THE MOON

Photograph of extended plains of the lunar Mare Tranquillitatis, showing crater Cauchy in between

a shallow rille and a low cleft - with a typical dome near the centre of the field of view - taken by

U.S. Astronaut James A. Lovell, Jf. from Apollo 8 spacecraft on 24 December 1968 in orbit around

the Moon (reproduced by courtesy of NASA). Compare this view with a terrestrial photograph of

the same region, as reproduced on Figure 16.20 on p. 278 in the text.

ZDENEK KOPAL Department of Astronomy, University of Manchester

THE MOON

D. REIDEL PUBLISHING COMPANY

DORDRECHT-HOLLAND

The first edition of this work has been published in 1966 under the title

AN INTRODUCTION TO THE STUDY OF THE MOON

ISBN-13: 978-94-010-3410-4 e-ISBN-13: 978-94-010-3408-1 DOl: 10.1007/978-94-010-3408-1

© 1969. D. Reidel Publishing Company, Dordrecht, Holland No part of this book may be reproduced in any form, by print, photoprint, microfilm,

or any means, without written permission from the publisher

To the Memory of my Father

PROFESSOR JOSEF KOPAL

(1883-1966)

PREFACE

A favourable reception of the first edition of this book - due no doubt to the nature of its subject - which went out of print in 2 years, gave its author a welcome oppor­tunity to update at present its contents. This was all the more necessary, as seldom in the annals of science has our knowledge of the physics and astronomy of the Moon made greater progress than during this time. The real heroes of this advance have, of course, been the spacecraft - 33 of which have now been sent out since 1959 to reconnoiter our satellite at a close range. The hard-landers among them just about delivered their message by the time when the first edition of this book went to the press; but it was the soft-landers and orbiters, which followed in their wake between 1966-67, that became really the principal contributors to lunar research. By now their programmes have likewise been completed; so that it may be both timely and opportune to take stock of the present state of our subject now - on the eve of the next stage of lunar exploration by manned landings on the surface of our satellite -which can be expected to take place in the very near future. *

The aim of the second edition of this book will be to provide the requisite infor­mation, brought up to date in an organized manner. Its form remains essentially similar to that of the first edition. However, the influx of new knowledge between 1966-68 necessitated that Chapters 16-17 and 22-24 be completely rewritten, as well as provided with new illustrations; and shorter additions incorporating recent work were interspersed throughout the whole book. The Bibliographical Notes accompa­nying each Part were likewise brought up to date as of the summer 1968. As a result, the size of this volume had to be increased somewhat if it was to fulfill its purpose of accompanying the reader to the front lines of current research. However, in response to a general demand for a simplified version of the same material, a parallel edition is to appear shortly in abridged form, in which the methodological or bibli­ographical material - of primary interest for the working scholar - will be very largely omitted; and attention focused on a survey of the results of principal interest for a more general reader.

If, however, the reader expects that recent experimental exploits of our spacecraft have solved all lunar problems outlined in the first edition, he is bound to be dis­appointed. For not only - as everywhere in science - one solved problem gives rise usually to many new ones; but it has also become evident that increasing arrays of facts which became available to us in the past 2 years do not often admit of simple

... Succesfully accomplished 20 July 1969 (see postscript).

vm THE MOON

explanation. In so far as the intimacy of our acquaintance with the lunar surface is concerned, hard-landing spacecraft (Rangers 7-9) of1964-65 have increased thousand­fold the spatial resolution previously attainable with the aid of telescopes from the distance of the Earth; and the soft-landers of 1966-68 have gained us a further resolution factor of a thousand. But, in spite of these giant strides, the meaning of new facts seems sometimes to possess an elusive ability of escaping from our out­stretched hands like a mirage.

It was aptly remarked, in this connection, by Thomas Gold that only too many investigators of the Moon have been inclined to seek in the new evidence - which spacecraft have so suddenly thrust upon us - but confirmations of their preconceived opinions. The range between too dogmatic an approach on one side, and reluctance to draw conclusions from the facts on the other, is often quite wide; and the extent to which the present writer may have succeeded in threading the middle road will be for future readers to judge.

