Impact Melt Research Articles

A Cryogenian impact structure lurking in the shadows of northern Sweden

AbstractHere we report on findings for four rock samples with melt texture found in a gravel pit within a glaciofluvial deposit near the small town of Kitkiöjärvi in northernmost Sweden. The samples are comprised of granitic clasts embedded in a brown fine‐grained melt matrix. The samples all contain quartz grains and/or clasts exhibiting multiple sets of planar deformation features oriented parallel to crystallographic planes characteristic of shock metamorphism. The samples also contain Former Reidite In Granular Neoblastic (FRIGN) zircon. We therefore conclude that the investigated samples represent impact melt rock. We interpret a U‐Pb concordia age of 658.9 ± 6.9 Ma (Cryogenian) derived using secondary‐ion mass spectrometry analysis of shocked zircon, as the best estimate for the age of the impact event that formed the melt rocks. Zircon grains from two of the samples yield younger lower intercept ages, raising the possibility that the samples came from multiple impact events of different ages. Although we cannot exclude this possibility, we interpret the younger ages from the clast‐rich melt rocks to reflect non‐impact‐related Pb loss events and suggest that all samples likely came from the same structure. Analysis of the glaciofluvial history of the region, along with the relatively high frequency of finds (five in total, as one similar melt rock was found in the pit in 2018), points to a short‐distance glacial transportation of the samples from the southwest. Since there are no known impact structures in Sweden within that area and/or of similar age, we conclude that an old (the oldest known yet) impact structure in Sweden potentially is yet to be discovered somewhere in the vicinity of the gravel pit.

Shock petrographic and numerical modeling constraints on the morphology and size of the Morokweng impact structure, South Africa

AbstractThe 369 m deep M4 drill hole, located ~18 km NNW of the center of the 146 Ma Morokweng impact structure (MIS), intersects shocked Archean granitoid gneisses and subsidiary dolerite intrusions that are cut by faults, cataclasites and mm‐ to m‐wide suevitic and pseudotachylitic breccia dikes. The shock features in quartz in the gneisses and breccia dikes include decorated planar deformation features (PDFs), planar fractures, feather features, and toasting. Other minerals show features that may be shock‐related, such as multiple sets of planar features and alternate twin ladder structures in feldspars, kink bands in biotite, and planar features in titanite, apatite, and zircon; however, these are variably annealed and/or overprinted by hydrothermal alteration effects, and confirmation of their origin awaits further study. Universal Stage measurements of PDF sets in quartz from 12 gneissic target rocks and from lithic and mineral clasts in three suevitic and three pseudotachylitic breccia dikes reveal four dominant sets: (0001), {}, {} and {}. Based on these observations, the average peak shock pressure in these rocks is estimated at ≤16 GPa, which supports the original proximity (within one transient cavity radius) of these rocks to the point of impact. No discernible depth‐dependent shock attenuation was noted in the core. These shock levels and the elevated structural position of the rocks in the M4 core relative to the impact melt sheet intersected in drill holes closer to the center of the MIS suggest that the M4 lithologies represent part of the parautochthonous peak ring volume that subsequently experienced 1.5–2 km of post‐impact erosion before it was buried beneath younger sediments. Numerical modeling using the iSALE‐2D code suggests that the original Morokweng crater had a rim‐to‐rim diameter of between 70 and 80 km, and that the rocks in the M4 core were originally located at a depth of 7–8 km and a radial distance of 8–9 km from the point of impact.

