Mapping Pit Crater Chains in the Bell Regio Quadrangle on Venus

Mapping Pit Crater Chains in the Bell Regio Quadrangle on Venus By William C. Sawford A thesis submitted to the Faculty of Science in partial fulfillment of the requirements for the degree of Bachelor of Science Department of Earth Sciences Carleton University Ottawa, Ontario March, 2014
1 The undersigned recommend to the Faculty of Science acceptance of this thesis Mapping Pit Crater Chains in the Bell Regio Quadrangle on Venus Submitted by William C. Sawford In partial fulfillment of the requirements of the degree of Bachelor of Science Thesis Supervisor
Thesis Co-Supervisor
Chair, Department of
Earth Sciences
i Abstract A 160,000 km! area within the Bell Regio Quadrangle on Venus (28°-‐32°N, 47°-‐
51°E) was mapped focusing on pit crater chains and their relation to an underlying graben-‐fissure systems. Surface images were acquired during the 1990-‐1992 Magellan mission using synthetic aperture radar (SAR) technology. These images were obtained through the NASA USGS website. The purpose of the research is to map pit crater chains on Venus and to examine their spatial relationship in relation to underlying graben-‐fissure systems and the mechanical competency of the surficial layer. A total of 143 pit crater chains were identified, described and categorized. A previously unobserved style of pit crater chain was observed: it consists of an oblong angular shaped pit with a diameter ranging from 2-‐10 km that is located at one end of a chain and a tail consisting of smaller pits uniform in shape and diameter. The new style of pit crater chain has been termed “tadpole-‐type” pit crater chains. Possible mechanisms for the formation of the tadpole-‐type pit crater chains are being investigated as part of this study. ii Acknowledgements I would like to thank my co-‐supervisors for allowing me to work on the Venus project, and specifically, Dr. Richard Ernst for meeting with me on a weekly basis and providing me with valuable insight on my thesis topic, and Dr. Claire Samson for her valuable feedback and helping me remain focused and on topic. I would also like to thank my external examiner Dr. Richard Herd for providing valuable feedback on my thesis. iii Table of Contents Title Page -‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ i Statement of Approval -‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ ii Abstract -‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ iii Table of Contents -‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ iv List of Figures -‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ vi Chapter 1 – Introduction 1.1 Venus: An Overview-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 1 1.2 Current Mapping Progress on Venus -‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 2 1.3 Research Objectives-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 3 Chapter 2 – Background Information 2.1 Previous Venus Missions-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 5 2.2 Magellan Mission -‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 6 2.3 Synthetic Aperture Radar-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 7 2.4 Geological History of Venus -‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 8 2.5 Regional Geology of Bell Regio Quadrangle-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 11 2.6 Pit Craters and Pit Crater Chains-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 12 Chapter 3 – Methods 3.1 Using SAR images on ArcGIS 10.1-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 15 3.2 Identification of Geological Structures-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 18 Chapter 4 -‐ Observations 4.1 Lithology-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 22 4.2 Pit Crater Chains-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 24 iv 4.3 Volcanic Flooding -‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 29 4.4 Graben Fissure Systems -‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 30 Chapter 5 -‐ Discussion 5.1 Interpreted Geological History of Study Area -‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 30 5.2 Lithological and Extensional Controls on the Formation of Pit Crater Chains -‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 31 5.3 Proposed Mechanisms for the Formation of “Tadpole-‐type” Chain-‐-‐-‐-‐-‐-‐-‐ 32 5.4 Comparison to Previous Mapping of Pit Crater Chains-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 35 Chapter 6 -‐ Conclusion 6.1 Future Research-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 36 References-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 37 Appendix-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐40 v List of Figures Figure 1. Study area of thesis shown on SAR image of Bell Regio and geoid anomaly map -‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 4 Figure 2. Cross Section Showing Pit Crater Formation Through Dyke Emplacement -‐-‐
-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 13 Figure 3. Magellan SAR image of Study Area -‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 17 Figure 4. An example of a graben-‐fissure system -‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 18 Figure 5. An example of a category 1 “even sized-‐type” pit crater chain-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐
-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 19 Figure 6. An example of a category 2 “trough-‐type” pit crater chain-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 20 Figure 7. An example of a category 3 “tadpole-‐type” pit crater chain-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 20 Figure 8. An example of flooding that has filled in pits within the study area -‐-‐-‐-‐-‐-‐-‐ 21 Figure 9. Geological Map of Study Area -‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 23 Figure 10. Distribution of Pit Crater Chain Type in Study Area -‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 25 Figure 11. Topographic Profile of Study Area along 30.1° N-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 25 Figure 12. Study area showing sub-‐areas where most pit crater chains are located-‐-‐-‐
