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Depleted shergottites record unique information about the primary composition and differentiation of the mantle of Mars. Their petrology, geochemistry, and cosmic ray exposure and crystallization ages suggest that most of them were excavated by a single young impact in the Amazonian‐aged lava flows of the Tharsis and Elysium volcanic provinces. However, the difficulties of deriving consistent model ages for individual craters and inadequate evaluation of 3–7 km craters capable of ejecting martian meteorites have not been settled. Here we perform detailed geological investigations and crater statistics in patches of impact melt deposits for potential source craters of depleted shergottites with D > 3 km, especially those in the Tharsis and Elysium volcanic provinces. By excluding the effect of heterogeneous textures across ejecta deposits, which hinder straightforward extraction of superposed production populations, our systematically updated model ages reveal that Chakpar crater at the northern flank of Ascraeus Mons is the best‐fit candidate. The local context of this crater permits establishing a link between the meteorites and specific lava flows. The long‐lived volcanic center here may experience an eruption and/or local deposition hiatus for about 1.8 billion years, and abundant subsurface water existed when the impact occurred at about 1.1 million years. [ABSTRACT FROM AUTHOR]

The collapse of large impact craters requires a temporary reduction in the resistance to shear deformation of the target rocks. One explanation for such weakening is acoustic fluidization, where impact‐generated pressure fluctuations temporarily and locally relieve overburden pressure facilitating slip. A model of acoustic fluidization widely used in numerical impact simulations is the Block model. Simulations employing the Block model have successfully reproduced large‐scale crater morphometry and structural deformation but fail to predict localized weakening in the rim area and require unrealistically long pressure fluctuation decay times. Here, we modify the iSALE shock physics code to implement an alternative model of acoustic fluidization, which we call the Melosh model, that accounts for regeneration and scattering of acoustic vibrations not considered by the Block model. The Melosh model of acoustic fluidization is shown to be an effective model of dynamic weakening, differing from the Block model in the style of crater collapse and peak ring formation that it promotes. While the Block model facilitates complex crater collapse by weakening rocks deep beneath the crater, the Melosh model results in shallower and more localized weakening. Inclusion of acoustic energy regeneration in the Melosh model reconciles required acoustic energy dissipation rates with those typically derived from crustal seismic wave propagation analysis. Plain Language Summary: Large "complex" impact craters differ in appearance from their smaller, bowl‐shaped counterparts. They are shallower in aspect, with terraced rim walls and an uplifted central mountainous peak or ring of mountains. These differences are the result of dramatic collapse of an initially bowl‐shaped cavity, which only happens in large craters, and is facilitated by temporary weakening of the target rocks. The physical mechanism for this weakening, however, is still debated. One potential weakening mechanism is acoustic fluidization: where impact‐generated vibrations around the crater reduce resistance to deformation until the vibrations dissipate. Computer simulations of large crater collapse that employ this idea have successfully reproduced many observations of large craters on rocky and icy planetary bodies. However, those simulations are yet to replicate rock fracturing in the crater rim region and require that the vibrations persist for much longer than is typical for analogous vibrations produced by earthquakes. By implementing an alternative model of acoustic fluidization we show that the regeneration of vibrations during crater formation facilitates more localized deformation during collapse and explains how vibrations can be sustained around the crater for such a long time. This paves the way for more realistic simulations of crater formation in the future. Key Points: Acoustic fluidization is a mechanism to explain temporary weakening of target rocks during the collapse of large impact structuresThe Melosh model of acoustic fluidization that accounts for scattering and regeneration of acoustic energy is implemented in the iSALE codeRegeneration of acoustic energy facilitates localized deformation and sustains vibrations for the duration of crater collapse [ABSTRACT FROM AUTHOR]

The formation of complex craters requires some form of transient weakening of target rocks. Acoustic fluidization is one proposed mechanism applied in many numerical simulations of large crater formation. In a companion paper, we describe implementing the Melosh model of acoustic fluidization in the iSALE shock physics code. Here, we explore the effect of Melosh model parameters on crater collapse and determine the range of parameters that reproduce observed crater depth‐to‐diameter trends on the Earth and Moon. Target viscosity in the Melosh model is proportional to the vibrational wavelength, λ $\lambda $, and the longevity of acoustic vibrations is ∝λQ $\propto \lambda Q$ (Q $Q$—quality factor). Our simulations show that λ $\lambda $ affects the size of the fluidized region, its fluidity, and the magnitude of the vibrations, producing a variety of crater collapse styles. The size of the fluidized region is strongly affected by the Q $Q$. The regeneration factor, e $e$, controls the amount of (re)generated acoustic energy and its localization. We find that a decrease in e $e$ leads to less crater collapse and that there are trade‐offs between e $e$ and Q $Q$. This trade‐off contributes to the more realistic Q $Q$ values than those used in the Block model. The diffusion of vibrations in regions with high stress and strain is controlled by the scattering term, ξ $\xi $. Compared to the Block model, the Melosh model results in a shallower zone of weakening in complex craters and enhanced strain localization around the crater rim. The parameter set that produces best depth‐diameter trends is λ $\lambda $ = 0.2× ${\times} $impactor radius, Q $Q$ = 10–50, e $e$ = 0.025–0.1, and ξ $\xi $ = 10–105 ${10}^{5}$m2s−1 ${\mathrm{m}}^{2}{\mathrm{s}}^{-1}$. Plain Language Summary: Impact craters are the most common geological features on planetary surfaces. As craters get larger in diameter, they express more complex morphology. Complex crater morphology results from target rocks behaving fluidly upon hypervelocity impact. The rocks behave as fluids because of the temporary reduction in strength, which results from vibrations that remain in the target after the passage of the shock wave. These vibrations cause pressure fluctuations and reduce the rock's strength. This concept is called acoustic fluidization. Here, we explore how vibrations' wavelength, dissipation, scattering, and (re)generation affect crater collapse. Our models show a larger subsurface deformation variety, which the previous simplified model lacked. We determine bounds for each explored parameter, show trade‐offs between them, and their success in reproducing depth‐to‐diameter trends on Earth and the Moon. Key Points: We explore the parameters of the Melosh model of acoustic fluidization (full model) and their effect on crater collapseWe find that the final crater size is a product of the trade‐off between the quality (Q) $(Q)$ and regeneration (e) $(e)$ factors and that vibrational wavelength, λ $\lambda $,affects the style of crater collapseConsidering the full model of acoustic fluidization produces a variety of subsurface deformation in the complex craters lacking in the hitherto applied models [ABSTRACT FROM AUTHOR]

Impact craters are a unique tool not only for inferring ages of planetary surfaces and examining geological processes, but also for exploring subsurface properties. We use ejecta blankets as proxies to obtain insights into the subsurface characteristics and the vertical stratification of Ganymede's icy crust. We investigated 36 prominent ray and halo craters using images acquired during the Voyager, Galileo, and Juno spacecraft missions. These craters exhibit diverse characteristics, including dark rays, bright rays, or their combination, in both continuous and discontinuous patterns as well as dark and bright halos. Dark halo craters (DHCs) have the smallest radial extents of their dark ejecta deposits, while dark ray craters (DRCs) have the largest. DRCs in dark terrain suggest a thickness of less than ∼2 km based on their excavation depths. DRCs and DHCs craters located in light terrain (LT) reveal significant heterogeneity in the uppermost portions of icy crust at various locations. DRCs and DHCs in the LT require the presence of at least one layer of dark material. This is the case if the LT is formed by tectonic rifting and graben formation. In contrast, if the LT is formed by tectonic spreading, bright halo and ray craters are expected to form. Key Points: We utilized ray and halo craters on Ganymede to determine the vertical stratigraphy of its icy crustThe excavation depths of dark ray craters in Ganymede's dark terrain suggest kilometer thick dark terrain layersThe ejecta facies of impact craters allow to narrow down the tectonic regime of crustal extension (rifting or spreading) [ABSTRACT FROM AUTHOR]

Lunar bases for long-term human habitation are a topic of growing interest to engineers. Structures to be built on the surface of the Moon must be designed to resist extreme environmental hazards such as hard vacuum/no atmosphere, low gravity, ionizing radiation, large temperature variations, moonquakes, and meteoroid impacts. Protective regolith covers may provide adequate shielding from some of these hazards. This paper presents a methodology that utilizes established physics-based models to determine the immediate effects of hypervelocity impacts (HVI), including crater geometry and shock pressure, for engineering design of lunar structures. First, empirical equations and nondimensional scaling laws for computing geometric features of HVI craters are discussed. These damage estimation tools are adopted from the fields of spacecraft design and planetary impact cratering. Next, the planar impact approximation model is discussed, which utilizes one-dimensional shock wave physics and impedance matching conditions to estimate the peak initial pressures generated in HVI. Comprehensive parametric analyses, encompassing a wide range of impact velocities and impactor masses as well as a variety of impactor and target materials, are performed to demonstrate applications of the models to both regolith-shielded and unshielded structures. Crater geometry descriptors and peak shock pressures in HVI scenarios involving various impactor-target combinations are computed with in-house programs and compared with computational results reported in the literature. The analyses indicate that for a given target, the main factors determining cratering damage are impactor mass, impactor density, and impact velocity, while the initial shock pressure is influenced mainly by impact velocity and impactor density. A comparison of impact pressures experienced by monolithic unshielded targets and granular dry sand layers highlights the potential importance of providing regolith-like shielding to attenuate HVI effects. Practical implications of the study are discussed from an engineering design perspective, underscoring the considerable value of such models to practitioners. Practical Applications: A practical methodology for determining cratering and peak shock pressure due to hypervelocity meteoroid impacts on unshielded and regolith-shielded lunar structures is proposed in this work. The methodology utilizes well-established physics-based models of hypervelocity impact effects. These models have several advantages that make them suitable for inclusion in lunar structural design guidelines: (1) the models presented are relatively simple yet realistic, empirical, and analytical models; (2) the models do not rely on computationally expensive numerical methods and have been successfully implemented using a commonly available scientific computing platform; and (3) the models consider several essential aspects of the complex physics of hypervelocity meteoroid impacts, which is essential to ensure accurate prediction of the immediate effects of such impacts for lunar structure design. Additionally, this work also includes a wide-ranging parametric analysis that explores and compares the predictions of the physics-based models from an engineering design perspective, encompassing a variety of impact scenarios and impactor-target combinations. The results highlight the practical importance of providing a protective shield, composed of a regolith-like granular material, to mitigate the peak shock pressure sustained by lunar structures in hypervelocity meteoroid impacts. The models, applications, and results presented herein are of immediate relevance to practicing engineers. [ABSTRACT FROM AUTHOR]

The formation of straight impact crater rims is widely accepted to be influenced by preexisting lithospheric structure. We investigate the distribution and orientations of straight crater rim segments across Mercury. We devise a mapping workflow aimed at minimizing distortions of length and orientation that arise when working on projected image mosaics on a global scale and produce a global map of the rims of 7,145 impact craters with diameters between 20 and 400 km. We extract straight rim segments that maintain consistent orientations for at least 10 km to assess their frequency and orientation. Our dataset shows that 83% of craters have straight rim segments that show strong east–west orientations at the poles and weak north–south or random orientations at lower latitudes. This emphasizes the importance of lithospheric structure for impact cratering on Mercury. Our map dataset provides valuable insights for future investigations into the tectonic evolution of Mercury.

