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Astrobiology is a very broad field, and it can be difficult to grasp all the topics and fascinating questions that it addresses. This textbook is focused on providing a comprehensive overview of some of the major strands of science that underpin the study of life in its cosmic context, while hopefully achieving an appropriate depth of understanding in key subjects such as physics, biology, chemistry, and geosciences. The textbook is based on six years of teaching an astrobiology course at undergraduate level at the University of Edinburgh. During that time, I have been able to observe what undergraduates find interesting and the things for which they have less enthusiasm. The content of the chapters is based around these experiences. In particular, concepts that are important and sometimes more tricky to grasp have more pages devoted on them. The textbook should by no means be regarded as exhaustive, and there are many other texts that can provide ancillary information and deeper discussion on particular subjects.

There is a point I would like to make right away. If you bought this textbook to read all about aliens, you are going to be disappointed. The matter of whether life exists elsewhere in the Universe is certainly one of astrobiology's major questions. “Does life exist beyond Earth?” sometimes expressed as “Are we alone in the Universe?” or similar formulations of the question of whether we are the only type of life in the Universe grips the public as well as the scientific imagination. It justifiably finds itself center stage whenever the word “astrobiology” comes into view. In fact, one of the challenges of being an astrobiologist is to explain to people that the subject is not just about searching for alien life! Perhaps one reason why the question of whether alien life exists is so pervasive is that it is a statistically reasonable question to ask. Earth is one planet in a galaxy which might have about 200 billion stars (the exact number is not known) in a Universe with possibly something like 150 billion galaxies (the exact number is also not known). These numbers tell us that there are about 10²² stars in the Universe (give or take orders of magnitude), which we could explore for the presence of habitable worlds. There is no shortage of planetary bodies for the attention of astrobiologists. It's quite reasonable that anyone who looks into the night sky should wonder whether we are alone. As soon as you start to talk about life in its cosmic context, the question of alien life therefore comes into full view.

However, astrobiology is concerned with many other fascinating questions that relate to life on our own planet and its future. In Table 1.1, you can see a summary of the textbook, its chapters, their content, and the fields they cover. This will give you an instant glimpse of what those topics are.

Table 1.1 The content of the textbook.

Chapter number	Subjects covered	Chapter title	Summary of content
1	Introduction	Astrobiology	Summary of the content of the textbook, the field of astrobiology and its history
2	B, H	What Is Life?	Discussion on what life is and the history of attempts to define life
3	B, C	Matter and Life	The basic structure of matter, major bonding types, and relevance to life
4	B, C	The Molecular Structure of Life	How molecules are assembled into the major molecules of life
5	B	The Cellular Structure of Life	The cellular structure of life and the major components and characteristics of cells
6	B	Energy for Life	How life gathers energy from the environment and the major types of metabolism
7	B	The Limits of Life	Life in extremes and its astrobiological significance
8	B, G	The Tree of Life	How phylogenetic trees are used to order biological information and test scientific hypotheses
9	A, B	The Universe, the Solar System, and the Elements of Life	The origin of the Universe, stellar evolution, the formation of planets, and the origin of the elements important for life
10	A, C	Astrochemistry: Carbon in Space	Introduction to chemistry in space and how complex carbon compounds can be formed in space
11	B, G	The Early Earth: The First Billion Years	The environment of the early Earth and the important characteristics for life
12	B, C, G	The Origin of Life	Hypotheses about how and where the first living things may have originated
13	B, G	Early Life on Earth	Evidence for life on the early Earth and the complexities in finding evidence for early life
14	B, G	The Geology of a Habitable World	The long-term geological history of Earth and its relationship to habitability and life
15	B, G	The Co-Evolution of Life and a Planet: The Rise of Oxygen	How life caused the rise of oxygen and the mechanisms and consequences of the rise of oxygen
16	B, G, H	Mass Extinctions	Discussion on the mechanisms and consequences of the major mass extinctions in the Earth's history
17	A, B, G	The Habitability of Planetary Bodies	Discussion on the general factors that determine whether planetary bodies can support life
18	A, B, C, G, H	The Astrobiology of Mars	In-depth discussion on the habitability of Mars, past and present
19	A, B, C, G, H	Ocean Worlds and Icy Moons	In-depth discussion on the habitability of icy moons hosting liquid water oceans
20	A, B, G, H	Exoplanets and the Search for Life	Methods to search for exoplanets, examine their habitability, and search for evidence of life
21	A, G, H	The Search for Extraterrestrial Intelligence	Methods and attempts to search for evidence of extraterrestrial intelligence
22	A, B, G, H	Our Civilization	The history and potential of our civilization beyond Earth

For each chapter, a brief summary of its subject matter is provided. Also shown are the key scientific disciplines covered by the chapters, where A is astronomy/astrophysics, B is biological sciences, C is chemistry, G is geosciences, and H denotes chapters that contain material that intersects with humanities/social sciences.

