~ LIBRARY

ASTRONOMY AND ASTROPHYSICS LIBRARY

Series Editors: M. Harwit, R. Kippenhahn, V. Trimble, J.-P. Zahn

Advisory Board:

J. N. Bahcall P. L. Biermann S. Chandrasekhar S. M. Faber L. V. Kuhi P. G . Mezger P. A. Strittmatter

[N\] LIBRARY

Series Editors:

ASTRONOMY AND ASTROPHYSICS LIBRARY

M. Harwit, R. Kippenhahn, V. Trimble, J.-P. Zahn

Tools of Radio Astronomy By K. Rohlfs

Physics of the Galaxy and Interstellar Matter By H. Scheffler and H. Elsasser

Galactic and Extragalactic Radio Astronomy 2nd Edition Editors: G . L. Verschuur, K. 1. Kellermann

Observational Astrophysics By P. Lena

Astrophysical Concepts 2nd Edition By M. Harwit

The Sun An Introduction By M. Stix

Stellar Structure and Evolution By R. Kippenhahn and A. Weigert

Relativity in Astrometry, Celestial Mechanics and Geodesy By M. H. Soffel

The Solar System By T. Encrenaz and J.-P. Bibring

Physics and Chemistry of Comets Editor: W. F. Huebner

Supernovae Editor: A. Petschek

Walter F. Huebner (Ed.)

Physics and Chemistry of Comets

With a Foreword by Fred L.Whipple

With 147 Figures

Springer-Verlag Berlin Heidelberg New York London Paris Tokyo Hong Kong Barcelona

Dr. Walter F. Huebner

Southwest Research Institute, Div. 15, P.O. Drawer 28510, 6220 Culebra Road, San Antonio, TX 78228-0510, USA

Series Editors

Martin Harwit The National Air and Space Museum, Smithsonian Institution 7th St. and Independence Ave. S.W Washington, DC 20560, USA

Rudolf Kippenhahn Max-Planck-Institut fiir Physik und Astrophysik Institut fiir Astrophysik Karl-Schwarzschild-StraBe 1 D-8046 Garching, Fed. Rep. of Germany

Virginia Trimble Astronomy Program University of Maryland College Park, MD 20742, USA and Department of Physics University of California Irvine, CA 92717, USA

Jean-Paul Zahn Universite Paul Sabatier Observatoires du Pie-du-Midi et de Toulouse 14, Avenue Edouard-Belin F-31400 Toulouse, France

Cover picture: The nucleus of Comet Halley as observed from the Giotto spacecraft. The pseudocolor image is composed of six images with increasing resolution toward the brightest part, and the visibility of the dark side is enhanced by a masking technique.

ISBN-13:978-3-642-74807-3 e-ISBN-13 :978-3-642-74805-9 DOl: 10.1007/978-3-642-74805-9

Library of Congress Cataloging-in-Publication Data. Physics and chemistry of comets / Walter F. Huebner, ed. p. cm. - (Astronomy and astrophysics library). Includes bibliographical references (p.) and index. ISBN-13:978-3-642-74807-3 1. Comets. 2. Astrophysics. I. Huebner, W F. (Walter F.), 1928-. II. Series. QB721.P49 1990 523.6 - dc20 90-10100

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Foreword

As this excellent book demonstrates, the study of comets has now reached the fas­cinating stage where we understand comets in general simple tenns while, at the same time, we are uncertain about practically all the details of cometary nature, structure, processes, and origin. In every aspect, even including dynamics, a choice among several or many competing theories is made impossible simply by the lack of detailed knowledge. The space missions, snapshot studies of two comets, partic­ularly the one that immortalizes the name of Sir Edmund Halley, have produced a huge mass of valuable new infonnation and a number of surprises. Nonetheless, we face the tantalizing realization that we have obtained only a fleeting glance at two of perhaps a hundred billion (lOll) or more comets with possibly differing natures, origins, and physical histories.

To my personal satisfaction, comets seem to have discrete nuclei made up of dirty snowballs, as I concluded four decades ago, but perhaps they are more like frozen rubbish piles. Almost certainly they are debris left over from the building of the solar system, but just how? And just where? Are comets frozen froth or a mixture of solids and voids? How big are the hunks, if there are hunks? Were they fonned from silicate dust cores with icy mantles or are they just a mishmash of colliding atoms and ions? Have the centers of the biggest ones been melted by radioactivity? Do some or many end up as inert Earth-crossing asteroids, a perpetual hazard to life on Earth? Even the density of the Comet P/Halley nucleus still remains uncertain although it is our best detennination.

