Terrestrial Craters Research Articles

Atmospheric Interactions of Ejecta on Earth and Mars Including the Effect of Vaporization

Atmospheres play an important role in ejecta deposition after an impact event. Many impact experiments and simulations neglect the effect of atmospheres. We simulate ejecta plumes created by craters with transient diameters of 2 and 20 km on Mars and Earth, to show the effect atmospheric density and crater size have on the strength of the interaction. The interaction of ejecta with an atmosphere is explored in this study using a two-fluid hydrocode that simultaneously simulates ejecta and atmospheres as coupled, continuum fields to correctly capture the transfer of mass, energy, and momentum between the two. Here, we study the effect of vaporization of plume material as well as the effect of the bow shock. We find that only the fastest ejecta is vaporized with a peak vaporized mass of 2.5 × 105 kg, 3.5 s after the impact in our 2 km diameter terrestrial crater. Terrestrial meteorites are preferentially formed from the fastest ejecta. However, that fastest ejecta is mostly vaporized in our simulations, so to form a terrestrial meteorite, there must be a sufficiently large impact for solid material to be ejected and not vaporize. Thus, we place a lower limit of 33 km on the size of crater needed to generate terrestrial meteorites, but the crater size needed could be substantially larger. The bow shocks in our simulations result in lofting of ejecta, especially vaporized material, in the wake of the impactor.

A Revision of the Formation Conditions of the Vredefort Crater

AbstractThe Vredefort impact structure, located in South Africa and formed 2.02 Ga, is the largest confirmed remnant impact crater on Earth. The widely accepted impactor diameter and velocity to form this crater are 15 km and 15 km/s, respectively, which produce a crater diameter of 172 km. This is much smaller than the most commonly cited estimates (250–280 km), and while previous results were able to match the geologic evidence known at that time, these impact parameters are not consistent with more recent geological constraints. Here, we conduct impact simulations to model the Vredefort crater formation with the shock physics code impact Simplified Arbitrary Lagrangian Eulerian (iSALE). Our numerical simulations show that combinations of the impactor diameter and impact velocity of 25 km and 15 km/s or 20 km and 25 km/s are able to recreate the larger crater size of ∼250 km. Moreover, these configurations can reproduce shock‐metamorphic features present in the impact structure today, including the distributions of breccia, shatter cones, planar deformation features in quartz and zircon, and melt. Our model also predicts that Vredefort and Karelia, Russia, where an ejecta layer from the impact was found, were approximately 2,000–2,500 km apart based on the layer thickness. Additionally, we use this model to predict the potential global effects of such a large impact by estimating the amount of climatically important gases released to the atmosphere at the time. Our work demonstrates the need to revisit previously estimated impactor parameters for large terrestrial craters in order to better characterize impact events on Earth and elsewhere.

Meteorite impact craters as hotspots for mineral resources and energy fuels: A global review

The ever-increasing recovery rate of natural resources from terrestrial impact craters over the last few decades across the globe offers new avenues for further exploration of mineral and hydrocarbon resources in such settings. As of today, 60 of the 208 terrestrial craters have been identified to host diverse resources such as hydrocarbons, metals and construction materials. The potential of craters as plausible resource contributors to the energy sector is therefore, worthy of consideration, as 42 (70%) of the 60 craters host energy resources such as oil, gas, coal, uranium, mercury, critical and major minerals as well as hydropower resources. Among others, 19 craters are of well-developed hydrocarbon reserves. Mineral deposits associated with craters are also classified similar to other mineral resources such as progenetic, syngenetic and epigenetic sources. Of these, the progenetic and syngenetic mineralization are confined to the early and late excavation stage of impact crater evolution, respectively, whereas epigenetic deposits are formed during and after the modification stage of crater formation. Thus, progenetic and syngenetic mineral deposits (like Fe, Ni, Pb, Zn and Cu) associated with craters are formed as a direct result of the impact event, whereas epigenetic deposits (e.g. hydrocarbon) are hosted by the impact structure and result from post-impact processes. In the progenetic and syngenetic deposits, the shock-wave induced fracturing and melting aid the formation of deposits, whereas in the epigenetic deposits, the highly fractured lithostratigraphic units of higher porosity and permeability, like the central elevated area (CEA) or the rim, act as traps. In this review, we provide a holistic view of the mineral and energy resources associated with impact craters, and use some of the remote sensing techniques to identify the mineral deposits as supplemented by a schematic model of the types of deposits formed during cratering process.

