 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to talk about the possibilities of life beyond Earth. So where in the universe might we find life? And let's start off looking in our solar system. So where could we look for life? Well, one of the most prominent places we've always considered is the planet Mars. Why? Because we know that liquid water existed there in the past. That is a key. We know it is the one other place in the solar system in the universe that we know of that actually had liquid water on its surface in the past. Now we've explored it regularly and the Viking landers in 1976 showed no organic molecules or any evidence of biological activity but could not completely rule it out. Curiosity rover in 2016 found mudstone. Why is this evidence that Mars was habitable? Because this would be something that had to form in water. So we do know for sure that Mars was habitable in the past. Does that mean life existed? No. It does not mean that life necessarily existed only that it was possible for life to have formed. Life formed quickly on Earth? Could it have formed quickly on Mars? So where might we find life? Could there be any life there today? We do know that it was warmer and wetter in the past on Mars. And liquid water could still exist underground today. We know that maybe some kind of very salty liquid water that could flow on the surface of Mars in some of the craters. But we do also know that pressures and temperatures absolutely prohibit liquid water to exist for long on the surface. It would very quickly either freeze or vaporize into a gas depending on the temperature. There is insufficient pressure for that to be there. So here we see some images of what Mars, drawings I should say, of what Mars would have looked like three to three and a half billion years, sorry three and a half to four billion years ago in the top section here. And that would have had an ocean on the top. So we do see a great ocean up there as well as some water filled areas. And we see that over by three and a half billion years they were really decreasing. And by two billion years they were almost gone. And after that the last few billion years pretty much nothing. So based on what we know of Mars these are our estimates. Now this was not a great deep ocean of miles and miles like ours. It may have been only a couple of meters deep. But there could have been a lot of water on Mars in the past. And since life formed easily on Earth early on, by this kind of time frame could it have done the same on Mars? Right now Mars also does lack any kind of atmosphere meaning harmful radiation which sterilizes the surface. But it is still one of our best bets and we do continue searching for the possibility of life on Mars. How about the outer solar system? Well the planets we can rule out. No life as we know it could exist on the planets of the outer solar system. The moons are a possibility. So Europa pictured here has an ocean of liquid salt water below the surface ice. So if you could go down many miles below this ice there would be water there. And that means could life form not on the surface but below the surface. Could there be some kind of simple life below the surface? And could we be able to detect that from the surface? As surface materials mix with that ocean of ice below when there are cracks that material flows up could we detect that from exploration of Europa? And that's one of the places that we do want to explore in the outer solar system is to take a closer look at Europa just for that purpose. We also have Enceladus which is one of the moons of Saturn which has material which has been seen to vent from the south polar region meaning that yes it too has liquid water below the surface. We also know that the moon Titan also around Saturn has an atmosphere. Now it's the only moon with an atmosphere and it does have a liquid on the surface which is methane. So methane is the very simplest organic compound just one carbon atom and four hydrogen atoms. So could there be a more complex organic chemistry going on here which could that lead to life? The difficulty? Very, very cold temperatures. And if you've studied chemical reactions the colder the temperatures lower the reactions go. So would it take a lot longer for some kind of life to form on Mars? I'm sorry on Titan Earth was much, much warmer than this. What if we look outside our solar system? Well we do know that there are thousands of planets known outside our solar system some of which are in the habitable zone of the star which is the region where liquid water could exist because the temperatures are sufficient. So really where this zone is depends on the specific temperature of the star. So if we look at this we could see that for a very massive star shown at the top that you could have a very wide habitable zone shown in the green here. Red would be too hot, blue would be too cold and in the green area water could exist. For an intermediate mass star like our own Sun the zone is smaller and closer to the star and for a very low mass star it's very small and very close to the star. So we tend to look at sun-like stars. Why? Because the cooler stars have a very small habitable zone meaning you have to have the planet in just the right place for it to be considered habitable. Hotter stars have a very short lifetime and may only live 10, 20 million years which may not be a sufficient amount of time for life to form if it takes as long as it did on Earth. So what kind of evidence do we have for life? Well, what do we search for? We're going to search for Earth-like planets around Sun-like stars. Does that mean they're the only things? No, but they're certainly our best bet to start off with for any life like our own. Is there oxygen in the atmosphere? And how can we see that? Well, we can look at stars where the planet passes in front of the star. When that occurs and we're used to those for transits when we detect the planets but if we put the light through the spectrograph when the planet is there we can then see the spectrum of the atmosphere of the planet overlaid on the spectrum of the star. So anything that is different there could be attributable to the planet's atmosphere. Could we find oxygen? And does that imply life? Remember that for most of Earth's history we did not have a lot of oxygen in the atmosphere. So a lack of oxygen may not mean that there is no life, simply that maybe life has not developed to the point where it is on land and needs the oxygen. That if everything is still in the oceans then there still could be life. It is difficult to directly study the planets. Why we can't explore them, we can't see them directly and we have to use indirect methods such as the one here to look at their atmospheres to be able to learn more about them. So while there may be life on planets outside the solar system it is going to be something very difficult to determine and we will look at communications in another lecture. So let's go ahead and finish up with our summary and what we looked at so far we have life only in one place, only here on Earth. Other possibilities within our solar system include Mars and Solotis around Saturn and Titan around Saturn. Those are probably the three most likely places we would expect to find life if not in our solar system. Outside of the solar system well we do see our biases here yes we look for Earth-like planets around Sun-like stars but those are the ones where we are most likely to find life like we have here on Earth. So that concludes this 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.