Conference report: Large Meteorite Impacts and Planetary Evolution VI

Special issue of <i>MAPS</i> in honor of Wolf Uwe Reimold on occasion of his 65th birthday

The Fourth Conference on Large Meteorite Impacts and Planetary Evolution (LMI IV) was held near the town of Parys in the Vredefort Dome, the center of Earth's oldest and largest preserved impact structure. The Vredefort Dome, approximately 120 kilometers southwest of Johannesburg, South Africa, presents a superb cross section through deep levels of the impact structure. The Dome also provides exposures of the exceptionally well preserved Archean and Paleoproterozoic (&gt;3.1 to 2.1 billion year old) rocks of the Kaapvaal craton. In July 2005, the northwestern part of the Dome was declared a World Heritage Site. Work is under way to strengthen the tourism infrastructure at the site, including construction of a visitor center.

The impact pseudotachylitic breccia controversy: Insights from first isotope analysis of Vredefort impact-generated melt rocks

The Duolun basin, which is located in Inner Mongolia, China, has been proposed to be an impact structure with an apparent rim diameter of about 70, or even 170 km. The designation as an impact structure was based on its nearly circular topography, consisting of an annular moat that surrounds an inner hummocky region, and the widespread occurrences of various igneous rocks, polymict breccias, and deformed crustal rocks. Critical shock metamorphic evidence is not available to support the impact hypothesis. We conducted two independent reconnaissance field surveys to this area and studied the lithology both within and outside of the ring structure. We collected samples from all lithologies that might contain evidence of shock metamorphism as suggested by their locations, especially those sharing similar appearances with impact breccias, suevites, impact melt rocks, and shatter cones. Field investigation, together with thin‐section examination, discovered that the suspected impact melt rocks are actually Early Cretaceous and Late Jurassic lava flows and pyroclastic deposits of rhyolitic to trachytic compositions, and the interpreted impact glass is typical volcanic glass. Petrographic analyses of all the samples reveal no indications for shock metamorphic overprint. All these lines of evidence suggest that the Duolun basin was not formed through impact cratering. The structural deformation and spatial distribution pattern of the igneous rocks suggest that the Duolun basin is most likely a Jurassic–Cretaceous complex rhyolite caldera system that has been partly filled with sediments forming an annular basin, followed by resurgent doming of the central area.

The melt rocks of the Vredefort impact structure – Vredefort Granophyre and pseudotachylitic breccias: Implications for impact cratering and the evolution of the Witwatersrand Basin

Recent investigations indicate the importance of meteorite impact as a process which has operated throughout geologic time to produce numerous originally circular structures as much as 50 km in diameter. One such structure, at Sudbury, Ontario, is associated with large volumes of internally derived igneous rock. Geological and experimental studies have demonstrated that rocks subjected to intense shock waves produced by hypervelocity meteorite impacts and by nuclear or chemical explosions develop distinctive and uniqueshock-metamorphic features, including: (1) high-pressure minerals such as coesite and stishovite; (2) crystal lattice deformation features such as isotropic feldspar (maskelynite) and « planar features » (shock lamellae) in quartz; (3) ultra-high-temperature reactions not produced by normal geological processes, such as decomposition of zircon to baddeleyite and melting of quartz to lechatelierite. These petrographic features, currently regarded as unequivocal evidence for meteorite impact, can be preserved and recognized even in very old and deeply eroded structures. Such features have now been observed in more than 50 « crypto-explosion » structures ranging in size from 2 km to more than 60 km in diameter. The recent discovery of shock-metamorphic features in rocks of the Sudbury structure, Ontario, indicates that this old and complex structure was also produced by a large meteorite impact. Petrographic shock effects are widespread in inclusions of « basement » rock in the Onaping « tuff », a unit now regarded as afallback breccia deposited in the original crater immediately after impact. Similar shock effects also occur in the footwall rocks around the basin, associated with shatter cones and unusual Sudbury-type breccias. Study of Sudbury specimens has establishedgrades of progressive shock metamorphism comparable to those recognized at younger impact structures (Brent, Ontario; Ries basin, Germany). Igneous activity associated with known meteorite impact structures takes two forms: The inferred development of the Sudbury structure was a complex process involving: (1) impact of an asteroidal body, forming a large (100-km) diameter crater with a central uplift; (2) subsidence of the central uplift and simultaneous emplacement of the Nickel Irruptive; (3) metamorphism, deformation, and erosion to its present appearance. The post-impact history of the Sudbury structure thus corresponds closely to that established for many ring-dike complexes and caldera subsidences. Similar compound impact-igneous structures, in which internal igneous activity is superimposed on a large impact crater, probably exist on both the earth and the moon. Future examination of « roofed lopoliths » and « ring-dike structures » for shock-metamorphic effects, combined with serious consideration of the geophysical effects produced by large-energy meteorite impacts, will be a productive field for cooperative studies by astrogeologists and igneous petrologists.

