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Physics of Comets (2Nd Ed.) K 7537tp.indd 1 8/3/10 3:25 PM WORLD SCIENTIFIC SERIES IN ASTRONOMY AND ASTROPHYSICS Editor: Jayant V. Narlikar Inter-University Centre for Astronomy and Astrophysics, Pune, India Published: Volume 1: Lectures on Cosmology and Action at a Distance Electrodynamics F. Hoyle and J. V. Narlikar Volume 2: Physics of Comets (2nd Ed.) K. S. Krishna Swamy Volume 3: Catastrophes and Comets* V. Clube and B. Napier Volume 4: From Black Clouds to Black Holes (2nd Ed.) J. V. Narlikar Volume 5: Solar and Interplanetary Disturbances S. K. Alurkar Volume 6: Fundamentals of Solar Astronomy A. Bhatnagar and W. Livingston Volume 7: Dust in the Universe: Similarities and Differences K. S. Krishna Swamy Volume 8: An Invitation to Astrophysics T. Padmanabhan Volume 9: Stardust from Meteorites: An Introduction to Presolar Grains M. Lugaro Volume 10: Rotation and Accretion Powered Pulsars P. Ghosh Volume 11: Find a Hotter Place!: A History of Nuclear Astrophysics L. M. Celnikier Volume 12: Physics of Comets (3rd Edition) K. S. Krishna Swamy *Publication cancelled. Lakshmi - Physics of Comets (3rd Ed).pmd 2 7/12/2010, 2:46 PM World Scientific Series in Astronomy and Astrophysics – ol. 12 K S Krishna Swamy retired from the Tata Institute of Fundamental Research, Mumbai, India World Scientific N E W J E R S E Y • LONDON • SINGAPORE • BEIJING • SHANGHAI • H O N G K O N G • TAIPEI • CHENNAI 7537tp.indd 2 8/3/10 3:25 PM Published by World Scientific Publishing Co. Pte. Ltd. 5 Toh Tuck Link, Singapore 596224 USA office: 27 Warren Street, Suite 401-402, Hackensack, NJ 07601 UK office: 57 Shelton Street, Covent Garden, London WC2H 9HE British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. PHYSICS OF COMETS (3rd Edition) World Scientific Series in Astronomy and Astrophysics — Vol. 12 Copyright © 2010 by World Scientific Publishing Co. Pte. Ltd. All rights reserved. This book, or parts thereof, may not be reproduced in any form or by any means, electronic or mechanical, including photocopying, recording or any information storage and retrieval system now known or to be invented, without written permission from the Publisher. For photocopying of material in this volume, please pay a copying fee through the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, USA. In this case permission to photocopy is not required from the publisher. ISBN-13 978-981-4291-11-8 ISBN-10 981-4291-11-0 Printed in Singapore. Lakshmi - Physics of Comets (3rd Ed).pmd 1 7/12/2010, 2:46 PM July 12, 2010 9:59 World Scientific Book - 9in x 6in comets Foreword Only in recent years has the true nature of comets become evident. With this knowledge has come the realization that comets are almost cer- tainly debris left over from the building of the outermost planets. This material must be typical of the gas and dust in the interstellar cloud from which the Sun and planetary system evolved. Thus, comets appear to offer us an opportunity to study primitive matter involved in the origin of the solar system, material that has been stored in deep freeze for 4.6 × 109 years. At the same time, the comets provide a link with interstellar solids, of which they are probably largely composed. The physical and chemical study of comets has now replaced classical celestial mechanics as the major focus of observational and theoretical research. Familiar areas of astronom- ical spectroscopy are approached from a somewhat unusual point of view in cometary studies, while comets provide an entirely new laboratory for an in- creased understanding of magneto-hydrodynamics. On the other extreme, the physics of amorphous ices and other solids can be applied to probe the internal structure, origin and activity of the comet nuclei. In summation, the study of comets has now become a manifold discipline with fascinating potential, particularly as the space age provides new observational input over the entire electromagnetic spectrum, coupled with the expectation of direct in situ studies and even the eventual return of samples from cometary nuclei. The marked progress in this direction is represented by the new re- sults described in this book's revised (second) edition. The several missions to Halley's comet have filled in many lacunae of comet knowledge. Fred L. Whipple v This page intentionally left blank July 12, 2010 9:59 World Scientific Book - 9in x 6in comets Preface We are in an exciting era of cometary science. This has come about due to space mission to comets, launch of several satellites and ground based studies. The first major space adventure was in 1986 when six spacecraft passed