Episode 1: Tessa’s Interests
This week’s episode is available from our podcast host here: Episode 1
We’re also listed on:
- Tessa’s page on ResearchGate, to see her publications
- Concepts referenced
- The SETI institute
- One article on finding the earliest form of life on earth: “Why It’s So Difficult to Find Earth’s Earliest Life” (Smithsonian Magazine)
- Viking Missions (NASA)
Tessa: So, I guess for starters, astrobiology is the study of the science of how life originated and developed on earth and how it might originate and develop elsewhere in the universe. Hence the astro part. Um, so basically it’s a very fancy way of saying my job is mostly focused on figuring out if there are aliens out there or not.
Uh, a lot of people when they hear about astrobiology and they’re think of looking for, you know, microbes on Mars or maybe the SETI project. Um, and I know people who are involved in both of those things, as well as a whole host of other chunks of my discipline. Um, there’s also a very large contingent, which is primarily focused on trying to unravel how life originated since that is still an unsolved problem.
We have some good guesses, um, but you know, it, no one’s been able to recreate it in the lab thus far. Um, beyond that my particular focus, however, is on trying to figure out better ways of detecting if there’s life on planets, around other stars. So, you know, we are basically looking at most classes of stars.
The only ones we haven’t really been paying attention to are the very short lived super giants, because the general theory is they don’t last long enough for life to really get a foothold on any planets that might orbit those stars.
Charles: Actually that’s interesting. So you say last long enough, is there a sort of a general idea of a timeline that would be generalizable across, you know, the universe in terms of when life might develop?
Tessa: Well, we only have an n of one here. You know, we have one planet where life has originated that we know of and that’s earth. Now on earth, it’s worth noting that life sprung up so far as we can tell pretty much as soon as the planet was habitable enough to support it.
I mean, it was very early on… Earth is 4.5 billion years old. The oldest uncontested fossils for life are I think around 4.1, 4.2. Um, there are some that are even earlier that are still being debated. We know for certain it was there by about 3.9 to 3.8 billion years ago. Um, so it originated very early on in earth’s history.
Um, so the general theory is that we assume that life isn’t too difficult to develop, ’cause otherwise it probably would have taken longer. On the other hand, and with these like super hot, super giant stars, they only last on the order of hundreds of millions of years. Um, so, well, it’s possible for life to get established, to develop in that time, it’s unlikely it could get established quite to the level of sort of the sophisticated global biosphere that we have here on earth, just cause there’s time, but there’s not enough time for that sort of thing to happen. Um, Especially since here on earth, they did take awhile for… while life originated pretty early on, it did take a while to develop sort of a complex biosphere. I mean, for the first billion years or so, earth was pretty much only inhabited by microbes. It was a non oxygen dominated atmosphere. Um, and while I’m sure the microbial ecology was fascinating, there wasn’t a whole lot going on beyond that, um, for a variety of reasons.
Um, generally the biggest one is that compared to most other fuels for life, um, oxygen has a lot of energy per you know, cubic amount compared to most other metabolites that are available out there. Um, so you’re kind of energy limited until you end up with an oxygen dominated atmosphere, at least to the best of our knowledge.
So, anyway, as a result, we haven’t really paid much attention to the sort of the short-lived stars. Um, Because comparatively speaking sort of sun-like stars, or even the more common, the red dwarf stars, which are much cooler and much smaller than our sun are probably better candidates just because they last so much longer.
Charles: You said something a little bit earlier that I would love to go back to, which is about the fossil record of life on earth. And the very first, like the very earliest fossils that we have. You said that there are some which are still in debate. What does that debate look like?
Tessa: Basically, because parcels from that era are, one, very rare because there aren’t very many places where the rock record goes back that far, because you basically had to have avoided most geological processes that would have destroyed them.
Um, which over 4 billion years is difficult. Um, but also because, you know, you’re dealing with very primitive organisms that can be kind of difficult to differentiate from just nonliving organic chemistry. I mean, we’re not even talking about necessarily, um, you know, fossil microbes that you can see we’re talking about no unusual organic compounds or traces.