Abstract Impact craters are a unique tool not only for inferring ages of planetary surfaces and examining geological processes, but also for exploring subsurface properties. We use ejecta blankets as proxies to obtain insights into the subsurface characteristics and the vertical stratification of Ganymede's icy crust. We investigated 36 prominent ray and halo craters using images acquired during the Voyager, Galileo, and Juno spacecraft missions. These craters exhibit diverse characteristics, including dark rays, bright rays, or their combination, in both continuous and discontinuous patterns as well as dark and bright halos. Dark halo craters (DHCs) have the smallest radial extents of their dark ejecta deposits, while dark ray craters (DRCs) have the largest. DRCs in dark terrain suggest a thickness of less than ∼2 km based on their excavation depths. DRCs and DHCs craters located in light terrain (LT) reveal significant heterogeneity in the uppermost portions of icy crust at various locations. DRCs and DHCs in the LT require the presence of at least one layer of dark material. This is the case if the LT is formed by tectonic rifting and graben formation. In contrast, if the LT is formed by tectonic spreading, bright halo and ray craters are expected to form.

Most impact basins are believed to have formed during the early epochs of planetary evolution. The planet's gravity, internal structure, and thermal regime have the strongest control over their formation. Because of this, we can use the geophysical constraints on Mars' interior composition, structure, and geophysical evolution derived from the InSight mission to better understand the formation of impact basins on the planet. To achieve this, we performed numerical simulations of large impacts using the iSALE shock physics code. We investigated the effects of temperature and crustal thickness variations on impact basin size and morphology. Our scaling relationships indicate that: (a) basins formed in a warmer crust have larger final diameters in comparison to basins formed in a colder crust, a difference that is further accentuated as basin size gets bigger; and (b) the largest impact basins on Mars were created by impactors ranging from 35 to 680 km in diameter, up to ∼32% larger than estimates based on classical scaling. Our results expand the current understanding of the extent of early and large impact bombardment on Mars and provide a more comprehensive knowledge of impact basin formation on planetary surfaces. Plain Language Summary: The recent advancements in the understanding of Mars' that resulted from the InSight mission can be used to better understand the early large bombardment that took place 4.4 to 3.7 Ga ago. This bombardment formed impact basins, the largest and most complex type of craters. Their size and shape depend on the interior structure and temperature of the planet when they formed. We can better understand basins by simulating their formation and comparing the results with observations. Here, we used advanced interior structure and temperature evolution models of Mars to simulate the formation of impact basins as accurately as possible. We simulated basins of various sizes forming at different locations at multiple stages during Mars' evolution. Based on that, we derived equations referred to as scaling relationships that express the connection between basin size and impact conditions for different epochs and locations on Mars. We concluded that: (a) basins formed in a warmer crust are larger than basins formed in a colder crust, and (b) the largest reported basins on the planet were created by impactors much larger than previously thought. Our results provide a more comprehensive knowledge of impact basin formation and valuable insights into the early large bombardment. Key Points: We numerically simulated impact basin formation on Mars using recent internal structure and temperature modelsWe described how thermal evolution and crustal thickness variations could have affected impact basin formation on MarsWe derived new scaling relationships for impact basins and provided an insight into the early large bombardment on Mars [ABSTRACT FROM AUTHOR]

Lunar impact basins are critical markers to understand the early bombardment history. This study focuses on isostatic compensation processes at 16 lunar farside basins, exploring how long‐term alteration processes affect estimates of impactor sizes and timing. Using a lithospheric flexure model, we analyze the isostatically compensated gravity signature of these basins. Our approach is based on previously published basin formation models using the iSALE‐2D shock physics code. Here we investigate how isostatic compensation processes alter the gravity signature using models of basin formation as initial conditions. We compare our results with the present day observed gravity data. We assume that isostatic compensation is the result of flexural deformation of the crust‐mantle boundary with varying lithospheric elastic thicknesses. Our results indicate that the effect of isostatic compensation on the gravity signature varies depending on the thermal conditions at the time of impact. We find that despite this variability, the overall influence of isostatic compensation on the gravity signature is generally moderate to minor. Additionally, our analysis is consistent with previous studies that have shown that the elastic thickness varies across the Moon. Our findings contribute to our understanding of the relationship between basin formation and the thermal evolution of the Moon. The variations in elastic thickness found provide nuanced insights into lunar geological processes and improve our understanding of the early history of the Moon. Plain Language Summary: This study addresses the early bombardment history of the Moon, particularly the basin forming asteroid impacts in the first 700 million years. The large crater structures and basins serve as historical markers that make it possible to estimate the size of the cosmic bodies that formed them and to estimate the thermal conditions of the Moon at the time of their formation. As the surface expression of some basins is poorly preserved, making size measurements uncertain, gravity anomalies, caused by subsurface mass displacements and changes in density, are valuable for evaluation. Based on previously published numerical simulations of basin formation, we correlate our results with gravity data from the Gravity Recovery and Interior Laboratory mission. In the period between the formation of the basin and today's observation, isostatic compensation processes probably caused changes that we are trying to reconstruct using models. Our findings emphasize the significant role of lithospheric elasticity, determining whether isostatic compensation occurs locally or regionally. Regarding the lunar farside, we observed spatial variations in elastic thickness, dependent on the cooling history of each basin. Importantly, our study revealed that previously determined impactor sizes exhibit no to minimal change with the introduction of isostatic compensation through our lithospheric flexure model. Key Points: Isostatic compensation: We use the elastic thickness of the lithosphere to account for isostatic compensation in basin formation modelsIsostatic compensation of lunar farside basins varies with impactor size and thermal state, affecting basin's gravity signaturesThe effects of isostatic compensation on the estimated impactor sizes for farside basins are limited [ABSTRACT FROM AUTHOR]

Knowledge of the shock remanent magnetization (SRM) property is crucial for interpreting the spatial change in a magnetic anomaly observed over an impact crater. This study conducted two series of impact‐induced SRM acquisition experiments by varying the applied field intensity (0–400 μT) and impact conditions. Systematic remanence measurements of cube‐shaped subsamples cut from shocked basalt containing single‐domain titanomagnetite were conducted to investigate the effects of changes in pressure and temperature on the SRM acquisition. The peak pressure and temperature distributions in the shocked samples were estimated using shock‐physics modeling. SRM intensity was proportional to the applied field intensity of up to 400 μT. SRM intensity data for peak pressure and temperature of up to 8.0 GPa and 530 K, respectively, clearly show that it increases with increasing pressure and decreases with increasing temperature. The SRM has unblocking temperature components up to a Curie temperature of 510 K, and it easily demagnetizes with alternating field demagnetization. The observed SRM properties can be explained by the pressure‐induced microcoercivity reduction and temperature‐induced modification of the blocking curve. Although the remanence acquisition efficiency of the SRM is significantly lower than that of the thermoremanent magnetization (TRM), the magnetic anomaly originating from the SRM distribution in a broader region may show a contribution comparable to that of the impact‐induced TRM distribution in a narrow region. Plain Language Summary: Crustal rocks on terrestrial planets acquire remanent magnetization at the time of an impact event as shock remanence. Magnetic field data observed over impact craters play a key role in reconstructing the magnetic field histories of terrestrial planets. Knowledge of shock remanence is crucial for interpreting the spatial changes in the magnetic field observed over an impact crater. In this study, a suite of shock remanence acquisition and evaluation experiments were conducted to investigate the effects of pressure and temperature changes during shock wave propagation on shock remanence acquisition. The experimentally observed characteristics of shock remanence, such as the stability with respect to thermal and alternating field demagnetization and the systematic intensity changes with respect to pressure and temperature changes, are qualitatively explained by the pressure‐ and temperature‐induced changes in magnetic properties during shock wave propagation. The shock remanence intensity with respect to impact conditions was evaluated using newly obtained experimental data, which enabled us to quantitatively evaluate the shock remanence distributions and their contribution to the magnetic field over impact craters. The shock remanence contribution is estimated to be comparable to that of the impact‐induced thermoremanent magnetization, considering the intensity and volume of these remanences. Key Points: Shock remanence acquisition and evaluation experiments are conducted to evaluate the effects of pressure and temperature changesCharacteristics of shock remanence can be explained by a pressure‐induced microcoercivity reduction and a temperature effect on itContribution of shock remanence to magnetic anomaly over crater is estimated to be comparable with that of impact‐induced thermoremanence [ABSTRACT FROM AUTHOR]

The formation of straight impact crater rims is widely accepted to be influenced by preexisting lithospheric structure. We investigate the distribution and orientations of straight crater rim segments across Mercury. We devise a mapping workflow aimed at minimizing distortions of length and orientation that arise when working on projected image mosaics on a global scale and produce a global map of the rims of 7,145 impact craters with diameters between 20 and 400 km. We extract straight rim segments that maintain consistent orientations for at least 10 km to assess their frequency and orientation. Our dataset shows that 83% of craters have straight rim segments that show strong east--west orientations at the poles and weak north--south or random orientations at lower latitudes. This emphasizes the importance of lithospheric structure for impact cratering on Mercury. Our map dataset provides valuable insights for future investigations into the tectonic evolution of Mercury. [ABSTRACT FROM AUTHOR]

