Crater Population Research Articles

Dependence of secondary crater characteristics on downrange distance: High‐resolution morphometry and simulations

AbstractOn average, secondary impact craters are expected to deepen and become more symmetric as impact velocity (vi) increases with downrange distance (L). We have used high‐resolution topography (1–2 m/pixel) to characterize the morphometry of secondary craters as a function of L for several well‐preserved primary craters on Mars. The secondaries in this study (N = 2644) span a range of diameters (25 m ≤D≤400 m) and estimated impact velocities (0.4 km/s ≤vi≤2 km/s). The range of diameter‐normalized rim‐to‐floor depth (d/D) broadens and reaches a ceiling of d/D≈0.22 at L≈280 km (vi= 1–1.2 km/s), whereas average rim height shows little dependence on vi for the largest craters (h/D≈0.02, D > 60 m). Populations of secondaries that express the following morphometric asymmetries are confined to regions of differing radial extent: planform elongations (L< 110–160 km), taller downrange rims (L < 280 km), and cavities that are deeper uprange (L< 450–500 km). Populations of secondaries with lopsided ejecta were found to extend to at least L ∼ 700 km. Impact hydrocode simulations with iSALE‐2D for strong, intact projectile and target materials predict a ceiling for d/D versus L whose trend is consistent with our measurements. This study illuminates the morphometric transition from subsonic to hypervelocity cratering and describes the initial state of secondary crater populations. This has applications to understanding the chronology of planetary surfaces and the long‐term evolution of small crater populations.

Crater density differences: Exploring regional resurfacing, secondary crater populations, and crater saturation equilibrium on the moon

The global population of lunar craters >20 km in diameter was analyzed by Head et al., (2010) to correlate crater distribution with resurfacing events and multiple impactor populations. The work presented here extends the global crater distribution analysis to smaller craters (5–20 km diameters, n = 22,746). Smaller craters form at a higher rate than larger craters and thus add granularity to age estimates of larger units and can reveal smaller and younger areas of resurfacing. An areal density difference map generated by comparing the new dataset with that of Head et al., (2010) shows local deficiencies of 5–20 km diameter craters, which we interpret to be caused by a combination of resurfacing by the Orientale basin, infilling of intercrater plains within the nearside highlands, and partial mare flooding of the Australe region. Chains of 5–30 km diameter secondaries northwest of Orientale and possible 8–22 km diameter basin secondaries within the farside highlands are also distinguishable. Analysis of the new database indicates that craters 57–160 km in diameter across much of the lunar highlands are at or exceed relative crater densities of R = 0.3 or 10% geometric saturation, but nonetheless appear to fit the lunar production function. Combined with the observation that small craters on old surfaces can reach saturation equilibrium at 1% geometric saturation (Xiao and Werner, 2015), this suggests that saturation equilibrium is a size-dependent process, where large craters persist because of their resistance to destruction, degradation, and resurfacing.

Investigating target versus impactor influences on Martian crater morphology at the simple‐complex transition

AbstractComparing craters of identical diameter on a planet is an empirical method of studying the effects of different target and impactor properties while holding total impact energy nearly constant. We have analyzed the Martian crater population within a narrow diameter range (7 km < crater diameter < 9 km) at the simple‐complex crater transition using three approaches. We looked for correlations of morphology with surface geology using a global crater database and global geologic map. We examined selected regions in detail with high‐resolution images to further understand the relationship between crater morphology and bulk target properties. Finally, we examined craters in close proximity to each other in order to hold target properties constant, so that we could isolate impactor effects on crater morphology. We found a strong correlation between target properties and interior crater morphology, and we found little evidence that impactor properties (other than impact angle) affect crater appearance. Central uplift and wall slumping are enhanced for less consolidated targets. Layered targets affected both the excavation and modification stages of complex crater formation; the resulting craters have pseudoterraces, flat floors, and central pits.

