Meteorite hunters may have found the largest known chunk of the Bronze Age Kaali Meteorite.

Two Polish meteorite hunters, Filip Nikodem and Andrzej Owczarzak, have recovered what they believe to be a fragment of the Kaali Meteorite, a large body which impacted the Estonian island of Saaremaa about 3500 years ago. The piece is reported to weigh about 40 kg, with a second fragment weighing 15 kg also found. The largest potential fragment of the meteorite discovered to date weighed 5.7 kg, and was found by Filip Nikodem in the spring of 2024.

A 40 kg potential fragment of the Kaali Meteorite found in Estonia by two Polish Meteorite Hunters. Z głową w gwiazdach/Facebook.

The Kaali Impact Structures are a series of nine circular crater lakes on the island of Saaremaa, the largest of which has a diameter of 110 m and a depth of 22 m. These are thought to have been caused by an object with a mass of between 20 and 80 tonnes entering the atmosphere at a velocity of somewhere between 10 and 20 km per second, and exploding in a fireball between 10 and 5 km above the ground. This explosion is thought to have largely vaporised the bolide, as well as removing about 81 000 m³ of rock on the ground, and incinerating vegetation up to 6 km from the impact site.  

The lake in the main Kaali Meteorite Crater on Saaremaa Island. The crater is about 110 m across, but the lake only occupies the central part of it. Wikimedia Commons.

This is thought to have happened about 3500 years ago, when the island was occupied by a Bronze Age population, although attempts to date the site and meteorite fragments from it have produced dates of between 5600 and 400 BC. A Bronze Age arrowhead made from meteoric iron which was found in Switzerland in the 1870s has been linked to the Kaali meteorites on the basis of its metallurgy, making a date of around 1500 BC more plausible.

(a) Overview of the Mörigen arrowhead (A/7396). Note adhering bright sediment material. Remnants of an older label on the left of the sample number. Total length is 39.3 mm. ( b) Side view of the Mörigen arrowhead. Layered texture is well visible. Point is to the right. Thomas Schüpbach in Hofmann et al. (2023).

Prior to 2023, the largest fragment of meteorite found at the Kaali sites weighed 6.21 g. However, in the autumn of that year Filip Nikodem obtained a search permit for the area, and, using a metal detector, found a series of pieces of iron which he believed to be meteoric in origin. These were handed to the Estonian National Heritage Board, who sent them to the University of Tartu, where a rapid X-ray fluorescence analysis found that the iron pieces contained between 0.6% and 4.8% nickel, as well as traces of titanium and vanadium, which is typical of meteoric iron.

The four pieces of meteoric iron handed to the Estonian National Heritage Board  by Filip Nikodem in 2023. Kristo Oks/University of Tartu.

In the spring of 2024, Filip Kikodem returned to the Kaali Lakes, collecting several more fragments with a total mass of 10.5 kg, the largest of which weighed 5.7 kg, making it (at that time) potentially the largest chunk of the Kaali bolide ever discovered. However, despite requests from the Estonian National Heritage Board, these pieces have never been surrendered to them. Instead, the Estonian authorities believe that the pieces were taken to Poland, where they are being analysed at a Polish university. Furthermore, the National Heritage Board received a letter from a lawyer in Poland, asserting Nikodem's ownership rights over the meteorite fragments.

A 5.7 kg possible meteorite fragment found by Filip Nikodem at Kaali Lakes in the spring of 2024. Filip Nikodem.

In the autumn of 2024 Filip Nikodem returned to Kaali Lakes, this time accompanied by meteorite hunter Andrzej Owczarzak, who did not posses a permit to search in Estonia, and was therefore in breach of Estonian heritage laws. At this time concerns were raised by the Estonian newspaper Saarte Hääl, which observed that the original search permit had been given to Filip Nikodem in relation to a two week project run by the University of Tartu in 2017, in which metal detectorists were given a short training course by the university then allowed to collect fragments for a display at a visitor centre, with the collectors being allowed to keep some fragments as a reward.

Saarte Hääl also noted that suspiciously large chunks of 'Kaali Meteorite' had begun to be offered for sale in Poland, and that some Polish enthusiasts had raised concerns that these might be fragments of the more common Morasko Meteorite, which fell near Potsdam about 5000 years ago. The newspaper also noted that Estonian law provides no specific protection for meteorites found in the country, although there are some restrictions upon where a metal detector can be used. 

Following the announcement of these discoveries, Jüri Plado of the University of Tartu applied for funding for a study on the feasibility of changing the law to protect meteorites found in Estonia, reasoning that such large objects would be a significant piece of national heritage and ought to be studied at an institute within the country, but this application was rejected. 

Local farmers talked to by Saarte Hääl stated that the Poles had shown them permits and promised to take any material found to the University of Tartu. However, several expressed doubts that the iron found was in fact meteoric, as it came from shallow depths on farmland, likely to have been disturbed since the Bronze Age, and resembled bog iron (impure iron deposits which precipitate out of solution in boggy soils). The newspaper contacted Kristo Oks of the Estonian National Heritage Board, who confirmed that it would be impossible to tell the difference between bog iron and meteorites without laboratory analysis. 

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Images from Japan's Smart Lander for Investigating Moon (SLIM).

The Japanese Aerospace Exploration Agency's Smart Lander for Investigating Moon (SLIM) probe has sent back it's first images, following a not-quite-to-plan landing on 19 January 2024. These images show an area of lunar landscape, and are built up by the synthesis of 257 individual low-resolution images from the probe's Multi-Band Camera (MBC). A number of potentially interesting rocks have been identified for future investigations.

A lunar surface scan mosaic image captured by the SLIM-mounted MBC (left) and its enlarged view (right). The grey area on the right of the mosaic lacks data due to the discontinuation of scanning operation. Japan Aerospace Exploration Agency.

