Welcome to unit four, our last unit in the science of the solar system. And this one's going to be quite a bit different. We're going to move from primarily considering geology and physics to chemistry, to thinking hard about the biology of life. In the universe, in the solar system, in the rest of the universe. I say biology although, in real life I am not a biologist, as you probably have figured out. And so, I'm not going to think of biology in the way that a biologist thinks of biology. I'm going to think of biology in the way that a planetary scientist thinks of biology. In general in Planetary Science, we, we tend to try to look at the big picture. We tend to try to distill the important points out and understand the general system. And in many ways, we're going to take that same approach with biology. It's a, it's an odd way to do biology, because biology is an, an incredibly mechanistic science. There are incredible numbers of details that you really have to understand well to be a good biologist. And, for the most part, we're going to ignore those. I think that's an okay thing to do. Because, when we're thinking about life in the solar system, life in the universe we don't know anything, and so in, in the face of our ignorance it's better to stay as broad as possible. In thinking through what we might be interested in looking for. Rather than be specifically guided by what's on the earth. It is however, a strange science to be doing. It's a strange science because of the answer to the following two questions. Question one. Is there evidence for life, anywhere outside of the earth in the entire universe? The answer to that, as I'm sure you know, is no, there is zero evidence. It's not even that there's evidence that people are arguing about, there's zero evidence at all that there's life anywhere, other than on the earth. Question number two. Is there any evidence that there isn't life, somewhere other than the earth? Also, the answer to that question is no. There's no evidence that there's no life. Now, actually here, people do sometimes try to argue that the fact that we have not been obviously visited by, by alien civilizations tells you something. You could, you could try to make that argument. I don't think it takes you very far since we, you know so little, so I think I'm going to go down on the road of saying there is absolutely no evidence that there isn't life elsewhere in the universe. When you're in a region of science where there's no evidence for something, and there's no evidence against something, and, the idea of even figuring out how to get the evidence is a pretty hard thing to do. You could find yourself in a region where it's a purely speculative science, and, and I will tell, I think that this field, this, this general field of thinking about life in the universe often called Astrobiology. There are times when it has a tendency to go down to. Some pretty wild speculation. And not wild speculation as in, I think there are giant green things that look like this. And so, maybe I shouldn't say speculation, but maybe the science of possibility. Rather than probability. You know, it's possible that if conditions are exactly right on this planet, that we just found that there could be little things living right here and right here. But the problem with the science of possibility are there are so many things that are possible, that, that figuring out the things that are, that are possible is, is so much less interesting than actually figuring out what's really going on out there. What's, what's probable out there. Figuring out what's probable. That's hard, because we don't know. We don't have any evidence for what these things are. So I understand the bind that astrobiology finds itself in, and it wants to be able to talk about life in the universe. And yet, the only way to do it is with this vocabulary of possibility. It is possible that these sorts of things could survive in these sorts of habitats. They could live in these sorts of places. It's just that, I don't think that gets you very far. So what are we going to do that, that I, that I think is different? We're going to have to do some of that talking about possibilities, about where life could possibly exist. What we're really going to do, is we're going to examine life on the earth, understand what it takes for things to, to live on the earth, and think about if those sorts of conditions are in existent in other places, so, we're going to be talking less about life itself when we're talking about other places in the solar system, other places in the universe, we're going to be talking about habitability, habitable areas, places that could possibly be. Supportive of life rather than definitely having life itself. Now, I just used that word, possibly supporting life. It's not great, but it will be the best that we can do. And in doing this, after we've looked at the Earth and understood what it's, what's required for life here on Earth, we're going to look for the solar system and see if there are any of the general conditions that exist, that might allow similar sorts of, or maybe even very different sorts of life. To exist in the solar system. And from there, we're going to move on into planets in the universe and think about if they might have conditions that could allow life to flourish there. We're going to try and stay as grounded as possible in, in the facts and the planetary science, in the, the physics and chemistry that we learn about these, these planets, the chemical conditions in them. And then we're going to try to stay as possible to