Extraterrestrial cratering was a critical component in the evolution of the early Earth, but discovery of very ancient and deeply exhumed impact structures remains elusive, as identification tools are in short supply. The recognition of such structures is hindered by post-cratering geological processes, whereby impact-induced features common in younger, upper-crustal craters may be lost. In this study, we make a detailed analysis of planar microstructures in zircons from four large, confirmed impact structures (Manicouagan, Rochechouart, Sudbury, Vredefort) preserved at different crustal levels, from two previously described non-impact seismites in SW Norway and northern Italy, and from the 3.0 Ga Maniitsoq structure in West Greenland. A total of ~3400 zircon grains were studied using exterior and interior scanning and transmission electron microscopy. We show for the first time that shocked zircons contain two successive, principally different types of planar microstructures, only one of which is diagnostic of impact. Closely spaced, contiguous microplanes (CP) are formed first, presumably by the shock wave. In Manicouagan and Rochechouart zircons the exterior CPs have ultrathin interior counterparts of straight dislocation arrays, as identified in Manicouagan zircon using transmission electron microscopy. They have the same close spacing and orientations as the exterior CPs and are abundantly decorated with tiny pores down to less than 50 nm across. These interior CPs are identical to shock-induced decorated, partly annealed amorphous planar microstructures in quartz (planar deformation features, PDFs) and are interpreted as such. The second type is open planar fractures (PF). They are widely and irregularly spaced and texturally younger than the CPs. They re-use and displace the CP orientations, which they cut in stepwise fashion. We interpret these PFs as formed by impact-induced seismic shaking in the wake of the shock wave. We confirm two previous reports of isolated planar fractures in zircons from non-impact seismites, showing that PFs per se are not impact-diagnostic. There are no CPs in any of these zircons. Zircons from different parts of the Maniitsoq structure contain CPs in various states of preservation besides PFs, corroborating that this very large and very deeply exhumed structure resulted from an extraterrestrial impact. [ABSTRACT FROM AUTHOR]

In the Earth's crust, Ni is generally concentrated in mafic and ultramafic rocks and is coupled with Mg in Mg-olivine, Mg-pyroxene and spinel. Whether the Ni-rich, and in general, the mafic component of Australasian tektites and microtektites is terrestrial or meteoritic is still debated. To test the origin of the Ni-rich component, we studied the Ni versus Mg distribution in a large geochemical database of Australasian tektites (n = 208) and microtektites (n = 238) from the literature. Nickel contents of up to 428 µg/g in tektites and 678 µg/g in microtektites covary with Mg in tektites and in most (∼85%) of the microtektites defining a mixing trend between crustal and chondritic values, thereby documenting the chondritic origin of the Ni-rich component in Australasian tektites/microtektites. Mixing calculations indicate up to 4% and up to 6% by weight chondritic component in tektites and microtektites, respectively. A possible mafic component of terrestrial origin is observed in a minority of tektite and microtektite specimens. This finding is consistent with previous works suggesting a possible occurrence of a chondritic signature in high-Ni tektites, based on the study of highly siderophile elements and Os isotopes, and high-Ni microtektites, based on Ni, Co, and Cr ratios. The combined geochemical and isotopic analyses of high-Ni tektites and microtektites in collections worldwide may thus reveal the chondritic impactor type that generated one of the presumably largest impacts in the Cenozoic. [ABSTRACT FROM AUTHOR]

The evolution of regolith thickness induced by continuous impact events on the lunar surface provides valuable information about lunar geology and the bombardment history of the inner Solar System. In this study, we develop a Lunar Topography and Regolith Evolution Model based on the production and distribution of lunar regolith resulting from individual impact craters. Considering the changing impact flux, evolving target properties, topographic degradation process, and crater superposition, our model simultaneously simulates both the spatial and temporal evolution of lunar cratered topography and regolith thickness. The modeled regolith thickness generally aligns with results estimated by in situ seismic experiments and measurements of small crater morphology. Through our simulations, we establish a novel quantitative relation between regolith growth rate and time, providing new constraints on the impact flux trend and buffering trend in the regolith growth process. By comparing this new relation with that predicted by our analytical regolith evolution model, we reveal the significant influences of evolving target properties and regolith distribution on the regolith growth rate. Our modeled results also show that the regional regolith thickness follows a characteristic long‐tailed distribution with substantial lateral variations. Both the root‐mean‐square fluctuation and correlation length of lunar regolith increase with lateral scale and the surface age, which are ∼4.7 and 18 m on the hectometer scale at 3.5 Ga. All these results contribute to a better understanding of the regolith evolution process and can provide valuable insights into the lunar surface dynamics. Plain Language Summary: Lunar regolith refers to the fine‐grained surficial layer of fragmental debris on the Moon that is primarily formed by repeated impact events. The evolution of regolith thickness can provide important information about lunar geology and the impact history with the lunar surface. In this study, we develop a Lunar Topography and Regolith Evolution Model (LTREM) that numerically simulates the spatiotemporal evolution of lunar cratered terrain and regolith thickness. Our model considers changing impact flux, the evolution of the target's bulk density and strength, topographic degradation, and crater overlapping, and can therefore reveal the variation of regolith thickness across the surface and provide a better understanding of the regolith growth process. The modeled regolith thickness generally aligns with results estimated by in situ seismic experiments and measurements of small crater morphology. Through our modeled results, we quantitatively characterize the decreasing regolith growth rate due to changing impact flux and the shielding effect of preimpact regolith. Our results also show that regional regolith thickness exhibits a long‐tailed distribution with substantial spatial variations at different lateral scales. All these results contribute to a better understanding of the growth process and spatial distribution of lunar regolith. Key Points: A Monte Carlo based Lunar Topography and Regolith Evolution Model for a two‐layer target is developedWe establish a novel quantitative relation between regolith growth rate and time, reflecting both the impact flux trend and buffering trendOur simulation results show that regolith thickness exhibits a long‐tailed distribution pattern with substantial lateral variations [ABSTRACT FROM AUTHOR]

Impact cratering is associated with extreme physical conditions with temperatures and pressures far exceeding conditions otherwise prevailing at the surfaces of terrestrial planets. As a consequence, shock‐metamorphosed rocks contain unique deformation features such as planar deformation features in quartz, high‐pressure mineral polymorphs and melted rock. While the physical conditions of formation for impact‐induced melting following the highest pressure and temperature conditions is relatively well understood, aspects of the formation of melt‐veins in otherwise seemingly relatively low shock material has been the topic of discussion. In a new study, Hamann et al. (2023, https://doi.org/10.1029/2023JE007742) are able to largely reproduce the current classification of progressive shock metamorphism of felsic rocks using a modern experimental set up that eliminates multiple shock wave reflections at sample containers and excavation and ejection of target material. Importantly, however, they find that shear deformation results in the formation of melt veins at pressures as low as 6 GPa. The authors recover stishovite in melt veins formed at low‐moderate (<18 GPa) shock pressure, lower than most previous studies. These results have bearing on our understanding of the conditions of progressive shock metamorphism at terrestrial impact structures. However, since the results are similar to data obtained from experiments on basaltic rocks, the results also have broader implications for understanding the shock histories of meteorite parent bodies. Hamann et al. show the importance of experimental impact cratering for bridging the gap between observations in shocked rocks from terrestrial impact structures, in meteorites, and in returned samples, and their formational conditions. Plain Language Summary: A new publication refines understanding of the conditions and processes required to melt rock during impact cratering. Impact cratering represents one of the most extreme geological processes there is, in terms of pressures and temperatures reached. The process is ubiquitous throughout the Solar System, today and in the past, and therefore understanding of the history of impacts on Earth and in meteorites and returned samples from other planetary bodies is essential in order to interpret Solar System evolution. Hamann et al. (2023, https://doi.org/10.1029/2023JE007742) show that experimental impact cratering is a critical link between observations of shock features and the pressure‐temperature conditions that formed those features. Specifically, Hamann et al. see that low shock pressures (6 GPa) is required for generation of small amounts of localized melt and they recover shock‐formed stishovite, a mineral with the same chemical composition as quartz, but with a different crystal structure, in their experiment at unusually low shock pressures compared to previous studies. Key Points: Intense shear deformation during impact experiments leads to formation of melt veins in granite at pressures as low as 6 GPaMelt veins formed in quartz grains subjected to >10–12 GPa during the experiments contain stishoviteAn update to the classification system of shock metamorphism of felsic rock is proposed by the authors [ABSTRACT FROM AUTHOR]

South China belongs to the northern geographic branch of the Australasian strewn field (AASF) of tektites and microtektites, and this area is assumed to be part of the uprange region of the putative impactor trajectory that formed the yet undiscovered source crater. Ferromagnetic minerals in impact glass may record the magnetization process and thermal history of impact melt, but the possible identity of ferromagnetic minerals in AASF tektites from South China is elusive. Here, we perform a rock magnetism and crystallography study of iron‐sulfur spherules in Muong Nong‐type tektites from South China, revealing abundant single‐domain and pseudo single‐domain nano‐grains of magnetite within the spherules. This result is consistent with the detection of magnetic signals caused by single‐domain and pseudo single‐domain magnetite in these samples. Compared to the large value ranges of magnetic properties of tektites in the entire strewn field, individual specimens of both Muong Nong‐type and single splash‐form tektites have relatively homogeneous magnetic properties in terms of magnetic susceptibility, natural remanent magnetization (NRM), saturation isothermal remanent magnetization (SIRM) and NRM/SIRM ratio. The results indicate that the impact melt that formed each tektite specimen experienced similar thermal history, and most AASF tektites from South China were cooled in an ambient magnetic field that is the same order of magnitude as the Earth's magnetic field. Plain Language Summary: The vast Australasian strewn field (AASF) of tektites and microtektites was formed at 0.788 million years ago by an enormous impact that may have occurred somewhere in the Indochina Peninsula. The magnetic properties of tektites may record the cooling history and post‐ejection magnetization process of the tektite‐forming melt, which could provide additional insights into the possible impact location. AASF tektites from South China are a unique portion of this strewn field regarding the hypothesized impactor trajectory, and our earlier rock magnetism study suggested that tektites in this region exhibit systematically different magnetic properties. This work performs detailed geochemical and crystallographic analyses for iron‐sulfur spherules in Muong Nong‐type tektites from South China, revealing abundant nano‐sized magnetite grains in the interior that are consistent with being single‐domain and pseudo single‐domain magnetite. This result is consistent with the detection of magnetic signals caused by single‐domain and pseudo single‐domain magnetite in these samples. Most tektites from South China have relatively homogeneous magnetic properties in the interior. Their paleointensity has the same order of magnitude compared to the Earth's magnetic field. Key Points: Nano‐sized euhedral magnetite grains were discovered in Muong Nong‐type tektitesIndividual tektite specimens from South China usually exhibit relatively homogenous magnetic propertiesAustralasian tektites in South China have been magnetized in an Earth‐strength magnetic field [ABSTRACT FROM AUTHOR]