The SLIM probe landed on the edge of Shioli Crater, a small lunar impact crater that is located within the much larger Cyrillus Crater in the Moon's Southern Hemisphere, on 19 January 2024. However, due to a problem with its engines it landed nose-down, then toppled so that its solar panels face to the west, a sub-optimal position which leave it receiving more sunlight than would be ideal. It was able to deploy one of the two small landers it carried, the Lunar Excursion Vehicle (LEV-1), which carried out a series of operations, including becoming the smallest ever independent probe to communicate directly with the Earth from the Moon (LEV-1 weighs only 2.1 kg, of which 90 g is its communication system), before powering down.

The SLIM landing site is now entering the lunar night, which will last for 14 days, and the probe which, which is reliant on solar power, will power down for this period. However, due to the unplanned nature of its landing orientation, it is not completely clear if it will be able to re-awaken at the end of this period.

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NASA's Lunar Reconnaissance Orbiter detects Luna 25 impact crator.

NASA's Lunar Reconnaissance Orbiter has detected a new crater on the Moon, thought likely to have been caused by the impact of the lost Russian probe, Luna 25, according to a press release issued on 31 August 2023. The crater is located on the inner rim of Pontécoulant G Crater, a position along Luna 25's planned trajectory, but about 400 km short of its intended landing site. This is approximately where Russia's Roscosmos space agency has predicted the probe impacted the Moon.

Lunar Reconnaissance Orbiter views from 27 June 2020, and 24  August 2023; before and after the appearance of a new impact crater likely from Russia’s Luna 25 mission. NASA.

Roscosmos, has lost contact with its Luna-25 spacecraft on Saturday 19 August, after a thruster rocket fired for 127 seconds instead of 84 as was planned, and believes that it has been destroyed after crashing into the Moon. The probe was intended to mark Russia's return to the Moon for the first time since the Soviet Luna 24 probe in 1977, which was the first robotic probe to land on the surface of another Solar System body, collect a sample, and return to Earth.

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The Nadir Crater: A possible second End Cretaceous Impact Structure off the coast of West Africa.

High velocity impacts by large asteroids and comets are a known, but poorly understood, threat to life on Earth. Impactors with diameters of about 50 m (the estimated size of the object which caused the Tunguska explosion) are thought to hit the Earth roughly once every 900 years, and objects with diameters of 1 km or greater, likely to cause devastation on a regional or global scale, to impact the Earth roughly every million years. The largest impacts cause devastation on a global scale, with the best known of these being the Chicxulub impact 66 million years ago, an event thought to have caused the End Cretaceous Extinction Event. Despite the importance of such events, very little evidence of their having occurred is known, with only about 200 large impact craters known on Earth, of which only 15-20 are interpreted as having happened in a marine environment, despite 71% of the Earth's surface being covered by water. This lack of impact craters, and in particular well preserved impact craters, severely hampers our ability to understand the effects of the events which cause them.

In addition to this general lack of impact craters in the rock record, there is a proportional lack of double impact craters. About 15% of the asteroids in near-Earth orbits are thought to be binaries (i.e. pairs of gravitationally locked asteroids, whereas only 2-4% of known impact craters on Earth have ever been suggested as potentially being parts of pairs. Furthermore, most of these suggestions have either been disproven or are disputed. On Venus, where meteor impacts are generally recorded as 'dark splotches' rather than true craters, due to the much more destructive nature of the Venusian atmosphere, about 14% of impacts appear to be binary. Even allowing for a considerable degree of inaccuracy in these numbers, binary impacts seem to be significantly under-represented on Earth.

Another phenomenon seen elsewhere in the Solar System but largely absent on Earth is impact clustering, i.e. the formation of a series of impact craters over a relatively short period of time. This is typically caused by the breakup of a large object due to a gravitational encounter with a planet, followed by the debris from this breakup remaining on an orbit which intersects that of the planet, resulting in a series of impacts over a drawn out period of time. The only observed instance of such an event was the breakup of Comet Shoemaker-Levy 9 in 1992, which was followed by a series of impacts in 1994 as the debris from this breakup collided with Jupiter, but it is thought that the breakup of larger bodies can result in strings of impacts which could continue for several million years. Only a single such event has been recorded on Earth; a series of linked craters in the Ordovician, which is combined in a shift in the ratio of argon isotopes in the Earth's atmosphere at that time, and evidence for an increase in the amount of extraterrestrial dust raining down on the Earth at that time. Other proposals have been made for clusters of impacts in the Cretaceous and Eocene, but the evidence in support of such events is sparse. This is despite the Moon showing clear evidence of a number of periods of impact clustering, events which must surely also have affected the Earth.

In a paper published in the journal Science Advances on 17 August 2022, Uisdean Nicholson of the School of Energy, Geoscience, Infrastructure and Society at Heriot-Watt University, Veronica Bray of the Lunar and Planetary Laboratory at the University of Arizona, Sean Gulick of the Institute for Geophysics, Department of Geological Sciences and Center for Planetary Systems Habitability at the University of Texas at Austin, and Benedict Aduomahor, also of the School of Energy, Geoscience, Infrastructure and Society at Heriot-Watt University, announce the possible discovery of a second End-Cretaceous impact structure on the Guinea Plateau of the coast of West Africa, based upon evidence from two-dimensional seismic surveys.

The Guinea Plateau is an area of submerged continental crust extending southwest from the coasts of Guinea and Guinea Bissau for about 400 km, where it meets with the Guinea Fracture Zone, beyond which lies the deep ocean. The plateau can be divided roughly into two parts, an inner zone which is about 400 m deep across most of its extent, and an outer zone, known as the Guinea Terrace, which deepens gradually from about 400 m to about 1200 m. The Guinea Fracture Zone is home to a series of seamounts (inactive submarine volcanoes), including the Nadir Seamount.

