Search

Mars Orbiter Laser Altimeter (MOLA) elevation measurements for 23 different impact craters and 12 different long runout landslides show rampart ridges on Martian fluidized ejecta flows are higher relief than those on Martian landslides. We propose a conceptual model to explain this height difference that is based on the effects of the impact and ejecta emplacement process on the development of ejecta ramparts. Our model explains the relatively high relief of the distal ramparts as well as the particle size distribution that is inferred from thermal inertia measurements. In this model, impact events produce ejecta curtains that advance radially at increasingly higher velocity outward excavating and roughening the surface as they impact. This produces an inertia-driven, ground-hugging ejecta flow composed of primary and secondary ejecta. This flow moves rapidly across the surface following closely behind the impacting ejecta curtain. The ejecta curtain continuously adds impact-generated debris to the flow front that includes large particles, some of which are overridden by the flow, but some accumulate at the flow front. These particles are pushed forward at the flow front as a high-friction "dam" with the accumulating material growing into a relatively high rampart as the ejecta curtain adds more large particles to it. In addition, the impact-roughened surface cause substantial vibrations and shear in the flows moving behind the ejecta curtain. This roughness results in kinetic sieving in the flows that brings large particles to the surface and transports them to the flow front where some are also overridden or accumulate to add the ones already at flow front. We propose that these processes combine to produce the observed high ejecta ramparts. The relatively high velocity of the ejecta flows pushes the load of coarse-grained debris to the top of even high developing ramparts. When the flow halts, it drains back from the accumulated coarse debris at the flow front, leaving a high rampart dominantly composed of large particles. • Ramparts on Martian ejecta flows are higher than ramparts on Martian landslides. • Martian ejecta flows are considerable thinner than Martian landslides of a given volume. • Ejecta emplacement processes are likely to be the cause the high relief ejecta ramparts. [ABSTRACT FROM AUTHOR]

Three anomalously young, ∼120 Ma old lunar mare pyroclastic beads have recently been reported (Wang et al., 2024) from Chang'e-5 (CE-5 soils, particularly distinguished from impact melt beads by sulfur isotope (34S/32S) composition and correlations with sulfur concentration. We examine lunar pyroclastic eruption theory and candidate eruption conditions in order to locate the vent and assess its geological context, finding that the estimated maximum pyroclast dispersal range from a candidate source vent is likely to be ∼200 km, placing it within the area of the CE-5–2.0 Ga sampled Em4 unit. The greatest predicted dispersal distances are associated with an explosive eruption from a stalled dike several kilometers below the surface, creating an elongated, multi-km-scale pit crater potentially surrounded by a dark pyroclastic ring. We assessed the Chang'e-5 region for such candidates and found none. This raises the possibility that the ∼120 Ma pyroclastic beads might have been delivered to the site from an impact crater outside Em4, but the most likely candidates are sufficiently large and at such great distances that they are likely to have reset the ages of any young pyroclastic beads thus delivered. Lacking a clear source for extraordinarily young pyroclastic beads, we reassess the possibility that the ∼120 Ma beads may be of local impact melt origin. Evidence favoring this hypothesis includes the abundant CE-5 impact glass bead ages in the 100–200 Ma year range previously reported (Long et al., 2022), and the similarities in composition and characteristics of the three beads and those of local impact origin. To address these conundra, further regional searches for a source vent and continued geochemical characterization and dating of CE-5 regolith glass beads should be undertaken. • Three ∼120 Ma old Chang'e-5 lunar basalt pyroclastic beads were recently reported. • This anomalously young age challenges the existing lunar thermal evolution paradigm. • We establish maximum pyroclastic bead emplacement ranges from the vent, ∼200 km. • We are unable to locate a source vent within this distance from the Chang'e-5 site. • We explore other origins for the ∼120 Ma glass beads, including local impact melt. [ABSTRACT FROM AUTHOR]

According to several orbital reflectance spectroscopy studies, numerous widely scattered lunar surface locales feature remarkably high (>98%) abundance of plagioclase. These "pure" anorthosite claims are based on an absorption band at ∼1.25 μm, associated with minor Fe2+ within plagioclase. But utilization of the 1.25 μm band as a direct gauge of plagioclase abundance requires an underlying assumption that plagioclase FeO is uniform across all measured materials. Available data for FeO in lunar plagioclase are in many cases suspect because electron-probe FeO measurements may be inflated by secondary fluorescence from nearby mafic phases. We studied plagioclase FeO in a set of 13 anorthositic lunar rocks, taking care to avoid proximity to mafic phases; or in cases where complete avoidance was not possible without sacrificing representativeness (i.e., fine-grained impact melt rocks with zoned silicates), correcting close-to-mafic analyses for secondary fluorescence (Sugawara, T. [2001], Japanese Mag. Mineral. Petrol. Sci. 30, 159–163). Results for rock-average plagioclase FeO range from 0.030 wt% in plutonic troctolitic anorthosite 76335, to 0.23 wt% in impact-melt rock 14310. In general, as shown by exsolution of mafic silicates from plagioclase within several samples, final plagioclase FeO in lunar plutonic anorthosites is determined by reequilibration during slow postigneous cooling and/or metamorphism. In contrast, four fast-cooled anorthositic impact melt rocks have consistently higher plagioclase FeO, averaging 1.9 times higher in comparison to average plutonic anorthositic rock. Thus, impact melted lunar anorthosite will have far more prominent 1.25 μm absorption than plutonic anorthosite of the same plagioclase abundance. In view of the inconstancy of plagioclase FeO, orbital spectral reflectance using the 1.25 μm absorption band, or the ratio between that band and one or more mafic silicate bands, cannot be precise enough to justify claims of ability to resolve "purest" (98 vol% +) anorthosite from compositions that are high-plagioclase but not that extreme. • Measuring abundance of lunar plagioclase using its 1.25 μm iron band is problematic. • Plagioclase Fe content in lunar highland rocks is highly diverse. • New electron probe data show plutonic plagioclase Fe ranging from 0.030 to 0.137 wt%. • Impact melted (fast-cooled) highland rocks have even higher plagioclase Fe. • Depth of the plagioclase 1.25 μm band is a function of both abundance and Fe content. [ABSTRACT FROM AUTHOR]

The surface conditions of terrestrial bodies strongly reflect their geological evolutionary processes and vary among various terrestrial bodies. This diversity is attributed to variations in the timescales of boulder formation through processes such as impact cratering, rockfalls from crater walls, seismic motion, and boulder fragmentation caused by micrometeoroid impacts and thermal stress. In this study, we examined boulders on Ceres using high-resolution images with a resolution of approximately 5 m/px obtained during the Ceres Extended Mission 2 Orbit 7 of the Dawn mission. Almost all boulders were present around impact craters, even at a resolution of 5 m/px, thus indicating that the boulders on Ceres were created by impact cratering alone. The maximum boulder size on Ceres is approximately 200 m, even around large craters, which may indicate the upper size limit determined by the mechanical strength of the boulders, such as the tensile strength, the scale effect, and/or shattering strength. The slope of the size-frequency distribution of boulders on Ceres varied significantly across the range of boulder sizes, thus making it difficult to describe it using a single function of a power-law relationship; in particular, it changed at approximately 100 m, thus indicating that destructive or formation mechanisms may be different for large boulders >100 m and for small boulders <100 m. There may also be a subsurface structure that prevents the formation of small boulders, although this is difficult to argue conclusively. We estimated that the lifetime of boulders larger than 50 m was equivalent to or shorter than 100 Myr. This lifetime is consistent with a theoretical estimation assuming that micrometeoroid impacts are the primary destructive mechanism. • We counted boulders (> 15 m) to understand the surface evolution of Ceres. • Almost all boulders on Ceres were formed by impact cratering. • The largest boulders on Ceres (approximately 200 m) implied surface strength limits. • The slope of the boulder size-frequency distribution changed at approximately 100 m. • The lifetime of boulders on Ceres was equivalent to or shorter than 100 Myr. [ABSTRACT FROM AUTHOR]

This study identifies 140 distal impact melt features with definitive flow morphology and determines them to originate from Tycho crater. The furthest of these impact melt deposits are located ∼450 km from Tycho (approximately ten crater radii). The impact melt deposits that we report in this study exhibit definitive melt flow features such as surface cracks, ridges, and block-rich margins. We typically found melt features with these morphologies on steep, Tycho-facing slopes. However, this population does not represent the full extent of distal melt that is likely to have been produced in the Tycho impact. Pond and veneer morphologies on nearby shallow slopes often suggest the presence of additional melt without either elevated Lunar Reconnaissance Orbiter Diviner instrument Rock Abundance fractions or distinctive melt flow morphologies in high-resolution imagery. We propose that topography is a controlling factor for the development of definitive melt flow morphologies in distal impact melts. The melt features that we identified in this work highlight the capacity for a single, large cratering event to resurface terrains at great distances from the crater. Modeling the production and distribution of distal impact melts, therefore, is an important next step in understanding the role that they play in the resurfacing history of airless bodies like the Moon. • Identifies 140 distal Tycho impact melt features with definitive flow morphologies. • Most distal melt flow features detected on steep, Tycho-facing slopes. • Distal impact melt is more difficult to detect when located on shallow slopes. • There is potentially much more distal impact melt than is currently detected. [ABSTRACT FROM AUTHOR]

