O. RICHARD NORTON

METEORITES

THE CAMBRIDGEENCYCLOPEDIA OF

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© O.R. Norton 2002

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First published 2002

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Library of Congress Cataloguing-in-Publication Data

Norton, O. Richard.The Cambridge encyclopedia of meteorites / O. Richard Norton.

p. cm.Includes bibliographical references and index.ISBN 0 521 62143 71, Meteorites. I. Title.QB755.N65 2002523.5�1�03–dc21 2001035621

ISBN 0 521 62143 7 hardback

Preface xvForeword xix

11 Cosmic dust: interplanetary dust particles 1

The smallest meteoroids – interplanetary dust particles (IDPs) 1The zodiacal light 2A solar connection 3Lifetime of interplanetary dust particles 3Density of interplanetary dust particles 4Comets as sources of interplanetary dust particles 4Atmospheric collection of interplanetary dust particles 5Collection of deep sea interplanetary dust particles 8Impacts close to home – a practical matter 8

Meteors 11Sporadic meteors 11Origin of sporadic meteors 11Shower meteors 12The Leonid meteor storm 13

Harvesting interplanetary dust particles in the space environment 18

To touch a star 19Celestial diamonds 19Celestial silicon carbide 20Interstellar graphite 21PAHs in graphite 21

A final word 24

References 25

12 The fall of meteorites 27

Orbits of meteoroids 28The fireball networks 28

Characteristics of a meteoroid’s atmospheric passage 32The true trajectory 32Height and angle of descent 32Meteoroid velocity 33Atmospheric drag 33Light phenomena 34Sound phenomena 36Electrophonic sounds 37

Meteoroids to meteorites: a lesson in survival 39Ablation 39The distribution ellipse 40“Fossil” strewn fields 42

vii

Contents

The temperature myth – hot or cold? 42The final resting place 44

References 46

13 External morphology of meteorites 47

Some general characteristics 48Falls and finds 48Density 48Size 49Meteorite shapes 50Oriented meteorites 52Fusion crust 53

Weathering of meteorites 58Mechanical weathering 58Chemical weathering 58The myth of lawrencite in iron meteorites 61A weathering scale 63

Terrestrial lifetimes of meteorites 65

“Fossil” meteorites 67The Brunflo “paleo” fossils 67The oldest “paleo” iron? 67A K–T boundary meteorite? 67

References 69

14 Classification of meteorites: a historical viewpoint 71

The early years 72

The Rose–Tschermak–Brezina classification 73

The Prior classification 75

References 77

15 Primitive meteorites: the chondrites 79

Petrologic–chemical classification of the chondrites 81Cosmic abundances 82

Chemical classification of the chondrites 83

The Van Schmus–Wood criteria for the petrologic–chemicalclassification of chondrites 86

1. Chondrule texture 862. Matrix texture 863. Homogeneity of olivine and pyroxene compositions 894. Structural states of low-calcium pyroxene 905. Degree of development of secondary feldspar 906. Igneous glass in chondrules 907. Metallic minerals (maximum weight percent nickel in FeNi) 908. Sulfide minerals (average nickel in FeS) 909. Weight percent carbon 90

10. Water content 91

viii Contents

Shock metamorphism and shock stage classification 93The Stöffler–Keil–Scott shock classification system 93

S1 – unshocked 93S2 – very weakly shocked 93S3 – weakly shocked 93S4 – moderately shocked 93S5 – strongly shocked 95S6 – very strongly shocked 95

Brecciated chondrites 96Regolith breccias 96Impact-melt breccia 98

References 100

16 Chondrites: a closer look 101

Chondrite density and porosity 103

Chondrules 106Chondrule sizes and abundances 106Chondrule textures and compositions 106Porphyritic chondrules 108Granular olivine–pyroxene chondrules (GOP) 110Nonporphyritic or droplet chondrules 110

The other chondrites 115Enstatite chondrites (E) 115“Rumuruti” chondrites (R) 116

Summary 117

References 118

17 Primitive meteorites: the carbonaceous chondrites 119

Ordinary vs carbonaceous chondrites 120

Carbonaceous chondrite groups 121CI chondrites (C1) 121

A new CI chondrite? 122CM chondrites 123

Are CI and CM chondrites related to comets? 125CV chondrites 128

Matrix 128Chondrules 129Olivine aggregates, dark inclusions 131Calcium–aluminum inclusions (CAIs) 131