Map and regional seismic sections showing location of Nadir Crater. (A) Regional bathymetry map of the Guinea Plateau and Guinea Terrace showing location of 2D seismic reflection and well data used in the study. JS, Jane Seamount; NS, Nadir Seamount; PS, Porter Seamount. The white dashed line shows the NE extent of high-amplitude discontinuous seismic facies at the top Maastrichtian interpreted as ejecta deposits and associated tsunami deposits. The north-east limit of this facies closely corresponds with the Maastrichtian shelf-slope break at the landward margin of the Guinea Terrace. Inset map shows a palaeogeographic reconstruction of the Atlantic near the end of the Cretaceous, about 66 Ma ago. Ch, Chicxulub Crater; Nd, Nadir Crater; Bo, Boltysh Crater. (B). Regional composite 2D seismic reflection profile extending from the GU-2B-1 well in the east to the deep Atlantic basin in the west, showing the structural and stratigraphic character of the Guinea Plateau and Guinea Terrace. (C) North-South seismic profile from the salt basin in the north to the Nadir Seamount, south of the Guinea Fracture Zone. Nicholson et al. (2022).

The Guinea Plateau formed by extensional rifting in the Triassic-Jurassic, as the opening of the Atlantic Ocean pulled the North American landmass away from the west coast of Africa. The plateau remained part of the passive (i.e. seismically inactive) margin of the Central Atlantic until about 100 million years ago, when the its southern margin underwent further rifting, as South America separated from Africa.

The crater which Nicholson et al. propose is located on the southwestern part of the Guinea Terrace, roughly 60 km to the north of the Guinea Fracture Zone and 100 km northwest of the Nadir Seamount. The crater is covered by about 400 m of sediment, and a water column about 900 m deep.

The stratigraphy of the area has been determined from boreholes and seismic surveys. Cretaceous deposits here can be divided into a Lower Cretaceous sequence, roughly 145-100 million years old, and an Upper Cretaceous sequence, 100-66 million years old, separated by an unconformity known as the Top Albian. The deposits making up the Upper Cretaceous sequence are largely undeformed, and extend laterally across the Guinea Terrace. These deposits can be further subdivided by a series of (seismically) reflective surfaces, numbered KU1-KU4, which are interpreted as representing major flooding events. The Upper Cretaceous sequence can be divided into a Lower Unit, which is about 300 m thick and contains a series of high amplitude reflective surfaces, and an Upper Unit, which is about 500 m thick and contains moderate-to-low amplitude reflective surfaces.

Seismic characteristics of the Nadir Crater. (A) Seabed depth map of crater showing seismic line locations and the mapped extent of the crater rim and damage zone.  (B)  West-east  seismic  section  (pre-stack  depth  migration  –  depth  domain)  across  the  crater,  highlighting  the  crater  morphology  and  damage  zone,  and  the  extent  of  subsurface deformation. (C) Detailed seismic stratigraphic and structural elements of the crater. KP, Cretaceous-Paleogene sequence (KP1 equivalent to Top Maastrichtian); KU, Upper Cretaceous seismic horizons. KU1 and KP1 'regionals' are schematic reconstructions of these seismic horizons before formation of the crater at the end of the Cretaceous and have been used to reconstruct a conceptual model of crater formation. (D) Southwest-northeast seismic section (pre-stack time migration – time domain) across the crater, showing crater morphology and seismic facies outside the crater, including high-amplitude seismic facies sitting above a roughly 100-ms-thick unit of chaotic reflections, interpreted to have formed as a result of seismic shaking following the impact event. Nicholson et al. (2022).

The end of the Cretaceous sequence here, identified as the Top Maastrichtian, is clearly identifiable, and well defined, but the age of other reflective surfaces in the sequence is less certain. The KU-1 reflective surface may mark the Top Turonian, making it about 90 million years old, and the high amplitude surfaces between this and the underlying Top Albian surface may represent black shale deposits, with elevated levels of organic carbon. The lower amplitude reflective surfaces above KU-1 probably represent marine shales and marls.

The Top Maastrichtian Surface lies at the top of another set of distinctive, high amplitude reflective surfaces, and is marked by an erosional unconformity on the southwest part on the Guinea Terrace, which is not present to the north and east. The reflective surfaces at this level extend laterally across the Guinea Plateau, although they are eroded away on both the continental slope and the inshore shallows. On the Guinea Terrace these surfaces overlie a chaotic seismic surface about 100 m deep.

Overlying the Top Maastrichtian, the Cainozoic deposits begin with a thick sequence of carbonates and clastic sediments, although these are considerably thinner on the Guinea Terrace than elsewhere on main Guinea Plateau. Precise dating of these sediments is again difficult, though sediments younger than about 23 million years old are apparently absent on the Guinea Terrace; strat here comprise a Palaeocene-Eocene sequence about 300 m thick, overlain by a series of Oligocene mass transport deposits (debris from submarine landslides).

The feature which Nicholson et al. identify as the Nadir Crater is a depression at least 8.5 km wide in the Top Maastrichtian. This can be seen where two separate seismic profiles cross one-another, suggesting a rounded or elliptical shape. Since there is no particular reason to believe that the intersection of the two seismic profiles lies at the centre of the depression, 8.5 km wide must be taken as a minimum measurement. The crater bed is 200 m below the 'seafloor' represented by the Top Maastrichtian, while the crater rim is raised 20-40 m above this level. There appears to be a slight uplift in the centre of the crater at the Top Maastrichtian level; this is more pronounced at the Top Albian level, where it rises 350 m above the surrounding terrain. On either side of this central uplift the strata below the crater floor and rime are intensely deformed, with a column of extensively folded and fractured material extending downwards 700 m from the Maastrichtian surface. A wider area of faulting extends 10-12 km from the crater in all directions, with all faults dipping towards the crater.

All of these phenomena are consistent with large impact craters elsewhere on Earth and other Solar System bodies. The rim is raised  above the surrounding terrain approximately one tenth as much as the crater floor is depressed beneath it. The crater is approximately one twentieth as deep as it is wide. The damage zone around the crater is approximately twice as wide as the crater itself. The crater has a terraces structure and a raised central peak.

