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CHARLES S. COCKELL is Professor of Astrobiology at the University of Edinburgh, United Kingdom.
Acknowledgments xvii
About the Companion Website xix
1 Astrobiology 1
1.1 Introductory Remarks 1
1.2 The Major Questions of Astrobiology and the Content of the Textbook 3
1.3 Some Other Features of the Textbook 9
1.4 A Brief History of Astrobiology 10
1.5 Conclusions 15
Bibliography 15
2 What is Life? 17
2.1 The Concept of "Life" 17
2.2 What is Life? The Historical Perspective 17
2.3 Spontaneous Generation 19
2.4 More Modern Concepts 23
2.5 Schrödinger and Life 27
2.6 Life as a Dissipative Process 27
2.7 Life: Just a Human Definition? 28
2.8 Does It Matter Anyway? 30
2.9 Conclusions 30
Questions for Review and Reflection 30
Bibliography 31
3 Matter and Life 33
3.1 Matter and Life 33
3.2 Life is Made of "Ordinary" Matter 34
3.3 The Atomic Nucleus 34
3.4 Electrons, Atoms, and Ions 35
3.5 Types of Bonding in Matter 37
3.6 Ionic Bonding 38
3.7 Covalent Bonding 39
3.7.1 Covalent Bonds and Life 40
3.8 Metallic Bonding 41
3.9 Van der Waals Interactions 42
3.10 Hydrogen Bonding 44
3.11 An Astrobiological Perspective 46
3.12 The Equation of State Describes the Relationship Between Different Types of Matter 47
3.13 Other States of Matter 50
3.14 The Interaction Between Matter and Light 53
3.15 Conclusions 57
Questions for Review and Reflection 57
Bibliography 57
4 The Molecular Structure of Life 59
4.1 Building Life 59
4.2 The Essential Elements: CHNOPS 59
4.3 Carbon is Versatile 62
4.4 The Chains of Life 62
4.5 Proteins 63
4.6 Chirality 66
4.7 Carbohydrates (Sugars) 68
4.8 Lipids 71
4.9 The Nucleic Acids 72
4.10 The Solvent of Life 76
4.11 Alternative Chemistries 78
4.13 Conclusions 84
Questions for Review and Reflection 85
Bibliography 85
5 The Cellular Structure of Life 87
5.1 From Molecules to Cells 87
5.2 Types of Cells 88
5.3 Shapes of Cells 90
5.4 The Structure of Cells 90
5.5 The Structure of Cellular Membranes 91
5.6 The Information Storage System of Life 96
5.7 Eukaryotic Cells 105
5.8 The Reproduction of Cells 107
5.9 Why Did Sexual Reproduction Evolve? 108
5.10 The Growth of Populations of Cells 110
5.11 Moving and Communicating 111
5.12 Viruses 116
5.13 Prions 118
5.14 Conclusions 118
Questions for Review and Reflection 119
Bibliography 119
6 Energy for Life 121
6.1 Energy and Astrobiology 121
6.2 Life and Energy 122
6.3 The Central Role of Adenosine Triphosphate 123
6.4 Chemiosmosis and Energy Acquisition 125
6.5 What Types of Electron Donors and Acceptors Can Be Used? 128
6.6 Aerobic Respiration 129
6.7 Anaerobic Respiration 132
6.8 Fermentation 134
6.9 Chemoautotrophs: Changing the Electron Donor 134
6.10 Energy from Light: Photosynthesis 142
6.11 Oxygenic Photosynthesis 142
6.12 Anoxygenic Photosynthesis 145
6.13 Rhodopsins and Photosynthesis 148
6.14 Evolution of Photosynthesis 149
6.15 Global Biogeochemical Cycles 150
6.16 Microbial Mats - Energy-Driven Zonation in Life 152
6.17 The Thermodynamics of Energy Acquisition and Life 154
6.18 Energy and Life in Extremes 156
6.19 Conclusions 158
Questions for Review and Reflection 158
Bibliography 158
7 The Limits of Life 161
7.1 The Limits of Life 161
7.2 The Importance of the Limits of Life for Astrobiology 162
7.3 The Most Extreme Conditions are Dominated by Microbes 163
7.4 Life at High Temperatures 165
7.5 Life at Low Temperatures 167
7.6 Salt-Loving Organisms 170
7.7 pH Extremes 173
7.8 Life Under High Pressure 174
7.9 Tolerance to High Radiation 176
