Paul J. Thomas Christopher F. Chyba Christopher P. McKay Editors Comets and the Origin and Evolution of Life With 47 Illustrations Springer
Contents Contributors xi Introduction: Comets and the Origin of Life 1 P.J. Thomas, C.F. Chyba, and C.P. McKay 1 Comets and the Origin and Evolution of Life 3 J. Oro and A. Lazcano 1.1 Introduction 3 1.2 Comets and the Origin of Life: An Idea with a Long History.. 4 1.3 Chemical Evolution of Cometary Nuclei 6 1.4 The Collisional History of the Early Solar System 8 1.5 A Cometary Origin for the Terrestrial Volatiles? 10 1.6 Comets and Prebiotic Synthesis 13 1.7 Cometary Collisions and Biological Evolution 15 1.8 References 19 2 The Origin of the Atmosphere and of the Oceans 29 A. Delsemme 2.1 Introduction 29 2.2 Hypothesis of the Volcanic Origin 29 2.2.1 The Missing Primary Atmosphere 30 2.2.2 The Origin of the Solar System 31 2.3 Existence of Accretion Disks 32 2.4 _Numerical Models for a Protosolar Accretion Disk 33 2.5 The Chondrites as Clues on Planetary Formation 34 2.6 From Dust to Planets 37 2.7 Temperature History of the Earth's Material 37 2.8 Thermochemical Equilibrium in Solar Nebula 38 2.9 Discussion: Was the Earth Outgassed? 40 2.10 Formation of the Giant Planets 43 2.11 Orbital Diffusion of Comets 45 2.12 Chronology 48 2.13 Chronology Discussion 50 2.14 Observational Confirmations 52 2.14.1 Cratering Record 52 2.14.2 Geochemistry 53 2.14.3 Geochemical Model 53 2.14.4 Noble Gases 55
vi Contents 2.14.5 Deuterium 56 2.15 Nature of the Early Atmosphere 57 2.16 Prebiotic Organic Syntheses 58 2.17.Summary 58 2.17.1 Verified Predictions of the Model 61 2.17.2 Unverified Predictions of the Model 62 2.18 Conclusion 62 2.19 References 62 3 Organic Chemistry in Comets From Remote and In Situ Observations 69 J. Kissel, F.R. Krueger, and K. Roessler 3.1 Introduction 69 3.2 Radiation and Hot Atom Chemical Processes in Space 71. 3.2.1 Radiation Sources in Space 71 3.2.2 Hot Species in Space 72 3.2.3 Target Systems in Space 76 3.2.4 Dualism of Radiation Chemistry and Hot Atom Chemistry 77 3.2.5 Fundamental Suprathermal Reaction Mechanisms... 79 3.2.6 Complex Organic Matter in Space 86 3.3 In Situ Measurements 90 3.3.1 Overview 90 3.3.2 Impact Formation of Molecular Organic Ions 99 3.3.3 Results 102 3.3.4 Possible Implications 104 3.4 References 107 4 Polymers and Other Macromolecules in Comets 111 W.F. Huebner and D.C. Boice 4.1 Introduction Ill 4.2 The Mass Ratio of Dust-to-Gas Emission in Comets 113 4.3 The Molecular Abundances 115 4.4 The Atomic Abundances 120 4.5 The Organic Component of Dust Particles 122 4.6 Laboratory Experiments 123 4.7 Conclusions 124 4.8 References 125 5 Numerical Models of Comet and Asteroid Impacts 131 P.J. Thomas and L. Brookshaw 5.1 Introduction 131 5.2 Atmospheric Passage of Impactors: Ablation 132 5.3 Hydrodynamic Simulation of Impact 134 5.3.1 Selection of an Appropriate Equation of State 134 5.3.2 Results of Impact Simulations 135 5.4 Airbursts and Organic Survivability 141
