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A series of small-scale impact cratering experiments were performed on basalt at velocities of 3.5–6.0 km s−1. The formation and motion process of ejecta were observed by high-speed and ultra-high-speed cameras. The momentum transfer coefficient values and cratering size were measured. Hydrodynamic simulations in 2D were carried out to replicate the impact conditions of the basalt and calibrate model parameters. The contribution of jet, central cratering, and spalling ejection to momentum enhancement was quantitatively analyzed by simulation. Simulation results show that: the mass and the momentum of the jet are less than 1 % of the projectile mass, and the initial momentum, can be ignored. For craters in small-sized basalt, the momentum enhancement mainly comes from the spall region ejecta. The formation mechanism of ejecta at different stages was analyzed based on the shock wave and jet theory. An analytical model is developed for estimating the velocity and motion of ejecta formation at different stages. The model calculation results of the speed of the ejecta formed in different stages are consistent with the numerical results. The maximum velocity of the ejecta formed in the jet and cratering stage depends on the initial impact velocity and the Hugoniot parameters of the projectile/target material. The minimum velocity of the spallation ejecta is linearly related to the spalling strength of the target. The model provides a fast and simple method to calculate the ejecta velocity and estimate the influence range of ejecta under different impact conditions. • The speed of the jet is greater than the initial impact speed. • The contribution of jet to momentum enhancement can be ignored. • For craters in small-sized, the momentum enhancement and mass mainly come from the spall region ejecta. • A model was developed for estimating the velocity and motion of ejecta. [ABSTRACT FROM AUTHOR]

The morphology of fresh lunar craters contains information about the physical properties of both the impactors and the lunar surface, and is therefore crucial to our knowledge of the impact cratering process. Spectral analysis is a powerful tool to study crater morphology, as it can reveal the topographic variation on different scales. In this study, we calculate the power spectral densities of the radial distance and elevation of the rim crest, floor, and rim flank outlines of fresh lunar craters. The resulting power spectral density can be decomposed into an average component and a natural variability component. For the average component, we derive the classic morphometric parameter‐crater diameter relations that are consistent with previous studies. For the natural variability component, we find that in general the spectral power increases with wavelength, which can be fitted by a piecewise function with four breakpoints. Among the four breakpoints, the power of the third breakpoint (i.e., the degree‐2 power) is of particular interest, as it determines the ellipticity of the outline. The power of the third breakpoint is found to have a diameter dependence with a peak at 20 km, which indicates that transitional craters are more elliptical than simple and complex craters. The diameter dependence of the power spectral density enables us to generate the synthetic outlines of a crater of a particular size, which can be used to develop a preliminary 3‐dimensional shape model for fresh lunar craters that is useful for improving Monte Carlo modeling of cratered surfaces on the Moon. Plain Language Summary: The shape of young impact craters on the Moon can tell us information about the impactor and the lunar surface, which further helps us to better understand how craters are formed on a planetary body. Unlike previous studies that investigated the overall shape of the crater in the spatial domain, in this study we look into more detailed variations of the crater shape in the frequency domain. For our selected young lunar craters, we first extract their rim crest, floor, and rim flank outlines, which are the three key components that make up a lunar crater. For the obtained outlines, we then determine on average how wide and deep they are, and also analyze how variable they are on smaller scales. The correlation between the crater shape and the crater size is investigated next, and we find that the outlines are more elongated for craters with diameters around 20 km. By reconstructing the variability of the outlines, we are able to develop a preliminary, synthetic 3‐dimensional shape model for young lunar craters, which can be used to model the evolution of the lunar surface. Key Points: The rim crest, floor, and rim flank outlines of fresh lunar craters are vectorized in the elevation dataThe radial distance and elevation of the outlines are linked to surface topography via their power spectral densitiesThe degree‐2 power points to the ellipticity of the outline, and transitional craters are more elliptical than simple and complex craters [ABSTRACT FROM AUTHOR]

We investigated the age and origin of 41 light plain deposits around the lunar south pole, using LRO data sets and successfully dated 22 of them using crater size‐frequency distribution measurements. We find that deposits related to Schrödinger basin are ∼3.8 Ga old, including deposits on the Amundsen crater floor, which we interpret as Schrödinger basin ejecta. Six study areas date to ∼3.7 Ga, which reflect an additional large‐scale or basin impact event post‐Schrödinger ‐ possibly Orientale. An additional two areas with younger isolated ages likely represent local impact sources. Finally, smooth, light plains‐like, ejecta deposits around Shackleton crater were found to be Eratosthenian in age. Our findings show that light plain deposits originate from a combination of both basin and local ejecta materials. Thus, samples collected in the region will exhibit a diverse range of basin materials and ages representative of the ejecta from multiple impact events. Plain Language Summary: The southern polar region of the Moon is the focus of future human exploration missions due to the presence of essential resources like water and the possible capability to sustain a prolonged human presence on the Moon. Thus, it is vital to understand the geologic context of the region for a correct interpretation of returned samples. In this paper, we used the Lunar Reconnaissance Orbiter data sets to investigate the age and origin of 41 smooth deposits around the lunar south pole. Of these, 22 could be accurately dated using the size‐frequency distribution of craters, and we found groups of smooth deposits of similar ages that can be related to a one or more source impacts. Some of them are related to the Schrödinger basin impact, while others are likely related to the Orientale basin on the farside of the Moon. We also found two deposits of younger age, likely related to fresh nearby craters, and other ejecta deposits that represent the formation age of the Shackleton crater. Therefore, these deposits appear to have formed from both basin/crater ejecta and local materials. Samples collected in this region will exhibit a diverse range of materials and sources from multiple events. Key Points: Forty‐one light plain deposits in the lunar southern polar region are investigated for origin and absolute model age determinationSchrödinger basin materials date to ∼3.8 Ga, Orientale materials to ∼3.7 Ga and Shackleton impact to ∼2.4 GaMultiple impact events and local sources contributed to light plain deposits across the lunar south pole [ABSTRACT FROM AUTHOR]