A lot of it comes down to isotope fractionation, basically of … the different flavors of carbon, you have life tends to preferentially use a version, that has less mass because they take less energy to move around. Um, and there are some deposits of organic material from that period, which seemed to have that sort of signature of being dominated by these lighter isotopes of carbon.
But there are ways you can get that, that don’t involve life. So that’s kind of what makes it difficult is that when you’re dealing with rocks that old, that processed, um, potentially harboring life, that primitive, it becomes difficult to distinguish what’s a fossil and what’s not a fossil.
Charles: So it sounds like a lot of the work that’s done in astrobiology involves a lot of highly specialized equipment. And probably a lot of methodologies that have only been available, relatively speaking, for a very short time. So like, what are the modern origins of astrobiology? Like when did the discipline really become a discipline?
Tessa: Um, it really kind of began relatively recently, probably within the seventies and eighties. Let’s start at Genesis was the early Mars missions, which were the first time people really scientifically thought about, okay, how could we potentially detect life on another planet?
Um, the Viking experiments on the Viking mission are probably the best known example of that sort of early work. Ultimately their results were inconclusive, uh, which generally was assumed to ultimately be a negative result, but there has been debate about that.
Charles: Could you describe what the Viking missions were?
Tessa: So the Viking missions were some of the first U S Landers since they were sent in the mid 1970s. And they carried a package of experiments. That would sample the soil. And then for example, uh, inoculate it with a glucose solution and see if any carbon dioxide side was produced as a result, you know, from metabolism, they would place it under a light bath to see if photosynthesis occurred.
Um, they would inject it with radio isotope labeled carbon and see if it got incorporated into the soil via a metabolic process. So on and so forth. Um, and some of the results didn’t produce anything. Some of the results did produce, for example, carbon dioxide, but only for a short amount of time, which was interpreted probably as being a chemical reaction rather than a biological one.
Um, generally speaking, just the results we got back were such a mixed bag that they were ultimately judged inconclusive and the general consensus amongst the community was that it was a negative result overall. And that if there had been living things there, we would have seen a less ambiguous signal and a less ambiguous results, um, from those experience, but that’s basically where it started.
Um, and then, you know, some of the forerunners, Carl Sagan, Lynn Margulis… SETI got started with Frank Osmo and Jill Tarter starting first conceptually in the 1960s. And then in practice and the 1970s, that was sort of the other part of it. Um, and kind of evolved from there. Not a whole lot happened during the eighties, cause there was sort of a, um, partially due to the negative results from Viking, and also just because there was sort of a downturn and planetary exploration. Um, a lot of NASA’s funding went into the space shuttle instead, as well as some of their, um, astronomical missions like the Hubble Sspace Telescope. Um, and then kind of really had a renaissance during the nineties when we started sending missions to Mars again.
Um, and then also we finally developed … sensitive enough instruments that we’re able to start detecting planets around other stars. Uh, the first exoplanet being 51 Pegasus, which was ’94, I think ’94, ’95, somewhere in there, that also sort of to a renewed interest and eventually led to the establishment of the NASA Astrobiology Institute, that was a funding program, um, that poured a lot of money into sort of developing the field and then with, uh, Kepler, we had sort of an explosion of exoplanets, Kepler being a space telescope that was specifically designed to look for a planets around other stars. And, you know, we went from knowing maybe a few dozen planets outside our solar system existed to having thousands of them.
Um, many of them potentially Earth-like. Um, and that kind of really changed the game because now all of a sudden, there’s this vast variety of worlds that we can actually start looking at and examining empirically and statistically, you know, unless life is extremely rare in the universe. Odds are at least one of them’s inhabited.
Right. At least primitive life, if not more complex life.
Charles: I guess it would question if we have only recently developed technologies that are sensitive enough, even just to detect all of these different planets, how would astrobiology as a field be able to meaningfully like take records? Um, and investigate those planets on a more- on the level where you would be able to determine whether life was or had been present.
Tessa: Um, so it depends on where you’re looking. If you’re looking within our solar system, you either look for extant life. Um, whether that’s through, um, the byproducts of biological metabolism, you may recall that people got very excited about discovering trace amounts of methane on Mars a couple of years ago.
