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This paper investigates the directional distribution of extraterrestrial objects (meteors, fireballs) impacting Earth's surface and forming craters. It also introduces a novel directional statistical mixture model to analyze their falls, validated through rigorous testing. First, we address whether these falls follow non-uniform directional patterns by explicitly employing directional statistical tools for analysing such data. Using projection techniques for longitude and latitude and more importantly, a general spherical statistical approach, we statistically investigate the suitability of the von Mises distribution and its spherical version, the von Mises–Fisher distribution, (a maximum entropy distribution for directional data). Moreover, leveraging extensive data sets encompassing meteor falls, fireball showers, and craters, we propose and validate a novel mixture von Mises–Fisher model for comprehensively analysing extraterrestrial object falls. Our study reveals distinct statistical characteristics across data sets: fireball falls exhibit non-uniformity, while meteor craters suggest a potential for both uniform and von Mises distributions with a preference for the latter after further refinement. Meteor landings deviate from a single-directional maximum entropic distribution; we demonstrate the effectiveness of an optimal 13-component mixture von Mises–Fisher distribution for accurate modelling. Similar analyses resulted in 3- and 6-component partitions for fireball and crater data sets. This research presents valuable insights into the spatial patterns and directional statistical distribution models governing extraterrestrial objects' fall on Earth, useful for various future works. [ABSTRACT FROM AUTHOR]

The Ordovician-aged Lawn Hill Impact Structure is perhaps unique among documented craters, in that it is largely filled with slumped, allochthonous Cambrian carbonates partly fluidised by the impact shock. It is a complex geological feature, with a thin horizon of suevite beneath carbonate sheets. The bulk of the impact ejecta material was either deposited on top of the mobilised carbonates, and not preserved owing to erosion, or is present beneath the base of current drilling. The geometry of the crater in its final form has been modelled in three dimensions using gravity data and dipole–dipole-induced polarisation geophysics, with control from an extensive drill-hole library. Post-impact modification, including terrace formation and carbonate fill, has resulted in an asymmetric ring syncline, which extends to depths >600 m. In the data-rich area of the Century zinc–lead–silver deposit, on the southwest margin of the crater, both intact and remnants of the crater rim and impact ejecta blanket were preserved above the deposit. The Proterozoic ore deposit is located on a detached terrace, which slumped into the crater and was buried by allochthonous carbonates from the Georgina basin. The sequence of events can be shown to comprise: impact with crater formation and injection of carbonate dykes and inter-thrust wedges in the crater walls with concurrent suevite deposits inside and surge deposits outside the crater; this was rapidly followed by slumping of carbonate mega-blocks transported by a fluidised carbonate slurry into the crater, concomitant with collapse of Proterozoic terraces; finally, collapse of the central crater uplift generated imbricate slices of Cambrian carbonate among mainly Proterozoic rocks inside the ring syncline. After impact, the Ordovician-aged Lawn Hill Impact Structure crater was filled with slumped terraces of Proterozoic sediments and Pb–Zn ore, fluidised Cambrian carbonate, plus mega-clasts of both of those, on top of a thin but persistent hematitic suevite. Outside the crater, remnants of base surge and debris deposits represent the ejecta blanket. The Century Zn–Pb deposit was preserved on a disrupted terrace; the crater rim was partially preserved above the deposit and the mineralised sequence truncated by the crater wall. Geophysical and drilling data show a terraced, asymmetric ring syncline, up to at least 600 m thick in places. [ABSTRACT FROM AUTHOR]

Since the impact ∼50,000 yr ago, surface runoff has entrained and transported sediment from the walls to the floor of Meteor Crater (Arizona, USA). Previous work interpreted this erosion and deposition to be due to predominantly fluvial (i.e., dilute water transport) processes. However, light detection and ranging (LiDAR)-derived topographic data and field observations indicate that debris flows dominated, which were likely generated by runoff that entrained the talus that borders bedrock cliffs high on the crater walls. The low gradient of the crater floor caused debris flows to stop, leaving lobate deposits, while fluvial processes delivered sediment toward the center of the crater. Cosmogenic radionuclide dating of levee deposits suggests that debris-flow activity ceased in the late Pleistocene, synchronous with regional drying. Assuming a rock-to-water ratio of 0.3 at the time of transport by mass flows, it would have taken ∼2 × 106 m3 of water to transport the estimated ∼6.8 × 106 m3 of debris-flow deposits found at the surface of the crater floor. This extensive erosion would require ∼6 m of total runoff over the 0.35 km2 upslope source area of the crater, or ∼18 mm of runoff per debris-flow event. Much more runoff did occur, as evidenced by crater lake deposits, Holocene fluvial activity (which produced little erosion), and contemporary rainfall rates. Rarely on Earth is the total amount of water that creates and runs through a landscape estimated, yet such calculations are commonly done on Mars. Our analysis suggests that erosional and depositional landforms may record only a small fraction of the total runoff. [ABSTRACT FROM AUTHOR]

Compared to intensive research on km‐sized meteorite impact craters, fewer studies focus on smaller craters. The small craters are often hard or impossible to recognize using "classical" criteria like the presence of shatter cones, shocked quartz, and geochemical indicators. Therefore, a long list of candidate structures awaiting approval/disapproval of their origin has been formed over the last decades. One of them is the Tor structure in central Sweden. To test a hypothesis of an impact origin of this structure, we have performed topographical analysis, geophysical studies, 10Be exposure dating of boulders, and 14C dating of Tor‐associated charcoal. None of the methods gave us a reason to claim the Tor structure is of impact origin. Thus, we support a recently suggested idea of Tor being formed by a grounded iceberg within a glacial lake. [ABSTRACT FROM AUTHOR]

The frequency of crater-producing asteroid impacts on Earth is not known. Of the predicted Holocene asteroid impact craters of <200 m diameter, only -30% have been located. Until now there has been no way to distinguish them from "normal" terrestrial structures unless pieces of iron meteorites were found nearby. We show that the reflective properties of charcoal found in the proximal ejecta of small impact craters are distinct from those produced by wildfires. Impact-produced charcoals and wildfire charcoals must derive from different heating regimes. We suggest that charcoal with specific reflective properties may help to recognize the meteoritic origin of small craters. [ABSTRACT FROM AUTHOR]

