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Charles Cockellis Professor of Astrobiology at the University of Edinburgh. His research interests are focused on the study of life in extreme environments and understanding the habitability of planets. After a doctorate at the University of Oxford he worked at the NASA Ames Research Centre, the British Antarctic Survey and The Open University. He has published widely in astrobiology and carried out fieldwork in locations from the Arctic to the Antarctic.

About the CompanionWebsite xvii

1 Astrobiology and Life 1

1.1 About this Textbook 1

1.2 Astrobiology and Life 4

1.3 What is Astrobiology? 4

1.4 History of Astrobiology 6

1.5 What is Life? 9

1.6 Conclusions 12

Further Reading 12

2 Matter, the Stuff of Life 13

2.1 Matter and Life 13

2.2 We are Made of ‘Ordinary’ Matter 13

2.3 Matter: Its Nucleus 14

2.3.1 Isotopes 14

2.4 Electrons, Atoms and Ions 15

2.5 Types of Bonding in Matter 15

2.6 Ionic Bonding 15

2.6.1 Ionic Bonds and Life 16

2.7 Covalent Bonding 17

2.7.1 Covalent Bonds and Life 17

2.8 Metallic Bonding 19

2.9 van der Waals’ Interactions 19

2.9.1 Dipole–dipole (Keesom) Forces 19

2.9.2 Dipole–induced Dipole (Debye; Pronounced deh-beye) Forces 19

2.9.3 Dispersion Forces 20

2.9.4 van der Waals’ Interactions and Life 20

2.10 Hydrogen Bonding 20

2.10.1 Hydrogen Bonds and Life 21

2.11 The Equation of State Describes the Relationship between Different Types of Matter 21

2.12 Phase Diagrams 22

2.12.1 Matter and Mars 23

2.12.2 Phase Diagrams and Life 24

2.13 Other States of Matter 25

2.13.1 Plasma 25

2.13.2 Degenerate Matter 25

2.14 The Interaction between Matter and Light 27

2.14.1 The Special Case of the Hydrogen Atom 29

2.14.2 Uses to Astrobiology 29

2.15 Conclusions 30

Further Reading 30

3 Life’s Structure: Building the Molecules 33

3.1 Building Life 33

3.2 The Essential Elements: CHNOPS 33

3.3 Carbon is Versatile 34

3.4 The Chains of Life 35

3.5 Proteins 35

3.6 Chirality 37

3.7 Carbohydrates (Sugars) 38

3.8 Lipids 39

3.9 The Nucleic Acids 39

3.9.1 Ribonucleic Acid 40

3.10 The Solvent of Life 43

3.10.1 Water as the Best Solvent 43

3.11 Alternative Chemistries 44

3.11.1 Alternative Core Elements 44

3.11.2 Alternative Solvents 47

3.12 The Structure of Life and Habitability 48

3.13 Conclusion 49

Further Reading 49

4 Life’s Structure: Building Cells from Molecules 51

4.1 From Molecules to Cells 51

4.2 Types of Cells 51

4.3 Shapes of Cells 53

4.4 The Structure of Cells 53

4.5 Membranes 55

4.5.1 Gram-negative and Positive Prokaryotic Membranes 56

4.5.2 Archeal Membranes 58

4.6 The Information Storage System of Life 58

4.6.1 Transcription – DNA to RNA 59

4.6.2 Translation – RNA to Protein 60

4.6.3 A Remarkable Code 62

4.6.4 DNA Replication 62

4.6.5 Plasmids 64

4.6.6 eDNA 65

4.7 Cell Reproduction 65

4.8 The Growth of Life 67

4.9 Moving and Communicating 68

4.9.1 Movement in Prokaryotes 68

4.9.2 Communication in Prokaryotes 68

4.10 Eukaryotic Cells 70

4.10.1 Endosymbiosis 72

4.11 Viruses 72

4.12 Prions 74

4.13 Conclusions 74

Further Reading 74

5 EnergyforLife 77

5.1 Energy and Astrobiology 77

5.2 Life and Energy 78

