 Greetings and welcome to the Introduction to Astronomy. In this video, we are going to talk about the possibilities of life existing beyond the Earth. At this point, the Earth is the only planet that we know of in the universe where life exists. However, we do know that the organic molecules that form life are very easy to form and are found on other planets, on asteroids, comets, and even in molecular clouds out in space. So it leads us to think that perhaps not only can the organic molecules form easily, but maybe life can as well. So let's start off looking here in our own solar system. Some places where might we find life? Well, Mars is one of always been one of the best bets for looking for life. Why Mars? Well, because we know that it is the only place other than Earth that had liquid water in the past. So knowing that liquid water was there and that liquid water on Earth was vital for forming life leads us to believe that if there is going to be some place else in the solar system, that it would have been on Mars. Now we've looked for life on Mars for a long time and even thought, remember, that there were canals and possibly civilizations on Mars over a hundred years ago. But we looked specifically with the Viking landers in 1976. Viking I and Viking II both went with material to test the Martian soil and look for signs of organic molecules, evidence of biological activity, but nothing conclusive was ever found, so no signs of living materials on Mars. The Curiosity rover has found, looked at mudstone, again finding evidence of liquid water and really gives evidence that Mars would have been habitable in the past. But have to say for sure, that means it was habitable, not that it was habited. So that does not mean necessarily that life existed. It does not rule out the possibility of life, but we certainly know now that Mars would not have had any kind of significant civilization. So could there be life on Mars today or what did Mars look like perhaps long ago? Mars might have looked something like this and had a great ocean here of liquid water on its surface, and we see remnants of some of that today and of some of water flows that have existed. So we do know that Mars was warmer and was wetter in the past than it is today. Could liquid water still exist underground? We know that there is a salty liquid water, not water as we know it, but a very briny, salty water that does flow intermittently on the surface today on the walls of some of the craters, and we can see evidence of that. So it's not water as we would think about it, but there is evidence that still to this day that there is some kind of liquid flowing on the surface of Mars. Typical liquid water, the stuff we're used to, because of the pressures and temperatures on Mars, does not exist. So you could not have liquid water on Mars today. And we also know that there's the lack of an atmosphere. And that leads to things like no ozone, which we have here on earth, which helps protect us from ultraviolet radiation. The Martian surface would be exposed to ultraviolet radiation from the sun, and that could help sterilize the surface. What might exist down below the surface is still a good question. We've dug down with some of the landers and the rovers looking for signs but have not been able to find anything to be able to confirm that life ever existed or still exists on Mars. Elsewhere in the inner solar system, we're unlikely to find any kind of life, neither Venus nor Mercury, show any signs of being habitable. But how about the outer solar system? In the outer solar system, we see that we're not going to be able to form life on the planets of the outer solar system, at least not in life as we know it. Because they do not have a solid surface. However, the moons are another story. So things like Europa, pictured here, does have an ocean of liquid salt water below the surface ice. So it does have water down below. And could there be mixing of that with surface materials? Could there be any kind of basic life down below? We know that there's not going to be any life on the surface. There is no atmosphere. So it is exposed to radiation and cosmic rays that would sterilize the surface. However, down below this surface, which would protect it, we could possibly have life. And there are missions that have been discussed to try to go to Europa to be able to try to sample and see if there is any kind of life down below the surface ice. Elsewhere in the outer solar system, Europa isn't the only one. We do look at other places. Enceladus around Saturn is another example. We do know that there is also liquid salt water below the surface there. And we have material that has been seen venting out from below the ice. So we know that there is currently liquid water there down below the surface. So similar to Europa, is there a possibility of life existing here? One of the most thought about objects would be Titan. Titan is the large moon of Saturn. And it is unique in that it has an atmosphere. And it has a liquid on the surface, not water. There's no liquid water on the surface. It's much too cold for that. The water would all be frozen solid into ice. But methane does exist in the form of lakes and rivers that we have seen by the lander that landed on Titan a number of years ago. So it is quite possible it has organic materials, things like methane, and could it have some kind