The main evidence in the Vredefort Dome and surrounding parts of the Witwatersrand Basin is presented, which indicates that this terrain represents the deeply exhumed root zone of a large, 2023 ± 4 Ma, complex impact structure. Shock metamorphic features such as impact melt, shatter cones, high-pressure quartz polymorphs and shock microdeformation features are restricted to the Vredefort Dome, which constitutes the central uplift of the impact structure. High strain rate brittle deformation features, including pseudotachylites and clastic breccias, are found over a larger radial distance of at least 100 km from the center of the structure. Low-temperature (∼300 °C) hydrothermal effects occur up to a similar radial distance but, in the central uplift, metamorphic mineral parageneses overprinting the shock and brittle deformation features in pelites indicate a strong radially-inward increase in temperature to between 700 and 900 °C. Pressure estimates of 0.2–0.3 GPa obtained from these mineral parageneses indicate that the Vredefort structure has been eroded by between 7 and 10 km. The original depth of burial of these rocks after the impact and their elevated temperatures explain the anomalous appearance and restricted distribution of many of the shock features compared to other, less eroded, structures. The metamorphism and hydrothermal activity are attributed to the combined effects of exhumation of deep (hot) crustal levels by the cratering event and formation of the central uplift, and shock heating, possibly with a minor component of heating caused by an overlying impact melt volume. The structural, shock and thermal impact-related features in the Vredefort Dome and Witwatersrand Basin provide a case study that should assist in the identification of other deeply exhumed impact structures.