Long-term isostatic relaxation of large terrestrial impact structures: structural characteristics inferred from scaled analogue experiments

Crater floor fractures are prominent post-cratering structural vestiges that are known from large impact craters on rocky celestial bodies. Two mechanisms have been proposed to explain the formation of crater floor fractures: emplacement of horizontal igneous sheets below crater floors and isostatic re-equilibration of crust underlying target rocks, i.e., crustal relaxation. Here, we use two-layer analogue experiments to model the deformation of lower and upper crust following crater formation, scaled to the physical conditions on Earth, to explore the structural and kinematic consequences of crustal relaxation. Specifically, the structural evolution of model upper crust was systematically analysed for various initial depths and diameters of crater floors, gleaned from previous numerical models for average continental crust. The analogue modelling results provide quantitative estimates of the duration, geometry and distribution of deformation zones in the upper crust and, for the first time, a quantitative relationship between the diameter, depth and fracture geometry of crater floors. The experiments also show that crater floor uplift is accomplished by long-wavelength subsidence of the crater periphery, which may operate on time scales of hundreds of thousands of years in nature. We conclude that patterns of natural crater floor fractures, including impact melt rock dikes known from the Sudbury and Vredefort impact structures, can be caused by long-term uplift of the crater floor, compensated by lateral crustal flow toward the crater centre.

Nature of the lunar far-side samples returned by the Chang'E-6 mission.

The Chang'E-6 (CE-6) mission successfully achieved return of the first samples from the far side of the Moon. The sampling site of CE-6 is located in the South Pole-Aitken (SPA) basin-the largest, deepest and oldest impact basin on the Moon. The 1935.3g of CE-6 lunar samples exhibit distinct characteristics compared with previous lunar samples. This study analyses the physical, mineralogical, petrographic and geochemical properties of CE-6 lunar scooped samples. The CE-6 soil has a significantly lower bulk density (0.983g/cm3) and true density (3.035g/cm3) than the Chang'E-5 (CE-5) samples. The grain size of the CE-6 soil exhibits a bimodal distribution, indicating a mixture of different compositions. Mineralogically, the CE-6 soil consists of 32.6% plagioclase (anorthite and bytownite), 19.7% augite, 10% pigeonite and 3.6% orthopyroxene, and with low content of olivine (0.5%) but high content of amorphous glass (29.4%). Geochemically, the bulk composition of CE-6 soil is rich in Al2O3 (14%) and CaO (12%) but low in FeO (17%), and trace elements of CE-6 soil such as K (∼630ppm), U (0.26ppm), Th (0.92ppm) and rare-earth elements are significantly lower than those of the lunar soils within the Procellarum KREEP Terrane. The local basalts are characterized by low-Ti (TiO2, 5.08%), low-Al (Al2O3 9.85%) and low-K (∼830ppm), features suggesting that the CE-6 soil is a mixture of local basalts and non-basaltic ejecta. The returned CE-6 sample contains diverse lithic fragments, including local mare basalt, breccia, agglutinate, glasses and leucocrate. These local mare basalts document the volcanic history of the lunar far side, while the non-basaltic fragments may offer critical insights into the lunar highland crust, SPA impact melts and potentially the deep lunar mantle, making these samples highly significant for scientific research.

Impact Craters on Earth with a Diameter of More than 200 km: Numerical Modeling

The three largest impact craters, the remains of which have been found on Earth to date, had diameters of about 200 km immediately after formation. The search for traces of larger impact structures continues. This paper presents the results of numerical modeling of the formation of terrestrial impact craters larger than those already found. It is shown that the inferred geothermal gradient significantly influences the initial geometry of the impact melt region, which may facilitate the search for the remains of deeply eroded ancient impact structures.

A failed search for concordancy across multiple isotopic systems in lunar impactites: Implications for testing the Late Heavy Bombardment hypothesis

