Impactor Population Research Articles

Asymmetrical distribution of 1–20 km craters on the Moon

Previous studies have provided evidence for the synchronous rotation induced cratering asymmetry on lunar surface through numerical simulations and statistical analysis of a limited number of fresh craters. In this study, we reevaluated cratering asymmetry in lunar highland from (70°W, 60°N) to (70°E, 60°S) region using a new crater catalogue with diameters (D) ranging from 1 to 20 km. By utilizing a depth-to-diameter (d/D) ratio constraint to exclude the interference of degraded and secondary craters, we observed significant asymmetry in craters with d/D > 0.15. Moreover, leveraging the characteristic that larger diameter craters (D > 7 km) are less susceptible to degradation, we observed a more pronounced asymmetry with increasing diameter. Particularly, impact craters with larger D and d/D ratios (D > 7 km, d/D > 0.15) displayed an asymmetrical longitudinal distribution, aligning with predictions from the theoretical model. In the diameter range of 10 km–20km, for craters with d/D > 0.15, we observed that new crater influx occurring after 4.0 Ga years ago contributed little to this particular crater population. Therefore, we suggest that the cratering asymmetry was already present before 4.0 Ga. Due to the non-uniform ejecta from the Orientale Basin onto the highland regions, a significant number of smaller impact craters (1–5 km) have degraded or disappeared in the leading region, thereby diminishing the manifestation of the cratering asymmetry. The pronounced asymmetry exhibited in our statistical results might suggest the existence of a significant population of low-velocity impactors in early impact period (>4Ga) around the cis-lunar space.

Impact structures on Mercury from MESSENGER data: Implications on their formation processes and crustal structure

In this study high-resolution stereo images and Digital Terrain Models (DTMs) from MErcury Surface, Space ENvironment, Geochemistry and Ranging (MESSENGER) mission were utilized to detect impact structures in regions not covered by Mercury’s Laser Altimeter (MLA), while gravitational data was utilized as a supported data set. We have established an inventory of 314 impact structures ≥150 km, classified on their morphological and gravitational characteristics. 24 basins ≥300 km have been newly discovered. Additionally, we have identified significant surface modifications in impact structures of smooth material infill, which can be either impact-induced or volcanic in origin. The Bouguer anomaly and crustal thinning in the center are displaying an interplay of predominant change of crustal structure in impact basins. Further, this study reveals a common impact history of Mercury and the Moon. Nevertheless, cumulative density distributions suggest the possibility of either a divergence in impactor populations responsible for forming large basins on both celestial bodies or a significant shift in impactor rates. This work holds important implications not only for understanding impact structure formation and evolution processes but also for interpreting the crustal structure. It presents an updated and expanded catalog of impact structures on Mercury, encompassing buried basins, and identifies new areas of interest, potentially serving as target sites for the forthcoming BepiColombo mission.

Binary craters on Ceres and Vesta and implications for binary asteroids

Context. Airless planetary objects have their surfaces covered by craters, and these can be used to study the characteristics of asteroid populations. Planetary surfaces present binary craters that are associated with the synchronous impact of binary asteroids. Aims. We identify binary craters on asteroids (1) Ceres and (4) Vesta, and aim to characterize the properties (size ratio and orbital plane) of the binary asteroids that might have formed them. Methods. We used global crater databases developed in previous studies and mosaics of images from the NASA DAWN mission high-altitude and low-altitude mapping orbits. We established selection criteria to identify craters that were most likely a product of the impact of a binary asteroid. We performed numerical simulations to predict the orientation of the binary craters assuming the population of impactors has mutual orbits coplanar with heliocentric orbits, as the current census of binary asteroids suggests. We compared our simulations with our survey of binary craters on Ceres and Vesta through a Kolmogorov-Smirnov test. Results. We find geomorphological evidence of 39 and 18 synchronous impacts on the surfaces of Ceres and Vesta, respectively. The associated binary asteroids are widely separated and similar in diameter. The distributions of the orientation of these binary craters on both bodies are statistically different from numerical impact simulations that assume binary asteroids with coplanar mutual and heliocentric orbits. Conclusions. Although the identification of binary craters on both bodies and the sample size are limited, these findings are consistent with a population of well-separated and similarly sized binary asteroids with nonzero obliquity that remains to be observed, in agreement with the population of binary craters identified on Mars.

