Introduction
Only larger asteroids and comets produce impact craters, whereas most smaller bolides produce airbursts many kilometers above the Earth, usually with little effect. Our investigation focuses on a small subset of bolides that burst at less than ∼700 m above Earth’s surface. To describe this specific type of near-surface airburst, we adopt the term “touch-down” events (also called Type 2 airbursts [1, 2]) from Van Ginneken et al. [3] Although the characteristics of these objects (e.g., diameter, density, entry angle, and velocity) may be only slightly smaller than those of a typical crater-forming impact, they can still interact with and create extensive damage to Earth’s surface [4, 5]. Van Ginneken et al. [3] state that airbursts are the most frequent and most hazardous type of impact event, and yet, such airbursts leave little evidence in the geologic record and, thus, have rarely been studied.
Study objectives
This study is preliminary, and to our knowledge, no one has previously attempted to model touch-down airbursts in this detail with as many parameters. Our primary goal in this study was to use hydrocode to model the potentially damaging effects of touch-down airbursts, mainly by quantifying temperature, pressure, and shock speed. Although no low-altitude touch-down airbursts have been observed in modern times, two high-altitude ones have occurred with damaging ground effects: one over Tunguska, Russia in 1908 and one over Chelyabinsk, Russia in 2013.
Our contribution explores more energetic events that occur at altitudes low enough to produce hypervelocity jets of vapor and impactor fragments that strike Earth’s surface. We chose specific modeling parameters to investigate whether touch-down airbursts can (i) occur just above Earth’s surface, (ii) eject fragments that strike the ground, (iii) cause significant damage to Earth’s surface, (iv) create shallow craters, (v) produce shock metamorphosis in some minerals, (vi) melt surficial sediments to produce meltglass, and (vii) create high-temperature microspherules. We explore bolides of three different sizes (an 80-m asteroid, a 100-m comet, and a 140-m comet) and compare their modeled effects and damage to the Trinity nuclear detonation. In addition, we present 15 other examples of touch-down airbursts using a more comprehensive range of parameters.
It is important to note that the models investigated are based on a very narrow set of parameters. Impactors with the diameters studied here do not have unique solutions and, instead, can have nearly infinite combinations of density, velocity, and angle of entry that will produce significant variations in burst heights and surface damage. We explored only a few of the numerous parameters that can potentially result in near-surface touch-down airbursts.
Even though relatively infrequent, touch-down airbursts are potentially dangerous and more common than typical crater-forming impact events. Boslough [2] concluded that airbursts can produce more surface damage than nuclear explosions of the same yield and, therefore, are more dangerous than previously assumed. The increased damage is partially due to the difference between a moving bolide with significant downward momentum and a stationary nuclear detonation, such as the Trinity atomic detonation. We explore this issue with our hydrocode models.
This preliminary investigation uses a limited suite of parameters, and we acknowledge considerable uncertainty about the effects of touch-down airbursts, especially since none have been scientifically documented in modern times. However, it is crucial to understand these types of touch-down airbursts and the evidence they may leave in the geological record. The primary purpose of this investigation is to test whether these observations are consistent with modeling experiments.
Airbursts: bolide fragmentation
Most studies conclude that during airbursts at all altitudes, the incoming comet or asteroid is mostly vaporized [6, 7]. However, some fragments from many airbursts commonly reach the ground surface, as in the following examples, most of which occurred at high altitudes and were not touch-down airbursts. On the other hand, for touch-down or Type 2 airbursts, the kinetic energy of the airburst may be high enough to eject fragments at hypervelocities, producing shallow craters. In this study, we explore bolide fragmentation and crater formation.
In the first high-altitude example, the airburst of the Tagish Lake meteorite (4 to 6 m in diameter, weighing 56 tonnes) occurred ∼29 km high above British Columbia, Canada, in January 2000. It produced an elliptical strewn field 5 × 16 km long containing >10,000 fragments up to ∼2.3 kg, each reaching the surface at terminal velocities [8, 9].
In another example, fragments of the Sikhote-Alin iron meteorite struck Siberia in 1947, distributing ∼8500 pieces totaling more than 23,000 kg across 1.6 square km. This event produced more than 100 impact craters ranging from 0.5–26 m in diameter [10].
Also, Argentina’s Campo del Cielo meteorite field contains >100 meteorites that formed craters up to 26.5 m in diameter and 6 m in depth. For the Sikhote-Alin and Campo del Cielo airbursts, shock pressures were sufficiently energetic to produce shock-generated craters in unconsolidated surficial sediments.