The deformation beneath the crater is also consistent with the predictions for large craters of this sort. The central uplift seen in such craters is caused by the elastic rebound of shock waves passing through the rock, causing material beneath the crater to flow upwards, initially flowing freely the brecciating as the rock regains its brittleness. The deformations seen in the zone 10-12 km from the crater are consistent with these areas being subjected to a significant shockwave, then flowing back towards the crater to fill a partial void.

Based upon examination of other impact craters on Earth, the zone of uplift beneath a crater 8.5 km wide would be expected to extend downwards to between 630 and 780 m beneath the impact surface. That of the Nadir Crater extends down about 800 m, with some deformation extending downwards into underlying Jurassic Sediments. However, this does not necessarily imply a much larger crater; rather it may be a consequence of a bolide impacting soft, unconsoidated marine sediments. The Mjølnir Crater, in the Barents Sea off the coast of Norway, shows a similar pattern of deformation beneath the crater, although here there is less deformation outside the crater rim, as is the case at Chicxulub. This deeper pattern of damage at sites where large, high velocity objects impact shallow marine sediments may be caused by the sudden decrease in the porosity of the strata impacted, resulting greater brecciation, and the formation of hydrothermal systems which could potentially last for millions of years.

The Top Maastrichtian layer within the crater is overlain by a series of high amplitude reflective layers identified as KP-1 to KP-3, with KP-1 marking the boundary between the Maastrichtian and the Palaeocene. Between the Top Maastrichtian and KP-1 is a layer about 100 m thick and relatively transparent to seismic waves, which Nicholson et al. interpret as a possible suevite layer (i.e melt materia), similar to that seen as the Chicxulub Crater. Above this, between KP-1 and KP-2 is a unit made up of a series of low-amplitude reflective surfaces onlapping onto the inner crater walls, which Nicholson et al. interpret as reworked ejecta material. 

The crater is surrounded by a blanket of material made up of a series of high amplitude reflective surfaces, which Nicholson et al. also interpret as reworked ejecta, combined with material deposited by tsunamis triggered by the impact. Beneath this is a layer of chaotic material, which may represent older strata re-organised by the shock wave from the impact.

The Nadir Crater a deep inner crater with in inner peak and a flat outer zone, surrounded by a raised rim, features also seen in confirmed marine craters such as the Chesapeake Crater (off the east coast of North America), Lockne Crater (in northern Sweden), and Flynn Creek Crater (in Tennessee), strongly supporting the idea that this is an impact crater, although this hypothesis will only be confirmed by drilling into the structure.

There are other processes which can form structures resembling impact craters, such as the dissolution of salt deposits, escape of gas or fluids, volcanic activity, deformation due to tectonic stresses, or any combination of these. However, none of these possible alternatives appears consistent with the geology of the Guinea Terrace,

Salt diapers (domes formed when salt intrudes into overlying rocks) can collapse as the salt within them dissolves into the water column, leaving a circular depression surrounded by radial faults. However, such structures do not have uplifted central zones, nor would they be surrounded by a ring of high amplitude material of the type Nicholson et al. interpret as ejecta at the Nadir Crater. Furthermore, while there are salt diapers on the Guinea Terrace, these are in a zone far to the northwest of the Nadir Crater, with the closest diaper being over 250 km from the crater. 

Craters left by fluids escaping from mud layers as they are buried and then subjected to pressure, are common features on the seafloor, being particularly common on the continental margins, where large amounts of methane are generated. Most of these are a few metres to a few hundred metres across, but the largest can exceed 10 km, making ruling our such an origin for the Nadir Crater impossible on the basis of size alone. However, such structures are almost always found in clusters, and no structures similar to the Nadir Crater can be seen anywhere else on the Guinea Plateau, nor are there any other signs of gas or fluid escape in this region. Furthermore, craters formed by fluid escape do not typically produce crater rings, central uplift, or ejecta, all of which can be seen at the Nadir Crater. 

A variety of depressions, typically bounded by fault zones, can be formed when slip-strike deformation leads to areas of seafloor being pulled apart. However, such depressions are rarely circular, typically being more than twice as long as they are wide. Nor do such structures have rims or centrally uplifted zones. More importantly, the Nadir Crater is located on an area of the African continental margin where slip strike activity is thought to have ceased about 44 million years before it formed, making such an origin highly unlikely.

The most plausible alternative hypothesis for the origin of the Nadir Crater is volcanism. Phreatic explosions, which occur when hot magma comes into contact with water, can form maars (broad, low relief volcanic craters) similar in form to the Nadir Crater. Furthermore, there clearly was volcanic activity nearly contemporary to the formation of the crater in the area; the Nadir Seamont is only 100 km away from the Nadir Crater, and 7.4 million years younger, i.e. close enough to be part of the same volcanic complex, along with the nearby Grimaldi Seamounts. However, maar structures seldom exceed 2-3 km in diameter, and their three dimensional structure has a distinctive funnel-shape unlike anything revealed by the seismic surveys at the Nadir Crater. Furthermore, maars do not have a central uplift structure, making it highly unlikely that the Nadir Crater is a volcanic maar.

Next Nicholson et al. used a hydrocode model to simulate the impact of a 400 m wide object (the estimated size of the Nadir impactor) into soft sediments covered by a range of water depths. All of the models produced a crater between seven and nine kilometres in diameter, with elevated rims and significant central uplift. The amount of subsequent rim collapse varied with water depth, with impacts into deeper water showing more collapse. This lead to more infill into the crater, resulting in a flatter profile and a less prominent central uplift zone. At depths of greater than 1 km, the rim completely collapsed into the crater, creating a very flat profile. In all cases, regardless of depth, the impact resulted in a crater surrounded by a network of faults.

Numerical model results of iSALE hydrocode simulations of final crater architecture for different water depths (200, 500, 800, 1100, and 1500 m, as indicated on the figure) Models assume a 400 m diameter impactor with an impact angle of 90°. The water layer has been removed from each image to better highlight the final crater morphology. Model outcomes show total plastic strain on the left and deformed lithologies on the right - the gray unit represents the assumed Cenomanian-Turonian black shale deposits as a marker horizon. Nicholson et al. (2022).