Recalibration of the lunar chronology due to spatial cratering-rate variability

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.

Crater Populations of the Saturnian Satellites Mimas, Rhea, and Iapetus

AbstractThe Saturnian system has been explored by four spacecraft: Pioneer 11, Voyager 1 and 2, and Cassini. Only the last three took images suitable for photogeologic analysis of the surfaces of Saturn's moons, and over the decades, several research groups have published data about the crater distributions on the Saturnian satellites. These groups have used those data to draw conclusions about the impactor populations and resurfacing histories of the moons, but no one has examined how well the different data agree between the researchers. We present independent mapping of the crater populations of Saturn's moons Mimas, Rhea, and Iapetus, and compare them with many published crater populations. We found that Mimas data are the most consistent between different researchers, and Rhea data are the least consistent. We attribute these differences to (a) data biases where there are fewer images upon which to map Mimantean craters but a large variety exist for Rhean, and (b) Rhea likely has different terrains with different impact crater populations which have not been generally recognized before. We also found that Iapetus' small craters appear to have a shallow branch, as others have found, and that shallow branch is not attributable to completeness limitations. Other bodies have shallow branches at small diameters, too, but they are not as shallow as Iapetus's, which suggests varying impacting populations as one moves closer to Saturn, in line with others' work on planetocentric impactors.

A Recent Impact Origin of Saturn’s Rings and Mid-sized Moons

We simulate the collision of precursor icy moons analogous to Dione and Rhea as a possible origin for Saturn’s remarkably young rings. Such an event could have been triggered a few hundred million years ago by resonant instabilities in a previous satellite system. Using high-resolution smoothed particle hydrodynamics simulations, we find that this kind of impact can produce a wide distribution of massive objects and scatter material throughout the system. This includes the direct placement of pure-ice ejecta onto orbits that enter Saturn’s Roche limit, which could form or rejuvenate rings. In addition, fragments and debris of rock and ice totaling more than the mass of Enceladus can be placed onto highly eccentric orbits that would intersect with any precursor moons orbiting in the vicinity of Mimas, Enceladus, or Tethys. This could prompt further disruption and facilitate a collisional cascade to distribute more debris for potential ring formation, the re-formation of the present-day moons, and evolution into an eventual cratering population of planetocentric impactors.

A Theory about a Hidden Evander-Size Impact and the Renewal of the Intermediate Cratered Terrain on Dione

The study introduces a theory about an Evander-size impact on the surface of Dione. Our study suspects a relatively low-velocity (≤5 km/s) collision between a ca. 50–80 km diameter object and Dione, which might have resulted in the resurfacing of one of the satellite’s intermediate cratered terrains in various ways, such as surface planing by “plowing” by ricocheting ejectiles, ejecta blanket covering, partial melting, and impact-triggered diapir formation associated with cryotectonism and effusive cryo-slurry outflows. Modeling the parameters of an impact of such a size and mapping the potential secondary crater distribution in the target location may function as the first test of plausibility to reveal the location of such a collision, which may be hidden by younger impact marks formed during, e.g., the Antenor, Dido, Romulus, and Remus collision events. The source of the impactor might have been Saturn-specific planetocentric debris, a unique impactor population suspected in the Saturnian system. Other possible candidates are asteroid(s) appearing during the outer Solar System’s heavy bombardment period, or a collision, which might have happened during the “giant impact phase” in the early Saturnian system.