Geology and Geochemistry of Noachian Bedrock and Alteration Events, Meridiani Planum, Mars: MER Opportunity Observations

AbstractWe have used Mars Exploration Rover Opportunity data to investigate the origin and alteration of lithic types along the western rim of Noachian‐aged Endeavour crater on Meridiani Planum. Two geologic units are identified along the rim: the Shoemaker and Matijevic formations. The Shoemaker formation consists of two types of polymict impact breccia: clast‐rich with coarser clasts in upper units; clast‐poor with smaller clasts in lower units. Comparisons with terrestrial craters show that the lower units represent more distal ejecta from at least two earlier impacts, and the upper units are proximal ejecta from Endeavour crater. Both are mixtures of target rocks of basaltic composition with subtle compositional variations caused by differences in post‐impact alteration. The Matijevic formation and lower Shoemaker units represent pre‐Endeavour geology, which we equate with the regional Noachian subdued cratered unit. An alteration style unique to these rocks is formation of smectite and Si‐ and Al‐rich vein‐like structures crosscutting outcrops. Post‐Endeavour alteration is dominated by sulfate formation. Rim‐crossing fracture zones include regions of alteration that produced Mg‐sulfates as a dominant phase, plausibly closely associated in time with the Endeavour impact. Calcium‐sulfate vein formation occurred over extended time, including before the Endeavour impact and after the Endeavour rim had been substantially degraded, likely after deposition of the Burns formation that surrounds and embays the rim. Differences in Mg, Ca and Cl concentrations on rock surfaces and interiors indicate that mobilization of salts by transient water has occurred recently and may be ongoing.

Precise radiometric age establishes Yarrabubba, Western Australia, as Earth\u2019s oldest recognised meteorite impact structure

The ~70 km-diameter Yarrabubba impact structure in Western Australia is regarded as among Earth’s oldest, but has hitherto lacked precise age constraints. Here we present U–Pb ages for impact-driven shock-recrystallised accessory minerals. Shock-recrystallised monazite yields a precise impact age of 2229 ± 5 Ma, coeval with shock-reset zircon. This result establishes Yarrabubba as the oldest recognised meteorite impact structure on Earth, extending the terrestrial cratering record back >200 million years. The age of Yarrabubba coincides, within uncertainty, with temporal constraint for the youngest Palaeoproterozoic glacial deposits, the Rietfontein diamictite in South Africa. Numerical impact simulations indicate that a 70 km-diameter crater into a continental glacier could release between 8.7 × 1013 to 5.0 × 1015 kg of H2O vapour instantaneously into the atmosphere. These results provide new estimates of impact-produced H2O vapour abundances for models investigating termination of the Paleoproterozoic glaciations, and highlight the possible role of impact cratering in modifying Earth’s climate.

Conference report: Large Meteorite Impacts and Planetary Evolution VI

Conference report: Large Meteorite Impacts and Planetary Evolution VI

Contrasting meteoritic signatures within the Clearwater East and Clearwater West impact structures: The view from osmium isotopes

Osmium isotopes provide a powerful tool for identifying meteoritic signatures in impactites. We apply the osmium isotope method to impact melt and country rocks from the Clearwater East and Clearwater West craters located in Quebec, Canada. Impact melts from Clearwater East have 187Os/188Os ratios of 0.1281–0.1285. These values indicate a significant meteoritic component, which exceeds that in impact melts at all terrestrial craters studied to date, except Morokweng. Such findings align with earlier results from chromium isotopes and platinum-group elements. In contrast, impact melts from Clearwater West have 187Os/188Os ratios between 6.604 and 59.12. These highly radiogenic ratios are indistinguishable from the 187Os/188Os ratios in country rocks. Hence, osmium isotopes provide no evidence for a meteoritic component in impact melts at Clearwater West. The Clearwater craters formed in almost identical targets. Therefore, target effects cannot readily explain the stark difference between the two Clearwater craters. If melt sheet heterogeneity is similar at the two craters, the probability that melts at Clearwater West host an undetected chondritic component is < 0.2%. Multiple scenarios may explain the non-detection of a meteoritic signature at West; the possibility of a differentiated achondrite impactor could be tested using chromium isotopes. At Clearwater East, a low impact speed (<10 km s−1) may best explain the unusually strong meteoritic signature. Although the signature (or its nondetection) at each crater may be related to asymmetric preservation of the impactor component, the results presented here provide further evidence that Clearwater East and Clearwater West were temporally separate impact events.