what we know about biology, and about life here on the earth. It will be a difficult task. The first question, we might want to start with, before we go on this quest for understanding where there might be life in the universe. First question really should be, what is life? You know, it's an interesting question, if you ask 100 different people a definition of the word life. [UNKNOWN] broadly applied. You'll get maybe 200 different answers. I'm going to come up with a couple of important concepts, that I think we would all agree are things that might involve life. When I think about the question, what is life? I, I really think instead, what does life do? So let's think of a couple things that life does. Like, takes energy from its environment. And what does it do with this energy? Well, it grows, it reproduces. People sometimes think that they want to put the word manipulates in there but then. You often, remember, that things like, like, plants you can maybe argue that they are manipulating their environment. But not really. They're growing and reproducing. So let's not, let's not put manipulating. You don't need intelligent life that's able to manipulate an environment. [SOUND] Let's think of, of, if there are any other common, characteristics that you could think of. One is, it, this is a vague concept, but it. It's complexity. There are many things that, that individual molecules or collections of molecules can do that you might say are like growing, reproducing, maybe you could make some arguments. But, there's some level of complexity that you want before you consider something to be alive. It doesn't have to be complex like a human. Bacterium is certainly alive it's small it's, but it's exceedingly complex, even in that small package. Once you get past these two, I don't actually have any other good general characteristics, that I would say, life has to follow. Now, if you take just these things, take energy, complex the problem with these two as a, as a definition of life, is that you could easily come up with things that. Fill the, this, these characteristics, which most people would not say are alive. What about a robot that is capable of getting solar panels. Takes energy from the solar panels, it grows by, by mining and building itself. It reproduces by building more robots. Certainly it's very complex. actually, you could even argue if you wanted to, that it's alive. I think most people. Would would probably not call this thing alive, and yet it fulfills a lot of our definitions. Fulfills a lot of our definitions because, maybe it's coming close to being alive. You can even think of some clearly inanimate objects that do some of these things, like a, like a crystal. A crystal grows, crystals in some ways reproduce, they make other crystals form around them. Crystals are very complex. Crystals are alive? Mm, no. Crystals, no one's going to argue that crystals are alive. So do we need work harder to make sure we have a definition that keeps animals, and plants, and bacteria, excludes crystals, maybe robots? no. I don't think so. Our goal is not to define life. So that we can go out and then find it, and know what it is when we find it. Our goal, not only in this class, but our goal as scientists, is to explore the universe, and see what's there, and understand it. We will find things, if we ever find other life. On other planets in their solar system or, outside of our solar system. We will learn so much more than we know right now that, that our silly attempts to write down definitions will, will clearly be wrong. I'm, I'm reminded of a, of the first ideas that people had about planets surround other stars, back before there were any planets around other stars. But they certainly thought that there were planets around other stars, and general theories had been developed, about what those planets would be like. And not surprisingly, those planets were going to be a lot like the planets in our solar system. Stars would have things like Jupiter at places like five AU, they would have things like terrestrial planets at places like one AU. They would have ice giants out at the outer edge. That was just the natural outcome, and we had good physical theories that could describe precisely why it was going to be that way, and they were all wrong. we have Jupiter's closer than Mercury, we have tiny, tiny terrestrial planets in, baked in really hot with a couple of day orbits. Planetary systems. The only, the only sort of planetary systems we have yet to find, are ones that are a lot like our own solar system. I suspect, that if we ever get to that stage, where we understand anything about life in the universe, that we will be feeling the exact same way. We will learn so much more, that all this seems just a little bit silly. So, we are not going to try to define what life is. We're going to take these as general characteristics, and this important one, that it takes energy, grows, reproduces. We're not going to worry so much about the complexity, because I don't know how to worry about that. But takes energy and has the right conditions where it can grow, and can reproduce. That's what we're going to worry about. When, as a society, as a species, we are able to explore these places, where you have the energy, that you could grow, that you could reproduce, and we see things that are life or look like life. We'll be able to develop this general theory of life in the universe. Until then, we're in these baby, baby exploratory phases, and let's step back, not over-define our parameters, and let's just go see what's out there.