Table Of ContentOXFORD MONOGRAPHS ON
GEOLOGY AND GEOPHYSICS NO. 11
Impact Cratering
A Geologic ProCess
H.J. Melosh
OXFORD MONOGRAPHS ON GEOLOGY AND GEOPHYSICS NO. 11 ‘ ' '
Series editors
H. Chamock
J. F. Dewey
_ S. Conway Morris .
A. Navrotsky
E. R. Oxburgh
R. A. Price
B. J. Skinner
OXFORD MONOGRAPHS ON GEOLOGY AND GEOPHYSICS
1. De Verle P. Harris: Mineral resources appraisal: mineral endowment, resources, and
potentialsupply: concepts, methods, andcases
J. J. Veevers (ed.): Phanerozoic earth history ofAustralia
Yang Zunyi, Cheng Yuqi, and Wang Hongzhen: Thegeology ofChina '
”
5
5 Lin-gun Liu and William A. Bassett: Elements, oxides, andsilicates: high-pressure
:
phases with implicationsfor theEarth’s interior
Antoni Hoffman and Matthew H. Nitecki (eds.): Problematicfossiltaxa
n
s
S. Mahmood Naqvi and'John J. W. Rogers: Precambrian geology ofIndia
n
s
Chih-Pei Chang and T. N. Krishnamurti (eds.): Monsoon meteorology
s
o
Zvi Ben-Avraham‘ (ed.): The evolution ofthePacific Ocean margins
p
e
Ian McDougall and T. Mark Harrison: Geochronology andthermochronology by the
r
40Ar/"Mr method '
' 10.
Walter C. Sweet: Theconodonta: morphology, taxonomy, paleoecology, and
evolutionary history ofa long-extinct animalphylum
11. H. J. Melosh: Impact cratering: a geologicprocess
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Impact Cratering ;
A- Geologic. Process,
H. J. MELOSH _
Lunar andPlanetary Laboratory, University ofArizona
OXFORD UNIVERSITY PRESS - New York
CLARENDON PRESS - Oxford - 1989
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Library ofCongress Cataloging-in-Publication Data
Melosh, H. J.
Impactcratering:ageologicprocess/H. J. Melosh.
p. cm.-—(Oxfordmonographsongeologyandgeophysics;no. I1)
Bibliography:p. _ .
Includesindex.
ISBN0—19-504284—0(alk. paper):$45.00(est)
1. Cratering. 2. Planets—Geology. 3. Impact. 1. Title.
II. Series.
QB603.C7M45 1989 > 88-5353
551.4’4—dc19 , . CIP
9 8 7 6 5 4 3 2
Printedin the UnitedStatesofAmerica
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PREFACE
Impact cratering has been recognized as an im- sibleway. Mostofourdetailed knowledge on im- '
portantgeologic process for only the lastfewdec- pactcrateringisspreadthroughalarge numberof.
ades. As recently as 1950 most astronomers be- papersandreportsthatappearedinawidevariety '
lieved that the lunar craters were giant volcanos, ofjournals and symposia. Only a few reviews of
and all but a few geologists derided the idea that theentirefieldareavailable,andtheyarenotonly '
the earth’s surface has been scarred by impact obsolete, buttheyalso do notgo into the individ- _
structureskilometersindiameter.Avigorouspro- ual aspects of cratering in sufficient depth for a
gram ofplanetary exploration in the Apollo era professional reader. The most comprehensive ~
and continued geologic research on earth has source ofinformation on impact cratering in re- '
changed these views profoundly. It is now recog- centyearshasbeenthe 1977bookImpactandEx-
nized that the cratered landscapes ofthe moon, plosion Cratering, which is a collection ofpapers ’
Mercury, Mars, and many ofthe solar system’s thatwerepresentedata 1976meetinginFlagstaff,
satellites are sculptured predominantly by re- Arizona. The importance ofthis book is empha- ~
peated impacts ofall sizes. Morethan 100impact sized by the fact that it is known informally as
Structures with diameters ranging up to 140 km “thebluebible” amongworkersonimpactcrater-
have been identified on earth. More recently,ev- ing. Forthe noninitiate, however, assimilation of
idence has been found that strongly suggests that the many papers in thisbookisboth diflicultand
the Cretaceous era was ended by the impact ofa 1 time-consuming. Furthermore, manyadvances1n.