The data on the concentrations of major and trace elements, including platinum group elements (PGE) and Re, in suevite samples collected in different parts of the Popigai meteorite crater are reported. The suevite studied shows the identity of the bulk composition; gneiss of the Anabar Shield is identified as the protolith. It was found that the total concentrations of PGEs in suevite are not uniform: they decrease successively in the samples from the western to the southern, and further to the eastern part of the crater. The results of this study showed that the enrichment of suevite on PGEs is a characteristic of meteoritic contamination and is not associated with either crustal or mantle rocks of mafic or ultramafic composition. [ABSTRACT FROM AUTHOR]

The JAXA Hayabusa2 mission accomplished the formation of an artificial crater on the asteroid Ryugu. The aim of this work is to analyse the area surrounding the artificial crater and reveal spectral variability compared to the same region before the crater formation, to mineralogically and physically characterize the subsurface exposed material. The crater's investigation focused on the analysis of two regions corresponding to the inner part of crater (the pit and the crater wall/floor), two areas related to ejecta deposited close to the crater, two areas of ejecta moved far from the crater, and two external areas. Each area was investigated both before and after the crater formation, by the study of the photometrically corrected spectral parameters: the 1.9  |$\mu$| m reflectance, the near-infrared spectral slope, and the depth of the bands at 2.7 and 2.8  |$\mu$| m. The subsurface material of the post-crater areas shows deeper absorption bands, a decrease in reflectance, and a reddening in spectral slope with respect to the surface material of pre-crater areas. The subsurface regolith could have experienced a lower OH devolatilization due to space weathering and/or could be composed of finer dark grains than the surface layer. The ejecta reached distances of |$\sim $| 20 m from the impact point, mainly moving in the northern direction; nevertheless, a few ejecta also reached the south-eastern part of crater. [ABSTRACT FROM AUTHOR]

We performed cratering experiments on targets composed of glass beads with a power‐law size distribution that simulated the surface of rubble‐pile asteroids, and we improved the previously studied reduction factor on the crater size scaling relationship including the armoring effect using the momentum transfer efficiency. Cratering experiments were conducted using light gas guns at Kobe University and ISAS/JAXA to control the impact velocity, vi ${v}_{\mathrm{i}}$, from 50 to 4,400 m s−1. Two kinds of mixed targets were prepared by mixing glass beads with diameters of 0.1, 1, 3, and 10 mm—one with the smallest diameter beads, and one without. The size ratio of the target bead to the projectile (diameter of 1–3 mm), ϕ $\phi $, changed from 0.03 to 10. The crater size scaling relationships for the mixed targets were found to depend on the first contacted bead size. Notably, first contact with a 10 mm‐sized bead reduced the crater radius by 35% in maximum. The reduction factor due to this armoring effect on the crater size scaling relationship is written as follows: f(ϕ)=1+8.99×10−3vi0.630.34ϕ−0.19 $f(\phi)={\left[1+\left(8.99\times {10}^{-3}\right){v}_{\mathrm{i}}^{0.63}\right]}^{0.34}{\phi }^{-0.19}$; it decreased with the increase of the size ratio of the target bead to the projectile, while it increased with the increase of the impact velocity and approached unity. Our improved crater size scaling relationship that includes the reduction factor could be used to reconstruct the crater size frequency distribution on rubble‐pile asteroids such as Ryugu, and it may lead to a revision of the crater chronology of asteroids. Plain Language Summary: A rubble‐pile asteroid, 162173 Ryugu, was explored by Hayabusa2, and the crater size frequency distribution on the surface was examined. Ryugu's surface age can be estimated from this size distribution using the impact crater chronology, but the size of the asteroids forming the craters is necessary for this estimate. The crater size scaling law is used to estimate the impactor size, but this law was established using homogeneous grains such as granular sand, so it might not be applicable for rubble‐pile asteroids covered with boulders of various sizes. Moreover, because it is well known that cratering efficiency is significantly reduced by an armoring effect when a smaller asteroid collides with a larger boulder, a scaling law that includes the armoring effect is required. In this study, we performed cratering experiments on the surface of simulated rubble‐pile asteroids and examined the armoring effect on the crater size. Based on the results, we improved the previously studied armoring effect and introduced the momentum transfer efficiency, β $\beta $, into the effect, instead of a restitution coefficient, which was assumed to be zero. Using this improved scaling relationship, a more accurate chronology can be established for the asteroid craters. Key Points: We performed cratering experiments on glass bead targets with a power‐law bead size distribution at 50–4,400 m s−1The size of craters formed in the mixed bead target was speculated to be changed with the size of the first contacted beadThe armoring effect is less efficient for granular targets with a size distribution, compared with equal‐sized granular targets [ABSTRACT FROM AUTHOR]

Abstract—The formation of impact craters on the Earth's surface is accompanied by the effect of shock waves on rocks. The shock wave compression results in rocks heating up to the point of melting and evaporation during unloading. The direct mechanical action of shock compression and residual heating change the magnetic properties of rocks. Geophysical modeling is used to determine the sources of the magnetic anomaly by interpreting the fields measured on the surface, but such modeling does not take into account the impact demagnetization of rocks. This work gives an example of analysis of the magnetic anomaly over the well-studied Bosumtwi crater (Ghana, 10.5 km diameter, 1 million years old), including the numerical modeling of the crater formation process and the construction of a magnetic anomaly model based on the simulated shock compression parameters and crater drilling data. It is shown that the morphological features of the crater— the crater rim and the central uplift—form positive magnetic anomalies around and inside the crater. [ABSTRACT FROM AUTHOR]