A great deal of this book is unapologetically focused on a basic understanding of biology. Indeed, even if alien life is your focus, you still need to know about the one type of life we currently know – life on Earth – before you can embark on any discussion about the conditions and possibilities for life elsewhere.

The textbook therefore begins with a study of life on Earth (Figure 1.4). There is another reason for starting with biology. If we want to investigate how the elements required for life were produced during and after the Big Bang or why a habitable environment needs certain characteristics, or why certain molecules and environments might have been needed for life to emerge, we need to know about biology first. We need to understand its structure, its requirements, and what conditions it can subsist under in order to question how those characteristics were made possible. In that sense, the first part of this book looks very much like standard biology textbook material and, at its core, it is. However, I have written it specifically to provide you with the necessary understanding of biology to enable you to think about the major questions of astrobiology. Throughout these chapters, I address some of the topics in biology that relate to questions about the conditions and possibilities for life elsewhere. These chapters are biology with an astrobiology flavor.

Figure 1.4 The one example we have of a planet that harbors life: Earth. Astrobiology seeks to understand how the phenomenon of life came about and whether it is unique in the Universe. Here, Earth rises over the lunar landscape in this iconic image taken by Apollo 8 in 1968. The image is sometimes called “Earthrise”.

Source: Reproduced with permission of NASA.

We start this tour of life by looking at the fundamental properties of matter and how those properties underpin the structure of the molecules of life. We then consider how these molecules are assembled into the major components of living cells. With a knowledge of the structure of life, we can then move on to think about how these cells can get the energy they need to grow and reproduce, all the time being mindful of those factors that might be specific to terrestrial life or from which we could learn something about life anywhere.

Once life did become established on Earth, what were its limits? We will investigate the physical and chemical boundaries of life that might define how diverse or extensive this life can become in extreme environments at the limits of planetary habitability. If we can find out what the physical and chemical extremes of life are, i.e. the most extreme conditions it can tolerate, we can begin to assess the habitability of other planetary bodies as locations for life. This knowledge even helps us to assess what the impact of human activity and industry might be on the biosphere. Questions that fascinate astrobiologists in this area of research include: What are the limits of life? How does life survive at physical and chemical extremes? Are these limits universal? What do these limits tell us about habitable conditions or the possible presence of life elsewhere? These probing lines of thought drive us to study life in unusual environments, from the deep oceans to the freezing wastes of Antarctica.

Supported by our knowledge of living things, we then explore how all life on Earth is related or linked into a “tree of life.” Buried within this tree of life are profound questions. The diversity of life on Earth is extraordinary. But what unites organisms and what is the relationship between them? How has this diversity changed over time? (Figure 1.5). Astrobiologists want to gain a better understanding of the evolutionary links between diverse organisms. We take an introductory look at the tree of life and see how biologists can construct phylogenetic trees to make sense of all the diversity of life on the planet, and to address certain scientific hypotheses.

Figure 1.5 A schematic of the history of Earth. Understanding this history and the co-evolution of life and the planet constitutes a key objective in astrobiology.

Source: Reproduced with permission of William Crochot.

At this point, we will be equipped with a solid understanding of the structure, interrelationships, and capabilities of the life that we know on Earth. It is time to put this into a more cosmic context.

In the next part of the textbook, we rewind and return to the beginning of the Universe and investigate how stars and planets formed. This distinctly astronomical turn of events in the book is a good way to address the question: How did the elements required for life (which we identified in the first part of the textbook) form, and where did they form? In particular, we examine the conditions for the formation of carbon compounds in the Universe, since carbon-containing compounds are the most important class of compounds for life.

I should stress that one could tackle astrobiology in the opposite direction. We could start at the beginning of the Universe and work our way through to the emergence of life. Chronologically, 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.