When we search for life beyond Earth, we have to figure out what we could measure that would tell us that life was, or is, there. And the starting place for that search must be Earth itself, the only place where we know for sure that life has lived. Every rock tells a story, and so does each fatty acid called a lipid. Your cholesterol, which is part lipid and part protein, could last for billions of years, and contains the information that you are a mammal. Could we find lipids beyond Earth? NASA astrobiologist Lindsay Hays explores this and other topics in her research. She also discusses places interesting for the search for life in our solar system and beyond.

Jim Green: When we go look for life in the universe, what are we looking for? Did you know your cholesterol could last for billions of years?

Lindsay Hays: It’s a really great way to understand, sort of, the history of the microbial life on this planet, since they don’t always leave fossils in the way that dinosaurs do.

Jim Green: Hi. I’m Jim Green, Chief Scientist at NASA, and this is Gravity Assist. This season is all about the search for life beyond Earth. I’m here with Dr. Lindsay Hays, and she’s the deputy program scientist for the astrobiology program at NASA. Welcome, Lindsay.

Lindsay Hays: Jim, I’m really glad to be here.

Jim Green: One of the things that the astrobiologists do is they really look at the history of life here on Earth. Now, why is that important for us to do that instead of just go looking for life beyond our Earth?

Lindsay Hays: It’s our one example of life that we know of. And so it’s really important that we understand not just what life looks like on this planet today, because it’s our only example, but also what life looked like in the past on this planet, how we got to this point is a really important way… it’s a really important thing to understand, how mass extinction events or mass radiation events, where you go from a couple of different families into many, many groups of families, how those kinds of things happen on this planet or on any planet.

Lindsay Hays: And the other really interesting thing about looking at the history of life on Earth is that past Earths are almost entirely different environments. There was a time on this planet where there was almost no oxygen on the surface of the planet. And it was still teeming with life. So what does a planet that has no oxygen, what does that look like? What does the life on that planet look like? And by studying the past Earth and the history of life on Earth, you can get an idea for some other, sort of, states of habitability that we don’t see today but that we might be able to see as we’re looking to bodies within our solar system, or extra solar planets. Past Earth can be the clue, it can teach us about not just this planet but a lot of other places as well.

Jim Green: Well, let’s see, about 3.8 billion years ago we believe life really started here on Earth, and it was really simple for long periods of time. And then it got to be more complex, meaning cells were getting together and forming much more complex structures. What happened that made that change? Do we know?

Lindsay Hays: Well, I mean, one of the things, one of the interesting things about studying the history of life is we can only, sort of, see the winners. We see today how things worked. And so when you look back, when you look back at Earth, and you look back at Earth’s history, you could understand, sort of, how we got to where we are. But we don’t necessarily know all of the different things that we’re acting on that life, all of the different factors.

Lindsay Hays:But one of the things that we always see as a way to, sort of, drive new innovations and drive new evolution is competition, things for resources, place where you can get energy, understandably the places where energy is easy to get and abundant, those are places that life probably started and probably started inhabiting very quickly. And as those areas, those niches filled out, you would expect evolution to become more complex in a way to get our energy that’s harder to get at.

Lindsay Hays:So we think about the abundance of life on this planet today that uses sunlight for energy, but that’s actually a relatively difficult thing to evolve. It’s much more easy to get chemical energy and then the ability, evolving the ability to create what we call photosystems, complexes of proteins and metals and things that allow you to take energy from sunlight, that’s just one stage of complexity as you evolve.

Lindsay Hays: The next thing is multi-cellularity. And as you were sort of alluding to, more complex structures, those sort of thing. It’s really hard to try and understand exactly what drove those evolutions, but actually there’s a recent study that came out that showed that predation, single cells that eat other cells, can actually drive those prey cells to create more complex structures, to become multi-cellularity as a way to sort of protect themselves from predation. Think animals living in herds makes it harder for any one animal to get attacked.

Jim Green: Early on when plants came on this Earth, what kind of plants do you think started here first?

Lindsay Hays: Well, so we’re definitely talking, early on, we’re talking single-celled organisms. So, cyanobacteria. These used to be called blue-green algae. But as we have understood them better we recognize that they’re not algae at all. They’re in fact a single-celled organism called cyanobacteria. They live in communities. But you probably know them as chloroplasts. So, at some point in the past couple billion years a different organism ate a cyanobacteria. It became incorporated in the cell. It wasn’t digested for some reason or another, and became a chloroplast.

Lindsay Hays: And that’s where we start to see algae and all those kinds of things. And then algae evolved into bigger things as life — first water, plants, and things like that. And then plants on land have only been around the past couple hundred million years. So early on, we’re definitely talking about, not even algae, but even single-celled bacteria that are photosynthetic.