Investigations of Apollo-returned samples radically altered our understanding of lunar history which has important implications for terrestrial habitability and Solar System evolution. Radiometric dating of those samples inspired the hypothesis that Moon experienced a Late Heavy Bombardment (LHB) at ∼3.9 Ga. The LHB concept has come under several recent challenges, including the concern that 40Ar/39Ar step-heating dates of Apollo impactites had been misinterpreted. Ultraviolet laser ablation (UVLAMP) 40Ar/39Ar dates – with their capacity for much higher spatial resolution and thus potential to avoid dating near-ubiquitous clasts in impact melt rocks – should in principle provide more interpretable results. Here we compare new ion microprobe 207Pb/206Pb accessory mineral dates for two Apollo 17 impactites for which UVLAMP 40Ar/39Ar dates had been previously obtained. Our results are consistent with a single accessory phase growth event for each sample, though the two samples yielded statistically different mean ages of ca. 3.974±0.013 and 3.928±0.003 Ga. Both can reasonably be interpreted as dating an impact event, but the 207Pb/206Pb dates are older than the associated 40Ar/39Ar dates by several hundred million years. We interpret that the age differences result from subsequent thermal disturbances. The discordancy between impact ages inferred from lunar impactites using two different radiometric systems suggests caution in acceptance of the LHB hypothesis without the benefit of both larger lunar datasets and more multichronometric studies. Even with such information, our capacity to know the lunar bombardment history is likely limited by compositional and thermal effects which appear to restrict growth of impact-produced accessory minerals to a small fraction of the lunar surface. Using currently available datasets, the LHB hypothesis may be effectively untestable.

Impact Momentum Transfer—Insights from Numerical Simulation of Impacts on Large Boulders of Asteroids

Asteroids pose potential hazards to Earth. The recent NASA Double Asteroid Redirection Test mission successfully demonstrated the change of an asteroid’s orbit by a kinetic impactor. This study focuses on impact-induced vertical momentum transfer efficiency (β − 1) considering various impact angles and subsurface boulder arrangements. Utilizing the iSALE-3D shock physics code, we simulate oblique impacts on different subsurface boulder configurations. Our results show that vertical ejecta momentum decreases with obliquity, with buried boulders inducing an anti-armoring effect. We define the direct impact-contacted boulder as the primary boulder and the surrounding boulders as secondary. The anti-armoring effect is most pronounced when the primary boulder is just below the surface, amplifying β – 1 by 50%. Impact angles between 60° and 75° exhibit a critical drop in ejecta momentum. An in-depth exploration of subsurface boulder arrangements reveals that secondary boulders have a minimal effect on vertical momentum transfer efficiency. Varying the size and separation of secondary boulders suggests that these subsurface features can either enhance or diminish the overall β − 1, providing insights into the dynamics of rubble-pile asteroids. In addition, impact melting is explored in our simulations, which suggests a minimal melt retention on Dimorphos’s surface. Volumes of retained melt differ by an order of magnitude for impacts on the homogeneous regolith and on targets with buried boulders. In summary, this study provides insights into the effect of subsurface boulders and impact angles on vertical momentum transfer efficiency, which is crucial for understanding asteroid deflection by a kinetic impactor.

Elemental analyses of feldspathic to basaltic soils and rocks on the moon using laser-induced breakdown spectroscopy