-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 26 Figure 13. Frequency of Pit Crater Chain Type in Sub-‐Area 1 -‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 27 Figure 14. Frequency of Pit Crater Chain Type in Sub-‐Area 2 -‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 28 Figure 15. Frequency of Pit Crater Chain Type in Sub-‐Area 3 -‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 29 Figure 16. Cross section of “tadpole-‐type” pit crater chain formation-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 34 vi Chapter 1: Introduction 1.1 Venus: An Overview Our knowledge and understanding of extraterrestrial planets has improved substantially over recent decades. With the technical improvements in instrumentation aboard spacecraft, high-‐resolution data and images of other planets in our Solar System have become readily available. With increased knowledge of the geology of other planets we can better understand the geological history of Earth. Venus is the second closest planet to the sun and the second of the inner terrestrial planets. It is the most comparable planet to Earth in our Solar System. Venus is approximately 81.5% the Earth’s mass, 85.7% of its volume and 95.1% of its density (Williams, 2010). The gravity one would experience on Venus is about 90.5% of what one would find on Earth. It was hypothesized that Venus’s surface was comparable to Earth’s before the development of satellite technologies. When surficial data of the Venusian surface became available, it was apparent that Venus has its own geological history and characteristics (Basilevsky and Head, 2003). The atmospheric composition of Venus is considerably different from what is observed on planet Earth. Earth’s atmosphere is composed of 78.1% N! , 20.9% O! and 0.9% Ar with trace amounts of other gases like CO! , Ne and He (Seinfeld and Pandis, 2006), while Venus’s atmosphere is composed of 96.5% CO! and 3.5% N! 1 and has clouds containing sulphuric acid (Williams, 2010), The average atmospheric pressure of Venus is 9200 kPa when compared to Earth 101.325 kPa. Venus also has a much higher surface temperature of approximately 450 °C. In comparison to Earth, Venus also lacks any a moon, a magnetic field and due to the high surface temperature, lacks atmospheric water (Williams, 2010). 1.2 Current Mapping of Venus Initially, very little was known about the surface of Venus due to the yellow sulphurous clouds within the Venusian atmosphere that obscured traditional photographic imaging. The Venera 7 spacecraft provided the first images of the Venusian surface. The spacecraft was equipped with a landing unit that successfully transmitted images to Earth. The development of synthetic aperture radar (SAR) imaging used in the Venus 9 mission of 1975 and the Pioneer Venus Mission of 1978 provided sufficient data for exploration of the Venusian Surface (Tanaka, 1994). The Magellan mission of 1990 is the most recent mission dedicated to mapping the surface of Venus. The surface was mapped using SAR imaging at a resolution of 75m/pixel (Tanaka, 1994). The resolution from the Magellan Mission was ten times greater than previous missions. Currently images are available through the USGS planetary website.1 With the abundance of Magellan SAR images there is a current initiative to do systematic mapping of Venus’s surface with small areas in progress (Williams, 2010). 1 See http://www.mapaplanet.org 2 1.3 Research Objective For this study, 142 pit crater chains in a 160 000 km! area found in the Bell Regio Quadrangle on Venus were mapped (Figure 1). An emphasis was put on examining the spatial distribution of pit crater chains with respect to a systems of underlying graben-‐fissures. In addition to mapping pit crater chains, the geological contacts and graben-‐fissure systems were also mapped. This study tries to understand whether pit craters chains form randomly or are linked to the stratigraphic, structural and/or magmatic characteristics of the area. This study is also focusing on a previously unobserved style of pit crater chains with the main goal of hypothesizing its mechanism of formation. 3 Figure 1. Study area location on Venus: (Top) SAR image from Magellan Mission of the Bell Regio – Study area bounded by black square. Image from Campbell and Campbell (2004). (Bottom) Study area bounded by blue square. Background image from Herrick (1999). 4 Chapter 2: Background Information 2.1 Previous Venus Missions The knowledge of Venus has grown substantially due to the planetary missions to collect data. An interesting retrospective can be found within the National Space Science Data Center (NSSDC) research from 2012 with the earliest attempts to gain information of Venus began on February 6th 1961 when the Sputnik 7 satellite was launched into Earth’s orbit by the U.S.S.R. The goal of the mission was for the satellite to impact the Venusian surface. The satellite was not properly designed to withstand the vacuum of space and was not able to escape Earth’s orbit (NSSDC, 2012). The first successful mission to Venus was launched on August 27th 1963 by the United States. The Mariner 2 mission was designed to fly by Venus and gather data on the atmosphere, magnetic field, charged particle environment and mass of the planet. The Mariner 2 mission was considered a resounding success and resulted in several future missions to gather additional information on the planet (NSSDC, 2012). The Venera 9-‐14 (1975-‐1982) missions to Venus measured the planet’s atmosphere and surface conditions. Venera 9 was also the first mission to provide images of Venus’s surface (NSSDC, 2012). The spacecraft used a TV panoramic view deployed from the lander to capture pictures of the surface. The Venus Pioneer Orbiter began orbiting Venus in December of 1978 and was the first spacecraft 5 capable of SAR imaging. The spacecraft was able to capture radar images at a resolution of 50-‐140km/pixels. Higher resolution SAR images were captured during the Venera 15 and 16 missions and improved resolution to 1.2-‐2.4 km/pixel. The most recent SAR images of Venus were taken during the Magellan mission launched in 1989 and these have a resolution of 75m/pixel. (NSSDC, 2012). Currently there are three ongoing missions: the MESSENGER, Akatsuki mission and Venus Express. The MESSENGER mission is primarily focusing on Mercury but has made two flybys of Venus, the Akatsuki mission is focusing on the dynamic principles of the Venusian atmosphere while the Venus Express is investigating the nature of the Venusian atmosphere in relation to surface geology (NSSDC, 2012). 2.2 Magellan Mission The Magellan spacecraft was launched on May 4th, 1989 and began orbiting Venus on August 10th 1990 (NSSDC, 2012). The primary mission objectives were to map the surface of Venus using SAR, determine the topographic relief of the surface and to obtain high-‐resolution global gravimetric data (NSSDC, 2012). The Magellan spacecraft was designed to utilize available components from the Voyager and Galileo missions. The spacecraft was assembled by the Martin Marietta Astronautics Group in Denver, Colorado and radar sensor components were designed by the Hughes Aircraft Company in El Segundo, California (Johnson, 1991). 