 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to discuss astrobiology and the possibilities of life in the universe. So we can look, first of all, what do we mean by life here on Earth and how does that lead us to consider the possibilities that life might exist elsewhere in the universe. So what kind of things are important for life? Well one thing we have to remember is that the Big Bang created only two elements, only hydrogen and helium. Everything else was created in stars. So that means that the very earliest planets to form couldn't have looked like the Earth because there would have been no heavier elements anything other than hydrogen and helium from which to form them. So what do we mean so we need other heavier elements we need things like carbon, we need things like oxygen and other elements things like iron that make up our blood we need those and those all had to have been created in stars. We also want to qualify what we mean when we say organic molecules. An organic molecule does not mean that it is living. It simply means that it is a carbon compound and carbon forms some very complex compounds giving rise to its own chemistry but it does not mean just because a molecule is organic means that it is based on carbon does not mean that it is necessarily living. And the other thing we want to look at is liquid water. Is liquid water a very important and this is the only known object with liquid water on the surface just this one and we picture it here that is our Earth. We know that Mars maybe had liquid water in the past or did have liquid water in the past and there may be other planets outside our solar system that have water but this is the only one that is known to have liquid water in the on its surface currently. So let's look a little bit here what do we mean one of the things we sometimes talk about is what we call the Copernican principle and what that means is that the Earth is not unique and it goes back to the time of Copernicus where Copernicus said that we were not the center of the solar system but the Sun was at the center instead. So we're not the center of the solar system we're not the center of the galaxy we're not the center of the universe we're not the center of anything. So when we talk about the Copernican principle we're really saying that the Earth is not something unique that there are lots of habitable planets out there and there could be more Earth-like planets out there in the universe in fact we've detected a number of Earth-type planets we just don't know whether life has evolved elsewhere. And one of this leads rise to a paradox that we call the Fermi paradox which was proposed by Enrico Fermi and he suggested that you know if other civilizations formed why haven't we found them yet? Why are they not here visiting us if other civilizations formed? Because what would happen very quickly kind of shown in the map here is that when a civilization formed on a star and developed space travel it should be able to colonize other planets very quickly and then those would colonize other planets around other stars and over relatively short times galactically speaking meaning many millions or even hundreds of millions of years that civilization would expand out and cover the and cover the galaxy so there should be civilizations all over the place. So what we may mean is that maybe this travel is a lot harder than we think it might be or that science fiction would lead us to believe that you know maybe there are easier ways to travel maybe there are not maybe it is very difficult to travel between the stars but even things that would be technologically capable now are not something and would still be able to explore the stars at least the nearby stars over things like millions of years you would be able to travel that kind of distance there. So it is a big what we call a paradox as to why these other civilizations have not yet been detected. So let's go and look at what we mean by astrobiology or the biology of of astronomy and one of the things that we look at one of the things we talk about are amino acids. Amino acids are the building blocks of life they make up our DNA that is inside each of ourselves that tells everything about a living creature. So the amino acids are the building blocks of that and they are very common they are found in meteorites and other organic compounds are found in things like molecular clouds dark regions here that we see we can actually detect the emissions of organic compounds. So we know that organic compounds form very easily in regions of star formation leaving the possibility that they could become parts of newly forming planets. But how about looking within a planet how easy is it to form these amino acids which would then possibly lead you to DNA? And one thing we look at that is an experiment that was done in the 1950s was the Miller-Urie experiment and what that did was to simulate the primordial atmosphere of the earth so not the atmosphere that it's made up of today nitrogen and oxygen primarily but what would have been present things like methane and ammonia and water and hydrogen that would have been present in the simulated in the sorry in the early atmosphere of the earth. So we simulate that here we put the atmosphere those gases that we believe would have been present we subject them to heat and electrical discharges as would have occurred in the early earth that we would have had you know lightning storms we would have had more heat and what we do is then run the water run you run water vapor through it so the water vapor runs through and as it condenses out there's a trap here where we can contain contain anything that forms so what we see in this trap is that we find amino acids and organic molecules that form from these basic compounds that we know would have been present in the early earth's atmosphere so what we see is not necessarily that life is easy to form but that the basing basic building blocks of life can't could have formed on the conditions present in the early earth and these compounds are not unique to earth so they likely could have formed elsewhere so we see that the building blocks of life are very common what we don't know is how common it is for life itself to actually form so how do we do that jump it's a very big step from organic compounds which are very common to life life is something that we don't know of other than here on the earth so what we want to look at is what was the early earth like at the time the earth might have looked something like this very hot molten impact constantly being impacted and that was sterilizing the surface so life was unable to form very early on however over time over that first few hundred million years the impact slowed and the oceans began to form and in the oceans we would find organic molecules much as we saw with the with the Miller-Urie experiment so what we see uh one one of the things that we find is that early fossilized life actually occurred very early on in the history of the earth we believe life itself may have formed earlier but we have to see what was actually able to form and one of the things that we see are these stromatolites and these objects here are actually formed very early on in fossilized remnants of them have been found dating back to three and a half billion years these are some of the very oldest objects that we see various oldest evidence of living objects now likely life had to exist before these so life formed if the earth is four and a half billion years old it had to have formed very shortly after the earth formed itself so it seems like there has been life on earth for the vast majority of the time that the earth has been around so we see some of this