Revised constraints on absolute age limits for Mercury's Kuiperian and Mansurian stratigraphic systems

AbstractFollowing an approach similar to that used for the Moon, Mercury's surface units were subdivided into five time‐stratigraphic systems based on geologic mapping using Mariner 10 images. The absolute time scale originally suggested for the time periods associated with these systems was based on the assumption that the lunar impact flux history applied to Mercury. However, we find that the duration and onset of corresponding periods in the stratigraphic sequences on Mercury and the Moon are not the same. Using high‐resolution and multiband image data obtained by the MErcury Surface, Space ENviroment, GEochemistry, and Ranging (MESSENGER) spacecraft, we identify and catalog fresh impact craters interpreted to have formed during Mercury's two most recent periods, the Mansurian and Kuiperian. We use the densities of the inferred Kuiperian‐ and Mansurian‐aged crater populations to estimate new limits for the age boundaries of these time intervals. Results suggest that both the Mansurian and Kuiperian periods began more recently and extended for significantly shorter durations of time than previously suggested. The Kuiperian is estimated to have initiated as recently as ~280 ± 60 Ma and the Mansurian as recently as ~1.7 ± 0.2 Ga.

Evidence for self-secondary cratering of Copernican-age continuous ejecta deposits on the Moon

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 (< 300m 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 1km 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. < 300m on the Moon) for absolute model age determination.

Evidence for stabilization of the ice-cemented cryosphere in earlier martian history: Implications for the current abundance of groundwater at depth on Mars

The present-day martian mean annual surface temperature is well below freezing at all latitudes; this produces a near-surface portion of the crust that is below the freezing point of water for > 2 consecutive years (defined as permafrost). This permafrost layer (i.e., the cryosphere) is a few to tens of km thick depending on latitude. Below the base of the permafrost (i.e., the cryosphere), groundwater is stable if it exists, and can increase and decrease in abundance as the freezing isotherm rises and falls. Where water is available, ice fills the pore space within the cryosphere; this region is known as the ice-cemented cryosphere (ICC). The potential for a large reservoir of pore ice beneath the surface has been the subject of much discussion: previous studies have demonstrated that the theoretical thickness of the martian cryosphere in the Amazonian period ranges from up to ∼9 km at the equator to ∼10–22 km at the poles. The total thickness of ice that might fill the pore space within the cryosphere (the ICC), however, remains unknown. A class of martian crater, the Hesperian-Amazonian-aged single-layered ejecta crater, is widely accepted as having formed by impact into an ice-cemented target. Although the target structure related to the larger multiple-layered ejecta craters remains uncertain, they have recently been interpreted to be formed by impact crater excavation below the ice-cemented target, and here we tentatively adopt this interpretation in order to infer the thickness of the ice-cemented cryosphere. Our global examination of the excavation depths of these crater populations points to a Hesperian-Amazonian-aged ice-cemented cryosphere that is ∼1.3 km thick at the equator, and ∼2.3 km thick at the poles (corresponding to a global equivalent water layer of ∼200 m assuming ∼20% pore ice at the surface). To explore the implications of this result on the martian climatic and hydrologic evolution, we then assess the surface temperature, atmospheric pressure, obliquity, and surface heat flux conditions under which the downward-propagating cryosphere freezing front matches the inferred ice-cemented cryosphere. The thermal models which can best reproduce the inferred ice-cemented cryosphere occur for obliquities between 25° and 45° and CO2 atmospheric pressures ≤600 mbar, but require increased heat fluxes and surface temperatures/pressures relative to the Amazonian period. Because the inferred ice-cemented cryosphere is much thinner compared with Amazonian-aged cryosphere thermal models, we suggest that the ice-cemented cryosphere ceased growing when it exhausted the underlying groundwater supply (i.e., ICC stabilization) in a more ancient period in Mars geologic history. Our thermal analysis suggests that this ICC stabilization likely occurred sometime before or at ∼3.0–3.3 Ga (during or before the Late Hesperian or Early Amazonian period). If groundwater remained below the ICC during the earlier Late Noachian period, our models predict that mean annual surface temperatures during this time were ≥212–227 K. If the Late Noachian had a pure CO2 atmosphere, this places a minimum bound on the Late Noachian atmospheric pressure of ≥390–850 mbar. These models suggest that deep groundwater is not abundant or does not persist in the subsurface of Mars today, and that diffusive loss of ice from the subsurface has been minimal.