CO chondrites 135CK chondrites 136CR chondrites 138CH chondrites 139

Organic carbon 140

Summary 142

References 143

Contents ix

18 Differentiated meteorites: the achondrites 145

Major characteristics of differentiated meteorites 146

Differentiation 147

Fractional crystallization and the igneous process 148

Asteroidal achondrites 151The HED association 151

Eucrites 151Diogenites 153Howardites 156

The mesosiderite connection 157Aubrites – the enstatite achondrites 159Ureilites 161Angrites 162Brachinites 163

Primitive achondrites 165Acapulcoites and lodranites 165Winonaites 165

Martian meteorites – the SNC group 166Shergottites 166Nakhlites 167Chassignites 168Origin of the SNC meteorites 170The saga of martian meteorite ALH 84001 172

Lunar meteorites – lunaites 175History’s lesson 176Baldwin’s Moon 177A history of discovery 177Anorthositic regolith breccias 178Mare basalts 179

References 182

19 Differentiated meteorites: irons and stony-irons 183

Mineralogy of meteoritic iron–nickel 184Crystal structure of meteoritic iron–nickel minerals 185Iron–nickel stability phase diagram 185Formation of the Widmanstätten structure 187

Classification of iron meteorites 190Structural classification of iron meteorites 190

Hexahedrites (H) 190Octahedrites (O) 190Ataxites (D) 191Anomalous irons 191

Chemical classification of iron meteorites 191

Chemical vs structural classification 197

Silicate inclusions in iron meteorites 199

Cooling the core 201

Stony-irons: the pallasites 203

x Contents

Texture 203Composition and classification 203Formation of pallasites 205

Metallic inclusions in iron meteorites 207Swathing kamacite 207Troilite 207Schreibersite 208Cohenite 209Graphite 209

References 212

10 Meteorites and the early Solar System 213

Radioisotopes – some basics 215

Decay constant and half-life 216Rubidium/strontium method 217Types of meteorite ages 218

In the beginning 220The solar nebula 221The first-condensed refractory minerals 224Formation of the first chondrules 226Multiple melting and recycling of chondrules? 227

The age of the Solar System 234Aluminum-26 and the age of chondrules and CAIs 234Absolute age of the Solar System 236Impact metamorphism 236Cosmic-ray exposure ages 237Terrestrial age 238

References 239

11 Asteroid parent bodies 241

The asteroid belt 242Titius–Bode Rule 242Discovery of the first asteroids 242Modern discoveries 243Numbering and naming asteroids 243Main belt asteroids 246Trojan asteroids 247Near-Earth and Earth-crossing asteroids 247

Asteroids as parent bodies of meteorites 249Meteoroid orbits and the asteroid belt 249Classifying asteroids through infrared reflectance spectrophotometry 249Distribution of asteroid types 250

Matching meteorites with parent asteroids 252C-type carbonaceous asteroids 252S-type asteroids and ordinary chondrites 252M-type metal asteroids 254

Physical characteristics of asteroids 255Space weathering 256

Contents xi

Onion shell vs rubble pile asteroid models 256Asteroid 4 Vesta and HED meteorites 258Where are the mantle rocks? 259Close encounters: flyby of asteroids 951 Gaspra and 243 Ida 260

Finale: NEAR encounter with 433 Eros 262Flyby of 253 Mathilde 263NEAR-Shoemaker orbits 433 Eros 263The final approach 266

References 269

12 Terrestrial impact craters 271

An impact crater misjudged 273

Impact cratering in the Solar System 276

Mechanism of impact cratering 279Stage 1: contact/compression 279Stage 2: excavation 279Stage 3: modification 281Effects of changing parameters on crater size 281

Morphology of impact craters 285Simple craters 285Complex craters 285

Criteria for the definitive recognition of terrestrial impact structures 291

Surface structures 291Remnants of the meteorite 291Shatter cones 292Impactites 293Coesite and stishovite 296Shocked minerals 297

Catastrophic impacts on Earth 300The Tunguska event 300When catastrophism had its day 301The Chicxulub crater 304Asteroid or comet? 305The Permian–Triassic extinction 305

References 306

Appendices

Appendix A Classification of meteorites (A.E. Rubin, 2000) 307

Appendix B Formation ages of selected meteorites 309

Appendix C Minerals in meteorites 311

Appendix D Preparing and etching iron meteorites 315

Appendix E Testing a meteorite for nickel 319

Appendix F Meteorite museum collections and selected researchfacilities 321

Appendix G Known terrestrial impact craters 325

xii Contents

Appendix H Summary of meteorites by classification(compiled by Bernd Pauli, June 2001) 331