Quantitatively evaluating the debris cloud-induced pitting damage in the AL-Whipple structure of spacecraft, is still a challenge, let alone microstructural plastic deformation usually accompanied by geometric cratering defects, which may decrease material stress-carrying capacity and structural life. With this motivation, a dedicated modelling technique is proposed to gain an insight into various modalities (i.e., geometric and plastic defects) of pitting damage-induced thermal modulation for infrared inspection. Then, a sequence of infrared images is obtained and analysed using the gap smoothing algorithm and naive Bayes classification methods. On this basis, a quantitative characterization framework is proposed to characterize the damage modalities and their morphologies. This method is validated using a 3D numerical model which contains pitting damage acquired from the real-world structure. Modality classification and morphology evaluation results are well consistent with the artificial defects, in terms of the modality, diameter, and depth. Then they‘re experimentally corroborated, in which the severity and distribution of pitting damage are precisely characterized, in terms of the crater number and morphologies, as well as the size of plastic regions around these craters, which are consistent with the actual observations. This work proposes a non-destructive strategy for pitting damage evaluation, which is engineering-friendly. [ABSTRACT FROM AUTHOR]

Surface unloading due to impact cratering results in lava filling the crater floor. Elevation differences in the crater floor, a common geological phenomenon on the Moon, represent direct evidence of cratering processes. However, few studies have been conducted on mare-filled craters on the Moon. Al-Biruni (81 km) is a farside impact crater with an inclined topographic profile on its floor. We quantitatively measure the morphology of Al-Biruni and model the basaltic lava emplacement to depict the cratering process. Differential subsidence due to melt cooling, wall collapse, impact conditions and structural failure were assessed as potential factors influencing the formation of the elevation differences on the floor. The results suggest that pre-impact topography is a plausible cause of the differences in floor elevation within Al-Biruni. Other factors may also play a role in this process, affecting lava flow by altering the topography of the crater floor after the impact. Thus, regardless of whether the lava inside the crater is impact-generated or comes from outside the crater, altering topography at different stages of the cratering process is an essential factor in creating the sloped terrain on the crater floor. [ABSTRACT FROM AUTHOR]

Rock abundances on the Moon represent both an opportunity to understand the history of the surface and of the regolith and a hazard to lander missions. While rock erasure by meteoroid bombardment is known to modify rock size˗frequency distributions, the interplay between rock erasure and rock exposure by impact cratering, and the resulting net rock abundance, is not known. Leveraging a coupling between modeling and optical imagery from the lunar orbit, we calculate new rock lifetimes that consider the specific shattering energy and the fragments produced by boulder shattering. We find differences between the estimated and expected specific shattering energy (Qs*), likely suggesting incomplete understanding of the scaling of the shattering energy with velocity and size. We find that the decrease in rock abundances with time on crater ejecta occurs faster than previous estimates based on thermal infrared data. Plain Language Summary: The surface of the Moon is littered with floating rocks ranging in size from centimeters to decameters. These rootless rocks are formed by impact cratering and are reduced to fine grained soil through fragmentation by meteoroid bombardment. Here we combine a model of rock formation and fragmentation with measurements of rock sizes extracted from images obtained from an orbiting spacecraft. We look at the effects of repeated fragmentation events. This gives us a better idea of the energy required for rock erasure as well as the abundance of centimeter‐scale rocks. We obtain an improved estimate of the time it takes for rocks to produce soil around impact craters. Key Points: The specific shattering energy of lunar boulders is estimatedThe decrease in rock abundance with time is modeled around impact cratersThe rock size‐frequency distribution covering the decameter to centimeter scale is modeled [ABSTRACT FROM AUTHOR]

Linear gravity anomalies (LGAs) on the Moon have been interpreted as ancient magmatic intrusions formed during the lunar expansion. The composition of such ancient subsurface intrusions may offer hints for the lunar thermodynamic state in the initial stage of lunar history. To pose a first compositional constraint on magmatism related to lunar expansion, this study analyzed the spectrum and gravity around craters on LGAs, such as Rowland, Roche, and Edison craters. Using reflectance spectra around the craters, we first surveyed non‐mare basaltic exposures. To test the LGA excavation scenario as a possible origin of the discovered exposures, we then compared the Gravity Recovery and Interior Laboratory data and post‐cratering gravity simulation with the iSALE shock physics code. Our spectral analysis reveals no basaltic exposure around the Rowland crater. Further, the observed termination of LGA at the crater rim contradicts the gravity simulation, which assumes that LGA predates the Rowland crater. These results suggest that LGA formation might postdate the Rowland formation and that lunar expansion lasted even after the Nectarian age. On the other hand, we found that both Roche and Edison craters possess basaltic exposures in their peripheries. Because the gravity reduction inside Roche crater can be reproduced in our simulation, the discovered basaltic exposures are possibly LGA materials ejected from these craters. The composition of those exposures shows that the LGA intrusions at the two locations are composed of low‐titanium magma, indicating that ancient magma during the expansion did not contain ilmenite‐rich melt, perhaps resulting from the low‐ilmenite content of the ancient upper mantle. Plain Language Summary: The Moon has positive, narrow, and long gravity anomalies, so‐called linear gravity anomalies (LGAs), which are thought to be magmatic intrusions formed during the ancient lunar expansion. The composition of these intrusions may provide insights into the early lunar thermal state. This study analyzed the reflectance spectra and gravity data around craters located on LGAs, specifically Rowland, Roche, and Edison craters, which are postulated to have ejected the LGA material. By analyzing the reflectance spectra, we first identified non‐mare basaltic exposures that likely originated from these subsurface formations. We next examine if these exposures are composed of the LGA materials by comparing the observed gravity with cratering simulation results. We found no basaltic exposures around Rowland and that gravity inside Rowland is not consistent with the excavation hypothesis, perhaps suggesting that LGA formation and lunar expansion occurred after the Rowland formation. On the other hand, we discovered basaltic exposures around both Roche and Edison craters, which can be interpreted as ejecta from the LGAs consistent with the gravity inside Roche crater. The composition of these exposures indicates that LGAs are composed of low‐titanium magma, which provides a new constraint on the ancient thermal state of the Moon. Key Points: High‐calcium pyroxene exposures and gravity reduction around Roche crater imply an excavation of lunar linear gravity anomaly materialsLunar subsurface dikes formed during the ancient expansion stage are estimated to be composed of low‐titanium magmaA linear gravity anomaly possibly postdates the Rowland crater, suggesting a potential prolongation of lunar expansion beyond the Nectarian age [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]

