Impact Cratering Process Research Articles

Spectral Analysis of the Morphology of Fresh Lunar Craters I: Rim Crest, Floor, and Rim Flank Outlines

AbstractThe morphology of fresh lunar craters contains information about the physical properties of both the impactors and the lunar surface, and is therefore crucial to our knowledge of the impact cratering process. Spectral analysis is a powerful tool to study crater morphology, as it can reveal the topographic variation on different scales. In this study, we calculate the power spectral densities of the radial distance and elevation of the rim crest, floor, and rim flank outlines of fresh lunar craters. The resulting power spectral density can be decomposed into an average component and a natural variability component. For the average component, we derive the classic morphometric parameter‐crater diameter relations that are consistent with previous studies. For the natural variability component, we find that in general the spectral power increases with wavelength, which can be fitted by a piecewise function with four breakpoints. Among the four breakpoints, the power of the third breakpoint (i.e., the degree‐2 power) is of particular interest, as it determines the ellipticity of the outline. The power of the third breakpoint is found to have a diameter dependence with a peak at 20 km, which indicates that transitional craters are more elliptical than simple and complex craters. The diameter dependence of the power spectral density enables us to generate the synthetic outlines of a crater of a particular size, which can be used to develop a preliminary 3‐dimensional shape model for fresh lunar craters that is useful for improving Monte Carlo modeling of cratered surfaces on the Moon.

Holocene impact craters on Earth

Abstract Impact craters are formed by collisions of cosmic bodies moving with hypervelocity. The formation of these features is not restricted to the distant geological past; new structures are constantly being created and at least 13 confirmed impact craters and crater fields have formed during the Holocene alone. This short review paper: (1) introduces the basics of the impact cratering process to physical geographers and Quaternary geologists; (2) provides a short description of representative examples of such features (Morasko, Kaali, Kamil, Ilumetsa); and (3) discusses the similarities and differences among very small craters, and contrasts these with larger impact structures. This manuscript may be useful to researchers planning to test whether a small Quaternary depression in the ground may be of impact origin.

Morphological characteristics of impact craters with diameters of 5–20 km on the Moon

The morphology of impact craters on the Moon plays an important role in understanding impact cratering process, surface and subsurface properties, and their evolutions. In this study, we systematically analyzed the depth, rim height, slope, and rock abundance of 15,135 impact craters between 5 and 20 km in diameter on the Moon, covering the simple and simple-to-complex transitional crater regimes. We found that the decline rates of inner average slopes of mare craters are generally larger than those of non-maria craters and that the degradation rate of inner upper wall slopes around rim crest is not uniform, which decreases rapidly when the depth/diameter ratio (d/D) lies in 0.25–0.15 but slowly when d/D is in 0.15–0.05. However, we found that there is no difference in crater rim height between different terrains (e.g., maria and highlands). In addition, we also found the decrease in rock abundance is more rapid than the ejecta infilling of the crater floor, especially for large simple craters. Moreover, we investigated the diameter-dependent morphological features of fresh craters in different terrains and revised the onset of transitional crater diameters based on the 180 freshest craters. We found that crater initial depth depends on target properties (e.g., strength and porosity) whereas that the degradation rate of the crater depth is independent of terrains. Furthermore, we found that the power-law relations for depth and diameter of fresh simple craters are d = 0.293D0.865 (mare), d = 0.297D0.884 (highland), d = 0.221D0.959 (north pole) and d = 0.209D0.981 (south pole). The onsets of transitional crater diameters are ∼14.2 km (mare), ∼15.7 km (highland), ∼16.6 km (north pole), and ∼ 16.8 km (south pole). These morphological characteristics and their differences in different terrains suggest that target properties (e.g., strength, layering, and porosity) affect the formation and degradation of larger simple and transitional craters.