Thus, fragmentation during airbursts appears to be common, regardless of whether the bolide is a comet or an asteroid and whether it occurs at a high or low altitude. The above evidence shows that bolide fragments from airbursts can reach the Earth’s surface, sometimes causing surface damage and shallow craters. This observation is one of the critical issues that we test with this modeling investigation.
Airbursts: meltglass and microspherules
Meltglass and microspherules occur in many common terrestrial and anthropogenic formations and thus, it is not always easy to differentiate them from impact examples, especially in the case of touchdown impacts without significant cratering. To distinguish the difference, shocked quartz and diaplectic glasses are decisive criteria. The production of meltglass and microspherules in airbursts, a central issue explored here, is described in the examples below.
Boslough and Crawford [5] described the theoretical process by which airbursts can produce meltglass. This process is important, so we quote their comment in its entirety. For some airbursts “with a kinetic energy above some threshold, the hot jet of vaporized projectile (the descending ‘fireball’) makes contact with the Earth’s surface, where it expands radially. During the time of radial expansion, the fireball can maintain temperatures well above the melting temperature of silicate minerals, and its radial velocity can exceed the sound speed in air. We suggest that the surface materials can ablate by radiative/convective melting under these conditions, and then quench rapidly to form glass after the fireball cools and recedes.”
The first meltglass-producing example is proposed at Tall el-Hammam in the Jordan Valley northeast of the Dead Sea, where a 50-m to 75-m touch-down airburst is proposed to have destroyed an ancient city ∼3600 years ago [11]. The high-temperature event produced meltglass, melted pottery, and melted mudbricks, some of which contain the imprints of plant material.
In another proposed airburst example, Osinski et al. and others [12–14] reported large quantities of meltglass produced ∼145,000 years ago along the shores of the Dakhleh Oasis in Egypt. The glass was discovered near the oasis at six locations separated by >40 km across ∼400 km2. This glass contains melted silica (lechatelierite, which forms at >1700 °C) that co-occurs with shock-fractured quartz and microspherules [12, 15]. Notably, Dakhleh glass contains the preserved imprints and fossil remains of plants onto which the molten glass fell. These impressions are similar to those observed on the melted roofing clay from Tall el-Hammam [11].
Similarly, a large meltglass strewn field from a proposed low-altitude airburst has been reported in a 75-km-long narrow corridor of Chile’s Atacama Desert [16–18]. The studies report twisted and folded silicate glasses up to 4 m in diameter and up to 15–25 cm thick [19], containing meteoritic grains [18] and shocked quartz grains [19]. Boslough et al. [20] attribute the 75-km-long field of meltglass to multiple low-altitude airbursts from fragments of a single comet. As with other airburst sites, this glass contains fossil imprints of plants on the surface at the time of the airburst.
Meltglass and microspherules produced by multiple hemispheric airbursts has been proposed for several sites at the 12,800-year-old Younger Dryas boundary (YDB) in Syria [21–23], Venezuela [24, 25], Pennsylvania [26], South Carolina [26], New Jersey [27], and New York [28]. The site in Syria at Abu Hureyra also displays plant imprints in meltglass, similar to imprints found at other YDB airburst sites.
In another potential airburst example, the Libyan Desert glass field is attributed to a cosmic event ∼29 million years ago. The field extends across ∼6500 km2 of the desert in Egypt [29] and is estimated to contain 1400 tonnes of meltglass [30], microspherules [31, 32], and shocked quartz [33, 34].
To explore a possible airburst origin of Libyan Desert glass, Svetsov et al. [30] developed a hydrocode model demonstrating that the glass could have formed during an airburst rather than a typical crater-forming impact. Likewise, Boslough and Crawford [5] developed a hydrocode airburst model for Libyan Desert glass using a 120-m-diameter sphere to represent an asteroid with kinetic energy of about 108 megatons and a maximum temperature of 5800 K. The caption of Boslough and Crawford’s Fig. 8 [5] states that the model produced a low-altitude airburst during which the high-temperature jet descended to Earth’s surface, and their figure also shows that the airburst excavated a multi-m-deep crater [5]. Importantly, Boslough and Crawford state: “…the resulting fireball (which contains air and ablated meteoritic material at temperatures exceeding the melting temperature of quartz) makes direct contact with the surface over a 10 km diameter area for more than 10 s after the explosion.” [5]. Thus, their model shows that low-altitude airbursts can produce craters and meltglass. We further investigate this meltglass-forming process using our hydrocode models.
Hydrocode modeling
Hydrocode programs are commonly used for impact simulations [1, 6, 35–41]. For this study, we first modeled airbursts using the Earth Impact Effects Program (EIEP) by Marcus et al. [42] and Collins et al. [39, 40] It is important to note that those authors caution that the methodology implemented in the EIEP is simplistic. Furthermore, Marcus et al. [42] emphasized that the results have significant uncertainties due to the limited understanding of impacts and airbursts. Even so, the EIEP and other modeling work by Collins et al. [39, 40, 43] is cited and used in hydrocode simulations for impact modeling [37, 44–47] and has been reported to produce consistent results [47].