Glossary 341

General index 349

Meteorite index 353

Contents xiii

for the secrets it holds. H.H. Nininger, the great meteoritehunter of the twentieth century, in his public lectures and inhis writings often lamented the surprising fact that meteor-ites were not part of the study of rocks in geology classes,either elementary or advanced. Yet, ironically, meteorites,more than terrestrial rocks, tell us much about Earth’sorigins. They are as old as the Solar System and considerablyolder than the crustal rocks of the Earth. Except for special-ized graduate courses in large universities with planetaryscience curricula, the situation has changed very little. Thischapter (and those to follow), then, is as much for buddinggeologists (and their teachers) as it is for the meteorite col-lector.

Chapter Three

External morphology ofmeteoritesAsk anyone you know to describe the most general charac-teristics they expect to see in a meteorite and they invariablygive you three: heavy, black, and full of “holes”. Althoughthese characteristics are quite true for some meteorites(actual holes are rare in meteorites but many iron meteoritesare deeply pitted), they are also true for Earth basalt. Othersmight say that meteorites are primarily made of iron and areattracted to a magnet. But then so is terrestrial magnetite (fre-quently mistaken for a meteorite), and, in fact, most stonymeteorites. How then can you recognize a meteorite? Therecognition and recovery of meteorites in the field is funda-mental to everything else that follows. We must first be ableto identify a fallen meteorite before we can probe its interior

48

Put in the simplest of terms, meteorites are rocks. Like ter-restrial rocks the majority of meteorites are made of aggre-gates of crystalline minerals. For the most part the mineralsof meteorites are common to terrestrial rocks but the way inwhich they are displayed in meteorites is unique to meteor-ites. The internal texture of meteorites is an important diag-nostic characteristic discussed in detail in the chapters tofollow. This chapter deals with the exterior characteristics ofmeteorites as we find them in the field. No one has ever seena meteorite as a meteoroid in its natural space environment sowe do not know what a truly pristine meteoroid looks like.We found in Chapter 2 that meteoroids are modified byheating, mechanical stresses and ablation. What we find onEarth has therefore been terrestrialized to some degree.Fortunately, most of the modifying effects occur on the exte-rior of the meteorite. The interior remains relatively pristinemuch as it was in its former space environment provided itdoesn’t remain undiscovered for too long.

In the most general terms, meteorites are classified intoone of three broad categories: stones; irons; and stony-irons. Thestones are rocks made up of common silicate minerals. Theseminerals are usually in a crystalline form small enough torequire a magnifier or microscope to see them. Theirarrangements within meteorites are quite different thaninside most terrestrial rocks which suggests radically differ-ent origins for them. The irons are, as the name implies,made primarily of iron alloyed with nickel. Most ironmeteorites are at least 90% iron with nickel, cobalt, sulfurand other elements making up the remaining 10%. Thestony-irons are a mixture of both silicate minerals andiron–nickel alloy. The ratio of stone to iron can vary consid-erably from meteorite to meteorite or even within a singlemeteorite but is roughly two parts stone to one part iron byvolume.

Falls and finds

Meteorites are designated as either falls or finds. The distinc-tion between them is important, statistically. Meteorites clas-sified as falls are those observed to fall and then picked upshortly thereafter. Finds are meteorites that were not seen tofall, remaining undiscovered for a period of years and thensubsequently recognized and recovered. If we divide allknown meteorites into these two groups, an important con-clusion is reached. Of the observed falls, 94% are stones, 5%are irons and 1% are stony-irons. Therefore, we correctlyconclude that the vast majority of meteorites falling to Earthare stones. Yet, if we look at the total inventory of meteorites

worldwide which includes both falls and finds, we find only69% are stones, 28% are irons, and 3%, stony-irons. Why thecontradiction? Simply because stony meteorites, especiallyterrestrially weathered specimens, are more difficult to dis-tinguish from ordinary terrestrial stones than either irons orstony-irons; the numbers reflect the ease with which ironmeteorites are recognized and recovered by the general pop-ulation. This is strongly substantiated if we look at the totalsamong the meteorites classified as finds alone. Here, stonesdrop to 56%, irons increase to 40% and stony-irons increaseto 4%. I must add here that scientists seldom search randomareas for meteorites (Antarctic and Sahara Desert meteoritesexcepted) and fewer yet see them actually fall. Most finds aremade by ordinary people who work and play out-of-doors:farmers plowing their fields; highway construction crews;and an army of weekend rock and gold hunters scatteredacross the countryside.