6 The Magellan mission consisted of six cycles, with each cycle lasting approximately 243 twenty-‐four hour days. The first three cycles were devoted to the surface mapping of Venus (Tanaka, 1994; Williams, 2005). The first cycle mapped 83.7% of the Venusian surface using the left look position. The second cycle mapped 54.5% of the surface using the right look position. The third cycle mapped 22.8% of the surface once again using the left look position. The first three cycles successfully mapped 98.3% of the Venusian surface. The fourth and fifth cycles focused on the global gravimetric data of Venus. The final cycle of the Magellan mission conducted an experiment termed the Windmill experiment. The experiment turned the solar panels on the satellite so it experienced atmospheric drag and put torque on the spacecraft in order to yield information on Venus’s upper atmosphere. Previous missions to Venus had satellites whose orbits were nearly circular. The Magellan mission used an elliptical orbit that reduced the cost and complexity of the spacecraft (Williams, 2005)(Tanaka, 1994). 2.3 Synthetic Aperture Radar Synthetic Aperture Radar (SAR) was used to map the surface of Venus. SAR was used instead of basic photograph imaging because the radar waves emitted from the satellite were able to penetrate the dense, optically opaque cloud layer (Johnson, 1991). The Magellan spacecraft was equipped with an altimeter antenna and a high-‐gain antenna (HGA) that was a spare part from the Voyager Mission; the 7 HGA was a 3.7m diameter parabolic dish antenna and had a SAR sensor behind the dish that could detect wavelengths of 12.6 cm (Johnson, 1991; Tanaka, 1994). The HGA was fixed to the spacecraft’s structure so that redirection of the antenna required rotation of the entire spacecraft (Johnson, 1991). The HGA and altimeter transmitted radar pulses towards the surface of Venus (Young, 1990). The radar sensor received images by sensing the backscatter of the waves (echoes) reflected against the planetary surface (Tanaka, 1994). SAR involves emitting radar pulses in rapid succession and receiving echoes while the spacecraft is in motion (Johnson, 1991). All the radar data collected during each orbit was processed on onboard recorders and was then transmitted to the Deep Space Network on Earth where the data were transferred on to magnetic tape (Johnson, 1991). 2.4 Geological History of Venus Prior to the Magellan mission there were three end member ideas proposed for the dominant planetary heat loss mechanism; plate tectonics, hot spots and conduction (Basilevsky and Head, 1998). It was discovered that Venus was quite different from Earth. While the Earth exhibits a bimodal distribution of ages for surface geological units (e.g. corresponding to young seafloors and continents with ages back to >4 Ga) Magellan images revealed that Venus exhibited a relatively young surface (Basilevsky and Head, 1998). Based on crater counting, the majority of the surface is approximately 300-‐600 MA (Nimmo and McKenzie, 1998). There are approximately 1000 impact craters on Venus whose areal distribution is 8 random (Basilevsky and Head, 1998). The data suggested that Venus had undergone a global resurfacing event caused by volcanic and tectonic processes. The rate of resurfacing had to be rapid in comparison to the rate of accumulation of impact craters to maintain the apparent randomness of areal crater distribution. Unlike Earth, whose global geodynamic processes are dominated by plate tectonics, Venus shows no signature of plate tectonics (Basilevsky and Head, 1998). Data gathered from the Venera landers determined that Venus is primarily composed of basaltic rocks (Basilevsky and Head, 1998). Further interpretations of SAR images show that the surface of Venus is covered with volcanic, extensional and deformed features. Venus exhibits volcanic features that are similar to features found on Earth but also has features that are unique to Venus (Basilevsky and Head, 1998). Earth’s history suggests that its geological processes were dominated by plate tectonics characterized by the balance between extension at divergent plate boundaries and compression at convergent plate boundaries (Basilevsky and Head, 1998). Venus shows no signature of plate tectonics but does show both extensional and compressional events (Basilevsky and Head, 1998). Initially Venus was controlled by a dominance of compressional features and has undergone two cycles altering between compressional and extensional dominance. Currently Venus is controlled by extensional features with the density of deformational structures and the strain rate decreased with time (Basilevsky and Head, 1998). 9 There are four geological time units associated with the history of Venus: the Pre Fortunian Period: Fortunian Period, Guineverian Period, and the Atlian Period (Ivanov and Head, 2011). The observable geological history starts with the Fortunian Period; during this period there was intense deformation of crustal material and the building of locally to regionally areas of thick crust. The Fortuna Formation is composed of tessera structural material that is defined by a unique pattern of deformation (Ivanov and Head, 2011). The exposed area of the Fortuna Formation comprises of about 7.3% of the total surface area of Venus. In places where the Fortuna Formation is in contact with other units, the other units cut through the tessera material suggesting that the Fortuna Formation is older (Ivanov and Head, 2011). The Guineverian period involved intense volcanic activity and the global formation of extensional structures and the tectonic annuli of coronae (Ivanov and Head, 2011). The Guineverian Unit comprises about 70% of the surface area of Venus. The Altian period comprised two major events, the formation of large and prominent rift zones and extensive lava flows. The Altian rift structures and lava flows cut all structures and units in the Fortunian and Guineverian periods suggesting that the Altian formations are the youngest in age. The Altian formations comprise about 19% of the surface area on Venus (Ivanov and Head, 2011). 