Astrobiology

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