All models show the development of a central uplift, although the extent of this varies with depth, with larger central uplift zones in impacts at greater depth, possibly representing the rocks rebounding more when the weight of the overlying water is removed by the flash-vaporization cause by the impact. These simulations showed material from as deep as 2 km below the surface being moves upwards, with material from up to 1 km deep reaching the subsurface within the central peak; based upon the available data Nicholson et al. estimate that material from as deep as 750 m beneath the Nadir Crater is present within its central peak.

Snapshots of numerical model results of an iSALE hydrocode simulation for 800 m target water depth. Considered this to be the most likely water depth for the impact, based on crater morphology. Model snapshots show transient crater formation and generation of a rim-wave tsunami after 3 seconds; rebound of central uplift and propagation of a rim-wave tsunami away from crater at 38 seconds; crater collapse, resurge, and central jet formation at 85 seconds; and further resurge at 245 seconds. Nicholson et al. (2022).

All of the simulated impacts produced zones of brecciation beneath the crater, with the depth to which this extended ranging from about 1 km up to about 2.5 km. Areas of damage were also present around all the simulated craters, with faults and fractures extending at least 5 km from the craters and reaching depths of at least 1 km.

The models are consistent with the observed features at the Nadir Crater, insofar as all of the craters forming at depths shallower than 1 km produced raised rims, uplifted central areas, terraces made up of debris, and areas of damage outside the crater. The models which produced craters most similar to the Nadir Crater had water depths of about 800 m.

Based upon the data gained by running these simulations, Nicholson et al. developed a conceptual model for the chain of events around the impact. It is assumed that the initial stratigraphy of the area comprised a series of flat, horizontal bedding plains, overlain by about 800 m of seawater. The initial impact would have formed a transient crater about 1 km deep, followed rapidly by the formation of the central uplifted zone, and the collapse of the initial crater rims. This would produce the current observed structure; a crater with a marked central uplift and observable rims surrounded by terraces of collapsed material, and a wider area of fallen ejecta, a hypothesis which they propose could by tested by drilling in the area, in order to confirm the actual nature of the deposits observed in the seismic profiles.

Conceptual model of the impact sequence at the Nadir impact site, based on seismic observations and analog models.  (A) At time zero, the impactor hits the water surface at a velocity of about 20 km per second, initiating a rim-wave tsunami in its wake. (B) Several seconds later, the transient crater forms, as the impactor and a substantial body of water are vaporised. Impactites (melt rock and breccias) line the transient cavity. Tsunami waves propagate away from the evacuated crater. Shock waves cause substantial damage below and around the impact site, and seismic waves propagate across the plateau; (C) major uplift (roughly 400 m relative to pre-impact regional) occurs in the 'rebound' crater modification phase, resulting in the formation of a raised crater peak; (D) radial collapse of the subsurface damage zone results in further modification of the crater, including the formation of terraces at the surface. Resurge of water transports substantial volume of ejecta and other sediment into the crater, deposited above the impactites. Nicholson et al. (2022).

Taking the data from the simulations, Nicholson et al. used the Earth Impacts Effect Program to assess the environmental damage caused by the event which formed the Nadir Crater. The initial impact would vaporise vast amounts of seawater and seafloor sediment and release an amount of energy roughly equivalent to 5000 megatons of TNT. This in turn would lead to the formation of a fireball over 5 km in diameter, an an air blast which would be travelling at about 470 km per hour at a distance of 50 km from the impact site. 

The event would release seismic waves equivalent to a Magnitude 7.0 Earthquake, fluidising seafloor sediments for hundreds of kilometres in all directions. Following fluidisation of the sediments they would likely be reworked extensively by the tsunami waves caused by the impact. The seismic waves would also be likely to trigger massive landslides along the Guinea Terrace.

The initial impact would cause an 'ejecta curtain', made up of water and sediment, over 2 km high, which would then collapse back onto the sea surface, triggering the formation of a tsunami wave over 500 m high, which would spread at about 400 m per second. As well as spreading out from the rim of the crater, this tsunami would spread inwards, forming a central water jet rising to over 2 km. This would then collapse back into the crater, forming a new tsunami, a process which might repeat itself several times. These repeated tsunamis would cause extensive reworking of sediments along the Guinea Terrace, at depths of up to 800 m. They would also cause extensive scouring of the West African coastline, and to a lesser extent that of South America, which, despite being about 1000 km away, would still be hit by waves about 5 m high. 

The impact would lead to large volumes of water entering the atmosphere, as well as black carbon dust derived from the black shale deposits laid down on the Guinea Plateau during the Cenomanian  and  Turonian.  The high temperatures and pressures unleashed by the impact would turn much of the organic material present in the area directly into methane, with global climatic implications, albeit short lived ones. The seismic shock wave could potentially release a lot more methane from gas hydrate deposits on the Guinea Plateau, and adjacent areas of the deep sea floor.

The impact which caused the Nadir Crater appears to have happened at, or very close to, the Cretaceous-Palaeocene boundary. This places the Nadir Impact very close to the Chicxulub Impact, chronologically speaking, raising the question as to whether the two events were connected. Potentially the Nadir and Chicxulub impactors might have been two parts of a binary object, two fragments of an object which broke up as it came within the Earth's gravitational field, two objects from a longer impact cluster, or two unrelated objects which happened to fall at about the same time. 

Impacts by binary asteroids have previously been suggested as the cause of other crater pairs, including the Lockne and Målingen craters in  Sweden, the East and West Clearwater craters in Quebec, and the Suvasvesi craters in Finland, although more refined dating has shown that neither the Clearwater and Suvasvesi craters are in fact binaries, and no there is no evidence in favour of the Lockne and Målingen craters being related, and therefore no evidence of a binary impact anywhere on Earth. 