Lindsay Hays: With the evolution of photosynthesis, we sort of see two stages. The first stage in the evolution of photosynthesis is just the ability to take in light at all. But then as those photosystems, those complexes became more complicated and started grouping together, a group of organisms that are probably like modern day cyanobacteria, or the ancient historical version of them, evolved the ability to split water. And in the process they started creating oxygen, which fundamentally changed the chemical composition of the atmosphere and sort of the surface environments on Earth.

Lindsay Hays: And the really neat thing about oxygen is that it allows you, it’s a really high-energy molecule, and it allows you break down more complex compounds. When you can consume oxygen and sugars, you can, sort of, get the full amount of energy out of those sugars. And that allowed things to become more complex because they can sort of get more energy out of the food they are eating. So, those processes and, sort of, a whole series of evolutionary steps allowed us to take the steps from being, sort of, simple life to this very multiple complex organisms we see today, all of the different kinds of singled-celled through elephants and whales and these huge enormous things that we see on our planet.

Jim Green: Well, you know, can you tell us about some of the places in the solar system that you’re excited about for looking for life?

Lindsay Hays: Ooh. Okay. Do I have to stay within the solar system?

Jim Green: No, actually. Beyond Earth, where would you go look for life beyond Earth?

Lindsay Hays: So, of course, there’s a lot of interesting things outside of the solar system. Whatever you can imagine, there’s probably some really cool planet outside of the solar system that’s like that. But I like thinking about our solar system because it’s a much more approachable environment. These are places that even in the farthest regions of the solar system… We’ve been to Pluto. It took us a long time to get to Pluto, but we’ve been there and we’ve been able to do that. So I’d say that there are a lot of places that I find really interesting in the solar system.

Lindsay Hays: And most of them are, sort, of in the sub-surface. Right? Deep under the surface of Mars in the rocks, under the oceans and the moons in the outer solar system. The things that I’m most excited about in our quest to look for life is to understand what makes an environment habitable on a planet or on some other body in the solar system, a moon or something that’s potentially less habitable as a whole than the Earth.

Jim Green: Well, you already mentioned a couple of really great places. You mentioned Mars and you mentioned the icy moons. If life is in both of those places, would they be similar or how different would they be, do you think?

Lindsay Hays: You’d have to see different evolution, different systems that would have evolved to live in those different places. When we look at our planet here, extremophiles are very different depending on the environments that you look at. When you look at things that live in deep sea environments, like the kinds of environments that we imagine we would see in some of the subsurface oceans on the moons of the outer planets. We see organisms that are evolved to live in high pressures and high temperatures.

Lindsay Hays: When we see things that live in rocks on the earth, we see things that are evolved to take advantage of tiny little bits of energy over very long lifetimes. Those things on the Earth eat hydrogen and other things like that that come from radioactive decay. But those two organisms have evolved into, sort of, very different types of systems. And so I would imagine that you would expect to see different kinds of lifes if you’re looking at different environments.

Jim Green: Well, we know a lot about Mars, and I think we could say there’s probably not life on the surface of Mars, at least we haven’t found it yet. But that gives us the opportunity to think about life below the surface. Would we rule out life below the surface on Mars?

Lindsay Hays: Would I rule out life on the surface, in the subsurface below Mars? Any time that we look for life, almost any place that we’ve looked on this planet for life we have found it, which tell me that life is incredibly robust. It stays wherever there is energy to be had. So I think that if there was ever life on Mars, it may be somewhere on the surface today. And I think ruling out life… I’m an astrobiologist, so I never want to rule out the potential for there to be life somewhere. I think that life is clearly not abundant on the surface of Mars today. And so I think looking in the subsurface is exactly the kind of thing that we should be doing.

Jim Green: But do we know enough about Mars to rule it out, or is it still a possibility that there might be life on Mars below the surface?

Lindsay Hays: I think that there’s, I think that there are enough tantalizing hints that I think it would be interesting to go and look for life in the subsurface. I definitely don’t think there’s anything that we’ve been able to rule out with regards to Mars at this point, other than, like you said, the fact that life is clearly not abundant on the surface of Mars today. So where could it be and what would it look like? Those are really interesting questions.

Jim Green: Well, what are some of the signatures of life that we should be looking for, then?

Lindsay Hays: Well, on Mars or anywhere?

Jim Green: Yeah. On Mars or any place. Let’s start with Mars.