through Comet Halley as close as 500 km from the nucleus. More recently, the far more adventures venture of the Deep Impact Mission to Comet Tempel 1 and Stardust Mission to Comet Wild 2 made it possible for the first time to look at the material in the deeper layers of the cometary nuclei and to study in the laboratory the collected cometary dust. All these studies have given rise to exciting and unexpected results which have raised some fundamental issues about comets and in turn on the solar system itself. For example, the detection of X-rays from comets was a real challenge in its understanding. The observed diversity in chemical composition among comets has direct bearing on the formation conditions in the solar nebula. The mineralogy of dust in IDPs, comets, meteorites, asteroids and circumstellar shell of stars are quite similar showing generic relationship between them. In view of these exciting developments it is an appropriate time to revise and update the second edition of the book \Physics of Comets" published in 1997. Therefore extensive revision has been carried out in most of the chapters. Emphasis has been to include all the new results with the hope that the flavour of excitement may be conveyed to the reader. The general arrangement, purpose and level of the updated and revised edition of the book remains the same as that of second edition. I am especially grateful to my colleagues, H.M. Antia who helped me in various ways and especially in the preparation of figures and S. Ramadurai for a thorough reading of the manuscript. I would also like to express my gratitude to my family and parents for their support and encouragement. vii July 12, 2010 9:59 World Scientific Book - 9in x 6in comets viii Physics of Comets I would like to thank all the publishers and authors who have given per- mission for the reproduction of figures and tables. The help of Mr. Vivek Vengurlekar in the preparation of the manuscript is highly appreciated. My interaction with several astronomers have been very helpful and especially, the cometary experts like, M.F.A'Hearn, P.D. Feldman, W.F. Huebner, M.J. Mumma, C.R.O'Dell and J. Watanabe. K S Krishna Swamy 204, Sigma Tower Plot 32, Gorai Road Borivali West Mumbai 400 091, India July 12, 2010 9:59 World Scientific Book - 9in x 6in comets Contents Foreword v Preface vii 1. General Introduction 1 1.1. Historical Perspective . 1 1.2. Discovery . 4 1.3. Appearance . 5 1.4. Statistics . 6 1.5. Importance . 9 1.6. Brightness . 10 1.7. Main Characteristics . 13 1.8. Spacecraft Encounters with Comets . 19 1.9. An Overall View . 23 Problems . 30 References . 30 2. Dynamics 33 2.1. Orbital Elements . 33 2.2. Orbit in Space . 35 2.2.1. Relevant equations . 35 2.2.2. Orbital elements from position and velocity . 40 2.2.3. Orbital elements from observations . 41 Problems . 45 References . 46 ix July 12, 2010 9:59 World Scientific Book - 9in x 6in comets x Physics of Comets 3. Physical Aspects 47 3.1. Black Body Radiation . 47 3.2. Perfect Gas Law . 49 3.3. Dissociative Equilibrium . 49 3.4. Doppler Shift . 50 3.5. Spectroscopy . 51 3.5.1. Atomic spectroscopy . 51 3.5.2. Molecular spectroscopy . 54 3.5.3. Chemical subgroups . 57 3.6. Isotopic Effect . 58 3.7. Franck-Condon Factors . 59 3.8. Intensity of Emitted Lines . 61 3.9. Boltzmann Distribution . 64 3.10. Λ-Doubling . 64 3.11. Photochemistry of Water . 65 3.12. Silicate . 67 3.13. Annealing . 68 3.14. Carbon . 69 3.15. Solar Radiation . 70 3.16. Solar Wind . 72 Problems . 73 References . 74 4. Spectra 75 4.1. Main Characteristics . 76 4.2. Forbidden Transitions . 90 4.3. Line{to{Continuum Ratio . 92 Problems . 93 References . 93 5. Spectra of Coma 95 5.1. Fluorescence Process . 95 5.1.1. Rotational structure . 98 5.1.2. Vibrational structure . 101 5.1.3. Comparison with observations . 102 5.1.4. Case of C2 molecule . 108 5.1.5. Prompt emission lines of OH . 114 5.1.6. Molecules other than diatomic . 118 July 12, 2010 9:59 World Scientific Book - 9in x 6in comets Contents xi 5.1.7. OH radio lines . 118 5.1.8. Oxygen lines . 121 5.1.9. Forbidden transitions . 121 5.1.10. Molecular band polarization . 123 5.2. Excitation Temperature . 125 5.2.1. Rotational temperature . 125 5.2.2. Vibrational temperature . 128 5.3. Abundances of Heavy Elements . 128 5.4. Isotopic Abundances . 130 5.5. Summary . 137 Problems . 137 References . 138 6. Gas Production Rates in Coma 141 6.1. Theoretical Models . 142 6.1.1. From the total luminosity . 142 6.1.2. From surface brightness distribution . 144 6.1.3. From number densities . 149 6.1.4. Semi-empirical photometric theory . 151 6.2.
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