In-situ analysis of major elements using laser-induced breakdown spectroscopy (LIBS) is essential for future lunar landing missions, yet its performance under lunar conditions remains not fully understood. This uncertainty arises from the absence of an atmosphere and the diverse range of surface materials, which vary in chemical composition from anorthosites to basalts, and in physical properties from fine regolith to boulders. To address these challenges, we developed and cross-validated a multivariate LIBS calibration model by measuring 169 compressed fine powders of geologic samples under vacuum. These samples fully encompass the bulk composition range of lunar meteorites. We investigated the applicability of the model to a wider range of samples by measuring lunar meteorites, terrestrial anorthites, and lunar simulants in various physical forms, including rock chips and soils with different grain sizes and bulk densities. For powder samples, the quantification accuracy, assessed using root mean squared error (RMSE), resulted in 2.5 wt% SiO2, 0.25 wt% TiO2, 1.2 wt% Al2O3, 1.3 wt% MgO, 1.2 wt% CaO, 0.33 wt% Na2O, 0.47 wt% K2O (0.060 wt% K2O in the <1 wt% range), and 1.5 wt% T-Fe2O3. For rock chip samples, the RMSEs were 3.1 wt% SiO2, 0.32 wt% TiO2, 2.2 wt% Al2O3, 2.5 wt% MgO, 2.0 wt% CaO, 0.33 wt% Na2O, 0.089 wt% K2O, and 2.1 wt% T-Fe2O3. Despite significant differences in physical conditions between powders and rocks, their RMSEs remained consistent within a factor of two. Changes in grain size or bulk density of soils had relatively minor effects on the RMSE. These RMSEs confirm that the quantification accuracy of LIBS is sufficient to distinguish the subgroups within the lunar anorthosite suite (e.g., anorthosites vs. norites) and basalts (e.g., high-Ti vs. low-Ti) across a range of soil types, from coarse to fine and from loose to compact, as well as rocks. Furthermore, our analysis shows that LIBS can differentiate between “purest” and “pure” anorthosites (98 and 95 vol% plagioclase, respectively) based on the 3σ detection limits of Mg and Fe lines. These capabilities of LIBS align well with the goals of future lunar exploration, such as locating ilmenite-rich soils for resource extraction, detecting purest anorthosites to understand early lunar evolution, and identifying noritic impact melts to refine lunar chronology. Overall, our results demonstrate that LIBS serves as a versatile tool for rapid geochemical characterization on the Moon.

Shock melt in the Cold Bokkeveld CM2 carbonaceous chondrite and the response of C‐complex asteroids to hypervelocity impacts

AbstractMany of the CM carbonaceous chondrites are regolith breccias and so should have abundant evidence for collisional processing. The constituent clasts of these fragmental rocks frequently display compactional petrofabrics; yet, olivine microstructures show that most CMs are unshocked. To better understand the reasons for this contradiction, we have sought other evidence for hypervelocity impact processing of CM chondrites using the Cold Bokkeveld meteorite. We find that this regolith breccia contains rare particles of vesicular shock melt that are close in chemical composition to bulk CM chondrite. Transmission electron microscopy of a melt bead shows that it is composed of silicate glass with inclusions of pentlandite, pyrrhotite, and wüstite. Characterization of shards of another bead by atom probe tomography reveals nanoscale clusters of sulfur that represent sulfide inclusions arrested at an early stage of growth. These glass particles are mineralogically comparable to micrometeoroid impact melt described from the Cb‐type asteroid Ryugu and melt that has been experimentally produced by pulsed laser irradiation of CM targets. The glass could have formed by in situ shock‐melting, but petrographic evidence is more consistent with an origin as ballistic ejecta from a distal impact. The scarcity of melt in this meteorite, and CM chondrites more broadly, is consistent with the explosive fragmentation of hydrous asteroids following energetic collisions. Cold Bokkeveld's parent body is likely to be a second‐generation asteroid that was constructed from the debris of one or more earlier bodies, and only a small proportion of the reaccreted material had been highly shocked and melted.

Geochemical evidence for a chondritic impactor in altered impact glass from the Stac Fada Member impactite, NW Scotland

The Stac Fada Member (Stoer Group) is a ∼1.2 Ga melt-rich impact breccia whose source crater and, therefore, proximity remains debated. We present a detailed in situ geochemical dataset for altered impact melt clasts within Stac Fada samples from Bay of Stoer and Second Coast. These altered impact melt clasts are now predominantly composed of clinochlore. Geothermometry of this clinochlore indicates formation temperatures within the ejecta blanket at 188–231°C, in agreement with previous estimates. A positive correlation between platinum group elements Ir, Pt and Rh – apparently independent of Ni – indicates two geochemical sources for platinum group elements enrichment. We propose that these represent an Mg–Ni-rich chondritic impactor and a mafic/ultramafic-layered body within the target lithology. Spatial variation in Ni content suggests that geochemical data, in combination with field observations, may be instrumental in assessing the likely location of the impact crater.