Cratering chronologies are used to derive the history of planetary bodies and assume an isotropic flux of impactors over the entire surface of the Moon. The impactor population is largely dominated by near-Earth-objects (NEOs) since ∼3.5 billion years ago. However, lunar impact probabilities from the currently known NEO population show an excess of impacts close to the poles compared to the equator as well as a latitudinal dependency of the approach angle of impactors. This is accompanied by a variation of the impact flux and speed with the distance from the apex due to the synchronicity of the lunar orbit around the Earth. Here, we compute the spatial dependency of the cratering rate produced by such variabilities and recalibrate the lunar chronology. We show that it allows to reconcile the crater density measured at mid-latitudes around the Chang'e-5 landing site with the age of the samples collected by this mission. Our updated chronology leads to differences in model ages of up to 30% compared to other chronology systems. The modeled cratering rate variability is then compared with the distribution of lunar craters younger than ∼1 Ma, 1 Ga and 4 Ga. The general trend of the cratering distribution is consistent with the one obtained from dynamical models of NEOs, thus potentially reflecting a nonuniform distribution of orbital parameters of ancient impactor populations, beyond 3.5 Ga ago, i.e., planetary leftovers and cometary bodies. If the nonuniformity of the cratering rate could be tested elsewhere in the Solar System, the recalibrated lunar chronology, corrected from spatial variations of the impact flux and approach conditions of impactors, could be extrapolated on other terrestrial bodies such as Mercury and Mars, at least over the last 3.5 billion years. The modeled cratering rate presented here has strong implications for interpreting results of the Artemis program, aiming to explore the South Pole of our satellite, in particular when it will come to link the radiometric age of the samples collected in this region and the crater density of the sampled units. • The spatial variability of the lunar cratering rate is computed from the updated NEO population. • The surface located at ±60°N, 90°W is 1.77 more cratered than at the equator and the antapex of motion (0°N, 90°E). • The new lunar chronology leads to differences in model ages of up to ∼30% compared to previous models, depending on location. • The cratering rate variability is in accordance with the distribution of craters formed over the last ∼3.5Ga on the Moon. [ABSTRACT FROM AUTHOR]

Comet nuclei are the most primitive small bodies in the solar system, and would contain highly volatiles and amorphous water ice. However, comet nuclei may lose their primordial properties after high-velocity impacts with other small bodies. Such high-velocity impacts might also cause the characteristic circular structures and pinnacles on the comet-nucleus surface. However, the true origin of these geological structure remains uncertain. We here sought to clarify the shape of impact craters and the effects of post-shock heating on the comet surfaces. We conducted high-velocity impact experiments at velocities of 3–5.8 km s−1 on porous-ice targets with 40%, 50% and 60% porosity, and measured the post-shock temperature around the impact crater. The hard, dense refrozen layer formed on the crater wall resembled the pinnacles and sharp pit rims observed on comet nuclei, suggesting that such structures on comets may originate from impacts. In addition, the crater size scaling law for porous ice targets was successfully obtained in the strength-dominated regime. From the temperature profiles obtained, the energy partition coefficient of the projectile kinetic energy to the post-shock heat, γ , was estimated as follows: γ =0.08, 0.27 and 0.63 for 40%, 50% and 60% porosity at 4.2 km s−1 impact velocity, and γ =0.58 and 0.22 for 50% porosity at 3 km s−1 and 5.8 km s−1 impact velocities. These results, together with our crater size scaling law for porous ice, were used to estimate the temperature distribution induced by post-shock heat on the comet-nucleus surface. Volatile depletion and crystallization of amorphous water ice can occur at distances less than twice the crater radius from the crater center—that is, the post-shock heat effects are confined to the area around the crater. This might suggest that the comet nucleus surface remains relatively primitive after impacts. • Cratering experiments were conducted on porous ice simulating the comet surface. • The post-shock temperature around the impact crater was measured. • Melted H 2 O ice was refrozen to form a hard and dense crater wall. • The energy partition coefficient changed from 0.08 to 0.63. • Effects of post-shock heating may be confined to the area around the impact crater. [ABSTRACT FROM AUTHOR]

The impact site of the Apollo 12 Lunar Module Ascent Stage is identified in Lunar Reconnaissance Orbiter Camera (LROC) images at 3.920° S, 338.828° E. The form of the impact feature and its ejecta, its proximity to the expected location and the absence of any other obvious candidate make the identification compelling. • The impact site of the Apollo 12 Lunar Module Ascent Stage is identified in LRO images. • The impact formed a linear depression 30 m long and a field of dark streaks visible for 3 km to the west. • The impact location from QuickMap is 3.920° S, 338.828° E. [ABSTRACT FROM AUTHOR]

Hundreds of new impact craters have been observed to form on Mars since spacecraft began imaging that planet. New impact craters produced visible ejecta deposits and many of them also have visible rays, similar to lunar and mercurian craters. However, some of the new martian impact craters have a circular feature of relatively low reflectance that we call a "halo." This feature is distinct from the usual visible ejecta deposits or ray patterns. In this paper we present an observational study of this halo feature and we discuss the results of this study with respect to the nature of the halos: what they are and how they may have formed. To address these questions, we measured diameters of both halos and their central craters. We found a strong correlation between halo diameter and crater diameter, which indicates that the nature of the halos is fundamentally governed by the amount of impact energy available at their formation. Specifically, halo size is controlled by impact energy according to the non-linear relationship D H ∝ E2/3, where D H is the diameter of the halo and E is the impact energy. We also found that certain factors may influence the formation of the halos: a thicker dust layer and lower elevations are both correlated with larger halos. From these correlations we conclude that the local surface characteristics as well as local atmospheric pressure influence the formation of the halos. Our description and analysis of the martian halo features provide a framework upon which specific halo formation mechanisms can be developed and tested in the future. • Some new martian impact craters are surrounded by a dark circular feature: a halo. • Halo diameter strongly correlates with crater diameter. • Halo formation depends on the amount of available impact energy. • Halo formation depends on local surface properties and atmospheric pressure. [ABSTRACT FROM AUTHOR]

Abstract Small craters of the lunar maria are observed to be in a state of equilibrium, in which the rate of production of new craters is, on average, equal to the rate of destruction of old craters. Crater counts of multiple lunar terrains over decades consistently show that the equilibrium cumulative size-frequency distribution (SFD) per unit area of small craters of radius > r is proportional r −2, and that the total crater density is a few percent of so-called geometric saturation, which is the maximum theoretical packing density of circular features. While it has long been known that the primary crater destruction mechanism for these small craters is steady diffusive degradation, there are few quantitative constraints on the processes that determine the degradation rate of meter to kilometer scale lunar surface features. Here we combine analytical modeling with a Monte Carlo landscape evolution code known as the Cratered Terrain Evolution Model to place constraints on which processes control the observed equilibrium size-frequency distribution for small craters. We find that the impacts by small distal ejecta fragments, distributed in spatially heterogeneous rays, is the largest contributor to the diffusive degradation which controls the equilibrium SFD of small craters. Other degradation or crater removal mechanisms, such cookie cutting, ejecta burial, seismic shaking, and micrometeoroid bombardment, likely contribute very little to the diffusive topographic degradation of the lunar maria at the meter scale and larger. Highlights • New analytical and numerical models show why the number of countable lunar craters reaches a certain limit. • The lunar surface is shaped by the creation of very long crater rays in a surprisingly strong way. • The soft appearance of the lunar surface is a result of numerous tiny impacts of ejecta fragments from distant craters. [ABSTRACT FROM AUTHOR]

Abstract The formation of impact craters in unconsolidated granular materials is a topic of enduring interest in solid-earth geophysics, planetary science, and several branches of engineering science. In particular, a general relationship between crater size, impact parameters, and target material properties is often sought. This paper presents a new empirical relationship, based on dimensional analysis and inspired by gas-dynamic shock physics, for the diameters of low- and high-speed impact craters in dry granular materials based on the hypothesis that surface-gravity- and shock-wave phenomena primarily set crater size. The final relationship involves the impacting object's kinetic energy and speed; the target material's density, angle of repose, and sound speed; and the gravitational acceleration at the impact location. It is formulated in terms of a dimensionless crater diameter, an algebraic combination of Froude number, Mach number, and the tangent of the target material's angle of repose, using an analogy to gas dynamics and an empirical power law for the dependence of granular-material sound speed on gravitational acceleration. The coefficient of determination for the final fit is 0.969 based on experimental impact data from 325 individual impacts spanning parametric ranges of >400 in crater diameter, 1010 in impact energy, 500 in gravitational acceleration, and 40 in target material density for two different angles of repose. The final formula provides insight into how impact energy conversion depends on Mach number and may be useful for predictive and forensic analysis of planetary impact craters and for granular-flow code validation. Highlights • Single impact-crater scaling formula uniting low- and high-speed impact regimes • Insight into dependence of impact energy conversion on impactor Mach number • Combines effects of Froude number, grain-to-grain friction, and Mach number • Encompasses 325 impacts from 12 experimental studies with vast parametric ranges [ABSTRACT FROM AUTHOR]

Abstract The surfaces of many planets and asteroids contain coarsely fragmental material generated by impacts or other geologic processes. The presence of such pre-existing structures may affect subsequent impacts, particularly when the width of the shock is comparable to or smaller than the size of pre-existing structures. Reasonable theoretical predictions and low-speed (<300 m s−1) impact experiments suggest that in such targets the cratering process should be highly dissipative, which would reduce cratering efficiencies and cause a rapid decay in ejection speed as a function of distance from the impact point. In this study, we assess whether these results apply at impact speeds between 0.5 and 2.5 km s−1. This study shows little change in cratering efficiency when 3.18 mm diameter glass beads are launched into targets composed of these same beads. These impacts are very efficient, and ejection speed decays slowly as a function of distance from the impact point. This slow decay in ejection speed probably indicates a correspondingly slow decay of the shock stresses. However, these experiments reveal that initial interactions between projectile and target strongly influence the cratering process and lead to asymmetries in crater shape and ejection angles, as well as significant variations in ejection speed at a given launch position. Such effects of asymmetric coupling could be further enhanced by heterogeneity in the initial distribution of grains in the target and by mechanical collisions between grains during excavation. These experiments help to explain why so few craters are seen on the rubble-pile asteroid Itokawa: impacts into its coarsely fragmental surface by projectiles comparable to or smaller than the size of these fragments likely yield craters that are not easily recognizable. Highlights • Little change in cratering efficiency occurs when a particle is fired at km/s into targets of identical sized particles. • Broad asymmetries in crater morphology and complex ejecta trajectories are produced by impacts into coarse-grained targets. • Crater asymmetry and anomalous ejecta trajectories decrease as impact speeds increase for identical projectile and target particle diameters. [ABSTRACT FROM AUTHOR]

