Bombardment History Research Articles

Evidence of a 4.33 billion year age for the Moon’s South Pole–Aitken basin

AbstractThe Moon’s farside South Pole–Aitken (SPA) basin is the largest and oldest visible impact basin in the inner Solar System. Determining the timing of this catastrophic event is key to understanding the onset of the lunar basin-forming epoch, with implications for understanding the impact bombardment history of the inner Solar System. Despite this, the formation age of the SPA basin remains poorly constrained. Here we show that the chemical composition of the lunar meteorite Northwest Africa 2995 is in good agreement with lithologies exposed within the southern region of the SPA basin. Radiometric dating of a range of mineral and rock components in Northwest Africa 2995 yielded consistent dates of ~4.32–4.33 billion years old. We interpret these dates as the age of SPA basin formation, inferring that this event occurred ~120 million years before the formation of the main cluster of lunar impact basins between ~4.2 Ga and 3.8 Ga. This weakens support for a narrow period of lunar late heavy impact bombardment and also implies that the earliest formed impact basins on the Moon (that is, >4.33–4.5 Ga old) were erased either by the SPA impact itself when its formation caused massive resurfacing of the lunar surface or through other geological processes.

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

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

Geological evidence for extensive basin ejecta as plains terrains in the Moon’s South Polar Region

Water ice and other volatiles that accumulated in the Moon’s polar regions are among the top priority targets for lunar exploration, due to their significances in both lunar geology and extraterrestrial resource utilization. Locating suitable landing sites and determining the provenance of sampled/measured surface materials are critical for future landed missions. Here, we map over 800 sites of plains terrains in the Moon’s south polar region, with a total surface area of ~46,000 km2. Orbital measurements and analog studies show that most of these plains have apparently higher albedo and lower iron content than volcanic mare plains, suggesting an origin of ejecta-induced debris flows from distant impact craters, especially from the Schrödinger basin. Our findings suggest that the entire lunar south polar region probably have experienced contributions from distant basin materials. We recommend these plains as priority landing sites for future exploration of lunar polar volatiles and early bombardment history.

Evolution of Impact Basin Gravity Signatures on the Lunar Farside: A Long‐Term Alteration Process

AbstractLunar impact basins are critical markers to understand the early bombardment history. This study focuses on isostatic compensation processes at 16 lunar farside basins, exploring how long‐term alteration processes affect estimates of impactor sizes and timing. Using a lithospheric flexure model, we analyze the isostatically compensated gravity signature of these basins. Our approach is based on previously published basin formation models using the iSALE‐2D shock physics code. Here we investigate how isostatic compensation processes alter the gravity signature using models of basin formation as initial conditions. We compare our results with the present day observed gravity data. We assume that isostatic compensation is the result of flexural deformation of the crust‐mantle boundary with varying lithospheric elastic thicknesses. Our results indicate that the effect of isostatic compensation on the gravity signature varies depending on the thermal conditions at the time of impact. We find that despite this variability, the overall influence of isostatic compensation on the gravity signature is generally moderate to minor. Additionally, our analysis is consistent with previous studies that have shown that the elastic thickness varies across the Moon. Our findings contribute to our understanding of the relationship between basin formation and the thermal evolution of the Moon. The variations in elastic thickness found provide nuanced insights into lunar geological processes and improve our understanding of the early history of the Moon.

Modeling the Evolution of Lunar Regolith: 2. Growth Rate and Spatial Distribution

AbstractThe evolution of regolith thickness induced by continuous impact events on the lunar surface provides valuable information about lunar geology and the bombardment history of the inner Solar System. In this study, we develop a Lunar Topography and Regolith Evolution Model based on the production and distribution of lunar regolith resulting from individual impact craters. Considering the changing impact flux, evolving target properties, topographic degradation process, and crater superposition, our model simultaneously simulates both the spatial and temporal evolution of lunar cratered topography and regolith thickness. The modeled regolith thickness generally aligns with results estimated by in situ seismic experiments and measurements of small crater morphology. Through our simulations, we establish a novel quantitative relation between regolith growth rate and time, providing new constraints on the impact flux trend and buffering trend in the regolith growth process. By comparing this new relation with that predicted by our analytical regolith evolution model, we reveal the significant influences of evolving target properties and regolith distribution on the regolith growth rate. Our modeled results also show that the regional regolith thickness follows a characteristic long‐tailed distribution with substantial lateral variations. Both the root‐mean‐square fluctuation and correlation length of lunar regolith increase with lateral scale and the surface age, which are ∼4.7 and 18 m on the hectometer scale at 3.5 Ga. All these results contribute to a better understanding of the regolith evolution process and can provide valuable insights into the lunar surface dynamics.

