Astrobiology

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.

Acknowledgements xv 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