This book is a thorough presentation of our current knowledge of comets, cov­ering all ramifications, including the one area in which comets are actually useful to modem physics. Comets are, indeed, space laboratories for the study of magne­tohydrodynamics, a frightful word standing for the processes occurring in hot gases or plasmas containing charged particles or ions along with magnetic fields such that the clouds of gas are partly or primarily controlled by the magnetic fields. Because most of the known universe consists of plasmas, the subject is vitally important in modem astronomy.

Nearer to home, comets are intimately related to the origin of the Earth, our Solar System, and probably to star fonnation generally. Perhaps they made life possible on Earth, but perhaps not. They may be prevalent in galaxies, possibly next to stars in total mass although no one seriously suspects them of providing the mysterious extra mass needed to hold our universe together.

When I say that this book is a thorough presentation, I note that the serious student who wishes to proceed farther with cometary studies will be able, if he or

VI Foreword

she chooses, to consult nearly eight hundred references in the literature, listed in an appendix where they do not clutter the clear-cut subject presentations. For the hasty reader a concise summary at the end provides an overview which may also be an alert for topics of individual special interest.

We all hope that the next big step in understanding comets will be space missions, probing not only their tails and comae but the actual nuclei themselves.

Fred L. Whipple

Director Emeritus Smithsonian Astrophysical Observatory,

Phillips Professor of Astronomy Emeritus Harvard University

Preface

This book provides a comprehensive review of our understanding of comets, fol­lowing the spacecraft investigations of Comets Giacobini-Zinner and Halley. The relevant physical and chemical processes are discussed, starting with the quasi­permanent celestial body, the comet nucleus. This is followed by discussions, ordered according to the ephemeral development of a comet, of the neutral coma, the dust coma and tail, and the plasma components in the coma and tail, including the solar wind interaction. Furthermore, the properties of a comet nucleus are deduced. The traditional description of the Oort cloud is supplemented by much-debated theoreti­cal discussions connecting the measured physico-chemical data of comets via their observed orbits to the more speculative origin and evolution of comet nuclei. The book concludes with an overview of the conditions of the nebula in which comets formed, implications for the origins of life, and prospects for future research.

Many of the processes described are interlinked so that parts of descriptions may be found in several places. However, cross references and an exhaustive index aid the reader in finding his or her way. Although researchers will find the book useful as a reference text, it is intended for advanced undergraduate and graduate students in space science, planetary science, astrophysics, and astrochemistry. New entrants to the field of comet observations and comet science will appreciate the treatment of conceptual material. The book will be useful for the scientific implications and the planning of new comet missions. It can serve as a guide to the type of observation and work that are still needed, with some suggestions on how to approach them and where to find additional data.

Special thanks go to all the members of the Halley Multicolour Camera team for the exciting cooperation and discussions in the preparation of the chapter on "The Nucleus"; W. Curdt, R. Kramm, and N. Thomas helped to produce most of the images of the nucleus and its dusty environment. Thanks go also to the many colleagues at the Max-Planck-Institut flir Aeronomie who provided useful information for the chapter on "The Plasma" and engaged in many stimulating discussions with the authors. Indebtedness is expressed to E. van den Heuvel for a number of valuable suggestions on the chapter on the "Orbital Distribution of Comets".

On a personal note, it is a pleasure to thank my colleagues and coauthors for their patience and their collaborative spirit, without which the coordination of the material for this book would not have been possible. I wish to express my sincere appreciation to the many dedicated "behind-the-scenes" reviewers for their invaluable counsel and for correcting errors and misconceptions in the text. Comments and critical

VIII Preface

remarks on especially difficult sections were made by the authors on each other's chapters as well as by Claude Arpigny, Daniel C. Boice, Bertram D. Donn, Tamas I. Gombosi, Ichishiro Konno, and Stuart J. Weidenschilling. Sincere thanks are also due to the many colleagues who supplied figures from their own research and made special efforts to revise them for inclusion in this book. Hermann U. Schmidt and R. Wegmann deserve special mention in this regard.

Very special thanks are due to Mrs. loge Gehne for her extraordinary efforts in preparing the manuscript on "The Plasma" and to Richard Spinks for his skill and good humor in proofreading and correcting the text and for his help in setting it into Tpc.