. lO-kmdiametercometorasteroid,andthenewest the field have been made since 1976. ‘
and mostsuccessful theories ofthe moon’s origin This book is an attempt to bring together the
suggest that it formed when a Mars-sized proto- currentknowledge on impactcratering ina single
planet struckthe protoearth 4.5 billion years ago. and comprehensive treatise. The field ofimpact
The planets themselves are believed to have ac- cratering is so large at present, however, that I
cretedfromdustanddebrisintheearly solarneb- must restrict the scope to topics that seemto‘bc -
ula by repeated impacts of smaller objects onto, of major importance to planetary geologists. V
growing planetary embryos, and geochemists are Thus, I have excluded large amounts ofmaterial
just nowbeginningto realizethat nearlyall ofthe on microcraters, low velocity impacts, petrology,
material now residing in planets has been pro- andgeochemistry. Ido nottreatthegeologyofin-
cessed through high velocity impacts. In the dividualterrestrialorplanetarycratersindetail— ’
minds of many geologists, impact cratering has booksincludingthese topics are in progressby ‘
risen from complete obscurity to become one of V otherauthors. My primaryfocus1son the impact
the “most-fundamental” geologic processes. One crateringprocess, anddetails ofspecificterrestrial
meteoriticist has even suggested that future his- and extraterrestrial craters are discussed only to
torians will accord the recognition ofimpact cra- theextentthattheyilluminatethatprocess. Ihave
teringan equal importancewith the development attemptedtogivethereaderafeelfortheessential
ofplate tectonics. physicsofimpactcrateringandtoestablishathe-
In spite ofthese major strides forward in the oreticalframeworkthatheor-shecanapplytothe
understanding of impact cratering, many geolo- particularcratering problem at hand. ~
gists, and even planetary scientists, remainigno- My intendedaudience, however, is'not primar-‘
rant ofvital facts about the crateringprocess and ily physicists. I have tried to aim the book at ge-
its resulting craters. Many planetary geologists ologists and planetary scientists, including both
were surprised and disappointed when Apollo 14 professionals working in other areas who find
failed to return samples of the moon’s mantle they need to know something about impact cra-
from the Fra Mauro formation, even though it ters, and graduate students who wish to acquaint
was well known among experts onrcratering that themselves with this important process. I origi-
the impact that created Imbrium basin was un- nally intended, therefore, to minimize the num-
likely to have excavated material from mantle ber ofequations. I soon found this to be impos-
depths..l have long felt that the reason for this sible without totally trivializingthe subject. Most
widespread ignorance is the lack ofany one place ofthe equations, however, are statedasformulas,
wherethemajorfactsaboutimpactcrateringhave from which the reader may deduce a quantity of ' -
been collectedtogetherand presentedinan acces— interest on the left-hand sideby substituting the
. PREFACE 1
vi H
values of some other well-defined quantities on geologicallyorientedreadermoregradually, start- ‘
the right-hand side. Although I was unable to ing from weak pressure and elastic waves, with OH
maintain this scheme throughout the book, the which I assume mostgeologistsare reasonablyfa- F
sections where more mathematical sophistication miliar,todescribingprogressivelystrongerplastic ' OP
is required are clearly marked and can be skipped waves, and finally shock waves. The Hugoniot -F
by most readers without losing the essential equations thus appear in the middle ofthe chap- o
thread ofthe exposition. . ter. Readers with physics backgrounds may wish Hm
Because my goal is largely expository, I have to skip this chapter, although the material in the -
tried to keep the number ofreferences in the text last halfofthe chapteris notpart ofmost physics » AH
tbroisatlminignwiimthumcitoantiotnhseisgrbooutnhddsistthraatctaintgeaxtndpangoet - curCrhiacupltae.rs 4, 5, and 8 are the'core ofthe book. - H-r
very enlightening to someone encountering the These chapters each deal with one of the three n‘
material for the first time. This was agdiflicult de— main stages recognized in cratering mechanics: A
cision, as some discoveries or insights may thus contact and compression, during which the me- n—
become divorced from the name of the worker teorite first contacts the target planet(Chapter4); a
.who first attained them. However, I have tried to crater excavation, when the crater itself opens am
provide an entry into the literature at the end of (Chapter 5); and crater modification, when the r
each chapter and theassiduous reader who pur- crater may partially collapse (Chapter 8). Ejecta .
sues these selected references should eventually deposits are treated immediately afterexcavation _.
be able to track down the authority behind each in Chapter 6, since the crater’s internal collapse ..
discovery. The more important discoveries are, has little effect on ejecta that has already been _._
however, explicitly referenced in the text, as are launched. Similarly, scaling (Chapter 7) is treated .