In contrast to multiring basins, megabasins lack clear ring structures and display crustal profiles characterized by a thin layer of crust at their center that gradually increases in thickness to the surrounding region. The 1,550‐km‐diameter Caloris basin on Mercury is often speculated to be a multiring basin; however, its crustal structure is indicative of a megabasin, perhaps explaining the lack of discernible rings. Here, we model the formation of Caloris basin using iSALE‐2D under a range of preimpact thermal conditions to determine whether the processes responsible for its megabasin crustal structure could lead to the formation of basin rings. We find that thermal gradients between 22 and 30 K/km—hotter than previously inferred—facilitate the reproduction of Caloris basin's crustal structure through crustal flowback, though such preimpact thermal structures preclude its formation as a multiring basin. Instead, repeated thrusting and necking events during transient crater collapse induce instances of fault reactivation, which diminish initial fault offsets and produce a series of discrete crustal blocks. Models assuming cooler thermal gradients lead to overly thin or non‐existent crust at the basin center, in contrast to observations. Even if crustal deficits were made up via a differentiating melt pool, the crust elsewhere would be overly thick, supporting the idea that the crustal structure of megabasins is primarily associated with transient crater collapse. We conclude that the transition to a megabasin morphology on terrestrial planetary surfaces, like the multiring transition, is dependent upon the size of the basin compared to the lithospheric thickness. Plain Language Summary: Impact craters are the most pervasive geologic features in our Solar System. At the largest scales, aptly termed "impact basins" can present with varying surface and crustal morphologies. For example, some impact basins contain multiple concentric topographic "rings," while others do not. Additionally, basins with and without rings exhibit contrasting crustal structures. Such differences are owed to the properties inherent to the impacted body at the time the basin‐forming impact occurred. On Mercury, the Caloris basin has frequently been suggested to have formed multiple rings, though its crustal structure suggests otherwise. Using numerical techniques, we model the formation of Caloris basin with an emphasis on reproducing its inferred crustal structure to determine whether these processes can also produce basin‐concentric rings. We find that a warm and weak interior structure allows the impact‐fractured crust to flow back toward Caloris' center during the impact event. The fracture patterns in the crust, and its fragmentation into several blocks, however, do not suggest it formed a series of rings. These results suggest that Mercury may have been warmer than previously thought, and that a planetary body's internal temperature compared to the size of the basin is what dictates whether rings will form. Key Points: The formation of the Caloris megabasin is marked by fault reactivation and the segmentation of its crust into several rafted blocksCrust at Caloris' center can be explained by the inward flow of crustal material weakened during transient crater collapseNear‐surface temperature gradients determine whether impact basins form with multiring or megabasin morphologies [ABSTRACT FROM AUTHOR]

The production, distribution, and evolution of lunar regolith are critical in deciphering the lunar bombardment history and comprehending the transport of materials across the lunar surface, which are still not well understood. In this study, we conducted a comprehensive investigation of factors influencing the production and distribution of lunar regolith by individual simple craters. Combining our results for the impact‐generated regolith volume with a lunar production function, we developed an analytical model to describe the regolith growth process. We found that the strength of bedrock significantly affects the crater size and hence the volume of regolith produced especially for subdecameter impactors. The regolith volume produced by an individual impact crater is quantitatively characterized as a function of crater diameter and preimpact regolith thickness. This regolith production is primarily determined by how much bedrock is shattered, followed by the impact‐induced volume change of target material and lastly by the regolith volume created by secondary cratering processes. When a single crater forms, preimpact regolith thickness greatly affects the regolith distribution pattern; a larger fraction of the regolith will be distributed outside the crater rim for a deeper preimpact regolith layer. Our regolith evolution model can serve as a good first‐order estimation of the regolith growth process that provides better constraints on the regolith buffering trend than previous studies. This model also suggests that, when ignoring the contribution from large, distant impacts, the regolith growth process is dominated by impact craters at scales from a meter to a few hectometers. Plain Language Summary: Lunar regolith refers to the fine‐grained surficial layer of fragmental debris on the Moon that is primarily formed by continuous impact events. Knowledge of regolith production, distribution and evolution is important for revealing the impact history of the Moon and the mixing and transport processes of materials on the lunar surface. In this study, we investigated how individual impact craters produce and distribute regolith and developed an analytical regolith evolution model to describe the process of regolith growth. We found that the strength of bedrock greatly affects the regolith production especially for subhectometer craters. The regolith volume is mainly determined by how much bedrock is shattered by impact cratering, but it also depends upon the impact‐induced volume change of target material and secondary cratering processes. The results also show that the distribution of postimpact regolith is greatly influenced by the preimpact regolith layer, with more regolith distributed outside the crater rim with increasing preimpact regolith thickness. Compared with previous studies, our analytical regolith evolution model better describes how preimpact regolith protects bedrock from bombardment, hence suppressing regolith production. The model also suggests that the local regolith growth process is primarily controlled by meter‐ to hectometer‐scale impact craters. Key Points: We conducted a comprehensive study of the factors influencing regolith production and distribution by individual lunar simple cratersAn analytical regolith evolution model is developed based on regolith volume produced by individual craters and lunar production functionOur model suggests that regolith growth process is dominated by impact craters at scales smaller than a few hectometers [ABSTRACT FROM AUTHOR]

Planetary impacts have shaped the surfaces and interiors of planets. They were particularly critical in the last stage of planetary accretion, as they have eventually formed terrestrial planets. During these large supersonic collisions, shock waves melted the impactor and the target, and formed silicate magma oceans. Because the propagation of shock waves and the melting is faster than the excavation of an impact crater, the cratering stage can be considered as a purely hydrodynamic process. Here, we use both laboratory impact experiments in water and numerical simulations to investigate the crater dimensions resulting from the impact of a liquid impactor onto a liquid target. We show that our numerical models reproduce the laboratory experiments at subsonic impact velocities. We then explore the effect of both the Froude number, which is the ratio of the impactor kinetic energy to gravity, and the Mach number, which is the ratio of the impact speed to the sound speed. We vary these two parameters independently in impact simulations, going from subsonic to supersonic conditions. We obtain a new scaling law for the crater dimensions that describes the transition from subsonic to supersonic impacts. Our results indicate that the transition between these two regimes results from a change in the partitioning of the impactor kinetic energy into potential energy in the crater and internal energy. Finally, our scaling suggests that, in the limit of large Mach numbers, the crater depth depends only on the sound velocity and gravity, and is independent of the impact speed. Plain Language Summary: Planetary formation involved a large number of very energetic collisions. Such impacts generated shock waves which led to widespread melting and the formation of magma oceans. Understanding the dynamics of impacts into magma oceans is of great importance as these collisions set the initial temperature and composition of terrestrial planets and satellites. Laboratory experiments and numerical simulations have been used to investigate large impacts. However, each approach has pros and cons. Liquid impact experiments can produce the small scales responsible for the mixing between the impactor and the target, but they fail to reproduce shock waves and supersonic speeds. In contrast, current numerical simulations reach supersonic conditions but produce a limited amount of turbulence and mixing. In this study, we bridge the gap between these two methods and improve our understanding of the effect of the impact velocity on the cratering process. Using the code iSALE, we numerically reproduce water impact experiments at low subsonic velocities. We then explore supersonic conditions in impact simulations. We obtain a new scaling law predicting the crater depth in more realistic impact conditions and show that it is limited only by the sound speed for large impact velocities. Key Points: The shock physics code iSALE is successfully benchmarked against subsonic water impact experimentsA scaling law is proposed for the crater depth as a function of the Mach and Froude numbers which are varied as independent parametersIn the limit of high Mach numbers, our scaling suggests that the maximum crater depth is controlled by the sound velocity and gravity, but not by the impact speed [ABSTRACT FROM AUTHOR]

Abstract South China belongs to the northern geographic branch of the Australasian strewn field (AASF) of tektites and microtektites, and this area is assumed to be part of the uprange region of the putative impactor trajectory that formed the yet undiscovered source crater. Ferromagnetic minerals in impact glass may record the magnetization process and thermal history of impact melt, but the possible identity of ferromagnetic minerals in AASF tektites from South China is elusive. Here, we perform a rock magnetism and crystallography study of iron‐sulfur spherules in Muong Nong‐type tektites from South China, revealing abundant single‐domain and pseudo single‐domain nano‐grains of magnetite within the spherules. This result is consistent with the detection of magnetic signals caused by single‐domain and pseudo single‐domain magnetite in these samples. Compared to the large value ranges of magnetic properties of tektites in the entire strewn field, individual specimens of both Muong Nong‐type and single splash‐form tektites have relatively homogeneous magnetic properties in terms of magnetic susceptibility, natural remanent magnetization (NRM), saturation isothermal remanent magnetization (SIRM) and NRM/SIRM ratio. The results indicate that the impact melt that formed each tektite specimen experienced similar thermal history, and most AASF tektites from South China were cooled in an ambient magnetic field that is the same order of magnitude as the Earth's magnetic field.

Quantifying the ejecta thickness distribution from complex craters is key to understanding surface‐evolving processes on Ceres. Using the Park et al. (2019, https://doi.org/10.1016/j.icarus.2018.10.024) shape model, we estimated the ejecta thickness of five complex craters located in Ceres' equatorial region by analyzing 1,778 smaller, simple craters in their continuous ejecta deposits. In addition, we constrained their rim‐crest ejecta thickness following Sharpton (2014, https://doi.org/10.1002/2013JE004523). The ejecta thicknesses range from ∼3 to 73 m and ∼96–223 m around complex craters and at their rim crest localities, respectively. We find that ejecta thicknesses on Ceres are thinner than those on the Moon. Meltwater likely facilitates thin ejecta deposits on Ceres, given that fluid pressure conditions allow transient liquid water stability at shallow depths (∼1.8 m). Such water must be short‐lived because the atmospheric pressure on Ceres is too low (≳2.09 × 10−8 Pa) to allow a stable liquid phase. Our findings are consistent with previous work that ascribes fluidized appearing ejecta morphologies to the melting of subsurface water ice. Plain Language Summary: Understanding how thin ejecta are deposited and redistributed is important to provide insights into crater erasure processes that occur throughout Ceres' geological history. Using high‐resolution surface elevation data, we estimated the thickness of ejecta deposits from five complex craters around Ceres' equator. Comparing our results with cases estimated for ejecta deposits on the Moon suggests that ejecta deposits are thinner on Ceres. Water ice present at depth may be melted upon impact, which causes ejecta to act as a fluid and form thin deposits. This is possible given that on Ceres, pressures allow stable conditions for liquid water at shallow depths. Meltwater present at shallow depths is short‐lived because surface atmospheric pressures are too low to allow a stable liquid phase. Key Points: On Ceres, ejecta thickness ranges from ∼3 to 73 m on ejecta deposits and ∼96–223 m at crater rimsBased on both covariance ellipse statistical analysis and outcrop measurements, ejecta deposits are thinner compared to the MoonFluidization of ejecta causes thin deposits due to transient meltwater stability at shallow‐depth (∼2 m‐deep) fluid pressure conditions (611 Pa) [ABSTRACT FROM AUTHOR]