10 2.5 Regional Geology Bell Regio Quadrangle The Bell Regio quadrangle (V-‐9) is bounded by latitudes of 25° and 40° N and longitudes of 30° and 60° E. Bell Regio is a broad rise approximately 1500 km in diameter and is characterized by extensive volcanism centered on several edifice complexes. Within the main highland of Bell Regio there are five major volcanic sources: Tepev Mons, Nefertit corona, Nyx Mons and two small, steep edifices on the southeast flank of Tepev. The shield volcano Api Mons and several coronae are also associated with flow fields in the area (Campbell and Rogers, 1994). There are five broad lithological units within Bell Regio. The oldest is a tessera unit that is highly deformed and rises above surrounding plains. The second unit is plains material that originated from lava flows. Plains material can further be divided into three sub units: radar dark plains, radar bright plains and ridged plains. Both radar bright and dark plains units contain east-‐west or southeast-‐northwest fracture patterns (which may represent parts of graben-‐fissure systems) and differ only by their relative brightness on radar images. Ridged plains material is more heavily deformed than the other two varieties of plains material and has two or more overlapping patterns of ridges. The third unit is composed of lava flows that originated from the five major volcanic sources in Bell Regio. The fourth and fifth units are composed of volcanic material from coronae and impact craters respectively (Campbell and Rogers, 1994). 11 The study area of this research is centered around Nyx Mons. Nyx Mons is characterized by gentle flank slopes (typically less than 1°) and a central bulge surrounded by a topographically high region hosting a radial graben-‐fissure system and pit crater chains. Circumferential fractures surround the edifice to the east, south and northwest (Campbell and Rogers, 1994). Previous mapping efforts by Campbell and Campbell (2004) indicate one broad lithological unit is present within the study area. The unit is composed of flood material that originated from Nyx Mons. 2.6 Pit Crater and Pit Crater Chains Pit craters are bowl-‐shaped steep-‐sided circular to elliptical features found on Venus and other celestial bodies in our solar system (Earth, the Moon, Phobos, Eros, Gaspra, Ida and Europa) (Davey et al., 2013). The diameter of pit craters can range from 75m to a few kilometers. Pit craters can be found isolated or in long chains than can range from a few to thousands of kilometers (Wyrick et al., 2004, 2010; Davey et al., 2013). Along a chain, pits can be isolated and irregularly spaced, contiguous, or coalescing, where the pits, following collapse, will form a trough (Davey et al., 2013). On Earth, pit craters chains are found in Hawaii, along the East Pacific rise, along the coastline of the Dead Sea in Israel and Jordan, and in Iceland. In Iceland it was discovered that pit crater chains are the result of dilation faulting of basalt that is overlain by unconsolidated sediments (Ferrill et al., 2011). 12 On Venus, pit crater chains can cluster on regional and local scales. Regionally, clusters of pit crater chains coincide with an extensional system (Davey et al., 2013). On a local scale, pit crater chain clusters are often inhomogeneously distributed throughout a graben-‐fissure system and are thought to be a product of lithology and extension (Davey et al., 2013). Extensional features on Venus are believed to be associated with subsurface laterally propagating dykes that are expressed on the surface as graben-‐fissure systems. As magma rises in the crust it reaches a neutral buoyancy level and begins to propagate both laterally and vertically as a dyke. As the dyke propagates upwards, the strain is concentrated at the tip of the dyke and produces fracturing in the host rock (if mechanically weak enough) and subsides above the dyke in the form of a graben-‐fissure system (Wyrick et al., 2004). The host rock can further fracture if the magma releases volatile material as it rises (Wyrick et al., 2004). Figure 2. Cross section showing pit crater formation through dyke emplacement. Image from Wyrick et al., 2004. 13 According to Davey et al. (2013), there are two lithological environments that may be conducive for the formation of pit crater chains: radar dark haloes from bolide impacts or volcaniclastic material from volcanic eruptions. The two lithological units could be mechanically weak because the material is unconsolidated or poorly welded (Davey et al., 2013). The radar dark halo unit consists of fine-‐grained, unconsolidated material approximately 1 m thick and is unlikely to produce pit crater chains that are detectable by the current resolution from the Magellan Radar images (Ghent et al., 2010). Ash fall or pyroclastic material from shield volcanoes would be mechanically weak enough to allow the formation of pit crater chains. Pit crater chains may preferentially form within shield plain material if there are enough shield volcanoes in the region to provide the necessary thicknesses of pyroclastic material (that can locally collapse into younger graben-‐
fissures which provide the necessary extension). There are areas on Venus where there are no pit crater chains but there are abundant extensional features (graben-‐fissure systems) throughout the unit. To explain the occurrence of pit crater chains in one area but not another there are two other aspects to consider (Davey et al., 2013). The first is that current unit descriptions give incomplete descriptions of the physical properties of lithological units that are relevant to the formation of pit crater chains (degree of welding, thickness, amount of vesiculation). The second is that a lithological unit that can facilitate pit crater chain formation may be buried. Relevant structures within the 14 buried unit that would aid in pit crater chain formation would be obscured from view and not described by surface mapping (Davey et al., 2013). As mentioned above, pit crater chain formation relies on an extensional controls (e.g., underlying graben-‐fissures) and lithological controls (e.g. a surface unit that is weak enough to collapse into the extensional space produced during graben-‐fissure formation). There are several other proposed mechanisms for the formation of pit crater chains in other bodies throughout our solar system. For example lava tubes, dyke with ground water interaction, collapsed magma chambers, karst dissolution, extensional faulting and dilational faulting have all been proposed as mechanisms for pit crater chain formation (Wyrick et al., 2004). Chapter 3: Methods 3.1 Using SAR images on ArcGIS 10.1 The study area is bounded by latitudes of 28° N and 32° N and longitudes 47° E and 51° E (Figure 2) and was chosen because it is abundant in pit crater