7.10 Life in Toxic Brews 176
7.11 Rocks as a Habitat 177
7.12 Polyextremophiles - Dealing with Multiple Extremes 180
7.13 Life Underground 181
7.14 Dormancy in Extreme Conditions 183
7.15 Eukaryotic Extremophiles 184
7.16 Are There Other Biospheres with Different Limits? 185
7.17 The Limits of Life: Habitability Revisited 186
7.18 Conclusions 186
Questions for Review and Reflection 186
Bibliography 187
8 The Tree of Life 189
8.1 A Vast Quantity of Life 189
8.2 Evolution and a "Tree of Life" 190
8.3 Classifying Organisms 192
8.4 The Tree of Life and Some Definitions 194
8.5 Problems with Classification: Homology and Analogy 196
8.6 Building a Phylogenetic Tree Using Genetic Material 198
8.7 Types of Phylogenetic Trees 202
8.8 A Modern View of the Tree of Life 202
8.9 Using Phylogenetic Trees to Test Hypotheses 204
8.10 Complications in Building Trees 206
8.11 Origin of Eukaryotes 210
8.12 The Last Universal Common Ancestor 211
8.13 Multiple Origins of Life? 212
8.14 Alien Life 213
8.15 Conclusions 214
Questions for Review and Reflection 214
Bibliography 214
9 The Universe, the Solar System, and the Elements of Life 217
9.1 Our Cosmic Situation 217
9.2 In the Beginning: The Formation of the Universe 218
9.3 Stellar Evolution: Low-Mass Stars 222
9.4 Stellar Evolution: High-Mass Stars 224
9.5 The Elements of Life 228
9.6 The Hertzsprung-Russell Diagram 228
9.7 The Sun is a Blackbody 232
9.8 The Formation of Planets 233
9.9 Types of Objects in Our Solar System 236
9.10 Meteorites and Their Classification 239
9.11 Laws Governing the Motion of Planetary Bodies 243
9.12 Conclusions 245
Questions for Review and Reflection 246
Bibliography 246
10 Astrochemistry: Carbon in Space 249
10.1 Astrochemistry: Carbon Molecules in Space 249
10.2 Observing Organics 249
10.3 In the Beginning 250
10.4 Different Environments for Chemistry 251
10.5 How Do Chemical Reactions Occur? 254
10.6 Forming Carbon Compounds 256
10.7 Formation of Water 257
10.8 Interstellar Grains 258
10.9 Polycyclic Aromatic Hydrocarbons 258
10.10 Even More Carbon Diversity 261
10.11 Comets and Organic Molecules 261
10.12 The Origin of Chirality 262
10.13 Laboratory Experiments 263
10.14 Observing Organic Molecules 264
10.15 Conclusions 265
Questions for Review and Reflection 265
Bibliography 266
11 Early Earth: The First Billion Years 267
11.1 The First Billion Years of Earth 267
11.2 Earth Forms and Differentiates 267
11.3 The Formation of the Moon 268
11.4 The Early Oceans 270
11.5 The Early Crust 273
11.6 The Early Atmosphere 273
11.7 The Temperature of Early Earth 275
11.8 The Late Heavy Bombardment 275
11.9 Implications of the Early Environment for Life 278
11.10 Conclusions 280
Questions for Review and Reflection 280
Bibliography 281
12 The Origin of Life 283
12.1 The Origin of Life 283
12.2 The Synthesis of Organic Compounds on Earth 284
12.3 Delivery from the Extraterrestrial Environment 288
12.4 The RNA World 291
12.5 Early Cells 294
12.6 Where Did the Origin of Life Occur? 295
12.7 A Cold Origin of Life? 301
12.8 The Whole Earth as a Reactor? 301
12.9 Conclusions 302
Questions for Review and Reflection 302
Bibliography 302
13 Early Life on Earth 305
13.1 Early Life on Earth 305
13.2 Early Life - Metabolisms and Possibilities 305
13.3 Isotopic Fractionation 308
13.4 Measuring the Isotope Fractionation: The Delta Notation 311
13.5 Sulfur Isotope Fractionation 311
13.6 Using Isotopes to Look for Ancient Life 312
13.7 Morphological Evidence for Life 315
13.8 Biomarkers 321
13.9 Contamination is a Problem 322