Introduction to the special issue

This special issue is the result of efforts by the Planetary Crater Consortium to engage the crater and broader planetary science community regarding the state of knowledge about impact craters. Impact craters are the most abundant landform on almost all solid surfaces in the solar system, observed on all bodies imaged closely to date except Jupiter's moon Io. Crater populations form the only known metric for modeling the surface ages of planetary bodies, and they are used to inform the underlying dynamics of the objects that generate them and the physics of the impacts themselves. Individual craters are used to understand local geology and geophysics, the environment in which they exist, and other important processes on the bodies upon which they form. In May 2015, the first-of-its-kind “Workshop on Issues in Crater Studies and the Dating of Planetary Surfaces” was convened at The Johns Hopkins University's Applied Physics Lab, where 50 community members presented talks and participated in discussions about the history of impact crater studies, where the field currently stands, and the most pressing questions and issues that are critical to advancing the field. This special issue arose from that conference. In this volume are 14 papers that span a wide range of topics. There are three review papers describing the state of knowledge of secondary impact craters, how large a counting surface needs to be for adequate crater statistics, and a survey of techniques and results of crater depth measurements for the last >50 years on solar system bodies. There are also new science papers, detailing crater morphometry and new methods and recommendations to measure them, terrestrial crater field work, models for types of craters and related features on the Moon and Mars, and implications of crater studies to Martian climate. Finally, there are three papers that describe new techniques and methods to analyze craters: a fast and robust way to determine crater saturation, a method to determine secondary crater clusters from those produced by atmospheric fragmentation, and a re-examination of classic size-frequency distribution techniques with new recommendations on how to display and analyze crater populations. Readers and editors alike thank the contributing authors and reviewers for their hard work on this valuable volume on impact craters. The editors and authors hope that the work in this volume will help pave the way for future research and studies into impact craters for many years to come.

Evidence of impact melt sheet differentiation of the lunar South Pole‐Aitken basin

AbstractThe South Pole‐Aitken (SPA) basin is the largest basin on the Moon. The basin‐forming impact likely melted the upper part of the mantle and formed an impact melt sheet. Impact melt of large terrestrial craters differentiated following a general magmatic differentiation sequence. However, it is still debated whether or not the SPA melt sheet underwent differentiation. To address this, we investigated the vertical and lateral variations in mineral composition of the SPA impact melt sheet area by analyzing the surface mineralogy including 277 post‐SPA craters using remote sensing reflectance data. We identified a 7 km thick, high‐Ca pyroxene‐rich layer and below that, a &gt;8 km thick, low‐Ca pyroxene‐rich layer. Both filled the entire impact melt sheet area (630 km in diameter). Considering the vast distribution of these two layers, their thicknesses, their FeO abundances (around 13.5 wt %), which are lower than most of the lunar mare basalt but within the estimated range of the differentiated SPA impact melt, and that the observed lithologies are consistent with a differentiation model, we conclude that the SPA impact melt was differentiated. Moreover, based on the lunar magma ocean differentiation model, Al2O3 and CaO abundances of the upper part of the mantle would be much lower after the mantle overturn. If the SPA impact occurred after overturn, these elemental abundances of the initial impact melt would be much lower than that of the before‐overturn impact case and result in different solidified layers. Our observation matches only the after‐overturn case, which may imply that the SPA impact occurred after overturn.