chains and is in part of the larger area currently being mapped by Wenzhe Lu.. Full resolution SAR images were obtained through the USGS planetary website and downloaded in TIFF format. The TIFF format was used to remain consistent with previous mapping efforts on Venus, such as Davey et al. (2013). The images were 15 imported into ArcGIS 10.1 and georeferenced using the geodetic parameters for Venus. In ArcGIS, lines were used to draw over pit crater chains and graben-‐fissure systems, and to define contacts. SAR images are different from ordinary photographs in the fact that radar waves are used as the source of illumination instead of a light source. The echo strength tends to increase with decreasing incidence angle (Tanaka, 1994). Therefore slopes facing the radar antenna are stronger and appear brighter in radar images compared to those of slopes that face away from the antenna. Smooth features will often appear dark because radar waves are reflected in directions away from the antenna (Tanaka, 1994). However, rough features are brighter than smooth features because of radar scattering, unless the smooth surface is oriented approximately perpendicular to the incoming radar wave, in which case the radar return will be particularly strong. 16 Legend Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 3. Study area: SAR image acquired by the Magellan Mission. Centered around 49° E, 30° N. 17 3.2 Identification of Geological Features Graben-‐Fissure Systems Graben-‐fissures are long, narrow, extensional, linear features that form on the surface of Venus (Figure 4). Graben have an observable floor while fissures are trough or V-‐shaped. Graben-‐fissure features can be organized in both radiating and linear systems and are often found intersecting other sets of graben-‐fissures. Those with a north-‐south trend will be illuminated on the right hand side and those with east-‐west trend will appear darker on left-‐looking SAR images (Ernst et al., 2003). Figure 4. An example of a NNW-‐trending graben-‐fissure system (47.2° E, 31.2° N) 18 Pit Craters and Pit Crater Chains Pit craters can be identified by the fact that depressional collapse features are illuminated on the right hand size on left-‐looking SAR images (Davey et al., 2013). Pit crater chains have been further classified into three categories. The first includes pits with uniform size and shape (Figure 5) and have been termed even sized-‐type chains. The second exhibits at least one trough feature somewhere throughout the chain, often the trough is more oval and stretched compared to other circular pits within the chain (Figure 6); these chains have been termed trough-‐type chains. The third category has been termed a “tadpole-‐type chain”. The “tail” consists of small circular pits while the “head” consists of a large, angular pit at the end of the chain (Figure 7). Figure 5. An example of a category 1, “even sized-‐type” pit crater chain, within the study area (49.4° E, 29.5° N). 19 Figure 6. An example of a category 2, “trough-‐type” pit crater chain (47.5° E 30.2° N) Figure 7. An example of a category 3 “tadpole-‐type” pit crater chain (50.1° E 30.7° N) 20 Volcanic Flooding On SAR images, local volcanic flooding appears darker than lithological units hosting pit crater chains. The dark nature of the flooded region suggests that the flooded zones are much smoother than those which host pit crater chains. The flooding appears to have filled in several of the bordering pit craters indicating that the flooded material is younger than the pit crater chains (Figure 8). Figure 8. An example of flooding that has filled in pits within the study area (49.1° E 29.5° N) 21 Chapter 4: Observations 4.1 Lithology Previous mapping efforts by Campbell and Campbell (2004), indicated only one lithological unit within the study area. Pit crater chains are hosted in a topographically high region surrounding the central bulge of Nyx Mons. Flood boundary contacts would suggest that there are two lithological units within the study area (Figure 9). The oldest unit comprises the topographically high region that hosts pit crater chains that surrounds the central bulge of Nyx Mon. It is hypothesized that this region is composed of radar-‐bright plains material. The second unit is composed of the flooded material from Nyx Mons. 22 Legend Pit Crater Chains Graben-‐fissure System Volcanic flooding Radar-‐bright Plains Material Representing an Older Generation of Volcanic Flooding Geographic Sub-‐Center of Radiating Graben-‐fissure System Figure 9. Geological map of study area. Location of map area same as in Figure 3 23 4.2 Pit Crater Chains Within the study area a total of 143 pit crater chains were mapped. The pit crater chains are all associated with a single radiating graben-‐fissure system. Pit crater chains within the study area have an average of 10.9 pits per chain and an average length of 9.3 km. Even sized-‐type pit chains (category 1) are the most frequent in the area, followed by trough-‐type (category 2) then tadpole-‐type chains (category 3). The study area was further divided into three sub-‐areas where the majority of pit crater chains are found (Figure 12). Each sub-‐area indicates a topographic high surrounding the central bulge of Nyx Mons (Figure 11). Within the three sub-‐areas there are a total of 137 pit crater chains. Another six pit crater chains were found hosted in areas of the younger volcanic flooding. 24 Frequency of Pit Crater Chain Category in Study Area Category 1 Chain Category 2 Type Category 3 0 0.1 0.2 0.3 0.4 0.5 0.6 Frequency Figure 10. Distribution of Pit Crater Chain Categories in Study Area Figure 11. Topographic profile of study area along 30.1° N. Arrows indicate sub-‐
