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Asteroid Regolith Weathering: a Large-Scale Observational Investigation

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Asteroid Regolith Weathering: a Large-Scale Observational Investigation University of Tennessee, Knoxville TRACE: Tennessee Research and Creative Exchange Doctoral Dissertations Graduate School 5-2019 Asteroid Regolith Weathering: A Large-Scale Observational Investigation Eric Michael MacLennan University of Tennessee, [email protected] Follow this and additional works at: https://trace.tennessee.edu/utk_graddiss Recommended Citation MacLennan, Eric Michael, "Asteroid Regolith Weathering: A Large-Scale Observational Investigation. " PhD diss., University of Tennessee, 2019. https://trace.tennessee.edu/utk_graddiss/5467 This Dissertation is brought to you for free and open access by the Graduate School at TRACE: Tennessee Research and Creative Exchange. It has been accepted for inclusion in Doctoral Dissertations by an authorized administrator of TRACE: Tennessee Research and Creative Exchange. For more information, please contact [email protected]. To the Graduate Council: I am submitting herewith a dissertation written by Eric Michael MacLennan entitled "Asteroid Regolith Weathering: A Large-Scale Observational Investigation." I have examined the final electronic copy of this dissertation for form and content and recommend that it be accepted in partial fulfillment of the equirr ements for the degree of Doctor of Philosophy, with a major in Geology. Joshua P. Emery, Major Professor We have read this dissertation and recommend its acceptance: Jeffrey E. Moersch, Harry Y. McSween Jr., Liem T. Tran Accepted for the Council: Dixie L. Thompson Vice Provost and Dean of the Graduate School (Original signatures are on file with official studentecor r ds.) Asteroid Regolith Weathering: A Large-Scale Observational Investigation A Dissertation Presented for the Doctor of Philosophy Degree The University of Tennessee, Knoxville Eric Michael MacLennan May 2019 © by Eric Michael MacLennan, 2019 All Rights Reserved. ii I would like to dedicate this entire work to my beloved grandparents Mary, James, Haroldine, and William; and to my parents, who have given me the ability to achieve my dreams. iii Acknowledgments I am grateful for everyone who has contributed their ideas, advise, and help into making this work possible. Without them, I would not have accomplished what I have in my 6 1/2 years as a graduate student. Firstly, this work would not have been made possible without the help and mentorship of my PhD advisor, Joshua Emery. I am very grateful of the time and energy he has dedicated in training and mentoring me in preparation for my future. The Emery Research Group of Excellence (EMERGE) has included so many exceptional researchers who have fostered a supportive, cooperative, and fun environment to perform research and I am better for having them by my side. A special thank you to, Michael Lucas and Noemi Pinilla-Alonso, who offered up their valuable IRTF telescope observing time in order to observe some of my targets and for showing me their remote observing skills. In addition, Noemi and Vania Lorenzi arranged time on the TNG for collecting spectra of a few of my target asteroids. I also wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. I am most fortunate to have had the opportunity to conduct remote observations using the IRTF, which lies atop this mountain. From 2014 to 2017 I was supported by the NASA Earth and Space Sciences Fellowship (NESSF) grant (# NNX14AP21H), which provided a sizable stipend and an allowance that was applied towards miscellaneous supplies, textbooks, computer software, and travel expenses. This dissertation makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration. iv Abstract As bodies lacking an atmosphere or significant protection from solar wind particles, asteroids are subject to processes that modify the physical state and spectral properties of their regoliths. By investigating the relevant factors that contribute towards asteroid regolith modification, this work will provide crucial insight into the nature of these processes. I propose and test two major hypotheses: 1) that physical (mechanical) breakdown is caused by both meteoroid bombardment and thermal fatigue cycling, and thus regolith grain size depends on asteroid size and rotation period, and 2) changes in spectral properties (space weathering) are due to solar wind bombardment and depend on an object's mineralogy, sun-distance, and surface age. I develop and validate a thermophysical modeling (TPM) approach that analyzes multi- epoch (pre- and post-opposition) thermal infrared observations for asteroids without prior shape or spin