Modeling the Evolution of Lunar Regolith: 1. Formation Mechanism Through Individual Simple Impact Craters

AbstractThe production, distribution, and evolution of lunar regolith are critical in deciphering the lunar bombardment history and comprehending the transport of materials across the lunar surface, which are still not well understood. In this study, we conducted a comprehensive investigation of factors influencing the production and distribution of lunar regolith by individual simple craters. Combining our results for the impact‐generated regolith volume with a lunar production function, we developed an analytical model to describe the regolith growth process. We found that the strength of bedrock significantly affects the crater size and hence the volume of regolith produced especially for subdecameter impactors. The regolith volume produced by an individual impact crater is quantitatively characterized as a function of crater diameter and preimpact regolith thickness. This regolith production is primarily determined by how much bedrock is shattered, followed by the impact‐induced volume change of target material and lastly by the regolith volume created by secondary cratering processes. When a single crater forms, preimpact regolith thickness greatly affects the regolith distribution pattern; a larger fraction of the regolith will be distributed outside the crater rim for a deeper preimpact regolith layer. Our regolith evolution model can serve as a good first‐order estimation of the regolith growth process that provides better constraints on the regolith buffering trend than previous studies. This model also suggests that, when ignoring the contribution from large, distant impacts, the regolith growth process is dominated by impact craters at scales from a meter to a few hectometers.

Inconsistency between the Ancient Mars and Moon Impact Records of Megameter-scale Craters

Impact crater populations provide a record of the bombardment history of a planetary body. These craters reveal an intense bombardment history on Earth's Moon that included significant, large impacts, at least four of which created craters larger than ∼1000 km across more than 3.7 Ga. Scaling the lunar impact crater history to Mars using generally well-established scaling rules indicates Mars should have ∼40–80 similarly sized, ancient craters. However, Mars has roughly seven. This is a nontrivial mismatch between observation and expectation. Possible methods to resolve the mismatch are discussed in this work: small number statistics, incorrect scaling laws, heat flow produced larger-than-expected craters, the ratio of Mars:Moon impactors was different in early solar system history, Mars’s ancient crater chronology is wrong, and/or Mars has hidden a large fraction of its large impacts. None of these scenarios are mutually exclusive. This work details how the different scenarios could work to bring observations more in-line with the scaling expectations, or vice versa. It is posited that the most likely sources of the mismatch are that the initial bolide rate was different (this is a noncontroversial supposition), that lunar heat flow produced larger craters than expected, and the formation of the Martian Borealis basin could have kept the surface warm enough for long enough to prevent large features from forming for an extended period of time. The primary purpose of this work is to present the issue with possible ways to solve it through future efforts.

Melt Production and Ejection From Lunar Intermediate‐Sized Impact Craters: Where Is the Molten Material Deposited?

AbstractDifferently aged impact melt in lunar samples is key to unveiling the early bombardment history of the Moon. Due to the mixing of melt products ejected from distant craters, the interpretations of the origin of lunar samples are difficult. We use numerical modeling for a better quantitative understanding of the production of impact‐induced melt and in particular its distribution in ejecta blankets for lunar craters with sizes ranging from 1.5 to 50 km. We approximate the lunar stratigraphy with a porosity gradient, which represents the gradual transition from upper regolith via megaregolith to the solid crustal material. For this lunar setting, we quantify the melt production relative to crater volume and derive parameters describing its increasing trend with increasing transient crater size. We found that about 30%–40% of the produced melt is ejected from the crater. The melt concentration in the ejecta blanket increases almost linearly with distance from the crater center, while the thickness of the ejecta blanket decreases following a power law. Our study demonstrates that if in lunar samples the concentration of a melt with a certain age is interpreted to be of a nonlocal origin, these melts could be the impact products of a large crater (>10 km) located hundreds of kilometers away.

Bombardment history of the Moon constrained by crustal porosity

The formation and evolution of the terrestrial planets were shaped by a bombardment of large impactors in a cluttered early Solar System. However, various surface processes degrade impact craters, and the early impact history of the Moon and the ages of its ancient impact basins remain uncertain. Here we show that the porosity of the lunar crust, generated by the cumulative crustal processing of impacts, can be used to determine the Moon’s bombardment history. We use a numerical model constrained by gravity data to simulate the generation of porosity by basin-forming impacts and the subsequent removal by smaller impacts and overburden pressure. We find that, instead of steadily increasing over the history of the Moon, lunar crustal porosity was largely generated early in lunar evolution when most basins formed and, on average, has decreased after that time. Using the Moon as a proxy for the terrestrial planets, we find that the terrestrial planets experienced periods of high crustal porosity early in their evolution. Our modelled porosities also provide an independent constraint on the chronological sequence of basin-forming impacts. Our results suggest that the inner solar system was subject to double the number of smaller impacts producing craters exceeding 20 km in diameter than has been previously estimated from traditional crater-counting analyses, whereas the bombardment record for the lunar basins (>200 km in diameter) is complete. This implies a limited late delivery of volatiles and siderophile elements to the terrestrial planets by impactors.