It is a great honor for us to deliver the citation for Prof. Dr. Thomas Kenkmann, a multitalented geologist, who has made remarkable scientific contributions in the field of impact geology over the last 20 years. With a background in structural geology, Thomas is one of the leading field geologists who has worked at numerous terrestrial impact structures making significant contributions to a better understanding of impact crater formation. However, considering Thomas just as an exceptionally talented field geologist falls way too short as he is also involved in laboratory impact cratering and shock wave experiments, remote sensing of terrestrial and extraterrestrial impact structures, microanalysis of impactites and shock features, experimental rock mechanics, and numerical modeling. This long list is probably still incomplete and we mention only those activities in which Thomas became a distinguished expert himself or where he initiated important developments by his particular ability to collaborate with colleagues of different scientific disciplines. Due to his extremely broad methodological background and his multidisciplinary approach, Thomas also continues to act as a bridge builder to close the gap between impact researchers using field observations, laboratory experiments, and numerical modeling to gain a better understanding of impact processes on Earth and other planets. Most scientists in the field of impact geology became involved in this topic early in their academic career, often already as M.Sc. or Ph.D. students. To our knowledge, Thomas was initially fascinated by tectonic structures on Earth and, coupled with his interest in architecture, he wanted to study how nature has created the given landforms on Earth—in fact, we always found it very inspiring to experience Thomas interpreting geological (impact) structures with the eyes of an architect. Like many other geologists, he initially did not consider the impact of cosmic bodies as an important geological process and, thus, he focused on tectonic processes as a student of geology at the University of Cologne and during his Ph.D. thesis at the GeoForschungsZentrum (GFZ) in Potsdam, Germany. It was Dieter Stöffler who recognized Thomas’s exceptional talent, and who got him interested in impact geology. In 1998, Thomas became part of Dieter's multidisciplinary research team at the Natural History Museum in Berlin, an experience which was certainly formative for Thomas’s holistic research philosophy later on in his career. Soon after, with his first publication (jointly with Boris Ivanov and Dieter Stöffler, both Barringer medalists) on faults in the basements of ancient impact structures, Thomas’s rapid rise into the ranks of the top researchers in impact geology began. Since then he has consistently made benchmark contributions based primarily on field work at numerous impact structures, as well as work coupled with numerical modeling, laboratory experiments, or remote sensing data of impact structures on Earth and Mars. Looking through the long list of papers, conference contributions, and popular scientific articles that Thomas has produced in only 20 years of research activity in this area, there is hardly any aspect of impact cratering and shock wave processes he has not dealt with. Among his numerous talents, Thomas is an outstanding field geologist and, speaking for myself (KW) as somebody who works mostly on the theoretical side, also an excellent field guide. I have been on several excursions with Thomas where he initiated vibrant discussions. He was able to put across his often highly innovative ideas to non–field experts, too. Furthermore, both students and colleagues alike who have joined him for fieldwork can account for his almost uncanny ability to recognize and unravel structures in the field, along with his general love of the outdoors. Early on in his impact cratering career, Thomas started to study the fault systems at impact structures (e.g., Ries crater, Germany; Upheaval Dome, USA) in order to reconstruct the mechanics of crater formation, especially the crater collapse and formation of central peaks. Over the years and through numerous field trips to different structures (e.g., Jebel Waqf as Suwwan, Jordan; Serra da Cangalha, Brazil; several impact craters in Australia and the United States), he started to notice deviations from circular geometry, which he interpreted as an indication of the direction of impact. Although his catalog of criteria for oblique impact is still debated, this example demonstrates Thomas’s exceptional ability to develop new ideas from detailed field mapping and observations serving as basis for the development of quantitative models that can be tested by computer models or laboratory experiments. Thomas has been active in both disciplines through collaboration with modelers and by initiating experimental campaigns. The impact modeling community owes a great deal to Thomas for inspiring modeling studies, many of which have led to the confirmation of processes that Thomas had suggested from observations ranging from the macroscale of terrestrial and extraterrestrial craters down to the microscale of specific shock features and the formation of shatter cones. Collaboration with Boris Ivanov, Natasha Artemieva, Alex Deutsch—all Barringer medalists—Gareth Collins, and my (KW) group has led to pioneering studies demonstrating how large-scale morphological characteristics and microscale shock metamorphic features can be linked to numerical modeling. While models are a satisfactory approach to account for the exceptional dynamics of impact cratering, they do not consider the heterogeneous nature of rocks. So it was a logical step for Thomas to become involved in laboratory impact experiments with natural and thus more complex materials. In a very early publication, he addressed the effect of heterogeneities on shock wave propagation using experiments to study the formation of local melts. Several years later, Thomas initiated the large “Multidisciplinary Experimental and Modeling Impact Research Network” (MEMIN) research unit of German impact researchers, where the very important aspect of rock heterogeneities on the entire impact process has been investigated in a quite holistic way. Thomas has been the speaker of this research unit since 2009, which has become a highly successful and productive research consortium in the field of impact research. In addition to his multiple research activities, Thomas has served the impact community by organizing two conferences (Large Meteorite Impacts III, Nördlingen, 2003; Bridging the Gap III, Freiburg, 2015), by editing four special volumes on impact cratering (GSA Special Paper No. 384, 2005—LMI; M&PS 48(1), 2013—MEMIN I; M&PS 52(7), 2017—BtG III; and the upcoming M&PS 53, 2018—MEMIN II), as a team leader of the ICDP deep drilling of the Chesapeake Bay impact structure, and as a reviewer of highly ranked journals and for international funding organizations. His dedication to impact cratering research was honored by the Governor of Utah (2009) for his work at the Upheaval Dome structure and by the Arab Union of Astrophysics and Astronomy for his work at the Waqf as Suwwan impact structure. Besides his outstanding abilities as a researcher, Thomas has also been an excellent teacher, Ph.D. supervisor, and promoter to the public. As curator of the impactites and petrographical and geological rock collections at the Natural History Museum in Berlin and of the ZERIN (Center of Ries Crater and Impact Research in Nördlingen) he was extensively involved in public outreach, he published several popular science articles, and he led the conception of an exhibition on earth science and impact cratering. In 2010, he was appointed professor and head of the Geology Department at the Albert-Ludwigs University Freiburg, Germany, honoring his scientific status and collaborative strength. His teaching talent was awarded by the Ministry of Science and Education of the federal state of Baden-Württemberg, especially for the student project “Screening Earth” to search for new impact craters. Numerous bachelor and master students have since experienced Thomas’s unbridled and youthful enthusiasm for teaching geology both in the classroom and in the field with sometimes rather unconventional methods. He has helped many young scientists to start their careers, several of which are now outstanding researchers in their own right. I (MHP) certainly would not be the researcher I am today without his ongoing support. We could carry on listing all the outstanding scientific achievements, skills, and services of Thomas, but the list would remain incomplete. Above all, we do want to emphasize that Thomas is one of the best colleagues and friends we have ever had in this business, and we owe Thomas special thanks for his friendship. His advice and support was vital in many projects we carried out together. Finally, we would like to thank all the colleagues who contributed to Thomas’s nomination. The enormous support speaks for the high reputation the awardee enjoys among the community. On behalf of all of them we sincerely congratulate Thomas on winning the Barringer Medal and Award.