Reconstructions show that the Nadir Crater site would have been about 5500 km from the Chicxulub Crater site at the end of the Cretaceous; significantly less than the 8000 km that separates them today, but still to far apart to have been caused by the impacts of two parts of a single object which broke up within the Earth's Roche limit (the minimum distance to which a large satellite can approach a larger body without tidal forces overcoming the internal gravity holding the satellite together), or within the Earth's atmosphere. 

This does not, however, rule out two fragments of a single object which was broken into fragments by an earlier encounter with the Earth's tidal field, then impacted the Earth on a subsequent encounter, in the way that Comet Shoemaker-Levy 9 was torn apart by Jupiter's gravitational field, then impacted Jupiter in several fragments on a subsequent pass several years later. There is some evidence of other large impacts around the End of the Cretaceous, most notably at Boltysh in Ukraine (currently dated to 65.4 million years ago, or 650 000 years younger than the Chixulub impact), as well as preserved fossil meteorites from Poland and the North Pacific. This could imply that the Earth was not just hit by a single large impact, but a protracted asteroid shower, lasting from months to one or two million years, with each impact triggering earthquakes and tsunamis, and contributing to rising atmospheric levels of greenhouse gasses and aerosols.

Estimates as to how often objects the size of the Nadir impactor hit the Earth vary from about once every 100 000 years to about once every 700 000 years. Thus even if it can be demonstrated that the Nadir Impactor fell within a million years of the Chicxulub impactor, it would be impossible to demonstrate that the two events were part of a single asteroid shower (Near Earth Asteroid (101995) 1999 RQ36 is roughly equivalent in size to the Nadir impactor, and is estimated to have a 1 in 1750 chance of hitting the Earth within the next few hundred years). However, if it can be demonstrated that the Chicxulub, Nadir, and Boltysh events all happened within a relatively short period of time, then it would present a much stronger case for the events being related, with important implications for our understanding of events at the End of the Cretaceous.

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Secondary cratering from the Early Permian of Wyoming.

Many Solar System bodies, including our Moon, are covered by enormous numbers of impact craters. On Earth, in contrast, the total number of such craters discovered stands at 208. This is largely due to the Earth's active surface, with the continents and continental shelves being subjected to constant erosion and deposition of sediments, and the ocean floors being constantly recycled through subduction and seafloor spreading. However, the situation is more complicated than it seems at first sight; many of the craters seen on the Moon and other Solar System bodies are in fact secondary craters, formed by debris ejected from larger impact events. Once understood, such craters have been relatively easy to identify, frequently being elliptical rather than circular in shape, shallower than primary craters, and arranged radially around the initial impact crater. The implications of this are debatable, with some planetary scientists arguing that as many as 95% of small craters on some bodies may be secondary in origin, whereas others see them as a insignificant proportion of the total number.

In a paper published in the journal GSA Bulletin on 11 February 2022, Thomas Kenkmann and Louis Müller of the Institute of Earth and Environmental Sciences at Albert-Ludwigs-Universität Freiburg, Independent Consultants Allan Fraser and Doug Cook, Kent Sundell of the School of Science at Casper College, and Auriol Rae, also of the Institute of Earth and Environmental Sciences at Albert-Ludwigs-Universität Freiburg, describe the discovery of a secondary impact field comprising at least 31 craters, and possibly as many as 60 more, in southeastern Wyoming, USA. 

The presence of a field of impact craters in the Rocky Mountains in Wyoming was first reported in 2018 by Thomas Kenkman, Kent Sundall, and Douglas Cook. At the time they reported about 40 circular-to-elliptical structures on a tilted Permian exposure on the northeast flank of the Sheep Mountain, eight of which showed sufficient grain deformation to be confirmed as impact structures. These were initially interpreted as the result of a single large object which broke up as it entered the atmosphere, resulting in a group of closely clustered craters.

The new paper by Kenkman et al. describes the presence of several other craters on Sheep Mountain Ridge, and other exposures of the same age at Wagonhound Ridge, Mule Creek, Fetterman Ridge, Fetterman Road, and Palmer Canyon Road, as well as possible craters at several other locations. These range from 10 to 80 m in diameter, and while many are circular, some are as much as 1.7 times as long as they are wide. All of the structures which are firmly established as being craters lie at the top of the Casper Formation, which is immediately overlain by the Opeche Shale member of the Permian Goose Egg Formation, dating the impacts to about 280 million years ago, making them late Early Permian in origin.

 

Digital elevation model map of southeastern Wyoming, USA, and adjacent areas showing the exposure of Casper Formation and the locations of the secondary craters. Based on the intersection of trajectory fans, the proposed site of the possible primary crater is reconstructed. SM, Sheep Mountain; MC, Mule Creek; FR, Fetterman Ridge; FRX, Fetterman Road; PCR, Palmer Canyon Road; WR, Wagonhound Ridge; BE, Box Elder Canyon; MR, Manning Ridge. Kenkman et al. (2022).

The Casper Formation is made up primarily of aeolian sandstones (i.e. sands laid down in a terrestrial desert or dune environment), although the uppermost portion of the section, where the craters are preserved, represents a marine transgression into this environment, forming a lagoon or sabkha environment (a sabkha being a coastal saltpan regularly refilled by tidal waters and emptied by evaporation). The craters sometimes form pedestals standing above the eroded surface of this formation, resulting from lithification processes of associated with the impacts, such as shock fusion (welding together of particles by a sudden impact), and cementing by a glassy melt formed within the crater.