The Yutu‐2 rover of the Chang'E‐4 spacecraft observed many meter‐sized shallow pits fully covered with small fragments, distinct from the typical rock‐free impact craters with comparable sizes in the landing area. The unique morphology of the pits and the visible and near‐infrared spectra of the fragments suggest that the fragments are broken pieces of impact melt‐conglutinated regolith breccia projectiles, which were excavated from preexisting craters. The rareness of rock on the landing area surface suggests that the preexisting craters were probably small in size (e.g., <60 m in diameter), not large enough to penetrate the thick regolith (~12 m) and to excavate the beneath rock breccia and/or bedrock. The presence of the impact melts was confirmed by the glass‐like spectra of some fragments with unusually high albedo and blue‐green tint in the centers of the pits. These observations reveal the gardening and consolidating processes on the Moon. Plain Language Summary: There are very rare outcrops of bedrock on the lunar surface because the Moon is covered by the lunar regolith. The detailed formation processes of the lunar regolith are important for understanding the orbiting and landing exploration data, and for planning future sample return missions. The lunar rover Yutu‐2 discovered fragment‐filled pits and identified in situ impact glass in the fragments. The glass‐bearing and regolith‐like fragments were probably broken pieces of lunar regolith breccia projectiles, which were excavated from the consolidated bottom and walls of preexisting small craters within the lunar regolith (<12–15 m in thickness). These observations contribute to our understanding of the formation processes of the lunar regolith. Key Points: Unique fragment‐filled pits distinct from the meteorite impact craters were observed at the Chang'E‐4 landing siteImpact glass in the fragment‐filled pits was identified in situ for the first timeThe discoveries of the Yutu‐2 rover provide important evidence for the recycling of the lunar regolith [ABSTRACT FROM AUTHOR]

Unexploded ordnance (UXO) pose a significant threat to post-conflict communities, and current efforts to locate bombs rely on time-intensive and dangerous in-person enumeration. Very high resolution (VHR) sub-meter satellite images may offer a low-cost and high-efficiency approach to automatically detect craters and estimate UXO density. Machine-learning methods from the meteor crater literature are ill-suited to find bomb craters, which are smaller than meteor craters and have high appearance variation, particularly in spectral reflectance and shape, due to the complex terrain environment. A two-stage learning-based framework is created to address these challenges. First, a simple and loose statistical classifier based on histogram of oriented gradient (HOG) and spectral information is used for a first pass of crater recognition. In a second stage, a patch-dependent novel spatial feature is developed through dynamic mean-shift segmentation and SIFT descriptors. We apply the model to a multispectral WorldView-2 image of a Cambodian village, which was heavily bombed during the Vietnam War. The proposed method increased true bomb crater detection by over 160 percent. Comparative analysis demonstrates that our method significantly outperforms typical object-recognition algorithms and can be used for wide-area bomb crater detection. Our model, combined with declassified records and demining reports, suggests that 44 to 50 percent of the bombs in the vicinity of this particular Cambodian village may remain unexploded. [ABSTRACT FROM AUTHOR]

We use observations of Bennu's impact craters, and measurements of particle sizes seen in images from the OSIRIS-REx (the Origins, Spectral Interpretation, Resource Identification Regolith Explorer sample-return mission) sampling event, to conclude there is a ∼ global sub-surface layer of finer-grained material on Bennu. The layer is evident in particle measurements from areas inside and around craters, roughness assessments of crater interiors vs. surrounding terrain, crater colors, crater depth-to-diameter (d/D) ratios, and the global crater size-frequency distribution. All data sets show a transition in behavior at crater diameters between 20 m and 40 m, with the most abrupt changes at ∼25 m diameter. Based on the depth of ∼25 m diameter craters, we infer that the layer is 1–4 m thick. The reddest craters are the youngest craters, and there is a trend to older ages as redness decreases. The decrease in redness is characterized by an initial, rapid fade, followed by a slower fade. The reduction in expected crater abundance with diameters ∼40–80 m, when extrapolating from larger sizes, is consistent with impact armoring caused by deeper, large sub-surface boulders depressing crater diameter. The existence of the layer appears to be a natural byproduct of impacts across a wide energy range and can be described as a "punctuated equilibrium": small, frequent impacts contribute to the local production of finer-grained particles and locally mix smaller particles into the subsurface; larger, less-frequent large impacts eject the fine-grained material, exposing interior boulders, and resetting the finer-grained particle population in that area. Other processes, e.g. thermal fracturing, could also contribute to the production of fine-grained material on rapid timescales. Extrapolating to the broader asteroid population and considering the production and preservation of small particles is related to target strength, surface acceleration, and impact speeds, we expect to find such layers on other asteroids, and that the layer thickness and depth will vary with composition, size, and orbital parameters. • Impact crater features on asteroid (101955) Bennu indicate subsurface layering. • Particles visible during the sampling event at Bennu support this conclusion. • A near-surface layer consists of finer-grained particles, and is 1–4 m thick. • We predict a layer of finer-grained particles should exist on other asteroids. • Layering has implications for the evolution of asteroid surfaces and interiors. [ABSTRACT FROM AUTHOR]

This paper proposes the use of Transfer Learning from the Moon to Ceres for real-time autonomous crater detection operations on asteroids. This approach, based on the use of a U-Net for crater segmentation, starts with the network training on a Moon Digital Elevation Model; the trained model is then refined using two different strategies, one based on data augmentation and one based on fine tuning with small sets of Ceres images. Data from the DAWN mission was used, while the reference crater catalogue for Ceres is the one created by Zeilnhofer from LAMO data, containing 44,594 craters with diameter greater than 1 km in a latitude range of 84.66°S–89.62°N, full longitude. The loss function chosen during network training is the Focal Tversky Loss, which takes into consideration the imbalanced distribution of class pixels in the ground truth (pixels corresponding to crater rims represent approximately only 3% over the whole dataset). For both the Moon and Ceres, the U-Net output is post-processed with a template matching algorithm. The detected craters are counted and compared to the crater catalogue locations in order to compute metrics performances (precision, recall, F1 score) in the crater domain. Post-processing on the Moon reached performance of 84.12% F 1 score on the baseline model. The baseline and augmented models gave a lower performance on Ceres data, although comparable with the outcomes found in other works, with a best F 1 score of 70.17%. However, when the model is completely retrained starting from the baseline, it ends up with a best F 1 score of 79.96%. Finally, a strategy for the actual on-board implementation is proposed considering the NVIDIA Jetson TX2 accelerator. We demonstrated that the inference time of the model on the accelerator is on the order of few milliseconds without loss of performances, thus proving the possibility to consider this approach for future real-time on-board navigation systems. • U-Net training on Moon DEM data and TL on Ceres for autonomous crater detection. • Loss-based imbalanced class training for segmentation performance optimization. • Application of U-Net for asteroid crater detection for the first time. • On-board algorithm implementation in real-time on the Nvidia GPU TX2. [ABSTRACT FROM AUTHOR]