13.10 Instruments Used to Look for Life 323
13.11 A Brief Summary 326
13.12 The Search for Extraterrestrial Life 327
13.13 Conclusions 327
Questions for Review and Reflection 327
Bibliography 327
14 The Geology of a HabitableWorld 329
14.1 The Geological History of Earth: A Habitable World 329
14.2 Minerals and Glasses 330
14.3 Types of Rocks 331
14.4 The Rock Cycle 334
14.5 The Composition of Earth 336
14.6 Plate Tectonics 338
14.7 Dating the Age of the Earth (and Other Planetary Bodies) 344
14.8 Age-Dating Rocks 345
14.9 Geological Timescales 352
14.10 The Major Classifications of Geological Time 352
14.11 Some Geological Times and Biological Changes 353
14.12 Conclusions 360
Questions for Review and Reflection 360
Bibliography 360
15 The Co-evolution of Life and a Planet: The Rise of Oxygen 363
15.1 Dramatic Changes on Earth 363
15.2 Measuring Oxygen Through Time 364
15.3 It Was Not a Simple Rise 368
15.4 Summarizing the Evidence for the GOE 370
15.5 The Source of Oxygen 371
15.6 Sinks for Oxygen 371
15.7 Why Did Atmospheric Oxygen Concentrations Rise? 372
15.8 Snowball Earth Episodes 373
15.9 Other Biological Consequences of the Rise of Oxygen 376
15.10 Oxygen and the Rise of Animals 377
15.11 Oxygen and the Rise of Intelligence 379
15.12 Periods of High Oxygen 379
15.13 Conclusions 380
Questions for Review and Reflection 380
Bibliography 381
16 Mass Extinctions 383
16.1 Extinctions 383
16.2 What is Extinction? 383
16.3 Five Major Mass Extinctions 385
16.4 Other Extinctions in Earth History 386
16.5 Causes of Mass Extinction 386
16.6 The End-Cretaceous Extinction 388
16.7 The Other Four Big Extinctions of the Phanerozoic 392
16.8 Do Microorganisms Go Extinct? 396
16.9 Recovery from Extinction 396
16.10 Can We Avoid Extinction? 398
16.11 The Sixth Mass Extinction? 400
16.12 Conclusions 401
Questions for Review and Reflection 401
Bibliography 401
17 The Habitability of Planetary Bodies 403
17.1 What is "Habitability"? 403
17.2 The Habitable Zone 405
17.3 Maintaining Temperature Conditions on a Planet Suitable for Water and Life 408
17.4 Plate Tectonics and Habitability 414
17.5 Does the Moon Play a Role in Habitability? 416
17.6 Other Planetary Factors that Influence Habitability 417
17.7 Surface Liquid Water, Habitability, and Intelligence 418
17.8 Habitable Environments Need Not Always Contain Life 418
17.9 Worlds More Habitable than Earth? 420
17.10 The Anthropic Principle and Habitability 420
17.11 The Fate of Earth 420
17.12 The Galactic Habitable Zone 421
17.13 The Right Galaxy? 422
17.14 Conclusions 422
Questions for Review and Reflection 423
Bibliography 423
18 The Astrobiology of Mars 425
18.1 Mars and Astrobiology 425
18.2 Martian Geological History: A Very Brief Summary 426
18.3 The Environmental Deterioration of Mars 427
18.4 Missions to Mars 429
18.5 Mars and Life 435
18.6 Trajectories of Martian Habitability 451
18.7 The Viking Program and the Search for Life 455
18.8 Searching for Life by Investigating Gases 458
18.9 Martian Meteorites 458
18.10 Mars Analog Environments 460
18.11 Panspermia: The Transfer of Life between Planets? 463
18.12 Conclusions 467
Questions for Review and Reflection 467
Bibliography 468
19 Ocean Worlds and Icy Moons 471
19.1 The Astrobiology of Moons 471
19.2 The Moons of Jupiter: Europa 472
19.3 The Moons of Jupiter: Ganymede and Callisto 477
19.4 The Moons of Jupiter: Io 479
19.5 The Moons of Saturn: Enceladus 479
19.6 The Moons of Saturn: Titan 484
19.7 Other Icy Worlds 490
19.8 Planetary Protection 494
19.9 Conclusions 496