Morphological Characterization of Decimeter‐ to Hectometer‐Scale Impact Craters at the Chang’E‐3/4/5 Landing Sites

AbstractThe morphology of impact craters on the Moon is important for understanding impact cratering process and lunar surface properties. Though the morphology of lunar impact craters at hectometer scales and larger is extensively investigated and well understood, the morphology of very small craters is rarely studied due to the lack of high‐resolution data. In this study, we investigated the morphology of 1,513 craters at decimeter to hectometer scales using the 5 mm/pixel–0.5 m/pixel topographic data of the Chang’E‐3/4/5 (CE‐3/4/5) landing regions. Our results show that crater depth, rim height, and inner wall slope increase with the diameter and that the mean values of depth/diameter ratio, rim height/diameter ratio, and inner wall slope are 0.065, 0.017, and 10.5°, respectively. These morphometric parameters are smaller than those of large craters above hectometer scales, which is probably due to low‐strength surficial regolith and rapid topographic degradation. The comparison of the morphometric parameters among the three regions suggests that the CE‐4, CE‐3, and CE‐5 landing regions have the highest, intermediate, and lowest regolith strengths, respectively. In addition, we first observed widespread meter‐scale concentric and flat‐bottomed craters in optical images, which are most probably formed due to strength heterogeneity within the regolith layer. Our results can help to understand near‐surface properties of the regolith layer, lunar surface evolution, and impact cratering process in the strength regime.

Mars as a time machine to Precambrian Earth

As Mars transitioned from an early Earth-like state to the cold desert planet it is today, it preserved a near pristine record of surface environments in a world without plate tectonics and complex life. The records of Mars’ Earth-like surfaces have remained largely untouched for billions of years, allowing space exploration to provide critical insights about the early days of our own planet. Here, we first review what Mars has taught us about volcanic, tectonic and metamorphic processes in the absence of discrete plates, drawing comparisons with the terrestrial and venusian records. Then, we summarize advances in understanding its early surface environments, including impact cratering, hydrological, sedimentary and geochemical processes. Altogether, the martian record provides a picture of early environments that were similar to modern terrestrial ones in many respects, with sediment and geochemical cycling, hydrothermal systems capable of hosting life, but with the exception that topography, sediment and heat sources were provided by volcanoes and impact cratering rather than plate tectonics. Mars thus offers a lens through which one might catch a glimpse of Earth's infancy, provided exploration efforts continue to refine our understanding of the similarities between Earth and Mars as well as the specificities of each planet.

Effects of Regional Thermal State on the Crustal Annulus Relaxation of Lunar Large Impact Basins

AbstractThe Moon reveals striking asymmetries in the crustal structure and chemical composition between its nearside and farside. With the gravity‐based crustal thickness model, we find that most large impact basins with rim diameters greater than 450 km on the nearside have no crustal annulus, which is instead common for basins on the farside. Previous impact hydrodynamic simulations show that the thermal state has significant influences on the impact cratering process. However, the effects of the thermal state on the long‐term post‐impact relaxation, not well studied, may be equally important for explaining the different crustal annulus structures between the nearside and farside basins. In this work, we export crustal structure and impact‐induced thermal anomaly from impact hydrodynamic simulations to subsequent post‐impact viscoelastic relaxation models, and vary the near‐surface temperature gradient and radiogenic element content (represented by Th content as a proxy) to explore influences of the crustal thermal state on the post‐impact relaxation of the annulus. Our modeling results indicate that as the basin structure starts with a relatively hot state and cools afterwards, a prolonged high‐temperature (>1,200 K) duration of the crustal annulus is required for its effective relaxation, which corresponds to a high near‐surface temperature gradient or Th content of the crust. More specifically, a near‐surface temperature gradient greater than 30 K/km in combination with a Th content of 4 ppm for the bulk crust (or a Th content of 10 ppm for a 10 km‐thick KREEP layer underlying the crust), likely representing the early thermal state of the nearside, can produce the complete annulus relaxation. In contrast, a near‐surface temperature gradient lower than 20 K/km with a crustal Th content of 1 ppm, possibly representing the thermal state of the lunar farside, produces limited relaxation. Our modeling results show that the thermal state of the lunar crust controls the final crustal annulus structure of impact basins, which can explain the different crustal annuli of large impact basins between the nearside and farside of the Moon.

