Comets and the Origin and Evolution of Life

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1 Paul J. Thomas Christopher F. Chyba Christopher P. McKay Editors Comets and the Origin and Evolution of Life With 47 Illustrations Springer

2 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 Comets and the Origin of Life: An Idea with a Long History Chemical Evolution of Cometary Nuclei The Collisional History of the Early Solar System A Cometary Origin for the Terrestrial Volatiles? Comets and Prebiotic Synthesis Cometary Collisions and Biological Evolution References 19 2 The Origin of the Atmosphere and of the Oceans 29 A. Delsemme 2.1 Introduction Hypothesis of the Volcanic Origin The Missing Primary Atmosphere The Origin of the Solar System Existence of Accretion Disks _Numerical Models for a Protosolar Accretion Disk The Chondrites as Clues on Planetary Formation From Dust to Planets Temperature History of the Earth's Material Thermochemical Equilibrium in Solar Nebula Discussion: Was the Earth Outgassed? Formation of the Giant Planets Orbital Diffusion of Comets Chronology Chronology Discussion Observational Confirmations Cratering Record Geochemistry Geochemical Model Noble Gases 55

3 vi Contents Deuterium Nature of the Early Atmosphere Prebiotic Organic Syntheses Summary Verified Predictions of the Model Unverified Predictions of the Model Conclusion 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 Radiation and Hot Atom Chemical Processes in Space Radiation Sources in Space Hot Species in Space Target Systems in Space Dualism of Radiation Chemistry and Hot Atom Chemistry Fundamental Suprathermal Reaction Mechanisms Complex Organic Matter in Space In Situ Measurements Overview Impact Formation of Molecular Organic Ions Results Possible Implications References 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 The Molecular Abundances The Atomic Abundances The Organic Component of Dust Particles Laboratory Experiments Conclusions References Numerical Models of Comet and Asteroid Impacts 131 P.J. Thomas and L. Brookshaw 5.1 Introduction Atmospheric Passage of Impactors: Ablation Hydrodynamic Simulation of Impact Selection of an Appropriate Equation of State Results of Impact Simulations Airbursts and Organic Survivability 141

4 Contents vii 5.5 Conclusions 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 The Uninhabitable Habitable Zone A Procrustean Model for Lunar Cratering Lunar Cratering and Geochemical Evidence Are the Earth's Oceans Extraterrestial? Mass Flux Through Time Cautions and Uncertainties Endogenous Production of Prebiotic Organics Impact Delivery of Intact Exogenous Organics Interplanetary Dust Particles (IDPs) Interstellar Dust Meteorites Catastrophic Airbursts Big Impacts Atmospheric Shock Synthesis of Organic Molecules Shocks from Meteors Shocks from Airbursts Shocks from Giant Impact Plumes Post-Impact Recombination Implications of the K/T Organics An Inventory of Organic Production on Early Earth Organic Concentrations and Sinks Episodic Events Prebiotic Organics on the Early Earth References 168 Impacts and the Early Evolution of Life 175 K.J. Zahnle and N.H. Sleep 7.1 Prologue Introduction An Ocean-Vaporizing Impact Rock Vapor Steam Salt Survival? Imbrium on Earth Filters The Lunar Record Energies of Basin-Forming Impacts Crustal Contamination by Chondrites 190

5 viii Contents Impact Stirring Impacts on Earth Impactor Mass Distribution Scaling the Lunar Impact Record to Earth Conclusion Appendix References Cometary Impacts on the Biosphere 209 D. Steel 8.1 Introduction Astronomical Considerations The Nature of the Impactors Meteoroids and Dust The Impact of Impacts Impacts: Links to Boundary Events and Environmental Effects Extinction/Impact/Geologic Upheaval Periodicity Phase Relation Between Biological/Geological Upheavals and the Solar Motion The Contemporary Environment Panspermia References The Contemporary Hazard of Cometary Impacts 243 D. Morrison 9.1 Introduction Impactor Population Nature of the Hazard Penetration Through the Atmosphere Globally Catastrophic Impacts Threshold for a Globally Catastrophic Climate Perturbation Hazard Analysis Risk Reduction and Mitigation Impact Prediction Deflection or Destruction Problems with Comets Summary and Conclusions References A1 and Liquid Water Environments in Comets 259 M. Podolak and D. Prialnik 10.1 Introduction A1 as a Heat Source Melting of Cometary Interiors Simple Model 262

6 Contents ix 10.4 Numerical Models I: The Effect of Amorphous Ice Numerical Models II: The Effect of Trapped Gas and Porosity Conclusions References Life in Comets 273 C.P. McKay 11.1 Introduction The Origin of Life in Comets Comets as Vectors for Life Conclusion References Comets and Space Missions 283 C.P. McKay 12.1 Introduction Missions Conclusion 286 Index 289

Comet Science Goals II

Comet Science Goals II {questions for goals} Don Brownlee Did the events postulated by the Nice Hypothesis really happen? Were there wide-spread solar system wide impact events that were coeval with the