Highlights • We used Cassini data to infer the composition of Titan's impact craters. • In RADAR radiometry data, degraded craters have higher emissivity than fresh ones. • In VIMS spectral data, craters in the dunes regions differ from those in the plains. • We suggest there are active surface processes occurring on Titan, like wind or rain. • These processes clear off its surface, while filling subsurface cracks with organics. Abstract Titan, the only moon in the solar system with a considerable atmosphere, is host to a variety of exogenic processes that shape its surface. These processes form features that are quite similar to features on Earth, including sand dunes, rivers, and lakes. The combination of a thick atmosphere and active surface processes also leads to a scarcity of impact craters on the surface of Titan. The compositions of these craters vary and may relate to the type and extent of erosion occurring at their location. In this work, we examined the composition of 12 impact features on Titan using Cassini's Visible and Infrared Mapping Spectrometer (VIMS) and 2.18 cm emissivity data from the RADAR radiometer on board the probe. Comparisons were made between crater composition as inferred from VIMS and composition as inferred from emissivity data with corresponding crater characteristics such as latitude, longitude and erosional state. We see a correlation between crater subsurface composition as inferred from the emissivity data and its erosional state and location. Well-preserved craters typically are more enriched in water-ice than degraded, organic-rich craters, suggesting that variations in composition are partially controlled by erosion and infilling. Moreover, craters located in dune fields show more subsurface organic enrichment than craters within the plains, which indicates that the efficiency of erosion and infilling varies with location and geologic context. VIMS data provide complementary information about crater surficial composition. We note that VIMS spectra do not change with erosional state, but rather appear to depend on the crater location on the moon. We suggest that there are active surface processes occurring on Titan, such as wind or rain, which are actively clearing off its surface and filling in subsurface fractures with organic materials. These processes would act to change the emissivity of the craters over time but leave the surface sensed by VIMS unchanged. [ABSTRACT FROM AUTHOR]

Highlights • The European lunar orbiter SMART-1 end of mission impact location was identified in LRO images. • The impact feature is a linear depression 30 m long with a fan of ejecta extending downrange. • The location is 34.261+/−0.002° S, 313.807+/−0.001° E. Abstract The impact site of the European Space Agency SMART-1 lunar orbiter was identified in Lunar Reconnaissance Orbiter (LRO) Camera (LROC) images. No pre-impact image of adequate resolution is available for confirmation, but the form of the impact feature and its ejecta, its proximity to the expected location and the absence of any other obvious candidate in the vicinity make the identification compelling. [ABSTRACT FROM AUTHOR]

Highlights • Absolute model formation ages of Occator crater range between 1.6 and 63.7 Ma. • Previous studies of Occator crater suggest age errors that are underestimated. • Impactor/crater scaling parameters have the largest influence on age dating results. • Ballistics on low gravity, quick rotating bodies allow for self-secondary cratering. Abstract The 90.5-km Occator crater, with its peculiar and unique bright spots, is one of the most prominent and renowned feature on Ceres. Occator attracted broad public attention in scientific media as it is proposed to exhibit signs of post-impact cryovolcanic activity. In order to understand the time sequence of deposition, several attempts were made during DAWN's primary mission by different research groups to date geomorphologic key units using superposed crater densities. Resulting absolute model formation ages for Occator's ejecta and its interior lobate deposits range from 200 Ma to 78 Ma and about 100 Ma to 6.9 Ma, but were based on different cratering chronology models, measurements on image data at varying resolution, and different statistical and methodological approaches. Here we present the results of a comprehensive approach of determining absolute model formation ages for Occator. This is achieved by using the best resolved Framing Camera image data, by careful treatment of secondary crater admixture and the natural variability of crater detection and sizes by different crater analysts, and by applying appropriate, objective criteria for count area selection. In this context, we evaluate previously published model ages and explain why our results are likely to yield more consistent and robust information about the formation age of Occator. We also show that, in contrast to previous publications, CSFDs measured on Occator's less competent ejecta blanket, and its more competent cryovolcanic- or impact melt related interior lobate deposits (ILDs), are so similar to each other, that the slight differences might rather be explained by different scaling parameters in different target materials or that the ILDs have formed almost contemporaneously or only shortly after the Occator forming impact. [ABSTRACT FROM AUTHOR]

Highlights • We test hypotheses for the formation of craters within martian glacial deposits. • Nine crater types and multiple degradation sequences observed. • There is a small size difference between ring-mold craters and bowl-shaped craters. • Flow lineations cross-cut older ring-mold craters but not other crater types. • Observations most consistent with formation by mantle emplacement and dissection. Abstract Lobate debris aprons (LDA), lineated valley fill (LVF), and concentric crater fill (CCF) on Mars, interpreted to be debris-covered glaciers, possess craters with a suite of distinct interior landforms (called "ring-mold craters") that have been attributed to the presence of glacial ice at depth or surface modification processes. We tested current hypotheses for the formation of ring-mold craters by conducting a comprehensive analysis of the size and morphology of 16,457 impact craters ≥125 m in diameter formed within glacial deposits in Deuteronilus Mensae. Two major groups, bowl-shaped craters and ring-mold craters, are found, with at least nine distinct crater types. While there is statistical difference in median diameters between these crater types, this difference is relatively small and is within the estimated uncertainty in diameter measurements and may be related to more enhanced erosion of the rims of ring-mold craters. Clear degradation sequences are observed, supporting a role for post-impact modification in producing at least some of the diversity in crater landforms. The spatial density of ring-mold craters is also directly correlated with the development of LDA, LVF, and CCF surface textures. Flow lineations cross-cut two ring-mold crater types but they maintain their circular planforms in some cases, suggesting that the craters initially formed completely within mantling layers deposited after glacial flow had ceased. We also find analogous craters in non-glacial units; glacial ice is therefore not required to form the observed morphologic diversity. Our observations are most consistent with formation of crater landforms by emplacement and modification of at least two depositional episodes of icy dust (i.e., "mantle"). This mantle was initially tens of meters in thickness to support crater formation, and has experienced much downwasting and erosion since emplacement. Derived crater retention ages of 460 Ma for LDA, LVF, and CCF features in the region therefore reflect deposition of mantle units and only give a very minimum age for the formation of LDA, LVF, and CCF. [ABSTRACT FROM AUTHOR]

Highlights • We constrain the physical characteristics of debris that covers glaciers on Mars. • Two main layers are observed: icy dust airfall superposes headwall-derived sediment. • In total, the supraglacial debris is a few meters to tens of meters in thickness. • Absence of near-surface radar reflectors may imply gradation in debris and ice. • Craters penetrate tens of meters into glacial ice yet maintain shape over ∼10 Ma. Abstract Geomorphic and geophysical evidence supports a debris-covered glacier origin for a suite of landforms at the mid-latitudes of Mars, including lobate debris aprons (LDA), lineated valley fill (LVF), and concentric crater fill (CCF). These large reservoirs of ice and their near-surface structure provide a rich record for understanding the planet's climate and history of global volatile exchange over the past billion years. LDA, LVF, and CCF are also potential sites for future robotic and human missions but the accessibility of glacial ice for direct sampling and in situ resource utilization depends largely on the geotechnical properties of the surface debris ("supraglacial debris"), including its thickness, grain sizes, and density structure. The physical properties of this supraglacial debris layer have been poorly constrained. We use images of morphology, digital elevation models, thermal inertia data, and radar sounding data to probe the near surface of LDA, LVF, and CCF in Deuteronilus Mensae in order to place constraints on the sources, grain sizes, thickness, and stratigraphy of supraglacial debris. We find evidence for at least a two-layer stratigraphy. Layered mantle consisting of atmospherically emplaced dust and ice superposes boulder-rich sediment sourced by rockfalls glacially transported downslope. High thermal inertia, boulder-rich termini and debris bands reminiscent of medial moraines are found throughout the study region, supporting a rockfall origin for at least a fraction of the debris exposed at the surface. This supraglacial debris layer would have thickened with time from sublimation of glacial ice and liberation of englacial sediment and dust. At present, the entire supraglacial debris package is a minimum of a few meters in thickness and is likely tens of meters in thickness in many locations, possibly thinning regionally at lower latitudes and locally thinning toward the headwalls. The lack of terracing or interior structures in craters formed within LDA, LVF, and CCF and the absence of near-surface reflectors in SHARAD radar data further suggest that no strong contrasts in permittivity or strength occur at the interface of the layers. [ABSTRACT FROM AUTHOR]

