Once again, another place where the Universe is going to make this difficult for us. Proving, once and for all that there’s alien life on another world. It should be straightforward, look for biosignatures, but it looks like there are natural sources that could explain almost any chemical we could hope to search for.
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Astronomy Cast, Episode 544
Weird Issues: Biosignatures
Fraser: Welcome to Astronomy Cast, our weekly fact-based journey through the cosmos where we help you understand not only what we know, but how we know what we know. I’m Fraser Cain, publisher of Universe Today. With me, as always, Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the Director of CosmoQuest. Hey Pamela. How you doing?
Pamela: I’m doing well. How are you doing, Fraser?
Fraser: Good. Are there any updates on Bennu mapping?
Pamela: Well, we’re in phase 2 mapping right now, which means that all of the wonderful, 6000-some odd people that have come to our site succeeded in mapping out the majority of the surface of Bennu, and we now have a specially select group of eight who each marked over 1000 images during the initial mapping phase. And that group of eight is doing a detailed measurement of the four potential sample sites, and their data is going into the pool to help figure out exactly where Bennu is gonna well, grab some stuff to bring back to Earth.
Fraser: That’s awesome. Please let us know when the final decision has been made.
Pamela: We will.
Fraser: Once again, another place where the universe is gonna make this difficult for us, proving once and for all that alien life is on another world. To be straightforward, look for biosignatures, but it looks like there are natural sources that could explain almost any chemical we could hope to search for.
I – and I think I’ve admitted this in previous episodes – that I have been a victim of the oversimiplification of the search for life that – we’ve even said this in previous episodes, I know, of Astronomy Cast – that all you have to do – it’s easy, Fraser. All you’ve gotta do is search for oxygen in the atmosphere of another planet, and boom, you know there’s life there because how else could you get oxygen. Turns out, you could get oxygen all kinds of ways. So, let’s talk about this hunt for biosignatures and how it’s actually been a lot more tricky. And you put in the Viking experiments, which I think is just – there was a great article. I don’t know if you saw this one that came out this week in Scientific American.
Fraser: So, it’s a perfect time to sort of go over this again and just show how sticky this problem is and how it’s still – it’s not – the universe is going to unveil its secrets to us unwillingly. So, let’s talk about the Viking experiments.
Pamela: So, the Viking experiments, back in the 1970s, sought to find four different ways that through four different processes would potentially be able to uncover the reality or the lack of reality of life on Mars. Each of the experiments had a control. Each of the experiments was run multiple times. And most interestingly, in some ways, each of the experiments was run by two different spacecraft that were separated by a fairly good distance across the surface of Mars.
Fraser: Right. So, can you explain sort of the setup of the experiment and how they worked, and how these different variables were tested for? It’s actually pretty incredible.
Pamela: So, back in the mid-1970s, we launched Viking 1 and Viking 2. Each of these spacecraft consisted of an orbiter that would relay the signal back to Earth and a lander that would run the experiments. In addition to having a whole variety of cameras that were used to look around their location, each of them had four experiments. The first of these experiments was a combination of gas chromatograph and mass spectrometer. And the goal of this particular experiment was to determine are there organic materials. And it was this particular experiment that led Nasa to say all the other results were probably faulty because when they went looking for organics, they didn’t find any. Now, the problem –
Fraser: So, how did the experiment work?
Pamela: They literally scooped up a bunch of soil. He did it and measured what the composition of the soil was.
Fraser: And I think they fed it some nutrients as well, so they tried to – or they fed it some – I think they warmed it up and I think they fed it some water.
Pamela: Well, that’s a different experiment. We’re gonna get there.
Fraser: Oh, different experiment. There’s like the limited –
Pamela: So, there’s a gas chromatograph mass spectrometer. There was a gas exchange experiment. There was a labeled release experiment.
Fraser: Labeled release, yeah.
Pamela: And there was a pyrolytic release experiment. So, there are four different experiments. So, the first one that should have been the most straightforward, except this is Mars, the first one – they just scooped up some soil, looked to see what the soil was made of, looked for some organic materials, and found none. And based on this, they were like can’t be life.