Much of my own research has been generously supported by the Planetary Atmo­spheres Program of NASA's Solar System Exploration Division. I especially wish to acknowledge the enthusiastic support I received from the management and staff of the Southwest Research Institute. Considering all the help that I received, I wonder how much I really contributed to this book.

San Antonio, June 1990 Walter F. Huebner

Table of Contents

1. Introduction By Walter F. Huebner (With 3 Figures) . . .. . . . . . . . . . . . . . . . . . .. . . 1 1.1 Plan of the Book ....................................... 1 1.2 General Concepts ....................................... 2 1.3 Orbits of Some Comets for Spacecraft Missions .............. 3 1.4 The Oort Cloud Compared to the Planetary Orbits ............ 5 1.5 Summary of Spacecraft Data from Comets .................. 6 1.6 Summary of New Data from Remote Sensing ................ 7 1.7 Reasons for Planned and Proposed Comet Missions ........... 8 1.8 Laboratory and Space Experiments and Simulations ........... 11

2. The Nucleus By H. Uwe Keller (With 14 Figures) ............................ 13 2.1 Introduction ........................................... 13

2.1.1 Early History .................................... 14 2.1.2 The Nucleus as Source of the Ephemeral Activity ...... 14 2.1.3 Space Missions .................................. 16

2.2 Observational Techniques ................................ 16 2.2.1 Size Determination from Photometry at Large

Heliocentric Distances ............................ 17 2.2.2 Gas and Dust Production .......................... 18 2.2.3 Nongravitational Forces ........................... 21 2.2.4 Rotation Parameters from Variability bf Brightness ..... 22 2.2.5 Infrared Observations ............................. 22 2.2.6 Radar Observations ............................... 23 2.2.7 Direct Observations ............ . . . . . . . . . . . . . . . . . . 23

2.3 Models of the Nucleus ................................... 23 2.3.1 The Sand Bank Model ............................ 24 2.3.2 Nongravitational Forces ........................... 24 2.3.3 Icy Conglomerate Nucleus . . . . .. . . . . . . . . . . . . . . .. . . 24 2.3.4 Nucleus Formation ............................... 25

2.4 The Nucleus of Comet P/Halley ........................... 26 2.4.1 Expectations .................................... 26 2.4.2 The Giotto Encounter .. . . . . . . . . . . . . . . . . . . . . . . . . . . 26 2.4.3 Imaging Properties and Orientation of the Nucleus ..... 28 2.4.4 Shape and Size .................................. 30

X Table of Contents

2.4.5 Spin of the Nucleus ......... '. . . . . . . . . . . . . . . . . . . . . 34 2.4.6 Surface ........................................ 35 2.4.7 Surface Features and Morphology ................... 38 2.4.8 Activity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 2.4.9 The Near Source Region .......................... 41 2.4.10 Global Dust Distribution .... . . . . . . . . . . . . . . . . . . . . . . 43

2.5 Modeling and Laboratory Experiments ...................... 45 2.5.1 Sublimation of an Icy Surface ...................... 45 2.5.2 Sublimation Through an Inert Layer ................. 45 2.5.3 Implications from the Restricted Activity ............. 46 2.5.4 The Outflow of Dust ............................. 48 2.5.5 Two-Dimensional Gas Dynamics .................... 50 2.5.6 The Extended Surface Source Model ................ 52 2.5.7 Comparison of the HMC Observations with Models .... 53 2.5.8 Laboratory Experiments to Simulate Surface Activity ... 54 2.5.9 Laboratory Experiments for Nucleus Formation ........ 55 2.5.10 Evolution of the Core ............................. 56 2.5.11 Computer Experiments ............................ 56

2.6 Physical Parameters of the Nucleus ........................ 57 2.6.1 Temperature Distribution within the Nucleus .......... 57 2.6.2 Surface Temperature .............................. 60 2.6.3 Perihelion Asymmetry ............................ 61 2.6.4 Mass .......................................... 61 2.6.5 Comets, Meteoroids, and Meteor Showers ............ 63 2.6.6 Density ........................................ 64 2.6.7 Splitting and Decay .............................. 65

2.7 Summary ............................................. 66 2.7.1 A New Model of Comet Nuclei .................... 66 2.7.2 Composition and Structure ......................... 67 2.7.3 Formation ...................................... 68 2.7.4 Sublimation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

3. The Neutral Coma By Michael F. A'Hearn and Michel C. Festou (With 17 Figures) 69 3.1 Introduction ........................................... 69 3.2 The Coma: A Transient Phenomenon ....................... 70