Integrated geophysical modelling of a giant, complex impact structure: anatomy of the Vredefort Structure, South Africa

The Slate Islands in northern Lake Superior represent the eroded remains of a complex impact crater, originally approximately 32 km in diameter. New field studies there reveal allogenic crater fill deposits along the eastern and northern portions of the islands indicating that this 500-800 Ma impact structure is not as heavily eroded as previously thought. Near the crater center, on the western side or Patterson Island, massive blocks of target rocks, enclosed within a matrix of fine-grained polymict breccia, record the extensive deformation associated with the central uplift. Shatter cones are a common structural feature on the islands and range from less than 3 cm to over 10 m in length. Although shatter cones are powerful tools for recognizing and analyzing eroded impact craters, their origin remains poorly constrained.

The Siljan ring structure (368 +/- 1.1 Ma) is the largest known impact structure in Europe. It isa 65-km-wide, eroded, complex impact structure, displaying several structural units, including a central uplifted region surrounded by a ring-shaped depression. Associated with the impact crater are traces of a post-impact hydrothermal system indicated by precipitated and altered hydrothermal mineral assemblages. Precipitated hydrothermal minerals include quartz veins and breccia fillings associated with granitic rocks at the outer margin of the central uplift, and calcite, fluorite, galena, and sphalerite veins associated with Paleozoic carbonate rocks located outside the central uplift. Two-phase water/gas and oil/gas inclusions in calcite and fluorite display homogenization temperatures between 75 degrees C and 137 degrees C. With an estimated erosional unloading of approximately 1 km, the formation temperatures were probably not more than 10-15 degrees C higher. Fluid inclusion ice-melting temperatures indicate a very low salt content, reducing the probability that the mineralization was precipitated during the Caledonian Orogeny. Our findings suggest that large impacts induce low-temperature hydrothermal systems that may be habitats for thermophilic organisms. Large impact structures on Mars may therefore be suitable targets in the search for fossil thermophilic organisms.

Associations between impact structures and meteorite occurrences are rare and restricted to very young structures. Meteorite fragments are often disrupted in the atmosphere, and in most cases, meteorite falls that have been decelerated by atmospheric drag do not form a crater. Furthermore, meteorites are rapidly weathered. In this context, the finding of shatter cones in Jurassic marly limestone in the same location as a recent (105 ± 40 ka) iron meteorite fall near the village of Agoudal (High Atlas Mountains, Morocco) is enigmatic. The shatter cones are the only piece of evidence of a meteorite impact in the area. The overlap of a meteorite strewn field with the area of occurrence of shatter cones led previous researchers to consider that the meteorite fall was responsible for the formation of shatter cones in the context of formation of one or several small (&lt;100 m) impact craters that had since been eroded. Shatter cones are generally not reported in association with subkilometer‐diameter impact craters. Here, we present new field observations and an analysis of the distribution and characteristics of shatter cones, breccia, and meteorites in the Agoudal area. Evidence for local deformation not related to the structural High Atlas tectonics has been observed, such as a vertical to overturned stratum trending N150‐N160. New outcrops with exposures of shatter cones are reported and extend the previously known area of occurrence. The area of in situ shatter cones (~0.15 km2) and the strewn field of meteorites are distinct, although they show some overlap. The alleged impact breccia is revealed as calcrete formations. No evidence for a genetic relationship between the shatter cones and the meteorites can be inferred from field observations. The extent of the area where in situ shatter cones and macrodeformation not corresponding to Atlas tectonic deformation are observed suggest that the original diameter of an impact structure could have been between at least 1–3 km. For typical erosion rates in the Atlas region (~0.08 cm yr−1), the period of time required for the erosion of such a structure (1.25–3.75 Ma) is much larger than the age of the meteorite fall. This line of reasoning excludes a genetic link between the shatter cones and the meteorite fall and indicates that the observed shatter cones belong to an ancient impact structure that has been almost entirely eroded.