The NASA Artemis program will send humans to the lunar south polar region, in part to investigate the availability of water ice and other in situ resources. While trace amounts of ice have been detected at the surface of polar permanently shadowed regions (PSRs), recent studies suggest that large ice deposits could be stable below cold traps in the PSRs over geologic time. A recent study modeling the rate of ice delivery, ejecta deposition and ice loss from cold traps predicted that gigatons of ice could be buried below 100s of meters of crater ejecta and regolith. However, crater ejecta vigorously mix the target on impact through ballistic sedimentation, which may disrupt buried ice deposits. Here, we developed a thermal model to predict ice stability during ballistic sedimentation events. We then modeled cold trap ice and ejecta stratigraphy over geologic time using Monte Carlo methods. We found that ballistic sedimentation disrupted large ice deposits in most cases, dispersing them into smaller layers. Ice retention decreased in most cases, but varied significantly with the sequence of ejecta delivery, particularly from basin‐forming events. Over many model runs, we found that south polar craters Amundsen, Cabeus, and Cabeus B were most likely to retain large deposits of ice at depths up to 100 m, shallow enough to be detectable with ground‐penetrating radar. We discuss these findings in the context of the imminent human exploration activities at the lunar south pole. Plain Language Summary: Some craters near the south pole of the Moon contain permanently shadowed regions (PSRs) which stay cold enough to trap water vapor as ice. Recent studies have predicted that large amounts of ice could be buried under thick protective layers of lunar soil in the PSRs. Lunar soil is mainly transported by large impacts which launch soil and boulders to distances up to hundreds of kilometers. However, when these projectiles land they have destructive effects and may melt or redistribute buried ice. We simulated this process, called ballistic sedimentation, and predicted the amount of ice it removes. We also simulated ice and soil deposition over billions of years to test how much ice is lost to ballistic sedimentation over time. We predicted which PSRs are most likely to have ice near enough to the surface to detect in future missions. The upcoming Artemis program will send crewed and robotic missions to the lunar south pole region, and our work will help with planning where to land, what instruments to bring, and how much ice we might find. Key Points: Ballistic sedimentation of cold traps can disrupt buried ice depositsCold traps in Amundsen, Cabeus, and Cabeus B are most likely to retain ice in the upper 100 mLunar cold trap ice retention is sensitive to the timing and sequence of impact events [ABSTRACT FROM AUTHOR]

Abstract We conducted impact experiments using targets composed of particles with size distributions and projectiles with a size larger than or comparable with the maximum size of particles in targets. The pattern and particle concentration in the ejecta curtain were investigated. The results show three types of ejecta curtain features: (i) filament pattern extending throughout the entire curtain and high concentration, (ii) filament pattern and low concentration, and (iii) mesh-like pattern with a structure on smaller scales than the entire curtain and low concentration. When the target consists of particles using a bimodal size distribution with size differences of more than one order of magnitude, the filament pattern appears, exhibiting case (i). If the target consists of particles with various sizes with size differences of more than one order of magnitude, the filament pattern appears, but the concentration decreases, appearing the features of case (ii). Case (iii) occurs when the target consists of particles with a single size or when the mass of particles with a certain size is dominant. Thus, the size distribution of the particles in the targets determines the pattern and particle concentration in the ejecta curtain. Based on these results, we confirm that the pattern in the ejecta curtain caused by the impact of the Small Carry-on Impactor (SCI) in the Hayabusa2 mission showing case (i) is consistent with the evaluated sizes and masses of grains and boulders in the ejecta curtain. Graphical Abstract

Seismic activity generated by impacts depends on impact conditions and properties of the impact site. Here, we combined mapping of the regolith thickness with numerical impact simulations to better estimate the seismic efficiency and seismic moment generated in small impact events in the uppermost crust on Mars. We used mapping of crater morphology to determine the regolith thickness that craters formed in. We found that local regolith thickness in the late Amazonian units is between 4 and 9 m. Combined with previous estimates for the NASA InSight landing site, we composed a more realistic uppermost crust analog and implemented it in numerical impact simulations. We estimated the seismic efficiency and seismic moment for small craters on Mars impacting a non‐porous or fractured bedrock overlaid by 5, 10, or 15 m thick regolith. Seismic energy showed more dependence on target properties. Three orders of magnitude more energy were produced in stronger targets. The seismic moment does not depend on target properties, and we confirm that seismic moment is almost proportional to impact momentum. The resulting seismic moment is in agreement up to a factor of 4 between different target types. We improved the scaling relationships developed from numerical simulations used in seismic moment approximations by constraining its dependence on more realistic target properties. Plain Language Summary: Small impacts form in the top few meters of the crust of Mars. From the NASA InSight mission data, we learned that the uppermost crust of Mars is layered. Here, we mapped and used rocky ejecta craters as a proxy to estimate the thickness of this top layer. We demonstrated that a younger, late Amazonian geological unit, has a thinner top regolith layer (4–9 m) compared to an older, Hesperian unit (3–17 m). We modeled several target scenarios, constrained by mapping results, and performed a suite of numerical simulations of small impacts. From these simulations, we determined a relationship between impact‐generated seismic energy and the seismic moment for different types of Martian upper‐crust analogs. Our models show that impact‐generated seismic energy strongly depends on target properties (three orders of difference among different analogs investigated here). On the contrary, seismic moment shows agreement up to a factor of 4 in different targets. We discussed our modeling results to the newly detected impacts on Mars, and confirmed that our models are reliable for predicting seismic moments of small impacts. Our results contribute to the understanding of the size and energy of impact‐generated seismic sources in different target analogs. Key Points: Using rocky ejecta craters, we mapped regolith thickness on Mars and found that in the late Amazonian volcanic unit, it is 4–9 mSeismic efficiency shows larger sensitivity on target properties (up to three orders of magnitude) than the seismic moment (factor of four)A comparison of numerical simulations with the new seismic detections of craters on Mars showed agreement with our models [ABSTRACT FROM AUTHOR]

South China, the northern part of the Australasian strewn field (AASF) of tektites and microtektites is located at the uprange of the hypothesized source crater that is still unconfirmed. Magnetic properties of impact glasses are an important indicator of their source materials and thermal history. Extensive magnetic investigations have been performed on various AASF tektites sampled from both the Indochina Peninsula and further south, but such information remains sparse for those sampled from South China. Here, we present a detailed rock magnetism study for both Muong Nong‐type and splash‐form AASF tektites from South China, showing that all the tektites have rather weak remanent magnetization, but ferromagnetic information can be extracted from their dominating paramagnetic signal. Signals caused by superparamagnetic particles are elusive, but those of single domain magnetite are detected in the splash‐form tektites, and signals of pseudo‐single domain magnetite are discovered in the Muong Nong‐type tektites. Each morphological type of tektites from a same geographic area exhibits large dispersions in their magnetic properties. Across the entire AASF, the Muong Nong‐type tektites from South China exhibit the lowest average magnetic susceptibility, and the splash‐form tektites exhibit the smallest average natural remanent magnetization and ratio of equivalent magnetization. The observed heterogeneous magnetic properties are mainly caused by the different contents and sizes of magnetic particles, which can be explained by the different shock level and/or cooling history of the tektite melts. Micro‐sized immiscible Fe‐S spherules in the Muong Nong‐type tektites from South China were likely originated from the pre‐impact target. Plain Language Summary: About 0.78 million years ago, an about 1 km diameter asteroid or comet ended its trajectory on Earth. Instantaneously, massive continental sediments were molten and ejected, and natural glasses (tektites) were quenched and deposited across vast areas over 1 × 108 km2, forming the largest known Cenozoic strewn field of tektites and microtektites, the Australasian strewn field (AASF). The impact site is still unconfirmed. Magnetic properties of impact glasses are indicative to their source materials and thermal history. This study presents the first rock magnetic measurements for AASF tektites from South China. We find that while the samples are dominated by paramagnetic signals, signals caused by single domain magnetite are detected, and the Muong Nong‐type tektites contain pseudo‐single domain magnetite. Signals of superparamagnetic grains are elusive. With heterogeneous magnetic properties, tektites from South China generally exhibit lower magnetic susceptibility, natural remanent magnetization, and ratios of equivalent magnetization than the rest of AASF. The observed magnetic properties can be explained by different shock levels and cooling histories of the tektite melts, but strong magnetic field(s) recorded by some Muong Nong‐type tektites have an unknown origin. The first occurrence of Fe‐S spherules in tektites from South China might originate from target materials. Key Points: The first comprehensive rock magnetism study is performed for both splash‐form and Muong Nong‐type tektites from South ChinaSamples from China generally exhibit lower magnetic susceptibility, natural remanent magnetization, and ratio of equivalent magnetizationDifferent shock levels and cooling histories of tektite melt can explain the observed magnetic properties, and Fe‐S spherules are found [ABSTRACT FROM AUTHOR]

The suevite (polymict melt rock-bearing breccia) composing the upper peak ring of the Chicxulub impact crater is extremely heterogeneous, containing a combination of relict clasts and secondary minerals. Using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM/EDS) and electron probe microanalysis (EPMA), we investigated the nature and occurrence of primary and secondary Fe-oxide and Fe-sulfide minerals to better understand hydrothermal trends such as mineral precipitation and dissolution, and to document the remobilization of Fe and associated siderophile elements within suevites. Large primary Fe-oxides (~20–100 µm) reveal decomposition and dissolution patterns, forming sub-micrometer to micrometer Fe-oxide phases. Secondary sub-micrometer Fe-oxide crystals are also visibly concentrated within clay. The occurrence of Fe-oxide crystals within clay suggests that these likely formed at temperatures ≤100 °C, near the formation temperature of smectite. The formation of Fe-oxide minerals on clay surfaces is of interest as it may form a micro-setting, where free electrons (from the oxidation of Fe2+) and the adsorption of simple organic molecules on the surface of clay could generate reactive conditions favorable to microbial communities. Primary and secondary Fe-sulfide minerals exhibiting a variety of morphologies are present within samples, representing different formation mechanisms. Secondary Fe-sulfide minerals occur within rims of clasts and vesicles and in fractures and voids. Some secondary Fe-sulfide grains are associated with Ni- and Co-rich phases, potentially reflecting the post-impact migration of siderophile elements within the suevite of the Chicxulub crater. [ABSTRACT FROM AUTHOR]