Lindsay Hays: Things like chemical fossils. The simplest things, really, are looking for evidence of disequilibria. I mean, fundamentally life takes advantage of a place where there’s energy to be exploited and takes that energy for itself. So something that indicates that there’s been some kind of chemical reaction that’s going on there. I am a organic chemist, so I always want to look for organic fossils, things like lipids or amino acids, things like that. Micro fossils, you might see traces of single-celled organisms or sometimes those single-celled organisms live in, sort of, communities and create macro-fossils.

Lindsay Hays: On Earth, we see these things as stromatolites where evidence of, sort of, sticky microbial mats that haven’t gotten broken up in storms and things like that and redeposited and indicate that there was something there that was doing something.

Lindsay Hays: There’s a whole range of biosignatures. And some things, you may be more likely to see them on Mars. Some things, you’d be more likely to see them in the subsurface ocean or in a plume from a subsurface ocean, those kinds of things.

Jim Green: Well, we’ve been talking about the building blocks of life for a while. And everyone knows about amino acids. But you mentioned one thing, lipids. And so, what are lipids and how would we look for a lipid as a biosignature?

Lindsay Hays: Sure. So, I like to think about lipids in this way, and that is if you are very, very, very lucky, the parts of you that may last for billions of years, that says that you were here, is actually your cholesterol. Lipids are the things that make up our cell membranes. They are the things that sort of make up the envelopes around our cells. And unlike DNA that would say, “This was Jim Green who was here,” these lipids are produced by, sort of, whole families of organisms. They can tell you this was a human or this was a mammal who was living here.

Lindsay Hays: But they last a really, really long time. These are basically oils. One reason that we can find oil is that it can be preserved for millions or even billions of years on this planet. And these hydrocarbons, they last for a really, really long time. They can be a really good record of the deep past, and different families make different compounds that allow us to say, “Hey, look. Something that was making that compound lived here at this time in the past.” And you can see whole groups of lipids. It’s a really great way to understand, sort of, the history of the microbial life on this planet, since they don’t always leave fossils in the way that dinosaurs do.

Jim Green: Yeah. That’s really fascinating. I mean, you know, and most of history of Earth, animals didn’t have a skeletal structure. They were much more cellular in nature. So as you say, we’re going to have to find the right pools of chemicals to be able to see that these are markers of ancient life. Well, what kind of instruments or technologies do we really need to develop to be able to make those kind of measurements?

Lindsay Hays: Well, as I said, I’m partial to lipids. They last for really long periods of time. So, I’m always interested in things like the spectrometer.

Jim Green: What is a spectrometer and how would it work?

Lindsay Hays: So there’s a couple of different ways. A spectrometer ultimately is something that is looking for wavelengths of light. And so we can think about a spectrometer as a way to look at how different compounds, what different compounds there are in a gas or something like that. We also have things like mass spectrometers which helps us to look at the ranges of mass that we get out of compounds that we have broken down. And these are all things that allow us to, sort of, do an inventory of the chemicals that we are looking at in a place.

Lindsay Hays:You can detect other compounds as well with a spectrometer, like amino acids, like you were talking about. or make atmospheric measurements depending on sort of how you set it up. Some people have argued for a camera as a way. to sort, of detect life. This could be a very hard thing to do.

Lindsay Hays: But if we send a camera, say, to a subsurface ocean, you’d have to do a lot of filtering of sea water to find microbes and stuff like that. And how would you even know what you were looking at? But the stuff that I’m most interested in looking at is in the deep subsurface. So, instruments, we have some idea of what kinds of instruments we may want to create. But really the technologies, I think, that are important are about how to get into that subsurface, how to get below the ice, how to get samples of stuff below of ice, how to get below the rocky surface of Mars.

Lindsay Hays: You have to think about getting sampling systems that are robust. Remember, we have no mechanics in outer space. You have to make sure that it’s going to be able to do what it’s going to be able to do, and you can’t fix it. And also how to keep them very clean. We don’t want to run the risk of bringing Earth life with us, detecting it, and saying, “We found life.” But really what we found was ourselves. So, I think that those are some really interesting technologies that when we combine them with the instrument development that some of our great teams are doing, can get us to not just being able to measure things but getting the samples that we may want to measure.

Jim Green: It just occurred to me as you were talking about it, getting below the surface, if you go to Mars, there are some really deep craters that really get below the surface. I mean, they can be hundreds of meters below the surface. Maybe that’s where we should land and begin our interrogation, because it’s down already at a low level.

Lindsay Hays: The rocks can always tell a story. We use the rocks on this planet to tell a story about the deep past. The most interesting thing I think about the history of Mars is that the rocks on the surface of Mars are on the whole even older or significantly older than the rocks on the surface of the Earth. And so, they also tell the history of Mars right there on the surface. And because they’ve got craters and because they’ve got other things like that, that’s exactly, that’s your window into those deep subsurface rocks.