Detrital shocked minerals can provide valua ble residual records of eroded impact structures. Recent studies have reported shocked minerals in modern alluvium in a subtropical climate from the deeply eroded 2.02 Ga Vredefort Dome impact basin in South Africa. To evaluate the detrital shocked mineral record at a large impact structure in a temperate setting with a Holocene glacial erosional history, we investigated 4000 detrital zircons and 20,000 quartz grains at the lesseroded 1.85 Ga Sudbury Basin in Ontario, Canada, for the presence of shocked sand grains. Modern alluvium from rivers within and outside the basin, and Holocene glaciofl uvial sands (eskers and outwash deltas ) across the basin were investigated for shocked minerals. Shocked zircon and/or quartz were found in all modern rivers and most Holocene glacial deposits within, but not outside, the basin. Petrography and scanning electron microscopy (SEM; back scattered electron [BSE]; cathodoluminescence [CL]) imaging and analysis (energy-dispersive X-ray spectroscopy [EDS], electron backscatter diffraction [EBSD]) were used to document shock microstructures. Of the total detrital zircons surveyed, 3% (118/3978) were identifi ed as shocked; Holocene samples contained higher average percentages of shocked zircon (63/1361, or 4.6%, with a high of 29%) compared to modern alluvium (55/2617, or 2.1%, with a high of 6%). EBSD analysis revealed a range of shock microstructures, including planar fractures, deformation microtwins, and crystal plastic deformation. At Sudbury, detrital shocked quartz is rare compared to zircon; only 15 grains ( 0.08%) were identifi ed, all with decorated planar deformation features (PDFs). These results demonstrate that a detrital shocked mineral record exists at a large impact basin that is in a "youthful" stage of erosion, despite its age. In addition to modern alluvium, our results also identify glaciofluvial eskers and deltas as reservoirs for detrital shocked minerals; glacial episodes thus enhance the dispersal and preservation of shocked detritus in sedimentary systems. Despite physical differences, the observation that the two largest Precambrian impact basins continue to contribute detrital shocked minerals 2 b.y. after impact suggests that a shocked mineral record of impacts on early Earth should reside in Precambrian siliciclastic rocks. ©2014 Geological Society of America.