Contents vii 5.5 Conclusions 144 5.6 References 144 Comets as a Source of Prebiotic Organic Molecules for the Early Earth 147 C.F. Chyba and C. Sagan 6.1 The Lost Record of the Origin of Life 147 6.2 The Uninhabitable Habitable Zone 148 6.3 A Procrustean Model for Lunar Cratering 150 6.4 Lunar Cratering and Geochemical Evidence 153 6.5 Are the Earth's Oceans Extraterrestial? 155 6.6 Mass Flux Through Time 155 6.7 Cautions and Uncertainties 156 6.8 Endogenous Production of Prebiotic Organics 157 6.9 Impact Delivery of Intact Exogenous Organics 157 6.9.1 Interplanetary Dust Particles (IDPs) 158 6.9.2 Interstellar Dust 159 6.9.3 Meteorites 159 6.9.4 Catastrophic Airbursts 159 6.9.5 Big Impacts 159 6.10 Atmospheric Shock Synthesis of Organic Molecules 160 6.10.1 Shocks from Meteors 161 6.10.2 Shocks from Airbursts 161 6.10.3 Shocks from Giant Impact Plumes 161 6.11 Post-Impact Recombination 162 6.12 Implications of the K/T Organics 162 6.13 An Inventory of Organic Production on Early Earth 164 6.14 Organic Concentrations and Sinks 166 6.15 Episodic Events 166 6.16 Prebiotic Organics on the Early Earth 167 6.17 References 168 Impacts and the Early Evolution of Life 175 K.J. Zahnle and N.H. Sleep 7.1 Prologue 175 7.2 Introduction 176 7.3 An Ocean-Vaporizing Impact 177 7.3.1 Rock Vapor 178 7.3.2 Steam 181 7.3.3 Salt 182 7.3.4 Survival? 183 7.3.5 Imbrium on Earth 185 7.3.6 Filters 186 7.4 The Lunar Record 187 7.4.1 Energies of Basin-Forming Impacts 188 7.4.2 Crustal Contamination by Chondrites 190
viii Contents 7.4.3 Impact Stirring 192 7.5 Impacts on Earth 193 7.5.1 Impactor Mass Distribution 193 7.5.2 Scaling the Lunar Impact Record to Earth 195 7.6 Conclusion 198 7.7 Appendix 201 7.8 References 204 8 Cometary Impacts on the Biosphere 209 D. Steel 8.1 Introduction 209 8.2 Astronomical Considerations 215 8.2.1 The Nature of the Impactors 215 8.2.2 Meteoroids and Dust 217 8.3 The Impact of Impacts 221 8.3.1 Impacts: Links to Boundary Events and Environmental Effects 223 8.3.2 Extinction/Impact/Geologic Upheaval Periodicity... 226 8.3.3 Phase Relation Between Biological/Geological Upheavals and the Solar Motion 231 8.4 The Contemporary Environment 232 8.5 Panspermia 232 8.6 References 233 9 The Contemporary Hazard of Cometary Impacts 243 D. Morrison 9.1 Introduction 243 9.2 Impactor Population 245 9.3 Nature of the Hazard 246 9.3.1 Penetration Through the Atmosphere 246 9.3.2 Globally Catastrophic Impacts 248 9.3.3 Threshold for a Globally Catastrophic Climate Perturbation249 9.4 Hazard Analysis 249 9.5 Risk Reduction and Mitigation 252 9.5.1 Impact Prediction 252 9.5.2 Deflection or Destruction 253 9.5.3 Problems with Comets 254 9.6 Summary and Conclusions 255 9.7 References 256 10 26 A1 and Liquid Water Environments in Comets 259 M. Podolak and D. Prialnik 10.1 Introduction 259 10.2 26 A1 as a Heat Source 261 10.3 Melting of Cometary Interiors Simple Model 262
Contents ix 10.4 Numerical Models I: The Effect of Amorphous Ice 263 10.5 Numerical Models II: The Effect of Trapped Gas and Porosity.. 265 10.6 Conclusions 270 10.7 References 271 11 Life in Comets 273 C.P. McKay 11.1 Introduction 273 11.2 The Origin of Life in Comets 275 11.3 Comets as Vectors for Life 277 11.4 Conclusion 279 11.5 References 279 12 Comets and Space Missions 283 C.P. McKay 12.1 Introduction 283 12.2 Missions 284 12.3 Conclusion 286 Index 289