Now, the thing is, the Phoenix lander, back during the existence of this show, determined that Mars has a chemical called perchlorate on it. And if you take perchlorates and you put them in a soil sample that contains organic molecules, and you heat this up, the perchlorates politely destroy all the organic molecules, leaving you with exactly what the Viking probe saw.
Fraser: Right. But with the one of the experiments, they did detect carbon dioxide released, but you’re saying that’s from the perchlorates. Right, right.
Pamela: So, from this first experiment – I’m taking these one at a time.
Fraser: Okay, okay. All right. I’m running to the end. I’m getting – spoiler alerts. No problem.
Pamela: Yes. You’re determined to jump ahead. You’re determined.
Fraser: No, please let this story unfold in its natural order of things.
Pamela: I will, thank you. So, in the first of the experiments, they looked for life. They did not find any life, and the result of not finding any life – or not life – not finding any organics was anytime they found anything else that signified “hey, there might be life here,” they were like “no, gonna ignore it.”
And this is kind of the key because well, I’m going to now go the gas exchange experiment. And in this particular exchange, they took a soil sample and they pulled all of the gas out of the sample and replaced it with inert helium that wouldn’t have any chemical reactions. And they also added nutrients to the soil. And the thinking was that if there were microbes in the soil, they would metabolize the nutrients and release, well whatever they released into the atmosphere of this vessel, and this would be measured in excess of the helium.
And both Viking 1 and Viking 2 saw the same concentrations of several gases released, including oxygen, carbon dioxide, nitrogen, hydrogen, and methane. And this is what you would expect if there were microbes. And so, here we have two different gas exchange experiments on two different probes, 4000 miles apart, that had the same result. Now, the thing was they also baked the bejesus out of that soil which they theorized would kill any microbes in the soil and then looked to see are there any –
Fraser: Yeah, they baked them and then they ran the experiment again, and they didn’t get the gases coming out, which is exactly what you would expect from dead microbes.
Pamela: Exactly. So, here we have the situation of this is doing exactly what you would expect. Now the argument was again, why didn’t they find organics, and so let’s not continue looking at this. And then, there was – also, when they tried redoing the experiment, the experiment didn’t run quite the way it was expected to run, and because it didn’t exactly match, they were like “oh, this must be some sort of strange chemistry.” And one of the critiques that has been put forward for this and other experiments they did is well, maybe they simply over-nutriented it, they overwatered it and they killed off the microbes with too much moisture.
So, Mars is a completely dry environment. We’ve all killed plants before. At least, if you’ve never killed a plant, that probably means you’ve never owned a plant. So, maybe the reason it didn’t look identical is they just over-nutriented it and killed it in the process.
Fraser: So, everybody agreed, came to terms with the fact that there was life on Mars, was not life on Mars – I’m, of course, being facetious here.
Pamela: We’re still going. There are still two more experiments.
Fraser: This fight is still happening. We are 40 years later, and the battle continues.
Pamela: Yeah. So, the other experiment that was done was a labeled release where they fed the theoretical microbes that might be there carbon-14, which is a radioisotope, and they looked to see if that radioisotope would be respirated, and lo and behold, the respiration was observed. They baked the nutrients. No respiration was observed. They’ve done hundreds and hundreds, and some would say thousands, of replications of this experiment using sterile soil, using microbe-rich soil. This is an experiment that no one has figured out how to produce false positives on or false negatives on.
And yet, even though this particular experiment showed the expected results on both probes, no life on Mars. So, this is the one where the scientists who ran that experiment is just like over it.
Fraser: Come on, people. What’s it gonna take?
Pamela: Yeah. That pretty much, pretty much – So then, we also get to the – this is the one I can never say correctly – the prolytic release experiment. So, in this case, they were looking to see if they could get photosynthesis to take place. So, they gave – I’m sorry. This one, they didn’t give nutrients. So, in this one, they illuminated the soil and they looked to see if respiration would take place. They looked to see if carbon-14 that they laced the atmosphere of the soil with would be absorbed by the microbes. They let respiration, photosynthesis, all of those things, take place.