After entering the results from EIEP, we made all other key calculations using Autodyn-2D (Ansys, Inc.), a hydrocode computer program previously used for modeling hypervelocity impact investigations [48–59]. We modeled airbursts by one asteroid (80 m in diameter) and two comets (100 and 140 m), each with different densities (∼1500, ∼1000, and ∼500 kg/m3, respectively) and different velocities prior to the airburst (17, 30, and 51 km/s, respectively). In addition, we produced a model of the Trinity nuclear detonation. The Trinity modeling was conducted only using Autodyn because the EIEP cannot model a static nuclear explosion. The input values (e.g., equations-of-state and material parameters) are provided below in Methods and in Supporting Information.
Autodyn is especially useful for modeling complex physical phenomena, including simultaneous interactions of liquids, solids, and gases, and for calculating phase transitions in multiple modeled materials. It is useful for modeling pressure, temperature, shock speed (defined here as the airburst-induced pressure wave that propagates through various materials, often at hypersonic velocities), and bulk material failure (defined as the point at which strain on a material exceeds a specified pressure value in the tensile direction).
In this study, temperatures are not fully quantitative because of the inherent difficulties in accurately determining extreme temperatures under chaotic conditions. The Autodyn program can account for some types of energy (e.g., kinetic, strain, and contact) but is unable to accurately calculate how some parameters affect temperature (e.g., plasma chemistry/physics and thermal radiation), potentially leading to modeled temperatures that are too high. Thus, for this study, Autodyn’s calculated temperatures are semiquantitative and primarily shown to illustrate the evolution of convection currents.
Concerning Autodyn’s utility, Pierazzo et al. [60] compared Autodyn with other commonly used modeling codes, including SOVA, SPH, iSALE, CTH, and ALE3D, and then compared those models with physical experiments. Those authors found Autodyn to agree well with the other code models and the experimental results [60]. They found that increasing the impact velocity adversely affected the accuracy of predicted peak pressures, but even so, they found Autodyn and other codes to be in good overall agreement with each other and with real-world experiments [60]. Baldwin et al. [49] concluded that Autodyn was able to replicate their laboratory impact experiments, and they used it to model large planetary impacts. However, both studies focused only on typical crater-forming impacts rather than airbursts.
Results and interpretations
Using the EIEP and Autodyn, we modeled the effects of four airbursts with a range of bolide diameters from 80–140 m, selectively chosen because the models produce touch-down airbursts, during which the hypervelocity jet intersects Earth’s surface. However, the effects of airbursts can vary widely depending upon a bolide’s diameter, density, velocity, angle of entry, and burst height [6, 7, 36, 61, 62].
We calculated multiple metrics (pressure, semiquantitative temperature, shock speed, visible representation, and material bulk failure) within a specific interval (ms) to illustrate a wide range of potential effects. The output includes images and videos (some shown below), which were generated to display the evolution of airbursts over time, including pressure, semiquantitative temperature as convection currents, shock speed, and visible conditions. Also illustrated is bulk failure, a metric indicating that some material has undergone damage exceeding the point at which material strain exceeds a specified pressure value in the tensile direction. This failure signifies that the formerly dense material has become hydrodynamic (i.e., pulverized, fractured, or amorphized).
Testing modeling accuracy
It is challenging to determine modeling accuracy related to touch-down airbursts because of a lack of scientifically recorded direct observations for comparison. However, there are two modern observed high-altitude airbursts to compare using the EIEP.
Tunguska, Russia (Table 1)
The blast yield of this 1908 airburst is estimated at ∼10–15 megatons of TNT equivalent with a burst altitude of ∼5–10 km [63–66]. The airburst generated a shockwave that toppled or snapped >80 million trees across ∼2000 km2 in a radial pattern from ground zero [63–66]. Estimated surface wind velocities were ∼40–70 m/s (144–250 km/h), greater than an EF-3 tornado [37], and the airburst ignited fires that consumed ∼500 km2 of forest [64]. The blast from this relatively small high-altitude airburst is reported to have killed ∼3 of the ∼30 people near ground zero, and nearly everyone experienced severe burns within an eight-km radius. [67] The comparison between the actual values and those modeled in the EIEP shows good correspondence (Table 1).
