorite. And you’ve just taken a deep look at the mineral structure, but what about all those organic molecules?

ANDREW STEELE: What we found was, in several of the meteorites, including Allan Hills, is a signal of a kind of complex organic material. It has several different names on Earth. If it’s from life, it’s called kerogen, If it’s in bacteria, it’s called insoluble organic material. I tend to call it macromolecular material or refractory material.

Because it’s trapped within the mineral, it’s not seen too much of the terrestrial conditions around it. And what you can do then is very minutely cut little slivers of that material out from the sample and put them on instruments that then go down to the point where you’re basically imaging or analyzing individual mineral lattices all the way down at almost the atomic level.

CHRISTIE TAYLOR: Oh, wow.

ANDREW STEELE: And at that point, you can see and use different techniques to kind of interrogate more of the nature of the organic material in there or the minerals in there and get the relationship between the organic material and the surrounding mineral matrix. Is this something that looks like, say, a bacteria sat on a mineral surface or a bacteria that’s kind of etched its way into a mineral surface? Or does this look like just a puddle of carbon with no real structure to it?

You start to pick up how the actual carbon relates to the mineral matrices around it. And from that you gain a lot of information as well, morphologically. And then we can use these really high resolution techniques that, many of which, weren’t available back in ’96, certainly not in the same resolution they are now, and really start to unpick the nature of the organic material there.

CHRISTIE TAYLOR: You know, you’ve mentioned all these techniques that weren’t available in 1996. If we haven’t found evidence of life in any of these meteorites, even with all these techniques, what have we learned about Mars as a result of looking at them?

ANDREW STEELE: Well, we do know, between this and the work on Allan Hills 84001 and the Tissent meteorite, and Curiosity and now Perseverance, we’re finding that Mars had quite an active organic chemical cycle and that Mars does do its own organic chemistry, which has major implications for early Earth or Enceladus or Europa and how organic chemistry could be expected to produce the building blocks of life on those bodies and on Mars.

My studies don’t necessarily negate that there is Martian life in these meteorites. What I look at is this complex organic material. Basically, if I want to find life, I assume there is no life and try and disprove that hypothesis. And I can’t disprove that nonlife processes made this material.

CHRISTIE TAYLOR: So the absence of evidence isn’t the evidence of absence.

ANDREW STEELE: Bingo. But it’s a stepwise journey. And some people, and rightfully so if you think about, it’s more difficult to prove a negative than a positive, right?

CHRISTIE TAYLOR: Mm-hmm.

ANDREW STEELE: But what this has done is– what the McKay group did and, effectively, over the last 25 years, have enabled the community to start thinking about this. They’ve enabled– the whole debate enabled a series of missions to Mars, the like of which our species had never undertaken before. It has enabled us to attack those problems and really think about those problems and develop instruments and protocols and procedures to understand how to find life on another planet.

And the spin-offs of that in the Astrobiology Institute and the NASA Funding Agency has spun out many programs to try and really unpick how to find life elsewhere. But that has led to a greater understanding of life on our own planet and how life could have possibly formed here.

It caused a real revolution in our understanding of where life could be in our own solar system. And I think the greatest legacy of this paper is in the search that we’re still on. And Dave and his team should be commended for having the courage to put that hypothesis out.

CHRISTIE TAYLOR: Well, and I think my last question, then, is, if you had to choose between more Martian meteorites or better instruments, what do you think the most important thing would be?

ANDREW STEELE: We have over 200, I think, Martian meteorites now. And I think I would like to see– all of the analysis we’re doing on Martian meteorites now, one of the spin-offs of doing that analysis is understanding the analysis chain that you have to go through on return samples. Understanding the measurements that we have to make, and how we make them, and how we begin to make them is the most important part of that.

And I think, for me, if you, in that question, assume that we are going to bring back samples from Mars, I think the way in which we look at those samples, the way in which we analyze them, the techniques we use, need to be not just developed, but the lessons learned from Allan Hills, the lessons learned from Tissent and the other Martians meteorites need to be put in practice to analyze the return samples.

And that’s not an easy thing to do if you think about the challenges associated with bringing a sample from another planet, planetary protection, where you store the sample, keeping it clean, how you analyze it, those things. It’s a really big unknown at the moment.

And NASA is working on this, so is the European Space Agency, the Japanese Space Agency, all trying to figure out how best to do this in a way that keeps the planet safe and also keeps the sample safe and allows these techniques and future generations going, oh, I’d like to do this analysis, this new instrument, I think it will do this. And how you ensure that legacy on the return samples with those instruments, I think is key.

And this debate started really with the ALH meteorite. And it’s still ongoing. And I think it’s, again, one of the legacies of this meteorite and the team’s analysis on this meteorite has been to really illustrate and illuminate the path forward in analyzing the samples as they come back from Mars.

CHRISTIE TAYLOR: Mm-hmm. Well, good luck in the meantime. Thank you so much, Dr. Steele.

ANDREW STEELE: It’s been a real pleasure. Thank you very much for having me on.

CHRISTIE TAYLOR: Andrew Steele is an astrobiologist at the Carnegie Institute of Science. He works on the Perseverance and Curiosity Mars rovers.

IRA FLATOW: Fascinating stuff. And Christie, we’re reading The Sirens of Mars all month, if folks want to continue on this Martian Book Club journey.

CHRISTIE TAYLOR: Step one, Ira, is always to go to our website, ScienceFriday.com/BookClub.

But this week, we have an extra special ask for our audience. We want to know what you think life on Mars looked like, if you think it happened at all. Carl Sagan famously thought giant turtles could have roamed the planet. Andrew Steele was just talking about microscopic life. But what do you think?

You can contact us a few ways. You can send us a voice memo with the SciFri VoxPop app wherever you get your apps. That’s the SciFri VoxPop app. And we have a phone number you can call at any time you want. That number is 646-767-6532. Again, 646-767-6532. Leave us a voicemail and tell us about Martians.

And again, for everything else we’re up to this month, check out ScienceFriday.com/BookClub.

IRA FLATOW: I love that factoid, that Carl Sagan thought there were turtles.

CHRISTIE TAYLOR: It’s amazing, right?

IRA FLATOW: Thanks so much, Christie.

CHRISTIE TAYLOR: Thank you.

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