Room temperature drop hammer impact and compression after impact (CAI) experiments were conducted on carbon fiber–epoxy resin (CF/EP) composites to investigate the variation in impact load and absorbed energy, as well as to determine the residual compressive strength of CF/EP composites following impact damage. Industrial CT scanning was employed to observe the damage morphology after both impact and compression, aiding in the study of impact-damage and compression-failure mechanisms. The results indicate that, under the impact load, the surface of a CF/EP composite exhibits evident cratering as the impact energy increases, while cracks form along the length direction on the back surface. The residual compressive strength exhibits an inverse relationship with the impact energy. Impact damage occurring at an energy lower than 45 J results in end crushing during the compression of CF/EP composites, whereas energy exceeding 45 J leads to the formation of long cracks spanning the entire width of the specimen, primarily distributed symmetrically along the center of the specimen. [ABSTRACT FROM AUTHOR]

Saturn's mid-sized icy moons have complex relationships with Saturn's interior, the rings, and with each other, which can be expressed in their shapes, interiors, and geology. Observations of their physical states can, thus, provide important constraints on the ages and formation mechanism(s) of the moons, which in turn informs our understanding of the formation and evolution of Saturn and its rings. Here, we describe the cratering records of the mid-sized moons and the value and limitations of their use for constraining the histories of the moons. We also discuss observational constraints on the interior structures of the moons and geologically-derived inferences on their thermal budgets through time. Overall, the geologic records of the moons (with the exception of Mimas) include evidence of epochs of high heat flows, short- and long-lived subsurface oceans, extensional tectonics, and considerable cratering. Curiously, Mimas presents no clear evidence of an ocean within its surface geology, but its rotation and orbit indicate a present-day ocean. While the moons need not be primordial to produce the observed levels of interior evolution and geologic activity, there is likely a minimum age associated with their development that has yet to be determined. Uncertainties in the populations impacting the moons makes it challenging to further constrain their formation timeframes using craters, whereas the characteristics of their cores and other geologic inferences of their thermal evolutions may help narrow down their potential histories. Disruptive collisions may have also played an important role in the formation and evolution of Saturn's mid-sized moons, and even the rings of Saturn, although more sophisticated modeling is needed to determine the collision conditions that produce rings and moons that fit the observational constraints. Overall, the existence and physical characteristics of Saturn's mid-sized moons provide critical benchmarks for the development of formation theories. [ABSTRACT FROM AUTHOR]

Meteorite impacts have proved to be a significant source of seismic signal on the Moon, and have now been recorded on Mars by InSight seismometers. Understanding how impacts produce seismic signal is key to the interpretation of this unique data, and to improve their identification in continuous seismic records. Here, we use the seismic Representation Theorem, and particularly the stress glut theory, to model the seismic motion resulting from impact cratering. The source is described by equivalent forces, some resulting from the impactor momentum transfer, and others from the stress glut, which represents the mechanical effect of plasticity and non linear processes in the source region. We condense these equivalent forces into a point-source with a time-varying single force and nine-component moment tensor. This analytical representation bridges the gap between the complex dynamics of crater formation, and the linear point-source representation classically used in seismology. Using the multiphysics modelling software HOSS, we develop a method to compute the stress glut of an impact, and the associated point-source from hypervelocity impact simulations. For a vertical and an oblique impact at 1000 m s−1, we show that the moment tensor presents a significant deviatoric component. Hence, the source is not an ideal isotropic explosion contrary to previous assumptions, and draws closer to a double couple for the oblique impact. The contribution of the point force to the seismic signal appears negligible. We verify this model by comparing two signals: (1) HOSS is coupled to SPECFEM3D to propagate the near-source signal elastically to remote seismic stations; (2) the point-source model derived from the stress-glut theory is used to generate displacements at the same distance. The comparison shows that the point-source model is accurately simulating the low-frequency impact seismic waveform, and its seismic moment is in trend with Lunar and Martian impact data. High-frequencies discrepancies exist, which are partly related to finite-source effects, but might be further explained by the difference in mathematical framework between classical seismology and HOSS' numerical modelling. [ABSTRACT FROM AUTHOR]

Wetumpka impact structure is a Late Cretaceous, marine‐target impact crater of about 5 km diameter. The apparent crater rim is mostly made of crystalline local basement, and the apparent crater floor consists of a mixed sediments of target lithology. These sediments are the provenance of the crater‐filling impactite sands, overlying trans‐crater slide unit, and the capping polymict impact breccia deposit, often referred to by previous workers as "central polymict breccia." The unit has been known to contain elongated mega‐clasts of up to tens of meters in size. This study attempted to understand the mode of emplacement of this polymict breccia, which occurs in some places on the apparent crater floor and resembles a polymict proximal ejecta deposit. This work also reports the first documentation of rare, potential impact spherules in the polymict impact breccia, interpreted to be a part of distal ejecta. The presence of large, decimeter‐sized clasts in the breccia can be best explained by the movement of overturned rim flap forming part of proximal ejecta from the crater rim to the apparent crater floor during early modification stage of impact cratering. Our work highlights the bimodal clast size distribution of the polymict breccia, and so we propose that the term "mega‐clast‐bearing impact breccia" be used for this unit. We attribute a generally steep orientation of the decameter sized clasts to primary imbrication during emplacement. The emplacement of this breccia is interpreted as associated with the ejecta emplacement process that occurred before the return of marine resurge. [ABSTRACT FROM AUTHOR]