Mimas' small size, lack of geologic activity, and high eccentricity suggest a frozen, inactive history. However, Cassini libration measurements are best explained by a present‐day liquid ocean under an ice shell 24–31 km thick, a configuration that tidal heating can support. These unexpected findings have motivated further study of Mimas' surface geology to constrain its interior evolution. Here, we model the formation of Mimas' large impact basin, Herschel, using iSALE‐2D. Our goal is to determine whether the current estimated ice shell thickness is sufficient to withstand the impact and produce the observed basin morphology. We find that, if Mimas had an ocean at the time of the Herschel‐forming impact, the ice shell had to have been 10s of km thicker than today. These findings are consistent with inferences from minimal tectonic activity that Mimas must be a young ocean world, if it indeed has an ocean today. Plain Language Summary: Mimas, a small moon of Saturn, is heavily cratered and lacks the typical characteristics of an ocean‐bearing moon, such as the active surface of its neighboring moon, Enceladus. However, measurements of Mimas, made by the Cassini mission, are best explained by an ocean under a relatively thick ice shell. Here, we try to understand how this ice shell and ocean may have changed with time by modeling the formation of Mimas' largest impact basin, Herschel. When such large impacts occur, the shapes and depths of the resulting basin can be affected by the presence of the ocean. We find that Herschel could not have formed in an ice shell at the present‐day thickness without obliterating the ice shell at the impact site. Rather, we conclude that, if Mimas has an ocean today, the ice shell has been thinning since the formation of Herschel, which could also explain the lack of fractures on Mimas. Mimas as an emerging ocean world places important constraints on the formation, evolution, and habitability of all of the mid‐sized moons of Saturn. Key Points: We performed numerical simulations of the formation of the Herschel impact basin to explore Mimas' evolutionThe Herschel impact basin could not have formed in an ice shell of the thickness indicated by Mimas' present‐day librationsHerschel's morphology and the lack of tectonics on Mimas are compatible with a thinning ice shell and geologically young ocean [ABSTRACT FROM AUTHOR]

Kamil crater (Egypt) is a natural laboratory for the study of processes and products associated with the impacts of small iron projectiles on the Earth's crust. In particular, because of the distinctive composition of the impactor (an ungrouped Ni-rich ataxite) and the target (Cretaceous sandstones and minor wackes) it offers a unique opportunity to study impactor–target physical–chemical interactions. Continuing the study of impact melt ejecta, we investigated the mineralogy and geochemistry of 25 Fe-Ni spherules - representative of a suite of 135 - recovered from the soil around the crater. Samples were collected during our 2010 geophysical expedition and investigated by combining scanning electron microscope imaging, electron probe microanalyzer and Raman spectroscopy analyses. Spherules range in size from 100 to 500 µm and show a variety of dendritic textures and mineral compositions dominated by Fe-Ni oxides of the wüstite – bunsenite and magnetite – trevorite series or Fe-Ni metal. All these features indicate quenching of high temperature (1600–1500 °C) oxide or metal liquid droplets under varying oxidizing conditions. A geochemical affinity with the iron impactor recorded by the Fe, Co, Ni ratios in the constituent phases (average Ni/Co element ratio of 25.1 ± 7.6; average Ni/(Ni + Fe) molar ratio of 0.21 ± 0.13), combined with target contamination (i.e., the ubiquitous occurrence of Si and Al from trace to minor amounts), document their origin as impact melt spherules formed through the physical and chemical interaction between metal projectile and silicate target melts and air. We propose a petrogenetic model that envisions formation as liquid droplet residues of immiscible projectile in a mixed silicate melt and their subsequent separation as individual spherules by stripping during hypervelocity ejection. We also argue that this model applies to all impact events produced by small iron projectiles and that such individual Fe-Ni oxide and metal spherules should be common impact products, despite little documentation in the literature. Our detailed mineralogical and geochemical characterization will facilitate their distinction from other, similar spherules of different origin (cosmic spherules, ablation spherules) often encountered in the geologic record. [ABSTRACT FROM AUTHOR]

We conducted impact experiments using targets composed of particles with size distributions and projectiles with a size larger than or comparable with the maximum size of particles in targets. The pattern and particle concentration in the ejecta curtain were investigated. The results show three types of ejecta curtain features: (i) filament pattern extending throughout the entire curtain and high concentration, (ii) filament pattern and low concentration, and (iii) mesh-like pattern with a structure on smaller scales than the entire curtain and low concentration. When the target consists of particles using a bimodal size distribution with size differences of more than one order of magnitude, the filament pattern appears, exhibiting case (i). If the target consists of particles with various sizes with size differences of more than one order of magnitude, the filament pattern appears, but the concentration decreases, appearing the features of case (ii). Case (iii) occurs when the target consists of particles with a single size or when the mass of particles with a certain size is dominant. Thus, the size distribution of the particles in the targets determines the pattern and particle concentration in the ejecta curtain. Based on these results, we confirm that the pattern in the ejecta curtain caused by the impact of the Small Carry-on Impactor (SCI) in the Hayabusa2 mission showing case (i) is consistent with the evaluated sizes and masses of grains and boulders in the ejecta curtain. [ABSTRACT FROM AUTHOR]

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. • A NE-directed oblique impact produced Theophilus crater, asymmetric distribution of secondary craters and impact melt ponds • Theophilus excavated a thin layer of high-Al olivine mare basalt underlain by Cyrillus ejecta and highland anorthositic rocks • An east-southeast directed oblique impact produced Shioli crater and its asymmetrically distributed ejecta and spall boulders • A hypervelocity impact (16 km/s) is responsible for the observed size and shape variation in the Shioli crater spall boulders • The Shioli boulders are mixtures of olivine basalt and highland materials, produced from Theophilus ejecta boulders [ABSTRACT FROM AUTHOR]

• Mimas shows a similar equatorially concentrated population of elliptical craters as observed on Tethys and Dione, suggesting a common origin or process. • Mimas has an additional population of north-south oriented elliptical craters at high latitudes. • Emplacement of the Herschel impact basin remains likely the most recent major geologic event in Mimas' history. • Mimas' regional crater distributions are well-matched by the Triton-based production function. Craters on the mid-sized moons of Saturn display different size-frequency distributions than those on the Jovian moons or at Pluto, indicating that material originating within the Saturn system likely played a key role in cratering the moons' surfaces. A challenge of so-called planetocentric debris is that little is known about the timing, extent, and distribution of the impacting material, adding large uncertainty to model-based surface ages derived from crater counts. By characterizing the planetocentric size-frequency distribution via the resulting craters, appropriate chronologies for dating Saturn's moons can be developed. Toward that end, we present a near-global map of elliptical craters on Mimas to further characterize the impactor populations that generated elliptical craters throughout the Saturn system. We also present crater counts within higher-resolution Cassini imagery to support continued efforts to date Mimas and refine our knowledge of cratering on the Saturnian satellites. Similar crater mapping studies have been conducted for Tethys and Dione, to which we compare Mimas' crater distributions. We find that Mimas' surface appears to be younger than the oldest regions of Tethys and Dione, and that the impactors that generated the extensive population of elliptical craters on Tethys and Dione likely affected Mimas as well. However, we identify an additional set of elliptical craters, so far found only on Mimas. We also find significant differences in the crater populations across the mid-sized moons, which affect the interpretation of crater-based relative ages, consistent with past work. While there are still considerable uncertainties, we find that Mimas' surface age is likely within a factor of two younger than the oldest terrains on Tethys and Dione, so Mimas is not the oldest surface among the mid-sized moons. However, Mimas' surface is close enough in age to those on Tethys and Dione to all but rule out it being only ∼100 Myr whilst the others are of order 1 Gyr old. [ABSTRACT FROM AUTHOR]

The Earth/Moon system likely results from a giant impact between a Mars-size object and the proto-Earth. This high-energy impact leads to extreme conditions under which volatile elements would not normally be preserved in the protolunar disk. However, recent measurements of lunar samples highlight the presence of a non-negligible amount of water in the Moon's interior (from 1.2 to 74 ppm). The aim of the present work is to quantify the water contribution of the late accretion on the early Moon. Here, we use a 2D axisymmetric model with the hydrocode iSALE-Dellen to study the fate of a large impactor on a target body similar to the early Moon with a crust, a magma ocean, and a mantle. For this purpose, we compute different models to monitor the depth to which the impacted material is buried at the end of the impact event and the degree of devolatilisation of the impactor. Three parameters are explored: the crustal thickness (ranging from 10 to 80 km), the impactor radius (ranging from 25 to 200 km) and the impactor velocity (ranging from 1 to 4 times the target escape velocity). Our models show that impactors with a radius greater than 25 km impacting a partially molten lunar body with a crust thinner than 40 km could significantly contribute to the water content of the lunar mantle even for impact velocities close to the lunar escape velocity. For impact velocities greater than 3 times the target escape velocity, the impactor material is significantly molten and its water content is devolatilised within the lunar atmosphere. Depending on the water content of the impactor material and the ability of the lunar magma ocean to maintain chemical heterogeneities, the late lunar accretion following the Moon-forming giant impact could explain the differences in water content among the lunar samples. • We model the ability of a large impactor to penetrate within a lunar magma ocean. • We use iSALE Dellen 2D axi-symmetrical hydrocode models. • We show that impactors larger than 25 km can penetrate a 40 km thick lunar crust. • Low velocity impactors melt and dehydrate within the deep lunar magma ocean. • Late accretion can contribute to the lunar mantle water content by 1–10 ppm. [ABSTRACT FROM AUTHOR]