Modeling the Distribution of Organic Carbon and Nitrogen in Impact Crater Melt on Titan

Abstract Titan is a chemically rich world that provides a natural laboratory for the study of the origin of life. Titan’s atmospherically derived C x H y N z molecules have been shown to form amino acids when mixed with liquid water, but the transition from prebiotic chemistry to the origin of life is not well understood. Investigating this prebiotic environment on Titan is one of the primary motivations behind NASA’s Dragonfly mission. One of its objectives is to visit the 80 km diameter Selk crater, where a melt sheet of liquid water would have formed during the impact cratering process. Organic molecules on Titan’s surface could have mixed with this water, forming molecules of prebiotic interest. Constraining how this material becomes trapped in the refreezing ice is necessary for Dragonfly to effectively target and interpret the samples it aims to acquire. In this work, we adapt the planetary ice model of Buffo et al. to Titan conditions to track how organic molecules will become trapped within the ice of the freezing melt sheet. We use HCN as a model impurity because of its abundance on Titan and its propensity to form amino acids in aqueous solutions. We show that without hydrolysis, HCN will be concentrated in the upper and middle portions of the resolidified melt sheet. In a closed system like Selk crater, the highest concentration of HCN appears 75% of the way into the frozen melt pond (relative to the surface), but HCN should be accessible at high concentrations nearer the surface as well.

Methane-saturated Layers Limit the Observability of Impact Craters on Titan

Abstract As the only icy satellite with a thick atmosphere and liquids on its surface, Titan represents a unique end-member to study the impact cratering process. Unlike craters on other Saturnian satellites, Titan’s craters are preferentially located in high-elevation regions near the equator. This led to the hypothesis that the presence of liquid methane in Titan’s lowlands affects crater morphology, making it difficult to identify. This is because surfaces covered by weak fluid-saturated sediment limit the topographic expression of impact craters, as sediment moves into the crater cavity shortly after formation. Here we simulate crater-forming impacts on Titan’s surface, exploring how a methane-saturated layer overlying a methane-clathrate layer affects crater formation. Our numerical results show that impacts form smaller craters in a methane-clathrate basement than a water-ice basement due to the differences in strength. We find that the addition of a methane-saturated layer atop this basement reduces crater depths and influences crater morphology. The morphology of impact craters formed in a thin methane-saturated layer is similar to that in a “dry” target, but a thick saturated layer produces an impact structure with little to no topography. A thick methane-saturated layer (thicker than 40% of the impactor diameter) could explain the dearth of craters in the low-elevation regions on Titan.

A review of the models of near-Earth object impact cratering on Earth

Near-Earth object (NEO) impact cratering is one of the frontier themes of planetary research. The cratering process and types, laboratory impact cratering phenomena, and cratering scaling are introduced. The hypervelocity impact-cratering process is conventionally divided into three successive stages: contact and compression, excavation, and modification. When large impact craters are formed in geological materials, shearing is the main deformation mode. At small scales, cratering in brittle materials is dominated by surface spalling; much of the crater volume consists of a wide, flat spall zone. According to the morphological characteristics of impact craters, impact craters are generally divided into two groups: simple and complex craters. The cratering mechanism of NEO impact cratering and the deficiency of point source model are analyzed. The cratering mechanism can be divided into strength regime and gravity regime. In the strength regime, the cratering results are controlled by strength, and in the gravity regime, the cratering results are dominated by gravity. Crater scaling laws have been established based on dimensional analysis, point source approximation and the results of experimental and numerical impact. The scaling law is a specific power rate form, which describes well the scaling of crater size, ejecta, and crater growth. But the scaling law of the point source model is not applicable to the experimental phenomenon in several impactor radii. The suggestions for future research of NEO impact cratering are pointed out:(1) scaling where the point source hypothesis is not applicable; (2) the effect of melting, gasification, atmosphere and temperature on the cratering process; (3) scaling law and model of oblique impact; (4) momentum enhancement effect of impact; (5) experimental and numerical methods to simulate the formation of impact craters.

