Open Research Online The Open University’s repository of research publications and other research outputs Space Weathering on Mercury: Laboratory Studies and Modelling Thesis How to cite: Bentley, Mark Stephen (2005). Space Weathering on Mercury: Laboratory Studies and Modelling. PhD thesis. The Open University. For guidance on citations see FAQs. c 2005 Mark Stephen Bentley Version: Version of Record Copyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyright owners. For more information on Open Research Online’s data policy on reuse of materials please consult the policies page. oro.open.ac.uk Space Weathering on Mercury: Laboratory Studies and Modeiiing A thesis submitted for the degree of Doctor of Philosophy by Mark Stephen Bentley September, 2004 Planetary and Space Sciences Research Institute The Open University, U.K. ProQuest Number: C820960 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a com plete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest ProQ uest C820960 Published by ProQuest LLO (2019). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code Microform Edition © ProQuest LLO. 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Box 1346 Ann Arbor, Ml 4 8 1 0 6 - 1346 Contents ii Contents C ontents .................................... ii List of figures ........................ v A bstract............................ viii Acknowledgments .............................................................................................................................x Chapter One - Regolith Evolution in the Solar System .............................................................1 1.1 Introduction........................................................................................................................1 1.2 W hy is regolith important? ....................... 6 1.3 Surface weathering agents .............................................................................................. 9 1.3.1 Meteorite bom bardm ent ................. 10 1.3.2 Ion sputtering ........................................................................................................ 14 1.3.3 Solar radiation effects ..................................................................... 19 1.3.4 Summary ................................................. 21 1.4 Regolith maturity indices .............................................................................................21 1.4.1 F erromagnetic resonance ............................................ 22 1.4.2 Magnetic susceptibility .......................................................................................24 1.4.3 Metallic/ferrous iron ratio .................................................................................. 25 1.4.4 Optical indices.. .....................................................................................................26 1.4.5 Other indices......................................................................................................... 28 1.5 Introduction to space w eathering ............................................................................... 29 1.5.1 Competing models o f space w eathering ......................................................... 29 1.5.2 The old paradigm versus the new ......................................................................31 1.5.3 Space weathering in the Solar System .............................................................34 1.6 Aims o f this w ork ...........................................................................................................36 1.7 Thesis roadm ap ................................................................................... .38 Chapter Two - Laboratory Simulations of Space Weathering on Mercury ........................ 40 2.1 Introduction ..................................................................................................................... 41 2.2 Simulating micrometeorite impacts ...........................................................................42 2.2.1 Requirements of an impact simulation technique ........................................ 42 2.2.2 Selection of an impact simulation technique ..................................................51 2.2.3 Pulsed laser simulation of hypervelocity impacts ..................... 55 2.2.4 Experimental configuration ................. 67 2.3 Simulating the surface environment of Mercury .................................................... 72 2.3.1 Pressure...................................................................................................................73 2.3.2 T emperature ...........................................................................................................76 2.3.3 Surface materials................................................................................................... 81 2.4 Experimental procedure .......................................................................................... 86 2.5 Conclusions ..................................................................................................................... 90 Contents iii Chapter Three - Analysis of artificially weathered regolith analogues ...............................92 3.1 Introduction .....................................................................................................................92 3.2 Initial experiments ......................................................................................................... 94 3.2.1 Visible observations .............................................................................................94 3.2.2 Initial magnetic susceptibility ........................................................................... 97 3.3 In search of metallic iron ............................................................................................108 3.3.1 Initial magnetic susceptibility ..........................................................................108 3.3.2 Mossbauer spectroscopy ...................................................................................109 3.3.3 Curie point determination .................................................................................109 3.3.4 Electron spin resonance spectroscopy. ....................................................... 117 3.4 Characterisation of metallic iron .............................................................................. 121 3.4.1 Frequency dependence of initial susceptibility ............................................122 3.4.2 Vibrating sample magnetometry .....................................................................128 3.5 The optical effects of weathering .............................................................................134 3.5.1 Reflectance measurements at the MSF ...................... 136 3.5.2 Reflectance measurements at RELAB...........................................................137 3.6 The temperature dependence of weathering ......................................................... 139 3.6.1 Initial magnetic susceptibility ..........................................................................140 3.6.2 Frequency dependence of initial susceptibility ............................................142 3.6.3 Reflectance spectra .............................................................................................143 3.6.4 Electron spin resonance spectra ...................................................................... 145 3.7 Regolith analogues vs. lunar soils ............................................................................146 3.8 Conclusions ...................................................................................................................147 Chapter Four - Modelling the surface environment on M ercury........................................151 4.1 Introduction ...................................................................................................................151 4.2 A planetary surface and sub-surface thermal model ............................................152 4.2.1 A review of Mercury thermal models ............................................................152 4.2.2 Theoretical overview ..........................................................................................154 4.2.3 Computational solution ..................................................................................... 159 4.2.4 Solar elevation on Mercury ...............................................................................161 4.2.5 Thermophysical parameters ............................................................................. 166 4.2.6 Application of the model ...................................................................................168 4.2.7 Implications for in situ instrumentation on Mercury ..................................176 4.2.8 Conclusions ................................................................................................... 183 4.3 Modelling the micrometeorite impact flux at Mercury .........................................184 4.3.1 An overview of micrometeorite distribution models ..................................185 4.3.2 The Divine