Crustal origin for olivine in the lunar Shioli crater ejecta boulders: Insights from the geological setting of Theophilus crater and Nectaris basin

On the Moon, impact craters and basins expose a wide range of crustal and mantle rocks that provide excellent opportunity for sampling them, understanding their origins and reconstructing spatial and temporal evolution of lunar interior. The previous studies detected olivine-bearing mantle rocks in and around large impact craters and basins. The Japanese SLIM mission landed on the ejecta of a ∼ 280-m-diameter Shioli crater that was emplaced on the ejecta blanket of ∼ 103-km-diameter Theophilus crater, for characterizing potential mantle-derived olivine in the Shioli crater ejecta boulders. To test this hypothesis, we studied the geological setting of Shioli crater, host Theophilus crater and Nectaris multi-ring basin using the orbiter data from Chandrayaan-1 and 2, Lunar Reconnaissance Orbiter, and Kaguya missions and the earth-based Arecibo radar observation. The asymmetrically distributed secondary craters and impact melt ponds around Theophilus crater suggests that a northeast-directed oblique impact produced this crater. Composition of Theophilus crater and surrounding region indicates that the crater excavated a heterogeneous target composed of a thin layer of high-Al olivine basalt (Mare Nectaris) underlain by anorthositic highland rocks possibly intruded by Mg-suite plutons; layers of Cyrillus crater ejecta blanket and Nectaris basin materials (both ejecta and impact melt sheets) were also present beneath the mare basalt flows. Hence, the Theophilus ejecta blanket is a mixture of all these materials. Our dating of Theophilus crater suggests that it is a ∼ 2 Ga Eratosthenian crater. Shioli is a fresh simple crater that was formed at ∼ 1 Ma on the uprange ejecta blanket of Theophilus, where the Arecibo radar data indicated the presence of abundant buried Theophilus ejecta boulders. An ESE-directed hypervelocity oblique impact event produced the elongated Shioli crater and its asymmetrically distributed bright ejecta. Shioli is a primary impact crater indicating the role of impact spallation processes associated with this hyper-velocity impact in producing thousands of ejecta (or spall) boulders around Shioli crater, displaying their asymmetric dispersal pattern and spatial variation of boulder sizes and shapes. The larger and elongated boulders are concentrated near the crater rim, while their size and axial ratio gradually decreases outward from the crater rim. The SLIM mission landed on a thin downrange ejecta of Shioli crater, where fewer large-size boulders are present. Our compositional study suggests that the Shioli ejecta boulders are composed of olivine basalt (Mare Nectaris) mixed with highland anorthositic fragments, including the reworked Cyrillus ejecta and Nectaris basin materials. The Shioli ejecta boulders were produced by complex impact fragmentation of already existing, buried Theophilus ejecta boulders. The regional crustal structure of Nectaris basin and its petrological composition suggest that both Nectaris basin and Theophilus crater did not excavate the lunar mantle. Therefore, the Shioli ejecta boulders are of crustal origin, including the olivine minerals present in them. Our results have important implications for the origin of olivine in the Shioli crater boulders being investigated by the SLIM mission.

Impacts on Ocean Worlds Are Sufficiently Frequent and Energetic to Be of Astrobiological Importance

Evidence for the beneficial role of impacts in the creation of urable or habitable environments on Earth prompts the question of whether meteorite impacts could play a similar role at other potentially urable/habitable worlds like Enceladus, Europa, and Titan. In this work, we demonstrate that to first order, impact conditions on these worlds are likely to have been consistent with the survival of organic compounds and/or sufficient for promoting synthesis in impact melt. We also calculate melt production and freezing times for crater sizes found at Enceladus, Europa, and Titan and find that even the smallest craters at these worlds offer the potential to study the evolution of chemical pathways within impact melt. These first-order calculations point to a critical need to investigate these processes at higher fidelity with lab experiments, sophisticated thermodynamic and chemical modeling, and, eventually, in situ investigations by missions.