It is a great honor for me to write this award citation for Prof. Dr. Gareth S. Collins and Prof. Dr. Kai Wünnemann, two extraordinary geophysicists, who both have devoted their careers to the study of impact craters and the role impact processes play in the solar system (Figure 1). The joint Barringer Medal is awarded for their outstanding and fundamental advancements in numerical modeling of shock physics and impact cratering. The award and medal honors the joint development of the iSALE shock physics code and its application to understanding impact crater formation at scales that range from the microscopic level to giant impacts. Thanks to Gareth and Kai, the iSALE computer code has become the numerical tool of choice for anyone interested in simulating impacts. The iSALE code is practically the only impact physics code with open access and has more than 200 users worldwide. Kai, Gareth, and their students continue to develop the code to cover more unresolved problems and to merge the gap between geological observations and physical models. Most of Gareth's and Kai's research involves the use of sophisticated computer codes. With an outstanding intuitive grasp of physics, they were able to efficiently implement relevant petrophysical parameters necessary for realistic simulations of natural impact processes. The iSALE shock physics code is based on the original SALE code (Simplified Arbitrary Lagrangian Eulerian; Amsden et al., 1980) that included an elasto-plastic constitutive model and allowed the use of various equations of state of multiple materials (Ivanov et al., 1997; Melosh et al., 1992). Gareth Collins and Kai Wünnemann improved the code in various ways by developing a modified strength and damage model (Collins et al., 2004), a porosity compaction model (Collins et al., 2011; Wünnemann et al., 2006) the 3-D version iSALE-3D (Elbeshausen et al., 2009; Elbeshausen & Wünnemann, 2011), and a dilatancy model (Collins, 2014). Having implemented porosity (Wünnemann et al., 2006), both were, for the first time ever, able to numerically derive the observed “scaling relations” between crater diameter and impact energy for small craters in porous materials such as sand, and showed that both porosity and friction between grains contribute almost equally to the observed scaling relation. They have also used this model to understand how porosity enhances melt production within impact craters and asteroidal collisions (Wünnemann et al., 2008). As preparations began for NASA's GRAIL mission to measure the Moon's gravity field, Gareth realized that the one critical piece missing from impact crater modeling is a reliable means for computing the volumetric expansion (dilatancy) due to fragmentation by a stress wave during impact. With the implementation of dilatancy (Collins, 2014), it is now possible to precisely correlate and match gravity signatures of craters with numerical simulations. Almost all craters are formed by oblique impacts. To adequately model obliquity, in particular highly oblique impacts, that show strong deviations from axial symmetry, full three-dimensional modeling is required. The development of the iSALE-3D version (Elbeshausen et al., 2009, 2013; Elbeshausen & Wünnemann, 2011) was the logical solution to address these issues. Numerical simulations must be rigorously tested for their validity. The iSALE hydrocode has been benchmarked against other hydrocodes (Pierazzo et al., 2008) and is validated against experimental data to assure the best performance (Pierazzo et al., 2008; Stickle et al., 2020). Based on these jointly developed and constantly improved iSALE hydrocodes, Kai and Gareth have investigated and solved many open questions in the field of impact cratering research, either together or with their students or other collaborating scientists. I would like to start with a few examples of common projects of Gareth and Kai. An important study was that of the buried and partly submerged Chesapeake Bay impact structure in Virginia, which is the largest impact crater in the United States. Gareth and Kai used a considerable strength contrast between the crystalline basement and the semi- to unconsolidated shelf sediments and were able to model a final crater that was consistent with the observational constraints (Collins & Wünnemann, 2005). This modeling was extremely important for the subsequent deep drilling project in the Chesapeake Bay impact structure in 2008 that was jointly funded by ICDP and the USGS. The predictions turned out to be so good that they were subsequently used to guide further geologic interpretation of the crater (Kenkmann et al., 2009b). Initially, Gareth and Kai concentrated on modeling mid-sized complex craters, such as the Ries (Wünnemann et al., 2005), Sierra Madera (Goldin et al., 2006), and Elgygytgyn and Haughton craters and compared their structural differences (Collins et al., 2008a). They explained the structural differences between them by the difference in thickness of the pre-impact sedimentary cover resting on crystalline basement. Kai and Gareth joined the impact cratering community at almost the same time at the beginning of the new millennium and they both have rapidly made it to the forefront of impact crater studies. Their careers run remarkably parallel. Kai Wünnemann graduated in 1998 at the Westfälische Wilhelms Universität Münster in Germany and started his career there at the Institute of Geophysics as a PhD fellow, later as a Research Associate. During this time, he got in touch with Barringer awardee (1998) Prof. B. A. Ivanov of the Russian Academy of Science, the developer of the SALE-B code. In the following years, Boris Ivanov became his mentor and teacher. Gareth received a Bachelor degree