Detailed Morphologic Mapping and Traverse Planning for a Rover-based Lunar Sample Return Mission to Schrödinger Basin

Abstract Schrödinger basin, a 312 km diameter complex impact structure located near the lunar south pole, has been widely cited as a prime target for future lunar exploration. In 2020 NASA identified Schrödinger as a high-priority landing site for a 2024 mission supported by the Payloads and Research Investigations on the Surface of the Moon solicitation and the Commercial Lunar Payload Services program. Schrödinger basin hosts an uplifted peak ring that would provide a surface mission with access to materials that originated deep within the lunar crust, as well as material ejected from the larger South Pole–Aitken basin. Schrödinger basin also hosts well-preserved mare and pyroclastic deposits that could provide valuable insight into volcanic processes on the Moon. This study used high-resolution Wide Angle Camera and Narrow Angle Camera images from the Lunar Reconnaissance Orbiter, elevation data from the Lunar Orbiter Laser Altimeter, and spectral data from the Clementine mission to produce a high-resolution morphologic map of the basin center consisting of 10 distinct morphologic units. This new map was used to plan traverse paths for a rover mission to the region. The design requirements for this traverse were based on those originally developed for the multiagency Human-Enhanced Robotic Architecture and Capability for Lunar Exploration and Science (HERACLES) mission and the Canadian Space Agency Precursor to Humans And Science Rover concept. The proposed traverse path includes up to 20 sample collection stops with the goal of better understanding lunar chronology, lunar volcanism, and the impact cratering process.

Emplacement conditions of lunar impact melt flows

Hypervelocity impacts on terrestrial bodies have the potential to rapidly heat and redistribute silicate target material to form impact melt flows. On the Moon, a subset of impact melt deposits exited the craters as laterally moving flows that moved away from the crater rim under the influence of gravity. These impact melt flows exhibit similarities to lava flows, but have the potential to be superheated by the impact cratering process. To estimate the initial temperature (T0) and thickness (H0) of these flows we combine new remote sensing analyses, experimentally-derived rheological relationships for three lunar analog materials, and numerical forward models of impact melt flow emplacement to identify combinations of parameters that yield flows matching the observed length of impact melt deposits on the Moon. We focus on the impact melt flows of four craters, which span a range of target materials and crater diameters (D): Giordano Bruno (D = 22.1 km), Necho (D = 36.9 km), Tycho (D= 85.3 km), and Copernicus (D= 96.1 km). We modeled three melt compositions—anorthosite, norite, and basalt—emplaced under cold ambient temperatures (Tambient= –180 °C) on the Moon, to place a minimum bound on cooling-limited flow emplacement. Results show that observed flow lengths can be achieved by subliquidus melts emplaced within a laminar flow regime. For high-density (0% vesicularity) melts, best-fit inversions for T0 and H0 have mean values of 1253.3 ± 149.8 °C and 21.6 ± 8.7 m, respectively; for low-density (64% vesicularity) melts, mean T0 and H0 values are 1343.3 ± 87.8 °C and 27.9 ± 5.7 m—with uncertainties reported at 1 standard deviation. Model results also show that under cold ambient temperatures, impact melt flows with 0 to 64% vesicularity would be expected to have mean emplacement durations of 76, 78, 174, and 186 h for Giordano Bruno, Necho, Tycho, and Copernicus, respectively. Therefore, initial impact melt temperatures do not have to be in the super-liquidus range to form gravity-driven flow deposits matching observed lengths, and emplacement times are long enough for the majority of self-secondary fragments to land before the impact melt flows are fully emplaced. This suggests that some impact melt flow may be lava-like in their emplacement dynamics and that differences in crater populations on impact melt flows and adjacent ejecta may differ significantly, with impact melts providing a more robust estimate of the emplacement age of the young craters due to the presence of fewer secondaries. Impact melts could still have super-liquidus temperatures at the time of their initial formation, but turbulence would facilitate the rapid entrainment of external clasts, quickly lowering melt temperatures to generate flows that are similar to the products of effusive volcanic eruptions.