Lindsay Hays: And a really cool way to look for either these chemical fossils, lipids, those kinds of things I was talking about or microfossils, as long as you can find rocks that, sort of, are sedimentary, they’re unaltered. They may have these fossils… If those fossils exist, those would be the right places to look for them.

Jim Green: Yeah. Now, the one thing about Mars, as you mentioned, is the surface rocks are older. And it’s because the rocks here on Earth have gone through a whole evolutionary stage of being modified by plate tectonics and wind

Lindsay Hays:Yeah

Jim Green:And weather and ocean. And so, we don’t have any of the old rocks on the surface anymore. It’s really, kind of, turned over.

Lindsay Hays: There’s a really active question within the habitability community, which is do we need plate tectonics for life to live on the surface and to be abundant on the surface? So, understanding not just local environments for habitability, but global environments for habitability. Plate tectonics are great because they, sort of, keep refreshing your stores of chemical energy. They turn it over and they reprocess it, they repackage it into new pieces that microbes or larger organisms can eat.

Lindsay Hays: But at the same time, they destroy all of those old signals. They destroy all of those old signs. And so it’s a two-fold kind of thing. What are you looking for? Are you looking for the rocks and the history of the old life? Or are you looking for something that’s active today?

Jim Green: So indeed, if life started on Mars at the same time it did Earth, then the way we could find how life started originally would be on Mars.

Lindsay Hays: Yeah.

Jim Green: What kind of studying and training does someone have to have to become an astrobiologist?

Lindsay Hays: So, astrobiology requires that we think about big questions. And so, to answer big questions, you have to get people who have a lot of different kinds of backgrounds. So, you know, the study and training, you have to be very interested in science. You have to be very interested in engineering, that kind of thing. But really, you’ve got to be thinking about how to work with other fields.

Lindsay Hays:So, I trained as a geologist and a biologist, but I also took a lot of chemistry classes. I took some classes on planetary science and understanding how planets form and what makes a planet habitable. It’s really… the kind of training that you need is really focused on teaching people to have an open mind. In astrobiology you have to know no matter how good you are in your field, the kind of questions we want to answer are the big questions. And so you’re going to need to be able to work with other people to figure those out. So, think lots of science, but also learning how to work with other people.

Jim Green: Well, Lindsay, I always love to ask my guests to tell me about what happened in their past. What person, place, or activity that got them so excited about being a scientist that, I call that a “gravity assist.” So Lindsay, what was your gravity assist?

Lindsay Hays: Ooh. Can I give two. Can I give some rocket going into the outer solar system, needs a couple of swing-bys to get me where I am?

Lindsay Hays: Well, the first thing is sort of a quirk of fate. I actually grew up in a little town called Jupiter, Florida. And Jupiter is cool not just because, of course, it’s named after the coolest planet in the solar system, but because it’s close enough to Cape Canaveral, to the Kennedy Space Center, that you… When I was growing up, you could see the space shuttle launches. Now, you couldn’t see the rocket, of course. You could just see the trails and the lights from the engines. But you could see them.

Lindsay Hays: And so a couple times of year if you knew when to look and you knew where to look, you could see people launching into space on a regular basis. And that was just a really cool thing to grow up in the shadow of. The other thing is I had a fantastic teacher in high school. I was not necessarily a great student in elementary school. And in middle school things started to sort of pique my interest. But my high school biology teacher really, really inspired me to get into science.

Lindsay Hays: She’s an amazing woman, very smart. She actually got her Ph.D. while teaching high school in biology and made the material really fascinating in a way that I hadn’t been able to engage with before. And so she’s actually now the vice principal of the school that I went to. But without her I probably would have been, I don’t know, a certainly actor or writer somewhere, because those are things I was always interested in but was never very good at. So, that was Dr. Raiford at Suncoast High School. I would list her as my second gravity assist.

Jim Green: Oh, that’s fantastic. Teachers are so important to all of us.

Lindsay:Definitely.

Jim Green:And it’s all about being receptive at the time that they’re teaching us. So I’m delighted that occurred for you. Well, Lindsay, thanks so much. It was really a joy talking to you about looking for life and your perspective on finding it out there.

Lindsay Hays: Sure. Thanks so much for having me, Jim.

Jim Green: Well, join me next time as we continue our journey to look for life beyond Earth. I’m Jim Green, and this is your “Gravity Assist.”

Credits:

Lead producer: Elizabeth Landau

Audio engineer: Emanuel Cooper