 

Remote sensing images of selected craters of the different crater fields. (A), (B), (D), and (E) are drone images; (C), (F), and (G) are Google Earth imagery. Crater locations: SM, Sheep Mountain; MC, Mule Creek; FR, Fetterman Ridge; FRX, Fetterman Road; PCR, Palmer Canyon Road. (A) Crater SM-1 has an elliptical outline, a pedestal morphology with a preserved proximal ejecta blanket, and a raised rim. The downrange (northwest) rim shows an overturned ejecta flap. The linear ejecta wall is interpreted as a herringbone pattern. (B) Craters SM-6-3-4-5 form a northwest-southeast–trending radial crater chain. The ovoid crater SM-2 shows an overturned ejecta flap downrange (northwest) and a linear ejecta wall. (C) Craters MC-1 and MC-2 represent eroded craters with very little topography but a concentric fracture pattern. (D) Strongly degraded craters FR-1 and FR-2 contain shock effects along their crater rims. (E) The deeply eroded crater SM-9 is circular and has a bright halo of quartzitic sandstone. (F) Crater FRX-20 is a strongly elliptical landform with a gently rising rim. (G) The western and northern rim of crater PCR-1 exposes steeply dipping rocks. PCR-2 shows an elliptical outline. Kenkman et al. (2022).

The Casper Formation, on the upper surface of which the craters are preserved, has very little surface exposure, 1% of in Converse County and only 4% in Albany County, suggesting that many more craters may be preserved buried beneath the centre. This area lies within the Laramide Mountains, an area within the wider Rocky Mountains where Late Cretaceous to early Eocene (~75-50 million years ago) reverse faulting and folding uplifted an area of Archean basement rocks and the material overlying it. Most of the known craters lie upon the Sheep Mountain flank of the Sheep Mountain anticline, an extended fold-ridge mountain running from northwest-to-southeast, where the Opeche Shale has eroded back revealing the upper surface of the Casper Formation; it has been calculated that about 2 km of overlying rocks have been eroded away here since the onset of the orogeny; strata that are still likely to be in place in other areas, covering up other craters associated with this field.

 

(a) Simplified geological map of the Wyoming state. (b) Aerial photograph of Sheep Mountain anticline (view from the NNW); (c) Geological cross-section through of Sheep Mountain anticline perpendicular to the strike of the average fold axis, and (d) Geological map of the Sheep Mountain anticline area. Amrouch et al. (2010).

Wagonhound Ridge is a similar, and associated, structure to the south of Sheep Mountain, showing slightly less uplift. The Mule Creek, Fetterman Ridge, Fetterman Road, and Palmer Canyon Road exposures are found on the southwestern slope of the basement uplift of the Laramie Mountains in transition to the Shirley Basin, where the Casper Formation has largely been eroded away, but is exposed on several remaining buttes.

The largest cluster of impact-related structures is found on the northeastern flank of the Sheep Mountain, where a series of circular, irregular-shaped, and ellipsoidal have been confirmed as impact craters. These vary in their preservation quality from pristine to heavily eroded, with erosion apparently linked to the recent exposure of structures which were rapidly buried after their formation; i.e. the most eroded structures are located higher on the flanks of the mountain. The most pristine structures show steep crater walls, raised rims with overturned ejecta flaps, and remains of proximal ejecta blankets. These craters have floors are covered by soil and filled with muds derived from the overlying Opeche shales, making it hard to establish their depth-to-diameter ratio. Many of these craters are elliptical-to-ovoid in shape, with their long axes having fairly consistent orientations of 315–328°. The distribution of ejecta around the craters is uneven, with well-developed overturned flaps on their northwestern sides, suggesting they were caused by debris thrown from a primary impact to the southeast. Four of the craters form a chain, with a similar orientation. The more eroded structures further upslope tend also to be more rounded, with an internal ring structures. These often stand proud of the eroded surface, being more resilient to erosion due to the shock-fusion of the sandstone.

Ten possible craters have been found on the exposed surface of the Casper Formation at Wagonhound Ridge. Two of these have been confirmed as definite craters due to elevated rims. These are again filled with soil, and slightly elliptical. 

At Mule Creek a large elliptical crater measuring 56 m by 44 m has an orientation of 284±5°. A second structure, measuring 30 by 27 m is adjacent to this. Neither of these are elevated above the surrounding rock surface, and neither preserves any rim structure or surrounding breccia. It is thought that these represent the lower portions of larger craters that have been mostly eroded away; the larger of them appears to be surrounded by a larger ring at a distance of about 100 m, possibly representing underlying rocks that were consolidated by the impact. This area is cut through by a north-south and northwest-southeast–trending tectonic joint system visible in remote sensing images, which would have served to hasten erosion in this area. This joint system extends about 300 m to the northwest of the main crater, and contains at least nine irregular, soil-filled depressions which might represent further impacts. Another cluster of possible craters, one of which has been confirmed as an impact structure, is found about 2.5 km to the southeast of the main crater at Mule Creek. These structures have crater rims composed of sandstone breccia sealed with chert matrix.

At Fetterman Ridge a series of erosional buttes have exposures of the upper surface of the Casper Formation which have been eroded away from much of the surrounding landscape. One of these, a hill measuring roughly 200 m by 100 m, hosts three impact craters, measuring 30 m by 22 m, 28 m by 17 m, and 10 m by 10 m. The long axes of the two elliptical craters trend west-north-west to east-south-east, although their southern rims are more eroded. The 28 m by 17 m is distinctly elevated on its northwestern rim, with a visible ejecta flap on its western side. Again, the brecciated and fragmented rocks are sealed by microcrystalline silica, making them resistant to erosion. Other buttes to the northwest and southeast show possible additional impact craters, although these are more heavily eroded.

The Fetterman Road cluster comprises six possible craters 7–30 m in diameter, some of which are distinctly eliptical. The most distinct of these is 25 m by 15 m and has an orientation of 296°. The rim of this crater is elevated about a metre above its interior.

Eight possible craters are present at Palmer Canyon Road, about 10 km to the southeast of the Fetterman Road cluster. The two most conspicuous of these measure 42 m by 40 m, and 28 m by 19 m. Again, elevated rims are composed of quartzitic breccia with microcrystalline chert fill.