The role of strength defects in shaping impact crater planforms

High-resolution imagery and digital elevation models (DEMs) were used to measure the planimetric shapes of well-preserved impact craters. These measurements were used to characterize the size-dependent scaling of the departure from circular symmetry, which provides useful insights into the processes of crater growth and modification. For example, we characterized the dependence of the standard deviation of radius (σR) on crater diameter (D) as σR ∼ Dm. For complex craters on the Moon and Mars, m ranges from 0.9 to 1.2 among strong and weak target materials. For the martian simple craters in our data set, m varies from 0.5 to 0.8. The value of m tends toward larger values in weak materials and modified craters, and toward smaller values in relatively unmodified craters as well as craters in high-strength targets, such as young lava plains. We hypothesize that m ≈ 1 for planforms shaped by modification processes (slumping and collapse), whereas m tends toward ∼ 1/2 for planforms shaped by an excavation flow that was influenced by strength anisotropies.Additional morphometric parameters were computed to characterize the following planform properties: the planform aspect ratio or ellipticity, the deviation from a fitted ellipse, and the deviation from a convex shape. We also measured the distribution of crater shapes using Fourier decomposition of the planform, finding a similar distribution for simple and complex craters. By comparing the strength of small and large circular harmonics, we confirmed that lunar and martian complex craters are more polygonal at small sizes. Finally, we have used physical and geometrical principles to motivate scaling arguments and simple Monte Carlo models for generating synthetic planforms, which depend on a characteristic length scale of target strength defects. One of these models can be used to generate populations of synthetic planforms which are very similar to the measured population of well-preserved simple craters on Mars.

Martian cratering 11. Utilizing decameter scale crater populations to study Martian history

AbstractNew information has been obtained in recent years regarding formation rates and the production size‐frequency distribution (PSFD) of decameter‐scale primary Martian craters formed during recent orbiter missions. Here we compare the PSFD of the currently forming small primaries (P) with new data on the PSFD of the total small crater population that includes primaries and field secondaries (P + fS), which represents an average over longer time periods. The two data sets, if used in a combined manner, have extraordinary potential for clarifying not only the evolutionary history and resurfacing episodes of small Martian geological formations (as small as one or few km2) but also possible episodes of recent climatic change. In response to recent discussions of statistical methodologies, we point out that crater counts do not produce idealized statistics, and that inherent uncertainties limit improvements that can be made by more sophisticated statistical analyses. We propose three mutually supportive procedures for interpreting crater counts of small craters in this context. Applications of these procedures support suggestions that topographic features in upper meters of mid‐latitude ice‐rich areas date only from the last few periods of extreme Martian obliquity, and associated predicted climate excursions.

Size-frequency distribution of different secondary crater populations: 1. Equilibrium caused by secondary impacts

Accumulation of impact craters is the major reason causing equilibrium of crater populations on airless planetary surfaces. Besides primary craters, the effect of widespread secondaries on the equilibrium of local crater populations is little studied. Here, the different secondary crater populations formed by the Hokusai crater on Mercury are systematically studied, and they are compared with those on the Moon to investigate their contribution to the evolution of local crater populations. Self-secondaries cause equilibrium on continuous ejecta deposits in a short time, and the equilibrium crater population has a differential SFD slope of about -3. Background secondaries are abundant on Mercury, and equilibrium caused by a combination of primaries and potential background secondaries follows the same pattern on the Moon and Mercury. The spatial dispersion of fragments that form both near-field and distant secondaries is the major factor affecting the degree of mutual destruction and thus the final crater SFD. Some clustered distant secondaries on Mercury are likely formed by individual fragments considering their large spatial dispersion and identical morphology with same-sized primaries, and the SFD rollovers of these secondaries possibly reflect the inherent SFD rollovers of the impact fragments. Near-field secondaries and many other distant secondaries have morphology and spatial distribution that are consistent with being formed by clustered fragments, and mutual destruction of secondaries may be the major reason causing the observed SFD rollovers. Heterogeneous secondary impacts are a potential explanation for both different crater densities within the equilibrium diameter range and different regolith thicknesses on coeval surfaces.

Determining proportions of lunar crater populations by fitting crater size distribution

We determine the proportions of two mixed crater populations distinguishable by size distributions on the Moon. A “multiple power-law” model is built to formulate crater size distribution N(D) ∝ D−α whose slope α varies with crater diameter D. This model is then used to fit size distributions of lunar highland craters and Class 1 craters. The former is characterized by α = 1.17 ± 0.04, 1.88 ± 0.07, 3.17 ± 0.10 and 1.40 ± 0.15 for D ranges ∼ 10 – 49, 49 – 120, 120 – 251 and ∼ 251 – 2500 km, while the latter has a single slope α = 1.96 ± 0.14 for about 10 – 100 km. They are considered as Population 1 and 2 crater size distributions, whose sum is then fitted to the global size distribution of lunar craters with D between 10 and 100 km. Estimated crater densities of Population 1 and 2 are 44 × 10−5 and 5 × 10−5 km−2 respectively, leading to the proportion of the latter being 10%. This result underlines the need for more thoroughly investigating Population 1 craters and their related impactors, the primordial main-belt asteroids, which dominated the late heavy bombardment.