Complex craters with diameters (D) ≥ 40 km on Callisto and Ganymede are shallower than would be expected from simply extrapolating the depth-diameter trend from smaller (D ≤ 40 km) craters. This unusual depth-diameter (d-D) trend, and associated changes in crater morphology, have been hypothesized to result from rheological transitions, including the existence of an ocean, within the moons' ice shell. Simulations of impact crater formation can reproduce the observed shallow depths but require heat fluxes roughly twice the maximum radiogenic flux to do so. Here we demonstrate that the d-D trends on Callisto and Ganymede can instead be explained as a direct consequence of viscous relaxation under radiogenic heating. We use numerical simulations of viscous relaxation to show that if craters form at the depth expected from an extrapolation of the complex crater d-D trend, they will evolve to the observed depths over timescales of 200 Myrs to 1 Gyrs. Large craters (e.g., D ≥ 80 km) younger than 200 Myrs, which would retain greater depths, should be relatively rare. If we instead assume that the craters formed at their observed depths, as proposed by previous impact modeling, they quickly become much shallower than observed. We find excellent agreement between observed crater depths on Ganymede and our simulated crater depths by assuming a pure-water ice composition and a diurnally averaged surface temperature of 120 K, but require either larger-grained or "dirty" ice with a modestly higher viscosity to match observations at Callisto, where the surface temperature is warmer (130 K). We favor the latter explanation because it is consistent with the existence of a dusty lag on Callisto's surface and the absence of a similar lag on Ganymede. Our results predict that, for a given crater diameter, post-relaxation crater depth should increase with increasing latitude, a hypothesis best tested on Callisto, whose relatively quiescent geologic history best preserves the signature of viscous relaxation under radiogenic heating. • Large, fresh craters on Callisto and Ganymede are observed to be unusually shallow. • We show that radiogenic heat causes rapid viscous relaxation of large craters. • Simulated depths match observations if craters are 200 Myrs to 1 Gyrs old. • Callisto's warmer crust is likely a mix of ice and dust to retard relaxation. [ABSTRACT FROM AUTHOR]

An automated system for counting small craters at lunar landing sites has been expanded to include selected locations on the Moon's far side and high latitudes. Highland areas were generally excluded from this new survey, because sloping terrain can have a significant effect on crater lifetimes. But some locations on highland light plains units were analysed, as were some young Copernican age crater ejecta blankets and impact melts. This study focuses on craters having diameters between 2.5 m and 50 m, for which counts are so high that they are best plotted as Crater Size Frequency Histograms (CSFHs), rather than the conventional cumulative plots (CSFDs). This histogram representation enables the finest details of the plots to be compared across sites. Using multiple precisely aligned images of each site enables false negatives to be included and false positives rejected, ensuring that no visually recognisable craters are missed, a capability that is not possible when analysing single images. There are some very distinct differences between the curves at different sites. Based on the known ages of boulder tracks and counts of craters formed over the last 50 years, the largest craters within the size range up to 50 m are thought to be no more that a few hundred million years old. A model for the evolution of small crater populations on an absolute time scale is presented and the absolute ages of young features are estimated. There is no evidence in the data for secondary cratering, nor is there any clear variation of counts and slopes with longitude or latitude, so other possible explanations for the variabilities between sites must be found. The results obtained by this project should prove useful for modelling crater formation and destruction and also as training data for machine learning systems. • Automated counting of small lunar craters. • Results plotted as Crater Size Frequency Histograms rather than CSFDs. • Confirmation of recent resurfacing of lunar maria. • Dating estimates for mare resurfacing and young Copernican age craters. • Model for evolution of cratered lunar maria. [ABSTRACT FROM AUTHOR]

Landslides are geomorphological features observed on many planetary bodies, formed in a wide range of geological, geomorphological and environmental conditions. Hence, study of their morphological and morphometric characteristics, along with their absolute or relative dating, can improve the understanding of geological and environmental conditions at the time of their emplacement. On Mars, landslides are present over a very large part of the planet, and the most spectacular and best studied are those in Valles Marineris where many mass wasting phenomena occurred between Hesperian and Late Amazonian periods. The present study aims at improving the knowledge of Martian geological and geomorphological evolution including environmental conditions through absolute dating and morphological analysis of selected large landslides outside of Valles Marineris. These mass wasting features were used as proxies for revealing how active the relatively recent (Middle to Late Amazonian) geological environment of Mars was through the information of their trigger timings and degradation histories. The five landslides of our study, which have never been analyzed previously, are located in the region of Hydraotes Chaos and Tiu Valles near the dichotomy boundary. The geomorphological and morphometric analyses show that the studied landslides, classifiable as rock avalanches, exhibit morphological features and mobility similar to those of other terrestrial and Martian rock avalanches. The results obtained from the absolute dating reveal that these landslides occurred within a time range spanning from 835 Ma (±290) to 252 Ma (±100) ago, consistent with occurrences of other landslides in the equatorial region of Mars. The most plausible triggering factors include ground shaking resulting from meteoric impacts, marsquakes and volcanic events related to the evolution of Tharsis and Elysium regions during the Amazonian period. Local resurfacing events acting on the studied landslides are probably related to aeolian deflation and deposition, minor mass wasting events and impact cratering. Finally, the geomorphological characteristics of these landslide bodies and their absolute dating reveal a possible recent (< 250–150 Ma) tectonic activity along the flanks of Tiu Valles and the southern Hydraotes Channel. • In the present work five rock avalanches in Hydraotes Chaos and Tiu Valles region were studied and dated • The analyzed landslides developed during the Amazonian period • The analyses show that these landslides have average mobility similar to already studied Martian landslides • The landslides may have been triggered by ground shaking caused by meteorite impacts or tectonic events • The analyses make it possible to identify tectonic structures potentially active in recent periods [ABSTRACT FROM AUTHOR]

The lunar cratering chronology function is based on the careful and systematic correlation of crater size-frequency distributions (CSFDs) with radioisotopic ages of samples from the Apollo, Luna, and Chang'e landing sites. The link between the samples and the CSFDs requires a detailed understanding of the landing site geology. Therefore, in this study, we have created a new detailed regional geological map of the Apollo 14 landing site using LRO, M3, and SELENE/Kaguya data. The geologic map allows us to define homogeneous areas for new CSFD measurements, from which updated N (1) values (i.e., cumulative number of craters with diameter ≥ 1 km) can be determined for the sampled geologic units. In particular, at the Apollo 14 landing site we measured the N (1) of the Imbrian Fra Mauro Formation (Ifm), which is interpreted to be ejecta from the Imbrium basin, and can be correlated with the Apollo 14 Imbrium ejecta samples. This new calibration point can be compared with the lunar chronology of Neukum (1983). We find that the new calibration point is consistent with the lunar cratering chronology of Neukum (1983). Together with our previous studies, this result supports the validity of this chronology. • We produced detailed geological map for the Apollo 14 landing site on Fra Mauro formation which is Imbrian basin ejecta. • The N (1) values of the Fra Mauro formation were compared with recently determined radiometric sample ages. • The newly obtained calibration points are consistent with the lunar cratering chronology of Neukum (1983). [ABSTRACT FROM AUTHOR]

The topographic slopes of the Moon host different types of mass wasting features, in which boulder falls are abundant globally. Boulder falls are characterized by trails that are produced by rolling, bouncing and sliding of boulders on the lunar surface, in response to ground shaking originating from shallow moonquakes, impact events and other surface processes. Our previous work showed evidence for moonquake-triggered boulder falls at hundreds of sites on the interior of Orientale multi-ring basin. In this study, we document new evidence for ejecta boulder falls around 135 fresh simple impact craters with diameters ranging from 0.1 to 2.5 km in the same basin. Our detailed characterization of ejecta boulder falls around eight selected impact craters reveal their modes of occurrences, morphological properties, formation ages and clues to their impact origins. The ejecta boulder falls have the characteristic boulder trails arranged radially around the host impact craters, shorter trail lengths, narrow trail widths and shallow trail depths, when compared to the moonquake-triggered boulder falls in the same basin. Some impact craters exhibit radial asymmetry of boulder trails around these craters because of oblique impacts, in which longer trails are abundant in down-range directions, while fewer and shorter trails characterize up-range directions. In some cases, local slopes also contribute to increased trail lengths and deviation from radial distribution of trails. Formation ages of a few host impact craters, determined based on conventional crater counting method, are in the range of 0.94–6.4 Ma, and the age of ejecta boulder falls is consistent with the host impact crater ejecta. These characteristics suggest that the trails were produced by the ejecta boulders, during the ejecta blanket formation stage of the host impact craters. The relationship between the host impact crater diameter and the average size of large ejecta boulders present in their ejecta blankets suggests that the host impact craters are primary impact craters. Hence, the initial ejecta boulder fall velocities are expected to be in the ejection velocity range of ∼5–100 m/s. During ballistic sedimentation, these ejecta boulders erode the local target materials including regolith and mix them along their trails. It is also observed that ejecta boulder falls occur around small fresh simple impact craters (<2–3 km) at the Schrödinger and Lorentz basins. Around large size impact craters and basins, ejecta boulder falls produced large-size radial grooves, lineaments and sculptures on the Moon. We conclude that the characteristics of ejecta boulder falls and moonquake boulder falls are distinctly different from each other, reflecting their distinct formation conditions, but these form contemporaneously on the Moon. • Hundreds of small fresh impact craters developed trail forming ejecta boulder falls at the Orientale, Schrödinger and Lorentz basins • The ejecta spall boulders produced trails radially around fresh lunar impact craters during the ejecta emplacement • Oblique impacts produced radial asymmetry in the ejecta boulder trail distribution and variation of trail lengths with respect to the impact trajectory • The ejecta boulder trails are characterized by shorter lengths, narrow widths, and shallow trail depths than the moonquake triggered boulder trails • The ejecta boulder falls are produced by ballistic sedimentation processes during ejecta emplacement of host impact craters [ABSTRACT FROM AUTHOR]