Ejecta velocity distribution is an important property for controlling asteroid surface evolution and for changing the size frequency distribution of asteroids and planetary dusts. Recent asteroid explorations revealed that boulders on an asteroid surface had a wide size frequency distribution. On the other hand, many studies on ejecta velocity distribution for cratering experiments used fine-grained homogeneous targets. Thus, to study the ejection process of various-sized boulders on rubble-pile asteroids, we conducted impact experiments using gas guns at impact velocities of 100 m s−1 to 4 km s−1 on targets with various-sized glass beads, and analyzed boulder trajectories in three dimensions to clarify the effect of grain size on ejection velocity distribution. The results showed that the ejection velocity, v 0 , decreased as the bead size increased, and the ejecta velocity scaling law was improved to v 0 gR = k 2 ′ r 0 + a R − 1 μ ′ including the bead radius, a ; r 0 is the initial position of the bead, g is the gravitational acceleration, R is the crater radius, and k 2 ′ and μ ′ are, respectively, 0.58 ± 0.02 and 0.62 ± 0.02 for the low-impact velocity range (<200 m s−1) and 0.61 ± 0.07 and 0.57 ± 0.04 for the high-impact velocity range (>1 km s−1). Using our improved ejecta velocity scaling law, we calculated the landing points of ejected boulders and concluded that boulders with radii >0.34 R could not be ejected outside the final crater. Moreover, when the Urashima crater on asteroid 162173 Ryugu was formed on the surface, boulders up to 64 m in diameter may have been ejected beyond the escape velocity of Ryugu to become tiny monolithic asteroids. • Cratering experiments were done on simulated rubble-pile surface on asteroids. • Ejecta velocity distributions for various sized boulders were obtained. • Crater scaling law for ejecta velocity was improved considering the boulder size. • The size of largest boulder ejected outside of the crater is 0.34 crater radius. • Urashima crater on Ryugu might eject a boulder <70 m to be a tiny asteroid. [ABSTRACT FROM AUTHOR]

The paleo-positions of terrestrial meteorite impact craters along with distance and displacement registered since formation due to plate tectonics were deciphered using GPlates, an interactive GIS-based plate tectonic reconstruction and modeling software. The results of the study are intriguing as several craters have traversed across the globe, both from the Eastern to Western hemisphere and from the Southern to Northern hemisphere, and vice versa. The oldest crater studied was Foelsche, which traversed from the Southern to Northern hemisphere and from the Western to Eastern hemisphere while covering a distance of 39080 ​km in the past 981 million years and recording a relatively shorter displacement of 10470 ​km. On the other hand, Jänisjärvi and Suvasvesi South have traveled longer distances (27781 and 29050 ​km, respectively) and are among the most displaced craters (17400 and 16988 ​km, respectively). Similarly, the paleo-position, distance, and displacement for all craters, with ages <1100 ​Ma, were computed in the study. Based on the derived paleo-position, we have accessed the possibility of any selective distribution of craters across different latitudinal segments. As Earth is a planet that recorded dynamic variations in the terrestrial surface area across different geological ages, calculating the same was an arduous task. The land area within each of the three latitudinal segments, viz. 0–30°, 30–60°, and 60–90°, in which a crater formed was calculated for the geological time corresponding to an impact cratering event. This calculated land area within the respective latitudinal zone at each instance of a crater's formation was then compared with the total land area on Earth. The results showed that 0–30° and 30–60° segments have equal crater frequencies whereas the 60–90° segment has a lesser frequency. The latitudinal crater distribution on Earth was then compared with Moon and Mars. The results revealed that there is a non-selective distribution of terrestrial impact craters across different latitudinal segments, indicating a non-perceivable latitudinal dependency for impact events. [Display omitted] • 192 Earth impact craters, formed after 1100 ​Ma, were reconstructed to its paleo-position. • The distance and displacement recorded by the craters due to plate tectonics is computed. • Latitudinal distribution of craters was studied and showed no major dependency on latitudes. • The latitudinal distribution was correlated with Moon and Mars. [ABSTRACT FROM AUTHOR]

Over the last decades, a significant number of small asteroids (diameter < 10 km) having a satellite in orbit around them have been discovered. This population of binary asteroids has very specific properties (secondary-to-primary diameter ratio of about 0.3, semi-major axis to primary diameter ratio around 2 and an obliquity of the system close to either 0 ∘ or 180 ∘) pointing at formation by YORP-induced spin-up and rotational fission. When impacting the surface of terrestrial bodies, those exotic objects lead to the formation of binary craters, exhibiting various morphologies depending on the configuration of the system at the moment of the impact. Planetary surfaces constitutes therefore the best (if not the only one) record of binary asteroid population through time. In contrast to the Moon or Mercury, a large fraction of impact craters on Mars exhibits thick ejecta layers due to the presence of volatile material at the moment of the impact (e.g., water ice). The martian surface represents thus the ideal case to survey for the existence of binary craters, as the ejecta morphology can attest of a synchronous impact. From a survey of 87% of Mars surface, we identify 150 binary craters (0.5% of the total), likely formed by the impact of binary asteroids. The properties of these craters contrast with those of the population of binary asteroids: size ratio close to unity, large separation, and isotropic orientation on the surface. We run numerical simulations of impacts to test whether tidal effects on the impact trajectory can explain these discrepancies. Our results suggest that a population of similarly-sized and well-separated binary asteroids with non-zero obliquity remains to be observed. [Display omitted] • First database of Martian binary craters with 150 entries compiled from a survey over 87% of the surface. • Comparison between properties of the binary craters and the population of binary asteroid systems. • Mismatch between observed and simulated properties of binary craters on Mars. • Unobserved population of binary asteroids suspected with large separation, near-similar size, and non-zero obliquity. [ABSTRACT FROM AUTHOR]