Questions for Review and Reflection 496
Bibliography 496
20 Exoplanets and the Search for Life 499
20.1 Exoplanets and Life 499
20.2 Detecting Exoplanets 500
20.3 Exoplanet Properties 508
20.4 Detecting Life 517
20.5 Surface Biosignatures 522
20.6 How Likely are These Signatures? 525
20.7 Other Ways to Find Life 525
20.8 Missions to Detect Biosignatures 526
20.9 Conclusions 527
Questions for Review and Reflection 527
Bibliography 528
21 The Search for Extraterrestrial Intelligence 529
21.1 The Search for Extraterrestrial Intelligence (SETI) 529
21.2 Methods in the Search for Extraterrestrial Intelligence 530
21.3 Communication with Extraterrestrial Intelligence (CETI) 533
21.4 The Drake Equation 537
21.5 The Fermi Paradox 538
21.6 Classifying Civilizations 542
21.7 Policy Implications 543
21.8 Conclusions 544
Questions for Review and Reflection 544
Bibliography 544
22 Our Civilization 547
22.1 Astrobiology and Human Civilization 547
22.2 The Emergence of Human Society 547
22.3 Threats to a Civilization 551
22.4 Climate Change and the Challenge to Civilization 553
22.5 The Human Future Beyond Earth 555
22.6 Settling the Solar System 556
22.7 Avoiding Extinction or Collapse: A Multiplanet Species 565
22.8 Environmentalism and Space Exploration as a Single Goal? 566
22.9 Sociology: The Overview Effect 567
22.10 Will We Become Interstellar? 568
22.11 Conclusions 569
Questions for Review and Reflection 569
Bibliography 569
Appendix 571
A.1 The Astrobiology Periodic Table 571
A.2 Units and Scales 571
A.2.1 Standard International Base Units 571
A.2.2 Basic Physical Constants 572
A.3 Temperature Scale Conversion 572
A.4 Composition of the Sun 573
A.5 Some of the Major Star Types, Temperatures, and Colors 573
A.6 Three- and One-Letter Designations of Amino Acids 573
A.7 Codon Table for the Genetic Code Associated with mRNA (also shown in Chapter 5; Figure 5.12) 574
A.8 Planetary Data 575
A.9 Geological Time Scale 576
Glossary 577
Index 601
If you've ever wondered about some of the most fascinating questions in science, such as how life originated, whether it could exist elsewhere, and how life has managed to evolve and persist on our planet for over three and a half billion years, then you've opened the right book.
Astrobiology is a remarkably diverse subject whose main objective is to investigate and understand the phenomenon of life in its cosmic context (Figure 1.1). Astrobiology might be said to address at least four large-scale questions:
Figure 1.1 Astrobiology seeks to understand the phenomenon of life in its cosmic context. This "ultra-deep field" view imaged by the Hubble Space Telescope includes nearly 10?000 galaxies across the observable Universe in both visible and near infrared light. The smallest, reddest galaxies are among the youngest, in existence when the Universe was just 800?million years old. How does life fit into this grand cosmic perspective?
Source: Reproduced with permission of NASA, ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI).
Astrobiology covers and integrates a wide diversity of subjects including astronomy, biology, chemistry, geosciences, planetary sciences, physics, and social sciences. In that sense, to the newcomer it may even look intimidating in its scope. However, as I hope you'll discover in this book, it is an enormously rewarding area of study and whether you come at this subject as an astronomer, biologist, or chemist, or from another discipline, the subject will encourage you to cross seamlessly into other areas of science.