That’s because in the atmosphere of Mars, methane is very short lived. So for it to be detectable, something has to be actively producing. And while there are, uh, geological processes that produce methane, which incidentally also involved liquid water, the vast majority of methane on earth is produced biology, so that got people very excited. There’s also potentially, at least for a planet like Mars, you could look for a fossilized life again, or examples from earth show that that can be. Very difficult to do unambiguously, but you know, if you find like the fossil of, say, something more complex, like a sponge, well, that’s pretty good evidence that there was at least like there at one point or another.
Um, if you’re talking about the icy moons of Jupiter and Saturn, you know, you might either send a probe down beneath the ice crust, into the oceans that lie underneath, which had been Italy poses, quite a number of. Engineering challenges. But beyond that, you know, that would be the most direct way to check if there’s life there.
Also some of those icy moons, um, occasionally, uh, have these plumes or geysers of liquid water that are erupted out of their IC crus, and there have been discussions to send probes through those plumes and sort of sample what’s in there. Cause it would be probably an easier way to get a sense of what the, uh, Sort of subsurface oceans or like without actually having to drill in.
And of course, if you detect DNA or something similar in those plumes, that’s also a pretty good indicator of life. Um, when you’re talking about planets around other stars, obviously we can’t easily send a probe there. Um, at least not anytime soon. And that’s where my research comes in instantly for that we are mostly focusing on.
How sort of, instead of looking for individual life forms, we are focusing on how biospheres affect the plants that host them as a whole, whether that’s looking for the sort of gases that are produced by biological activity on a planetary scale. So for earth, that would be oxygen, um, to a lesser extent, methane, um, some also nitrogen compounds that are primarily biologically produced, um, ozone has also been suggested potentially.
Um, or things like, more exotically, looking for the light, that’s absorbed by the presence of chlorophyll on a planetary scale, which you, at least in theory and detect. There are even people who are talking about looking for signs of bioluminescence on a planetary scale, cause you’ve got oceans and they’ve got a lot of bacteria in them that are bioluminescent.
Theoretically, you could attack that. Um, most of the work has been focused primarily, however, on looking at the atmospheres, cause that’s going to be the easiest for us to observe. Um, it’s a lot easier to detect the atmospheric composition of a planet using a tool called spectroscopy than it is to, say, image a planet directly and see, okay, you know, are there willing spots there that might be bioluminescence? For a long time it was assumed we could just look for oxygen. Cause again here on earth, the vast, vast, vast majority of oxygen is produced by plants, photosynthesis. However, as we have discovered, you know, a more, a wider diversity of worlds, um, we also discovered that the ways you can get an oxygen rich atmosphere are a lot more varied than we thought. And a lot of them involve completely non biological processes. For example, if you’ve got a planet that’s extremely water rich, which water’s pretty common in the universe, so that’s, you know, very plausible, um, orbiting that star that is very active, that produces a lot of UV and higher wavelength radiation, um, that radiation can split water molecules into hydrogen and oxygen. The hydrogen escapes into space because it’s very light, oxygen hangs around and you can end up with an extremely oxygen rich atmosphere with nary, a trace of life. Um, so we’ve discovered, you know, probably within the last 5, 10 years, that the assumptions that we had had about what, you know, a biosphere looks like at a distance, may be very prone to false positives and sort of the challenge for the last couple of years has been figuring out, okay, how do we deal with that?
Overall, there’s been slowly the shift from looking for soda, sort of these ideas of smoking gun molecules that, Oh, well we found oxygen and methane, so therefore this planet must be inhabited to thinking about it more in terms of probability, sort of almost a Bayesian approach of, okay, based off the data we have, what is the likelihood that the signals we’re seeing, you know, in terms of atmosphere, composition or whatever are the result of biological activity. And of course it can be a little difficult to sort of put boundaries on that, um, just because there’s a lot, we still don’t know, but it’s kind of what the focus of the field has been.
My particular area of expertise has been using a mathematical field graph theory, also known as network theory to try to analyze the behavior of planetary atmospheres, sort of, um, as a whole, as opposed to looking at individual molecules, basically, we’re looking more at the pattern of interactions between different molecules rather than the molecules themselves.