 

Outcrop-scale observations at the Wyoming crater field. (A) Panoramic view of crater WR-5 that shows very little relief. Note the outcrops along the rim. Persons for scale. (B) Brecciated ejecta. Fragments are partly angular and partly subrounded (SM-34). (C) The wind-scoured crater walls composed of quartzitic sandstone show abundant ventifacts (PCR-001). (D) Chert with flow textures and vesicles is very abundant at most of the craters (SM-36). (E) Breccia with quartzitic matrix (PCR-001). (F) The variegated contact of Casper sandstone and the Opeche Member of the Goose Egg Formation contains a few shocked quartz grains. Kenkman et al. (2022).

All of the discovered craters are on the upper surface of the Casper Formation, and all are in sandstones, but the nature of these sandstones varies slightly from site-to-site, reflecting an environment which was fully sub-aerial in the southwest, passing through a braided-river system into a shallow marine environment with some carbonate deposition in the northeast. The presence of water in the sands in some environments does not appear to have led to degradation of the crater rims. In all cases the craters were buried beneath Opeche Shale Member red beds of the Goose Egg Formation rapidly after their formation.

Brittle deformation, indicative of sudden physical shocks, can be seen on both large and small scales. The crater rims and ejecta all show brecciation (breaking into angular fragments) and brittle deformation, while individual grains are often intensely fractured.

Microstructures related to impact. (A) Crosscutting {1013} and (0001) PDF lamellae in sample from crater SM-19 are decorated by fluid inclusions. (B) {1013} PDF lamellae in sample from crater MC-1. A + B show that shock effects are restricted to the detrital grains while the overgrowth is undeformed. (C) Relatively wide-spaced planar fracture lamellae in sample from crater WR-4. (D) Concussion fractures in adjacent quartz grains emanate from initial grain contacts. (E) Crater SM-28 contains abundant chert layers and chert lumps that are embedded in the sandstone. Some of the chert lumps contain spherical lapilli. For interpretation, see text. (F) Close-up of a spherical lapillus that is interpreted as an accretionary lapillus. The concentric rings are composed of microcrystalline quartz around a dark-colored center. All photomicrographs were taken with crossed polarizers. Kenkman et al. (2022).

However, shocked grains are somewhat rare in the crater sediments, with slides made up from samples taken in the field typically showing only two-or-three shocked grains, and these usually being surrounded by unshocked grains rather than clustered together. Shocked grains were found in all parts of the crater structures, and at a much lower fequency level, outside the craters in undeformed sands, probably indicating their having been blown from craters by winds shortly after their formation. The degree of fracturing implies that these grains were subjected to pressures in excess of 10 gigapascals. 

Cherty (amorphous) silica is present at all sites, often forming the matrix which binds the sand grains together. Investigation of one of the craters at Sheep Mountain found a variety of structures within this chert, including elongated shapes and wavy layers. Within the chert were spherical structures resembling accretionary lapilli; glassy grains which are typically associated with violent volcanic eruptions, formed by the accretion of glassy siliica layers onto grains suspended in hot, turbulent air. 

The Wyoming Crater Field shows a number of features that help the reconstruction of the direction being travelled by impactors. The most obvious of these are oval or elliptical shaped craters, and craters arranged into chains, which gives the orientation of the direction of travel, but not the actual direction. However, direction of travel can be determined by using the following lines of evidence: (1) a steeper crater wall uprange; (2) a preserved overturned ejecta flap downrange, with a preferred deposition of ejecta downrange; (3) an ovoid crater shape with the strongest curvature downrange, and (4) V-shaped herringbone patterns of ejecta pointing up-range. 

The area has been subjected to some deformation since the impact craters formed, but none appears to have had its orientation changed by more than about 5%, enabling the use of craters from different sites to attempt to relocate the site of the original impact. Based upon this, Kenkman et al. suggest that the original impact happened at a site with map co-ordinates close to 41°28′N, 103°59′W; all of the craters lie between 150 km and 200 km from this site.

Modelling of possible trajectories of objects thrown from a primary impact crater suggest that boulders with a diameter of 2 m would need an initial velocity of 3-4 km per second to reach 150-200 km from the initial impact, while 4 m objects could reach this distance with an initial speed of 2 km per second would also fall within this zone. Such objects would deliver energy in the range of 12 to 400 gigajoules when they impacted, arriving at angles of between 45° and 60°.

Kenkman et al. calculate that an object with a density of 2500 kg per cubic meter (typical for many rock types), with a 4 m radius impacting at 1 km per second would create an impact crater about 45 m in diameter. A similar object 2 m in radius would create a crater 25-30 m in diameter, depending on its angle of approach. A range of objects between 1 and 4 m in diameter, travelling at between 500 and 1500 m per second, would generate craters between 8 and 55 m in diameter, with larger impacts releasing more energy and creating more shocked material as a proportion of the impactors mass.

Kenkman et al.'s findings reveal a series of clusters of craters across a wide area of southeast Wyoming, all of which appear to have formed simultaneously about 280 million years ago. These are best explained as secondary craters caused by material thrown from a large primary impact crater. No obviously foreign material was found in any of the craters, making it likely that the impacting material was similar in composition to the rocks of the areas impacted, and reconstructions of the direction of travel suggest the primary impact was between 150 and 200 km from the discovered craters.

Based upon the reconstructed size and impact speeds of the ejecta material, Kenkman et al. predict that the original impactor would have been 2.0-2.7 km in diameter, and to have hit the ground at about 20 km per second, creating a crater 50-65 km in diameter. 

This would place the location of the original impact in Goshen or Laramie counties in Wyoming, or Kimball, Banner, or Cheyenne counties in Nebraska. These areas fall within the Denver Basin, and Permian strata are deeply buried beneath subsequent Mesozoic and Cainozoic deposits. Despite this apparent lack of accessibility, these deposits have been heavily boreholed by hydrocarbons exploration companies, which may enable reconstruction of the original impact site. To date, Kenkman et al. have not found evidence of distorted or missing sedimentary strata which might be associated with such an impact in any well log data examined, but one drill core, 1–35 Hawk Fee, does show a breccia layer at 3023–3066 m below the surface, and Kenkman et al. are hoping to visually inspect this core in the near future. 

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