Hydrocode simulation of the impact melt layer distribution underneath Xiuyan Crater, China

In this research, we studied the distribution of impact melt layers underneath Xiuyan crater using hydrocode simulation. The target was modeled by granite based on the rock type distribution around the crater and projector by iron, because most small and isolated terrestrial craters are formed by iron projectile. The simulated crater diameter and depth are 1 710 and 320 m, respectively, which are in good agreement with observations of 1 800 and 307 m (except for the post-impact lacustrine sedimentation). The validated model shows that impact melt materials were first formed along the transient crater floor and wall by highshock pressure, and then refilled inward the crater along with collapse of the crater wall. The final style of impact melt materials is interbedded with shock breccia underneath the crater center, which is verified through two layers in the borehole located in the crater center.

A centennial reappraisal of the Vredefort pseudotachylytes: shaken, not stirred by meteorite impact

Major pseudotachylyte zones constitute a spectacular component of the renowned c. 2.023 Ga Vredefort impact structure, South Africa, but it has always been difficult to explain how they were formed. In his original account, in 1916, Shand interpreted the pseudotachylyte as due to cataclasis and frictional heating but pointed out two enigmas that have remained since: there were no associated major faults, and the pseudotachylyte volumes he observed were far greater than in similar rocks located within faults elsewhere on Earth. New observations show that the Vredefort pseudotachylyte zones were indeed formed by cataclasis and frictional heating, not by faulting but owing to impact-induced seismic shaking initiated around temporarily loosened blocks in dendritic fracture systems. Progressive cataclasis of such loose blocks by intense, high-frequency oscillations of the country rock at the beginning of the cratering process led to size reduction, rounding and comminution, and frictional melting of feldspars and biotite in the comminuted parts. Most pseudotachylyte was thus not injected from anywhere but produced in situ . The process of seismic shaking is well known from impacts on the Moon and asteroids, terrestrial earthquakes and nuclear tests but has largely been overlooked in terrestrial cratering, except in the theoretical concept of acoustic fluidization.

The number of impact craters on Earth: Any room for further discoveries?

Only 128 impact craters exposed at Earth's surface have been found so far, while new craters are discovered occasionally. Taking into account the permanent consumption of craters by erosion we present the first estimate on the number of impact craters that should be present at Earth's surface. Our study yields no evidence for any systematic incompleteness in the available inventory of the craters larger than about 6 km in diameter exposed at the surface. In contrast, more than 90 craters in the diameter range from 1 km to 6 km should still be waiting to be discovered, and even more than 250 between 0.25 km and 1 km diameter. The transition from a probably complete inventory above 6 km to a strongly incomplete record at smaller sizes may be related to the difference between simple and complex craters. Beyond these results on the terrestrial crater record, our findings tentatively suggest that erosion rates on the 10 to 100 million year scale may be closer to present-day erosion rates than previously assumed.

Discovery of stishovite in Apollo 15299 sample

High-pressure polymorphs recovered in terrestrial craters are evidence of meteoroid impact events on the Earth’s surface. Despite countless impact craters on the Moon, high-pressure polymorphs have not been reported to date in returned Apollo samples. On the other hand, recent studies report that the high-pressure polymorphs of silica, coesite, and stishovite occur in shocked lunar meteorites. We investigated regolith breccia 15299, which was returned by the Apollo 15 mission, using the combined techniques of focused ion beam (FIB), synchrotron X‑ray diffraction (XRD), and transmission electron microscopy (TEM). The regolith breccia 15299 studied here consists of a mafic impact melt breccia with millimeter-sized, coarse-grained, low-Ti basalt clasts. The mafic melt breccia consists of fragments of minerals (olivine, pyroxene, plagioclase, silica, and ilmenite) and glass. Several quartz, tridymite, and cristobalite grains of 10-100 mm across occur in the mafic impact melt breccia. Vesicular melt veins of less than ~200 mm wide cut across the mafic melt breccia matrix and mineral fragments. Some silica grains are entrained in the melt veins. One of the silica grains entrained in the melt veins consist of stishovite [a = 4.190(1), c = 2.674(1) Å, V = 46.95 Å 3 , space group P4 2 /mnm] along with tridymite and silica glass. This is the first report of high-pressure polymorphs from returned lunar samples. TEM images show that the stishovite is needle-like in habit, and up to ~400 nm in size. Considering the lithologies and shock features of 15299, it is inferred that the stishovite possibly formed by the Imbrium impact or subsequent local impact event(s) in the Procellarum KREEP Terrane (PKT) of the nearside of the Moon.