Crater Morphometry on Callisto

Impact crater formation is affected by the physical properties of planetary crusts, allowing comparison of crater dimensions to serve as a proxy for comparing the crustal properties of different planetary bodies. New topographic profiles of Callisto craters, derived from Galileo-based digital terrain models, are presented, and the crater dimensions recorded. These data were used to derive crater morphometry scaling trends, which were then compared to the established trends of Ganymede and the Moon. Our comparative study suggests that the upper brittle portion of Callisto’s ice crust allows for the retention of steep-sided and elevated rim scarps, while subsurface warmer ice leads to an enhanced uplift and shallowing of the crater bowl. Crater dimensions are similar between Callisto and Ganymede, suggesting that the bulk properties of their near-surface crusts are comparable. The most notable difference between craters on these two Galilean moons were the smaller central pit diameters on Callisto. This difference can be explained if the pit formation on these bodies is controlled by the presence and movement (drainage and/or volatile loss) of impact melt water: the lower impact velocity and/or lower expected crustal heat flow on Callisto will result in less impact melt generation, and thus smaller central pits.

Zircon U‐Pb provenance analysis of impact melt and target rocks from the Rochechouart impact structure, France

AbstractThe Rochechouart impact structure in the northwestern part of the French Massif Central (FMC) has a great diversity of impactites, including monomict impact breccias, suevite, and impact melt rocks (IMRs). The structure is strongly eroded, which allows the study of impactites of the crater fill and the transition into the crater floor. The FMC has had a multistage geological evolution from the late Neoproterozoic to the Ordovician (600–450 Ma) until the later stages of the Variscan orogeny (~300 Ma). Previous geochronological work on Rochechouart has been focused mainly on the impactites and constraining the impact age, and scarce work has been done on the FMC‐related target rocks. Here, U‐Pb isotope analysis by LA‐MC‐ICP‐MS has been conducted on zircon from two IMRs from the Recoudert and Montoume localities, and from a monzodiorite, a paragneiss, and two amphibolite samples of the basement to the impact structure. Zircon from the target rocks yielded mainly Neoproterozoic to Carboniferous ages (~924 to ~301 Ma) that can mostly be correlated to different stages of the geological evolution of the FMC. The monzodiorite also yielded a Permian age of 272 ± 12 Ma. Zircon from the IMRs, and especially from the Montoume sample, gave a comparatively higher diversity of Neoproterozoic to Jurassic ages (~552 to ~195 Ma). Provenance analysis for the zircon age populations of the impactites compared to those of the basement rocks shows overall poor correlation between the two age groups. This suggests that other target lithologies were involved in the formation of these impact melts as well. Post‐Variscan and preimpact ages (281–226 Ma) obtained for both melt rocks probably reflect a previously unconstrained event in the evolution of the regional geological history. Ages similar to the currently most widely accepted impact age of ~204–206 Ma were obtained from both IMR samples. In addition, the Montoume melt rock yielded several post‐204 Ma ages, which might reflect a to date unconstrained, about 194 Ma postimpact thermal/hydrothermal event.

High-precision U-Pb zircon dating identifies a major magmatic event on the Moon at 4.338 Ga.

The Moon has had a complex history, with evidence of its primary crust formation obscured by later impacts. Existing U-Pb dates of >500 zircons from several locations on the lunar nearside reveal a pronounced age peak at 4.33 billion years (Ga), suggesting a major, potentially global magmatic event. However, the precision of existing geochronology is insufficient to determine whether this peak represents a brief event or a more protracted period of magmatism occurring over tens of millions of years. To improve the temporal resolution, we have analyzed Apollo 14, 15, and 17 zircons that were previously dated by ion microprobe at ~4.33 Ga using isotope dilution thermal ionization mass spectrometry. Concordant dates with sub-million-year uncertainty span ~4 million years from 4.338 to 4.334 Ga. Combined with Hf isotopic ratios and trace element concentrations, the data suggest zircon formation in a large impact melt sheet, possibly linked to the South Pole-Aitken basin.