papersthat, in my opinion, offerimportantexten- beforecollapsebecauseitgenerallyappliesonlyto
sions of the text presentation. The decison of the transient crater formed at the end ofthe ex-
whether to include a direct reference or not is cavation stage. Multiring'basins are described in
somewhat subjective, and I apologize in advance a separate chapter (Chapter 9) to clearly distin-
to any of my colleagues who do not see them- guish them from‘complex crater types, including
selves cited as often as they deserve. > peak-ring craters. In the past, many authors have
Oneoftheadvantagesofwritingthefirstmono- groupedpeak-ringcratersandmultiringbasinsto-
graph on a subject (although it adds considerably gether, but, for reasons spelled out in the text, I
tothe effortofwriting!) is the freedom to organize believethat they are distinct. '
the material withoutbeing influencedbyprevious Chapters 4 through 9 encompass the classic ap-
prejudiceson howitshould fittogether. I havede- plication ofplanetary.impact cratering studies to
parted from the usage established in earlier re- single craters formed on airless bodies. The first
viewsin a number ofways. The first chapter is a half of Chapter 10 goes beyond this traditional
brief history of the study of impact craters that area to discuss craters as elements of planetary
provides both background for the facts ex- landscapes and offers an introduction to crater
pounded later and a rationale for much of the statistics. InthesecondhalfofChapter 10thissta-
workthat has been performed todate. It is partic- tistical background is applied to the growth and
ularly interesting tome that the nineteenth cen- maintenance ofregoliths on airless bodies. This
tury astronomers’ dismissal ofthe impact origin latter portion requires somewhat more mathe-
oflunarcraterswaswell-foundedatthetime; only matical sophistication than the rest ofthe book,
an understanding of high-velocity impact me- but I believe that anyone with some background
chanics was able to answer their cogent objec- in elementary statistics and calculus can work
tions. It was a similar lack ofappreciation ofcra- throughit. Chapter 11 thendiscusseswhatlittle is
tering mechanics that in 1896 led the eminent known about impacts on planets with dense at-
geologistG. K. Gilbertto rejectimpactas thepro- mospheres, and howthe crateringprocessmaybe
cessthatcreated MeteorCrater, Arizona. The sec- modified in the presence ofan atmosphere. The
. 0ndChapterisabrieftourofimpactcratersat size final chapter describes the connections between
scales ascending from a few microns to a few impactcrateringand otherterrestrial orplanetary
thousand kilometers. This chapter is meant to ac- geologic processes and attempts to show impact
quaint the complete novice with the general ap- cratering’s place in geology and planetary science
pearanceofimpactcratersand providesaconcep- asawhole. Mathematicaldetailsofthederivation
tual framework for organizing crater morphology ofthe Hugoniot equations are treated in the first
as a function ofsize. The third chapter is an eleé appendix, while a second appendix describes
mentary introduction to stress wave physics with equations ofstate thathave been found usefulfor
anemphasis on those processes important for im- impact cratering computations, including tables
pact cratering. Most expositions of shock wave of equation of state parameters for a variety of
physics begin with the Hugoniot equations and materials ofgeologic interest. '
derive the properties ofstrong stress anes from The reader should be warned that many details
them. In this book I have chosen to acquaint the ofthe impact cratering process are still not fully
PREFACE vii
understood. In many cases this leaves room for trouble with his review of Chapter 7, and Bob
subjective interpretation. Naturally, I have Strom reviewed Chapters 6 and 10 and corrected
tended to favor my own interpretations in these some of my errors. I owe thanks to Bill Mc-
cases. Wheneverthis situation arises in the book, Kinnon for unpublished information on the
however, I have tried to point out the areas of strength-gravity transition, Gene Shoemaker for .
controversy and have provided references to al- unpublished data on the primary meteoroid flux,
ternativepoints ofview. Ann Vickery, who read and Ewen Whitakerforhistorical information on
every portion of the manuscript and provided lunar studies, hisvmarvelous prints of Galileo’s
many helpful comments, has expressed the opin- original lunar maps (the Frontispiece of this
ion thateveryexpertonimpactcrateringwill find book), and the LickObservatoryphoto ofthefull
somethingtodisagreewithinthisbook. Thatisas moon.EdGafi‘neyprovidedreferencestomanyof
it should be, and by putting these areas of dis- the Tillitson equation of state parameters, and
agreement,“on the table” I hope to stimulate fur- . Pete Schultz, Ron Greeley, Arthur Frankel, Dick
ther Work to clarify these uncertainties. I have Pike, and Dave McKay all provided. sorely.
also tried to indicate areas, such as oblique im- neededphotographicprints. MaryEllenByersex-
pact, that need furtherwork, again in the hope of cavatedlargenumbersoforiginalphotosfromthe
stimulatingfurther study. depths ofthe LPL plate vault and the image li-
Researching and writing this book have occu- brary, andTerrySchemenauerdidanoutstanding
piedseveralyears. DuringthistimeIhavehadthe jobturningthemintocamera-readycopy.Mostof
benefitofadviceandassistancefromalargenum- the line drawings in the bookare the work ofSue '
ber of people without whom this book would Angelon, although George Feltovic helped in fin-
’ never have been completed. Ofall these people, ishing off the last of them as the deadline
Ann Vickery has provided the most consistent approached. NASA’sPlanetaryGeologyandGeo-
support and sometimes the strongest criticisms. physics program has supported my research on
This book would have had a very different form impact cratering for the past ten years. Finally, I
without her aid. Several graduate classes at the . mustthank Joyce Berry ofthe Oxford University
Lunar and Planetary Lab have tested the success Press, whose encouragement changed my vague
ofmy pedagogic schemes. Don Gault kindlycon- ideasaboutabookonimpactcrateringintoacon-
sentedtoreadtheentiremanuscriptandprovided crete writingproject, andwhose patient “remind-
. many helpful criticismsalong with agreat deal of ers” helpedpush the project into completion.
information. He has allowed me to publish a
number ofphotographs, some ofwhich are from Tucson . H, J_ M_
' his unpublished work. Tom Ahrens read and im- April 1988
provedChapter4, RobertSchmidtkeptmeoutof