Complex Crater Formation by Oblique Impacts on the Earth and Moon

AbstractAlmost all meteorite impacts occur at oblique incidence angles, but the effect of impact angle on crater size is not well understood, especially for large craters. To improve oblique impact crater scaling, we present a suite of simulations of complex crater formation on Earth and the Moon over a range of impact angles, velocities and impactor sizes. We show that crater diameter is larger than predicted by existing scaling relationships for oblique impacts; there is little dependence on obliquity for impacts steeper than 45° from the horizontal. Crater depth, volume and diameter depend on impact angle in different ways—relatively shallower craters are formed by more oblique impacts. Our simulation results have implications for how crater populations are determined from impactor populations and vice‐versa. They suggest that existing approaches to account for impact obliquity may underestimate the number of complex craters larger than a given size by as much as one‐third.

A unique Saturnian impactor population from elliptical craters

The crater populations of Saturn's midsized icy moons are not well matched by the size-frequency distributions of impacting material inferred from other outer planet satellite systems, frustrating attempts to date their surfaces and constrain models of their formation. Elliptical craters record the trajectories of impacting materials and can thus be used to characterize the impactors' dynamics and facilitate crater interpretations. Here, we report evidence of a unique Saturn-orbiting impactor population, distinct from any previously described source population. This population is identified through global mapping and analysis of the elliptical crater populations on Saturn's moons Tethys and Dione. We report the presence of a strong signal of east/west oriented elliptical craters located in the equatorial latitudes of both satellites. Poleward of 30° the orientations of these craters become more widely distributed. On Tethys, a survey of a high latitude area (30°-60° N) reveals that the non-elliptical craters are better matched by the production function constructed from craters on Triton than from the Jovian moons, although the high likelihood of planetocentric material in both the Saturnian and Neptunian systems precludes us from determining an absolute age using the Triton derived production function. Rather, we can conclude that the mid-sized Saturnian satellites have been strongly affected by a potentially similar collisional environment to Triton, giving rise to a Triton-like crater population along with a unique population that is not observed at Triton. Origin scenarios for the elliptical craters on Tethys and Dione would have to explain the orientation and latitudinal clustering of east/west oriented craters, as well as the characteristics of the azimuthally isotropic population, which requires evenly distributed cratering across each satellite without a preferred impact direction. These characteristics can serve as constraints on the formation and evolution of the Saturnian satellites.

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.

Crater production on Titan and surface chronology

Context. Impact crater counts on the Saturnian satellites are a key element for estimating their surface ages and placing constraints on their impactor population. The Cassini mission radar observations allowed crater counts to be made on the surface of Titan, revealing an unexpected scarcity of impact craters that show high levels of degradation. Aims. Following previous studies on impact cratering rates on the Saturnian satellites, we modeled the cratering process on Titan to constrain its surface chronology and to assess the role of centaur objects as its main impactors. Methods. A theoretical model previously developed was used to calculate the crater production on Titan, considering the centaur objects as the main impactors and including two different slopes for the size-frequency distribution (SFD) of the smaller members of their source population. A simple model for the atmospheric shielding effects is considered within the cratering process and our results are then compared with other synthetic crater distributions and updated observational crater counts. This comparison is then used to compute Titan’s crater retention age for each crater diameter. Results. The cumulative crater distribution produced by the SFD with a differential index of s2 = 3.5 is found to consistently predict large craters (D > 50km) on the surface of Titan, while it overestimates the number of smaller craters. As both the modeled and observed distributions flatten for craters with D ≲ 25 km due to atmospheric shielding, the difference between the theoretical results and the observational crater counts can be considered as a proxy for the scale to which erosion processes have acted on the surface of Titan throughout the Solar System age. Our results for the surface chronology of Titan indicate that craters with D > 50 km can prevail over the Solar System age, whereas smaller craters may be completely obliterated due to erosion processes acting globally.

