A Global Survey and Regional Scale Study of Coronae on Venus A thesis submitted for the Degree of Doctor of Philosophy of the University of London By Simon W. Tapper Department of Geological Sciences University College London 1998 ProQuest Number: U643667 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 complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest. ProQuest U643667 Published by ProQuest LLC(2016). 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 LLC. ProQuest LLC 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 Abstract Coronae are large-scale geological structures on Venus normally consisting of a planimetrically circular topographic rim which encircles a basin. They are considered to have formed by plume activity. The thesis describes and examines the characteristics of coronae using a new and comprehensive database which is used to further understanding of corona properties and the geological evolution of Venus. Topographic data were surveyed to identify coronae which are not easily detectable in synthetic aperture radar (SAR) images because they lack the annulus of brittle scale fractures that were previously considered to characterise all coronae. Data used to describe the distribution, morphology, geological setting and associated volcanic and tectonic structures were obtained from altimetry, high resolution Synthetic Aperture Radar (SAR) images returned by the 1990 Magellan mission and synthetic stereo images generated from Magellan data. Detailed geological mapping of the Scarpellini Quadrangle was then used to examine coronae on a regional scale and study their geological context and history. The 229 coronae identified by the survey were found in a variety of geological settings and materials. They occur on topographic rises and their margins and on ridge belts north of the Beta-Atla-Themis region. They have widths comparable to those described by earlier surveys but tend to be lower in height. A range of morphologies were identified, including a new class which have complexly deformed interiors. Most of th^ coronae have irregular fractures aligned with their rim topography, but fracture networks and radial structure are also seen. The large numbers of additional coronae strongly imply that they are a far more important global heat loss mechanism than previously thought. The results indicate that the plume’s ability to deform lithospheric materials varies not only spatially with lithospheric thickness but also with other factors such as regional stress regime. The morphological and stratigraphie evidence presented here indicates that coronae can develop in episodes, rather than by a continuous process of formation as suggested by some authorities. Contents Abstract i Contents ii List of figures V List of tables v ii Acknowledgements v ii i Statement of originality ix Chapter 1: Introduction 1 Introduction 1 Mapping Venus 4 Previous investigations 5 Population and distribution 7 Corona evolution 8 Nomenclature 11 Terminology 11 Chapter 2: Global survey of coronae 14 The 1992 survey 14 Survey 16 Data Magellan altimetry 17 Radar interactions with geological surfaces 17 Incidence angle 18 Stereometry 22 Image processing 23 Procedure and survey design 25 Morphology 28 Stratigraphy 29 Volcanism 30 Tectonics 31 Chapter 3: Spatial and altitudinal distribution 32 Spatial distribution 32 Spatial analysis 41 Corona density distribution 1992 survey 41 1997 survey 43 All coronae 43 Nearest neighbour analysis 44 Clustering 46 Altitude 50 Distribution by morphology 53 Distribution by altitude 53 Chapter 4: Geological setting 55 Geological setting 55 Stratigraphy 58 Discussion 62 Chapter 5: Morphology and dimensions 67 Morphology 67 Dimensions 71 Height and width by morphology 75 Altitude and morphology 77 Altitude and relief 77 Discussion Morphology 79 Height 81 Width 81 Chapter 6: Volcanism associated with coronae 83 Volcanism and morphology 84 Volcanism and altitude 87 Discussion 90 Chapter 7: Tectonics associated with coronae 94 Stealth corona tectonics 94 Discussion 102 Chapter 8: Mapping the Scarpellini Quadrangle 105 Physiographic setting 105 Mapping techniques 111 Missing data and image processing artefacts 111 General geology 112 Material units and volcanic landforms 113 Tessera 113 Plains 116 Central volcanoes 118 Domes 120 Small volcanic edifices and edifice fields 120 Lava flow fields 121 Crater materials 121 Surface materials 123 Statistics 126 Rad iothermal properties 127 Structures Wrinkle ridges 128 Fractures 128 Graben 129 Geological history 130 Chapter 9: The coronae of Scarpellini 131 