Shocked quartz in distal ejecta from the Ries impact event (Germany) found at\u2009~\u2009180\xa0km distance, near Bernhardzell, eastern Switzerland

Impact ejecta formation and emplacement is of great importance when it comes to understanding the process of impact cratering and consequences of impact events in general. Here we present a multidisciplinary investigation of a distal impact ejecta layer, the Blockhorizont, that occurs near Bernhardzell in eastern Switzerland. We provide unambiguous evidence that this layer is impact-related by confirming the presence of shocked quartz grains exhibiting multiple sets of planar deformation features. Average shock pressures recorded by the quartz grains are ~ 19 GPa for the investigated sample. U–Pb dating of zircon grains from bentonites in close stratigraphic context allows us to constrain the depositional age of the Blockhorizont to ~ 14.8 Ma. This age, in combination with geochemical and paleontological analysis of ejecta particles, is consistent with deposition of this material as distal impact ejecta from the Ries impact structure, located ~ 180 km away, in Germany. Our observations are important for constraining models of impact ejecta emplacement as ballistically and non-ballistically transported fragments, derived from vastly different depths in the pre-impact target, occur together within the ejecta layer. These observations make the Ries ejecta one of the most completely preserved ejecta deposit on Earth for an impact structure of that size.

Physical and Chemical Evolution of Lunar Mare Regolith

AbstractThe lunar landscape evolves both physically and chemically over time due to impact cratering and energetic processes collectively known as space weathering. Despite returned soil samples and global remote sensing reflectance measurements, the rate of space weathering in the lunar regolith is not well understood. To address this, we developed a novel three‐dimensional landscape evolution model to simulate the physical processes that control the burial, excavation, and transport of regolith on airless bodies. Applying this model to the lunar mare, we find that over billions of years of surface evolution, material typically spends only a few million years on the surface where it is exposed to the effects of space weathering. The small surface residence times are a result of vigorous mixing by small‐scale impacts, predominantly driven by secondary crater formation. We deduce the rate of space weathering by comparing our modeled distribution of surface residence times on the lunar mare to measurements of space weathering maturity from Apollo soil samples and orbital surface reflectance datasets. These chemical constraints indicate that soil on the lunar mare reaches maturity in 7 Myr of cumulative surface exposure though due to uncertainties in the rate of small secondary crater production, this timescale could be 2–3 times higher. Weathering progresses more rapidly upon initial exposure to space but the surface residence time required to achieve maturity is realized over billions of years as regolith is repeatedly buried and exposed by small impacts.

The chemical composition of impact craters on Titan

We investigate the spectral behavior of nine Titan impact craters in order to constrain their composition. Past studies that have examined the chemical composition of impact craters on Titan have either used qualitative comparisons between craters or combined all craters into a single unit, rather than separating them by geographic location and/or degradation state. Here, we use Visual and Infrared Mapping Spectrometer (VIMS) data and a radiative transfer code to estimate the atmospheric contribution to the data, extract the surface albedos of the impact craters, and constrain their composition by using a library of candidate Titan materials, including essentially water ice, tholin, a dark component, and other possible ices at different grain sizes. Following a general characterization of the impact craters, we study two impact crater subunits, the “crater floor” and the “ejecta blanket”. The results show that the equatorial dune craters – Selk, Ksa, Guabonito, and the crater on Santorini Facula – appear to be purely composed of organic material (mainly an unknown dark component). Titan’s midlatitude plain craters – Afekan, Soi, and Forseti – along with Menrva and Sinlap, are enriched in water ice within an organic-based mixture. This follows the geographic pattern observed in our previous work with VIMS data, where the uppermost layers of the midlatitude alluvial fans, undifferentiated plains, and labyrinth terrains were found to consist of a mixture of organics and water ice, while the equatorial plains, hummocky terrains, and dunes were found to consist of a mixture of dark material and tholins. Furthermore, we found that the addition of some form of ice improves the fit in the ejecta spectra of Afekan and Sinlap craters. We find no indication for the presence of either NH3 or CO2 ice. Our main results agree with an existing Titan surface evolution scenario, wherein the impact cratering process produces a mixture of organic material and water ice, which is later “cleaned” through fluvial erosion in the midlatitude plains. This cleaning process does not appear to operate in the equatorial regions, which are quickly covered by a thin layer of sand sediment (with the exception of the freshest crater on Titan, Sinlap). Thus, it appears that active processes are working to shape the surface of Titan, and it remains a dynamic world in the present day.

