I A SYSTEMATIC STUDY OF THE SPECTRAL REFLECTIVITY CHARACTERISTICS OF THE METEORITE CLASSES WITH APPLICATIONS TO THE INTERPRETATION OF ASTEROID SPECTRA FOR MINERALOGICAL AND PETROLOGICAL INFORMATION by MICHAEL J. GAFFEY B.A., University of Iowa (1968) M.S., University of Iowa (1970) Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in the Department of Earth and Planetary Sciences Massachusetts Institute of Technology February, 1974 Signature of Author Department of Earth and Planetary Sciences, January 7, 1974 Certified by ,~, Thesis Supervisor Accepted by on A SYSTEMATIC STUDY OF THE SPECTRAL REFLECTIVITY CHARACTERISTICS OF THE IETEORITE CLASSES WITH APPLICATIONS TO THE INTERPRETATION OF ASTEROID SPECTRA FOR MINERALOGICAL AND PETROLOGICAL INFORMATION by Michael J. Gaffey B.A., University of Iowa (1968) M.S., University of Iowa (1970) Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in the Department of Earth and Planetary Sciences Massachusetts Institute of Technology January 7 , 1973 ABSTRACT The purpose of this thesis is to study the spectral re- flectivity of a natural, cosmically occurring series of solid materials (meteorites) and to discuss the applicability of this information to the interpretation of asteroid spectra for the mineralogical and petrological compositions of the asteroid surfaces. A detailed study of the transmission spectra of oriented olivine crystals shows that the interaction of a photon with the groundstate orbital of a transition metal ion is a simple function of the shape of the orbital (distribution of electron probability) and the vibration vector of the photon. The features in the reflection spectrum of a silicate material can be correlated with crystal field effects in the mineral phases. The compound features in the reflection spectra can be calculated using the centers and halfwidths of the absorptions due to each transition in the individual mineral phases. Physical mixtures of minerals are spectrally dominated by the dielectric phases with the highest optical density. Particle size varia- tions strongly affect the albedo of a particulate surface but only weakly affect the relative reflectivity curve over a wide range of particle sizes. Asteroidal surfaces should be fragmental mixtures of the underlying material. The particle size distribution and glass content of the surface should have minimal effect on the relative reflection spectra, such that variations between the spectral reflectivity curves of asteroids are primarily functions of petrological and mineralogical variations between the surfaces of these bodies. The meteorites represent a mineralogical and petrological series which should share many of the composi- tional characteristics of asteroidal material. Measurements were made of the spectral reflectivity curves of 156 meteorite specimens from essentially all meteorite classes. Examination of high quality spectra (un- altered specimens) from these data has shown that each meteorite class has a characteristic spectral reflectivity curve. Varia- tions between spectra of different materials can be understood in terms of the abundance and distribution of mineral phases, metamorphic grade and shock history of the specimens. The characteristic curves can be compared directly to narrow band- pass filter measurements of asteroid spectra to determine the mineralogy and petrology of the asteroid surfaces. A litho- logical interpretation of several asteroid spectra indicates a wide variety of materials are present. The relative propor- tions of these materials are totally unlike the proportions of meteorite types observed in terrestrial falls. Thesis Supervisors Thomas B. McCord Acknowledgements Scientifically, there are at least four individuals without whose active and expert cooperation, this thesis would not have been possible in anything like its present form or on the relatively short timescale of its completion. I would like to acknowledge the contribution of each of these person separately. Dr. Thomas B. McCord was my advisor throughout this effort and supplied not only the essential scientific and financial assistance, but also provided a great deal of advise and encouragement. Dr. McCord, perhaps more importantly, provided an astronomical framework within which this work could be developed and exploited to the utmost. He also provided valuable critical evaluations of the preliminary versions of this thesis. Dr. Roger G. Burns provided not only a great deal of insight into the physics of crystal field theory and the nature of comp- ositional data contained in transmission spectra, but also provided the wherewithall to undertake an experimental program to correlate the known physics of transmission spectra to the measured