5.3 The Central Role of Adenosine Triphosphate 78

5.4 Chemiosmosis and Energy Acquisition 80

5.5 What Types of Electron Donors and Acceptors can be Used? 83

5.6 Aerobic Respiration 83

5.7 Anaerobic Respiration 86

5.8 Fermentation 88

5.9 Chemoautotrophs 88

5.9.1 Methanogens and Methanotrophs 90

5.9.2 Sulfur Cycling 91

5.9.3 Iron Oxidisers 91

5.9.4 Nitrogen Cycling and the Chemoautotrophs 91

5.10 Energy from Light: Oxygenic Photosynthesis 92

5.11 Anoxygenic Photosynthesis 94

5.12 Global Biogeochemical Cycles 97

5.13 Microbial Mats – Energy-driven Zonation in Life 99

5.14 The Thermodynamics of Energy and Life 100

5.14.1 Gibbs Free Energy: The Energy in Reactants and Products 100

5.14.2 Gibbs Free Energy: The Concentration of Compounds 100

5.14.3 Gibbs Free Energy: Using Redox Reactions 100

5.15 Life in Extremes 103

5.16 Conclusions 103

Further Reading 103

6 TheTreeofLife 105

6.1 A Vast Diversity of Life 105

6.2 The Tree of Life 106

6.3 Some Definitions 106

6.4 Classifying Organisms 106

6.5 Homology and Analogy 109

6.6 Building a Phylogenetic Tree 110

6.7 Some Definitions and Phylogenetic Trees 112

6.8 Types of Phylogenetic Trees 113

6.9 Using Phylogenetic Trees to Test Hypotheses 113

6.10 Complications in Building the Universal Tree of Life 115

6.10.1 Endosymbiosis 116

6.10.2 Horizontal Gene Transfer 117

6.11 The Last Universal Common Ancestor 119

6.12 Molecular Clocks 120

6.13 Alien Life 121

6.14 Conclusions 121

Further Reading 121

7 The Limits of the Biospace 123

7.1 The Biospace 123

7.2 The Importance of the Biospace for Astrobiology 123

7.3 The Edges of the Biospace are Dominated by Microbes 124

7.4 Life at High Temperatures 126

7.4.1 Uses for Thermostable Molecules 127

7.5 Life at Low Temperatures 127

7.6 Salt-loving Organisms 129

7.6.1 Salt-in Strategy 130

7.6.2 Salt-out Strategy 130

7.6.3 Low Water Activity 130

7.7 pH Extremes 130

7.8 Life Under High Pressure 132

7.9 Tolerance to High Radiation 132

7.10 Life in Toxic Brews 134

7.11 Life on the Rocks 134

7.12 Polyextremophiles – dealing with Multiple Extremes 136

7.13 Life Underground 137

7.14 Dormancy in Extreme Conditions 138

7.15 Eukaryotic Extremophiles 139

7.16 Are there Other Biospaces? 140

7.17 The Limits of Life: Habitability Revisited 140

7.18 Conclusions 140

Further Reading 141

8 The Formation of the Elements of Life 143

8.1 In the Beginning 143

8.2 Low Mass Stars 147

8.3 High Mass Stars 149

8.4 The Elements of Life 150

8.5 The Hertzsprung–Russell Diagram 152

8.6 The Sun is a Blackbody 156

8.7 The Formation of Planets 157

8.8 Types of Objects in our Solar System 159

8.9 Laws Governing the Motion of Planetary Bodies 160

8.10 Meteorites 163

8.11 Conclusions 165

Further Reading 165

9 Astrochemistry – Carbon in Space 167

9.1 Astrochemistry: The Molecules of Life? 167

9.2 Observing Organics 167

9.3 In the Beginning 168

9.4 Different Environments 169

9.4.1 Diffuse Interstellar Clouds 169

9.4.2 Molecular Clouds 169

9.4.3 Protoplanetary Disc 170

9.4.4 Carbon-rich Stars 171

9.4.5 Shock Waves from Supernova Explosions and Other Astrophysical Violence 172

9.5 How are Compounds Formed? 172

9.6 Interstellar Grains 174

