Astrobiology. Charles S. Cockell

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      It's then time to take yet another step back and to think about how this early period fits into the whole history of our planet. We begin a chapter where we consider how geologists date rocks and order their understanding of the history of Earth, and we discuss some of the major transitions in life during the history of the planet, including the rise of multicellular animals. In a similar way to the first chapters, which may seem quite basic to a biologist, the geologists among you may feel that these chapters are very much in the tone of a standard geology textbook, and you'd be right. But remember, we are bringing scientists from many disciplines on this astrobiology journey, so for a biologist or chemist, for instance, this material may be new. However, as with the chapters on biology, I have written these chapters with an astrobiological flavor, so even if the core material is familiar to you, I hope you will consider it from a new angle.

      With this overarching view of the history of our planet, we might be tempted to think that all this geological and biological evolution has been smooth and orderly. Unicellular organisms evolved into animals, and then intelligence emerged. However, the next two chapters elaborate on why this isn't the case. By investigating rises in atmospheric oxygen that have occurred in our planet's past and the role of mass extinctions in changing biological diversity, we can see that the emergence of life on a planet, and its success over billion-year timescales, is fraught with difficulties, including astronomical perturbations such as asteroid and comet impacts.

      We will see that life itself is responsible for some of these changes, such as the rise of oxygen, but in other cases, such as the effects of an asteroid impact, it has been a hapless passenger. Are these challenges universal and were the opportunities that presented themselves during the co-evolution of the planet and life ones that we would expect to occur on any planet that has life? This question is discussed as we progress, but you might like to keep it in mind at any time you are thinking about the history of life on Earth. If there is life on other planets, is our own planet a universal template for how it too would evolve? What features of this planet's biological evolution are an idiosyncratic result of particular conditions here?

      At this stage, we have a more complete understanding of planet Earth, its history, its life, its geology. We have got to grips with a detailed understanding of the one planet we know that supports life, its characteristics and how life shaped, and was shaped by, its environment. So now we take this knowledge and expand further to the cosmic context: We leave Earth and head outwards.

In the following chapters, we take what we know about Earth and consider what might make a planet habitable for life and where else in the Universe such environments might exist. Taking a look close to home – our own Solar System – we investigate how Mars compares to Earth. We examine the icy moons of Jupiter and Saturn that host oceans beneath their surfaces. Are other planetary bodies in our Solar System habitable? We move on from this position to consider the billions of other planets in our Universe, looking at the methods used to search for planets around other stars, so-called exoplanets, how we determine their different physical characteristics (Figure 1.6), and how we might search for life on them.

       Figure 1.6 Habitable worlds orbiting other stars. As this artist's impression makes clear, the detection of rocky worlds around other stars offers us the possibility of a statistical assessment of how common Earth-like worlds are in the cosmos, an analysis of their diversity, and the possibility of determining whether they host detectable life.

      Source: Reproduced with permission of NASA/JPL-Caltech/R. Hurt (SSC-Caltech).

      In the final chapters of the book, we consider extraterrestrial intelligence and whether there are any other intelligences in the Universe with which we can communicate. Is intelligence inevitable and has it arisen elsewhere? If it has evolved elsewhere, can we communicate with it? What happens if we do?

Astrobiology is not just about non-human life on our planet. As a tool-building civilization that has the capacity to travel beyond Earth and even change the life support system of our own spaceship Earth, our own past and future are part of a complete investigation of the relationship between life and its cosmic environment. In the final chapter, we contemplate the future and fate of our own civilization. We can ask questions about ourselves such as: Will humans leave Earth permanently? How do we settle on other planets? How do we preserve Earth while settling in space? How will we adapt to space? Can society be successfully expanded to these environments? (Figure 1.7). These are not so much scientific questions, more technical questions, but they very much have a bearing on the applications of astrobiology to human society. These questions generate direct links between astrobiology and humanities and social sciences as they force us to confront our own place in the cosmos and the story of life.

       Figure 1.7 Astrobiology is concerned with the human future beyond the Earth. Can we establish stations on other planetary surfaces and will they eventually become self-sustaining?

      Source: Reproduced with permission of NASA.

      In summary, each chapter in this textbook is designed to present a text on a particular aspect of the link between life and the cosmos. I have attempted to explain some of the principles of astrobiology with respect to each subject area, so that you can read the book in a structured, directional way. Alternatively, you can pick and choose aspects of astrobiology that are of special interest for a whole astrobiology course or parts of a course by reading selected chapters.

1.3 Some Other Features of the Textbook

      There are a few other general points I'd like to make about the chapter contents of the textbook. You will notice that the units I use in the textbook are not consistent throughout. For example, growth temperatures of microorganisms are usually shown in Celsius. Temperatures of planetary surfaces are often expressed in Kelvin. Different scientific fields tend to use different units, and, rather than creating complete consistency (which would result in seemingly odd units being used for phenomena where they are not normally used), I have stuck with the normal conventions. These differences highlight the multidisciplinary nature of astrobiology.

      In all the chapters, I have included some other information shown in boxes. Some of the boxes present points of debate in astrobiology that are worth discussing with others or contemplating yourself. These “Discussion Points” are an opportunity to get you to think about ideas in different fields that link to astrobiological questions. I have also written them in places where the material might seem very conventional. I hope they will stimulate you to think about the material being described in new ways. For example, is the biochemical structure of life on Earth something universal or a very particular outcome of Earth's experiment in biological evolution? Such a question should encourage you to think about what the basics of biology might, or might not, tell us about life elsewhere, if it exists. I have attempted to provide some similar thoughts and questions in all the chapters. The content and questions in these boxes are by no means exhaustive, and you should use them to encourage other discussions or come up with new questions. In particular, try to think about questions that bridge different chapters and the different fields in the book.

      Here and there I have included some textboxes about some of the major facilities that astrobiologists use. There are a vast number of techniques that astrobiologists employ in the laboratory, but some large facilities, coordinated internationally, such as space telescopes, expand the reach of the science significantly. They also give you a flavor of the modern nature of international science.

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