Seismic observations of impacts on Mars indicate a higher impact flux than previously measured. Using six confirmed seismic impact detections near the NASA InSight lander and two distant large impacts, we calculate appropriate scalings to compare these rates with lunar-based chronology models. We also update the impact rate from orbital observations using the most recent catalog of new craters on Mars. The snapshot of the current impact rate at Mars recorded seismically is higher than that found using orbital detections alone. The measured rates differ between a factor of 2 and 10, depending on the diameter, although the sample size of seismically detected impacts is small. The close timing of the two largest new impacts found on Mars in the past few decades indicates either a heightened impact rate or a low-probability temporal coincidence, perhaps representing recent fragmentation of a parent body. We conclude that seismic methods of detecting current impacts offer a more complete dataset than orbital imaging. [ABSTRACT FROM AUTHOR]

This paper considers how space missions that fly through the plumes known, or suspected, to erupt naturally from some icy ocean worlds (IOW), such as Enceladus, or that aim to intercept icy ejecta from impact cratering processes on such bodies can sample the water and ice within the plumes. The mechanics of how grains (either in the plumes or the ejecta) would interact with a passing spacecraft (i.e., impact speeds, shock pressures, etc.) are introduced. The impact speeds are estimated and vary with both the mass of the IOW and the orbital parameters of a space mission. This can lead to large differences in impact speeds (and hence collection methods) at bodies such as Enceladus and Europa. The implications of these different impact speeds (a few hundred m s−1 to several km s−1, and even greater than 10 km s−1) for the collection of organic materials from the plumes are shown to be significant. [ABSTRACT FROM AUTHOR]

Plume-surface interactions (PSI) occur during the take-off and landing of interplanetary vehicles, leading to particle ejection and the formation of craters. This can be detrimental to the vehicle and any structures or infrastructure near the landing site. A major challenge in developing a comprehensive understanding of this three-dimensional phenomenon is the need to characterize the ejecta and cratering dynamics simultaneously. Here, experiments are conducted in a vacuum chamber at different nozzle heights and ambient pressure conditions using high-speed stereo-photogrammetry and planar particle tracking velocimetry to quantify the cratering and ejecta dynamics. Predictably, it was observed that the trajectory of ejecta with a large Stokes number was mostly unaffected by the nozzle flow after leaving the crater. Under rarefied conditions, the ejecta kinematics (velocity, ejection angle, range, and height) were significantly different compared to continuum conditions. Finally, the findings demonstrate a dependency between ejecta kinematics and crater topology for the current test cases, providing critical insights into particle ejection's initial characteristics. • Simultaneous cratering and ejecta measurements during plume-surface interaction. • Ambient pressure and nozzle height significantly affected crater and ejecta behavior. • Changes in the ejecta kinematics correlated with the changes in the crater shape. [ABSTRACT FROM AUTHOR]

Large asteroid impacts have long been attributed to imparting long‐lived heat in the upper crust, as well as widespread fracturing and porosity. When an impact occurs on or near an ocean environment, displaced water rushes back filling the crater and percolating down into the fractured crust. The significant heat and fracturing from impact allow for extensive hydrothermal activity. Hydrothermal alteration has been observed at the 180 km Chicxulub impact structure in Yucatán, Mexico. Previous studies estimate widespread hydrothermal transport and activity within the Chicxulub structure, but these processes are largely dependent on the assumed post‐impact permeability distribution. In this work, we present an impact simulation that tracks fragmentation as well as the generation of porosity and permeability during the Chicxulub impact event. The generation of porosity during tensile failure results in final porosities (up to 40%) that are up to a factor of 4 higher than previous models and in better agreement with drill core data. We find that both fragmentation and porosity contribute to overall permeability of the Chicxulub structure. Estimated permeabilities (up to 10−14 m2) are greater than measured values from drill cores because of the contribution from large‐scale fragmentation, which cannot be resolved in cm‐diameter drill cores. The larger porosities and permeabilities computed in this work suggest that the volume of hydrothermal activity generated by Chicxulub were 10 times more than previously estimated for Chicxulub and 100 times more than the Yellowstone caldera. Plain Language Summary: The Chicxulub impact event that led to the extinction of the dinosaurs 66 million years ago hit the Earth on a carbonate platform leaving a 180 km wide impact crater onto what is now Yucatán, Mexico. The impact caused widespread fracturing of the carbonate crust and underlying granite. The extent of this fracturing and what effect it had on hydrothermal activity is of great interest. Several drill cores have been unearthed from within the crater by scientific expeditions. In this work, we build on previous impact simulations of the event using an updated code to model and estimate the extent of fragmentation in the carbonate and granite. Specifically, we model the formation of porosity (void space) and fragmented materials and estimate permeability (the interconnectedness of void space or fractures) and compare with what has been measured and observed in the drill cores for various regions within the crater. We find that the impact likely created more permeable space than what is estimated from drill core data. More permeable space may suggest more hydrothermal activity could be sustained as water from the ocean rushed back into the fractured, warm crust. Hydrothermal systems from the Chicxulub impact may have been 100 times larger than the modern Yellowstone Caldera. Key Points: We simulate impact‐induced porosity with iSALE that is comparable to porosity measured in two different drill coresWe find the permeabilities introduced by the Chicxulub impact event may be greater than those measured from the M0077A and Yax‐1 drill coresThe output from this work will be useful and necessary for constraining the longevity and evolution of hydrothermal systems [ABSTRACT FROM AUTHOR]