Density

Most meteorites have densities greater than the average ter-restrial rock. This means that for a given size, meteorites areheavier than most terrestrial rocks. Most stony meteorites aretightly compacted with little pore space. The most commonstony meteorites, the ordinary chondrites, have a density range ofbetween 3.5 and 3.8 g/cm3. By comparison, granite, anigneous rock that makes up much of the Earth’s crust, has adensity of 2.7 g/cm3. Meteorite high densities are due pri-marily to the presence of iron. Some of this iron is locked iniron-bearing silicates but some contain as much as 20% inthe metal phase. There is sufficient elemental iron in most ofthe ordinary chondrites that they are attracted to a strongmagnet and metal detectors will detect them (as relic or trashindications!). Rare type chondrites, the so-called EH chondrites,can have as much as 35% iron metal. Of the chondriticmeteorites the carbonaceous chondrites show the greatest variationin density. Of the seven known groups, three contain abun-dant chemically bound water with little or no metal. Theother carbonaceous chondrite groups contain variableamounts of unaltered metal. Thus, the group as a whole varyin density between 2.2 and 2.9 g/cm3.

The irons, not surprisingly, are the densest of the meteor-ites. They contain variable amounts of iron alloyed to nickelwith accessory metal sulfides, carbides, and phosphides.They have densities twice that of the stones ranging between7.5 and 7.9 g/cm3. Their composition resembles no rocktype on Earth but they are probably similar to Earth’s corematerial. (Earth’s core density is considerably greater than

Some general characteristics

iron meteorites since the core is under a high state of com-pression. Uncompressed, Earth core material is probably verysimilar in density to iron meteorites.) There is no mistakingan iron meteorite. Its weight alone attracts attention. Thestony-irons, being roughly two-thirds silicate minerals toone-third iron–nickel alloy by volume have densities that liesomewhere between the stones and irons.

Size

Meteorites show an enormous variation in size from micron-sized dust particles filtering slowly through the atmosphereto giants weighing many tons. The average meteorite foundon Earth is about the size of a baseball. The size of a meteor-ite depends upon its impact history in space as well as itsfragmentation history in the atmosphere and its further frag-mentation when it impacts Earth. The vast majority ofmeteoroids fragment as they ablate in the atmosphere. It is nosurprise that the largest meteorites in the world are ironssince they have greater tensional and compressional strengththan stones and are better able to withstand the aerodynamicpressures encountered during atmospheric passage as well asthe forces at impact. A large iron meteorite has the bestchances of remaining in one piece (assuming no pre-entryfractures exist) if it enters the atmosphere at large zenithangles with a velocity of less than 12.3 km/s. A meteoritewith a diameter of 5 m entering at a 60° zenith angle and aspeed of 10 km/s may impact without fracturing, but impactspeeds of more than 2–4 km/s will produce a crater andvaporize the meteorite in the process.1

There is therefore a limit to the maximum size of aniron meteorite reaching Earth that depends upon the heat

generated at impact. This is determined by the initial mass ofthe meteoroid before it enters the atmosphere, its initialvelocity and its remnant cosmic velocity when it hits theground. We saw in Chapter 2 that meteorites over about 10tons retain some of their cosmic velocity right to the Earth’ssurface. For example, if a 1000 metric ton iron meteorite hitsthe top of the atmosphere at 20 km/s at a zenith angle of 45°,it retains about 63% of its cosmic velocity and about 90% ofits initial mass when it strikes the Earth. Given that it doesn’tbreak up in the atmosphere it retains a residual cosmic veloc-ity of about 12.6 km/s with 900 metric tons of its mass stillintact when it impacts. The meteoroid possesses enormouskinetic energy at impact, most of which is converted almostinstantly to heat. An iron meteoroid of this mass and speedwill generate about 8�1013 J of energy or almost 19000cal/g of heat on impact. Now, 19000 cal/g is an enormousamount of heat especially when we compare it to a commonvalue such as 540 cal/g necessary to turn water to a vapor at100°C. V.F. Buchwald points out that the element carbon hasone of the highest heats of vaporization of any knownmaterial, 13000 cal/g. In the example above, even if themeteorite were pure carbon, it would vaporize on impact.This heat of impact is more than sufficient to vaporize themeteorite on impact even though some of the energy is con-sumed in producing a shock wave in the ground and a cratera few hundred feet across. Many such craters are known onEarth. Meteor Crater, Arizona, is the classic example and thebest preserved impact crater on Earth. It was produced by aniron body estimated to be about 30 m in diameter. Nothingremains of this body today other than fragments off the mainmass (Fig. 3.1). Since meteorites heavier than 100 tons retainsubstantial amounts of their original cosmic velocity and