areas hosting pit crater chains. Sub-‐Area 2 is not shown because it is north of profile. Image from Campbell and Rogers (1994). 25 Centered around 49° E, 30° N. Legend Pit Crater Chains Graben-‐fissures Flood Boundaries Boundary of Sub-‐Area Figure 12. Study area: showing sub-‐areas where most pit crater chains are located. 26 Sub-‐area 1, located in the eastern region of the study area has the highest density of pit crater chains (0.00352 pit chains/km! ). These pit crater chains have a northeast trend (Figure 12). Within sub-‐area 1, there is a mixture of pit crater chains (Figure 13). The tadpole-‐type chains (category 3) within this region have their “heads” located at northeast end of the chain, with the exception of one chain that has a “head” on the southwest end of the chain. All of the pit crater chains in sub-‐area 1 are hosted along the eastern portion of the topographic high with the exception of bordering pits that have been partially flooded. Frequency of Pit Crater Chain Category in Sub-‐Area 1 Category 1 Chain Category 2 Type Category 3 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 Frequency Figure 13. Frequency of pit crater chain category in sub-‐area 1 Sub-‐area 2 is located approximately 19 km west of a boundary of the youngest volcanic flooding and has a density of 0.00236 pit chains/km! with pit chains that have a northeast trend (Figure 12). Pits in this sub-‐area are hosted in 27 radar-‐bright flood material at a topographic high. Category 1 even sized-‐type pit crater chains within the sub-‐area have the highest frequency followed by category 2 (Figure 14). Pit crater chains that are at a lower topography have trough-‐type and pits that have been partially flooded. Frequency of Pit Crater Chain Category in Sub-‐Area 2 Category 1 Chain Category 2 Type Category 3 0 0.1 0.2 0.3 0.4 0.5 0.6 Frequency Figure 14. Frequency of pit crater chain category in sub-‐area 2. Sub-‐area 3 contains pit crater chains along the western portion of the topographic high, has a density of 0.0088 pit chains/km! and has pit chains with a northwest trend (Figure 12). Category 1, even sized-‐type pit chains are the most frequent chain type within sub-‐area 3 (Figure 14). Tadpole-‐type chains (category 3) within the subarea have “heads” located at the southeast end of the chain. Pit crater 28 chains within each of the three sub-‐areas are inhomogeneously distributed with respect to the graben-‐fissure system. Frequency of Pit Crater Chain Category in Sub-‐Area 3 Category 1 Chain Category 2 Type Category 3 0 0.1 0.2 0.3 0.4 0.5 0.6 Frequency Figure 15. Frequency of pit crater chain category in sub-‐area 3 4.3 Volcanic Flooding The study area has been partially flooded by a younger generation of material from Nyx Mons. Mapping efforts by Campbell and Rogers (1994) indicated at least three generations of volcanic flooding within the study area. The first generation of flooding is composed of radar-‐bright plains material (light green material in Figure 9) that host pit crater chains at a topographic high. The second generation of flooding originated from Nyx Mons and is observable to the southeast of the study area. The third generation of flooding originated from Nyx Mons 29 (yellow unit in Figure 9) covering the second generation. Evidence for the relative age of flooding can be observed on the flood boundary where several pits have been filled in by flood material. 4.4 Graben-‐fissure systems In the study area pit crater chain formation is associated with one radiating graben-‐fissure system. Pit crater chains preferentially form superimposed or parallel to the graben-‐fissure system. The graben-‐fissure system has at least two well-‐defined sub-‐centers from which pit crater chains radiate. The pits in sub-‐area 1 radiate from the southern sub-‐center while the pits in sub-‐areas 2 and 3 radiate from the northern sub-‐center. The geographic sub-‐centers of the graben-‐fissure system have been flooded but isolated pits within the flooded material appear to follow the radiating pattern of the system. Chapter 5: Discussion 5.1 Interpreted Geological History of Study Area These observations can be explained by the following sequence of events. A sequence of lava flows topped by a poorly welded pyroclastic material would occur first. The pyroclastic material would be produced by small shield volcanoes (e.g. Davey et al., 2013). Next, there was the emplacement of the radiating dyke swarm 30 (marked by the radiating graben-‐fissure system) and the formation of pit crater chains in response to the extensional force of emplacement and collapse of the overlying poorly welded pyroclastic material. Finally, there were volcanic flow from Nyx Mons that caused the partial flooding of pits. An alternative scenario involving a different timing for the radiating dyke swarm would not work. Suppose dyke emplacement (and graben-‐fissure formation) occurred before the layer of pyroclastic material. Under this second scenario, the local collapse would have to happen during or after emplacement of this pyroclastic unit. However, emplacement of this pyroclastic unit would undoubtedly fill in any potential collapse space atop graben-‐fissures. So a subsequent collapse of pyroclastic material (to form pits) would seem unlikely. 5.2 Lithological and Extensional Controls on the Formation of Pit Crater Chains Pit crater chains within the study area are hosted in radar-‐bright plains material surrounded by flood material originating from Nyx Mons. The radar bright plains material features abundant small volcanic edifices and graben-‐fissure systems (Campbell and Campbell 2004). The shield volcanoes located within Bell Regio could provide a veneer of pyroclastic material that overlie the radiating graben-‐fissure system to allow for collapse and formation of pit crater chains. If the pit crater chains are hosted in radar-‐bright plains material this would be a lithological unit that contains enough extensional controls (graben-‐fissure systems) 31 to facilitate the formation of pit crater chains with the model proposed by Davey et al. (2013). 5.3 Proposed mechanisms for the formation of “tadpole-‐type” chains This study area contains tadpole-‐type chains, which don’t appear to have been described previously in the pit chain literature. A mechanism is proposed to explain the formation of these unique features (Figure 16). The first point to recognize is that the lateral magma flow through the dyke (underlying the graben-‐
fissure) is not continuous but rather moves in pulses. This is well established for dyke emplacement. (e.g., Ernst et al., 2001). So occasionally there will be a drop in magma supply at the starting end of the dyke (at the focus of the radiating system) and the leading edge of the dyke will temporarily stop propagating. As a new pulse of magma enters the dyke, then there is likely a pressure increase at the front end of the dyke, before the dyke overcomes the tensile strength of the host rock ahead and resumes lateral propagation. This temporary pressure increase has an important effect on the dyke size and shape at the leading edge of the dyke. The pressure increase causes the dyke to widen and the top of the dyke to reach to a shallower level. Furthermore when the dyke overcomes the resistance of the host rock ahead and resumes lateral propagation, then the pressure in the dyke (at its leading edge) drops and the top surface of the dyke also drops (to a greater depth). At the same time the