9.7 Forming Carbon Compounds 175

9.8 Polycyclic Aromatic Hydrocarbons 176

9.9 Even More Carbon Diversity 176

9.9.1 Prebiotic Compounds 177

9.10 Comets 178

9.11 Chirality 179

9.12 Laboratory Experiments 179

9.13 Observing these Molecules 180

9.14 Conclusions 181

Further Reading 182

10 The Early Earth (The First Billion Years) 183

10.1 The First Billion Years of the Earth 183

10.2 The Earth Forms and Differentiates 183

10.3 The Formation of the Moon 184

10.4 The Early Oceans 186

10.5 The Early Crust 187

10.6 The Early Atmosphere 188

10.7 The Temperature of the Early Earth 189

10.8 The Late Heavy Bombardment 189

10.9 Implications of the Early Environment for Life 192

10.10 Conclusion 194

Further Reading 194

11 The Origin of Life 197

11.1 Early Thoughts on the Origin of Life: Spontaneous Generation 197

11.2 Some Possible Ideas for the Origin of Life 200

11.3 The Synthesis of Organic Compounds on the Earth 200

11.3.1 Possible Reaction Pathways 202

11.4 Delivery from the Extraterrestrial Environment 204

11.5 The RNA World 206

11.6 Early Cells 208

11.7 Where did it Happen? 210

11.7.1 Deep Sea Hydrothermal Vents 210

11.7.2 Land-based Volcanic Pools 211

11.7.3 Impact Craters 211

11.7.4 Beaches 212

11.7.5 Bubbles 213

11.7.6 The Deep Sub-Surface 213

11.7.7 Mineral Surfaces 213

11.8 A Cold Origin of Life? 214

11.9 The Whole Earth as a Reactor? 214

11.10 Conclusions 214

Further Reading 214

12 Early Life on Earth 217

12.1 Early Life on the Earth 217

12.2 Early Life – Metabolisms and Possibilities 217

12.3 Isotopic Fractionation 220

12.3.1 Carbon Isotopes 221

12.4 Measuring the Isotope Fraction: The Delta Notation 221

12.5 Sulfur Isotope Fractionation 223

12.6 Using Ancient Isotopes to Look for Life 223

12.7 Morphological Evidence for Life 225

12.7.1 How are Microorganisms Fossilised? 225

12.7.2 Evidence for Fossil Microbial Life 225

12.7.3 Stromatolites 229

12.8 Biomarkers 230

12.9 The Search for Extraterrestrial Life 230

12.10 Conclusions 231

Further Reading 231

13 The History of the Earth 233

13.1 The Geological History of the Earth 233

13.2 Minerals and Glasses 233

13.3 Types of Rocks 235

13.3.1 Igneous Rocks 235

13.3.2 Sedimentary Rocks 235

13.3.3 Metamorphic Rocks 236

13.4 The Rock Cycle 237

13.5 The Composition of the Earth 239

13.6 The Earth’s Crust and Upper Mantle 239

13.7 Plate Tectonics 240

13.8 Dating Rocks 244

13.9 Age-dating Rocks 246

13.9.1 Absolute Dating of Rocks 246

13.9.2 Relative Dating 250

13.9.3 Unconformities 251

13.10 Geological Time Scales 252

13.11 The Major Classifications of Geological Time 252

13.12 Some Geological Times and Biological Changes 254

13.12.1 The Precambrian 254

13.12.2 The Phanerozoic: The Rise of Animals and Complexity 254

13.13 Conclusion 259

Further Reading 260

14 The Rise of Oxygen 261

14.1 Dramatic Changes on the Earth 261

14.2 Measuring Oxygen Through Time 262

14.2.1 Minerals that Form at Low Oxygen Concentrations 262

14.2.2 Changes in the Oxidation State of Elements 263

14.2.3 Banded Iron Formations and their Isotopes 264

14.2.4 Sulfur Isotope Fractionation 264

14.3 Summarising the Evidence for the Great Oxidation Event 265

14.4 The Source of Oxygen 266