Some general characteristics 49

Fig. 3.1. A 240 g fragment of a Canyon

Diablo iron meteorite from Meteor

Crater. The hole results from terrestrial

weathering within thin-walled ablation

cavities. Specimen dimensions are

approximately 75�67 mm. (Photo by

Michael Casper.)

mass, they tend to self destruct on impact. If the meteoriteenters at a zenith angle of 60° to near grazing incidence, itsspeed might be slowed sufficiently to reach Earth withoutdestroying itself or fragmenting. The fact we see no meteor-ites larger than the Hoba iron (60 tons) may simply meanthat very few large meteorites in the 100� ton categorysatisfy these criteria.

Meteorite shapes

Meteorite shapes are dictated primarily by ablative forces andfragmentation. The original shapes are never retained andremain unknown. Most stony meteorites have random shapessince they almost always explosively disrupt and fragmenthigh in the atmosphere. They break along deep, possibly pre-

existing fractures; and, like crystalline rocks on Earth, whenstony meteorites break, sharp edges result. If fragmentationoccurs high in the atmosphere at or near the beginning of theablation process, the sharp edges are rounded by ablation andthe rough broken faces often retain a remarkably flat surfacesmoothed over by the fusion process so that many stonymeteorites have obvious angularity (Fig. 3.2). The overallsurfaces of stony meteorites are smooth and slightly undu-lating, suggesting uneven ablation processes. Sometimesstones develop flattened depressions with raised edges.Superimposed over these depressions may be small pitswhere turbulent flow of superheated gas has ablated specificareas (Fig. 3.3). They range in shape from nearly circular tohighly elliptical. Hand specimens frequently show pits thesize and shape of a human thumb print. These “thumbprints” are called regmaglypts or sometimes piezoglypts. Theirshape and size relate to their location on the meteorite andto areas of differing ablation.

Iron meteorites have irregular shapes. If they are associatedwith impact craters, they tend to be flattened and fragmentedlike shrapnel from an exploded bomb. Meteorites from theSikhote-Alin fall that were produced by large individualsexploding on impact are typically jagged and distorted.Meteorites from the same fall that did not shatter on impactor were not involved in explosions are more rounded andshow remarkable deep pitting and flow structures. Somespecimens show characteristics of both types. Figure 3.4illustrates the two opposing sides of a Sikhote-Alin ironmeteorite. The left side shows shallow smoothed-over

50 External morphology of meteorites

Fig. 3.2. Angularity of a 920 g Gao-

Guenie H5 chondrite. Note the flat sur-

faces nearly at right angles to each

other. These are surfaces produced by

fragmentation in the atmosphere along

preexisting fracture lines acquired

while in space. The sharp edges have

been rounded by ablation. Specimen

dimensions are 113�94 mm. (Photo

by Michael Casper.)

Fig. 3.3. This 800 g Holbrook L6 chondrite shows a slightly concave face

(right) containing shallow cavities or regmaglypts produced during the abla-

tive process. The face measures 300 mm in the longest dimension. The

largest Holbrook specimen recovered weighs 6.6 kg and resides in the col-

lection of the Center for Meteorite Studies at Arizona State University.

(Photo by Michael Casper.)

pitting, many with elliptical shapes where melting has takenplace after the pitting formed. The right side shows deep,subcircular pitting on an obvious fragmented side. Thismeteorite was in the early stages of ablation when fragmen-tation took place off the main mass. The raw fragmented sideapparently turned, orienting itself toward the forward-proceeding side, producing deep pits and all but erasing thefragmented nature of the meteorite. What was once the sideof a large mass became oriented away from the forward end.Material accumulated on this now rear-facing side, flattenedand broadened the pits.