Introduction 131 M a 131 Juksakka 134 Thermuthis 136 Nabuzama 139 Mukylchin 141 Stealth coronae 143 111 Chapter 10; Stratigraphy and origin 151 Number of coronae 151 Type of coronae 152 Morphology and tectonics 160 Chronology 162 Stratigraphy 163 Global geology 167 Chapter 11: Summary and conclusions 169 Summary of results 169 Further work Further analysis of Magellan SAR 170 Laboratory experiments 171 New data 174 Numerical modelling 177 Coronae of the solar system 177 Appendixes 182 I 1997 Corona survey: data and notes 182 II 1992 Corona survey: data and notes 187 References cited 194 IV List of figures 1.1 SAR image of Aramaiti Corona 2 1.2 Quadrangles of the V-Map program 6 1.3 Squyres’s (1992) three stage model of corona evolution 9 1.4 Smrekar and Stofan (1997) model of corona evolution 10 2.1 The effect of surface roughness on radar backscatter 19 2.2 Geometric distortions in radar images 20 2.3 Radar backscatter variation with surface roughness 21 2.4 Directional filter matrices used to create shaded-relief images 25 2.5 Shaded relief image of the Scarpellini Quadrangle 26 2.6 Schematic illustrating terms used to describe corona morphology 27 3.1 Venus topography and names of major geographical features 33 3.2 Global distribution of coronae recorded by the 1992 and 1997 survey 34 3.3 Global distribution of different sized coronae 36 3.4-7 Distribution of coronae which have the same morphology 37-40 3.8 Density distribution of coronae of the 1992, 1997 and expanded corona database 42 3.9 Nearest neighbour distribution of coronae types 1-5 47 3.10 Nearest neighbour distribution of coronae types 6-8 48 3.11 Nearest neighbour distribution of coronae of the 1992, 1997 and expanded corona database 49 3.12 Base level of coronae identified by the 1992, 1997 surveys 51 4.1 Magellan image of Parga Chasma 56 4.2 Magellan altimetry of the Beta-Atla-Themis (BAT) region 56 4.3 SAR image of corona identified in tessera materials 59 5.1 Corona types. Topographic groups and percentage of the total population 68 5.2 Topographic profiles of coronae with different morphology 69 5.3 Graph showing the relative proportions of coronae identified by the 1992 and 1997 surveys 69 5.4 Three dimensional perspective view of Atete Corona 70 5.5 Three dimensional perspective view of Yavine corona 70 5.6 Corona height distribution 73 5.7 Corona width distribution 74 5.8 Mean width of coronae, ordered by morphology 76 5.9 Mean height of coronae, ordered by morphology 76 5.10 Mean altitude of coronae^ordered by type 78 5.11 Mean height of coronae plotted against altitude 78 6.1 SAR image of a corona dominated by volcanism 85 6.2 SAR image of a corona with a moderate amount of associated volcanism 85 6.3 Relative amount of volcanic activity associated with coronae ordered by morphology 86 6.4 Plot showing variation in the degree of volcanism with altitude 88 6.5 Interpretation of the relationship between volcanism and coronae at different altitudes 92 7.1 SAR image of Cerridwen and Neyterkob coronae 95 7.2 SAR image showing detail of the eastern rim of Neyterkob corona 95 7.3 Tectonic styles displayed by coronae identified in the new survey 96 7.4 SAR image of a stealth corona 97 7.5 SAR image of a stealth corona lacking tectonic structure 97 7.6 SAR image of a corona, previously classified as an arachnoid 100 7.7 SAR image of a corona with radial pattern of wrinkle ridges 101 7.8 Deformation patterns in materials surrounding coronae 101 8.1 SAR mosaic of the Scarpellini Quadrangle 106 8.2 Location map of named features within the Scarpellini Quadrangle 107 8.3 Geologic map of the Scarpellini Quadrangle 108-109 8.4 Magellan altimetry of the Scarpellini Quadrangle 110 8.5 Backscatter cross-section of geological units 114 8.6 Geological units and their chronological relationships 114 8.7 Geological cross section through the Scarpellini quadrangle 109 8.8 SAR image of tessera with a ridge and groove configuration 115 8.9 SAR image of tessera with convoluted configuration 115 8.10 SAR image of tessera with a radial configuration 115 8.11 SAR image of lineated tessera 117 8.12 SAR image of mottled and lineated plains material 117 8.13 SAR image of lineated plains material 119 8.14 SAR image of regional plains material 119 8.15 SAR image of craters Medhavi and Michelle 122 8.16 SAR image of wrinkle ridges in regional plains material 122 8.17 Map showing the distribution of surficial materials 124 9.1 SAR image of the Scarpellini Quadrangle showing the location of coronae 132 9.2 (a) SAR image and (b) topographic map of Ma Corona 133 9.3 (a) SAR image and (b) topographic