TERRESTRIAL IMPACT STRUCTURES

The idea for this atlas was born already several years ago. Then, a precise topographic atlas presenting terrestrial impact craters, the scars of the impacts of solid bodies onto the Earth's surface, had not been established yet. This was astonishing, as such impacts had long been considered as the "most fundamental process on the surfaces of the terrestrial planets", as the late Eugene Shoemaker, one of the pioneers of impact geology, had put it. But when the first digital elevation data had been processed from the early phase of the TanDEM-X mission, they immediately showed their great potential for mapping applications. We were particularly excited, because the objective of this space-borne undertaking of generating a precise high-resolution digital elevation model for the entire solid surface of Earth would allow, for the first time, to map the morphology of all terrestrial impact structures with a topographic expression, in detail. What had been the limiting factor of the existing datasets for such an exercise in the past - no global coverage, data gaps, or data artefacts - would no longer hamper the cartographic work. Only very small impact craters below the resolution of the TanDEM-X data would escape recognition in the TanDEM-X maps. Patience, however, was required before the final TanDEM-X Digital Elevation Model (DEM) was released and access to the data for science applications was granted. In the meantime, we had investigated the workflow to present the topography of an impact structure and its environs by using Raw DEM scenes. Because our intention was not to merely publish a sequence of high-quality maps, our atlas concept foresaw to additionally provide short and concise, illustrated text sections for each impact structure. As two of us, Thomas Kenkmann and Wolf Uwe Reimold, have throughout their careers as impact geologists confirmed the impact origin of a considerable number of structures and participated in the studies of many more, these so-called "fact sheets" also reflect personal experience. Clearly this is the reason why these texts sometimes differ in style - this is indeed intended. What is more, the fact sheets also reflect the varied degrees of detailed study that different impact structures have permitted to date. The individual texts also provide information about the location of a structure and how to access it. For those structures in very remote parts of the globe, the access notes can, however, be limited. A list of selected references completes each text. These are the main references that were used by the authors of the individual fact sheets - who are acknowledged at the top of each structure's first page. These references are also intended to provide the layperson with a strategic entry into the literature pertaining to any of these impact structures. The atlas consists of three parts: introductory chapters, the physical maps with corresponding texts for all impact structures covered herein, and an annexure with supplementary information. The first introductory chapter briefly explains why interplanetary space is filled with small bodies and why they sometimes approach Earth's orbit. Chapter 2 goes a bit deeper into the principles of hypervelocity impact. It explains crater formation, shock metamorphism, and specific impact-related lithologies. This chapter introduces impact related concepts that are addressed in more detail in the subsequent impact structure-related fact sheets. The third chapter describes the terrestrial impact crater record. A short account on its completeness - or rather lack thereof - is followed by its status at the time of the manuscript deadline as of February 2020. As the last two years have been very productive in identifying previously unknown impact structures, we have thrived towards a status that is as complete as possible. Our atlas, however, has a differentiated view on some alleged impact structures. Where we feel that the evidence for an impact origin is still incomplete or ambiguous, we treat these structures separately, even though other impact data bases may list these cases as confirmed - without further analysis of the evidence. The remaining introductory chapters explain the remote sensing principles behind data acquisition and processing, together with how the physical maps were generated. Chapter 4 illustrates radar remote sensing, particularly the concept of Synthetic Aperture Radar (SAR) that allows high-resolution space-borne imaging in the microwave spectral domain. While SAR yields 2D imagery, it is a pre-condition for SAR interferometry, which exploits elevation and therefore delivers a 3D view. This concept has been successfully implemented in the TanDEM-X mission, which is the topic of chapter 5. This section not only summarizes the characteristics of both radar satellites, but it also explains how the challenging task of interferometric data takes was pursued with subsequent data processing. Chapter 6 explains how the maps were produced. This demanded a stepwise approach with observation of various requirements, such as map projection, illumination, and dealing with the presence of water bodies. With chapter 7 the map-oriented part of the atlas - by far the major portion of our book - begins: Africa, North/Central America and South America are covered in Part 1, Asia, Australia and Europe in Part 2. All impact structures on a continent are listed alphabetically. Each section comprises a one-page physical map together with the illustrated fact sheet. Those structures where confirmation as impact structures has remained ambiguous or incomplete appear separately at the end of each continent chapter. An annex with the complete list of selected references, a glossary, a geologic timescale, and lists of acronyms and abbreviations completes the atlas. For whom, which readership, was this atlas compiled? Our atlas is intended to provide the professional expert with a reference book summarizing the terrestrial impact crater record as of February 2020 in a complete and coherent, as well as concise manner. For the interested layman our book can be a source of impact cratering information at different levels - basic in the introductory chapters and more specific in the fact sheets. The maps together with the indication how to access an individual impact site may even trigger visits to some of the more conveniently located structures. In this atlas our knowledge and expertise from different research areas - impact geology, a bit of astronomy, and remote sensing - has been combined. Thomas Kenkmann and Wolf Uwe Reimold assembled the fact sheets for the currently confirmed impact structures. Their ongoing work in the past years even helped to add several new impact structures to the terrestrial impact record - and to the atlas content. We believe that this work provides a status that is as up-to-date as possible. During their field work, they obtained a large repository of imagery, some of which is shared here in the book. Manfred Gott­wald dealt with the remote sensing related tasks, formulated the introductory chapters, and took care of the graphic artwork. This included the processing of the TanDEM-X digital elevation data, the production of the physical maps, and, where appropriate, the fusion of the TanDEM-X data with Sentinel-2 data. What we present in this atlas about the terrestrial impact structure record - its content and parameterization - reflects the status that, as we feel, provides a complete and consistent view as of February 2020. For several impacts, however, different results can be found in literature. As the study of impact structures is an ongoing and vivid research topic, for these structures only future work will reveal the final truth. Considerable effort has been put into the making of this reference work. We hope that our result will provide assistance to many, and enjoyment as well. It may be a reference volume for years to come. Manfred Gottwald, Garching bei München Thomas Kenkmann, Albert-Ludwigs-Universität, Freiburg Wolf Uwe Reimold, University of Brasília, Brasília February 2020