They then baked the sample to see if any absorbed carbon-14 that was turned into the microbes bodies – this is what all of us do, we breathe things in, we turn them in, we eat things, we turn them into ourselves. So, what they looked to do was see if that taking this C-14 and turning into itself, thing microbes can do, had occurred. And here, they had inconclusive but probably, and yeah.
So, you have three inconclusive experiments, and by inconclusive, I mean it looks like life. And the thing with the prolytic one that left them so confused is here they did bake the bejesus out of the sample, replicate the experiment, and still got signs of life, which could mean that simply whatever it was chose to live through the baking stage.
Fraser: Right. Life finds a way.
Pamela: Life finds a way, and we know that things like well, tardigrades, water bears – you bake the bejesus out of them. Like put them on a heat shield of a spacecraft and pass them to the atmosphere, and they still live. And so, the poor scientist, now retired, who was in charge of the experiment that looked to see if they could get the microbes to absorb nutrients and release them back out, he was certain that they’d found indications of life, and had spent the entire rest of his career furious. There’s no other word for it.
Fraser: Yeah. No, absolutely. That is the way to describe it. I mean, when you think about the path that NASA took after the Viking experiments, NASA’s takeaway was this is too inconclusive for us to say that we found anything. And so, therefore, let’s go back to the drawing board. Let’s send the opportunity rovers to find out if there was ever water on Mars. Let’s send Curiosity to find out if water was there for a long time. Let’s send Mars 2020 to see if there’s any evidence of any life activity ever in the history of Mars, past or present.
Like they spent 40 years getting back to the Viking experiments and they’re not even at redoing the Viking experiments. There’s not even anything on Mars 2020 to redo the Viking experiments.
Pamela: And this is where he gets so eloquently annoyed because Mars Curiosity could’ve replicated his experiment very easily. It has everything on board except for like the space to do that one particular experiment. If you could do it in the 1970s, you can do it today. And he’s just like “why won’t they replicate this?” And more than that, he also points out that we now have the technology to not only replicate the experiment, but to look and see if when things get metabolized, get respirated, is the chirality – the shape of the molecules – like the shape here on Earth, or is it something different.
And he goes through an entire litany in a “we didn’t know this was gonna come out when we scheduled recording this episode.” He goes through a whole litany of critiques that starts with “okay, we now know there’s enough water to sustain microorganisms on Mars.” We understand ultraviolet activation of the Martian surface, and how this could have expected the experiments. We understand now that there are complex organic molecules on Mars. It was the lack of those complex organics that was part of why they said all of these other potential positive results were actually negatives.
The Phoenix and Curiosity found evidence of ancient Martian environments that would have been habitable. The excess of carbon-13 over carbon-12 in the Martian atmosphere is indicative of biological activity. The Martian atmosphere is in disequilibrium. The cO2 should long ago have converted to cO in ultraviolet light. We now know that fresh organisms can survive in Mars-like environments. There is a rapid disappearance of methane from the Mars atmosphere, but we keep finding methane in Mars atmosphere. Formaldehyde and ammonia, each possibly indicative of biology, are claimed to be in the Martian atmosphere.
Six channel spectral analysis by Viking’s imaging system found terrestrial lichen and green patches on Mars rocks to have identical color, saturation, hue, and intensity. Here, he is actually arguing. And I found his original research paper.
Fraser: That’s amazing.
Pamela: There is lichen on Mars that the Viking probes could see, and there’s a paper from the 1970s describing how the patterns on the rocks change over time in a way that is completely consistent with lichen growing on rocks.