3.2.1 Cometary Forms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 3.2.2 The Nucleus as the Source of Coma Gas ............. 72 3.2.3 Grains as Distributed Sources of Coma Gas ........... 75 3.2.4 Hydrodynamic Flow .............................. 78 3.2.5 Photolytic Processes in the Coma ................... 82 3.2.6 Chemical Processes in the Coma .................... 88 3.2.7 The Exosphere: Escape from the Coma .............. 89

3.3 Excitation and Emission Mechanisms ....................... 91 3.3.1 Fluorescence. . . . . . . .. . . . . . . . . . . . . . . . .. . . . . . . . . . . 91

Table of Contents XI

3.3.2 Prompt Emission ..... . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 3.3.3 Collisional Effects ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 3.3.4 Some Specific Examples .......................... 96

3.4 Composition of the Coma ................................ 102 3.4.1 Identification of Species . . . . . . . . . . . . . . . . . . . . . . . . . . 102 3.4.2 Abundances in the Coma .......................... 105 3.4.3 Isotope Ratios ................................... 109

3.5 Summary and Prospectus ................................. 110

4. Dust By Eberhard Griin and Elmar Jessberger (With 47 Figures) . . . . . . . . 113 4.1 Introduction ........................................... 113 4.2 What is Comet Dust? .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 4.3 Dust Release from the Nucleus ............................ 120 4.4 Dust Emission ........... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 4.5 Dust Coma Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

4.5.1 Simple Models ... . . . . . . . . .. . . .. . . .. . .. . . . . . . . . . 126 4.5.2 Advanced Models ................................ 129 4.5.3 Observations and Measurements .................... 131

4.6 Dust Tails ............................................. 142 4.6.1 General Features ................................. 142 4.6.2 Fine Structures .................................. 147 4.6.3 Dust Trails ...... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148

4.7 Mass Distribution ....................................... 150 4.8 Optical and Infrared Properties ............................ 152

4.8.1 Scattered Light .................................. 152 4.8.2 Polarization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 4.8.3 Absorption and Thermal Emission .................. 158

4.9 Dust Composition ...................................... 159 4.9.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 4.9.2 Cometary Dust Composition Before the Halley Encounters 159 4.9.3 The PIA and PUMA Experiments ................... 160 4.9.4 Principal Particle Types ........................... 161 4.9.5 Sizes and Densities of Grains ...................... 163 4.9.6 The Organic Component .......................... 163 4.9.7 The Isotopes .................................... 165 4.9.8 The Average Chemical Composition ................. 168 4.9.9 Comparison to Carbonaceous Chondrites and the Sun ... 169 4.9.10 Variability of Mg, Si, and Fe ....................... 170

4.10 Dust Production Rate and the Dust to Gas Ratio .............. 172 4.11 Summary ............................................. 175

5. The Plasma By Wing-H. Ip and Ian Axford (With 45 Figures) .................. 177 5.1 Introduction ........................................... 177

xn Table of Contents

5.2 Large-Scale Processes of the Solar Wind-Comet Interaction 178 5.3 Ion Composition ........... . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 5.4 Ionospheric Processes ................................... 198

5.4.1 Ion Dynamics ................................... 199 5.4.2 Thermal Structure of the Ionosphere ................. 206 5.4.3 Stability of the Magnetic Cavity .................... 211

5.5 Plasma Wave Turbulence ................................. 212 5.5.1 Ultra Low Frequency Waves . . .. . . . . . . . . . . . . . . . . . . 214 5.5.2 High-Frequency Waves ........................... 220

5.6 Ion Acceleration ........................................ 223 5.6.1 Energetic Ions in the Distant Coma .................. 223 5.6.2 Energetic Electron Acceleration ..................... 230 5.6.3 Energetic Ions in the Inner Coma ................... 231 5.6.4 Structure of the Bow Shock ........................ 232

6. Orbital Distribution of Comets By Jan H. Oort (With 3 Figures) ............................... 235 6.1 Introduction ........................................... 235 6.2 Origin and Dynamical Evolution .......................... 238 6.3 Time Scales ........................................... 240 6.4 The Outer Part of the Presolar Nebula . . . . . . . . . . . . . . . . . . . . . 242 6.5 The Distribution of the Reciprocal Semimajor Axes ........... 242 6.6 Other Theories for the Origin and Evolution ................. 243 6.7 Short-Period Comets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 6.8 Differences Between Long- and Short-Period Comets ......... 244