in Geophysics with Mathematics at the University of Liverpool in 1998 and then started a PhD at Imperial College London in the group of Barringer awardee (2020) Prof. J. Morgan. This was also the time when Kai met Gareth and the intense collaboration of both began. After completing his PhD, Gareth became a Research Associate at the Lunar and Planetary Laboratory at the University of Arizona, USA, and joined the group of Prof. H. J. Melosh in 2002. Jay Melosh, Barringer Awardee of 1999, who sadly passed away in 2020, was an outstanding scientist of modern impact cratering who brought a high level of physical rigor to the field of impact cratering. During this time, Kai became post-doctoral fellow at the Imperial College London, and later on also at the Lunar and Planetary Laboratory (LPL) in Tucson, USA. In the scientifically inspiring environment at LPL, the iSALE code rapidly grew and has been transformed to the fundamental tool in impact science that we are facing today. In 2005, after his post-doctoral period, Kai went back to Germany and became head of a junior research group at the Museum of Natural History Berlin, Germany. I had the great luck to work with Kai in this institute at that time for a couple of years and we used the time for joint projects. In 2011, Kai became head of the Department Impacts and Meteorites Research and since 2017 he is a Professor of Impact and Planetary Physics at Freie Universität Berlin and the Museum of Natural History. When Gareth returned to the UK in 2004, he passed through the various stages of academia at Imperial College London and became at first a NERC Research Fellow (2004), then an Advanced Research Fellow (2007), a Senior Lecturer (2011), a Reader (2014), and, since 2018 he is a Professor of Planetary Science. Both Gareth and Kai are training young impact researchers to ensure that their knowledge is passed onto the next generation. The diligent supervision assured the high level of quality that characterizes their common publications. Some of the former PhD students and staff have since made names for themselves and are well known in the impact community: Dr. A. S. P. Rae, Dr. V. Bray, Dr. R. W. Potter, and Dr. T. M. Davison went through the “Collins” school, while Dr. M. Zhu, Dr. D. Elbeshausen, Dr. N. Güldemeister, and Dr. R. Luther passed the “Wünnemann” school. Of course, Gareth and Kai also tread separate paths and they have set their respective scientific priorities. In the following, I would like to list some scientific milestones that both have achieved so far in the middle of their careers. Even though these studies are not mutually co-authored, they are based on the common groundwork documented in the iSALE code and prosper from intense communication between both of them. To my opinion, one of the major achievements of Gareth is his modeling of the Chicxulub impact structure and proving the formation of peak rings by central-peak collapse (Collins et al., 2002, 2008b, 2020; Morgan et al., 2016). He showed in detail how a central uplift forms and how the inner ring of the crater results from the collision of the inward-collapsing rim with the outward-moving base of a collapsing central peak. This interpretation was consistent with the structures imaged by deep seismic sounding of the crater and is now largely confirmed by the results of the IODP/ICDP 364 drilling expedition. He has definitively laid to rest the melt cavity model of peak ring formation (Baker et al., 2016). He also applied these studies, for example, to craters on the Moon such as the Schrödinger basin (Kring et al., 2016). Gareth is a co-developer of a very popular interactive web program, called the Earth Impact Effects Program (impact.ese.ic.ac.uk) for estimating the environmental consequences of impact events (Collins et al., 2005). It has gotten a great deal of media attention. This website continues to receive more than 2000 “hits” per week (and up to 10 times more when asteroids or comets are in the news), with users ranging from elementary school children to professional scientists and the journalists for whom the website was actually designed. Finally, to make this website something more than the “cartoon” that such sites may become, Gareth took the lead in writing up a scientific paper in which the code, all of its approximations, and the actual equations evaluated by the program are described. Gareth has continued to improve the program, recently adding a link to Google Earth to produce area maps of impact damage as well as the ability to predict tsunami effects in the event of an impact into the oceans. This capability was recently called for by civil defense groups in the United States to aid in disaster response in the event of a large asteroid impact. Gareth is also involved in many planetological studies. He has contributed to determining the size and formation process of the largest impact basins on the Moon, the Orientale basin (Johnson et al., 2016; Potter et al., 2013) and the South Pole Aitken basin (Miljković et al., 2015; Potter et al., 2012). He was involved in understanding the positive gravity anomalies of the large impact craters on the Moon (mascons). The multi-author research group found out that while pre-impact target porosities of less than ~7% produce negative residual Bouguer anomalies, porosities greater than ~7% such as those of the Lunar regolith, produce positive anomalies (Milbury et al., 2015). Impact crater modeling on the icy satellites Ganymede and Europa (Bray et al., 2012) allowed Gareth and coworkers to estimate the thickness of the icy crust of these Jovian satellites. Together with F. Ciesla and his PhD student T. Davison, Gareth conducted a series of simulations to investigate the heating