14.5 Sinks for Oxygen 266

14.6 Why did Oxygen Rise? 267

14.7 Snowball Earth Episodes 268

14.8 Other Biological Consequences of the Rise of Oxygen 270

14.9 Oxygen and the Rise of Animals 271

14.10 Periods of High Oxygen 272

14.11 Conclusions 273

Further Reading 273

15 Mass Extinctions 275

15.1 Extinction 275

15.2 What is Extinction? 275

15.3 Five Major Mass Extinctions 277

15.4 Other Extinctions in Earth History 278

15.5 Causes of Mass Extinction 278

15.6 The End-Cretaceous Extinction 279

15.7 The Other Four Big Extinctions of the Phanerozoic 284

15.7.1 End-Ordovician Mass Extinction 284

15.7.2 Late Devonian Mass Extinction 285

15.7.3 The Largest of all Mass Extinctions: The End-Permian Extinction 285

15.7.4 End-Triassic Mass Extinction 287

15.8 Impacts and Extinction 287

15.9 Some Questions About Extinctions and Life 287

15.10 The Sixth Mass Extinction? 289

15.11 Conclusions 290

Further Reading 290

16 The Habitability of Planets 291

16.1 What is Habitability? 291

16.2 The Habitable Zone 292

16.2.1 Star Types 293

16.2.2 Continuously Habitable Zone 294

16.2.3 The Galactic Habitable Zone 294

16.2.4 The Right Galaxy? 295

16.3 Maintaining Temperature Conditions on a Planet Suitable for Water and Life 295

16.3.1 Effective Temperature and the Greenhouse Effect 295

16.3.2 The Carbonate–Silicate Cycle 297

16.4 Plate Tectonics 299

16.5 Do We Need a Moon? 300

16.6 Surface Liquid Water, Habitability and Intelligence 301

16.7 Uninhabited Habitats: Habitats Need Not Always Contain Life 301

16.8 Worlds More Habitable than the Earth? 302

16.9 The Anthropic Principle 303

16.10 The Fate of the Earth 303

16.11 Conclusions 304

Further Reading 304

17 The Astrobiology of Mars 307

17.1 Mars and Astrobiology 307

17.2 Martian History: A Very Brief Summary 308

17.3 The Deterioration of Mars 309

17.4 Missions to Mars 311

17.5 Mars and Life 314

17.5.1 Liquid Water and Mars 314

17.5.2 Basic Elements for Life on Mars 320

17.5.3 Trace Elements for Life on Mars 321

17.5.4 Energy and Redox Couples for Life on Mars 321

17.5.5 Physical Limits to Life: Radiation 324

17.5.6 Physical Limits to Life: pH 324

17.5.7 Physical Limits to Life: Salts 325

17.5.8 Habitat Space for Microbes on Mars 325

17.6 Trajectories of Martian Habitability 325

17.6.1 Trajectories for an Uninhabited Mars 326

17.6.2 Trajectories for an Inhabited Mars 328

17.7 The Viking Programme and the Search for Life 329

17.7.1 GCMS Analysis 329

17.7.2 Gas Exchange Experiment 329

17.7.3 Labelled Release Experiment 330

17.7.4 Pyrolytic Release Experiment 330

17.7.5 Viking: A Lesson in Science 331

17.8 Martian Meteorites 331

17.9 Mars Analogue Environments 333

17.10 Panspermia – Transfer of Life Between Planets? 333

17.10.1 Ejection from a Planet 334

17.10.2 Interplanetary Transfer Phase 335

17.10.3 Arriving at the Destination Planet 336

17.11 Conclusions 338

Further Reading 338

18 The Moons of Giant Planets 341

18.1 The Astrobiology of Moons 341

18.2 The Moons of Jupiter: Europa 342

18.2.1 A Sub-Surface Ocean? 344

18.3 The Moons of Jupiter: Ganymede and Callisto 347

18.4 The Moons of Jupiter: Io 348

18.5 The Moons of Saturn: Enceladus 349