Evidence from Arrokoth and comets strongly suggests a very low density for this and similar small Kuiper belt objects. Plausible compositions imply high porosities, in excess of 70%, and low compaction crush strengths. If so, impact craters on Arrokoth (especially Sky, its largest) formed largely by compaction of pore space and material displacement. This is consistent with geological evidence from New Horizons imaging. High porosity reduces cratering efficiency in the gravity regime whereas compaction moves it toward crush strength scaling and increased efficiency. Compaction also guarantees that most impactor kinetic energy is taken up as waste heat near the impact point, with momentum transferred to the rest of the body by elastic waves only. Monte Carlo simulations of Sky‐forming conditions indicate that the momentum imparted likely separated Arrokoth's two lobes, but displacement was limited by dissipation at the neck between them. Unusual strength properties are not required to preserve Arrokoth's bilobate configuration. Plain Language Summary: It has become apparent over the last few years that small asteroids and comets are very underdense compared with the materials they are made of. This means that their total porosities are likely quite high, in excess of 70%, both as tiny voids within particles (so‐called microscopic porosity) and spaces between particles (macroscopic porosity). But none are likely as porous as the distant denizens of the Kuiper belt such as Arrokoth (visited by the New Horizons spacecraft in 2019). This paper concerns impact craters on Arrokoth and similar small bodies, and the rather unusual effects expected. Imagine a fluffy (fine powder) snowball striking a much larger fluffy snowball, only that the snow is not pure ice but a mixture of porous icy, rocky, and carbon‐rich particles. Even at high velocities (>100 s of m/s) craters should mostly form by compacting pore space and pushing material away from the impact point, not the traditional blasting of ejecta back into space. Similar to crush‐up of an automobile bumper, compaction helps to protect from the potentially catastrophic effects of large impacts, such as complete disruption of the target or breakup of bilobate bodies like Arrokoth, and should be incorporated in future collisional evolution studies. Key Points: Arrokoth is likely a low density, highly porous, contact binary planetesimalImpact craters on Arrokoth, and particularly its largest, likely formed as compaction craters, with modest or little ejectaHigh porosity acted to protect Arrokoth from catastrophic disruption, ejecta recoil, and lobe dislocation and rearrangement [ABSTRACT FROM AUTHOR]

Recent dynamical modeling of the formation and evolution of the Saturnian satellites suggests that the ages of the mid‐sized inner moons (Mimas, Enceladus, Tethys, Dione, and Rhea) could be as young as 100 Myr. This estimate is in contrast to most previous modeling and observational work that suggest an age more contemporaneous with the formation of Saturn 4.5 Ga ago. Given the heritage of using craters to constrain surface ages, we examine the impact craters of Dione using imagery from NASA's Cassini ISS camera and analyze their size‐frequency distributions (SFDs) to understand impactor populations. We survey four areas across different geologic terrains and compare our crater counts to standard outer solar system production functions. In addition to crater counts, we study several crater types such as elliptical and polygonal to further examine the bombardment source for the craters. We find evidence for a Saturn‐specific planetocentric impactor source, as none of the standard production functions fit the data. We compare our Dione data with our work on Tethys and find similarly shaped SFDs between the satellites. However, Dione's surface has been extensively modified to remove smaller craters (D ∼ <5 km) and has been bombarded by larger impactors, creating more D ∼ >20 km on Dione than Tethys. In contrast, Tethys more generally represents an ancient unmodified surface within the Saturn system. More complete observations and assessment of the cratering records on the satellites of Uranus and Neptune's moon Triton would enable better constraints on the bombardment history of the Saturn system. Plain Language Summary: NASA's Cassini mission left a major unresolved question: How old are the mid‐sized inner moons (Mimas, Enceladus, Tethys, Dione, Rhea) of Saturn? Models that form the moons and change their orbits around Saturn over time have suggested that these moons may be as young as 100 million years old. In contrast, previous work inferred these moons formed at the same time as Saturn (age ∼4.5 Ga), or later in Saturn's history but still long ago (ages between 4.5 and 1 Ga). Because impact cratering is a common process throughout solar system history, crater distributions can provide additional insight as to the formation of these moon. However, we are missing a piece of information that is critical to interpreting craters: the source and frequency of objects that hit the moons over time. Here, we mapped 4002 impact craters on Saturn's moon Dione over four different areas of the surface. We analyzed the crater density versus crater size (i.e., size‐frequency distributions) for each region and compared them to predictions for what a surface in the outer solar system should look like when hit with Sun‐orbiting material. Our comparison indicates that our Dione data doesn't match previous predictions, which we interpret as showing that Saturn‐orbiting material is creating at least some craters. We find that the surfaces of Tethys and Dione have similar distributions of craters, but that previously active processes on Dione have erased some of the smaller craters. This information improves our ability to generate better predictions of the size and frequency of impacting material that can help future researchers date the moons. Key Points: We propose a local Saturnian impactor source as the primary component in creating the craters observed on the surface of DioneLack of craters with diameters of 1–4 km in all mapped areas implies resurfacing mechanisms acting on all mapped terrainsAnalysis of crater types shows trends in elliptical crater azimuths and evidence for more polygonal craters outside the wispy terrain [ABSTRACT FROM AUTHOR]

The prototype for investigations of formation mechanisms and related geological effects of large impact basins on planetary bodies has been the Orientale basin on the Moon. Its widespread secondaries, light plains, and near-rim melt flows have been well mapped in previous studies. Flow features are also widely associated with secondaries on planetary bodies, but their physical properties are not well constrained. The nature of flow features associated with large impact basins are critically important to understand the emplacement process of basin ejecta, which is one of the most fundamental processes in shaping the shallow crusts of planetary bodies. Here we use multisource remote sensing data to constrain the physical properties of flow features formed by the secondaries of the Orientale basin. The results suggest that such flows are dominated by centimeter-scale fine debris fines; larger boulders are not abundant. The shattering of target materials during the excavation of the Orientale basin, landing impact of ejecta that formed the secondaries, and grain comminution within the flows have substantially reduced particle sizes, forming the fine flows. The discovery of global-wide fine debris flows formed by large impact basins has profound implications to the interpretation of both previously-returned samples and remote sensing data.

The third stage of the Saturn IV rocket used in the five Apollo missions made craters on the Moon ∼30 m in diameter. Their initial impact conditions were known, so they can be considered controlled impacts. Here, we used the iSALE‐2D shock physics code to numerically simulate the formation of these craters, and to calculate the vertical component of seismic moment (∼4 × 1010 Nm) and seismic efficiency (∼10−6) associated with these impacts. The irregular booster shape likely caused the irregular crater morphology observed. To investigate this, we modeled six projectile geometries, with footprint area between 3 and 105 m2, keeping the mass and velocity of the impactor constant. We showed that the crater depth and diameter decreased as the footprint area increased. The central mound observed in lunar impact sites could be a result of layering of the target and/or low density of the projectile. Understanding seismic signatures from impact events is important for planetary seismology. Calculating seismic parameters and validating them against controlled experiments in a planetary setting will help us understand the seismic data received, not only from the Moon, but also from the InSight Mission on Mars and future seismic missions. Plain Language Summary: Observations of meteoroid strikes on the Moon, including artificial impacts made by the Saturn boosters from the Apollo missions, present valuable information for connecting impact conditions with seismic properties of the lunar and planetary crusts. These artificial impacts on the Moon were made by irregularly shaped spacecraft with low density. We numerically simulated these artificial impacts, using several different projectile geometries in order to represent the spacecraft's low density and shape. We found that the projectile representation affected the crater size. We calculated the amount of energy transferred into seismic waves and seismic moment created in these impacts to be consistent with previous studies. These parameters did not change with different projectile representations, but were affected by the material properties of the impact site. The aim is to use the results from the controlled lunar impacts to help understand the seismic signatures of impacts on Mars. Key Points: Numerical impact simulations of the Apollo rocket stage drop on the MoonProjectile geometry affects cratering process, cratering efficiency, and momentum transferEstimates of the seismic moment and seismic efficiency in these impacts [ABSTRACT FROM AUTHOR]

Understanding the morphological characteristics of craters that are indicative of their formation environment can provide insight into surface geology. Layered ejecta (LE) craters, found on Mars and some other planetary bodies, have been hypothesized to have formed as a result of either interaction with subsurface volatiles (volatile fluidization model) and/or with the atmosphere (atmospheric entrainment model). Formation of LE craters by either model should result in different grain size distributions throughout the ejecta deposit. Using thermal inertia to infer surface properties, we investigated LE craters and their ejecta deposits in an effort to distinguish between possible LE formation processes on Mars. We used thermophysical properties of crater ejecta to determine grain size distribution, to model horizontal mixtures and vertical layering, and to identify materials present within the ejecta. We assessed the thermal properties of 50 uniformly sampled LE craters using Mars Odyssey Thermal Emission Imaging System (THEMIS) and Mars Global Surveyor Thermal Emission Spectrometer (TES) data. Our THEMIS analysis identifies 12 craters with grain size distributions consistent with the volatile fluidization model, 3 craters that have characteristics potentially associated with either model, and 22 craters that do not exhibit characteristics matching either model. Our TES analysis identifies 11 craters with characteristics consistent with the volatile fluidization model and 8 craters that are consistent with the atmospheric entrainment model. While some observations of grain size distributions provide evidence for either or both models, there is not overwhelming support for either model, potentially due to uncertainties of derived thermal inertia data. Plain Language Summary: Impact craters are found throughout the solar system and can tell us about the geology around them. A specific type of crater, layered ejecta (LE) craters, may help us identify underground ice on Mars and other planetary bodies. LE craters are hypothesized to form when the energy of an impactor causes sub‐surface ice to become fluid or gas and flow outward. This hypothesis requires the presence of sub‐surface ice, and if it was found to be true, then identifying LE craters could be used to identify the locations of subsurface ice and water in the solar system. LE craters are identified by their ejecta deposits, which is the material that resettles around a crater following an impact. Grain size distributions throughout the ejecta deposits can tell us about their formation. We examined the grain size distribution of ejecta deposits surrounding 50 LE craters on Mars and looked at their thermal inertia, which is how well material holds onto heat, where smaller sand particles release their heat more quickly than large particles like boulders. Overall, the observed grain size distributions and thermal properties do not provide overwhelming support for the hypothesis that requires sub‐surface ice and thus further research is warranted. Key Points: The formation process of Martian layered ejecta (LE) craters could have important implications for the presence of subsurface volatilesIn a sampling of 50 LE craters, we find no distinct relationship between crater type and trends in apparent thermal inertiaResults provide no overwhelming support for either the atmospheric entrainment or the volatile fluidization model for LE crater formation [ABSTRACT FROM AUTHOR]