A NEW GLOBAL CATALOGUE OF LUNAR CRATERS (≥1 KM) WITH 3D INFORMATION AND PRELIMINARY RESULTS OF GLOBAL ANALYSIS

Abstract. Impact craters are the predominant geomorphological features on the lunar surface. They can be studied to infer the ages of the lunar surfaces, the generation processes of the geological units, and the sequences of its geological events. Lunar crater-related research is dependent on crater records, which are usually stored in the form of crater catalogues. In the past, various efforts have been dedicated to generating global lunar crater catalogues. All published global catalogues, however, either contain only relatively large craters or lack 3D morphological information. This paper first presents approaches for automatic crater detection and the extraction of crater morphological information. The approaches have been performed on the lunar global datasets, e.g., digital elevation models (DEMs) and images, resulting in a global catalogue of lunar craters. To guarantee the reliability of the crater detection results, intensive manual-checking processes have been performed to improve the correctness and completeness of the catalogue. The generated global catalogue contains entries on 1.31 million lunar craters. It extends the existing global catalogues to craters with diameters ≥1 km and enriched with 3D morphological information of craters. Global analyses of craters were conducted based on the newly generated catalogue, including the analysis of crater density and depth-to-diameter ratio. We re-examined the previously observed distributions and patterns to show its fidelity and further explored other global relationships, which have not been discovered in previous research. The results updated the clues on impact cratering process and terrain differences. The developed global catalogue of lunar craters can be utilised for different applications by the research community, and the relevant research will help to enrich the literature and facilitate the advancement of crater-related planetary science.

A plagioclase-rich rock measured by Yutu-2 Rover in Von Kármán crater on the far side of the Moon

The Chang'E-4 spacecraft successfully landed in Von Kármán crater inside the South Pole-Aitken basin on the lunar far side on January 3, 2019 and the Yutu-2 Rover has performed explorations on the lunar surface for nine lunar days as of September 2019. Our earlier analysis of the visible and near-infrared spectrometer measurements made by the Yutu-2 rover during the first two lunar days shows that the regolith of the landing site may have come from the nearby Finsen crater and is dominated by plagioclase with lesser amount of mafic minerals. During its third lunar day explorations, the Yutu-2 photographed a small piece of lunar rock and measured its reflectance spectra. Compared with the spectra of its surrounding regolith, this rock's spectrum has deeper absorption features, indicating its fresher nature. To obtain the mineralogy of the rock, we compared the rock's spectrum with the spectral library data of NASA's reflectance experiment laboratory of returned lunar rocks and lunar meteorites. We found that this rock is also plagioclase-rich with a possible plagioclase abundance of 60–80 vol%. A source region analysis using the Moon Mineralogy Mapper's remote sensing observations indicates this rock was ejected from the Zhinyu crater, about 30 km west of the landing site, rather than directly from the Finsen crater. Numerical simulations of the Zhinyu crater on the impact cratering process and ejecta thickness distribution confirmed our findings and imply that the surficial materials at the CE-4 landing site experienced a complicated evolution rather than simply retaining the pristine or primordial ejecta directly from the Finsen crater.

Surface gravity and crater diameter as proxies of extra-terrestrial impact

Impact cratering is a dominant geological process in the Solar system and is one of the frontier themes in planetary research. Here we explore the possible dependency of surface gravity and crater geometry in the natural and theoretical prediction from scaling factor of crater diameter using morphometric analysis. Theoretical model predicts a negative correlation between final crater diameter (Dfr) and gravitational acceleration of the target body (g). In our study, an inner solar system body reveals consistent correlation that demonstrates a fundamental relationship between Dfr and g, although such process may not be applicable if we consider outer solar system bodies. Our study indicates that impact-cratering process is primarily controlled by objects associated with asteroids belts. We also demonstrate that crater-diameter scaling can be used as the proxy for extra-terrestrial impact.