width of the overlying graben increases (the greater the depth to the top of the dyke, the wider the overlying graben), and a wide graben is less likely to support 32 overlying weak material than is a narrow graben. This means that as soon as magma propagation resumes there would be an increase in the graben width and the overlying pyroclastic material could collapse into this space, producing a pit. This model would require the pressure needed for dyke propagation to be greater after the dyke has stopped than while it is actively being emplaced. By this model the tadpole-‐type pits would result from the greatest pressure drops (either due to a larger magma pulse or host rock properties that resulted in a greater pressure increase during longer gaps between magmas pulses). So short gaps between pulses of magma would create the pits associated with tadpole “tails” while the head of the tadpole would be created by a longer interruption in magma emplacement 33 (a) (b) (c) (d) Figure 16. Cross section of “tadpole-‐type” pit crater chain formation. Arrow indicates direction of propagation. (a) Drop in magma at the center of the graben-‐
fissure system that causes the dyke to stop propagating. (b) New pulse enters the dyke causing it to widen at the leading edge. (c) As propagations resumes the level of the dyke drops. (d) Increased width and depth facilitates larger pit formation. 34 5.4 Comparison to previous mapping of pit crater chains An objective of this study was to compare pit crater chains mapped as part of this research project with the pit crater chains mapped by Davey et al. (2013). Davey et al. determined that pit crater chains are inhomogeneously distributed throughout a graben-‐fissure system with an inferred lithological and extensional control. Based on the data obtained from SAR images and published geological maps of the region (Campbell and Campbell, 2002) the pit crater chains within the study area support the conclusion of Davey et al. (2013). The pit crater chains seem to be randomly distributed over or near parallel to the radiating graben-‐fissure in the study area. The eastern side of the radiating graben-‐fissure system has a greater density of pit crater chains compared to the western side of the system. The random distribution of chains and changes in chain density suggest that slight changes in lithology over an area of approximately 150 km can drastically affect pit crater formation. The tadpole chains indicate that pit crater chain formation is not just a function of extensional control and lithology but also a function of width, depth and flow of magma in underlying dykes. 35 Chapter 6: Conclusion 6.1 Future Research A total of 143 pit crater chains were mapped in relation to one radiating graben-‐fissure system on Venus. Pit crater chains were inhomogeneously distributed throughout the graben-‐fissure system and had a higher density on the eastern portion of the fan than the western portion. A previously unobserved style of pit crater chain was described as a “tadpole-‐type” chain and exhibited a large, angular pit at the end of the chain. In the study area it was determined that pit crater chain formation is likely a process that includes extensional fracturing and lithological control of regional shield plain material that is rich in volcanic edifices and graben-‐fissure systems. A mechanism for emplacement of pits is proposed that links each pit to a new magma pulse during emplacement along the underlying dyke. Each pulse would result in a temporary pressure increase at the front of the dyke and the pressure drop once magma propagation began would be associated with a widening of the overlying graben and enhanced collapse of overlying weak pyroclastic material produced by local shield volcanoes. By this model the tadpole-‐
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NASA-‐JPL. http://www2.jpl.nasa.gov/magellan/guide.html 39 Appendix Data for Pit Crater Chains in Sub-‐Area 1 (A1) ID Length # of (m) Pits 2 16727 39 3 8531 6 4 7544 16 5 12747 12 6 2986 6 7 4416 7 8 7938 8 9 34945 30 10 15633 18 11 21767 28 12 4922 10 13 1825 3 14 4801 8 Description Circular pits -‐ Get larger towards NE Circular Tail -‐ Tadpole feature at NE head -‐ oblong head Circular Pits -‐ Mostly equal size -‐ large pit towards SE of chain Circular Tail -‐ Tadpole feature at NE head -‐ oblong head Circular Pits – Trough present in middle of chain Circular pits -‐ Equal Size Mix of circular and oblong pits -‐ roughly equal size -‐ large pit in middle of chain Circular pits -‐ Equal Size Tail -‐ Large head oblique Circular pits -‐ Equal Size Circular pits -‐ get larder towards NE -‐ Larger head Circular pits -‐ Equal Size Circular pits -‐ get larder towards NE Circular pits -‐ trough in middle of chain Length of Tadpole Tale (m) Length of Tadpole Head (m) Trend Type of Chain N/A N/A NE Even 5551 2137 NE Tadpole N/A N/A NE Even 6320 4485 NE Tadpole N/A N/A NE Trough N/A N/A NE Even N/A N/A NE Trough 19629 7946 NE Tadpole N/A N/A NE Even 18116 1916 NE Tadpole N/A N/A NE Even N/A N/A NE Tadpole 19629 7946 NE Tadpole 40 16 6064 7 17 5172 6 18 8197 9 19 9190 9 20 10913 8 21 31 8215 11 15409 11944 13019 8901 10 7 8 5 8642 16 9292 10 4054 8 32 39 40 41 42 43 44 45 46 2553 7805 6 9 5115 3 7136 7 47 48 14552 14 8042 7 8156 7 10508 6 6498 6132 10 5 49 50 61 71 72 Circular pits – trough towards ends of chain Circular pits -‐ trough in middle of chain Circular pits -‐ trough in middle of chain Circular pits -‐ Equal Size Circular tail -‐ Tadpole SE -‐ Oblong head Circular pits -‐ Equal Size Mix of circular and oblong pits -‐ trough in middle of chain Mostly Trough Mostly Trough Mostly Trough Circular pits -‐ tadpole ne -‐ oblong head Circular pits -‐ Equal Size Circular pits -‐ Equal Size Circular pits -‐ Equal Size Mostly Trough Circular pits -‐ Tadpole NE -‐ oblong head Circular pits – Trough towards end of chain Circular pits – trough towards end of chain Circular pits – Oblong head at NE end Mix of circular and oblong pits – Trough in middle of chain Circular pits -‐ vary in size Circular pits -‐ Equal Size Circular Pits – N/A N/A NE Trough N/A N/A NE Trough N/A N/A NE Trough N/A N/A NE Even 5330 4761 NE Tadpole N/A N/A NE Even N/A N/A N/A N/A N/A N/A N/A N/A N/NE N/NE NE NE Trough Trough Trough Trough 6683 2471 E/NE N/A N/A N/NE Tadpole Even N/A N/A NE Even Even N/A N/A N/A N/A NE NE 2760 2318 NE N/A N/A NE Trough Tadpole Trough Trough N/A N/A N/NE 3264 4644 NE Tadpole N/A N/A NE N/NE N/A N/A Trough Even N/A N/A N/A N/A N/NE NE Even Trough 41 73 9560 74 5783 6 4 75 15084 8 12433 17 86 87 8405 12 5112 5 88 89 6414 