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ISOSTATICALLY COMPENSATED EXTENSIONAL TECTONICS ON ENCELADUS by Scott Stuart McLeod A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Earth Sciences MONTANA STATE UNIVERSITY Bozeman, Montana May 2009 ©COPYRIGHT by Scott Stuart McLeod 2009 All Rights Reserved ii APPROVAL of a thesis submitted by Scott Stuart McLeod This thesis has been read by each member of the thesis committee and has been found to be satisfactory regarding content, English usage, format, citation, bibliographic style, and consistency, and is ready for submission to the Division of Graduate Education. David R. Lageson Approved for the Department of Earth Sciences Stephan G. Custer Approved for the Division of Graduate Education Dr. Carl A. Fox iii STATEMENT OF PERMISSION TO USE In presenting this thesis in partial fulfillment of the requirements for a master’s degree at Montana State University, I agree that the Library shall make it available to borrowers under rules of the Library. If I have indicated my intention to copyright this thesis by including a copyright notice page, copying is allowable only for scholarly purposes, consistent with “fair use” as prescribed in the U.S. Copyright Law. Requests for permission for extended quotation from or reproduction of this thesis in whole or in parts may be granted only by the copyright holder. Scott Stuart McLeod May 2009 iv DEDICATION I dedicate this work to my parents, Grace and Rodney McLeod, for their tireless enthusiasm, encouragement and support, and to my friends and colleagues who never stopped believing in me – you know who you are. v ACKNOWLEDGEMENTS I would like to acknowledge my committee, David R. Lageson, James G. Schmitt and David M. Klumpar for allowing and encouraging me to pursue such an arcane thesis topic; I also wish to thank the following individuals (in alphabetical order): Edward E. Adams, Jeff Banfield, Mike Cavaness, Stuart Challender, Martin Chapman, Tara Chesley- Preston, Mark Greenwood, Trent Hare, Donna Jurdy, Isaac Klapper, Dr. W. Locke, Falene Petrik, Carolyn Porco, Thomas Roatsch, Frank Scholten, Colin Shaw, Alice Stanboli, Michael Sulock, Ismael Talke, Kenneth Tanaka, Peter Thomas, B. William Turner, and Jeannette Wolak. Special thanks go to Monica Bruckner (proofreading) and Beth Helmke (GIS). Finally, I would like to quote verbatim from the note to the reader from the 1698 English edition of The Celestial Worlds Discover’d: “’Tis true there are not every where Mathematical Demonstrations; but where they are wanting, you have probable and ingenious Conjectures, which is the most that can reasonably be expected in such matters. What belongs to, or has any thing to do with Astronomy, you will see demonstrated, and rest ingeniously and shrewdly guess’d at, from the affinity and relation of the heavenly Bodies to the Earth. For your farther Satisfaction read on, and farewel.” vi TABLE OF CONTENTS 1. INTRODUCTION .............................................................................................................. 1 2. GEOLOGIC SETTING ........................................................................................................ 2 Historical background .................................................................................................... 2 Physical Properties .......................................................................................................... 5 Size, Shape, Mass, and Gravity ............................................................................ 6 Size and Mass Comparison with Other Active Bodies ......................................... 7 Surface Composition and Color ........................................................................... 8 Surface Temperature ......................................................................................... 10 Atmosphere ....................................................................................................... 10 Space environment ....................................................................................................... 12 Orbital Parameters and Resonances ................................................................. 13 Synchronous Rotation ....................................................................................... 14 Magnetospheric Interaction .............................................................................. 15 Generalized Geography ................................................................................................. 16 Sulci Explained ................................................................................................... 19 3. METHODS ...................................................................................................................... 21 Data Source ................................................................................................................... 22 Reference Grid Construction ............................................................................. 23 Accuracy and Scale ............................................................................................ 27 Distortion ........................................................................................................... 28 Terrain Types and Visual Interpretation ............................................................ 29 Kinematic Analysis ......................................................................................................... 31 Dynamic Analysis ........................................................................................................... 32 Crater Counting ............................................................................................................. 32 Geophysical Modeling ................................................................................................... 34 4. RESULTS ......................................................................................................................... 40 Surface Features: Descriptive Analysis ......................................................................... 40 Craters, Cratered Plains and Crater Counting ................................................... 40 Impact Craters........................................................................................ 41 Crater Counting ...................................................................................... 48 Ridged Plains ...................................................................................................... 51 Srp Regions ............................................................................................ 52 Crp Regions ............................................................................................ 54 vii TABLE OF CONTENTS – CONTINUED The South Polar Terrain ......................................................................... 56 Dorsa ...................................................................................................... 57 Faults, Fractures and Sulci ................................................................................. 58 Kinematic Analysis ......................................................................................................... 59 The Tiger Stripes as Possible Spreading Centers ............................................... 62 Tectonic Features Outside the South Polar Terrain .......................................... 67 Dynamic and Geophysical Analysis ............................................................................... 67 Enceladus Thermal Anomaly ............................................................................. 68 Properties of Water ........................................................................................... 70 Formation of a South Polar Basin ...................................................................... 72 Ice, Subduction and Spreading .......................................................................... 80 5. DISCUSSION ................................................................................................................... 81 A Subsurface Ocean ...................................................................................................... 81 Other Geologic Issues ................................................................................................... 82 Downslope Transportation on Low-mass Bodies .............................................. 82 Cryovolcanism as a Resurfacing Mechanism ..................................................... 83 Enceladus’ Internal Heat Source ....................................................................... 85 Ohmic Heating ....................................................................................... 85 Serpentinization ..................................................................................... 86 Diapir-induced Reorientation ............................................................................ 87 Ammonia ........................................................................................................... 88 Comparison with Miranda ................................................................................. 88 A Possible “Ancestral Antapical Venting System” (AAVS) ................................. 90 6. CONCLUSIONS ............................................................................................................... 91 REFERENCES CITED ...........................................................................................................