ABSTRACT Title of dissertation: LIGHT SCATTERING PROPERTIES OF ASTEROIDS AND COMETARY NUCLEI Jian-Yang Li, Doctor of Philosophy, 2005 Dissertation directed by: Professor Michael F. A’Hearn Associate Research Professor Lucy A. McFadden Department of Astronomy The photometric properties of asteroids and cometary nuclei, bodies important for understanding the origin of the Solar System, are controlled by the physical properties of their surfaces. Hapke’s theory is the most widely used theoretical model to describe the reflectance of particulate surfaces, and has been applied to the disk-resolved photo- metric analyses of asteroid 433 Eros, comet 19P/Borrelly, and asteroid 1 Ceres, in this dissertation. Near Earth Asteroid Rendezvous returned disk-resolved images of Eros at seven wavelengths from 450nm to 1050nm. The bidirectional reflectance of Eros’s surface was measured from those images with its shape model and geometric data. Its single- scattering albedo, w, was found to mimic its spectrum, with a value of 0.33±0.03 at 550nm. The asymmetry factor of the single-particle phase function, g, is -0.25±0.02, and the roughness parameter, θ¯,is28◦±3◦, both of which are independent of wavelength. The V-band geometric albedo of Eros is 0.23, typical for an S-type asteroid. From the disk-resolved images of Borrelly obtained by Deep Space 1 (DS1), the maps of its w, g, and θ¯ were constructed by modeling the reflectance of Borrelly terrain by terrain. w varies by a factor of 2.5, with an average of 0.057±0.009. g changes from -0.1 to -0.7, averaging -0.43±0.07. θ¯ is ≤35◦ for most of the surface, but up to 55◦ for some areas, with an average of 22◦±5◦. The 1-D temperature measurement from DS1 can be well described by the standard thermal model assuming a dry surface, except for one area, where the discrepancy can be explained by a sublimation rate that is consistent with the observed water production rate. HST images through three filters, covering more than one rotation of Ceres, were acquired. Its V-band lightcurve agrees with earlier observations very well. A strong absorption band centered at about 280nm is noticed, but cannot be identified. w of Ceres was modeled to be 0.073±0.002, 0.046±0.002, and 0.032±0.003 at 555nm, 330nm, and 220nm, respectively. The maps of w for Ceres at three wavelengths were constructed, with eleven albedo features identified. Ceres’ surface was found to be very uniform. LIGHT SCATTERING PROPERTIES OF ASTEROIDS AND COMETARY NUCLEI by Jian-Yang Li Dissertation submitted to the Faculty of the Graduate School of the University of Maryland, College Park in partial fulfillment of the requirements for the degree of Doctor of Philosophy 2005 Advisory Commmittee: Professor Michael F. A’Hearn, Chair Associate Research Professor Lucy A. McFadden Professor J. Patrick Harrington Professor Bruce Hapke Professor Roald Z. Sagdeev Doctor Anne J. Verbiscer c Copyright by Jian-Yang Li 2005 ACKNOWLEDGMENTS I own my gratitude to all people who made this thesis possible, and made my expe- rience in graduate school wonderful. First and the foremost, I would like to thank my two advisors, Mike A’Hearn and Lucy McFadden, for changing my view of the planetary system to a fantastic world that I enjoy staying in, looking at, and exploring for the rest of my life. Starting my graduate research work with some sort of language barrier as a foreign student, I was impressed by Mike’s prompt responses to my questions that sometimes even myself had trouble in understanding from what I said. I gained great confidence from the conversations with Mike at the beginning, and received invaluable scientific advises from Mike continuously since then. As a mentor, Mike guided me by not only his scientific advises, but also his confidence, infinite enthusiasm, and a great sense of humor. And thank you Mike for introducing me into and letting me work for the exciting Deep Impact project! Lucy, as one of the most important collaborators of Mike’s, has been a great advisor not only in my research, but also in my scientific personality and even everyday life. With her help, I never afraid of talking to anybody or presenting my work anywhere. She introduced me to so many people who have helped or will help me greatly in my career. I have been deeply impressed by her enthusiasm to astronomy, skills of communication, and team work. I am grateful to Lucy for her help in the way that is probably more important for me than my research. So thank you Lucy for sitting with me before my first iii presentation in the first DPS meeting I attended! I would like to thank Doug Hamilton for allowing me to work on a dynamics project in the summer of 2000, which was my first project in astronomy, allowing me to taste, for the first time, the exciting feeling of being in the field that has attracted me since my childhood. The plot of the stable time scale of test particles in the giant planet region is still on the