The first edition of this book was largely written while its author was on a sabbatical leave from the University of Manchester at the Boeing Scientific Research Labora­tories in Seattle, U.S.A.; and the preparation of this second edition has been carried out at the same institution in the summer of 1968. May the writer use this opportunity to reiterate his sincere appreciation to his friends Guilbert L. Hollingsworth, Director of the Boeing Laboratories; Burton H. Colvin, Head of Mathematics Research; and John C. Noyes, Head of Geo-Astrophysics Research, of their hospitality extended to him at all times, and for placing all facilities of their laboratories so generously at the disposal of his work. Miss Linda Baqui (having changed her name) was no longer available for the typing of the new manuscript; but this task was accomplished with equal skill by Miss Janet Sutherland.

The writer wishes, furthermore, to express his deep indebtedness to all colleagues at the Space Division of the Boeing Company in Kent - in particular, to Messrs. John H. Lincoln, Albert J. McDonald, and Martin C. Stickels - who facilitated his access to the original photographic data secured by the Lunar Orbiters between 1966 and 1968, many of which are reproduced in this book; and to his friends at the NASA Headquarters in Washington for similar access to the data secured by the Surveyors. The actual production of the photographic prints for the press was expertly ac­complished by Mr. Don Fenton of the Boeing Scientific Research Laboratories in Seattle, and Mr. Charlie Lowe of the Department of Astronomy, University of Manchester.

The skeleton maps of the Moon which accompany this volume were prepared with unrivalled skill and knowledge by Ing. Antonin Riikl of Prague, Czechoslovakia. The foldout map of the lunar front side is identical with the one which accompanied already the first edition of the book; but the corresponding map of the far side, based largely on the Orbiter photographs, is so far unique of its kind, and constitutes an original and significant contribution to lunar cartography.

Last, but not least, the present writer wishes to express his thanks and appreciation to his publisher, Mr. Anton Reidel of D. Reidel Publishing Company in Dordrecht,

PREFACE IX

as well as to his able assistant and collaborator, Miss N. M. van der Heijden, for their cooperation and care which they bestowed on the production of this volume; while, at Manchester, the unfailing assistance of Miss Ellen B. Finlay has been - as ever - invaluable.

Boeing Scientific Research Laboratories, Seattle, Wash., U.S.A. August, 1968

ZDENEK KOPAL

TABLE OF CONTENTS

PREFACE

PREFACE TO THE FIRST EDITION

PART ONE

MOTION OF THE MOON AND DYNAMICS OF THE

EARTH-MOON SYSTEM

VII

XIII

Chapter 1. Basic Facts: Distance, Size, and Mass 3 Chapter 2. The Motion of the Moon in Space 9 Chapter 3. Rotation of the Moon; Optical Librations 16 Chapter 4. Motion of the Moon about its Center of Gravity; Physical

Librations 24 Chapter 5. Photometric Consequences of the Moon's Motion: Phases and

Eclipses 44 Chapter 6. Dynamics of the Earth-Moon System 55 Bibliographical Notes 69

PART TWO

INTERNAL CONSTITUTION OF THE LUNAR GLOBE

Introduction Chapter 7. Hydrostatic Equilibrium and its Limitations Chapter 8. Thermal History of the Moon Chapter 9. Stress History of the Moon Chapter 10. Possible Convection in the Lunar Interior Chapter 11. Chemical Composition of the Moon Chapter 12. Exosphere of the Moon Bibliographical Notes

75 77 91

110 133 151 161 171

XII THE MOON

PART THREE

TOPOGRAPHY OF THE MOON

Introduction 177 Chapter 13. Global Form of the Moon and its Gravitational Field 178 Chapter 14. Relative Coordinates on the Moon and their Determination 205 Chapter 15. Mapping of the Moon 225 Chapter 16. Morphology of the Lunar Surface 258 Chapter 17. Origin of the Lunar Formations 292 Chapter 18. Lunar Surface as an Impact Counter, and its Stratigraphy 332 Bibliographical Notes 348

PART FOUR

RADIATION OF THE MOON

Introduction Chapter 19. Photometry of Scattered Moonlight Chapter 20. Thermal Emission of the Lunar Surface Chapter 21. Electromagnetic Properties of the Lunar Surface Chapter 22. Luminescence of the Lunar Surface Chapter 23. Structure of the Lunar Surface Chapter 24. Large-Scale Features of the Lunar Surface Bibliographical Notes

POSTSCRIPT

REFERENCES

INDEX OF NAMES

355 357 371 400 423 438 461 484

491

493

518

PREFACE TO THE FIRST EDITION

After several decades spent in astronomical semi-obscurity, the Moon has of late suddenly emerged as an object of considerable interest to students of astronomy as well as of other branches of natural science and technology; and the reasons for this are indeed of historical significance. For the Moon has now been destined to be the first celestial body outside the confines of our own planet to be reconnoitered at a close range by means of spacecraft built and sent out by human hand for this purpose. At the time of writing, not less than ten such spacecraft of American as well as Rus­sian origin landed already on different parts of the lunar surface; and some of these provided remarkable records of its detail structure to a spatial resolution increased thousandfold over that attained so far from our ground-based facilities.