early evidence from long ago how do we form more complex molecules we need to make a jump we need to jump from chemical evolution which is what we've been looking at in things like the Miller-Urie experiment to biological evolution so there's a big jump there you can have chemical reactions that can make these compounds but how do we get that into something that is actually living one of the things is the DNA which contains all of the information about the organisms it's built on the amino acids that we've already talked about so you put the amino acids together in the right pattern and this molecule is able to replicate itself one of the beginning signs of life is that there it is able to reproduce so when we talk about chemical evolution we don't have any reproduction going on here we actually have reproduction that DNA can split itself and replicate itself and make identical copies you also have the RNA which is a simpler molecule which aids in the flow of the genetic information could this be some kind of intermediate step between the organic molecules again we said that these are common to DNA and then to life so how do we get it could this be an intermediate step leading to life now one of the things that we see on earth and that we associate with life is oxygen what do we see about oxygen in the atmosphere and what we find is that for the vast majority of the earth's history there was very little oxygen in the atmosphere oxygen was tied up maybe in things like carbon dioxide or water but if we look at the percentage here very very small until about 650 or so million years before the present that's only the last half billion years of the earth's history so at the time of the very earliest life there was no oxygen in the earth's atmosphere now it took time and the oxygen content has then come up and then settled down to about the 21% we know of it today but this is about the time that photosynthesis began early on and oxygen began to accumulate so we had a very low level of oxygen even two and two point four billion years ago but it would not have been sufficient for us to be able to breathe this is only at about a three percent range of the earth's atmosphere so would not have been sufficient oxygen for us to breathe and it hasn't been till the last half billion years that we've actually gotten significant quantities of this the development of free oxygen meaning not tied up in other molecules meant that we actually formed the ozone layer which allowed life to move from water where life had existed for most of that to land and that was the development of the ozone layer which protected us from ultraviolet radiation ultraviolet radiation very damaging and sterilizing and would rip apart these organic molecules as they formed especially out on land without the water in the ocean to protect them but the development of the ozone layer absorbing a lot of that ultraviolet light then allowed life to move on to land and that's what we begin to see here shortly after life begins to move from the oceans on to land so while life existed on earth for the vast majority of earth's history life was only in the oceans now when we talk about habitable environments what do we mean they're based on what we're used to here on earth so we are opening our own form of bias we think perhaps that life in the universe has to be something like like us there are possibilities of alternative biochemistries science fiction is good about putting things like silicon based life life based on silicon instead of carbon because carbon silicon shares some chemical similarities to carbon can you have life that is based on some other liquid that is not water but these are really all speculation we have no idea until we can actually someday maybe get out there in the universe to see what is going on what other types of life there might be we like water as a solvent because it is needed some kind of liquid is really needed to facilitate biological and chemical reactions if you dry out the organic molecules they're not going to react together and form a great chains water happens to be liquid at a very narrow range of temperatures and pressures so it's not necessarily common as a liquid it is very common in the solar system either is a gas or a solid but liquid exists only that we know of here on the earth so other liquids things like methane exist on titan but the temperatures are much colder and when temperatures are colder reactions seem to go much slower so the the reactions would go slower would it be as likely for life to have formed some place where the temperatures are bitterly cold even if you did have a liquid so water is one thing we look at another is carbon and carbon again is the basis of life on earth we also use what we call the biogenic elements which include hydrogen nitrogen oxygen phosphorus and sulfur which are very primary elements needed for life are they required maybe not maybe we can imagine life built on something else but it's hard to figure out exactly how that's going to work maybe it's because we just haven't detected it yet maybe it's because it simply is not possible but it is something to be considered as a possibility now one of the types of life that we look at is called the extremophiles that there are types of life that can exist in very extreme conditions for humans it's not we have very strict limits as to what ranges we can live very high temperatures very low temperatures and human life will cease to exist ph the acidity is also very very narrow ranges so we cannot exist if our body chemistry gets thrown off will become ill and possibly even die depending on how far off the ph level or the acidity becomes extremophiles on the other hand can exist and even thrive in some of the most extreme conditions we have things like thermophiles which exists at high temperatures even up to the boiling point of water we have psychrophiles which can exist at low temperatures 25 degrees below zero Celsius so extremely bitterly cold temperatures temperatures acidophiles and alkylophiles exist at high and low ph levels and other organisms that can exist even with high levels of salt very high pressures or high levels of radiation so these things that would destroy human life still other other objects are able to exist and other creatures are able to exist with these one of those is what we call the tardigrade tardigrades were actually discovered back in the 1700s and they exist under a very wide variety of conditions this is an extremely enlarged image of one seen here but they can exist under extreme temperatures hot or cold pressures high or low radiation levels dehydration starvation in fact they can even tested that they can survive in the vacuum of space for up to 10 days so what it seems to show is that life is very hardy it's very good at finding a way to survive and the tardigrade and other extremophiles are one example of this so let's finish up here with our summary and what we've looked at in this section is we know life it's based on carbon it's based on water that's what we use is our basis for life elsewhere in the universe that does introduce a bias because we limit where we're going to look for life because we're going to look for place things based on carbon and we're going to look for areas that actually have water especially in a liquid form we know that life on earth formed very early within the first billion years after the earth formed so life has been here on earth for a very long time and we talked about some of the organisms that can survive extreme conditions including surviving in the vacuum of space for more than a week so that concludes our lecture on astrobiology 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