And there is some very preliminary evidence that suggests the, at least if you do the sort of analysis for all the planets in our solar system that have significant atmospheres, um, the pattern of the resulting chemical reaction network that you get, you know, using this approach, for most of the times in our solar system, it looks like this jumbled mess of spaghetti – it looks very random and very disorganized. If you do this sort of analysis on the atmosphere of earth, you end up with a network that looks highly organized. It’s hierarchical. You know, you have, um, points in the network that feed into one point, which then feeds into another point, you know, going up from layer to layer, to layer it’s modular, you know, it’s sort of very obviously divided into subsections and you don’t see the sort of pattern over the organization in the other atmospheres in our solar system. And we suspect that is probably due to the presence of a biosphere. Um, so that’s kind of where my focus has been on for the last five years or so.
Charles: That’s all fantastic. The thing that I have already said to you, but will say again is that astrobiology as a discipline, sounds a lot like a fake job that you give somebody in science fiction. Well, I was just gonna to say, do you, I don’t think though what you’re doing is fake, obviously, um, but do you, is there any kind of academic pushback against astrobiology ever, where people think this is barely a real field?
Tessa: Uh, there definitely has been, um, they’re less common now, but there have been critiques in the field saying almost that it’s pseudoscience because, you know, sort of using the philosophical definition of science, it’s very difficult for us to falsify any of our theories so far due to lack of data.
Charles: This is actually, I would say just quickly, this is an interesting and unexpected point of convergence between our two areas of focus where they’re quite different, but systematics, I think, has not infrequently a similar reaction of sort of an incredulity regarding whether anything we’re doing is actually real because systematics is inherently a historical field. And we can’t physically go back in time and confirm our hypotheses about how… about the point of time and the nature of different divergences against, across different lineages. And so there’s often that sense of, are you actually doing anything particularly when you then try to use phylogenetic trees to make evolutionary inferences, where I think a very common criticism is that we can’t actually know that the trees are real and it ends up being sort of a circular way of reasoning , we have this tree, so we think this about the evolutionary history, but we used these traits assigned to these different groups to create that phylogeny. I wonder if there’s anything similar that goes on in astrobiology?
Tessa: Oh yeah, absolutely. And you know, the thing that keeps me awake at night sometimes is, is what I’m doing actually really meaningful and significant? Are these measurements actually useful for anything? Um, you know, because at this point there is so little actual, empirical data to work with, um, that it’s, frankly, it’s hard to actually verify any of it. Um, so I guess, you know, that’s kind of a challenge we face, and like I said, you know, there has been some pushback from the field about whether or not this is even a real thing that we’re doing because you know, we can’t falsify any of it yet.
Um, that has been in decline, however, mostly because we are actually starting to get to the point where, while we haven’t discovered, you know, surefire proof of extraterrestrial life, we are discovering that a lot more of the universe is potentially hospitable to life than we thought. And so at the very right examining these environments, especially in terms of comparing to them, to known inhabited environments on earth on earth, seems a lot more useful.
And in some cases you can actually make falsifiable predictions, you know, we can talk about, is there going to be liquid water on Mars in any way? And if so, is it going to be in a state that’s actually habitable at least the life as we know it, um, you can make measurements in predictions and models based on that you can make measurements and predictions in the model space about whether or not some of the, um, Exoplanets discovered are going to be habitable or not based off of, you know, the temperature of the star they, uh, orbits and the length of the year and the length of the rotation period, and stuff like that. We can even make some inferences about their composition. Um, for example, a project that I was part of a year or two back that we finally got the paper out of published not too long ago, was modeling planets that were entirely covered by ocean.
Uh, cause there’s no reason to believe that would be uncommon. Um, you know, like I said, water is very, very abundant in the universe and there’s no reason you can’t get a planet that is largely if not entirely covered by water. And for a long time, it had been assumed that that would be a good place to look for life, cause hey, life is completely dependent on water here. Except when we actually did the modeling and check the numbers, it turns out it’s not because a ton of important nutrients, especially phosphorus here on earth are the result of surface rock getting eroded by rain. And obviously you don’t have that on an ocean covered planet because you don’t have any surface rock.
Um, and you know, that is something we can model and to an extent, even do experimentation on, you know, the availability of phosphorus and these little, you know, self contained ecosystems that we’ve got bottled up. Um, and you know, that is a testable prediction. So I think that has kind of helped the credibility of the field at any rate.