characteristics of the reflection spectra. Dr. Burns also provided invaluable critical assistance in the preparation of this manuscript, both in form and content. Dr. John B. Adams provided the equipment required to make the spectral reflectivity measurements of my specimens as well as providing a great deal of insight into the nature of the reflection spectrum of a material and the parameters which govern variations in these spectra. His previous work with the reflection spectra of rocks and minerals provide an excellent background for my work. Dr. Edward Olsen, of the Field Museum, provided the bulk of the major samples utilized in this study. A whole day of his time was spent going through the museum's collection, literally from Abee to Zhoutnevyi, allowing me to chose the specimens most needed for my work. Indeed, he supplied specimens which I would have probably found impossible to obtain from any other source. He also provided a great deal of information into the nature and relationships of meteoritic materials which helped greatly in my understanding of this subject. On a more personal basis, irreplacable and invaluable support was provided by Susan (Jenks) Gaffey, my fiance and wife during this period. The work could probably have been done without her aid and understanding, but it might not have been worth while. A great many other indiviuals provided scientific or technical input which greatly expedited this work. Their contributions to this thesis are many and varied. Dr. Clifford Frondel and Mr. David Cook provided a large number of meteorite specimens from the Harvard collection which provided an excellant starting selection. Dr. Carleton Moore of Arizona State University and Dr. Karl Turekian of Yale University also provided meteorite specimens to flesh out this work. Dr. John Lewis provided a great deal of insight into the nature of meteoritic materials and possible models for their 5 origins. Dr. Clark Chapman provided both a critical review of portions of this work and a large amount of practical information concerning asteroids and the asteroidal spectra. Dr. Robert Huguenin assisted my understanding by sharing his knowledge about charge tranfer effects in silicates. Ms. Carle Pieters and Mr. Michael Charette both provided a forum for discussing the work and various results which helped greatly in separating the wheat from the chaff. Mr. George Fawcett and Mr. Lawrence Bass both provided invaluable aid in my battles with the Big Blue Box (IBM 360-65). With their aid I managed to win the war. To all of the above individuals and to any whom I might have inadvertantly omitted, I offer my very sincere gratitude (and maybe even a trip to Cabots). Table of Contents Abstract Acknowledgements Table of Contents List of Figures List of Tables List of Appendices Chapter Is Asteroids and Meteorites: Background a) Introduction I-1 b) Purpose of 'this study I-2 c) Asteroids as members of the Solar System I-4 d) The choice of meteorites to represent the asteroidal surface materials I-7 e) Other materials for comparison I-11 References - Chapter I 1-14 Chapter II: Previous spectral reflectivity work a) Meteorites and natural silicates - Laboratory measurements II-1 b) Asteroids - Observational work II-4 c) Mineraloeic and petrologic interpretation of asteroid spectra II-7 References - Chapter II II-8 Chapter III: Spectral reflectivity of natural silicate materials: Internal Effects a) Introduction III-1 b) Crystal Field Theory III-1 c) Crystal field effects measured in minerals III-5 d) An ular variations of photon interaction III-8 e) Angular variation in photon interaction - Experimental III-10 f) Photon interaction: Implications and discussion III-18 g) Summary and conclusions III-22 References - Chapter III III-22 Chapter IV, Spectral reflectivity of natural materialss external Effects a) Introduction IV-1 b) The nature of light reflection - dielectric materials Iv-1 c) Particle size IV-2 d) Particle packing effects IV-5 e) Illumination angle effects IV-6 f) Mixing of mineral phases IV-7 g) Metal phases IV-10 h) Conclusion IV-12 References - Chapter IV IV-12 Chapter V: Atteroid surfacess Physical and litho- logical characteristics a) Introduction V-1 b) Surface texture and materials: Dynamic considerations V-1 c) Surface texture and composition - Obser- vational Data V-13 d) Conclusions V 14 References - Chapter V Chapter VI: Meteorites: Mineralogy and petrology a) Introduction VI-1 b) Meteoritic minerals VI-2 c) Meteorite types: Irons VI-5 d) Meteorite types: Stony-irons VI-8 e) Meteorite types: Stones - Chondrites VI-10 f) Meteorite types: Achondrites VI-21 g) Alternate meteorite compositions VI-27 References - Chapter VI VI-28 Chapter VI: Experimental Procedures a)