Unlike stony meteorites that are composed of mineralseach with individual crystal structures, irons are constructedaround a single cubic crystal form, either octahedral or hexa-hedral and they tend to break along natural crystal boundar-ies. Figure 3.5 shows two sides of a Sikhote-Alin specimen.On the left is a surface with many ablation pits. Near the endof its ablation period, the meteorite suffered fragmentationoff the main mass. The right picture shows the fragmentedside and a careful examination will reveal triangular-shapedstructures that are expressions of the octahedral dipyramidsaround which the nickel–iron alloy is structured.

Pits on iron meteorites are much deeper than on stones,suggesting differential melting of softer minerals. They tendto be edged by sharp ridges that in some cases terminate in

sharp points, a quality much valued by the collector.Commonly, nearly spherical inclusions or nodules of ironsulfide (troilite) are found in the interiors of many ironmeteorites that have a substantially lower melting point thanthe surrounding iron–nickel alloy. If these inclusions are nearthe surface, they can rapidly ablate away leaving round cav-ities. A most extraordinary case of pitting is found in theGoose Lake iron. Within this meteorite some of the pits havelarger diameters than their openings. Just how ablation couldhave formed them is something of a mystery. Many believethese pits were produced by terrestrial weathering of lessresistant materials (Fig. 3.6).

Some general characteristics 51

Fig. 3.4. This is an iron meteorite from the Sikhote-Alin fall showing opposing

sides. The left side shows a heavily ablated and melted face where the abla-

tion pits have been smoothed over. The right side shows deep pitting on a

fragmented side that su≈ered less ablation and melting. The specimen is

12 cm in length. (Photos by O. Richard Norton.)

Fig. 3.5. These two photos show front and back sides of a Sikhote-Alin iron

meteorite. The left image shows an ablated smooth surface with extensive

pitting. The right image shows the opposing side where fragmentation took

place near the end of the ablation period revealing parallel-running linear

features typical of octahedrite meteorites. The specimen measures

11.8�9.5 cm. (Specimen provided by Dr David Mouat, Desert Research

Institute, University of Nevada System. Photos by O. Richard Norton.)

Oriented meteorites

During their atmospheric flight, most meteoroids tumblerandomly. This tends to smooth out irregular meteorites intoroughly elliptical or subspherical shapes. Many mimic theappearance of rounded riverbed stones. About 5% of all stonymeteorites exhibit a symmetry that suggests they havesomehow oriented themselves as they ablated. Such meteor-ites are referred to as oriented, usually having the appearanceof a steep-sided cone or flattened shield. These shapesstrongly suggest that the meteoroid has maintained stabilizedflight through most of its visible trajectory. Meteoroids tendto orient themselves in a configuration that producesmaximum drag forces. This means that a meteoroid with aflattened shape or a large flat area on an otherwise randomshape will orient the flat side perpendicular to the directionof maximum drag (Fig. 3.7). This stabilizes the meteoroid

and prevents tumbling. To produce the oriented shaperequires that the stone remain stationary as it ablates or thatit rotate about an axis perpendicular to the flat face and alongits direction of motion. Once the meteoroid has attained itsorientation, the front end (end facing the direction of travel)begins to experience the greatest ablation, being sculpted bythe air currents flowing at hypersonic velocity around thefront end. Here the material melts rapidly and flows towardthe rear of the meteoroid where it accumulates, building abroad apron. At the same time, the front end or brustseite1

becomes narrow, forming a symmetrical cone or in somecases, a flattened shield shape.

Orientation is more common in iron meteorites if theydon’t disrupt in flight. About 28% show some signs of orien-tation and many are nearly perfect cones or shields. The greatWillamette meteorite is remarkably bell-shaped (Fig. 2.22).The stony meteorite from Adamana, Arizona, is nearly a

52 External morphology of meteorites

Fig. 3.6. View of a surface section of the

1169.5 kg Goose Lake iron meteorite

showing deep, rounded cavities. The

largest cavity shown is over 50 mm in

diameter. (Photo by O. Richard Norton.)

Fig. 3.7. A meteorite that stabilizes during atmospheric

flight may become oriented with respect to its direction

of motion. Here the flat side experiences maximum

drag, causing it to orient so that it spins around its

direction of travel with maximum ablation on the

forward-facing side. Ablation soon forms a cone or

shield shape.

II The word brustseite is German literally translated as breast side, or moreappropriately, front side. It is a seldom used term in meteoritics today but

still useful when referring to oriented meteorites which in many casesmimic the shape of the human female breast.