Introduction: Metal-rich asteroids are widely believed to be the exposed cores of differentiated planetesimals, although the process that leads to their formation is still not well understood. One of the leading hypotheses that explains an asteroid's rich metal composition supposes that most of the primitive crust and mantle of the differentiated progenitor body was stripped off by hit-and-run collisions, leaving behind a bare core [1]. However, these collisions are not able to strip off the entirety of the mantle, without disrupting the metallic core. Here we use numerical simulations to explore different formation scenarios for metal-rich asteroids. We also investigate the crater sizes and morphologies that may be found on such bodies today. Our results have important implications for the NASA's 'Psyche' mission at asteroid (16) Psyche [2]. Numerical Model: We use the iSALE-2D shock physics code [3, 4] to simulate high-velocity impacts into differentiated small bodies. iSALE-2D includes strength models suitable for impacts into geologic targets [5] and has been extensively validated against laboratory impact experiments [3], as well as benchmarked against other hydrocodes [6, 7]. We assume that the structure of a differentiated body has a layered configuration in which an olivine-rich mantle covers an iron core (e.g., [8]). Here we varied the mantle layer thickness between 10 and 50 km. Formation mechanism for metal-rich asteroids: We investigated impacts in the sub-catastrophic collision regime [9, 10] as a potential formation mechanism for metal-rich small bodies. Based on our results we propose a new mechanism of efficient mantle removal on small bodies, through such impacts (after the body already experienced hit-and-run collisions). Mantle material is removed by a process of back-side spallation with minimal loss of core material. According to our numerical simulations, we find that head-on sub-catastrophic collisions can remove up to 95% of the residual mantle material from mantle-poor differentiated asteroids. The specific impact energy required to remove a given fraction of the mantle is well fit by: Qcrit = agR3μg, where ag is a constant and μg is a coupling parameter to the target, in the gravity regime. Our proposed mechanism is applicable over a large range of scales, up to ≈ 1000 km. Previous studies of mantle-removing collisions have focused on much larger scales (e.g., [11, 12]). Crater sizes and morphologies on metal-rich asteroids today: Our numerical simulations of impacts less than a few km in diameter into Psyche-analogues show that craters on metal-rich asteroids can exhibit a variety of morphologies, depending on the surface material and asteroid structure [13]. For example, if Psyche's structure is homogeneous, then craters are expected to be simple, circular depressions. For iron targets, the craters exhibit tall curled rims, characteristic of impacts into ductile materials, and large depth-to-diameter ratios compared with craters in rocky targets. On the other hand, if Psyche has a layered structure, the craters could be concentric or have flat floors. Small craters, which are more susceptible to small scale variations in the target structure, can exhibit even more exotic morphologies. The morphologies of craters on Psyche today can be used to discriminate between different formation scenarios. Conclusions Our results suggest that metal-rich small bodies had a complicated evolutionary history and experienced more than one collision regime. We propose that sub-catastrophic, mantle-spalling impacts represent the missing link between the typical products of hit-and-run collisions and the observed metal-rich asteroids. Based on the crater morphologies on Psyche and together with our numerical simulations, we will be able to provide an insight into possible formation mechanisms for metal-rich asteroids. Acknowledgements: We gratefully acknowledge the developers of iSALE. This work has received funding from the UK's Science and Technology Facilities Council (STFC) (Grant ST/S000615/1) and from the European Union's Horizon 2020 research and innovation programme, under grant agreement No. 870377 (project NEO-MAPP). [ABSTRACT FROM AUTHOR]

The danger from very small asteroids: Impacts of even very small asteroids can detectably influence our society. In 2013, a 20 m in diameter body exploded over Chelyabinsk, seriously injuring >1500 people [1]. In 1947, formation of a strewn field in Sikhote-Alin caused the USSR army to go on alert [2]. An explosion of an extraterrestrial body over Tunguska river flattened 2000 km2 of Siberian forest [3]. Humankind can only prepare for this natural hazard if the historical frequency and environmental influence is understood. Based on the extrapolation of the currently measured impact rate of small bodies at the top of the atmosphere, >20 craters ~100 m in diameter are expected in the Holocene alone [4], yet only six are known [5]. Identification of small impact craters (<200 m in diameter) is problematic, because the methodologies for identification of impact craters have been developed for large impact structures. The most unequivocal criteria are the presence of shock metamorphic indicators (like planar deformation features in quartz or shatter cones) and, to lesser extent, detection of geochemical traces of extraterrestrial matter [6]. However, in the case of small impact craters in unconsolidated target rocks, such signatures are detectable only within a small volume of target material which have subsequently been distributed as ejecta over 10's of km2, diluting the measurable shock metamorphic signal [7]. As a result, out of 14 confirmed small impact craters, planar deformation features and high-pressure phases have been reported only from two recent impact craters developed in desert areas: Kamil [8] and Wabar [9]. A small number of potential shatter cones are reported from Kaali [10] and the suspected crater Sobolev [11], where the target rock consists of a couple of meters of unconsolidated sediments overlying lithified rocks from which the shatter cones were formed. Fragments of melted rocks have been reported from only 4 out of 14 cases. In practice, the only widely used impact indicator of confirmed non-witnessed (excluding Carancas [12]) impact craters <200 m in diameter is the association with iron meteorite fragments (13 out of 13 cases). Dating very small impact craters. The formation of some features was witnessed in modern times, such as Carancas [12] or Sikhote Alin [2]. In the case of Wabar, there are some historical records pointing to the moment of impact [13]. Establishing a crater age is straightforward if there is 14C datable (<50 ka) organic material associated with the impact structure, e.g., in the form of a paleosol (e.g., Morasko [14]), charcoals of organisms killed during the impact event (e.g., [15,16,17]) or dating of the oldest materials in a crater lake (Morasko: [18]). If no material for 14C dating is available, the age can be estimated based on palynology (Morasko: [19]), cosmogenic nuclides within meteorites associated with the craters, e.g., Haviland [20] or Henbury [21]. Exposure cosmogenic nuclide dating of the ejecta is only possible if craters are more than a couple tens of thousands years old and target rocks were at least partially consolidated [22]. Luminescence dating, such as at Odessa [23] or Wabar [24], may in many cases not yield precise or accurate results due to not complete resetting during impact event, such as at Kamil [25] or Morasko [26]. Often, different methods give drastically different age estimations, e.g., Kaali [17]. And sometimes, the only method is guestimate based on the level of crater preservation: Veevers, Dalgaranga [5]. Studies, including field work in very small impact craters especially those developed in unconsolidated materials should be designed specifically for those features by understanding both the impact cratering process and Quaternary geology techniques. This allows not only to describe and recognize new impact sites [27], but also to use those well preserved craters to better understand the impact process itself. For example, by mapping the ejecta blanket [14], and interaction of paleosol with ejecta we can learn about the stages of ejecta deposition [28] and better understand complexities of the double crater formation [29]. Additionally, even though a presence of features characteristic for ejecta blankets of young very small impact craters can not be treated as a proof of an impact origin of a specific circular structure, a lack of them is suggestive that an impact origin can be excluded, and other processes are more likely [30]. [ABSTRACT FROM AUTHOR]