wall in my office. Thanks Doug! I own many thanks to Pat Harrington, with whom I finished my second-year project on measuring the expansion of a planetary nebula. It was the fantastic HST images of the beautiful planetary nebula that allowed me finally realize that I did like astronomy, although I did not know if it was the “planetary” here that led me into the field of planetary sciences finally. I did not realize that I was going to use IDL to develop all software tools for my thesis work when Pat taught me some basic commands of IDL for the first time. Thank you Pat for the opportunity of working with you, and for your name in my first publication! Thanks also go to Casey Lisse, who helped me greatly with his quick mind and patience when I just started my thesis work. Thank Dennis Wellnitz for his clear expla- nations to any questions I asked. I really enjoyed talking with him. Thank Bruce Hapke for his invaluable advises and comments since I started to learn the theories of light scat- tering and apply it to my work. Thank Anne Verbiscer for reading my thesis with great detail, and for checking all the references. And thank everybody who has helped me in my research. In addition to all the important people in my career in astronomy, I am grateful to all the people who have helped me from other aspects. Thanks to the support from the iv department, especially John Trasco, Mary Ann Phillips, Linda Diamond, for shielding us with a peaceful and easy environment. Thanks to all my friends who signed either or both of the two graduation gifts I received. Thanks to my best friends, Jianglai, Yuanzhen, Qing, for painting my life with colors, and for giving me confidence. I owe a deep debt of thanks to my fiancee,´ Huaning, who witnesses the whole process of my thesis work, and every detail of the birth of this thesis. I would not have finished this thesis, and decided to devote my life to astronomy without her continuous support, understanding, encouragement, and forgiveness. I would not have been enjoying such a wonderful life that I am now without her. Thank you Huaning for the unique graduation gift you gave me, which was the best gift I ever received. And thank you for being with me anytime. I love you! Finally, thank the world for having so many mysteries and so much fun; thank modern technology for enabling us to go to the Moon, Mars, Jupiter, Saturn, and beyond. This thesis is dedicated to my parents! v TABLE OF CONTENTS List of Tables ix List of Figures xi 1 Introduction 1 1.1 History of Solar System Small Bodies ................... 1 1.2 Motivation . ............................... 9 1.3 Overview of Chapters . ........................... 16 2 Light Scattering Theory 18 2.1 Basic Concepts and Theoretical Preparation . ............... 18 2.2 Empirical Expressions of Reflectance ................... 24 2.3 Hapke’s Scattering Law ........................... 31 2.4 Phase Function and Planetary Photometry . ............... 42 2.5 Data Modeling Techniques . ....................... 45 3 Whole-Disk Phase Functions of Irregularly-Shaped Bodies 56 3.1 From Bidirectional Reflectance to Disk-Integrated Phase Function .... 56 3.2 Effects of Shapes . ........................... 59 3.3 Numerical Simulations with Ellipsoidal Shape ............... 67 3.4 Numerical Simulations with Eros’s Shape . ............... 73 3.5 Summary and Discussions . ....................... 78 vi 4 Asteroid 433 Eros 80 4.1 Background . ............................... 80 4.2 Ground-Based Phase Function ....................... 82 4.3 Disk-Resolved Photometry . ....................... 86 4.4 Discussions . ............................... 97 4.5 Summary . ...............................116 5 The Nucleus of Comet 19P/Borrelly 119 5.1 Background . ...............................119 5.2 Disk-integrated Phase Function .......................124 5.3 Disk-Resolved Photometry . .......................127 5.4 Disk-Resolved Thermal Modeling . ...................152 5.5 Discussions . ...............................163 5.6 Summary . ...............................172 6 HST Observations of Asteroid 1 Ceres 174 6.1 Background and Data Description . ...................174 6.2 Data Reduction ...............................177 6.3 Disk-Integrated Photometry . .......................182 6.4 Disk-Resolved Analysis ...........................188 6.5 Discussions . ...............................203 6.6 Summary . ...............................215 7 Summary and Future Work 217 vii 7.1 Summary . ...............................217 7.2 Future Work . ...............................223 Bibliography 236 viii LIST OF TABLES 2.1 A summary of five Hapke’s parameters. ............... 47 2.2 The Hapke’s parameters for the phase functions shown in Fig. 2.7. .... 52 3.1 Fitted parameters for the midpoint phase function and upper limit phase function. ............................... 71 3.2 The results of fitting the theoretical phase functions for Eros.