Fraser: Right. And not only that but there is an experiment on board the International Space Station where they took a whole bunch of Earth-based lifeforms, opened them up to the harshness of space, and then returned them to either Earth-based environment or Mars-based environment. And all of the lifeforms were able to recover in the Earth-based environment, and all, or almost all, of the lifeforms were able to recover mostly in the Mars-based environment. And the one that did great was cyanobacteria. So, we know of lifeforms that can make the journey through the vacuum of space, land on Mars, and get rolling again.
Fraser: Not land back on Earth. Land on Mars. We already have lifeforms that can thrive on Mars, and we have lifeforms that can handle going to space and then land on Mars and thrive on Mars. So, it’s like we’re learning how much tougher and better life is today. But I think the whole point, why this is all just in the weird issues category is it just shows that – obviously, if you saw a Martian run past the camera, then you would have proof positive that there’s life on Mars. But just – this is tricky.
Fraser: And you just – you ran through a bunch of them. You ran through the methane discovery by Curiosity of the methane in the atmosphere. That could’ve been conclusive evidence that there was life on Mars. Nope, nope, because volcanoes can also produce those levels, or can produce methane.
Pamela: Well, so the methane in particular is super annoying because we’re now finding that the methane has both seasonal variations and diurnal variations, which means it has a day-night cycle and it has a seasonal cycle. And that gets much trickier to explain as not being life, but if it happens to be that there are ices that have methane from an earlier period trapped in the soils, and those ices just happen to be getting melted and releasing the methane. But this is an ongoing process. And so, it keeps – the requirements necessary to eliminate this as being life are becoming extraordinarily complicated.
And one of the arguments that our poor, poor, much maligned Gilbert Levin has put forward that NASA would be covering this up, covering up the discovery of life, is if we are bound and determined to put human beings on Mars, we’re not going to do it if everyone knows there is life on Mars that is either going to contaminate our life and kill us or we’re gonna contaminate it and kill it.
Fraser: More likely.
Pamela: And so, his argument for why people are refusing to acknowledge this is it would delay timelines. Now, I don’t know what to even say to that.
Pamela: But I think this is going to be one of my favorite conspiracy theories.
Fraser: Right. Because it’s like pretty high up in – scientifically rigorous when you think about the people who are making this.
Fraser: So, this issue of we thought we had a slam-dunk detection of life on another world, turns out we didn’t. And so, now as we are getting far more powerful telescopes, as Cassini is detecting organic molecules and hydrogen gas in the geysers that are pouring out of Enceladus, as we are seeing these geysers on Europa, and as this next generation of telescopes is coming online to observe other worlds and directly observe the atmospheres of other planets, we once thought this would be relatively straightforward. And I am almost, like I said, I am almost certain you have said the words, something to the effect of “if you see oxygen in the atmosphere, you see life.” Would you say that today?
Pamela: I would say you probably see life. And this is where it gets trickstery. Chris McKay, who is someone whose research I really enjoy following, and he’s someone who uses chemistry to find situations that shouldn’t naturally exist, so he’s one of the ones that’s out there studying how so many of the elements in the atmosphere of Mars are not in the ratios that you would expect. He’s one of the ones who’s been out there studying the environment of Titan and noting that the elements are not in the ratios you would expect, and then looking at possible biological and possible geological reasons for this to happen.
And his research is some of the research that is working really hard to say “okay. So, here are all the complicated hoops that you can jump through to get these results, but also life.”
Fraser: Right. But as long as there are hoops that you can jump through, then you can’t say life. And when you think about the capability of these telescopes, if you point one of these telescopes and you determine the presence of say methane on a planet, you don’t know how much methane, you just know methane – check.
Pamela: Well, with Mars, we know how much.
Pamela: With Titan, we know how much.
Fraser: Right. But I’m talking about –
Pamela: Alien worlds.
Fraser: Yeah, yeah. Right. Kepler whatever, 629, right. Something that you’ll point James Webb at and determine the characteristics of its atmosphere. You can’t get that – you can get methane – check. You can get ozone – check. Oxygen – check. But again, there are natural processes that could theoretically create each one of these, and you don’t necessarily get the quantity of them in these atmospheres.