Charles: What kind of evidence is used to determine – you said that we have, we now know that there are many more planets that are potentially hospitable. Um, what determines whether a planet is considered hospitable or habitable?
Tessa: Mmm. That is also an area of debate right now. Um, obviously the general assumption is that it’s gotta be, it can’t be too hot or too cold that liquid water can’t exist.
That’s the first thing people look for in terms of habitability. Um, and there are ways of thinking about that, whether that in terms of, are you far enough away from your son that, you know, your oceans won’t get boiled off almost immediately. Um, you can also think about in terms of atmosphere, composition. Theoretically, Venus is in the it’s far enough way from the sun that it could have oceans of water on it. In fact, there’s some suspicion it might have early on in its history. Um, however, it also has an absurdly carbon dioxide, rich atmosphere, and it’s got a ridiculously high greenhouse effect. So instead the surfaces hot enough to melt lead, and obviously there’s no liquid water there. Um, so that is one of the primary ways we think about it.
Um, again, availability of nutrients, whether it’s talking about, you know, whether or not you can have a complete phosphorus cycle there, like I was discussing, you know, just a bit earlier, or if the lack of having an erodable surface land mass means that can’t happen. Um, if your planet is too depleted in, um, certain elements like carbon or nitrogen, you’re gonna have a hard time probably getting life started.
Um, so if your planet, if the star your planet is orbiting, is super active, and it’s just, you know, having solar flares all the time and spewing out high energy movie and x-ray radiation, then it’s probably going to sterilize anything that’s living there. So that’s probably out. Um, so yeah, there are multiple different factors that we can analyze and think about when it comes to terms of have ability.
Charles: Is there any sort of consensus among astrobiologists about… a feeling like there definitely is or isn’t life out there? And to what level of life it might be?
Tessa: The general assumption is that statistically there’s almost certainly life out there. The question is, you know, is it close by or not? How common is it?
Um, is it close enough that we can actually observe it in some form or another. Uh, the other general consensus is that we are probably going to be more likely to discover simple microbial life than we are to develop, discover, like, complex life. And the reasoning for that, like I said, is because microbial life has existed on earth for much, much longer than complex life has.
The, you know, first billion years of earth history, as I mentioned earlier, was completely dominated purely by microbial life. So as a result, you know, just again, based off of that, it’s apparently easier to develop microbial life than it is to develop multicellular life, so likelihood of us discovering microbial life is probably going to be higher as a result.
Again, we’re all inferring this from exactly one planet’s life history, so it’s hard to say how accurate that is, but I do believe both of them are reasonable assumptions. Um, as for whether or not you’re going to develop, discover, you know, technology-using life… that is a much trickier question in part, because there are a lot of unanswered questions about, okay.
It took 4 billion years for intelligence develop here on earth. Does that mean intelligence it’s hard to do or was that just your evolutionary chance? Um, how long do technological civilizations last, you know, obviously with climate change and pandemics and nuclear weapons there’s a lot of debate about whether or not intelligent civilizations tend to destroy themselves or outstrip their resources and then collapse, or, you know, do they tend to last a long time?
Um, and another part of that equation is going to be, is there a point where they advanced enough that it’s actually hard for us to recognize them as being the product of, you know, intelligent civilizations and not just natural phenomena. Um, you know, if, if you’ve got a civilization that has, I don’t know, uploaded some intelligence into, you know, the plasma occurrence outside its host star, would we actually recognize that as an intelligent civilization and not just a weirdly active star or something like that, that, um, you know, would we recognize the gods, when we see them? You know, or do we just see the results of what they do?
You know, planets changing or disappearing for reasons we can’t really determine, but which wouldn’t look more like a bizarre natural phenomenon then something intelligent. Um, so, you know, that kind of makes it difficult to make assumptions about how common technological life is and how easy it’s going to be for us to detect.
Charles: In thinking about how much astrobiology sounds like a science fiction thing, I’m wondering if there has been any influence on the field from science fiction, in the sense of, um, being a source, not of genuine theories, but of new avenues of thought in terms of, is there any sort of feedback loop from science fiction and speculation in science fiction about broadening people’s ideas about what kind of life, what life can be, what kind of life might be out there, where you might be able to find life?