Crater Distributions of Uranus's Mid-sized Satellites and Implications for Outer Solar System Bombardment

Abstract Outer solar system impact bombardment is largely unconstrained. Although recent data from the Jupiter, Saturn, and Pluto systems have produced new constraints, analysis is incomplete without inclusion of the Uranus system. We reanalyze Uranus system crater populations with recent improvements in processing of Voyager 2 imaging. No consensus in crater populations on mid-sized Uranian satellites, Miranda, Ariel, Umbriel, Titania, and Oberon, was resolved during the Voyager era. For satellites with available data, we find variability in crater size–frequency distributions (SFDs) for diameters (D) < 10 km. Most terrains on Miranda show a shallower slope (ratio of smaller to larger craters is smaller), while Inverness Corona on Miranda and Ariel's terrains show a steeper slope (ratio increases). For D > 10 km, satellites with available data show a steeper slope. Shallower-sloped SFDs for D < 10 km and steeper slopes for D > 10 km agree with Pluto system data—a proxy for the heliocentric impactor population originating from the Kuiper Belt—implying these SFDs represent heliocentric bombardment in the Uranus system. The shallow-sloped population for smaller diameters is also observed on Jovian satellites, but not on mid-sized, heavily cratered Saturnian satellites or Triton (Neptune), which have steeper slopes. This implies the heliocentric impactor population originating from the Kuiper Belt reaches throughout the outer solar system, but that the Saturnian, Neptunian, and maybe Uranian systems also might have their own planet-specific impactors. Finally, we find Ariel appears overall younger than the other Uranian satellites, supporting relatively recent geologic activity.

Warning of asteroids approaching Earth from the sunward direction using two Earth-leading heliocentric orbiting telescopes

Ground-based optical telescopes suffer from a blind zone surrounding the Sun. In this article, two telescopes were proposed to be deployed on an Earth‑leading heliocentric orbit approximately 10 million kilometers ahead of the Earth to warn of asteroids approaching Earth from the sunward direction. Considering the initial orbit determination, a discovery model for asteroids was established. Based on Granvik's impactor and near-Earth asteroid (NEA) populations, the warning efficiency of the Earth‑leading heliocentric orbiting telescopes for asteroids approaching Earth from the sunward direction was simulated. The simulation results show that under the designed search strategy, with a limiting apparent magnitude of 24, the warning efficiency of two Earth‑leading heliocentric orbiting telescopes for impactors and NEAs approaching Earth from the sunward direction reaches 99.0% and 94.3% (with 0.4% and 0.8% uncertainty, respectively) within 6 years, which is 95.8% and 57.5% (with 0.7% and 1.7% uncertainty, respectively) for one telescope deployed on an Earth‑leading heliocentric orbit and 22.1% (with 1.5% uncertainty) for two telescopes deployed on a Sun-Earth L1 orbit. Earth‑leading heliocentric orbiting telescopes thus provide a possible option to cover the blind zone of ground-based optical telescopes.

Collisional Evolution of the Inner Zodiacal Cloud

Abstract The zodiacal cloud is one of the largest structures in the solar system and strongly governed by meteoroid collisions near the Sun. Collisional erosion occurs throughout the zodiacal cloud, yet it is historically difficult to directly measure and has never been observed for discrete meteoroid streams. After six orbits with Parker Solar Probe (PSP), its dust impact rates are consistent with at least three distinct populations: bound zodiacal dust grains on elliptic orbits (α-meteoroids), unbound β-meteoroids on hyperbolic orbits, and a third population of impactors that may be either direct observations of discrete meteoroid streams or their collisional by-products (“β-streams”). The β-stream from the Geminids meteoroid stream is a favorable candidate for the third impactor population. β-streams of varying intensities are expected to be produced by all meteoroid streams, particularly in the inner solar system, and are a universal phenomenon in all exozodiacal disks. We find the majority of collisional erosion of the zodiacal cloud occurs in the range of 10–20 solar radii and expect this region to also produce the majority of pickup ions due to dust in the inner solar system. A zodiacal erosion rate of at least ∼100 kg s−1 and flux of β-meteoroids at 1 au of (0.4–0.8) × 10−4 m−2 s−1 are found to be consistent with the observed impact rates. The β-meteoroids investigated here are not found to be primarily responsible for the inner source of pickup ions, suggesting nanograins susceptible to electromagnetic forces with radii below ∼50 nm are the inner source of pickup ions. We expect the peak deposited energy flux to PSP due to dust to increase in subsequent orbits, up to 7 times that experienced during its sixth orbit.

