this does make sense. However, to know which elements that were formed in the early Universe are relevant for life, it's important to know something about biology in the first place, which is why I have started the textbook with biology. However, I have attempted to write each chapter as a stand-alone text. If you want to select chapters and work through them in a different order, such as from the beginning of the Universe in a more astronomically focused class, you can do that.
Once we have investigated the history of the Universe, the formation of the elements of life and the important role of carbon, the textbook then returns to Earth to examine the emergence of life on this habitable planet. We investigate the environmental characteristics of our planet during its first billion years to understand what sort of environments and habitats could have existed on Earth at that time.
The question of how life might have originated in this early environment is our next task. We consider the chemical reactions and environments in which life could have originated and discuss some of the ideas for the reactions that allowed simple precursor molecules to come together to make the macromolecules of biology – how chemical reactions led to the formation of the first self-replicating cell. This leads to questions such as: How did life originate? Where did life originate? Was it inevitable? When did it happen? All of these questions encompass the question of the origin of life on Earth and might tell us something about whether an origin of life could happen somewhere else.
We follow this up by considering the evidence of early life on Earth. We consider some of the complexities and controversies associated with the evidence of preserved life in the rock record. These problems make full use of our acquired knowledge about the structure of life, the energy sources it can use, and the environments in which it can persist.
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,