A renewed interest in our satellite, stemming from this source, on the part of the students of many branches of science and technology has also underlined the need for presenting the gist of our present knowledge in this field in the form that could serve as an introduction to the study of the Moon not only for astronomers, but also for serious students from other branches of science or technology. There is, to be sure, no lack of literature on the Moon today - perhaps the contrary is true - but very little of it has presented its subject as integral part of the underlying physical sciences, and as another example of the applicability of their general methods for its study. This represents a task, and a challenge, which we shall attempt to meet.

The aim of the present volume is thus very literally expressed by its title: namely, to provide an introduction to the study of the Moon, based on the broad background of astronomy, physics and chemistry, with which the reader will be expected to have some familiarity. The emphasis throughout will be on the development of basic meth­ods from (whenever possible) the first principles, rather than on a mere enumeration of facts; for a proper understanding of the methods of lunar research should, in the long run, prove more rewarding for the reader than a simple account of latest facts, the arsenal of which will change from year to year.

The four parts of which this book consists have been organized in accordance with this plan. The first contains a survey of our present knowledge of the motion of the Moon and dynamics of the Earth-Moon system, obtained mainly by astronomical methods. The second part will be concerned with the internal structure of our satel­lite; and the third, with the lunar surface features and their interpretation. In the latter, we shall firmly keep in mind that the surface of the Moon must be regarded as a 'boundary condition' of all internal processes discussed in Part II, as well as an 'impact counter' of all external collisions which disfigured the face of the Moon since

XIV THE MOON

the time of its origin; for in no other way can an interpretation of its prolific surface markings possess any meaning. The fourth and concluding part of the volume will deal with the radiation reaching us from the Moon in all domains of the spectrum; and its bearing on the microstructure of the lunar surface on a scale much smaller than that resolvable by telescopic means not only from the Earth, but also from orbit­ing spacecraft.

As the reader can easily verify by a glance at its table of contents, the present vol­ume attempts to cover almost the entire field of lunar studies; but the depth to which this will be done will vary somewhat from chapter to chapter - depending not the least on the previous coverage of their subjects in existing literature in the English language. For instance, the motion of the Moon in space will be outlined in Chapter 2 only briefly for the sake of completeness, as the student of this subject is well served by the available literature in both its fundamental and expository aspects. The same is also the case with the photometry of lunar eclipses, which has in recent years been extensively dealt with, e.g., by Link. On the other hand, this is not true yet of the motion of the Moon around its centre of gravity (on which the only available mono­graph by Habibullin exists in the Russian language); and for this reason, more space has been given to it in the present volume.

The subject of Part II - internal structure of the Moon - has not so far been mono­graphically treated in any language. This should explain the attention which we have paid to it in this book; and its author hopes that the reader may find our present introduction to it adequate. In Part III - concerned with the topography of our satel­lite and an interpretation of its surface features - the reader's attention will likewise be focussed on the methods of research rather than on any descriptive account of the lunar landscape - mainly because such accounts have already filled many a book of both old and recent date; and there is no need of repetition. On the other hand, the last part of our volume (concerned with lunar photometry and its interpretation) will bring astronomy and physics in common focus more closely than has been true of any other part of this book; and its concluding chapters - summarizing, in fact, the entire contents of this volume - will draw on the interpretation of observed facts by all meth­ods known to human science.

Each Part of this book is, moreover, accompanied with bibliographical notes on the chapters that it comprises, containing additional references and other points which would have made the main text too discursive. Such bibliographies are by no means complete - for any attempt at completeness would have occupied with bibliographical material a volume exceeding considerably in size this whole book. Special and more complete bibliographies of lunar research have in recent years been compiled by several writers *; and their existence renders any repetition superfluous. Besides a large part