Pluto's Sputnik basin is the only large impact basin in the outer Solar System that resembles basins in the inner Solar System in morphology and scale, with dimensions of ∼1800 × 1000 km and a depth of ∼2.5 km. Sputnik basin has been compared to giant impact basins without prominent topographic rings, such as the Hellas basin on Mars and South Pole-Aitken basin on the Moon. However, a north-south trending arcuate chain of water-ice mountain blocks within the western half of the basin resembles a peak-ring structure as found in impact basins on terrestrial bodies, suggesting that Sputnik basin may be more analogous to a peak-ring or multiring basin. Here, we perform a comparative analysis between Sputnik basin and 23 impact basins in the inner Solar System. We reconstruct Sputnik basin's topography prior to being partially filled with an N 2 -rich ice deposit. In our analyses, we measure key characteristics such as average radial topography, ring spacing and crustal/shell thinning beneath the basins. We find that Sputnik basin is morphologically, topographically, and structurally consistent with the peak−/multiring basins, having most similarities to Apollo and Humboldtianum basins on the Moon, rather than giant impact basins without prominent topographic rings. Sputnik's consistency with a peak−/multiring structure has implications for aspects of its formation, evolution, and structure. In particular, this new interpretation of the basin structure decreases the size of both the impactor and the possible mass anomaly contained within the basin by half relative to previous interpretations. • Sputnik basin is a large impact basin that resembles inner Solar System basins. • Newly reconstructed Sputnik pre-fill topography illuminates subsurface structure. • Sputnik is topographically and structurally consistent with peak−/multiring basins. • New interpretation decreases the predicted size of impactor and mass anomaly by half. [ABSTRACT FROM AUTHOR]

Highlights • Widespread cratered terrain in the Rongo quadrangle formed at about 1.8 Ga. • Two cryovolcanic edifices are identified within the quadrangle. • The 260 km diameter Yalode basin structure formed at approximately 580 Ma. • Yalode and Haulani ejecta could potentially serve as regional stratigraphic markers. Abstract The Dawn spacecraft arrived at dwarf planet Ceres in spring 2015 and imaged its surface from four successively lower polar orbits at ground sampling dimensions between ∼1.3 km/px and ∼35 m/px. To understand the geological history of Ceres a mapping campaign was initiated to produce a set of 15 quadrangle-based geological maps using the highest-resolution Framing Camera imagery. Here we present the geological map of the Ac-10 Rongo Quadrangle, which is located at the equator encompassing the region from 22°N to 22°S and 288° to 360°E. The total relief within the quadrangle is 11.1 km with altitudes ranging from about −7.3 km to +3.8 km. We identified nine geological units based on surface morphology and surface textural characteristics. The dominant and most widespread unit is the cratered terrain (crt) representing ancient reworked crustal material. Its consistent formation age across the quadrangle is 1.8 Ga. Two edifices (unit th), Ahuna Mons and an unnamed tholus within Begbalel Crater, are interpreted to be of (cryo)volcanic origin. The southwest portion of the quadrangle is dominated by ejecta material (Ye) emplaced during the formation of the 260-km diameter Yalode impact basin at about 580 Ma. Rayed crater ejecta material (cr) is dominant in the eastern part of the quadrangle but also occurs in isolated patches up to a distance of 455 km from the 34 km diameter source crater Haulani. The remaining five geological units also represent impact crater materials: degraded rim (crdeg), bright crater (cb), hummocky floor (cfh), talus (ta), and crater (c) materials. Widespread Yalode and Haulani ejecta materials can potentially be utilised as stratigraphic markers. Therefore, it is essential to consistently map their full areal extent and to date their formations using impact crater statistics. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

Highlights • We mapped the Ac-2 Coniraya Quadrangle of Ceres at a scale of 1:500,000. • We derived CSFD measurements for all major surface units and obtained theirAMAs. • The youngest features are the fresh craters like Oxo (∼4 Ma) or FC IV (∼12 Ma), while the oldest unit is the cratered terrain (3.1 Ga to 3.5 Ga). • We identified smooth impact melt-like deposits with pits in and around Ikapati crater. • Possible ice in the crust is heterogeneously distributed, as lobate flows, bright spots, and pits are only found at certain locations such as Ikapati and Oxo. Abstract Since its arrival at Ceres, Dawn's Framing Camera has been imaging the dwarf planet at different altitudes, using 8 different filters. Based on these images, global clear filter mosaics, digital terrain models, and global color mosaics were produced. These datasets are basis for the derived photo-geologic map of the Ac-2 Coniraya quadrangle, located between 0° and 90° eastern longitude and between 20° and 40° northern latitude. Crater size-frequency distribution (CSFD) measurements have been applied to derive the stratigraphic history of the identified geologic units in the Ac-2 Coniraya quadrangle. Based on these investigations, we found that the Ac-2 Coniraya quadrangle shows evidence for a long geologic history lasting over 3.5 Ga. Most of the identified geologic units are related to impact cratering at different periods of time. While the oldest unit identified in the Ac-2 Coniraya quadrangle, the cratered terrain, shows an absolute model age (AMA) of 3.3–3.5 Ga, the youngest units, including fresh impact craters and lobate flows, show AMAs of < 100 Ma. The large number of different sized impact craters excavated a variety of materials with different colors and albedo, indicative of a heterogeneous crustal composition, including water or volatiles. Pits identified at Ikapati crater's smooth deposits might further indicate the presence of volatiles in the subsurface. Furthermore, our mapping of the Ac-2 Coniraya quadrangle shows that possible H 2 O ice in the crust is heterogeneously distributed, as lobate flows, bright spots, and pits are only found at certain locations, related to relatively fresh impact craters. [ABSTRACT FROM AUTHOR]

Highlights • A photo-geologic map of Ceres North Pole Ac-1 Asari quadrangle has been produced with Dawn Framing Camera images at a resolution of 35 m/pixel. • Lobate materials are associated with impact cratering and are possibly formed by high speed flows. Their abundance does not appear to be latitude dependent. • Approximately 1-km large bright areas are found on the floor of few craters, within permanently shadowed regions, and might originate from infall of exogenic material. Abstract The Dawn Framing Camera repeatedly imaged Ceres' North Pole quadrangle (Ac-1 Asari, latitudes >66°N) at a resolution of ∼35 m/pixel through a panchromatic filter, enabling the derivation of a digital terrain model (DTM) and an ortho-rectified mosaic. Using this dataset, a photo-geologic map and stratigraphy, complemented with absolute model ages of impact craters, were produced. We identified the following key surface features: an ancient 4.5 km high isolated dome with a non-impact origin; recent lobate materials on crater interiors possibly formed as high-speed flows of collapsed rim material; and recent bright areas in permanently shadowed regions (PSRs), which we interpret as ice accumulation mostly by infall of exogenic material. Crater morphologies and dimensions do not differ from those in other quadrangles, suggesting the widespread influence of a rheologically weak target during the crater formation process. There is a paucity of lobate materials associated with impact cratering, in contrast to previous identifications with lower spatial resolution imagery. [ABSTRACT FROM AUTHOR]

In most of the previous studies related to collisional disruption of planetesimals in the gravity regime, Smoothed Particle Hydrodynamics (SPH) simulations have been used. On the other hand, impact simulations using grid-based hydrodynamic code have not been sufficiently performed. In the present study, we execute impact simulations in the gravity regime using the shock-physics code iSALE, which is a grid-based Eulerian hydrocode. We examine the dependence of the critical specific impact energy Q RD * on impact conditions for a wide range of specific impact energy ( Q R ) from disruptive collisions to erosive collisions, and compare our results with previous studies. We find that collision outcomes of the iSALE simulation agree well with those of the SPH simulation. Detailed analysis mainly gives three results. (1) The value of Q RD * depends on numerical resolution, and is close to convergence with increasing numerical resolution. The difference in converged value of Q RD * between the iSALE code and the SPH code is within 30%. (2) Ejected mass normalized by total mass ( M ej / M tot ) generally depends on various impact conditions. However, when Q R is normalized by Q RD * that is calculated for each impact simulation, M ej / M tot can be scaled by Q R / Q RD * , and is independent of numerical resolution, impact velocity and target size. (3) This similarity law for Q R / Q RD * is confirmed for a wide range of specific impact energy. We also derive a semi-analytic formula for Q RD * based on the similarity law and the crater scaling law. We find that the semi-analytic formula for the case with a non-porous object is consistent with numerical results. [ABSTRACT FROM AUTHOR]

GRAIL observations have been used to derive an updated catalog of lunar impact basins with diameters ≥ 450 km. In this study, we assess the age and relaxation state of these basins and find that the 13–15 stratigraphically younger basins are significantly less relaxed than the 8–16 identifiable older basins. The change in relaxation state is most likely a signature of the cooling of the base of the lunar crust below 1300–1400 K, which we infer happened between 4.21–4.45 Ga based on geochronological measurements. We compare the predicted number of basins before and after this reference point with several different modeled lunar cratering chronologies. While these models are sensitive to the ratio of 1km:450 km craters, we conclude that a model with a broad period of late heavy bombardment (a “sawtooth”) is more likely to fit both the number of relaxed and unrelaxed basins, and the mass of material delivered as a late veneer. [ABSTRACT FROM AUTHOR]

Impact craters across Mars, that are otherwise similar, commonly exhibit vastly different thermal inertia values on their rims. We focused on the Tisia Valles region because the terrain is mostly flat, and craters of various ages are present within the same target geologic unit. We present an investigation of two hypotheses for why these thermal inertia variations exist. The first hypothesis is that crater rim thermal inertia is affected by crater degradation state, and the second hypothesis is that thermal inertia is affected by the amount of regolith mantling the rims, regardless of degradation state. To investigate the first hypothesis, we used a series of multilevel regression analyses to test for correlations between crater rim thermal inertia and depth-diameter ratio, rim irregularity, and radii variation, which are indicators of crater degradation state. To investigate the second hypothesis, we tested for a correlation between thermal inertia and mantled crater rim percentage, which is indicative of the amount of regolith atop the rims. Our results did not support the first hypothesis but did support the second hypothesis. Therefore, we concluded that regolith rim mantling affects crater rim thermal inertia variations in the Tisia Valles region. Crater degradation may not have an effect on rim thermal inertia because regolith is transported down the crater rim slopes faster than local regolith is produced from erosion in this region. An implication of this work is that the spatial extent of regolith-rich regions near Tisia Valles can be mapped using the spatial extent of proximal craters with low thermal inertia rims. Additionally, the timing of mantling events can be quantified by comparing counts of craters with high thermal inertia rims to those of low thermal inertia rims within a region. [ABSTRACT FROM AUTHOR]

