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Spectroscopic and Theoretical Constraints on the Differentiation of Planetesimals a Dissertation Submitted to the Graduate Divis

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Spectroscopic and Theoretical Constraints on the Differentiation of Planetesimals a Dissertation Submitted to the Graduate Divis SPECTROSCOPIC AND THEORETICAL CONSTRAINTS ON THE DIFFERENTIATION OF PLANETESIMALS A DISSERTATION SUBMITTED TO THE GRADUATE DIVISION OF THE UNIVERSITY OF HAWAI‘I IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN ASTRONOMY AUGUST 2009 By Nicholas A. Moskovitz Dissertation Committee: E. Gaidos, Chairperson R. Jedicke N. Haghighipour D. Jewitt S. Krot We certify that we have read this dissertation and that, in our opinion, it is satisfactory in scope and quality as a dissertation for the degree of Doctor of Philosophy in Astronomy. DISSERTATION COMMITTEE Chairperson ii c Copyright 2009 by Nicholas A. Moskovitz All Rights Reserved iii Gei˘ wo˘ de jiar¯ en.´ iv Acknowledgements Whatever weaknesses may be present in the following pages, they undoubtedly would have been far greater without support, insight and inspiration from numerous individuals. First and foremost, I am deeply grateful to my advisor Eric Gaidos whose intelligence, patience and deep curiosity have helped to guide me through the intellectual rigors of completing this dissertation. I want to extend a special thanks to Robert Jedicke who, through his enthusiasm and thoughtful insight, has been an invaluable contributor to this work. Thanks are due to the rest of my committee members (David Jewitt, Nader Haghighipour and Sasha Krot) for their helpful perspectives and for tactfully listening to my sometimes outlandish ideas. Many of my peers have been instrumental (in small ways and large) to the completion of this dissertation. To those who fall into the ubiquitous category of “too many to mention”, I thank you for your support. Others have been absolutely essential in their contributions to this work and I would be remiss if I did not recognize them individually. Mahalo nui loa to Mark Willman for the multitude of conversations that have helped me to navigate around a host of scientific road blocks, and whose excellent stories and good humor have made many a night on Keck and the 88” vastly more entertaining. I am also indebted to Bin Yang, an always dependable source of incisive opinions and measured criticism. This dissertation has been significantly enriched thanks to a number of conversations with Ed Scott, Jeff Taylor, Gary Huss, Paul Lucey, Sam Lawrence and David Nesvorny. My thanks to Bobby Bus for being an excellent resource for observing v knowledge and who, along with Richard Binzel, has been exceedingly generous with the sharing of data. I also want to thank several other “experts on the mountain” who have helped me to become a competent observer: John Dvorak, Dan Birchall, John Rayner, Jana Pittichova, Greg Wirth and Greg Aldering. I am very appreciative of the help I have received from the staff and faculty at the IfA, but I would particularly like to thank Karen Meech for starting me down a path that has led to the realization that the smallest bodies in the universe are indeed the most interesting, and Narayan Raja whose genius has saved me from countless hours of frustration that otherwise may have resulted in the destruction of more than one desktop computer. I would also like to thanks the good folks at NASA who deemed my research sufficiently interesting to provide three years of funding. Both at work and away, Lisa Chien has shared with me her brilliance, humor and irrepressible spirit. I can not imagine having tried to complete this thesis without her; I am so lucky that she has been a part of this adventure. Lastly I must send thanks to my family: to my parents for always providing their love and encouragement, and impressing upon me at an early age a deep fascination for the natural world. I am so very grateful for their support and the opportunities that they have provided me. And to my sister whose joie de vivre and audacious pursuit of her dreams have been an inspiration to me. vi Abstract The differentiation of small proto-planetary bodies into metallic cores, silicate mantles and basaltic crusts was a common occurrence in the first few million years of Solar System history. In this thesis, observational and theoretical methods are employed to investigate this process. Particular focus is given to the basaltic, crustal remnants of those differentiated parent bodies. A visible-wavelength spectroscopic survey was designed and performed to constrain the population of basaltic asteroids in the Main Belt. The results of this survey were used to provide statistical constraints on the orbital and size-frequency distributions of these objects. These distributions imply that basaltic material is rare in the Main Belt (particularly beyond the 3:1 mean motion