Introduction: Meteorites in France have always been recovered by the general public. FRIPON, a fireball observation network [1] now covers France and is developing in Europe and in Canada (www.fripon.org). It detects fireballs and allows us to calculate meteorite strewn fields within 24 hours so that searches can be launched very early on and meteorites can be recovered before they alter significantly. Because of the need to search everywhere, including in private land, it is important that the general public be aware of the scientific value of meteorites and willing to contribute to helping us find them on the ground. Vigie-Ciel (www.vigie-ciel.org) is the citizen science counterpart of the FRIPON network. It also invites the general public to participate in the detection of fireballs (www.vigie-ciel.imo.net - a fruitful collaboration with the Amercican Meteor Society) and the identification of impact craters on Earth based on shaded relief images (www.vigie-cratere.org). As participation is more effective if the people involved are informed and trained, Vigie-Ciel also contributes to scientific education on meteors, meteorites and impact craters and, more generally, in planetology. Searching for meteorites: Searching for a meteorite on the ground is not an easy task. Several test campaigns have allowed us to identify the main issues that have to be dealt with. These are related to: (1) the respect of legislation, private property and the inhabitants of the area; (2) the logistical aspects in order to have well-coordinated groups of a reasonable size in the field; (3) the implementation of an efficient communication towards the general public, but also towards the local authorities and the inhabitants of the area. If, for the time being, no meteorites have been found in France through the efforts of FRIPON/Vigie-Ciel, three meteorites have been recovered in the neigboring countries (Cavezzo in Italy [2], Winchcombe in England [3] and Kindberg in Austria [4]) with a contribution of FRIPON equipment and/or through the search methods that were designed by the FRIPON/Vigie-Ciel teams. As in earlier times, all these discoveries involved curious citizens who reached out to scientists, but at least two new factors were also implicated: (1) observations by a professional and/or an amateur camera network which led to a computation of the strewn field and (2) multiple communication channels through the press, the social networks and specially designed leaflets. The Vigie-Ciel network and tools: Information and training of potential participants is an essential part of the Vigie-Ciel program and we are still working at improving it with the help of our national, regional and local partners who also play a major role in setting up the field search campaigns when a meteorite is deemed to have fallen in a neighboring area. Such campaigns could not be set up without their active help and their knowledge of the local territory. These partners, who come from various contexts, are often involved in scientific outreach. They comprise planetariums, natural history museums, astronomy clubs, but also schools, village or town halls... and a large fraction of them also host a FRIPON camera. The Vigie-Ciel National Team offers them training, support and tools - which are still being improved - to perform these tasks. The pedagogic tools we developed include exhibits, a field search training protocol and a set of suitcases containing meteorites, meteowrongs and various tools to study the properties of rocks, which has been reproduced 22 times and distributed in France in the former (smaller) administrative region. These pedagogic suitcase sets are designed to be used by our partners to train interested amateurs and/or to be lent to other structures who wish to become local partners of the project (after they have received a training). So far, the National Team has trained about three times as many instructors as the number of suitcase sets and these, in turn, have the ability to coach new trainers and animators and to organize public outreach events. Conclusion: Vigie-Ciel is a citizen science project designed in France to invite the general public to contribute to studying the flux, origin and nature of extraterrestrial materials that reach the surface of the Earth. Its original focus was on recovering meteorites whose fall was detected by the FRIPON camera network. However, citizens can now contribute in various other ways, which include searching for previously undetected terrestrial impact craters on the Vigie-Cratère website, reporting fireballs and hosting a FRIPON camera. Acknowledgements: The FRIPON project was initiated thanks to ANR (Agence Nationale de Recherche) funding, while the Vigie-Ciel project benefited from funding from the "Investissements d'Avenir" allocated by the Commissariat Général à l'Investissement. [ABSTRACT FROM AUTHOR]

Previous age estimates of the Imbrium impact range from 3770 to 3920 Ma, with the latter being the most commonly accepted age of this basin-forming event. The occurrence of Ca-phosphates in Apollo 14 breccias, interpreted to represent ejecta formed by this impact, provides a new opportunity to date the Imbrium event as well as refining the impact history of the Moon. We present new precise U-Pb analyses of Ca-phosphates from impact breccia sample 14311 that are concordant and give a reliable weighted average age of 3938 ± 4 Ma (2σ). Comparison with previously published U-Pb data on phosphate from Apollo 14 samples indicate that all ages are statistically similar and suggest phosphates could have been formed by the same impact at 3934 Ma ± 3 Ma (2σ). However, this age is older than the 3770 to 3920 Ma range determined for other samples and also interpreted as formed during the Imbrium impact. This suggests that several impacts occurred during a 20-30 Ma period around 3900 Ma and formed breccias sampled by the Apollo missions. [ABSTRACT FROM AUTHOR]