Where have all the craters gone? Earth’s bombardment history and the expected terrestrial cratering record

Here we make the first quantitative estimates of the maximum total number of large terrestrial impact craters that can survive until the current era on Earth. Our estimates of crater survival are made using the age of Earth’s crust and the expected flux of large extraterrestrial bodies striking Earth. This analysis neglects effects such as weathering and burial, which tend to preferentially erase smaller craters. Thus, our results represent the maximum number of craters we expect to exist on Earth today. Although the constant recycling of crustal material creates a strong bias toward younger ages, our model suggests that the terrestrial cratering record can be used to determine the size frequency distribution of the impactors that bombarded Earth over the past 3.5 G.y. However, our model suggests that the cratering record cannot be used to distinguish between scenarios with a constant flux of impactors over time and scenarios where the bombardment rate was considerably higher in Earth’s ancient history. Luckily, layers of distal impact ejecta may act as records of impacts even when the source craters have been tectonically recycled. We suggest that searches for these distal impact ejecta layers will be more fruitful for constraining the bombardment history of Earth than searches for large craters.

Quantifying the attenuation of structural uplift beneath large lunar craters

Terrestrial crater observations and laboratory experiments demonstrate that target material beneath complex impact craters is uplifted relative to its preimpact position. Current estimates suggest maximum uplift is one tenth of the final crater diameter for terrestrial complex craters and one tenth to one fifth for lunar central peak craters. These latter values are derived from an analytical model constrained by observations from small craters and may not be applicable to larger complex craters and basins. Here, using numerical modeling, we produce a set of relatively simple analytical equations that estimate the maximum amount of structural uplift and quantify the attenuation of uplift with depth beneath large lunar craters.

Morphology and population of binary asteroid impact craters

Observational data show that in the Near Earth Asteroid (NEA) region 15% of asteroids are binary. However, the observed number of plausible doublet craters is 2–4% on Earth and 2–3% on Mars. This discrepancy between the percentage of binary asteroids and doublets on Earth and Mars may imply that not all binary systems form a clearly distinguishable doublet crater owing to insufficient separation between the binary components at the point of impact. We simulate the crater morphology formed in close binary asteroid impacts in a planetary environment and the range of possible crater morphologies includes: single (circular or elliptical) craters, overlapping (tear-drop or peanut shaped) craters, as well as clearly distinct, doublet craters. While the majority of binary asteroids impacting Earth or Mars should form a single, circular crater, about one in four are expected to form elongated or overlapping impact craters and one in six are expected to be doublets. This implies that doublets are formed in approximately 2% of all asteroid impacts on Earth and that elongated or overlapping binary impact craters are under-represented in the terrestrial crater record. The classification of a complete range of binary asteroid impact crater structures provides a template for binary asteroid impact crater morphologies, which can help in identifying planetary surface features observed by remote sensing.

The theoretical plausibility of central pit crater formation via melt drainage

Central pit craters are seen in large craters on some icy satellites and on Mars. We investigate the hypothesis that central pits form when impact melt drains into fractures beneath the impact crater. For this process to occur, the volume of melt generated during the impact, the volume of void space in fractures beneath the impact crater, and the volume of melt able to drain before the fractures freeze shut all must exceed the volume of the observed central pits. We estimate the volume of melt generated using results from previous numerical modeling studies. The fracture volume is estimated using gravity anomalies over terrestrial craters. To estimate the amount of melt able to drain before freezing, we consider flow through plane parallel fractures. These calculations all suggest that enough liquid water could drain into fractured ice beneath a crater on Ganymede to form a central pit. On Earth and the Moon, silicate impact melt will freeze before a large volume is able to drain, so we do not expect to see central pits in impact craters in targets with no ice. In summary, we find our calculations are consistent with observed central pits in craters on Ganymede and the lack of central pits in craters on Earth and the Moon.