In this work we update the regolith mixing model presented by Gault et al. (1974) , including new input values and reworking key parameters. Much as Gault et al. did, we present a way to calculate the rate at which lunar regolith is overturned at depth. The model describes a mixing front that proceeds downward from the surface following a power-law function of time. Our most important update is the inclusion of secondary impacts. Our calculations show that secondaries are necessary to produce the reworking rate inferred from the depth distribution of surface-correlated material in Apollo cores (Fruchter et al., 1977; Morris, 1978; Blanford, 1980), from the rate at which splotches rework the top 3 cm of regolith (Speyerer et al., 2016), and from the rate at which Diviner cold spots (Bandfield et al., 2014) and crater rays (Pieters et al., 1985; Hawke et al., 2004; Werner and Medvedev, 2010) are reworked into background regolith. Overturn calculations that only consider the impact of primaries fail to describe observed reworking rates at all depths and timescales. We conclude that secondary impacts dominate mixing in the top meter of lunar regolith. [ABSTRACT FROM AUTHOR]

The relationship between the length of crater rays and primary crater radius is still poorly understood. In this study we mapped the ray systems of 27 lunar craters, ranging from 10 m to 84 km in diameter. For each ray system, we measured the number, length, and optical maturity index (OMAT) of the rays. Our mapping effort shows that ray length scales to primary crater radius (R) with a power-law of R 1.22 , except for the smallest and freshest rays (<10 m diameter craters, less than ∼40 years old), which were ∼10 radii longer than expected. We also undertook an analytical modeling exercise in an attempt to better understand the physical processes that control ray length. The model suggests that the empirical relationship (R 1.22 ) can be explained as resulting from the combination of 1) the relationship between ejecta fragment size and ejected distance, and 2) the scaling of secondary crater diameters, which create rays by excavating bright material from below the dark space-weathered layer, as a function of ejected velocity. When the ejecta fragments are large enough, they are able to excavate bright material from beneath the mature space-weathered layer, while smaller ejecta fragments simply mix the dark mature lunar soil. We then use this secondary crater scaling relationship to predict the ray length for different depths of well-weathered lunar soil. The anomalously long rays for the smallest, freshest craters may be due to the fact that their bright ejecta fragments are not traveling fast enough to excavate below the space-weathered surface layer, and simply become deposited on the surface. We suggest observations that will help refine many of the poorly constrained assumptions of our model. [ABSTRACT FROM AUTHOR]

Given rapid photodissociation and photodegradation, the recently discovered organics in the Martian subsurface and atmosphere were probably delivered in geologically recent times. Possible parent bodies are C-type asteroids, comets, and interplanetary dust particles (IDPs). The dust infall rate was estimated, using different methods, to be between 0.71 and 2.96 × 10 6 kg/yr (Nesvorny et al., 2011; Borin et al., 2017; Crismani et al., 2017); assuming a carbon content of 10% (Flynn, 1996), this implies an IDP carbon flux of 0.07 − 0.3 × 10 6 kg/yr. We calculate for the first time the carbon flux from impacts of asteroids and comets. To this end, we perform dynamical simulations of impact rates on Mars. We use the N-body integrator RMVS/Swifter to propagate the Sun and the eight planets from their current positions. We separately add comets and asteroids to the simulations as massless test particles, based on their current orbital elements, yielding Mars impact rates of 4.34 × 10 − 3 comets/Myr and 3.3 asteroids/Myr. We estimate the delivered amount of carbon using published carbon content values. In asteroids, only C types contain appreciable amounts of carbon. Given the absence of direct taxonomic information on the Mars impactors, we base ourselves on the measured distribution of taxonomic types in combination with dynamic models of the origin of Mars-crossing asteroids. We estimate the global carbon flux on Mars from cometary impacts to be  ∼ 0.013 × 10 6 kg/yr within an order of magnitude, while asteroids deliver  ∼ 0.05 × 10 6 kg/yr. These values correspond to ∼ 4 − 19 % and ∼ 17 − 71 % , respectively, of the IDP-borne carbon flux estimated by Nesvorny et al., Borin et al. and Crismani et al. Unlike the spatially homogeneous IDP infall, impact ejecta are distributed locally, concentrated around the impact site. We find organics from asteroids and comets to dominate over IDP-borne organics at distances up to 150 km from the crater center. Our results may be important for the interpretation of in situ detections of organics on Mars. [ABSTRACT FROM AUTHOR]

Using crater size-frequency distributions (CSFDs) and careful discrimination of primary and secondary impact craters in the mare region south of Aristarchus Crater, we reexamined the age of unit ‘P60’, determined previously by Hiesinger et al. (2003) to be one of the youngest basaltic surfaces on the Moon. Owing to the apparently young age of these basalts, this region is a potential site for future sample return missions. We investigated this 55,000 km 2 region with Lunar Reconnaissance Orbiter Camera (LROC) images to assess potential variations in CSFDs across the unit, particularly in light of heavy contamination by secondary craters from Aristarchus Crater, and to determine the age(s) of P60 over its full areal extent. We are able to match, within one standard error, the previously determined age using approximately the same counting area. In addition, we defined twelve regions between the rays of secondaries to determine if and how the ages of P60 might vary across the unit. For these inter-ray regions, we find a systematic progression of ages from west to east, ranging from youngest (∼1 Ga) in the west, to 1.8 Ga southwest of Aristarchus, to 2.2 Ga south of Aristarchus, and finally to 2.7 Ga southeast of Aristarchus. This variation in ages is not solely attributable to secondary cratering, indicating it must be at least partially due to volcanic resurfacing. The northwestern-most extension of P60 may belong to a different unit owing to topographic and crater distribution differences. Analysis of the summed CSFD for P60 provides evidence for emplacement of the younger basalts (<2.5 Ga) on top of an older surface of ∼3.6 Ga. We observe that ∼2–3 km diameter craters within an older mare unit are embayed by the younger lava flows, some of which are ‘ghost’ craters, with barely visible rims. In addition, we identified six volcanic vents that are possible sources for the younger P60 flow(s). This work indicates that volcanism spanned a significant range, from ∼3.6 Ga to 1.0 Ga ago across this region. [ABSTRACT FROM AUTHOR]

Over the last decade, observations acquired by the Shallow Radar (SHARAD) sounder on individual passes of the Mars Reconnaissance Orbiter have revealed the internal structure of the Martian polar caps and provided new insights into the formation of the icy layers within and their relationship to climate. However, a complete picture of the cap interiors has been hampered by interfering reflections from off-nadir surface features and signal losses associated with sloping structures and scattering. Foss et al. (The Leading Edge 36, 43–57, 2017, https://doi.org/10.1190/tle36010043.1 ) addressed these limitations by assembling three-dimensional data volumes of SHARAD observations from thousands of orbital passes over each polar region and applying geometric corrections simultaneously. The radar volumes provide unprecedented views of subsurface features, readily imaging structures previously inferred from time-intensive manual analysis of single-orbit data (e.g., trough-bounding surfaces, a buried chasma, and a basal unit in the north, massive carbon-dioxide ice deposits and discontinuous layered sequences in the south). Our new mapping of the carbon-dioxide deposits yields a volume of 16,500 km 3 , 11% larger than the prior estimate. In addition, the radar volumes newly reveal other structures, including what appear to be buried impact craters with no surface expression. Our first assessment of 21 apparent craters at the base of the north polar layered deposits suggests a Hesperian age for the substrate, consistent with that of the surrounding plains as determined from statistics of surface cratering rates. Planned mapping of similar features throughout both polar volumes may provide new constraints on the age of the icy layered deposits. The radar volumes also provide new topographic data between the highest latitudes observed by the Mars Orbiter Laser Altimeter and those observed by SHARAD. In general, mapping of features in these radar volumes is placing new constraints on the nature and evolution of the polar deposits and associated climate changes. [ABSTRACT FROM AUTHOR]

The 492 km-diameter Apollo impact basin post-dates, and is located at the inner edge of, the ∼2240 km-diameter South Pole–Aitken (SPA) basin, providing an opportunity to assess the SPA substructure and lateral heterogeneity. Gravity Recovery and Interior Laboratory gravity data suggest an average crustal thickness on the floor of SPA of ∼20 km and within the Apollo basin of ∼5 km, yet remote sensing data reveal no conclusive evidence for the presence of exposed mantle material. We use the iSALE shock physics code to model the formation of the Apollo basin and find that the observational data are best fit by the impact of a 40 km diameter body traveling at 15 km/s into 20–40 km thick crustal material. These results strongly suggest that the Apollo impact occurred on ejecta deposits and collapsed crustal material of the SPA basin and could help place constraints on the location, size and geometry of the SPA transient cavity. The peak ring in the interior of Apollo basin is plausibly interpreted to be composed of inwardly collapsed lower crustal material that experienced peak shock pressures in excess of 35 GPa, consistent with remote sensing observations that suggest shocked plagioclase. Proposed robotic and/or human missions to SPA and Apollo would present an excellent opportunity to test the predictions of this work and address many scientific questions about SPA basin evolution and structure. [ABSTRACT FROM AUTHOR]

A new class of wind streaks on Mars uniquely associated with impact craters is most clearly detected in nighttime thermal infrared imaging. Thermally bright streaks radiate from some well-preserved impact craters and are related to the impact process. Using laboratory experiments performed at the NASA Ames Vertical Gun Range, we test the hypothesis that these streaks are formed from either the winds within an air-blast or winds set up by expanding impact vapor interacting with the atmosphere. The experiments use a variety of tracers and instruments to document three interrelated processes occurring in the impact of a Pyrex projectile into an easily vaporized powdered dolomite target: (1) a surface roughening spreading outward from the impact point, (2) an expanding vapor plume, and (3) outward winds made visible by dust trails from vertically placed, dusty pipe cleaners. The clear connection between the surface roughening, vapor expansion, and outward winds implicate an expanding vapor interacting with the atmosphere as the controlling process. [ABSTRACT FROM AUTHOR]