Understanding the geological modification processes on asteroids is fundamental for elucidating their surface evolution. Images of small asteroids from spacecrafts show a depletion in smaller craters. Seismic shaking was considered to be responsible for erasing such small craters and the main driver modifying the geology of asteroids. However, a recent artificial impact experiment on asteroid Ryugu conducted by the Japanese Hayabusa2 spacecraft revealed minimal seismic efficiency on the asteroid. To investigate whether a standard seismic shaking model can reproduce the observed crater record, we analyzed the crater distributions on four asteroids, i.e., Eros, Itokawa, Ryugu, and Bennu, using crater production functions under cohesionless conditions. Crater retention ages were estimated as a function of crater diameter for the four asteroids using the crater size-frequency distribution and crater production function estimated for each asteroid. We obtained the relation between a crater retention age t and crater diameter D in the form of power-law function (t ∝ D a ).We found that the power-law indices a are inconsistent with diffusion processes (e.g., seismic shaking, where a = 2). This result suggests that seismic shaking models based on a linear diffusion equation cannot explain the crater distribution on the small asteroids. Alternative crater obliteration processes include surface flows suggested by geomorphological and spectral features of Ryugu. Using the crater statistics, we also show that the vertical mixing of the Ryugu material at the depths shallower than 1 m occurs in 103–105 yr by cratering. This rapid resurfacing and replacement of the surface layer in the short timescale would account for the decrease in space weathering rate suggested by previous studies. Furthermore, the timescale required for vertically transporting Ryugu materials to the depths of 2–4 m (104–106 yr), where cosmic rays would not reach, can be compared with the cosmic-ray exposure ages of returned samples. This comparison can be used to constrain the distribution of impactors that collide with Ryugu. • Crater obliteration process on Ryugu cannot be explained by standard seismic shaking models • Vertical mixing of materials shallower than 1 m occurs in 103–105 yr on Ryugu • CRE age of returned samples would reflect the mixing timescale to 2–4 m (104–106 yr) [ABSTRACT FROM AUTHOR]

We use Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC) images to analyze the boulder distribution around Vaughan crater, a small, fresh crater on the far side of the Moon. Using six similar lunar impact craters of known age for comparison, we determine the approximate age of the crater. Vaughan crater lies in a part of the South Pole-Aitken basin that is compositionally similar to lunar meteorite Dhofar 961, on the basis of Lunar Prospector Gamma Ray Spectrometer data, making Vaughan crater a potential candidate for the source of this meteorite and possibly others paired with it. Boulder distributions reveal that as impact craters age, the boulder population density in their ejecta decreases. This understanding allows us to use size-frequency distributions to compare crater ages on the basis of their boulder population density. We find that Vaughan crater is very young (<25 Ma) owing to its placement within the boulder size-frequency comparison plot and its population of large (>10 m diameter) boulders. Additionally, we compare LROC NAC distributions to thermophysically derived values for rock abundance from the Diviner Lunar Radiometer. We find that our LROC NAC count is consistent with Diviner rock abundance values, which further supports the inference that Vaughan is a very young impact crater and could plausibly be a source of lunar meteorite Dhofar 961. • Vaughan is a very young, rocky, lunar crater located in the northern South Pole-Aitken basin. • Boulder size-frequency distributions indicate an age < 25 Myr. • Imagery-based rock abundance is consistent with thermophysically derived data. • Compositional similarities suggest Vaughan as a potential lunar meteorite source crater in South Pole-Aitken basin. [ABSTRACT FROM AUTHOR]

Lunar meteorites provide information about areas of the Moon not sampled by the Apollo and Luna missions, so they can be used as more-representative geochemical datasets for evaluating global lunar evolution. However, the launching craters remain unknown for many lunar meteorites. This paper, therefore, aims to identify the launching sites of lunar meteorites with remote-sensing datasets. This work focuses on one lunar-meteorite group, the Northwest Africa 773 (NWA 773) clan, which has the very-low-Ti basaltic composition and one of the youngest crystallization ages (~3 Ga) among the lunar basaltic meteorites. We investigated the source of the NWA 773 clan by comparing their geochemical compositions and geochronological information with the following remote-sensing datasets: 1) the Lunar Prospector gamma-ray elemental maps (FeO, TiO 2 , Th) and the SELENE gamma-ray elemental maps (CaO), 2) the surface ages determined from imaging data by using the crater-counting method, and 3) high-spatial-resolution maps of the FeO and TiO 2 and spectral-ratio images made from the SELENE multi-band imager data. From these comparisons, the NWA 773 clan is most likely to have originated from the north part of Mare Imbrium inside the Procellarum KREEP Terrane (PKT). Furthermore, the eruption age of surface basalt surrounding the Le Verrier D crater in the north part of Mare Imbrium between 3.0 and 3.3 Ga, and the formation of the Le Verrier D crater, are consistent with the duration of igneous activity and timing of meteoroid impact of the NWA 773 clan. There is a fresh crater with bright ejecta on the edge of the Le Verrier D crater, and it is possibly the young launch crater of this meteorite group. • The source region of lunar meteorite is investigated with remote sensing datasets. • The basaltic meteorite group, NWA 773 clan originated from Mare Imbrium. • NWA 773 clan likely launched from young crater within the Le Verrier D crater. [ABSTRACT FROM AUTHOR]

The China National Space Administration (CNSA) today released a long list of researchers who would receive a second batch of Chang'e-5 samples - another 1% of the entire collection. Although researchers outside China are keen to analyze samples in their own labs, the samples so far have only been distributed to labs in China. [Extracted from the article]