Geophysical characterization of the Daskop granophyre dyke and surrounding host rocks, Vredefort impact structure, South Africa

AbstractGranophyre dykes in the central part of the Vredefort impact structure are believed to be the remnants of the impact melt sheet, which intruded downwards along the fractures in the crater floor. Little is known about their original penetration depth, dip, structural relationships with the host rocks, and their general geophysical characteristics. This information is critical to understand the emplacement history of the granophyre dykes, as it relates to the formation and modification of large impact structures. We conducted magnetic and resistivity surveys across the Daskop granophyre dyke (DGD), one of the impact melt dykes in the structure's core. The magnetic survey revealed that the DGD gives a strong magnetic response at positions where the dyke outcrop exceeds the surface topography, but a very weak response where the outcrop is nearly at the same elevation as the surrounding topography. The magnetic anomaly is thus predominantly due to the outcrop protruding above ground level, suggesting a limited volume of dyke material in the subsurface and a small penetration depth. The resistivity survey performed on two profiles, set perpendicularly across the DGD, indicated a shallow penetration depth (<3 m), consistent with the magnetic interpretation. Thus, our geophysical study demonstrates that the DGD is currently at the very bottom of its original emplacement. This may either be an erosional coincidence, or it may be controlled by a fundamental process of impact cratering. Further studies are warranted to determine if other granophyre dykes at Vredefort are similarly at their lowermost terminations.

Stress‐Strain Evolution During Peak‐Ring Formation: A Case Study of the Chicxulub Impact Structure

AbstractDeformation is a ubiquitous process that occurs to rocks during impact cratering; thus, quantifying the deformation of those rocks can provide first‐order constraints on the process of impact cratering. Until now, specific quantification of the conditions of stress and strain within models of impact cratering has not been compared to structural observations. This paper describes a methodology to analyze stress and strain within numerical impact models. This method is then used to predict deformation and its cause during peak‐ring formation: a complex process that is not fully understood, requiring remarkable transient weakening and causing a significant redistribution of crustal rocks. The presented results are timely due to the recent Joint International Ocean Discovery Program and International Continental Scientific Drilling Program drilling of the peak ring within the Chicxulub crater, permitting direct comparison between the deformation history within numerical models and the structural history of rocks from a peak ring. The modeled results are remarkably consistent with observed deformation within the Chicxulub peak ring, constraining the following: (1) the orientation of rocks relative to their preimpact orientation; (2) total strain, strain rates, and the type of shear during each stage of cratering; and (3) the orientation and magnitude of principal stresses during each stage of cratering. The methodology and analysis used to generate these predictions is general and, therefore, allows numerical impact models to be constrained by structural observations of impact craters and for those models to produce quantitative predictions.

Transitional impact craters on the Moon: Insight into the effect of target lithology on the impact cratering process

AbstractWe studied a data set of 28 well‐preserved lunar craters in the transitional (simple‐to‐complex) regime with the aim of investigating the underlying cause(s) for morphological differences of these craters in mare versus highland terrains. These transitional craters range from 15 to 42 km in diameter, demonstrating that the transition from simple to complex craters is not abrupt and occurs over a broad diameter range. We examined and measured the following crater attributes: depth (d), diameter (D), floor diameter (Df), rim height (h), and wall width (w), as well as the number and onset of terraces and rock slides. The number of terraces increases with increasing crater size and, in general, mare craters possess more terraces than highland craters of the same diameter. There are also clear differences in the d/D ratio of mare versus highland craters, with transitional craters in mare targets being noticeably shallower than similarly sized highland craters. We propose that layering in mare targets is a major driver for these differences. Layering provides pre‐existing planes of weakness that facilitate crater collapse, thus explaining the overall shallower depths of mare craters and the onset of crater collapse (i.e., the transition from simple to complex crater morphology) at smaller diameters as compared to highland craters. This suggests that layering and its interplay with target strength and porosity may play a more significant role than previously considered.