of organic chemistry. The difficulty is these very cold temperatures. When the temperatures are very cold, chemical reactions will go in a much slower pace. And that means that maybe the chemical reactions that formed life on Earth are still working. And they're still not going at a fast enough rate to have formed life on Titan. So again, further studies would be needed to see if any kind of simple basic life has ever formed on Titan. So within our solar system, if we're looking for life, we're looking for something very basic. Things like bacteria, things like very simple, simple, single-celled organisms that may possibly exist. If any other complex civilization had ever existed in our solar system, we would have long since been able to detect that. But we can also look outside our solar system to get a look for possibilities of life. We now know of thousands of planets outside our solar system. Some of these are in what we call the habitable zone of their star, which is the region where temperatures are appropriate for liquid water to exist. So we look at what kind of stars do we look at for these? Well, we look at stars like our sun. So stars like our sun here in the middle would have a good size habitable zone, whereas a smaller, cooler star would have a very small habitable zone very close to it. So here you're looking at the green region as the habitable zone. And inside, the red gets too hot, the blue gets too cold, the green is where it's just right. So these are the areas where liquid water could possibly exist. So for our solar system, just that third planet here, that would be us here at Earth, would be the one that is in the habitable zone, Mercury and Venus being too close, and Mars being a little bit too far away. Now what about larger stars? Larger, hotter stars, you wouldn't wanna be close to one, but they would have a larger habitable zone because they are higher temperatures. So there would be a larger region around them where temperatures would be appropriate for liquid water. However, these hot stars do not live very long. In fact, some of these only live millions of years, and it's quite likely that there are, that there would not have, life would not have time to form around them. So here, lifetime is short, and down here, the habitable zone is small. So a very small habitable zone, meaning unlikely to look at these. So when we wanna look at stars, we try to select stars that are similar to our own sun, giving enough time for life to have formed, and possibly even intelligent life, and having a large enough habitable zone to have a big enough region where possible planets could exist. So what would be some signs of evidence for life? And what we'd see, if we're searching again, Earth-like planets, we're looking for things like oxygen in the atmosphere. Does that mean life, and does a lack of oxygen mean no life? Right now we would think that if we were to find oxygen in an atmosphere, it would be a very good sign of life because that is one of the few ways that you can get oxygen into the atmosphere. Oxygen does not form naturally, as we've seen none of the other planets. Anyplace else with an atmosphere, we do not see free oxygen, or oxygen molecular oxygen. We don't see that anywhere except on the Earth. So it's very likely that if we could ever detect an other planet with significant quantities of oxygen in the atmosphere, it would be a good sign that it did have some kind of life. However, does a lack of oxygen mean no life? And the answer is no. On the Earth, we only had oxygen in the atmosphere for the last half billion years. So for the first few billion years, we had essentially very little oxygen in the atmosphere, but we still had life. Maybe very simple life or just ocean life, but life did exist. So just because we do not detect oxygen in an atmosphere does not necessarily mean that life does not exist on that planet. It is very difficult to directly study these planets. We can't explore them. There's no way to send spacecraft to them, especially those when we get outside of our solar system. We need to use indirect methods to be able to study these. These planets are too faint and too close to their stars to really be able to study them directly. So we need to use indirect methods to be able to study them. So let's finish up here with our summary. And what we found is certainly so far, life has been found only on one planet and that would be the Earth. That is the only place that we know of that any kind of life exists. We can look within our solar systems, Mars, and Salatus Titan as being several good examples as where life might exist, maybe some kind of simple life. And outside of our solar system, we wanna look for Earth-like planets around sun-like stars. Does that mean life couldn't exist under other conditions? Not necessarily, but it does mean that that's where we're going to be most likely to look for life because that's the kind of life that we think might be able to exist. Until we can actually detect life that forms under other conditions, we don't know for sure if life can form under those extreme conditions. We do know that life can form, and at least formed once, based on carbon and with liquid water. So that concludes our lecture on life beyond Earth. We'll be back again next time for another topic in astronomy. So until then, have a great day, everyone, and I will see you in class.