Tessa: Oh yeah, absolutely. Absolutely. Um, I mean, it’s no secret that a lot of us came up through science fiction. I know I, you know, come from a long line of sci-fi nerds. Um, and beyond that, uh, You know, I feel like it’s the genre as a whole has kind of helped us or gave us room rather to think about more speculative ways that life might manifest itself. You know, talking about completely different substrates, whether it’s talking about Silicon-based life, which could directly exist under radically different conditions than what we see on earth.
Um, to, you know, life as an information pattern rather than a biological phenomenon, uh, to, you know, looking for life in substrates that we would normally not consider living like in the geology itself or, you know, in the interstellar media or stuff like that. Um, so I think the genre of science fiction in general has really done astrobiology a boon, even though it may make us make it harder for us to be taken seriously at times. Um, but you know, just sort of giving us that space mentally to play around.
Charles: Yeah, I guess I would just say at the end, are there any questions that you wish people would ask astrobiologists that nobody does?
Tessa: That’s a good question. Um,
Oh, I got one. I got one. Okay. Um, so I do kind of wish people would ask us more about how astrobiologists conceptualize life. Cause I think that would be useful for people to know. Sort of something that’s been building, especially in my lab and someday my advisor and I are going to write an article about this and probably publish it in Slate or something is… We need to stop thinking about life binary terms, gender, um, you know, I think it’s more…
Charles: I love that casual… just very casual threading-in…
Yeah. Yeah. Because, you know, there’s been debate in the astrobiology community and also philosophically about what is a life form to begin with. How can we recognize life if we don’t really have a great definition for it?
Tessa: And we don’t, um, because are exceptions seemingly to every rule and so something my advisor and I have been advocating for is instead thinking about life as a spectrum, you know, a giraffe is going to be more alive than, I don’t know, say a lichen or something, which is going to be more alive than a virus, which is going to be more alive than a rock.
Um, that there’s a spectrum of being alive, that it’s not a black and white thing of living and nonliving. You have a range of features, um, and you can have, you know, phenomena anywhere up and down that range.
Charles: I love that conceptualization, not even within a scientific realm, but one thing that was enlightening for me was in a discussion of choice and reproductive rights, et cetera. I, I, I wanna, I want to attribute it to Willy Parker, but I’m not sure that I can, but the idea that life in the human context is not, um, a binary of, you’re alive once you’re born and before then you aren’t, but the life, as you said, is a process, um, and that there are different, there are gradations to how much you are alive and how people can relate to that relative to their own life.
Tessa: Right, exactly.
Charles: That’s fantastic. And yeah, I imagine that astrobiology sounds like a field that’s would be extremely ripe for a philosophy of science and in my own reading and philosophy of science, I don’t think I’ve seen astrobiology brought up a lot, but I don’t know if that’s because I’m just not looking for it or if it’s, because it’s not really been integrated into the mainstream as much.
Tessa: I would suggest it’s probably available… I mean, we are still a young field as these things go, but there’s been a lot of discussion about, you know, definitions. How do you define metabolism or how do you define information? Cause we talk a lot about how information can be encoded by a lot biologically and also about talking about life as a phenomenon of information, you know, very abstractly.
How do you define a living thing to begin with? Um, and you know, there are also a lot of debates about not only where did life come from, but, you know, what is, is its primary homework, you know, does life exist as a way to dissipate entropy? Um, is it something that arises spontaneously or are there laws of physics?
That are specific to living organisms or, you know, specific manifestations. Is it a purely emergent phenomenon, and there is no underlying, simple rule or unifying rule for it? Um, stuff like that, you know, that there have been very, very rich philosophical discussions within the field. And I think they’re slowly starting to trickle out, um, as we kind of become a more established field, at least I certainly hope so, but yeah, the, the sort of philosophical aspects of it are tremendously important.
And actually it was a, a pro a long time. It was a running joke in my lab that our grad meetings are only 90 minutes long at most, because anything longer than that, and eventually all of our discussions ended up turn up into an argument about whether or not free will really exists or not.
Charles: I… listen, it happens to all of us.