18.5.1 The Plumes of Enceladus 349

18.6 The Moons of Saturn: Titan 352

18.7 Other Icy Worlds 359

18.7.1 Triton 359

18.7.2 Ceres 359

18.7.3 Pluto 360

18.8 Planetary Protection 360

18.9 Conclusions 362

Further Reading 362

19 Exoplanets: The Search for Other Habitable Worlds 363

19.1 Exoplanets and Life 363

19.2 Detecting Exoplanets 364

19.3 Transit Method for Detecting Exoplanets 364

19.4 Doppler Shift/Radial Velocity Method of Detecting Exoplanets 366

19.5 Astrometry 368

19.6 Variations in Other Attributes of Stars 368

19.7 Orbital Brightness Changes 368

19.8 Gravitational Lensing 368

19.9 Direct Detection 369

19.10 Using Direct Detection to Study Protoplanetary Discs 369

19.11 Exoplanet Properties 371

19.11.1 General Properties 371

19.11.2 Hot Jupiters and Neptunes 371

19.11.3 Super-Earths and Ocean Worlds 371

19.11.4 Rocky Planets in the Habitable Zone 374

19.11.5 Planets in Binary and Multiple Star Systems 374

19.11.6 Strange Worlds 375

19.12 Detecting Life 376

19.12.1 Biosignature Gases 376

19.12.2 Surface Biosignatures 380

19.12.3 How Likely are These Signatures? 381

19.13 Conclusions 382

Further Reading 382

20 The Search for Extraterrestrial Intelligence 385

20.1 The Search for Extraterrestrial Intelligence 385

20.2 The Drake Equation 386

20.3 Methods in the Search for Extraterrestrial Intelligence 387

20.4 Communication with Extraterrestrial Intelligence 389

20.5 The Fermi Paradox 391

20.5.1 Civilisations are too Far Apart in Space 393

20.5.2 No Other, or Very Few, Civilisations have Arisen 393

20.5.3 Intelligent Life Destroys Itself 394

20.5.4 Life is Periodically Destroyed by Natural Events 394

20.5.5 It is the Nature of Intelligent Life to Destroy Other Civilisations 394

20.5.6 They Exist, But We See No Evidence of Them 394

20.5.7 They are in the Local Area, But Observing us Rather Than Attempting to Make Contact 395

20.5.8 They are too Busy Online 395

20.5.9 They are Here 395

20.5.10 The Evidence is Being Suppressed 395

20.6 Classifying Civilisations 396

20.7 Policy Implications 397

20.8 Conclusions 398

Further Reading 398

21 Our Civilisation 399

21.1 Astrobiology and Human Civilisation 399

21.2 The Emergence of Human Society 399

21.3 Threats to a Civilisation 402

21.4 Climate Change and the Challenge to Seven Billion Apes 404

21.5 The Human Future Beyond the Earth 407

21.5.1 The Rocket Equation 407

21.6 Settling the Solar System 408

21.7 Avoiding Extinction or Collapse: A Multiplanet Species 411

21.8 Environmentalism and Space Exploration as a Single Goal? 413

21.9 Sociology: The Overview Effect 414

21.10 Will We Become Interstellar? 414

21.11 Conclusions 415

Further Reading 415

Appendix 417

A.1 The Astrobiological Periodic Table 417

A.2 Units and Scales 417

A.2.1 Standard International Base Units 417

A.2.2 Basic Physical Constants 418

A.3 Temperature Scale Conversion 418

A.4 Composition of the Sun 419

A.5 Some of the Major Star Types and Temperatures and Colour 419

A.6 Three- and One-letter Designations of Amino Acids 419

A.7 Codon Table for the Genetic Code (also shown in Chapter 4; Figure 4.12) 420

A.8 Planetary Data 421

A.9 Geological Time Scale 421

Glossary 423

Index 443