[BLANK_AUDIO] In this lecture we'll address a somewhat intriguing question, could we be martians? And this is slightly flippant way of asking, are planets isolated islands? Or can they transfer life? In other words, could life have been transferred from Earth to Mars in meteorites? Is it even possible there could be an origin of life on Mars, and then life transferred from Mars to Earth? In which case, we'd all be Martians. We know that life travels around on the Earth. Birds for example can fly from one country to another, even one continent to another. And seeds can also drift through the wind from one continent to another. This is an area that's been of interest to ecologists for a very long time. It's called island biogeography. How life can be transferred from one island to another. The question, can life be transferred from one planet to another, is just an extension of this old question in ecology. It sometimes been called panspermia, the idea of transfer of life through interplanetary space. It was first considered by Svante Arrhenius, who thought about the idea of microbes, other small lifeforms, being pushed through space by the pressure of the sun's radiation. And Lord Calvin considered the possibility of rocks being transferred through space containing life, allowing other planets to be inoculated. It's a reasonable idea because we know that rocks can be transferred from one planet to another. We collect for example Martian meteorites in Antarctica, and we know these meteorites are from Mars because the rocks correspond to the rock composition on Mars and they also contain bubbles of gas that correspond to the composition, the chemical and isotopic composition of gases in the Martian atmosphere. So if Martian meteorites are on Earth, then it's probably true there are some Earth meteorites on Mars. And so the transfer of material has occurred between Earth and Mars. And the question is, could life have been transferred between planets? Well, for life to be transferred between two planets it has to survive three processes. First of all it's going to have to survive being launched from a planetary surface, in other words being ejected, from the surface of a planet in an asteroid or comet impact, and the intense pressures that that involves. It's then going to have to survive the journey through space, many years traveling through the extreme conditions of interplanetary space. And then finally, it's going to have to survive atmospheric entry through the atmosphere of the host planet to land on the new planet, and then take root on the new planet in which it has arrived. Let's have a look at these three processes and see what we know about the survival of life in these three different stages. First of all, launch. We can simulate the launch of microbes from a planet in an asteroid comet impact with this facility. This is a light gas gun and it accelerates pieces of rock at kilometers a second and slams them into other pieces of rock, much like an asteroid or comet impact. And we can ask the question, can microbes survive that process? On the right you can see a graph, and on the x-axis is shock pressures up to gigapascals of pressures. And, on the left-hand side, you can see an axis showing the survival fraction of microbes. What this graph shows you is that microbes can survive tens of gigapascals, the pressures required to survive being launched from a planetary surface at fast enough speeds to escape the gravity of a planet and be launched into space. So on the face of it, life can survive the pressures required to be launched in an asteroid impact into interplanetary space. What about survival in space? Well experiments have been done by the European Space Agency where microbes have been launched into space and left there for a long time, up to six years. This is a facility that you can see here called the exposed facility, that was bolted onto the outside of the International Space Station. And my lab, and some other European investigators as well, had microbes that they put into this facility to ask the question, can they survive in space for a year and a half? And indeed they can. In our own samples, a single microorganism managed to survive, the sign of bacterium, for a year and a half in space. Well, that's rather a short space of time, it may take many decades. At least the shortest length of time may be many decades to transfer between planets, but what these experiments do show us is that microbes can survive in space for long periods of time, possibly many years. Once they've traveled through space, what about atmospheric entry? Well one of the problems with coming through the atmosphere in a rock is the intense temperatures of those rocks as they hurtle through a planetary atmosphere. But the good news is that that temperature occurs for a very brief period of time. In fact, the rock on the outside of a meteorite melts and forms what's called a fusion crust. But deep inside the rock, the temperatures may be low enough for life to be able to survive. And in fact, there's some evidence in meteorites that the interior of the meteorite remains at well below 60 degrees. Which is low enough for life to be able to persist inside the rock. So the outside of the rock is melted. Life would certainly not survive that, but the rock is big enough, the interior re, remains cool enough for life to be able to survive that atmospheric transit. So, when we look at all three of those stages, launch, survival in space, and atmospheric entry, it seems that taken individually microorganisms can in fact survive each one of those three processes required to travel from one planet to another. Of course we have have no evidence that life has been transferred for example between Earth and Mars. But these experiments give some sort of perspective on the possibility of life to be transferred from one planet to another, and suggests that it might be possible. Of course it goes without saying that it's important that the conditions on the host planet are actually conducive to life. If a Earth rock landed on the surface of Mars today for example, it wouldn't have much luck in perpetuating life. The surface is very dry, high levels of ultraviolet radiation, and ionizing radiation. But that might have been different in the early history of Mars when there was more liquid water available. So what we have learned in this lecture? Well, we've learned that the idea that life can be transferred between planets is a very old one. It addresses the question of whether planets are biological islands. Microbes can apparently survive the three phases of transfer from one planet to another. They can survive the high pressures associated with launch. They seem to be able to survive in space for quite some reasonable periods of time. And it looks like they would also survive atmospheric entry if they were buried deep within a rock. We don't know what the maximum time for survival of microbes is in space and this is an on going question of astrobiology. How long can microbes really survive in the severe, desiccating conditions of outer space with high levels of radiation? This idea of life being transferred from one planet to another, panspermia, has not yet been shown to have occurred. But whether it can occur remains an important question in island biogeography and in astrobiology. And panspermia of course also has implications for the origin of life. The idea that life could originate on one planet and then be transferred to another planet. [BLANK_AUDIO]
