Physics and Chemistry of Comets

1. Introduction.- 1.1 Plan of the Book.- 1.2 General Concepts.- 1.3 Orbits of Some Comets for Spacecraft Missions.- 1.4 The Oort Cloud Compared to the Planetary Orbits.- 1.5 Summary of Spacecraft Data from Comets.- 1.6 Summary of New Data from Remote Sensing.- 1.7 Reasons for Planned and Proposed Comet Missions.- 1.8 Laboratory and Space Experiments and Simulations.- 2. The Nucleus.- 2.1 Introduction.- 2.1.1 Early History.- 2.1.2 The Nucleus as Source of the Ephemeral Activity.- 2.1.3 Space Missions.- 2.2 Observational Techniques.- 2.2.1 Size Determination from Photometry at Large Heliocentric Distances.- 2.2.2 Gas and Dust Production.- 2.2.3 Nongravitational Forces.- 2.2.4 Rotation Parameters from Variability of Brightness.- 2.2.5 Infrared Observations.- 2.2.6 Radar Observations.- 2.2.7 Direct Observations.- 2.3 Models of the Nucleus.- 2.3.1 The Sand Bank Model.- 2.3.2 Nongravitational Forces.- 2.3.3 Icy Conglomerate Nucleus.- 2.3.4 Nucleus Formation.- 2.4 The Nucleus of Comet P/Halley.- 2.4.1 Expectations.- 2.4.2 The Giotto Encounter.- 2.4.3 Imaging Properties and Orientation of the Nucleus.- 2.4.4 Shape and Size.- 2.4.5 Spin of the Nucleus.- 2.4.6 Surface.- 2.4.7 Surface Features and Morphology.- 2.4.8 Activity.- 2.4.9 The Near Source Region.- 2.4.10 Global Dust Distribution.- 2.5 Modeling and Laboratory Experiments.- 2.5.1 Sublimation of an Icy Surface.- 2.5.2 Sublimation Through an Inert Layer.- 2.5.3 Implications from the Restricted Activity.- 2.5.4 The Outflow of Dust.- 2.5.5 Two-Dimensional Gas Dynamics.- 2.5.6 The Extended Surface Source Model.- 2.5.7 Comparison of the HMC Observations with Models.- 2.5.8 Laboratory Experiments to Simulate Surface Activity.- 2.5.9 Laboratory Experiments for Nucleus Formation.- 2.5.10 Evolution of the Core.- 2.5.11 Computer Experiments.- 2.6 Physical Parameters of the Nucleus.- 2.6.1 Temperature Distribution within the Nucleus.- 2.6.2 Surface Temperature.- 2.6.3 Perihelion Asymmetry.- 2.6.4 Mass.- 2.6.5 Comets, Meteoroids, and Meteor Showers.- 2.6.6 Density.- 2.6.7 Splitting and Decay.- 2.7 Summary.- 2.7.1 A New Model of Comet Nuclei.- 2.7.2 Composition and Structure.- 2.7.3 Formation.- 2.7.4 Sublimation.- 3. The Neutral Coma.- 3.1 Introduction.- 3.2 The Coma: A Transient Phenomenon.- 3.2.1 Cometary Forms.- 3.2.2 The Nucleus as the Source of Coma Gas.- 3.2.3 Grains as Distributed Sources of Coma Gas.- 3.2.4 Hydrodynamic Flow.- 3.2.5 Photolytic Processes in the Coma.- 3.2.6 Chemical Processes in the Coma.- 3.2.7 The Exosphere: Escape from the Coma.- 3.3 Excitation and Emission Mechanisms.- 3.3.1 Fluorescence.- 3.3.2 Prompt Emission.- 3.3.3 Collisional Effects.- 3.3.4 Some Specific Examples.- 3.4 Composition of the Coma.- 3.4.1 Identification of Species.- 3.4.2 Abundances in the Coma.- 3.4.3 Isotope Ratios.- 3.5 Summary and Prospectus.- 4. Dust.- 4.1 Introduction.- 4.2 What is Comet Dust?.- 4.3 Dust Release from the Nucleus.- 4.4 Dust Emission.- 4.5 Dust Coma Structure.- 4.5.1 Simple Models.