resonance at 2.5 AU), however relic fragments of crust from multiple differentiated parent bodies are likely present. To provide insight on the mineralogical diversity of basaltic asteroids in the Main Belt, we performed a series of near-infrared spectroscopic observations. We find that V- type asteroids in the inner belt have spectroscopic properties consistent with an origin from a single parent body, most likely the asteroid Vesta. Spectroscopic differences (namely band area ratio) between these asteroids and basaltic meteorites here on Earth are best explained by space weathering of the asteroid surfaces. We also report the discovery of unusual spectral properties for asteroid 10537 (1991 RY16), a V-type asteroid in the outer Main Belt that has an ambiguous mineralogical interpretation. We conclude this thesis with a theoretical investigation of the relevant stages in the process of differentiation. We show that if partial silicate melting occurs within the vii interior of a planetesimal then both core and crust formation could have happened on sub-million year (Myr) time scales. However, it is shown that the high temperatures necessary to facilitate these processes may have been affected by the migration of molten silicates within these planetesimals and by chemical interactions between liquid water and silicate rock. Finally, a 1-dimensional model of heat conduction is used to explore whether differentiation would have occurred for planetesimals across a range of sizes (4 - 250 km) and times of accretion (0 - 3 Myr). viii Table of Contents Acknowledgements . v Abstract . vii List of Tables . xi List of Figures . xii Chapter 1: Introduction . 1 1.1 Introduction to Asteroids and Meteorites . 1 1.2 Physical Processes Relevant to Planet Formation and the Evolution of Small Bodies . 4 1.3 The Process of Differentiation . 6 1.4 Heat Sources in the Early Solar System . 8 1.5 Relics of Differentiated Bodies . 11 1.5.1 Core and Mantle Fragments . 11 1.5.2 Basaltic Crust: Meteorites . 14 1.5.3 Basaltic Crust: Vesta and the Vestoids . 16 1.5.4 Basaltic Crust: Non-Vestoid V-types . 20 1.6 Definitions . 22 1.7 Outstanding Questions . 24 Chapter 2: The Distribution of Basaltic Asteroids in the Main Belt . 29 2.1 Introduction . 29 2.2 Selecting Basaltic Asteroid Candidates . 31 ix 2.3 Observations . 40 2.4 Data Reduction . 43 2.5 Size and Orbital Distribution of Basaltic Asteroids . 48 2.5.1 Size-Frequency Distribution . 49 2.5.2 Masses of Basaltic Material . 54 2.5.3 Semi-major Axis Distribution . 59 2.6 Implications and Comparison to Other Work . 60 2.6.1 Basaltic Asteroids with a > 2:5 AU.................. 61 2.6.2 Basaltic Asteroids with a < 2:5 AU.................. 63 2.6.3 Total Basaltic Asteroid Inventory . 64 2.6.4 Comparison to Other Work . 66 Chapter 3: Mineralogy of Basaltic Asteroids . 71 3.1 V-type Asteroids in the Inner Main Belt . 71 3.2 Tools of Mineralogical Analysis . 75 3.3 Band Analysis of HED Meteorites . 80 3.4 Band Analysis of Inner Belt V-type Asteroids . 83 3.5 Spectroscopic Diversity of Inner Belt V-type Asteroids . 90 3.5.1 Asteroidal Versus Meteoritic BARs . 93 3.5.2 Band Parameters and Orbital Properties . 100 3.5.3 Summary of Inner Main Belt V-type Asteroids . 105 3.6 Extreme Case: Outer Main Belt Asteroid 10537 (1991 RY16) . 106 3.6.1 Observations . 107 3.6.2 Spectral Interpretation and Mineralogical Analysis . 109 3.6.3 Discussion of 10537 . 112 Chapter 4: The Thermal Evolution of Planetesimals . 116 4.1 Metal-Silicate Segregation . 117 4.2 Formation of a Basaltic Crust . 123 4.3 The Effects of Silicate Melt Migration . 128 x 4.4 The Effects of Hydration Chemistry . 139 4.5 One-Dimensional Thermal Model . 147 4.6 The Sizes of Differentiated Bodies . 150 4.7 Thermal Evolution Scenarios . 154 Appendix A: Asteroid and Meteorite Data . 160 A.1 Spectroscopically Confirmed V-type Asteroids . 160 A.2 Band Parameters of HED Meteorites . 163 A.3 NIR Spectra of Inner Main Belt V-types . 165 xi List of Tables 2.1 Vestoid Colors . 35 2.2 Non-Vestoid Basaltic Candidates . 36 2.3 Observation Summary . 41 2.4 Interlopers in Vestoid Dynamical Space . 57 3.1 Summary of NIR Spectroscopic Observations . 85 3.2 Band Parameters of V-type Asteroids . 86 3.3 Band Parameters for Eucrite Millbillillie . 98 3.4 Band Depths and ∆v for V-type Asteroids . 103 3.5 Spearman Rank Probabilities Between Band Parameters and Orbital Elements . 104 4.1 Symbols and Definitions . 118 4.2 Thermodynamic properties of hydration reactions . 141 A.1 Spectroscopically Confirmed V-type Asteroids . 160 A.2 Band Parameters of HED Meteorites . 163 xii List of Figures 1.1 Dynamical map of all known asteroids . 2 1.2 Energy per unit mass of planetesimal from the decay of 26Al and gravitational binding . 10 1.3 Visible to NIR spectra of basaltic material . 17 1.4 Dynamical map of spectroscopically confirmed V-types . 19 2.1 Visible through near-IR spectra of basaltic objects .
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