Impacts into layered targets may generate "concentric craters" where a wider outer crater in the top layer surrounds a smaller, nested crater in the basement, which itself may be complex or simple. The influence of target on cratering depends on the ratio of target strength to lithostatic stress, which, in turn, is affected by gravity, target density, and crater diameter. When this ratio is large, the crater size is primarily determined by target strength, whereas gravitational forces dominate when the ratio is small. In two‐layer targets, strength may dominate in one or both layers, whereby the outer crater develops in the weaker top layer and the nested crater in the stronger substrate. However, large natural craters that should be gravity‐dominated in both cover strata and substrate may be concentric, the reasons for which are not yet fully understood. We performed qualitative impact experiments at 10–502 G and 1.8 km/s with the Boeing Corp. Hypervelocity centrifuge gun, and at 1 G and 0.4 km/s with the CAB CSIC‐INTA gas gun into layered sand targets of different compositions and grain densities but similar granulometry to analyze gravity‐dominated cratering. The results are compared with iSALE‐2D numerical simulations and natural craters on Earth and Mars. We show that target layering also affects the excavation process and concentric crater formation in gravity‐dominated impacts. The most important factors are the density and internal friction of each target layer, respectively. We propose that this is also valid for natural craters of sizes that should make their formation gravity‐dominated. Plain Language Summary: Concentric impact craters show a "soup‐plate" or "inverted sombrero" shape that forms on planetary surfaces with distinct subsurface layers. Generally, a deep inner crater forms in the substrate and a shallower, broader outer crater forms in the upper layer. The shape is obtained either from extensive post‐impact collapse of the upper, often weaker layer, or already during crater excavation, which is the focus of this study. The ratio of outer to inner diameter in the latter craters can be used to probe the depth of the upper layer and the contrast in material properties to the substrate. However, the controls on the relative size of the inner and outer craters are not well understood. While the formation of natural concentric craters is often attributed to a change in cohesive strength between the upper and lower layers, we show through impact experiments and numerical simulations that concentric craters can also form in cohesionless targets with a contrast in both density and friction coefficient between the layers. This provides an additional mechanism for concentric crater formation that may explain the development of some large, gravity‐dominated, naturally occurring concentric craters on Earth, Mars and elsewhere. Key Points: Concentric craters are not only due to strength differences between layers but also observed in cohesionless, gravity‐dominated targetsThis may explain the occurrence of large gravity‐dominated concentric craters on Earth, Mars and elsewhereKey factors affecting the concentric growth in these cases are the density and internal friction of each target layer, respectively [ABSTRACT FROM AUTHOR]

Uniformitarian theories of Martian history present many problems. A Biblical alternative is proposed that includes the Day-4 cratering hypothesis and Mars' crustal magnetism. Greatly reduced magnetism in the centers of the four largest impact craters suggests that most large Mars impacts occurred during the Genesis Flood. These impacts, and subsequent volcanism generated by them, produced a climate with sufficient liquid water for the Martian floods. A young age for Mars is supported by several geological observations. Rapid flooding can explain the valley networks and the outflow channels and provide a tighter volume estimate of around a 100 m Global Equivalent Layer (GEL). The flood water was absorbed into the subsurface and into hydrated minerals. That temporary atmosphere disappeared as water vapor precipitated out and the atmosphere cooled. Precipitation also absorbed CO2 and SO2 which helps account for their atmospheric disappearance. [ABSTRACT FROM AUTHOR]

Craters on Mars are a window into Mars' past and the time they were emplaced. Because the crust is heated and shocked during impact, craters can demagnetize or magnetize the crust depending on the presence or absence of a dynamo field at the time of impact. This concept has been used to constrain dynamo timing. Here, we investigate magnetic anomalies associated with craters larger than 150 km. We find that most of those craters, independent of age, exhibit demagnetization signatures in the form of a central magnetic low. We demonstrate a statistically significant association between such signatures and craters, and hypothesize that the excavation of strongly magnetic crustal material may be an important contribution to the dominance of demagnetized craters. This finding implies that the simple presence or absence of crater demagnetization signatures is not a reliable indicator for the activity of the Martian dynamo during or after crater formation. Plain Language Summary: Craters on Mars allow studying the time at which they were emplaced and as such they are a window into Mars' past. Because the crust is heated and shocked during impact and thus recrystallization occurs, craters can demagnetize or magnetize the crust depending on the presence or absence of a dynamo field at the time of impact. A classic magnetization signature is expressed by a magnetic high in the crater interior and the youngest of those craters have been used to constrain dynamo timing. Here, we investigate magnetic anomalies associated with all craters larger than 150 km. We find that most of those craters and independent of age exhibit a demagnetization signature, and fewer a magnetization signature. In general, the largest craters show a demagnetization signature. To explain the dominance of demagnetization signatures we hypothesize that the excavation of strongly magnetic crustal material may be an important process. This finding implies that the simple presence or absence of crater magnetization signatures is not a reliable indicator for dynamo activity and magnetic signatures of craters are dependent on multiple parameters such as crustal thickness. Key Points: Craters on Mars dominantly show demagnetization signaturesLarge craters and thin crust promote magnetic lows in the crater interior irrespective of dynamo activityCrustal excavation is likely an important process that promotes demagnetization signatures [ABSTRACT FROM AUTHOR]

Polygonal impact craters (PICs) are unique geological features observed on various planetary bodies and constitute a small percentage of the impact crater population. This study focuses on PICs on Ganymede, where no such craters have been investigated so far. Here we present the distribution of PICs, examine their morphological characteristics, investigate the causes for their polygonal shapes, and discuss the factors that support their formation. We identified and analyzed 459 PICs on Ganymede with complex crater morphologies. They are widely distributed across the Moon and occur on both dark and light terrain. Our analysis revealed that the majority of orientations of straight rim segments align parallel or subparallel to adjacent tectonic linear features. There is at least one tectonic linear orientation adjacent to PICs, even in dark, cratered terrains. Based on the comparison of the number of PICs and tectonic linear features, Ganymede is believed to have undergone more intense tectonic activity than other icy bodies like Ceres and Dione, where PICs were described. The presence of numerous PICs on Ganymede suggests that surface lineations and grooves are expressions of fractures that form zones of weakness in the ice crust that got reactivated during impact cratering. [ABSTRACT FROM AUTHOR]