- 4.5.2 Advanced Models.- 4.5.3 Observations and Measurements.- 4.6 Dust Tails.- 4.6.1 General Features.- 4.6.2 Fine Structures.- 4.6.3 Dust Trails.- 4.7 Mass Distribution.- 4.8 Optical and Infrared Properties.- 4.8.1 Scattered Light.- 4.8.2 Polarization.- 4.8.3 Absorption and Thermal Emission.- 4.9 Dust Composition.- 4.9.1 Introduction.- 4.9.2 Cometary Dust Composition Before the Halley Encounters.- 4.9.3 The PIA and PUMA Experiments.- 4.9.4 Principal Particle Types.- 4.9.5 Sizes and Densities of Grains.- 4.9.6 The Organic Component.- 4.9.7 The Isotopes.- 4.9.8 The Average Chemical Composition.- 4.9.9 Comparison to Carbonaceous Chondrites and the Sun.- 4.9.10 Variability of Mg, Si, and Fe.- 4.10 Dust Production Rate and the Dust to Gas Ratio.- 4.11 Summary.- 5. The Plasma.- 5.1 Introduction.- 5.2 Large-Scale Processes of the Solar Wind-Comet Interaction.- 5.3 Ion Composition.- 5.4 Ionospheric Processes.- 5.4.1 Ion Dynamics.- 5.4.2 Thermal Structure of the Ionosphere.- 5.4.3 Stability of the Magnetic Cavity.- 5.5 Plasma Wave Turbulence.- 5.5.1 Ultra Low Frequency Waves.- 5.5.2 High-Frequency Waves.- 5.6 Ion Acceleration.- 5.6.1 Energetic Ions in the Distant Coma.- 5.6.2 Energetic Electron Acceleration.- 5.6.3 Energetic Ions in the Inner Coma.- 5.6.4 Structure of the Bow Shock.- 6. Orbital Distribution of Comets.- 6.1 Introduction.- 6.2 Origin and Dynamical Evolution.- 6.3 Time Scales.- 6.4 The Outer Part of the Presolar Nebula.- 6.5 The Distribution of the Reciprocal Semimajor Axes.- 6.6 Other Theories for the Origin and Evolution.- 6.7 Short-Period Comets.- 6.8 Differences Between Long- and Short-Period Comets.- 7. Comet Formation and Evolution.- 7.1 Introduction.- 7.1.1 The Nucleus.- 7.1.2 The Comet Cloud.- 7.1.3 Comet Evolution.- 7.1.4 Recent Advances.- 7.2 Chemical and Physical Clues from Observations.- 7.2.1 Chemistry.- 7.2.2 Shape of the Nucleus.- 7.2.3 Structure and Density of the Nucleus.- 7.3 Formation of Comet Nuclei.- 7.3.1 Models for Giant Planet Subnebulae and the Solar Nebula.- 7.3.2 Presolar Nebula.- 7.3.3 Formation Mechanisms.- 7.3.4 Long-Term Processes.- 7.4 Dynamical Evolution of Comets.- 7.4.1 Infeed into the Planetary Region.- 7.4.2 Planetary Perturbations of Comet Orbits.- 7.5 Physical Evolution of Comet Nuclei.- 7.5.1 Some Scenarios for Comet Evolution.- 7.5.2 Observed Evolutionary Effects.- 7.6 Summary.- 8. Implications of Comet Research.- 8.1 Introduction.- 8.2 Progress in Physics and Chemistry of Comets.- 8.2.1 Composition of the Nucleus.- 8.2.2 Structure of the Nucleus.- 8.2.3 Plasma Structures in Coma and Tail.- 8.2.4 Orbital Theories.- 8.3 Progress on the History of the Early Solar System.- 8.4 Comets and the Origins of Life.- 8.4.1 The Origins of Life.- 8.4.2 Organic Materials in Comets.- 8.4.3 Comet Transport of Volatiles.- 8.4.4 The Role of Comets.- 8.5 Unsolved and New Problems.- 8.5.1 Nucleus Rotation and Surface.- 8.5.2 Chemical Abundances in the Coma and the Nucleus.- 8.5.3 Dust Tail.- 8.5.4 Coma and Tail Plasma.- References.