Astrobiology

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