Central peaks of Copernican period complex craters of the Moon, provides the best examples to study impact melts morphologies alongwith the exposed subsurface, as they are better preserved and less affected by the space weathering. Crater Tycho, present towards nearside (SW) of the Moon, is one such example of young and fresh complex crater. Present study is high-resolution compositional investigation involving spectroscopic analysis coupled with morphological study of central peaks and floor of crater Tycho. Alongwith the other contemporary craters the study is intended to understand the nature of occurrence and distribution of compositionally distinct lithologies identified near their central peaks and crater floor that differ in colour and specific appearance. A detailed high-resolution analysis suggests that the clastic exposures associated with the melts have a mafic composition that have been observed at similar other contemporary craters. They represent the fragmental polymictic breccia clasts whose mineralogy and stratigraphic relation with the melts suggests them to be representative of subsurface anorthositic gabbro/noritic body. Their association with structural features, such as cooling cracks/fractures and breccias dikes indicate their formation at different stages of cratering and associated crustal modification. The formation mechanism of the polymictic breccia clasts causing lithological variability has been discussed. [ABSTRACT FROM AUTHOR]

Titan has an organic-rich atmosphere and surface with a subsurface liquid water ocean that may represent a habitable environment. In this work, we determined the amount of organic material that can be delivered from Titan's surface to its ocean through impact cratering. We assumed that Titan's craters produce impact melt deposits composed of liquid water that can founder in its lower-density ice crust and estimated the amount of organic molecules that could be incorporated into these melt lenses. We used known yields for HCN and Titan haze hydrolysis to determine the amount of glycine produced in the melt lenses and found a range of possible flux rates of glycine from the surface to the subsurface ocean. These ranged from 0 to 1011 mol/Gyr for HCN hydrolysis and from 0 to 1014 mol/Gyr for haze hydrolysis. These fluxes suggest an upper limit for biomass productivity of ∼103 kgC/year from a glycine fermentation metabolism. This upper limit is significantly less than recent estimates of the hypothetical biomass production supported by Enceladus's subsurface ocean. Unless biologically available compounds can be sourced from Titan's interior, or be delivered from the surface by other mechanisms, our calculations suggest that even the most organic-rich ocean world in the Solar System may not be able to support a large biosphere. [ABSTRACT FROM AUTHOR]

When making contact with an undercooled target, a drop freezes. The colder the target is, the more rapid the freezing is supposed to be. In this research, we explore the impact of droplets on cold granular material. As the undercooling degree increases, the bulk freezing of the droplet is delayed by at least an order of magnitude. The postponement of the overall solidification is accompanied by substantial changes in dynamics, including the spreading-retraction process, satellite drop generation, and cratering in the target. The solidification of the wetted pores in the granular target primarily causes these effects. The freezing process over the pore dimension occurs rapidly enough to match the characteristic timescales of impact dynamics at moderate undercooling degrees. As a result, the hydrophilic impact appears “hydrophobic,” and the dimension of the solidified droplet shrinks. A monolayer of cold grains on a surface can reproduce these consequences. Our research presents a potential approach to regulate solidified morphology for subfreezing drop impacts. It additionally sheds light on the impact scenario of strong coupling between the dynamics and solidification. [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]

Abstract Large asteroid impacts have long been attributed to imparting long‐lived heat in the upper crust, as well as widespread fracturing and porosity. When an impact occurs on or near an ocean environment, displaced water rushes back filling the crater and percolating down into the fractured crust. The significant heat and fracturing from impact allow for extensive hydrothermal activity. Hydrothermal alteration has been observed at the 180 km Chicxulub impact structure in Yucatán, Mexico. Previous studies estimate widespread hydrothermal transport and activity within the Chicxulub structure, but these processes are largely dependent on the assumed post‐impact permeability distribution. In this work, we present an impact simulation that tracks fragmentation as well as the generation of porosity and permeability during the Chicxulub impact event. The generation of porosity during tensile failure results in final porosities (up to 40%) that are up to a factor of 4 higher than previous models and in better agreement with drill core data. We find that both fragmentation and porosity contribute to overall permeability of the Chicxulub structure. Estimated permeabilities (up to 10−14 m2) are greater than measured values from drill cores because of the contribution from large‐scale fragmentation, which cannot be resolved in cm‐diameter drill cores. The larger porosities and permeabilities computed in this work suggest that the volume of hydrothermal activity generated by Chicxulub were 10 times more than previously estimated for Chicxulub and 100 times more than the Yellowstone caldera.

Surface unloading due to impact cratering results in lava filling the crater floor. Elevation differences in the crater floor, a common geological phenomenon on the Moon, represent direct evidence of cratering processes. However, few studies have been conducted on mare-filled craters on the Moon. Al-Biruni (81 km) is a farside impact crater with an inclined topographic profile on its floor. We quantitatively measure the morphology of Al-Biruni and model the basaltic lava emplacement to depict the cratering process. Differential subsidence due to melt cooling, wall collapse, impact conditions and structural failure were assessed as potential factors influencing the formation of the elevation differences on the floor. The results suggest that pre-impact topography is a plausible cause of the differences in floor elevation within Al-Biruni. Other factors may also play a role in this process, affecting lava flow by altering the topography of the crater floor after the impact. Thus, regardless of whether the lava inside the crater is impact-generated or comes from outside the crater, altering topography at different stages of the cratering process is an essential factor in creating the sloped terrain on the crater floor.

Abstract Craters on Mars are a window into Mars' past and the time they were emplaced. Because the crust is heated and shocked during impact, craters can demagnetize or magnetize the crust depending on the presence or absence of a dynamo field at the time of impact. This concept has been used to constrain dynamo timing. Here, we investigate magnetic anomalies associated with craters larger than 150 km. We find that most of those craters, independent of age, exhibit demagnetization signatures in the form of a central magnetic low. We demonstrate a statistically significant association between such signatures and craters, and hypothesize that the excavation of strongly magnetic crustal material may be an important contribution to the dominance of demagnetized craters. This finding implies that the simple presence or absence of crater demagnetization signatures is not a reliable indicator for the activity of the Martian dynamo during or after crater formation.