Araguainha impact structure, Brazil: New insights into the geology of the central uplift

AbstractExtensive, new outcrops along the MT‐100 state road in the northern part of the central uplift of the 40‐km diameter, 252–259 Ma old Araguainha impact structure, Central Brazil, have become available for investigation. They offer new insight into the contact relationships between the different lithologies and the genesis of different types of impact‐related rocks, as well as the current level of erosion of the structure. Three types of impact melt rock (IMR) with different field relationships and compositions can now be distinguished: (1) Type‐I of granitic composition and occurring mainly as veins and dikes, besides a few larger pods, in the central alkali granite core of the central uplift; (2) Type‐II in the form of plastically deformed clasts of mainly highly silicious compositions in polymict impact breccia; and (3) Type‐III, derived from partially melted conglomerate or sandstone precursors, and that occurs at selected sites in (meta)sedimentary strata of the basement in the immediate environs of the alkali granite core. Both polymict lithic and melt‐bearing (suevitic) impact breccias are recognized in the 110‐m thick integrated section through impact breccia directly overlying the crater floor. This crater floor is composed of (meta)‐sedimentary basement strata with granite injections and, locally, sandstones of the Devonian sedimentary Furnas Formation of the Paraná Basin. Main breccia components are (meta)‐pelites and (meta)sandstones of the basement that is currently favored to be related to the regional Paraguay Belt and to the lower sequence of the Paraná Basin sedimentary strata. Locally, breccia contains clasts of IMR Type‐II, and only very rarely are granitic fragments observed. Clasts of IMR Type‐I have never been observed in the breccia deposits. These new observations preclude significant involvement of alkali granite in the formation of the polymict breccia or in the production of shock melts. They also reveal the major role of the (meta)sedimentary precursors in the production of IMR by shock melting and provide essential information for better understanding the cratering processes involved in the formation of an impact structure in a sedimentary target, of the size of the Araguainha impact structure.

Effect of Target Properties on Regolith Production

Based on the measurements of regolith thicknesses on the lunar maria (basalts), the lunar regolith was determined to have accumulated at a rate of about 1 m/Gyr since the era of the late heavy bombardment. However, regolith production on porous targets (e.g., crater ejecta deposits) is less studied, especially for Copernican units, and how target properties affect regolith production is not well understood. Here, we measured regolith thicknesses on the ejecta blanket of the Copernicus crater, showing that the regolith production rate sensitively depends on the initial target properties. The regolith production rate of the Copernicus ejecta blanket (3.0 ± 0.1 m/Gyr) is significantly larger than that of the Copernicus impact melt, which was previously estimated to be 1.2 ± 0.2 m/Gyr. Although crater production varies with different targets, our observed crater density of the Copernicus impact melt is indistinguishable from that of the Copernicus ejecta because impacts fracture the melt, causing it to resemble the ejecta. However, due to the fact that the formation of crater ejecta had already caused them to undergo fragmentation, ejecta require fewer fragmentation times to become regolith compared to impact melt; thus, the growth of regolith on the ejecta is faster than the melt. This indicates that similar observed size–frequency distributions do not indicate similar regolith production, especially for the targets with significant differences in initial physical properties.

The “suevite” conundrum, Part 2: Re‐examining the type locality at the Ries impact structure, Germany