information, in order to determine various thermophysical properties { chiefly the thermal inertia. This TPM approach is applied to over 250 asteroids to determine their thermal inertia. Combining other thermal inertia datasets with mine, for a total of over 300 objects, a characteristic grain size is estimated for each object. Next, a multiple linear model is used to quantify the grain size dependence on asteroid diameter and rotation period, which are both shown to be statistically-significant model predictors. I also identify grain size differences between spectral groups { namely the M-types, which exhibit 4 times larger regolith grains, on average. Spectral data from meteorite and irradiated samples, spanning the visible and near- infrared regions, are used to develop an index to quantify the degree of space weathering. This space weathering index is applied to asteroid spectral observations and used in a multi- linear model to determine the predictor variables that increase the perceived amount of v asteroid space weathering. Perihelion distance, diameter, and the average sun distance are found as statistically-significant factors in the multiple linear model. I also present evidence that regolith grains smaller than 0.5 mm enhance the extent of space weathering. vi Table of Contents Introduction1 1 Asteroid Thermophysical Modeling4 1.1 Introduction....................................4 1.2 Thermal Modeling Background.........................6 1.2.1 Thermophysical Models.........................7 1.2.2 Diameter and Albedo...........................8 1.2.3 Thermal Inertia and Surface Roughness.................9 1.2.4 Shape and Spin Direction........................ 12 1.2.5 Motivation for this Work......................... 12 1.3 Thermophysical Model Description....................... 13 1.3.1 Smooth TPM............................... 13 1.3.2 Rough TPM................................ 15 1.3.3 Non-spherical Shapes........................... 16 1.3.4 Flux Calculation and Data-fitting Routine............... 18 1.4 Method Testing and Validation......................... 22 1.4.1 Synthetic Flux Data Set......................... 22 1.4.2 Model Validation Results......................... 24 1.4.3 Parameter Bias Analysis......................... 27 1.4.4 Other Possible Sources of Bias...................... 29 1.5 Application to WISE Observations....................... 31 1.5.1 Data Description............................. 31 1.5.2 Results and Discussion.......................... 33 vii 1.6 Summary and Future Work........................... 41 2 Mechanical Weathering of Regolith 44 2.1 Introduction.................................... 44 2.2 Background: Regolith Formation Mechanisms................. 47 2.3 Observations & Thermophysical Modeling................... 48 2.3.1 Data Description............................. 52 2.3.2 Sparse Lightcurve Sampling....................... 52 2.3.3 TPM Implementation........................... 73 2.3.4 TPM Results............................... 75 2.3.5 Comparison to Previous Works..................... 85 2.4 TPM Analysis................................... 88 2.4.1 Thermal Inertia Dependence on Diameter and Temperature..... 88 2.4.2 Spin Dependency on Size......................... 90 2.5 Methods: Grain Size Estimation......................... 92 2.5.1 Asteroid-Meteorite Connections..................... 92 2.5.2 Regolith Thermal Conductivity Model................. 102 2.5.3 Data and Model Limitations....................... 106 2.6 Grain Size Modeling Results........................... 109 2.6.1 Notes on Individual Objects....................... 109 2.7 Analysis...................................... 120 2.7.1 Thermal Skin Depth........................... 121 2.7.2 Multi-linear Regression.......................... 122 2.7.3 Compositional Groups.......................... 125 2.7.4 Asteroid Families and Weathering Timescale.............. 129 2.7.5 Near-Earth Asteroids........................... 131 2.8 Discussion..................................... 132 2.8.1 Regolith Weathering........................... 132 2.8.2 Regolith Retention & Loss........................ 139 2.9 Conclusions.................................... 144 viii 3 Regolith Space Weathering 146 3.1 Introduction.................................... 146 3.2 Space Weathering Background.......................... 150 3.2.1 Laboratory Studies............................ 150 3.2.2 Asteroid Observational Studies..................... 151 3.3 Methodology................................... 152 3.3.1 Telescopic Survey............................. 154 3.3.2 Other Data Sources............................ 158 3.3.3 Band Parameter Analysis........................ 162 3.4 Results: Meteorites
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