Secondary crater-initiated debris flow on the Moon

In recent work, radar circular polarization echo properties have been used to identify secondary craters without distinctive “secondary” morphologies. Because of the potential for this method to improve our knowledge of secondary crater populations—in particular the effect of secondary populations on crater-derived ages based on small craters—it is important to understand the origin of radar polarization signatures associated with secondary impacts. In this paper, we utilize Lunar Reconnaissance Orbiter Camera photographs to examine the geomorphology of secondary craters with radar circular polarization ratio enhancements. Our investigation reveals evidence of dry debris flow with an impact melt component at such secondary craters. We hypothesize that these debris flows were initiated by the secondary impacts themselves, and that they have entrained blocky material ejected from the secondaries. By transporting this blocky material downrange, we propose that these debris flows (rather than solely ballistic emplacement) are responsible for the tail-like geometries of enhanced radar circular polarization ratio associated with the secondary craters investigated in this work. Evidence of debris flow was observed at both clustered and isolated secondary craters, suggesting that such flow may be a widespread occurrence, with important implications for the mixing of primary and local material in crater rays.

Fates of satellite ejecta in the Saturn system, II

We assess the fates of ejecta from the large craters Aeneas on Dione and Ali Baba on Enceladus (161 and 39 km in diameter, respectively), as well as that from Herschel (130 km in diameter) on Mimas. The ejecta are treated either as ‘spalls’ launched from hard surfaces, or as ‘rubble’ launched from a weak rubble pile regolith. Once in orbit we consider the ejecta as massless test particles subject to the gravity of Saturn and its classical satellites. The great majority of escaped ejecta get swept up by the source moons. The best fit to the ejecta population decay is a stretched exponential with exponent near 1/2 (Dobrovolskis et al., Icarus 188, 481–505, 2007). We bracket the characteristic ejecta sizes corresponding to Grady–Kipp fragments and spalls. Based on this and computed impact velocities and incidence angles, the resulting sesquinary craters, if they exist, should have diameters on the order of a few meters to a few km. The observed longitude distribution of small craters on Mimas along with the findings of Bierhaus et al. that small moons should not have a secondary crater population (Icarus 218, 602–621, 2012) suggest that the most likely place to find sesquinary craters in the Saturn system is the antapex of Mimas.

Analysis of impact crater populations and the geochronology of planetary surfaces in the inner solar system

AbstractAnalyzing the density of impact craters on planetary surfaces is the only known technique for learning their ages remotely. As a result, crater statistics have been widely analyzed on the terrestrial planets, since the timing and rates of activity are critical to understanding geologic process and history. On the Moon, the samples obtained by the Apollo and Luna missions provide critical calibration points for cratering chronology. On Mercury, Venus, and Mars, there are no similarly firm anchors for cratering rates, but chronology models have been established by extrapolating from the lunar record or by estimating their impactor fluxes in other ways. This review provides a current perspective on crater population measurements and their chronological interpretation. Emphasis is placed on how ages derived from crater statistics may be contingent on assumptions that need to be considered critically. In addition, ages estimated from crater populations are somewhat different than ages from more familiar geochronology tools (e.g., radiometric dating). Resurfacing processes that remove craters from the observed population are particularly challenging to account for, since they can introduce geologic uncertainty into results or destroy information about the formation age of a surface. Regardless of these challenges, crater statistics measurements have resulted in successful predictions later verified by other techniques, including the age of the lunar maria, the existence of a period of heavy bombardment in the Moon's first billion years, and young volcanism on Mars.