Who is this booked aimed at? Its core audience is students taking astrobiology courses or lectures, but it can be read by anyone with an interest in the subject. The book might be useful for scientists in specific disciplines keen to see where their subject area intersects with other fields of science. For policy makers or others with a social sciences or humanities background, the book will provide a scientific backdrop to astrobiology and its links to space exploration. I would hope, given its structure and layout, that interested laypersons might find this a useful reference source to learn about astrobiology and get to grips with some of its key concepts.
You might ask: But why would we want to study biology from a cosmic perspective in the first place? And in answer: Even the most limited view of life on Earth forces us to consider the cosmic view. Our planet seems like a tranquil place. However, it is subjected to the vagaries of its astronomical environment. For example, a key hypothesis for the mechanism of the extinction of the dinosaurs is an asteroid impact 66?million years ago (Figure 1.2). This hypothesis underscores the fact that to understand past life on Earth, we need to understand how the astronomical environment may have influenced life. To know how the dinosaurs and about 75% of all animal life went extinct at the end of the Cretaceous geological period, you have to know how frequently asteroid impacts occur. That means knowing something about how many asteroids are in space, where they come from, and why they exist in the first place, a question itself linked to the formation of the Solar System. The death of the dinosaurs takes you into astronomy, and that means into the purview of astrobiology.
Figure 1.2 Astrobiology requires an understanding of astronomy. At the end of the Cretaceous, these flying reptiles (pterosaurs) and many other forms of life, including the dinosaurs, are thought to have been driven to extinction by the effects of a large asteroid impact. Therefore, to understand the history of life on Earth, we need to investigate our planet's astronomical environment, a key objective of astrobiology.
Source: Reproduced with permission of NASA.
Instead of looking back in time, we could look into the future. Eventually, when the Sun's luminosity increases to a sufficiently high value, the Earth's oceans will boil away, and the planet will suffer a runaway greenhouse effect, eventually turning into a Venus-like world (Figure 1.3). This will probably occur in approximately 1.5?billion years. Thus, to understand the future of life on Earth, we must also understand our astronomical environment. We need to know how stars are born and die and what the fate of planets is during this stellar evolution.
Figure 1.3 The future of the Earth is a topic in astrobiology. When the Sun turns into a Red Giant star in several billion years from now, the Earth will be a dead planet. However, the increasing luminosity of the Sun over the next one to two billion years will ensure that life on Earth will already have long since been extinguished and the oceans boiled away when the planet enters a greenhouse state. Therefore, to understand the future of life on our planet, we need to know about the evolution of stars. Biology and astronomy are inextricably linked in astrobiology.
Source: Reproduced with permission of Fsgregs, https://upload.wikimedia.org/wikipedia/commons/6/60/Red_Giant_Earth.jpg.
In summary, investigating the past and future of life on Earth means that we need to look beyond Earth to get answers. Astrobiology is about taking that journey and linking the diverse scientific fields needed to understand life on our own planet and, potentially, life beyond.
Let me address an interesting question that I often get asked: What's the use of astrobiology and how is it different from biology? Can it really claim to be a field in its own right? I hope that you've already got some understanding of why it is a useful area of science. Without a long digression into the philosophy of science, it is worth remembering that scientific fields are human constructions. It is true to say that the existing field of "biology" covers investigations into all the living things that we know, which might make you wonder why we need astrobiology at all. However, astrobiology is merely a way to think about living things in their astronomical or cosmic context, to address questions such as the origin of life or the existence of life elsewhere that requires linking biology with other fields such as planetary sciences or astronomy. Does that make astrobiology a subfield of biology or something wider? You decide! I don't think it really matters. What is important is that it is a useful term to encompass a set of scientific questions. It brings together a wide range of scientists, in textbooks like this or at scientific conferences, and of course in scientific papers. If a scientific word achieves nothing more than the advance of collaboration between thinking human beings, then it is an outstanding thing.
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...
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