High-resolution mapping of Ceres, Vesta and the icy satellites of Saturn, Uranus and Pluto reveals a rich variety of well-preserved impact crater morphologies on these low gravity bodies. These objects provide a natural laboratory to study effects of composition on crater formation processes under similar surface gravity conditions (though mean impact velocities vary by several factors). Simple craters occur on all these bodies but subtle differences in morphology on Ceres and Vesta are recognized. Immature complex craters (with large floor mounds but not terraces or conical central peaks) occur on Vesta and while smaller than predicted are consistent with its silicate composition. Asymmetric simple craters (with incomplete scarp development) on all bodies are likely related to differential overburden stresses in the rim, and their occurrence is consistent with lower crustal strength on icy bodies including Ceres. Immature and mature complex craters exhibit increasing degrees of complexity, including spiral floor deformation patterns (related to failure in converging floor material), central peaks, and impact melt. Cerean crater morphologic types and simple-complex transition diameters are smaller than on Vesta but similar to those on icy satellites, indicating a much weaker rheology for Ceres' outer layers under impact conditions. These are consistent with geophysical indications of a low-density water ice and probably clathrate rich outer shell. Fluidized floor deposits (impact melt or melt-solid mixtures) are significant in craters >25 km across on Ceres but absent on Saturn satellites. Central pit craters are common on Ceres (at diameters of ~75 to 150 km consistent with gravity scaling from the larger Galilean satellites) but are absent on Saturnian satellites and Charon. The contrasting impact melt and central pit behaviors on Ceres and Saturn's moons is contrary to expectation given the higher impact velocities at Saturn but might be related to lower internal temperatures, or the higher fraction of non-ice material on Ceres. The correlation or scaling of transition diameters to surface gravity is near −0.65 rather than −1, perhaps due to increased porosity on lower gravity bodies. The fundamental similarity of crater morphologies on Ceres and icy satellites, however, indicates that the weaker rheology of water ice results in similar craters even if the non-(ice+clathrate) components are as high as ~30 vol%. • Impact craters on mid-sized asteroids, dwarf planets and icy moons form under similar gravity but in icy to silicate crusts • Complex craters occur at very different diameters on Vesta and Ceres due to their very distinct compositions • Crater morphologies on Ceres are very similar to those on icy moons, indicating ice is the dominant crustal component • Impact melt deposits and central peaks on Ceres (but not mid-sized icy moons) may be related to temperature differences • Asymmetric simple craters are related to local overburden pressure on the rim scarp and crustal strength differences [ABSTRACT FROM AUTHOR]

Using data acquired by the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) mission, we investigate spectral properties of craters on the near-Earth asteroid (101955) Bennu. We compare Bennu's craters with its global average by means of four spectral parameters: (a) minimum position of the band at 2.7 μ m, (b) depth of the hydrated phyllosilicate absorption band at 2.7 μ m, (c) normalized spectral slope from 0.55 to 2.0 μ m, and (d) reflectance factor at 0.55 μ m. We examine 45 craters using spectral data obtained under various observing conditions. For 20 craters, we find a shortward shift of the 2.7- μ m band minimum relative to the global 2.7- μ m band minimum, which we attribute to the presence of relatively fresh (less space-weathered) material excavated from the sub-surface by crater-forming impacts. For three craters, we find an anti-correlation between spectral slopes and reflectance factor for a series of spectra acquired during a specific scan, where we observe that spectra become redder and darker towards the center of the crater. We attribute this to the presence of fine-particulate regolith. Localized spectral heterogeneities are apparent inside a prominent equatorial crater on Bennu, which is one of the asteroid's oldest geological features. We propose that such local spectral heterogeneities could be used as a tracer of mass movement on Bennu. We show that younger craters are redder, brighter, and have deeper 2.7- μ m bands. Comparing global average spectral values of Bennu and crater frequency distributions as a function of the chosen spectral parameters, we find that craters evolve to assume the global average spectral properties of Bennu. A positive correlation identified between the reflectance factor and 2.7- μ m band depth suggests that brighter craters tend to be more hydrated. Finally, we put into context, the results from the Small Carry-on Impactor experiment by the Hayabusa2 spacecraft, which created an artificial crater on the near-Earth asteroid (162173) Ryugu. • The shortward shift of the 2.7-μm band inside craters indicates the presence of freshly excavated sub-surface material. • A scan-dependentant anti-correlation between REFF0.55 μm and spectral slopes hints at the presence of fine particulates. • Younger craters are brighter and have deeper 2.7-μm hydration bands and redder spectral slopes. • Nightingale crater, the primary sample site of the OSIRIS-REx mission, is the reddest and the darkest crater [ABSTRACT FROM AUTHOR]

The near-Earth asteroid 162173 Ryugu, the target of the Hayabusa2 mission, is noted to be a spinning top-shaped rubble-pile. Craters are among the most prominent surface features on Ryugu. Their shapes, particularly their depth-to-diameter ratio (d/D), can provide an important proxy for probing both the internal structure and surface processes of planetary bodies. Here, we report d/D of every impact crater on Ryugu using a shape model derived from stereo-photoclinometry. We found that the average, standard deviation, and observed range of d/D for the entire set of craters are 0.09, 0.02, and 0.03–0.15, respectively. Except for possible pit craters, the maximum d/D of large craters on Ryugu (D > 50 m) is close to 0.13, which is comparable with those of fresh simple craters on rocky asteroids, such as Gaspra and Ida. Conversely, the d/D of small craters (D < 50 m) increases with the crater diameter. This behavior implies that a smaller crater on Ryugu is formed as a shallower crater. As on Itokawa, the surface environment on Ryugu likely inhibits craters becoming deep. This especially affects smaller craters, as their normal small depth decreases in the Ryugu environment and they become still more shallow. As a result, small craters rapidly degrade beyond the point where they can be identified as candidate craters. This is likely responsible for the apparent lack of small craters. The d/D has no reliable relationship with the types of crater classification in Hirata et al. (2020). Examination of latitudinal and longitudinal variation in d/D of craters on Ryugu revealed no statistically significant trends. • We measured the depth-to-diameter ratio (d/D) of 77 possible impact craters on Ryugu. • The max d/D of large craters is comparable to those of fresh craters on asteroids. • We found that the d/D of small craters (D < 50 m) increases with crater diameter. • The d/D has no reliable relationship with the crater type in Hirata et al., 2020. • There are no statistically significant trends in the regional variation of the d/D. [ABSTRACT FROM AUTHOR]