Craters of the Pluto-Charon system

NASA's New Horizons flyby mission of the Pluto-Charon binary system and its four moons provided humanity with its first spacecraft-based look at a large Kuiper Belt Object beyond Triton. Excluding this system, multiple Kuiper Belt Objects (KBOs) have been observed for only 20 years from Earth, and the KBO size distribution is unconstrained except among the largest objects. Because small KBOs will remain beyond the capabilities of ground-based observatories for the foreseeable future, one of the best ways to constrain the small KBO population is to examine the craters they have made on the Pluto-Charon system. The first step to understanding the crater population is to map it. In this work, we describe the steps undertaken to produce a robust crater database of impact features on Pluto, Charon, and their two largest moons, Nix and Hydra. These include an examination of different types of images and image processing, and we present an analysis of variability among the crater mapping team, where crater diameters were found to average ±10% uncertainty across all sizes measured (∼0.5–300km). We also present a few basic analyses of the crater databases, finding that Pluto's craters' differential size-frequency distribution across the encounter hemisphere has a power-law slope of approximately –3.1±0.1 over diameters D ≈ 15–200km, and Charon's has a slope of –3.0±0.2 over diameters D ≈ 10–120km; it is significantly shallower on both bodies at smaller diameters. We also better quantify evidence of resurfacing evidenced by Pluto's craters in contrast with Charon's. With this work, we are also releasing our database of potential and probable impact craters: 5287 on Pluto, 2287 on Charon, 35 on Nix, and 6 on Hydra.

The self‐secondary crater population of the Hokusai crater on Mercury

AbstractWhether or not self‐secondaries dominate small crater populations on continuous ejecta deposits and floors of fresh impact craters has long been a controversy. This issue potentially affects the age determination technique using crater statistics. Here the self‐secondary crater population on the continuous ejecta deposits of the Hokusai crater on Mercury is unambiguously recognized. Superposition relationships show that this population was emplaced after both the ballistic sedimentation of excavation flows and the subsequent veneering of impact melt, but it predated the settlement and solidification of melt pools on the crater floor. Fragments that formed self‐secondaries were launched via impact spallation with large angles. Complex craters on the Moon, Mercury, and Mars probably all have formed self‐secondaries populations. Dating young craters using crater statistics on their continuous ejecta deposits can be misleading. Impact melt pools are less affected by self‐secondaries. Overprint by subsequent crater populations with time reduces the predominance of self‐secondaries.

The missing large impact craters on Ceres.

Asteroids provide fundamental clues to the formation and evolution of planetesimals. Collisional models based on the depletion of the primordial main belt of asteroids predict 10–15 craters >400 km should have formed on Ceres, the largest object between Mars and Jupiter, over the last 4.55 Gyr. Likewise, an extrapolation from the asteroid Vesta would require at least 6–7 such basins. However, Ceres' surface appears devoid of impact craters >∼280 km. Here, we show a significant depletion of cerean craters down to 100–150 km in diameter. The overall scarcity of recognizable large craters is incompatible with collisional models, even in the case of a late implantation of Ceres in the main belt, a possibility raised by the presence of ammoniated phyllosilicates. Our results indicate that a significant population of large craters has been obliterated, implying that long-wavelength topography viscously relaxed or that Ceres experienced protracted widespread resurfacing.

Planetary surface dating from crater size-frequency distribution measurements: Poisson timing analysis

The predictions of crater chronology models have customarily been evaluated by dividing a crater population into discrete diameter intervals, plotting the crater density for each, and finding a best-fit model isochron, with the uncertainty in the procedure being assessed using 1/√n estimates, where n is the number of craters in an interval. This approach yields an approximate evaluation of the model predictions. The approximation is good until n becomes small, hence the often-posed question: what is the minimum number of craters for an adequate prediction? This work introduces an approach for exact evaluation of a crater chronology model using Poisson statistics and Bayesian inference, expressing the result as a likelihood function with an intrinsic uncertainty. We demonstrate that even in the case of no craters at all, a meaningful likelihood function can be obtained. Thus there is no required minimum count: there is only varying uncertainty, which can be well described. We recommend that the Poisson timing analysis should be preferred over binning/best-fit approaches. Additionally, we introduce a new notation to make it consistently clear that crater chronology model calibration errors are inseparable from stated crater model ages and their associated statistical errors.