Regional Impact Crater Mapping and Analysis on Saturn's Moon Dione and the Relation to Source Impactors

AbstractRecent dynamical modeling of the formation and evolution of the Saturnian satellites suggests that the ages of the mid‐sized inner moons (Mimas, Enceladus, Tethys, Dione, and Rhea) could be as young as 100 Myr. This estimate is in contrast to most previous modeling and observational work that suggest an age more contemporaneous with the formation of Saturn 4.5 Ga ago. Given the heritage of using craters to constrain surface ages, we examine the impact craters of Dione using imagery from NASA's Cassini ISS camera and analyze their size‐frequency distributions (SFDs) to understand impactor populations. We survey four areas across different geologic terrains and compare our crater counts to standard outer solar system production functions. In addition to crater counts, we study several crater types such as elliptical and polygonal to further examine the bombardment source for the craters. We find evidence for a Saturn‐specific planetocentric impactor source, as none of the standard production functions fit the data. We compare our Dione data with our work on Tethys and find similarly shaped SFDs between the satellites. However, Dione's surface has been extensively modified to remove smaller craters (D ∼ <5 km) and has been bombarded by larger impactors, creating more D ∼ >20 km on Dione than Tethys. In contrast, Tethys more generally represents an ancient unmodified surface within the Saturn system. More complete observations and assessment of the cratering records on the satellites of Uranus and Neptune's moon Triton would enable better constraints on the bombardment history of the Saturn system.

Lunar Crater Detection on Digital Elevation Model: A Complete Workflow Using Deep Learning and Its Application

Impact cratering process is the major geologic activity on the surface of the Moon, and the spatial distribution and size-frequency distribution of lunar craters are indicative to the bombardment history of the Solar System. The substantial efforts on the development of automated crater detection algorithms (CDAs) have been carried out on the images from the remote sensing observations. Recently, CDAs via convolutional neural network (CNN) on digital elevation model (DEM) has been developed as it can combine the discrimination ability of CNN with the robust characteristic of the DEM data. However, most of the existing algorithms adopt a traditional two-stage detection pipeline including an edge segmentation and a template matching step. In this paper, we attempt to reduce the gap between the existing DEM-based CDAs and the advanced CNN methods for object detection, and propose a complete workflow including an end-to-end deep learning pipeline for lunar crater detection, in particular for craters smaller than 50 km in diameter. Based on the workflow, we benchmark nine representative CNN models involving three popular types of detection architectures. Moreover, we elaborate on the practical application of the proposed workflow, and provide an example method to demonstrate the performance advantage in terms of the precision (82.97%) and recall (79.39%). Furthermore, we develop a crater verification tool to manually validate the detection results, and the visualization results show that our detected craters are reasonable and can be used as a supplement to the existing hand-labeled datasets.

Impact-related crystallization and modification of small zircons in Apollo 15 and 16 impactites at 4.2 Ga

Because of their robustness against resetting, in situ U-Pb ages of zircons in lunar impactites have the potential to provide constraints on the lunar bombardment history that may complement the more common K-Ar ages. Most previous work has focused on relatively large zircons that show growth zoning and ages were mostly interpreted as early igneous crystallization ages. Here we combine high-resolution mineralogical imaging and in situ U-Pb dating by ion microprobe to identify, characterize and date <20 μm size zircons in thin sections of lunar impact breccias. Several tens of grains of zircons of this size range were identified in thin sections of impactites from the Apollo 15 and 16 landing sites. Small zircons are more abundant in both noritic and evolved clinopyroxene, SiO2 or K-feldspar bearing lithologies compared to anorthositic bulk compositions. Both granular zircon aggregates and overgrowth on existing zircon or baddeleyite (in breccias 15455 and 67915) are interpreted to reflect high-temperature recrystallization of zircons or its high-temperature-pressure precursor phases, following shock heating events by impact. In contrast, conchoidal or poikilitic zircons <10 μm in Fe-Ni metal bearing noritic clasts or matrix (67915, 67955) crystallized in situ from impact melt. Most U-Pb ages of the 24 analyzed grains are either concordant or reverse discordant with 207Pb-206Pb ages ranging from 4.15 to 4.25 Ga. The small age range, combined with a large textural spectrum and the frequent presence of Fe-Ni metal suggest zircon crystallization from impact melt and recrystallization of pre-existing zirconium-bearing minerals by impact heating. Such ‘impact’ zircons with 4.2 Ga ages have now been reported from most Apollo landing sites, suggesting widespread formation and modification of zircons by basin-forming impacts at this time. The contrast between U-Pb zircon (predominantly 4.2 Ga) and K-Ar feldspar ages (predominantly 3.9 Ga) likely reflects resetting of the latter chronometer by impact heating.