Titan's few impact craters are currently many hundreds of meters shallower than the depths expected. Assuming these craters initially had depths equal to that of similar-size fresh craters on Ganymede and Callisto (moons of similar size, composition, and target lithology), then some process has shallowed them over time. Since nearly all of Titan's recognized craters are located within the arid equatorial sand seas of organic-rich dunes, where rain is infrequent, and atmospheric sedimentation is expected to be low, it has been suggested that aeolian infill plays a major role in shallowing the craters. Topographic relaxation at Titan's current heat flow was previously assumed to be an unimportant process on Titan due to its low surface temperature (94 K). However, our estimate of the thermal conductivity of Titan's organic-rich sand is remarkably low (0.025 W m −1 K −1 ), and when in thick deposits, will result in a thermal blanketing effect that can aid relaxation. Here, we simulate the relaxation of Titan's craters Afekan, Soi, and Sinlap including thermal effects of various amounts of sand inside and around Titan's craters. We find that the combination of aeolian infill and subsequent relaxation can produce the current crater depths in a geologically reasonable period of time using Titan's current heat flow. Instead of needing to fill completely the missing volume with 100% sand, only ∼62%, ∼71%, and ∼97%, of the volume need be sand at the current basal heat flux for Afekan, Soi, and Sinlap, respectively. We conclude that both processes are likely at work shallowing these craters, and this finding contributes to why Titan overall lacks impact craters in the arid equatorial regions. [ABSTRACT FROM AUTHOR]

Young asteroid families are unique sources of information about fragmentation physics and the structure of their parent bodies, since their physical properties have not changed much since their birth. Families have different properties such as age, size, taxonomy, collision severity and others, and understanding the effect of those properties on our observations of the size-frequency distribution (SFD) of family fragments can give us important insights into the hypervelocity collision processes at scales we cannot achieve in our laboratories. Here we take as an example the very young Datura family, with a small 8-km parent body, and compare its size distribution to other families, with both large and small parent bodies, and created by both catastrophic and cratering formation events. We conclude that most likely explanation for the shallower size distribution compared to larger families is a more pronounced observational bias because of its small size. Its size distribution is perfectly normal when its parent body size is taken into account. We also discuss some other possibilities. In addition, we study another common feature: an offset or “bump” in the distribution occurring for a few of the larger elements. We hypothesize that it can be explained by a newly described regime of cratering, “spall cratering”, which controls the majority of impact craters on the surface of small asteroids like Datura. [ABSTRACT FROM AUTHOR]

The orbital data products of Ceres, including global LAMO image mosaic and global HAMO DTM with a resolution of 35 m/pixel and 135 m/pixel respectively, are utilized in this research to create a global catalogue of impact craters with diameter ≥ 1 km, and their morphometric parameters are calculated. Statistics shows: (1) There are 29,219 craters in the catalogue, and the craters have a various morphologies, e.g., polygonal crater, floor fractured crater, complex crater with central peak, etc.; (2) The identifiable smallest crater size is extended to 1 km and the crater numbers have been updated when compared with the crater catalogue ( D  ≥ 20 km) released by the Dawn Science Team; (3) The d/D ratios for fresh simple craters, obviously degraded simple crater and polygonal simple crater are 0.11 ± 0.04, 0.05 ± 0.04 and 0.14 ± 0.02 respectively. (4) The d/D ratios for non-polygonal complex crater and polygonal complex crater are 0.08 ± 0.04 and 0.09 ± 0.03. The global crater catalogue created in this work can be further applied to many other scientific researches, such as comparing d/D with other bodies, inferring subsurface properties, determining surface age, and estimating average erosion rate. [ABSTRACT FROM AUTHOR]

Iapetus's equatorial ridge, visible in global views of the moon, is unique in the Solar System. The formation of this feature is likely attributed to a key event in the evolution of Iapetus, and various models have been proposed as the source of the ridge. By surveying imagery from the Cassini and Voyager missions, this study aims to compile a database of the impact crater population on and around Iapetus's equatorial ridge, assess the relative age of the ridge from differences in cratering between on ridge and off ridge, and test the various models of ridge formation. This work presents a database that contains 7748 craters ranging from 0.83 km to 591 km in diameter. The database includes the study area in which the crater is located, the latitude and longitude of the crater, the major and minor axis lengths, and the azimuthal angle of orientation of the major axis. Analysis of crater orientation over the entire study area reveals that there is no preference for long-axis orientation, particularly in the area with the highest resolution. Comparison of the crater size-frequency distributions show that the crater distribution on the ridge appears to be depleted in craters larger than 16 km with an abruptly enhanced crater population less than 16 km in diameter up to saturation. One possible interpretation is that the ridge is a relatively younger surface with an enhanced small impactor population. Finally, the compiled results are used to examine each ridge formation hypothesis. Based on these results, a model of ridge formation via a tidally disrupted sub-satellite appears most consistent with our interpretation of a younger ridge with an enhanced small impactor population. [ABSTRACT FROM AUTHOR]

This work revisits the hypothesis of the so-called ‘lunar terminal cataclysm’ suggested by Tera et al. (1973, 1974) as a strong peak in the meteorite bombardment of the Moon around 3.9 Ga ago. According to the hypothesis, most of the impact craters observed on the lunar highlands formed during this short time period and thus formed the majority of the lunar highland impact breccias and melts. The hypothesis arose from the observation that the ages of highland samples from all the lunar missions are mostly grouped around 3.9–4.0 Ga. Since those missions, however, radiometric dating techniques have progressed and many samples, both old and new, have been re-analyzed. Nevertheless, the debate over whether there was a terminal cataclysm persists. To progress in this problem we summarized results of 269 K–Ar datings (mostly made using the 40 Ar– 39 Ar technique) of highland rocks represented by the Apollo 14, 15, 16, 17 and Luna 20 samples and 94 datings of clasts of the highland rocks from 23 lunar meteorites representing 21 localities on the lunar surface, and considered them jointly with the results of our modelling of the cumulative effect of the impact gardening process on the presence of impact melt of different ages at the near-surface of the Moon. The considered results of K–Ar dating of the Apollo-Luna samples of lunar highland rocks confirmed a presence of strong peak centered at 3.87 Ga. But since the time when the hypothesis of terminal cataclysm was suggested, it has become clear that this peak could be a result of sampling bias: it is the only prominent feature at the sites with an apparent domination of Imbrium basin ejecta (Apollo 14 and 15) and the age pattern is more complicated for the sites influenced not only by Imbrium ejecta but also that of other basins (Nectaris at the Apollo 16 site and Serenitatis at the Apollo 17 site). Our modelling shows that the cataclysm, if it occurred, should produce a strong peak in the measured age values but we see in the considered histograms and relative probability plots not only the 3.87 Ga peak (due to Imbrium basin), but also several secondary peaks caused by the formation of other basins distributed between 3.87 and 4.25 Ga. The lunar terminal cataclysm hypothesis is in disagreement with the distribution of K–Ar ages for the highland rocks of the lunar meteorites. The population of lunar meteorites representing localities randomly distributed over the lunar surface, and thus free from the mentioned sampling bias, shows no ∼3.9 Ga peak as it should, if the cataclysm did occur. We conclude that the statistics of sample ages contradict the terminal cataclysm scenario in the bombardment of the Moon. We also see evidence for the formation of several impact basins between 3.87 and 4.25 Ga which is likewise incompatible with the hypothesis of a short interval cataclysm. There remain other basins, including the largest South Pole – Aitken, the ages of which should be determined in future studies to further clarify the impact history. Sample-return missions targeted to date several key basins need to be planned, and the continued study of lunar meteorites may also bring new details to the general view of the impact bombardment of the Moon. [ABSTRACT FROM AUTHOR]

Impact-cratering processes on small bodies are thought to be mainly controlled by the local material strength because of their low surface gravity, and craters that are as large as the parent bodies should be affected by the target curvature. Although cratering processes on planar surfaces in the strength-controlled regime have been studied extensively, the mechanism by which target curvature affects the cratering processes remains unclear. Herein, we report on a series of impact experiments that used spherical targets with various diameters. The resultant craters consisted of a deep circular pit and an irregular-shaped spall region around the pit, which is consistent with the features reported in a number of previous cratering experiments on planar surfaces. However, the volume and radius of the craters increased with the normalized curvature. The results indicate that the increase in the spall-region volume and radius mainly contributes to the increase in the whole crater volume and radius, although the volume, depth, and radius of pits remain constant with curvature. The results of our model indicate that the geometric effect due to curvature (i.e., whereby the distance from the equivalent center to the target free surface is shorter for higher curvature values) contributes to increases in the cratering efficiency. Our results suggest that the impactors that produce the largest craters (basins) on some asteroids are thus smaller than what is estimated by current scaling laws, which do not take into account the curvature effects. [ABSTRACT FROM AUTHOR]