In this work, numerical experiments were carried out in the framework of the molecular dynamics method to study the impact interaction of metallic nanoclusters of different sizes with a metallic substrate in a wide range of velocities. Analysis of the data obtained showed that the crater volume remains proportional to the impact energy and inversely proportional to the dynamic strength of the material, with the line slope being independent of the cluster size and the cluster–substrate material. [ABSTRACT FROM AUTHOR]

An experimental method is proposed to assess blasting requirements based on in-situ cratering behavior. The method relies on single hole blast (SHB) tests to derive burden-dependent relationships. A non-linear crater model of the form Y = C b s is calibrated from SHB test results to describe the cratering behavior. Parameters C and s represent characteristic blastability index parameters that depend on geomechanical and operational conditions. A series of experimental SHB tests were conducted at three hard rock mines. The craters were mapped to capture the breakout profile and calibrate the model. Results showed that coefficient C presents an inverse linear correlation with burden B , while exponent s is approximately constant for a considerable range of burden values. Experimental results are used to define burden dependent relationship for bench blast designs and blasthole placement in underground development rounds. A complementary analysis addresses the prevalence of multiple cratering mechanisms according to geology and burden dimension. [ABSTRACT FROM AUTHOR]

In a landscape with patchily distributed forage resources, an animal's distribution may reflect the distribution of the resources if the population is forage-limited in time or space. This may be particularly explicit in climatically extreme and seasonally variable environments, notably alpine and polar environments during winter. Sustainable management considers the amount of available alpine ground lichen in winter as a predictor of carrying capacity for the last remaining populations of wild European Mountain reindeer Rangifer tarandus tarandus in alpine environments in South Norway. Ground lichen growth is constrained to elevated, wind-blown, snow-free ridges. However, wild Mountain reindeer also persist in areas providing little lichen resources. Our alternative functional hypothesis is that the critical resource is the amount of snow-free feeding areas during winter, which provide direct access to forage, likely to be energy-profitable regardless of the type of vegetation. We sampled animal-borne videos from wild Mountain reindeer during maximum accumulation of snow in late winter/spring, in three contrasting areas providing variable amounts of ground lichen and alternative vegetation resources. Direct observations of reindeer foraging via videos document an active selection and strong preference for feeding on snow-free patches, regardless of type of vegetation. Active 'cratering' in snow was not observed. In contrast, walking behaviours occurred much more frequently across snow-covered areas. Remote sensing data and analyses corroborated these observations. In the sustainable management of wild Mountain reindeer amounts of vegetated snow-free areas is an important functional predictor of winter carrying capacity. [ABSTRACT FROM AUTHOR]

The S1222a marsquake detected by InSight on 4 May 2022 was the largest of the mission, at MwMa ${M}_{w}^{Ma}$ 4.7. Given its resemblance to two other large seismic events (S1000a and S1094b), which were associated with the formation of fresh craters, we undertook a search for a fresh crater associated with S1222a. Such a crater would be expected to be ∼300 m in diameter and have a blast zone on the order of 180 km across. Orbital images were targeted and searched as part of an international, multi‐mission effort. Comprehensive analysis of the area using low‐ and medium‐resolution images reveals no relevant transient atmospheric phenomena and no fresh blast zone. High‐resolution coverage of the epicentral area from most spacecraft are more limited, but no fresh crater or other evidence of a new impact have been identified in those images either. We thus conclude that the S1222a event was highly likely of tectonic origin. Plain Language Summary: During its time on Mars, NASA's InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission recorded over 1,300 seismic events, known as "marsquakes." Of these, a number were identified as coming from meteoroid impact cratering events on the surface. The largest event identified by InSight, labeled S1222a, bore some similarities to two large impact events recorded earlier in the mission. In order to investigate whether the S1222a event might also have been caused by an impact event, we undertook a comprehensive search of the region in which the marsquake occurred. We did not identify any fresh craters in the area, implying that the marsquake was likely caused by geological processes. Key Points: The S1222a marsquake detected by InSight on 4 May 2022 somewhat resembled previous impact‐generated eventsWe performed an image search in the estimated source region, using data from multiple Mars orbiter missionsNo new impact crater has been discovered in this area, pointing to a tectonic origin for the quake [ABSTRACT FROM AUTHOR]

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

Shock‐related calcite twins are characterized in calcite‐bearing metagranite cataclasites within crystalline megablocks of the Ries impact structure, Germany, as well as in cores from the FBN1973 research drilling. The calcite likely originates from pre‐impact veins within the Variscan metagranites and gneisses, while the cataclasis is due to the Miocene impact. Quartz in the metagranite components does not contain planar deformation features, indicating low shock pressures (<7 GPa). Calcite, however, shows a high density (>1/μm) of twins with widths <100 nm. Different types of twins (e‐, f‐, and r‐twins) crosscutting each other can occur in one grain. Interaction of r‐ and f‐twins results in a‐type domains characterized by a misorientation relative to the host with a misorientation angle of 35°–40° and a misorientation axis parallel to an a‐axis. Such a‐type domains have not been recorded from deformed rocks in nature before. The high twin density and activation of different twin systems in one grain require high differential stresses (on the order of 1 GPa). Twinning of calcite at high differential stresses is consistent with deformation during impact cratering at relatively low shock pressure conditions. The twinned calcite microstructure can serve as a valuable low shock barometer. [ABSTRACT FROM AUTHOR]

The test results for factors influencing flyrock have a positive correlation with the results obtained in the burden distance test of 71%, spacing distance of 61.25%, initial burden of 95.50%, stemming height of 97.73%, Powder factor of 24, 27%, filling hole depth 26.70%. The results of the comparative analysis of theoretical and actual flyrock throws by looking at the comparison of the theoretical and actual maximum throws using the Richard and Moore constant. Using the constant A, looking at the face burst constant value of 99.9 and createring 8.79, the furthest faceburst flyrock throw value was 212 m and cratering 192 m. [ABSTRACT FROM AUTHOR]