11 90 5097 7 103 9473 9 104 3410 3 105 2947 5 106 19633 17 11857 6 108 109 4056 3 6068 4 110 111 19150 9 112 5099 9 131 3556 138 20223 139 22514 4 5 10 Trough in middle of chain Circular pits – Equal Size Circular pits – equal size Circular pits -‐ equal size Circular pits -‐ tadpole SE -‐ oblong head Circular pits -‐ Equal Size Circular pits -‐ Equal Size Circular pits -‐ Equal Size Circular pits -‐ Equal Size Mix of circular and trough pits Circular pits -‐ Equal Size Circular pits -‐ Equal Size Circular pits -‐ Equal Size Circular pits -‐ Equal Size circular pits -‐ equal size Circular pits -‐ Equal Size Trough hosted in middle of chain Circular pits -‐ Equal size Circular pits -‐ Tadpole SW -‐ oblong head Tadpole head on NE Trough present in middle and end of chain N/A N/A NE Even NE Even NE Even N/A N/A N/A N/A 10285 2410 NE N/A N/A N/A N/A N/A N/A NE Tadpole Even NE Even NE Even NE Even N/A N/A NE N/A N/A N/A N/A NE Trough Even NE Even NE Even NE Even NE Even NE Even NE Trough NE Even NE NE NE Tadpole Tadpole Trough N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A 2133 13811 N/A 1706 5753 N/A 42 Data for Pit Crater Chains in Sub-‐Area 2 (A2) ID Length # of (m) Pits 0 16488 34 1 20189 38 38624 19 29 30 15606 30 7422 7 51 53 3796 6 29122 15 7016 13 54 55 60 10791 8 62 3817 5 63 4187 6 5318 6 66 67 6288 9 17722 25 68 69 70 6498 15084 102 5099 10 12 9 Description Circular Pits – Equal Size Circular Pits – Equal Size Circular pits – trough in middle of chain Circular pits -‐ Equal Size Circular pits -‐ Equal Size Circular pits -‐ Equal Size Circular pits -‐ tadpole NE -‐ oblong head Circular pits – Trough in middle of chain Mostly troughs – two small circular pits Circular pits -‐ equal size Circular pits -‐ equal size Mix of circular pits and troughs Circular pits -‐ equal size Circular pits -‐ equal size Circular pits -‐ Equal Size Mix of circular pits and troughs Mix of circular pits Length of Tadpole Tale (m) N/A Length of Tadpole Head (m) N/A Trend Type of Chain NE Even N/A N/A NE Even N/A N/A NE Trough N/A N/A NE Even N/A N/A NE Even N/A N/A NE Even 18431 N/A 11888 N/A NE NE Tadpole Trough N/A N/A NE Trough N/A N/A NE Even N/A N/A NE Even N/A N/A NE Trough N/A N/A NE Even N/A N/A NE Even N/A N/A NE Even N/A N/A NE Trough N/A N/A NE Trough 43 113 6088 17 13538 8 114 115 6992 116 7032 117 10402 118 2029 119 22128 120 2320 121 9373 122 6302 123 12342 124 9141 125 7304 126 2479 127 5669 130 3065 144 6652 145 22558 146 17405 147 30894 148 11220 149 8740 5 6 10 3 13 8 3 7 11 12 5 6 8 6 2 12 14 25 15 12 and troughs Circular pits -‐ Equal Size Mix of circular pits and troughs Circular pits -‐ Equal Size Mostly Troughs Mix of circular pits and troughs Mostly Troughs Circular pits -‐ Equal Size Mix of circular pits and troughs Circular pits -‐ Equal Size Circular pits -‐ Equal Size Circular pits -‐ tadpole NE -‐ oblong head Circular pits -‐ tadpole NE -‐ oblong head Circular pits -‐ Equal Size Circular pits -‐ Equal Size Circular pits -‐ Equal Size Circular pits -‐ Equal Size One small pit and one large trough Mix of circular pits and troughs Mix of circular pits and troughs Mix of circular pits and troughs Mix of circular pits and troughs Mix of circular pits and troughs N/A N/A NE Even N/A N/A NE Trough N/A N/A NE Even N/A N/A N/A N/A NE NE Trough Trough N/A N/A N/A N/A NE NE Trough Even N/A N/A NE Trough N/A N/A NE Equal N/A N/A NE Equal 9954 2276 NE Tadpole 5907 3073 NE Tadpole N/A N/A NE Even N/A N/A NE Even N/A N/A NE Even N/A N/A NE Even N/A N/A W-‐E Trough N/A N/A NE Trough N/A N/A NE Trough N/A N/A W-‐E Trough N/A N/A W-‐E Trough N/A N/A W-‐E Trough 44 Data for Pit Crater Chains in Sub-‐Area 3 (A3) ID Length # of (m) Pits 22 15122 10 23 21069 14 24 16592 26 25 10812 20 26 5443 10 27 11598 25 28 56 8906 10569 12 4 57 3273 3 58 59 76 77 78 79 17403 13758 14798 6685 6235 8571 8 11 21 6 4 6 Description Circular pits -‐ Tadpole SE -‐ Oblong head Mix of circular and oblong pits -‐ roughly equal size -‐ large pit in middle of chain Circular pits -‐ Tadpole SE -‐ Oblong head Circular pits -‐ Equal Size Circular pits -‐ Equal Size Circular pits -‐ tadpole se -‐ Circular head Mix of Troughs and Circular pits Circular pits forming a trough Circular pits -‐ Equal Size Mix of Troughs and Circular pits Mix of Troughs and Circular pits Circular pits -‐ Equal Size Circular pits -‐ Equal Size Circular pits -‐ Equal Size Mix of Troughs and Length of Tadpole Tale (m) Length of Tadpole Head (m) Trend Type of Chain 8165 7488 NW Tadpole N/A N/A NW Trough 12603 3916 NW N/A N/A NW Tadpole Even N/A N/A NW 9075 1140 NW Tadpole N/A N/A NW Trough N/A N/A NW Trough N/A N/A NW Equal N/A N/A NW Trough N/A N/A NW N/A N/A NW Trough Even N/A N/A NW N/A N/A N/A N/A NW W Even Even Even Trough 45 80 81 82 19502 6457 12262 15 6 16 83 12443 18 84 8405 12 85 91 5112 7579 6 10 92 11621 15 93 21212 17 94 7962 8 95 14446 23 96 9298 18 97 5299 10 107 10402 6 128 5895 6 129 11971 13 132 7983 12 133 5751 134 8022 16 13 Circular pits Mix of Troughs and Circular pits Mix of Troughs and Circular pits Circular pits -‐ Equal Size Tail of Circular Pits – Oblong angular head W Circular pits -‐ Equal Size Circular pits -‐ Equal Size Circular pits -‐ Equal Size Tail of Circular Pits – Oblong angular head NW Tail of Circular Pits – Oblong angular head NW Circular pits -‐ Equal Size Circular pits -‐ Equal Size Circular pits -‐ Equal Size Circular pits -‐ Equal Size Mix of Troughs and Circular pits Tail of Circular Pits – Oblong angular head NW Tail of Circular Pits – Oblong angular head NW Circular pits -‐ Equal Size Circular pits -‐ Equal Size Circular pits -‐ Equal Size N/A N/A W Trough N/A N/A NW Trough N/A 10209 N/A 2200 NW W-‐E Even Tadpole N/A N/A N/A NW N/A NW N/A 9029 N/A 2019 NW NW 14012 6615 NW N/A N/A N/A N/A N/A N/A NW NW NW Even Even Even Tadpole Tadpole Even Even Even Even N/A N/A NW N/A 3277 N/A 2367 NW NW Trough Tadpole 8070 3662 NW Tadpole N/A N/A NW N/A N/A NW N/A N/A W Even Even Even 46 135 1515 4 136 2549 9 137 3909 3 140 6214 7 141 3882 4 151 6031 152 12104 155 23442 6 9 22 Circular pits -‐ Equal Size Circular pits -‐ Equal Size Circular pits -‐ Equal Size Mix of Troughs and Circular pits Mix of Troughs and Circular pits Circular pits -‐ Equal Size Mix of Troughs and Circular pits Tail of Circular Pits – Oblong angular head NW N/A N/A N/A N/A NW NW Even Even Even N/A N/A NW N/A N/A NW Trough N/A N/A NW Trough N/A N/A NW Even N/A 19840 N/A 3360 NW NW Trough Trough 47

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Mapping Pit Crater Chains in the Bell Regio Quadrangle on Venus

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