Structural asymmetry in martian impact craters as an indicator for an impact trajectory

Impact crater formation is a highly dynamic and complex geological phenomenon. Methods of structural geology are capable of giving insights into deformation processes that occur during cratering. While most deformation observed in craters exhibits a generally radial symmetry, recent discoveries of non-radial structural elements in the central uplifts of terrestrial craters have led to the suggestion that obliquely impacting projectiles can cause non-radial deformation in the central subsurface area of the crater. The lack of ejecta blankets as accepted indicators of an oblique impact on Earth make it difficult to correlate the observed non-radial structures with the impact direction. Therefore, in this study three martian complex impact craters were selected that exhibit an oblique ejecta blanket and layered bedrock in the central uplift. The central uplifts were structurally mapped using High Resolution Imaging Science Experiment (HiRISE) imagery and additional high-resolution digital terrain models (DTMs). Structural data were evaluated and show a correlation between the orientation of at least two structural criteria and the direction of impact: (1) The strike of upturned bedrock layers is on average perpendicular to the impact trajectory and (2) the majority of faults show a preferred trend parallel to the impact trajectory. A fourth martian crater was examined as a counterexample, which had a radial ejecta blanket and thus no indication of an oblique trajectory. Strike in this crater showed no preferred orientation. These results will help to solidify the suggested correlation between non-radial structural features in impact craters and the horizontal component of momentum transferred from an obliquely impacting projectile to the target.

Geochemical signatures and magmatic stability of terrestrial impact produced zircon

Understanding the role of impacts on early Earth has major implications to near surface conditions, but the apparent lack of preserved terrestrial craters >2Ga does not allow a direct sampling of such events. Ion microprobe U–Pb ages, REE abundances and Ti-in-zircon thermometry for impact produced zircon are reported here. These results from terrestrial impactites, ranging in age from ~35Ma to ~2Ga, are compared with the detrital Hadean zircon population from Western Australia. Such comparisons may provide the only terrestrial constraints on the role of impacts during the Hadean and early Archean, a time predicted to have a high bolide flux. Ti-in-zircon thermometry indicates an average of 773°C for impact-produced zircon, ~100°C higher than the average for Hadean zircon crystals. The agreement between whole-rock based zircon saturation temperatures for impactites and Ti-in-zircon thermometry (at aTiO2=1) implies that Ti-in-zircon thermometry record actual crystallization temperatures for impact melts. Zircon saturation modeling of Archean crustal rock compositions undergoing thermal excursions associated with the Late Heavy Bombardment predicts equally high zircon crystallization temperatures. The lack of such thermal signatures in the Hadean zircon record implies that impacts were not a dominant mechanism of producing the preserved Hadean detrital zircon record.

Lightning remagnetization of the Vredefort impact crater: No evidence for impact‐generated magnetic fields

The Vredefort impact crater in South Africa is one of the oldest and largest craters on Earth, making it a unique analog for planetary basins. Intense and randomly oriented remanent magnetization observed in surface samples at Vredefort has been attributed to impact‐generated magnetic fields. This possibility has major implications for extraterrestrial paleomagnetism since impact‐generated fields have been proposed as a key alternative to the dynamo hypothesis for magnetization on the Moon and asteroids. Furthermore, the presence of single‐domain magnetite found along shock‐generated planar deformation features in Vredefort granites has been widely attributed to the 2.02 Ga impact event. An alternative hypothesis is that the unusual magnetization and/or rock magnetic properties of Vredefort rocks are the products of recent lightning strikes. Lightning and impact‐generated fields can be distinguished by measuring samples collected from below the present surface. Here we present a paleomagnetic and rock magnetic study of samples from two 10 m deep vertical boreholes. We show that the magnetization at depth is consistent with a thermoremanent magnetization acquired in the local geomagnetic field following the impact, while random, intense magnetization and some of the unusual rock magnetic properties observed in surface rocks are superficial phenomena produced by lightning. Because Vredefort is the only terrestrial crater that has been proposed to contain records of impact‐generated fields, this removes a key piece of evidence in support of the hypothesis that paleomagnetism of the Moon and other extraterrestrial bodies is the product of impacts rather than past core dynamos.