Large impact cratering during lunar magma ocean solidification

The lunar cratering record is used to constrain the bombardment history of both the Earth and the Moon. However, it is suggested from different perspectives, including impact crater dating, asteroid dynamics, lunar samples, impact basin-forming simulations, and lunar evolution modelling, that the Moon could be missing evidence of its earliest cratering record. Here we report that impact basins formed during the lunar magma ocean solidification should have produced different crater morphologies in comparison to later epochs. A low viscosity layer, mimicking a melt layer, between the crust and mantle could cause the entire impact basin size range to be susceptible to immediate and extreme crustal relaxation forming almost unidentifiable topographic and crustal thickness signatures. Lunar basins formed while the lunar magma ocean was still solidifying may escape detection, which is agreeing with studies that suggest a higher impact flux than previously thought in the earliest epoch of Earth-Moon evolution.

Formation of Small Craters in the Lunar Regolith: How Do They Influence the Preservation of Ancient Melt at the Surface?

AbstractThe impact melt that records the formation time of basins is essential for the understanding of the lunar bombardment history. To better understand melt distribution on the Moon, this study investigates mixing of melt by small impacts using a Monte Carlo numerical model. The obtained mixing behavior is then integrated into a larger scale model developed in previous work. While large impacts produce most of the melt volume in both the regolith and megaregolith, we find that the dominant source of melt near the surface is small impacts. Material in the top meter is affected mainly by impacts that form craters &lt;5 km in diameter. In the uppermost 10 cm, melt with age &lt;0.5 Ga is abundant; while as depth increases older melt is increasingly present. This may indicate that the excess of impact melt &lt;0.5 Ga in lunar samples from the near surface is caused by the cumulative mixing of small impacts. A comparison of the age distribution of melt derived from craters of different sizes with that of impact glass constrains the size of spherule‐forming impacts. Our model is consistent with observations if most impact glass spherules from the near surface are produced by &lt;100 m craters and &gt;100 m craters do not contribute abundant spherules. The distribution of the datable melt with depth is also analyzed, which is essential for future sampling missions. Excavated materials of young and large craters (&gt;100 m on highlands; &gt;10 km on maria) appear to be the most fruitful targets.

Exploring the Bombardment History of the Moon

Exploring the Bombardment History of the Moon

Repeated impact-driven plume formation on Enceladus over megayear timescales

Water plumes erupting from the ‘tiger stripe’ features on the south pole of Enceladus are thought to connect to a global subsurface ocean. Proposed origins for the initial stress necessary to form the ‘tiger stripes’ include a giant impact, which would require true polar wander to explain the location of the plumes if the impact did not occur at the South Pole, or tensile stresses, which would require volumetric expansion associated with partial freezing of the subsurface ocean. In addition to the requirement of a fine-tuned impact, true polar wander, or partial freezing of the subsurface ocean, a further issue with these hypotheses is that the ‘tiger stripes’ may be short-lived. We show here that impact resurfacing can seal off plumes and mass loss can lead to their compression and closure over ∼1 Myr. Since plumes are observed at present, a mechanism by which new plumes can be generated every ∼1 Myr and by which such plumes are most likely to form at the south pole is needed. We propose and investigate the possibility that impacts constitute an adequate repeating source for the continual instigation of fractures and plumes. We find that the rate of impacts on Enceladus suggests the formation of ∼103 independent plume systems per Gyr, the majority on the south pole, and is consistent with the Cassini-derived age of the south pole for a lunar-like bombardment history, our estimates of fracture lifetimes, and with the needed parameters for parallel fracture propagation. The model favors a bombardment history similar to that of Triton over one more similar to that of the Galilean satellites.