and melting of porous planetesimals and minor bodies upon hypervelocity collisions (Davison et al., 2010, 2012). Gareth also supports the recent InSight Mission (Daubar et al., 2018) and has detected some impact events based on their seismic signature. Major achievements of Kai Wünnemann include the modeling of oceanic impacts that are of eminent importance as 2/3 of the Earth is covered by water (Wünnemann & Lange, 2002). Together with his colleague Robert Weiss, Kai conducted pioneering work in modeling the generation, propagation, and shoaling of impact-generated tsunamis and published several landmark papers on this topic (Bahlburg et al., 2010; Weiss et al., 2006; Weiss & Wünnemann, 2007; Wünnemann et al., 2007, 2010; Wünnemann & Weiss, 2015), among them the modeling of the Eltanin oceanic impact event that took place in the Arctic Pacific Ocean (Weiss et al., 2015). Kai has been closely collaborating with field geologists to investigate terrestrial impact craters and, based on their findings and geophysical constraints, he has modeled various impact events numerically to achieve the best matching between nature and model. An important contribution was the numerical modeling of the formation of the Ries crater combining the iSALE and the SOVA hydrocodes to reproduce crater morphology, size, and composition and deposition of the ejecta. Two landmark papers led by the Barringer awardees Prof. D. Stöffler (1993) and Dr. N. Artemieva (2015) shed light on the formation of suevite (Artemieva et al., 2013; Stöffler et al., 2013). Moreover, he has investigated Carancas, Peru (Kenkmann et al., 2009a), Serra da Cangalha, Brazil (Vasconcelos et al., 2012), Morasko, Poland (Bronikowska et al., 2017), and Vista Alegre craters, Brazil (Vasconcelos et al., 2019). Prof. Wünnemann was also involved in the experimental impact cratering initiative MEMIN (2009–2018) (Kenkmann et al., 2018) as the principal investigator of its modeling part. He modeled the experimentally produced craters and used acoustic emissions of impact experiments to investigate the attenuation of shock waves and the seismicity of impact events (Güldemeister & Wünnemann, 2017; Moser et al., 2013). Scaling of experimentally formed craters is key for the application of the experimental results. Kai Wünnemann used the numerical codes as the principal tool to bridge the gap between experiment and nature (Güldemeister et al., 2015; Ormö et al., 2015; Prieur et al., 2017). A novel understanding of microstructures arose from his and his coworker's mesoscale models. He was able to simulate the heterogeneous distribution of shock pressures and temperatures at the grain scale that allowed us to understand shock effects in various minerals and rocks. Modeling led to the re-calibration of shock features in quartz at low shock pressures (Kowitz et al., 2013, 2016) and the quantification of the effect of porosity (Durr et al., 2012; Güldemeister et al., 2013). These mesoscale models were also used to understand the microstructure and heterogeneous distribution of shock effects in meteorites (Moreau et al., 2017, 2019). More recently, Kai Wünnemann switched the scale to focus on the role of impact bombardment in the late accretion history of the Moon (Marchi et al., 2013, 2014; Rolf et al., 2016; Zhu, Artemieva, et al., 2019; Zhu, Wünnemann, et al., 2019). Together with Menghua Zhu, he modeled giant, cataclysmic impact events and their influence on the thermal state of the early Moon (Zhu et al., 2017) and conducted systematic computational analyses to understand melt production and the formation of the lunar mega-regolith (Liu et al., 2019). Recently, both Gareth and Kai have been involved in the Impact Modeling Working Group of the DART-HERA mission of NASA and ESA. The rapid increase in newly discovered near-Earth asteroids document that asteroid impacts pose a real threat to the Earth. Gareth and Kai are the leading scientists to better understand the environmental effects of both, smaller, relatively frequent events and large, rare events (Morgan et al., 2022). For me, as a field structural geologist and experimentalist, the common work with Kai and Gareth has always been very educative, rewarding, and productive. I have learned so much on the mechanics of impact cratering from them and I really appreciated the excellent and intense collaboration and fruitful exchange between modelers and observers. Kai and Gareth have developed a new conference format to further improve the cooperation between modelers and observers and to understand each other better. The “Virtual Field Trip” at the Bridging the Gap III Conference in Freiburg, Germany, 2015, was designed in parallel to the “Ries crater field trip” and everyone got a chance to observe in person how capable impact crater models have become and what their limitations are. In turn, Gareth and Kai do also prove their theoretical concepts in the field. To conclude, Kai and Gareth together developed and applied the comprehensive computational tool iSALE to understand shock and impact phenomena at various scales ranging from the micro-scale to the mega-scale. With this universally applicable tool and its applications, they were able to push forward impact cratering studies extraordinarily. There is nobody that deserves the Barringer Award like these two awardees. I salute Kai and Gareth for their groundbreaking work that is key to understanding impact craters and that guides geologists! Congratulations to the 2022 Barringer Medal and Award. Open Access funding enabled and organized by Projekt DEAL. Data sharing not applicable—no new data generated, or the article describes entirely theoretical research.

A probable impact structure in Betic Cordillera, Almeria, SE Spain