7. Comet Formation and Evolution By Hans Rickman and Walter F. Huebner (With 17 Figures) . . . . . . . . 245 7.1 Introduction ........................................... 245

. 7.1.1 The Nucleus .................................... 246 7.1.2 The Comet Cloud ................................ 247 7.1.3 Comet Evolution ................................. 249 7.1.4 Recent Advances ................................ 250

7.2 Chemical and Physical Clues from Observations .............. 251 7.2.1 Chemistry ....................................... 253 7.2.2 Shape of the Nucleus ............................. 257 7.2.3 Structure and Density of the Nucleus ................ 259

7.3 Formation of Comet Nuclei .............................. 262 7.3.1 Models for Giant Planet Subnebulae and the Solar Nebula 264 7.3.2 Presolar Nebula .................................. 264 7.3.3 Formation Mechanisms ........................... 265 7.3.4 Long-Term Processes . . . . .. . . . .. . . . . . . . . . . . . . . . . . 268

7.4 Dynamical Evolution of Comets ........................... 269 7.4.1 Infeed into the Planetary Region .................... 272 7.4.2 Planetary Perturbations of Comet Orbits .............. 277

Table of Contents xm

7.5 Physical Evolution of Comet Nuclei ........................ 284 7.S.1 Some Scenarios for Comet Evolution ................ 284 7.S.2 Observed Evolutionary Effects ..................... 292

7.6 Summary ............................................. 302

8. Implications of Comet Research By Walter F. Huebner and Christopher P. McKay (With 1 Figure) 30S 8.1 Introduction ........................................... 305 8.2 Progress in Physics and Chemistry of Comets . . . . . . . . . . . . . . . 306

8.2.1 Composition of the Nucleus ........................ 306 8.2.2 Structure of the Nucleus ........................... 309 8.2.3 Plasma Structures in Coma and Tail ................. 310 8.2.4 Orbital Theories ................................. 312

8.3 Progress on the History of the Early Solar System . . . . . . . . . . . 313 8.4 Comets and the Origins of Life ........................... 318

8.4.1 The Origins of Life .............................. 318 8.4.2 Organic Materials in Comets ....................... 320 8.4.3 Comet Transport of Volatiles ....................... 323 8.4.4 The Role of Comets .............................. 325

8.5 Unsolved and New Problems ............................. 326 8.S.1 Nucleus Rotation and Surface ...................... 326 8.S.2 Chemical Abundances in the Coma and the Nucleus .... 327 8.S.3 Dust Tail ....................................... 329 8.S.4 Coma and Tail Plasma ............................ 330

References ..................................................... 333

SUbject Index 367

Index of Contributors

A' Hearn, Michael F. Astronomy Program, University of Maryland, College Park, MD 20742, USA

Axford, Wl. Max-Planck-Institut fiir Aeronomie, Postfach 20, D-3411 Katlenburg-Lindau, Fed. Rep. of Germany

Festau, Michel C. 'Observatoire de Besan~on, 41 bis avo de l'Observatoire, F-25044 Besan~on Cedex, France

Gran, E. Max-Planck-Institut fiir Kernphysik, Postfach 10 39 80, Saupfercheckweg 1, D-6900 Heidelberg, Fed. Rep. of Germany

Huebner, Walter F. Southwest Research Institute, Division 15, P.O. Drawer 28510, 6220 Culebra Road, San Antonio, TX 78228-0510, USA

[p, W.-H. Max-Planck-Institut fiir Aeronomie, Postfach 20, D~3411 Kadenburg-Lindau, Fed. Rep. of Germany

Jessberger, E.K. Max-Planck-Institut fiir Kernphysik, Postfach 10 39 80, Saupfercheckweg 1, D-6900 Heidelberg, Fed. Rep. of Germany

Keller, H.Uwe Max-Planck-Institut fiir Aeronomie, Postfach 20, Max-Planck-StraBe 2, D-3411 Kadenburg-Lindau, Fed. Rep. of Germany

MacKay, c.P. Ames Research Center, Life Science Div., Solar System Exploration Branch, Moffett Field, CA 94035, USA

XVI Index of Contributors

Oort, l.H. Sterrewacht Leiden, Postbus 9513, NL-2300 RA Leiden, Netherlands

Rickman, Hans Astronomiska Observatoriet, Box 515, S-75120 Uppsala, Sweden