The cold, northern high-latitudes around the Phoenix landing site feature a population of small and very shallow craters known as palimpsest craters or boulder halos that could give insight into the subsurface ice content of the region. Notably there is also a split in the population by the size of craters that feature blocky ejecta: the smallest crater sizes that are <200–350 m in diameter typically do not feature blocky ejecta, while craters larger than this size range do. This has been interpreted as there being a less consolidated layer of mainly ice that sits over a more consolidated layer containing more silicate material from which the larger craters are able to excavate the blocky ejecta. This work focuses on determining whether these 200 m - 1 km shallow craters could be indicative of topographic relaxation due to substantial ice in the subsurface, with goals to determine the subsurface structure and most importantly the amount of ice present through its abundance and depth extent. We use finite element modeling of axisymmetric simple craters, employing the most recent characterizations of solid-state creep for water ice, silicate-rich ice, and CO 2 ice, to determine the timescales and magnitudes of relaxation accommodated by an icy single or multiple material layered subsurface to determine the environment's properties. Our models include a 35 m thick layer of pure ice (water, or water and CO 2) over a mobile substrate of particulate rich water ice of variable thicknesses able to accommodate relaxation for larger crater sizes up to 1 km in diameter. Our findings demonstrate that relaxation is able to account for significant flattening of crater topography, though for smaller craters the relaxation is incomplete, leaving about half the depth. The accommodated relaxation of our modeled subsurface does imply associated increases in ice abundances of 4 to 10 times greater than is currently estimated for the region, dependent on the extent of the mobile layer. • Sub-km icy craters at high latitudes on Mars can partially relax over 10 Myr. • Observed flat crater profiles suggest substantial near-subsurface ice. • Depth extent of icy layer at northern high latitudes could be hundreds of meters. [ABSTRACT FROM AUTHOR]

Remote sensing observations made by the spacecraft Hayabusa provided the first direct evidence of a rubble-pile asteroid: 25143 Itokawa. Itokawa was found to have a surface structure very different from other explored asteroids; covered with coarse pebbles and boulders ranging at least from cm to meter size. The cumulative size distribution of small circular depressions on Itokawa, most of which may be of impact origin, has a significantly shallower slope than that on the Moon; small craters are highly depleted on Itokawa compared to the Moon. This deficiency of small circular depressions and other features, such as clustered fragments and pits on boulders, suggest that the boulders on Itokawa might behave like armor, preventing crater formation: the “armoring effect”. This might contribute to the low number density of small crater candidates. In this study, the cratering efficiency reduction due to coarse-grained targets was investigated based on impact experiments at velocities ranging from  ∼ 70 m/s to  ∼ 6 km/s using two vertical gas gun ranges. We propose a scaling law extended for cratering on coarse-grained targets (i.e., target grain size  ≳  projectile size). We have found that the crater efficiency reduction is caused by energy dissipation at the collision site where momentum is transferred from the impactor to the first-contact target grain, and that the armoring effect can be classified into three regimes: (1) gravity scaled regime, (2) reduced size crater regime, or (3) no apparent crater regime, depending on the ratio of the impactor size to the target grain size and the ratio of the impactor kinetic energy to the disruption energy of a target grain. We found that the shallow slope of the circular depressions on Itokawa cannot be accounted for by this new scaling law, suggesting that obliteration processes, such as regolith convection and migration, play a greater role in the depletion of circular depressions on Itokawa. Based on the new extended scaling law, we found that the crater retention age on Itokawa is 3–33 Myr in the main belt, which is in good agreement with the cosmic-ray-exposure ages for returned samples from Itokawa which may reflect the age of material a few meters beneath the surface. These ages strongly suggest that the global resurfacing that reset the 1–10 m deep surface layer may have occurred in the main belt long after the possible catastrophic disruption of a rigid parent body of Itokawa suggested by Ar degassing age ( ∼ 1.3 Gyr). [ABSTRACT FROM AUTHOR]

Many small bodies in the solar system have bulk density well below the solid density of the constituent mineral grains in their meteorite counterparts. Those low-density bodies undoubtedly have significant porosity, which is a key factor that affects the formation of impact craters. This paper summarizes the results of lab experiments in which materials with porosity ranging from 43% to 96% were impacted at ∼1800 m/s. The experiments were performed on a geotechnical centrifuge, in order to reproduce the lithostatic overburden stress and ejecta ballistics that occur in large-scale cratering events on asteroids or planetary satellites. Experiments performed at various accelerations, up to 514G, simulate the outcomes of impacts at size scales up to several tens of km in diameter. Our experiments show that an impact into a highly porous cohesionless material generates a large ovoid-shaped cavity, due to crushing by the outgoing shock. The cavity opens up to form a transient crater that grows until the material flow is arrested by gravity. The cavity then collapses to form the final crater. During collapse, finely crushed material that lines the cavity wall is carried down and collected in a localized region below the final crater floor. At large simulated sizes (high accelerations), most of the crater volume is formed by compaction, because growth of the transient crater is quickly arrested. Nearly all ejected material falls back into the crater, leaving the crater without an ejecta blanket. We find that such compaction cratering and suppression of the ejecta blankets occur for large craters on porous bodies when the ratio of the lithostatic stress at one crater depth to the crush strength of the target exceeds ∼0.005. The results are used to identify small solar system bodies on which compaction cratering likely occurs. A model is developed that gives the crater size and ejecta mass that would result for a specified impact into a porous object. [ABSTRACT FROM AUTHOR]

We identified and mapped 129 mare basalt deposits in and around the South Pole–Aitken (SPA) basin, and determined absolute model ages (AMAs) for 101 of these units by performing crater size-frequency distribution (CSFD) measurements. The derived AMAs range from 2.2 Ga to 3.7 Ga, with the youngest deposits within Antoniadi crater and the oldest deposits at Jules Verne crater. Our investigations indicate a major peak in volcanic activity between 3.6 Ga and 3.2 Ga, which is a similar time range as the major volcanic activity on the nearside, and the rest of the farside. However, a second peak in volcanic activity (2.2–2.5 Ga), as observed for the nearside and parts of the farside, is not observed for the mare deposits within the SPA basin. Combining all AMAs derived for farside mare basalts reveals that volcanic activity was more abundant and lasted longer on the nearside than on the farside. We propose that the stripping of insulating crust by the large SPA-forming impact event, in combination with lower amounts of heat producing elements like Th, might be responsible for the reduced volcanic activity in the SPA basin. In addition, we estimated the thicknesses and volumes of the investigated mare deposits. With thicknesses between ∼31 m and ∼273 m and volumes of ∼1 km³ to ∼2630 km³, the mare basalt deposits in and around the SPA basin show a wide range of dimensions, similar to other mare basalts of the near- and farsides. A trend between the AMAs and the estimated volumes was not observed, but the mare deposits within the large northern craters (e.g., Apollo, Ingenii, or Leibnitz) seem to be generally larger and more voluminous than the mare basalt deposits at the center of the SPA basin. [ABSTRACT FROM AUTHOR]

Crater size-frequency distributions on the ejecta blankets of Aristarchus and Tycho Craters are highly variable, resulting in apparent absolute model age differences despite ejecta being emplaced in a geologic instant. Crater populations on impact melt ponds are a factor of 4 less than on the ejecta, and crater density increases with distance from the parent crater rim. Although target material properties may affect crater diameters and in turn crater size-frequency distribution (CSFD) results, they cannot completely reconcile crater density and population differences observed within the ejecta blanket. We infer from the data that self-secondary cratering, the formation of impact craters immediately following the emplacement of the continuous ejecta blanket by ejecta from the parent crater, contributed to the population of small craters (< 300 m diameter) on ejecta blankets and must be taken into account if small craters and small count areas are to be used for relative and absolute model age determinations on the Moon. Our results indicate that the cumulative number of craters larger than 1 km in diameter per unit area, N (1), on the continuous ejecta blanket at Tycho Crater, ranges between 2.17 × 10 −5 and 1.0 × 10 −4 , with impact melt ponds most accurately reflecting the primary crater flux ( N (1) = 3.4 × 10 −5 ). Using the cratering flux recorded on Tycho impact melt deposits calibrated to accepted exposure age (109 ± 1.5 Ma) as ground truth, and using similar crater distribution analyses on impact melt at Aristarchus Crater, we infer the age of Aristarchus Crater to be ∼280 Ma. The broader implications of this work suggest that the measured cratering rate on ejecta blankets throughout the Solar System may be overestimated, and caution should be exercised when using small crater diameters (i.e. < 300 m on the Moon) for absolute model age determination. [ABSTRACT FROM AUTHOR]

Recent work on dating Copernican-aged craters, using Lunar Reconnaissance Orbiter (LRO) Camera data, re-encountered a curious discrepancy in crater size-frequency distribution (CSFD) measurements that was observed, but not understood, during the Apollo era. For example, at Tycho, Copernicus, and Aristarchus craters, CSFDs of impact melt deposits give significantly younger relative and absolute model ages (AMAs) than impact ejecta blankets, although these two units formed during one impact event, and would ideally yield coeval ages at the resolution of the CSFD technique. We investigated the effects of contrasting target properties on CSFDs and their resultant relative and absolute model ages for coeval lunar impact melt and ejecta units. We counted craters with diameters through the transition from strength- to gravity-scaling on two large impact melt deposits at Tycho and King craters, and we used pi-group scaling calculations to model the effects of differing target properties on final crater diameters for five different theoretical lunar targets. The new CSFD for the large King Crater melt pond bridges the gap between the discrepant CSFDs within a single geologic unit. Thus, the observed trends in the impact melt CSFDs support the occurrence of target property effects, rather than self-secondary and/or field secondary contamination. The CSFDs generated from the pi-group scaling calculations show that targets with higher density and effective strength yield smaller crater diameters than weaker targets, such that the relative ages of the former are lower relative to the latter. Consequently, coeval impact melt and ejecta units will have discrepant apparent ages. Target property differences also affect the resulting slope of the CSFD, with stronger targets exhibiting shallower slopes, so that the final crater diameters may differ more greatly at smaller diameters. Besides their application to age dating, the CSFDs may provide additional information about the characteristics of the target. For example, the transition diameter from strength- to gravity-scaling could provide a tool for investigating the relative strengths of different geologic units. The magnitude of the offset between the impact melt and ejecta isochrons may also provide information about the relative target properties and/or exposure/degradation ages of the two units. Robotic or human sampling of coeval units on the Moon could provide a direct test of the importance and magnitude of target property effects on CSFDs. [ABSTRACT FROM AUTHOR]