Pamela: And the thing this all reminds me of is back in the early 2000s, if you went to a series of Mars talks at a conference, like Lunar and Planetary Sciences Conference, you would hear a mix of people saying “here are all the ways that you describe the geological features on Mars using everything but water to justify their existence. Here is how you get valleys. Here’s how you get everything using dust processes, fluvial and Aeolian were the phrases that kept coming up. And it’s the fluvial, the ones that require water, that you also had a minority set of voices at that point going “but water, water.”
And over the past two decades, we’ve seen this literal sea-turn of people saying there were once oceans on Mars now. And so, the part of me that really wants there to be microbes out there – we’re gonna end it at microbes. The part of me that really wants there to be microbes out there really hopes that within our lifetime, all of these gymnastics that we’re going through right now to justify methane in all of these places becomes “oh, oops, there’s methanogens everywhere.” And we don’t know. This is why poor, beat up Gilbert Levin desperately wants these experiments repeated.
Fraser: Right, right. And as we said, Mars 2020 won’t be the machine that’ll do it. Humans that go there, filthy humans covered in microbes, covered in cyanobacteria and lichen, will –
Pamela: I hope they’re not covered in lichen. That’s really a horrifying idea.
Fraser: Covered in lichen. They are so gross. They will dig – they will have – in theory, that’s the kinds of tools that you really are gonna need. A sample return mission will go a long way, bringing some of those samples from Mars back to Earth, but I think it’s exactly it. You land a spacecraft and someone looks over, off to the horizon, and goes “hey, is that lichen on those rocks over there?” And then they go “well, let’s go check and let’s find out.” It’s really showing us that from the moment we think we’ve got the capability to detect some kind of life in this other place to the time when we’re sure, especially if it’s microscopic, is actually a lot more intensive and a lot more complicated than we ever thought.
And that if we ever thought that any of these secrets would be revealed to us quickly, we apparently have another thing coming – that it’s gonna take us decades. It’s gonna take us many decades. We’re at 40 years already with Mars. Buckle up. Obviously, unless SpaceX lands and while people are out there, I don’t know, making their lawns for the new Mars colony, they happen to dig up a Martian skeleton, it’s gonna take us a while still to do either the sample return mission or to have the people there with the proper experiments or the right spacecraft. And same problem, right?
You send a laboratory to Mars. It performs a bunch of experiments, and people will think that those experiments might be inconclusive. Back to square one.
Pamela: Right, right. And I just want this to be definitively proven while poor Gilbert is still alive.
Fraser: Yeah, yeah. He deserves a win.
Fraser: He really does. And then a Nobel prize.
Fraser: Yeah. I think that’d be right. Okay, Pamela. Do you have some names for us this week?
Pamela: I do. I have a variety of people I would like to thank for supporting us through patreon.com/astronomycast. It is you and all the things you do, all the money that you give that allows us to pay Susie to keep this show going, to keep hurting us, the cats that we are, and to do the weekly production of our episodes. This week I would like to thank Jordan Young, Burry Galwin, Ramjia Manthu, Andrew Polestra, David Trog, Brian Cagle – the giant nothing, Laura Kittelson, Robert Palesma, Corey Dovel, Les Howard, Paul Jarman, Joss Cunningham, Emily Patterson, Josh Hoy, Ed Ankschartensver.
Fraser: Thank you, everybody. Again, we couldn’t do this without you. See you next week, Pamela.
Thank you for listening to Astronomy Cast, a nonprofit resource provided by the Planetary Science Institute, Fraser Cain, and Dr. Pamela Gay. You can find show notes and transcripts for every episode at Astronomy Cast. You can email us at firstname.lastname@example.org, tweet us @astronomycast, like us on Facebook, and watch us on YouTube. We record our show live on YouTube every Friday at 3:00 p.m. Eastern, 12:00 p.m. Pacific, or 1900 UTC. Our intro music was provided by David Joseph Wesley. The outro music is by Travis Searle, and the show was edited by Susie Murph.
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Duration: 30 minutes