Crater size-frequency distribution measurements and age of the Compton–Belkovich Volcanic Complex

The Compton–Belkovich Volcanic Complex (CBVC) is a 25×35km feature on the lunar farside marked by elevated topography, high albedo, high thorium concentration, and high silica content. Morphologies indicate that the complex is volcanic in origin and compositions indicate that it represents rare silicic volcanism on the Moon. Constraining the timing of silicic volcanism at the complex is necessary to better understand the development of evolved magmas and when they were active on the lunar surface. We employ image analysis and crater size-frequency distribution (CSFD) measurements on several locations within the complex and at surrounding impact craters, Hayn (87km diameter), and Compton (160km diameter), to determine relative and absolute model ages of regional events. Using CSFD measurements, we establish a chronology dating regional resurfacing events and the earliest possible onset of CBVC volcanism at ∼3.8Ga, the formation of Compton Crater at 3.6Ga, likely resurfacing by volcanism at the CBVC at ∼3.5Ga, and the formation of Hayn Crater at ∼1Ga. For the CBVC, we find the most consistent results are obtained using craters larger than 300m in diameter; the small crater population is affected by their approach to an equilibrium condition and by the physical properties of regolith at the CBVC.

Asteroid 4 Vesta: Dynamical and collisional evolution during the Late Heavy Bombardment

Asteroid 4 Vesta is the only currently identified asteroid for which we possess samples in the form of meteorites. These meteorites revealed us that Vesta is a differentiated body and that its differentiation produced a relatively thin basaltic crust that survived intact over its entire collisional history. The survival of the vestan basaltic crust has long been identified as a pivotal constraint in the study of the evolution of the asteroid belt and the Solar System but, while we possess a reasonably good picture of the effects of the last 4 Ga on such a crust, little is known about the effects of earlier events like the Late Heavy Bombardment. In this work we address this gap in our knowledge by simulating the Late Heavy Bombardment on Vesta in the different dynamical scenarios proposed for the migration of the giant planets in the broad framework of the Nice Model. The results of the simulations allowed us to assess the collisional history of the asteroid during the Late Heavy Bombardment in terms of produced crater population, surface saturation, mass loss and mass gain of Vesta and number of energetic or catastrophic impacts. Our results reveal that planet–planet scattering is a dynamically favorable migration mechanism for the survival of Vesta and its crust. The number of impacts of asteroids larger than about 1 km in diameter estimated as due to the LHB is 31 ± 5, i.e. about 5 times larger than the number of impacts that would have occurred in an unperturbed main belt in the same time interval. The contribution of a possible extended belt to the collisional evolution of Vesta during the LHB is quite limited and can be quantified in 2 ± 1 impacts of asteroids with diameter greater than or equal to 1 km. The chance of energetic and catastrophic impacts is less than 10% and is compatible with the absence of giant craters dated back to 4 Ga ago and with the survival of the asteroid during the Late Heavy Bombardment. The mass loss caused by the bombardment translates in the erosion of 3−5 m of the crust, consistently with the global survival of the basaltic crust of Vesta confirmed by the Dawn mission. Our analysis revealed that the contribution of the LHB to the cratering of Vesta’ surface is not significant and is actually erased by the crater population produced by the following 4 Ga of collisional evolution of the asteroid, in agreement with the data provided by the Dawn mission.

Size‐frequency distribution of crater populations in equilibrium on the Moon

AbstractOverprinting of craters by subsequent impacts and topographic degradation complicates crater statistics, especially for old surfaces and small‐diameter crater populations. A crater population is regarded as in equilibrium at a particular crater size when smaller craters are being produced at the same rate at which they are being destroyed. Evaluating the equilibrium state of crater populations is challenging, and empirical equilibrium densities are frequently inferred. By performing careful crater counts and cross comparisons on several lunar surfaces, we study the size‐frequency distributions (SFD) for the crater populations, which have portions in equilibrium. The results are one of the few observational constraints on the SFD of crater populations in equilibrium, showing that referring to empirical equilibrium densities is not safe for evaluating the equilibrium states of crater populations. Equilibrium densities are not positively correlated with the ages of crater populations, and some populations in equilibrium have crater densities less than those previously believed to represent equilibrium conditions. Besides the SFD of the production population, different crater removal rates at different diameters also affect the SFD of crater populations in equilibrium. The equilibrium onset diameter (Deq) of a crater population can be translated to model ages because older populations have larger Deq, and those for same‐aged surfaces are comparable. We show that the crater populations studied here are in equilibrium at much smaller diameters than those predicted for same‐aged surfaces by crater degradation models, thus indicating lower crater degradation rates on the Moon, and/or younger ages of the counting areas.