Impact basins on the Moon can serve as a benchmark for timing and intensity of the impact flux in the inner solar system. The basin morphology and morphometry depend on impactor size, mass, and velocity as well as the thermal state of the lunar lithosphere which is a function of the cooling history. Erosion by superimposed impact bombardment has altered the surface expression of basin structures over time, making it difficult to determine the size unequivocally solely based on topographic expression. The gravity signature of basins is thought to be a less altered measure of the size of impact structures. By a systematic study of basin formation using the iSALE‐2D shock‐physics code, we investigate the influence of the lunar thermal state and different impactor properties on the transient crater and final basin size and on the resulting gravity anomaly. As constraints we use gravity data of 16 farside basins and their assumed formation ages to estimate the subsurface temperature related to the cooling history of the Moon. Our modeling results confirm that the thermal state affects the basin formation process and the basin sizes significantly. We provide quantitative relationships between the observed gravity signal, the different basin sizes, and the impactor diameter considering the thermal state of the Moon upon impact, which correlates with the formation ages or periods in the literature. Our study allows for estimating the impactor size from the observed gravity field if the formation age and, thus, the thermal state of the lithosphere is approximately known. Plain Language Summary: Large basin structures testify to the lunar bombardment history by bodies several tens to hundreds of kilometers in diameter. To reconstruct how often the Moon, and thus also Earth, was hit by large cosmic bodies of a given size, it is essential to relate the size of basin structures with impactor size. However, since the surface expression of most known basins on the Moon is not pristine, the definition of the actual basin size is often ambiguous. As an alternative, we use the observed pronounced positive gravity excursion across lunar basins as a measure of basin size. In addition, we consider the thermal evolution of the Moon which is thought to affect the characteristics of the gravity signature of basins. Therefore, the timing when a given basin was formed in the cooling history of the Moon plays an important role. We use the gravity signature and the lunar thermal state as constraints for modeling the formation of the lunar basins as a function of impactor size. Our results allow for estimating the impactor size and thermal state of the Moon for 16 lunar farside basins. The predicted thermal states are in line with proposed absolute ages of the basins. Key Points: We provide relationships between impactor size, gravity signal, and farside basin size as a function of cooling history and formation timeGravity anomalies serve as a measure of basin size as impacts altered morphology and crustal thickness models simplify subsurface structureOur basin formation models agree with the assumed formation time based on crater chronology and the estimated cooling history of the Moon [ABSTRACT FROM AUTHOR]

Abstract The third stage of the Saturn IV rocket used in the five Apollo missions made craters on the Moon ∼30 m in diameter. Their initial impact conditions were known, so they can be considered controlled impacts. Here, we used the iSALE‐2D shock physics code to numerically simulate the formation of these craters, and to calculate the vertical component of seismic moment (∼4 × 1010 Nm) and seismic efficiency (∼10−6) associated with these impacts. The irregular booster shape likely caused the irregular crater morphology observed. To investigate this, we modeled six projectile geometries, with footprint area between 3 and 105 m2, keeping the mass and velocity of the impactor constant. We showed that the crater depth and diameter decreased as the footprint area increased. The central mound observed in lunar impact sites could be a result of layering of the target and/or low density of the projectile. Understanding seismic signatures from impact events is important for planetary seismology. Calculating seismic parameters and validating them against controlled experiments in a planetary setting will help us understand the seismic data received, not only from the Moon, but also from the InSight Mission on Mars and future seismic missions.

Core from Hole M0077 from IODP/ICDP Expedition 364 provides unprecedented evidence for the physical processes in effect during the interaction of impact melt with rock-debris-laden seawater, following a large meteorite impact into waters of the Yucatán shelf. Evidence for this interaction is based on petrographic, microstructural and chemical examination of the 46.37-m-thick impact melt rock sequence, which overlies shocked granitoid target rock of the peak ring of the Chicxulub impact structure. The melt rock sequence consists of two visually distinct phases, one is black and the other is green in colour. The black phase is aphanitic and trachyandesitic in composition and similar to melt rock from other sites within the impact structure. The green phase consists chiefly of clay minerals and sparitic calcite, which likely formed from a solidified water–rock debris mixture under hydrothermal conditions. We suggest that the layering and internal structure of the melt rock sequence resulted from a single process, i.e., violent contact of initially superheated silicate impact melt with the ocean resurge-induced water–rock mixture overriding the impact melt. Differences in density, temperature, viscosity, and velocity of this mixture and impact melt triggered Kelvin–Helmholtz and Rayleigh–Taylor instabilities at their phase boundary. As a consequence, shearing at the boundary perturbed and, thus, mingled both immiscible phases, and was accompanied by phreatomagmatic processes. These processes led to the brecciation at the top of the impact melt rock sequence. Quenching of this breccia by the seawater prevented reworking of the solidified breccia layers upon subsequent deposition of suevite. Solid-state deformation, notably in the uppermost brecciated impact melt rock layers, attests to long-term gravitational settling of the peak ring. [ABSTRACT FROM AUTHOR]

Mars exhibits diverse surface changes at all latitudes and all seasons. Active processes include impact cratering, aeolian sand and dust transport, a variety of slope processes, changes in polar ices, and diverse effects of seasonal CO2 frost. The extent of surface change has been surprising and indicates that the present climate is capable of reshaping the surface. Activity has important implications for the Amazonian history of Mars: understanding processes is a necessary step before we can understand their implications and variations over time. Plain Language Summary: Many surface processes are active across the surface of Mars, producing changes that can be observed from orbit and by landed missions. The observed changes include new impact craters, sand and dust transported by the wind, a variety of flows on slopes, and many changes to perennial ice deposits. Many changes are driven by seasonal CO2 frost. These changes indicate that surface evolution on Mars is ongoing and not confined to past climate conditions. Key Points: Many different surface processes are actively shaping the geomorphology of Mars todayCurrent surface changes must be understood in order to correctly interpret changing environments over timeSeveral active Martian processes were not predicted and lack a terrestrial analog [ABSTRACT FROM AUTHOR]

Light plains on the Moon are distinctive smooth deposits formed by large impact basins. Featuring diverging morphology and frequent spatial association with large secondary craters, light plains were interpreted to be formed either by ground-hugging ejecta flows and/or debris flows triggered by landing of impact ejecta. Light plains located in closed depressions, term ponded light plains (PLPs) here, cover ∼3% of the lunar global surface. They lack diagnostic geomorphology that could be ascribed to secondary impacts or primary ejecta, suggesting a possibly different emplacement mechanism, which has not been investigated before. PLPs feature comparable morphology with both lunar mare that were mainly emplaced by effusive volcanism and large impact melt pools that were deposited by melt-rich ejecta in rims and terraces of large impact basins. For such smooth plain materials, topography roughness at small scales might be an indicative information for their different emplacement styles. This study analyses topography roughness for PLPs and similar-sized melt pools that were formed by the Orientale basin and also coeval mare units on the Moon. We find that mare units have smaller roughness than same-aged melt pools, because large entrained blocks are abundant in melt pools that likely yielded larger viscosity. We further notice that PLPs have a medium topography roughness between coeval mare units and large melt pools, and the topography roughness of PLPs is closer to that of mare units. Correlated observations using high-resolution images suggest that the smoother topography of PLPs than cogenetic melt pools are mainly due to the minor existence of entrained blocks in PLPs. This comparison suggests that while PLPs and melt pools are both basin-related smooth ejecta deposits, PLPs may feature a lower viscosity during the emplacement, and they are most consistent with being melt-rich primary ballistic ejecta that were accumulated in local depressions. Considering that the youngest multiple-ring basin on the Moon, the Orientale basin, is capable of forming numerous PLPs across most parts of the entire Moon, this work predicts that both disrupted and sandwiched melt-rich ejecta deposits should be widespread in the lunar highland crust due to earlier-formed impact basins. • Light plains and melt pools formed by the Orientale basin and contemporaneous mare units are compared. • Small-scale roughness of light plains in closed depressions is closer to that of mare units than melt pools. • Light-plains in closed depressions were emplaced by filling of low-viscosity melt-rich primary ejecta. [ABSTRACT FROM AUTHOR]

The Lidang circular structure in the center of the Guangxi Province is about 8 km in diameter. This structure appears as an abnormal shallow depression that has disturbed the rather harmonic regional joint systems. Its unique occurrence in the whole region, the circular morphology, negative topography, and the spatial distribution of interior and exterior strata are all consistent with those of impact craters that are formed by asteroidal or cometary collision. To test the impact hypothesis, we carried out both field investigation and remote sensing study of this structure. Regional geological history suggests that if the impact hypothesis were correct, the impact event should have occurred at or after the Early Permian. Field investigation found that the strata inside and outside the crater are dominated by parallel stacks of Lower and Upper Permian limestone that have various thicknesses and different mud contents. The layers of limestone within and outside the circular structure have identical attitudes; no structural disturbances were visible in the outcrops. Field investigations provide conclusive evidence against the impact cratering hypothesis. A high-resolution digital elevation model shows that the spatial distribution of rounded mountains within the structure is controlled by faint but continual extension of joints, suggesting that the crater interior has gone through a much higher degree of erosion. Therefore, regional joints that had once existed within the crater are preserved less well than exterior terrains, forming the abruptly disrupted circular depression. Differential erosion, as the possible formation mechanism of the Lidang structure, is consistent with the different mud contents found between the interior and exterior limestone. The circular outline of this structure may correspond to the shape of the original deposition basin. In conclusion, the Lidang circular structure is a polje formed by karstification, not an astrobleme.

Knowledge of the shock remanent magnetization (SRM) property is crucial to interpret the spatial changes in magnetic anomalies observed over the impact crater. This study reports the spatial distributions of SRM intensity and stability of single‐domain titanomagnetite‐bearing basalt based on the SRM acquisition experiments using a two‐stage light gas gun with Al projectiles, remanence measurements for divided subsamples, and impact simulations. The SRM properties systematically change with increasing pressure, and three distinctive aspects are recognized at different pressure ranges: (1) constant intensity below 0.1 GPa, (2) linear trend as intensity is proportional to pressure up to 1.1 GPa, and (3) constant intensity and increasing stability above 1.9 GPa. The SRM intensity and stability distributions suggest that the crustal rocks containing the single‐domain titanomagnetite originally had an SRM intensity distribution according to the distance from the impact point, which changed depending on the remanence stability after the impact. Plain Language Summary: Shock remanent magnetization (SRM) is acquired as a result of the shock wave propagation in a magnetic field. Knowledge of a three‐dimensional distribution of the SRM intensity and stability is crucial for interpreting the spatial change in magnetic anomalies observed over the impact crater and reconstructing the paleo‐planetary field based on the anomaly data. Here, we report the SRM intensity and stability structures of fine‐grained single‐domain titanomagnetite‐bearing basalt based on the newly designed study protocol such as the SRM acquisition experiments, remanence measurements for divided subsamples, and impact simulations. The SRM properties systematically change with increasing pressure, and three distinctive aspects are recognized at different pressure ranges. The systematic changes in the SRM intensity and stability suggest that the crustal rocks containing the single‐domain titanomagnetite originally had an SRM intensity structure at the time of impact, and this structure changed depending on the remanence stability after the impact. Key Points: The shock remanent magnetization (SRM) acquisition experiments and impact simulations were conducted to reveal the SRM intensity and stability distributionsThe SRM properties systematically change with pressure, and three distinctive aspects are recognized at different pressure rangesThe SRM intensity varied with the distance from the impact point, which changed depending on the remanence stability after the impact [ABSTRACT FROM AUTHOR]