The volcanic parent rock ("protolith") composing the lunar maria is likely to have developed diverse mechanical properties upon emplacement. The abundance of rocks ejected by impacts may be sensitive to protolith properties. We investigate the relationship between the possible diversity of mare protoliths and the abundance of rocks excavated and emplaced by impacts employing cumulative size‐frequency distributions (SFDs) of rocky craters in the maria. The SFDs of 15 mare units were calculated through varying degrees of Lunar Reconnaissance Orbiter Diviner‐derived rock abundance within crater ejecta blankets. By inspecting the frequency of high rock abundance craters, we find that the age of a surface does not control an impactor's ability to excavate rocks. Through analysis of SFDs, we find that differences in SFD behavior may act as probes for the mechanical strength of the protolith and the thickness of buried lava flows. Generally, regions with SFDs that have shallower slopes with increasing rockiness may be associated with thick, competent flow types, and regions with SFDs that have unchanging slopes with increasing rockiness may be associated with stacked, thin flows that are possibly more friable. The greater frequency of high rock abundance craters and the converging behavior of SFDs at two units within Mare Humorum and Oceanus Procellarum suggest that the surfaces are underlain by thick, competent lava flows. Finally, SFD slope analysis suggests that secondary impact craters are relatively ineffective at excavating rocks and the SFDs of rocky craters may reflect primary cratering populations. Plain Language Summary: When ancient lunar lava flows cooled and hardened into rock, they likely developed different properties depending on the conditions of eruption. The variations in these now‐buried volcanic rocks can be probed using impacts, which break up and eject material from the subsurface. Ejected pieces of lava flow can be measured in terms of their abundance, which we use to infer lava flow properties in this study. By inspecting the populations of craters with a high abundance of rocks in their ejecta, we determine that age does not control the frequency of highly rocky craters. Further, we determine the size‐distribution of craters with rocks in their ejecta and find different relationships between the rock‐free crater populations and the rocky crater populations. We find that changes in the rock‐free and rocky crater populations may represent variations in lava flow properties. Finally, secondary impact craters that are formed from high‐velocity debris made by primary impacts are ordinarily difficult to discern from the primary impact population; however, we determine that by inspecting the slightly rockier crater population, secondaries may be filtered out. Key Points: Rock excavating impact populations are probes of mare protolith propertiesSurface age does not control an impactor's ability to excavate rockThe size‐frequency distributions of rocky crater populations exhibit behaviors reflective of substrate competence [ABSTRACT FROM AUTHOR]

Dating from the lunar magma ocean solidification period, the Procellarum KREEP Terrane (PKT) occupies 16% of the surface but has a much higher thorium abundance compared to the rest of the Moon and is thus interpreted to carry 40% of the radioactive elements by volume in the form of an anomalously thick KREEP-rich layer. Subsequent research has focused on the processes responsible for PKT concentration and localization (e.g., degree-1 convection, farside impact basin effects, etc.), and the effect of PKT high-radioactivity localization on lunar thermal evolution (e.g., topography relaxation, mantle heating, late-stage mare basalt generation, etc.). Here we use a stratigraphic approach and new crustal thickness data to probe the nature of the PKT with depth. We find that most PKT characteristics can be explained by sequential impact cratering events that excavated and redistributed to the surface/near-surface a much thinner Th-rich KREEP layer at depth, implying that no anomalous conditions of PKT thickness, radioactive abundances, geodynamics, thermal effects or magma generation are likely to be required as in the previous studies. [ABSTRACT FROM AUTHOR]

Two impact basins on the Moon—Freundlich‐Sharonov and Hertzsprung—are nearly the same size but exhibit different surface morphologies and subsurface structures. Gravity data reveal a bench‐like transitional structure in the crust‐mantle interface between the outer ring and the inner basin cavity in Hertzsprung, unlike that beneath both Freundlich‐Sharonov and larger multi‐ring basins. We use iSALE‐2D to model the formation of impact basins into a 40‐km thick pre‐impact crust with a range of thermal conditions to understand the divergent development of these basins and gain insight into the factors affecting whether a basin forms with a peak‐ring or multiring basin structure. We find that thermal gradients of at least 30 K/km result in Freundlich‐Sharonov‐type basins, in agreement with previous work. Cooler thermal gradients of approximately 15–20 K/km are needed to develop Hertzsprung‐like multiring basins with observed bench‐like structures in the crust‐mantle topography. We find that for cooler models, the bench structure develops early in the cratering process as a rotated inner normal fault cutting the crust‐mantle interface, whereas models with higher thermal gradients instead develop diffuse deformation zones instead of a discrete inner fault. The peak rings of both basins develop later in the cratering process as the collapsed central uplift. These results highlight the complex interplay between a strong lithosphere needed to develop ring faults and accessible ductile rocks that facilitate multiring basin formation. The varying thermal conditions giving rise to Hertzsprung and Freundlich‐Sharonov impact basins may be a possible constraint on the lunar cooling rate and chronology. Plain Language Summary: Impact cratering is a complex process that depends on several factors describing a planetary body. We studied two large impact craters on the farside of the Moon, Freundlich‐Sharonov and Hertzsprung basins, to understand how the temperatures of the lunar subsurface may have affected their development. We focus on recreating observed crater dimensions and subsurface features to evaluate our models. Specifically, we model the formation of a distinct "bench" feature between the crust and mantle layers that exists beneath Hertzsprung basin but is not present beneath Freundlich‐Sharonov basin. We found that the temperatures at the time of impact play an important role in the nature of the structure that develops after a large impact. We successfully modeled basins that look like Freundlich‐Sharonov using relatively warm temperature conditions and we modeled Hertzsprung‐like basins using cooler temperature conditions. These varying thermal conditions directly relate to the character and location of faulting associated with each basin. These models underscore the effects derived from changing thermal conditions as a planetary surface cools and show that the impact craters that form on a planet or Moon are sensitive to these effects in different ways. Key Points: Highly localized gravity inversions show variability in the subsurface structure of two lunar basins, Hertzsprung and Freundlich‐SharonovDifferences in surface morphology and crust‐mantle structure arise from variation in near‐surface temperatures at the time of each impactRotation during crater collapse is fundamental after large impacts, inverting the orientation of faults formed early in the impact process [ABSTRACT FROM AUTHOR]