A New Martian Crater Chronology: Implications for Jezero Crater

Abstract Crater chronologies are a fundamental tool to assess the relative and absolute ages of planetary surfaces when direct radiometric dating is not available. Martian crater chronologies are derived from lunar crater spatial densities on terrains with known radiometric ages, and thus they critically depend on the Moon-to-Mars extrapolation. This extrapolation requires knowledge of the time evolution of the impact flux, including contributions from various impactor populations, factors that are not trivially connected to the dynamical evolution of the early Solar System. In this paper, we will present a new Martian crater chronology based on current dynamical models, and consider the main sources of uncertainties (e.g., impactor size–frequency distribution; dynamical models with late and early instabilities, etc.). The resulting “envelope” of Martian crater chronologies significantly differs from previous chronologies. The new Martian crater chronology is discussed using two interesting applications: Jezero crater’s dark terrain (relevant to the NASA Mars 2020 mission) and the southern heavily cratered highlands. Our results indicate that Jezero’s dark terrain may have formed ∼3.1 Ga, i.e., up to 0.5 Gyr older than previously thought. In addition, available crater chronologies (including our own) overestimate the number of craters larger than 150 km on the southern highlands, suggesting either that large craters have been efficiently erased over Martian history or that dynamical models need further refinement. Further, our chronology constrains the age of Isidis basin to be 4.05–4.2 Ga and that of the Borealis basin to be 4.35–4.40 Ga; these are predictions that can be tested with future sample and return missions.

Change in the Earth–Moon impactor population at about 3.5 billion years ago

The bombardment of impactors (leftover planetesimals, asteroids and comets) created numerous impact craters on the Moon. The giant planets in the outer Solar System are believed to have experienced a dynamical instability, in which the migration of giant planets delivers impactors to the inner Solar System bodies1,2. The difference between the population of large (diameter more than ~5 km) impact craters observed on heavily cratered lunar highlands and that on the lunar maria3 was thought to support the lunar Late Heavy Bombardment, which started ~0.6 billion years after planet formation and could have been caused by the late instability of giant planets4–6. However, large craters on various-aged lunar surfaces have similar size–frequency distributions when considering the preferential erasure of small craters7,8. In addition, dynamical and geochemical evidence favour an early instability of giant planets at ~4.5 Gyr ago2,5,9. Here, we report the evolution at geological scales of regolith thickness on the Moon, which is a proxy for the change of the size–frequency distribution slope for Earth–Moon impactors with diameters less than ~50 m (which generate craters with diameters less than ~1 km (ref. 10)). We found an abnormally slow growth of regolith thickness per unit of impact flux before $$3.5_{ - 0.6}^{ + 0.3}$$ Gyr ago (3σ uncertainty), which can best be explained by a population of craters of less than ~1 km whose size–frequency distribution had a shallower power-law slope ( $$- 2.57_{ - 0.16}^{ + 0.30}$$ ) than that afterward ( $$- 3.24_{ - 0.06}^{ + 0.03}$$ ). The transition time at ~3.5 Gyr ago supports the early instability of giant planets, in which dominant Earth–Moon impactors changed from leftover planetesimals to asteroids5. The value of −3.24 is consistent with the preferential delivery of small asteroids via Yarkovsky–YORP effects11. The change in growth of the lunar regolith thickness around 3.5 Gyr ago, a consequence of a change in population of the impactor bodies from planetesimals to asteroids, indicates that the instability of giant planets happened early.