AbstractOne of the most common types of allochthonous impactite produced in hypervelocity impact events is impact breccia that contains melt particles. In numerous terrestrial hypervelocity impact structures such melt‐bearing breccias have been termed “suevite,” after the type locality at the Ries impact structure, Germany. Despite its widespread occurrence, the origin, emplacement, and classification of suevite remains debated. In this contribution, we re‐examine the nature and origin of suevite at the Ries impact structure. The results of new field and laboratory investigations, when combined and synthesized with results from previous studies, lead to a multi‐stage model for the origin and emplacement of allochthonous impactites during the Ries impact event. Following the creation of a transient cavity the so‐called Bunte Breccia and “megablocks” were emplaced via ballistic sedimentation and subsequent radial flow during the excavation stage to form a continuous ejecta blanket. At the end of the excavation stage, a mixture of melt and lithic fragments formed a lining to the transient cavity and it is this material that later became the crater, dike, and outer suevite (OS) units. The crater suevite represents the material from the displaced zone of the transient cavity that was transported and mixed but never left the cavity. The emplacement of dike suevite occurred during the modification stage as the crater suevite was intruded into fractures in the underlying crater floor. The OS and rare impact melt rocks overlying the ballistic (Bunte Breccia) ejecta deposits were emplaced as outwards‐directed ground‐hugging flows largely during the modification stage of crater formation. The OS flows varied both spatially and temporally in terms of the flow characteristics, from being dominated by solid particles and gas (cf. pyroclastic density currents) to a mixture of solid particles, liquid (impact melt), and minor gases (i.e., particulate impact melt‐rich flows). These particulate impact melt‐rich flows dominated by far. Minor “fallback” of material from an ejecta plume is evidenced by accretionary lapilli in the Nördlingen 1973 core. In summary, allochthonous impactites at the Ries impact structure are not unusual but are consistent with observations from other terrestrial and planetary craters, where melt‐rich impactites overly ballistic ejecta deposits both outside and inside crater rims and where melt‐rich impactites occur in crater interiors.

Geological context of the Chang’e-6 landing area and implications for sample analysis

Research on returned samples can provide ground truth for the study of the geological evolution history of the Moon. However, previous missions all collected samples from the near side of the Moon, which is significantly different from the far side of the Moon in terms of the thickness of the lunar crust, magma activity, and composition. Therefore, the samples from the far side of the Moon are of great significance for a comprehensive understanding of the history of the Moon. China's Chang'e-6 (CE-6) probe has successfully landed on the lunar far side and will return samples in the coming days. With the precise location of the CE-6 landing site, a detailed analysis of the geological background is conducted in this research. The landing site of CE-6 is within the Apollo crater, which is inside the largest impact basin on the Moon, i.e., the South Pole-Aitken (SPA) basin. According to the numerical simulation of the formation process of the SPA basin, CE-6 landed at the edge of the SPA impact melting zone, which is presumably composed of impact melt of the lunar mantle. The Apollo crater subsequently excavated deep material again, which constitutes the basement of the CE-6 landing area. Later, erupted basalt covered these basement rocks, and they also constitute the main source of the CE-6 samples. Based on the dating method of crater size-frequency distribution, we find that the basalt is ∼2.50 Ga. The CE-6 samples also possibly contain basement rocks as excavated and ejected by craters, and they can provide crucial information for our understanding of lunar geological history along with the basalt samples.

A mineralogical and isotopic study of the historic monomict eucrite Padvarninkai

AbstractThe Padvarninkai meteorite is a relatively understudied eucrite, initially misclassified as a shergottite given its strong shock characteristics. In this study, a comprehensive examination of the petrology; mineral composition; major, minor, and trace element abundances; and isotopic composition (C, O) is presented. Padvarninkai is a monomict eucrite consisting of a fine to coarse‐grained lithology and impact melt veins. Pyroxene grains are typically severely fractured and mosaicked whilst plagioclase is either partially or totally converted to maskelynite. Based on shock features observed in pyroxene, plagioclase, and apatite, Padvarninkai can be given a shock classification of M‐S4/5. Despite the high shock experienced by this sample, some of the original igneous textures remain. Compositionally, Padvarninkai is a main group eucrite with a flat REE pattern (~10–12 × CI) and elevated Ni abundances. Whilst both new and literature oxygen isotopes are similar to other eucrites, however, Padvarninkai displays an anomalously high δ13C value. To reconcile the high Ni and δ13C value, impact contamination modeling was conducted. These models could not reconcile both the high Ni and δ13C value with the eucritic δ18O values, arguing against impact as a source for these anomalies.