Thickness of orthopyroxene-rich materials of ejecta deposits from the south pole-Aitken basin

The South Pole-Aitken (SPA) basin is the largest impact structure on the Moon and is believed to have excavated the orthopyroxene (Opx)-rich lower crust and/or upper mantle materials. For the complex craters outside of the SPA transient cavity, the origin of the Opx-rich central peaks is possibly from either the Opx-rich materials of the SPA ejecta deposits or the unexcavated lower crust and/or upper mantle. To estimate the thickness of the Opx-rich materials of the SPA ejecta deposits, this study investigated large complex craters (dozens of kilometers) that have penetrated the Opx-rich materials and exposed deeper mafic-poor crustal material based on spectra extracted from small fresh craters (sub-kilometer scale). The amount of foreign material introduced to these large complex craters by other lunar impact events was estimated to guarantee the least influence on the compositional analysis. The study results suggest that a 56 km-diameter crater at ~640 km northwest of the SPA center is large enough to penetrate the Opx-rich materials of the SPA ejecta deposits, which are thinner than ~4.7 km at this location. This result also indicates that the intense bombardment history of the large craters and basins outside of the SPA transient cavity excavated and redistributed a large amount of crustal material across the basin, possibly resulting in the heterogeneous distribution of mafic-rich and mafic-poor materials on the SPA surface.

Impact melting upon basin formation on early Mars

The early bombardment history of Mars may have drastically shaped its crustal evolution. Impact-induced melting of crustal and mantle materials leads to the formation of local and regional melt ponds, and the cumulative effects of the impact flux could result in widespread melting of the crust. To quantify impact-melt production, its provenance and final distribution as a function of impact conditions, we carried out a systematic parameter study using the iSALE shock physics code. In contrast to simplified scaling laws for estimating the amount of melt generated by shock compression, we take the planet's thermal state at the time of impact into account. In addition, we consider decompression melting as a consequence of lithostatic uplift of initially deep-seated material. We find that the geothermal profile has a strong effect on melt production, and that melt volumes are significantly increased by up to a factor of seven in comparison to existing analytical estimates. Enhanced melting occurs at impactor sizes (and velocities) that deposit most of their energy at a depth close to the base of the lithosphere. Impactors larger than 10 km penetrate through the lithosphere and can generate a significant amount of melt by decompression due to lithostatic uplift, which can make up to 40% of the total melt volume. In some cases, the total melt volume exceeds the volume of the transient (and final) crater and the surface expression of these collisions may resemble large igneous provinces rather than typical craters.

Evolution of the Earth’s atmosphere during Late Veneer accretion

ABSTRACT Recent advances in our understanding of the dynamical history of the Solar system have altered the inferred bombardment history of the Earth during accretion of the Late Veneer, after the Moon-forming impact. We investigate how the bombardment by planetesimals left-over from the terrestrial planet region after terrestrial planet formation, as well as asteroids and comets, affects the evolution of Earth’s early atmosphere. We develop a new statistical code of stochastic bombardment for atmosphere evolution, combining prescriptions for atmosphere loss and volatile delivery derived from hydrodynamic simulations and theory with results from dynamical modelling of realistic populations of impactors. We find that for an initially Earth-like atmosphere, impacts cause moderate atmospheric erosion with stochastic delivery of large asteroids, giving substantial growth (× 10) in a few ${{\ \rm per\ cent}}$ of cases. The exact change in atmosphere mass is inherently stochastic and dependent on the dynamics of the left-over planetesimals. We also consider the dependence on unknowns including the impactor volatile content, finding that the atmosphere is typically completely stripped by especially dry left-over planetesimals ($\lt 0.02 ~ {{\ \rm per\ cent}}$ volatiles). Remarkably, for a wide range of initial atmosphere masses and compositions, the atmosphere converges towards similar final masses and compositions, i.e. initially low-mass atmospheres grow, whereas massive atmospheres deplete. While the final properties are sensitive to the assumed impactor properties, the resulting atmosphere mass is close to that of current Earth. The exception to this is that a large initial atmosphere cannot be eroded to the current mass unless the atmosphere was initially primordial in composition.

The Thermal Evolution of Mercury Over the Past ∼4.2 Ga as Revealed by Relaxation States of Mantle Plugs Beneath Impact Basins

AbstractWe investigated the relaxation states of 11 large impact basins on Mercury based on an updated crustal thickness map, finding that the pre‐Tolstojan basins have comparable instead of varied relaxation states, suggesting that Moho temperature (TempMoho ) did not decrease substantially from ∼4.2 to 3.8 Ga. At the same time, mantle uplift beneath the Caloris basin is the least degraded, therefore implying a sharp decrease of TempMoho ∼ 3.8 Ga. These findings contrast with our thermal evolution models that predict a fast decrease of TempMoho between ∼4.2 and 3.8 Ga. Therefore, the discrepancies in the cooling rate suggest that the relatively elevated bombardment history between ∼4.2 and 3.8 Ga might have input additional energy to Mercury and substantially decreased the cooling rate.