Early impact chronology of the icy regular satellites of the outer solar system

The dating of the surfaces of the giant planets' regular satellites is hampered by two issues. The first is the lack of known samples from which radiometric ages can be measured. The second is a potentially different cratering chronology compared to those of the Moon and Mars, due to a difference in the impactor population and planetary system architecture, making a direct comparison and absolute ages derivation difficult. That leaves modelling as the only way to obtain something close to absolute surface ages. Here we applied and reported an improved chronological model of the outer solar system during an episode of dynamical instability caused by giant planet migration. We have combined a series of dynamical and Monte Carlo simulations with various crater scaling laws, impactor size-frequency distributions and observational crater densities to compute the expected crater densities on the surfaces of the regular satellites of Jupiter, Saturn and Uranus. From this, we derived their oldest surface ages. Our impactors are sourced from heliocentric planetesimals that originated from the disk of small bodies exterior to the orbit of the giant planets, and the irregular satellites, which were likely captured by the mutual encounters between the giant planets. With both crater scaling laws and the assumption that the giant planets began their migration at 4.5 Ga, most satellites' surfaces were dated to be older than 4.3 Ga, apart from the inner and small satellites, i.e. Mimas, Enceladus, Hyperion and Miranda. Their estimated surface ages are ca. 4 Ga, although these bodies' masses are expected to have been heavily eroded and if they existed prior to giant planet migration they were likely to be destroyed. Our prediction of old surface ages for the mid-sized Saturnian satellites has implications for their formation, their tidal evolution, the interior of Saturn and the ages of the rings.

Bennu's near-Earth lifetime of 1.75 million years inferred from craters on its boulders.

An asteroid's history is determined in large part by its strength against collisions with other objects1,2 (impact strength). Laboratory experiments on centimetre-scale meteorites3 have been extrapolated and buttressed with numerical simulations to derive the impact strength at the asteroid scale4,5. In situ evidence of impacts on boulders on airless planetary bodies has come from Apollo lunar samples6 and images of the asteroid (25143) Itokawa7. It has not yet been possible, however, to assess directly the impact strength, and thus the absolute surface age, of the boulders that constitute the building blocks of a rubble-pile asteroid. Here we report an analysis of the size and depth of craters observed on boulders on the asteroid (101955) Bennu. We show that the impact strength of metre-sized boulders is 0.44 to 1.7 megapascals, which is low compared to that of solid terrestrial materials. We infer that Bennu's metre-sized boulders record its history of impact by millimetre- to centimetre-scale objects in near-Earth space. We conclude that this population of near-Earth impactors has a size frequency distribution similar to that of metre-scale bolides and originates from the asteroidal population. Our results indicate that Bennu has been dynamically decoupled from the main asteroid belt for 1.75±0.75 million years.

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 ratio of hazardous meteoroids to orbital debris in near-Earth space

Orbital debris is known to pose a substantial threat to Earth-orbiting spacecraft at certain altitudes. For instance, the orbital debris flux near Sun-synchronous altitudes of 600–800 km is particularly high due in part to the 2007 Fengyun-1C anti-satellite test and the 2009 Iridium-Kosmos collision. At other altitudes, however, the orbital debris population is minimal and the primary impactor population is not man-made debris particles but naturally occurring meteoroids. While the spacecraft community has some awareness of the risk posed by debris, there is a common misconception that orbital debris impacts dominate the risk at all locations. In this paper, we present a damage-limited comparison between meteoroids and orbital debris near the Earth for a range of orbital altitude and inclination, using NASA’s latest models for each environment. Overall, orbital debris dominates the impact risk between altitudes of 600 and 1300 km, while meteoroids dominate below 270 km and above 4800 km.