'" Cf., e.g., J. W. Salisbury, R. A. van Tassel, J. E. M. Adler, R. T. Dodd, Jr., and V. G. Smalley, 'Bibliography of Lunar and Planetary Research, 1960--63' (AFCRL Notes No. 684, 62-676, 63-903, 64-885), Bedford, Mass., 1961-64; A. C. Mason, 'Bibliography of the Moon', Military Geol. Branch, U.S. Army, Washington, 1960; A. Pogo, 'Annotated Bibliography of Physical Observations of the Moon 1910--1960', Calif. Inst. of Tech., Pasadena, 1961; or L. R. Magnolia and J. R. Trew, 'The Lunar Problem - Bibliography' (STLI AB 61-5110--40), Space Technology Laboratories, Los Angeles,

PREFACE TO THE FIRST EDITION xv

of older literature, even though of historical significance, may be devoid of other interest for a modern critical reader of the subject. One of the aims of the present volume (and of the selection of references quoted herein) has been to assist the user in the choice of collateral reading by an attempt to separate the chaff from the wheat. However, the limitations of space alone collaborated also in keeping the list of refer­ences as given on pp. 493-517 to its present size. The latter can, therefore, claim neither completeness nor uniqueness in selection; but any significant omissions could easily be rectified by using the present bibliography as a guide.

The references are almost solely to investigations published in books or journals that should be freely available to any interested reader; references to mere abstracts or reports of limited circulation have been kept to a minimum. In order to introduce the subject to the reader as it stands today, we have selected the references mainly from the literature that has appeared in the past 10 years. Exception to this has been made only in Chapter 15 ('Mapping of the Moon'), where historical interests are in the forefront; but elsewhere these have been relegated to the 'Bibliographical Notes'.

In conclusion, it is scarcely necessary to emphasise that, in the years to come, our acquaintance with our only natural satellite is going to become very much more inti­mate than it has been as long as astronomical telescopes represented our principal link of communications; and many issues still in suspense should be resolved in the near future by direct exploration by means of spacecraft of increasing effectiveness. As a result of these changing means of research, the Moon may soon cease to be treated as an astronomical object, and be professionally annexed by other sciences more intimately connected with direct exploration. The present volume may, perhaps, be the last monograph devoted to our satellite to come out from the pen of an astron­omer. Nevertheless, it is equally certain that the present unprecedented advances in space research of the solar system have been made possible only on the basis of all knowledge which astronomers gathered so far largely from our terrestrial vantage point - a knowledge which, by itself, represents a remarkable achievement of the human mind - and which we aim in the present volume to introduce to the technical student of the subject.

The text of this book has originated in the courses on different aspects of lunar studies which the present writer has given to graduate students at the University of Manchester (and other institutions) in the past several years. A sabbatical leave in 1964-65, which he spent as honorary research fellow at the Boeing Scientific Research Laboratories in Seattle, Wash., offered a welcome chance for committing their con­tents to a written form. May I take this opportunity to express my sincere appreciation to Mr. G. L. Hollingsworth, Director of the Boeing Scientific Research Laboratories, to Dr. Burton H. Colvin, Head of the Mathematics Research Laboratory, to Dr.

1961. For earlier literature the reader can consult the relevant sections of the yearly volumes of Astro­nomische Jahresberichte, which have appeared since 1900. A good many references to earlier literature - but, unfortunately, mixing freely chaff with wheat - can be found also in Fielder's Structure of the Lunar Surface, Pergamon Press, London, 1961, or Baldwin's Measure of the Moon, University of Chicago Press, Chicago, 1963.

XVI THE MOON

John C. Noyes, Head of the Astro-Geophysics Research, as well as to all other friends for placing all facilities of these laboratories so generously at my disposal at all stages of this work, and for their unfailing courtesy and cooperation. A large part of the difficult MS. was expertly typed for the press by Miss Linda Baqui in Seattle; while, at Manchester, the unfailing assistance of Miss Ellen B. Finlay in all editorial matters has been - as always - invaluable.

Many illustrations accompanying Part III of this book have been secured in the course of an extensive Manchester Programme in lunar photography at the Observa­to ire du Pic-du-Midi, supported since 1959 by the Air Force Cambridge Research Laboratories and the Aeronautical Chart and Information Center of the U.S. Air Force. May I, at this opportunity, expreSi; once more to these institutions our sincere appreciation of the unfailing cooperation with which they have met all our needs.

Last, but not least, I wish to thank Professor C. de Jager, Editor of the Astrophysics and Space Science Library, for inviting me to publish this book in his series; and to the D. Reidel Publishing Company for the care which they have bestowed on the printing of the entire volume.

'Greenfield', Parkway, Wilmslow, Cheshire, England December, 1965

ZDENEK KOPAL