Chelyabinsk, Russia (Table 2)
In 2013, a ∼29.7-km-high, ∼500-kt airburst [6, 36, 68–70] occurred over Chelyabinsk, Russia. The blast ejected numerous meteorite fragments, two of which weighed 64.7 and 540 kg [71]. They struck Earth’s surface at low terminal velocities, with the largest creating a 9-m diameter hole in a frozen lake. Even though this airburst occurred high above the Earth’s surface, the shockwave injured nearly 1500 people on the ground, mostly from flying glass and other debris. The comparison between the modeled and actual values (Table 2) shows good correspondence. Given the lack of detailed information about touch-down airbursts, using EIEP in this study is acceptable as a first-order approximation.
Trinity : height, 31.4 m; 24.8 kt; plutonium, 19.8 g/cm3; tower, 9.7 g/cm3; ground, 2.64 g/cm3 (Figures 1–7).
Trinity, pressure at 3 ms. The resolution of 0.28 × 0.28 m is sufficient to model the Trinity bomb and tower legs. However, we modeled them several times larger for greater visibility but still used actual density and energy inputs. Thus, the model results are comparable to the observed results. #1) The Trinity bomb was mounted ∼31.4 m high on a metal tower 30.48 m tall (100 ft.). The nuclear fireball is ∼16 m wide at 3 ms. #2) Legs of the tower (not to scale) extend ∼6 m into the Earth beneath the tower. This model shows that blast pressures (red) are transmitted down the tower legs. The modeled pressure = ∼60 GPa in the fireball and ∼5 GPa at the top of the metal tower legs; shock speed = 25,000 m/s inside the fireball. Semiquantitative temperature = ∼4000 K. The field of view is 200 × 100 m. Click the bar below for the Pressure video .
https://vimeo.com/manage/videos/931733079
Trinity, pressure at 34 ms. #1) The touch-down nuclear fireball is ∼55 m wide at 34 ms. #2) High pressures >1 GPa are transmitted through the tower legs, and #3) into the Earth below the tower. The modeled pressures are sufficient to produce shock metamorphism. Maximum modeled pressure = 17 GPa; shock speed = 4600 m/s; temperature = ≥300,000 K. The field of view is 200 × 100 m.
Trinity, pressure at 64 ms. #1) The nuclear fireball is ∼82 m wide. Maximum shock speed = 4600 m/s. Upon contacting the ground, the blast wave produces #2) a reflection wave that rebounds upward and #3) a base surge that expands laterally. #4) High-velocity fragments from the bomb produce multiple craters up to 2 m deep, spread across ∼55 m of the ground surface. #5) The pressure damages the buried tower foundations. #6) Fragments of the bomb and tower create a ∼12-m deep crater under the tower. Maximum pressure on the surface is ∼15 GPa, sufficient to produce shocked quartz [73]. Maximum temperatures are higher than the vaporization temperature of quartz.
Trinity, pressure at 123 ms. #1) The fireball is ∼127 m in diameter. #2) Maximum pressure = 8 GPa within the fireball. #3) A pressure wave (0.2 GPa) propagates into the earth. The maximum shock speed is ∼4300 m/s, and the maximum model temperature was ∼300,000 K, matching the previously reported maximum temperature [81].
Trinity, semiquantitative temperature (shows convection currents) at 74 ms. #1) The touch-down nuclear fireball is ∼92 m wide. Maximum shock speed = ∼4500 m/s. #2) Molten fragments of the bomb and sediment (red) are ejected or drawn into the rising fireball plume. Only a few fragments are modeled for Trinity and the other airbursts described below; in reality, there would be thousands to millions of fragments. #3) Craters formed by the impact of fragments are up to 10 m deep and span ∼55 m. #4) Temperatures near the tower legs and #5) beneath the tower exceed the boiling point of quartz. Maximum pressure = ∼10 GPa, sufficient to shock quartz grains. Click the bar below for the Temperature Video .
https://vimeo.com/manage/videos/932493306
Trinity, density at 123 ms. #1) The fireball is 127 m wide. #2) Bomb and tower fragments produce ∼3-m-deep craters on the ground surface at the arrow. #3) The fragments create a >5-m-deep crater beneath the bomb. Below the tower, the high pressure compresses the sandy, unconsolidated sediment ∼3.5× from ∼2.64 g/cm3 to ∼9.34 g/cm3. #4) This densifying shockwave continues downward through the substrate, further compressing the sediment.
Trinity, bulk failure at 250 ms. #1) The fireball is 235 m wide (wider than the field of view). #2) The white areas at the fireball’s center and near the deepest part of the crater, listed as “void,” represent near-vacuum conditions. #3) The cratered area is ∼75 m wide with an upturned rim. #4) High pressures and temperatures cause the bulk sediment to become broadly plastic/elastic (green). Note that “elastic” means reversibly flexible with the potential to reheal; “plastic” means irreversibly flexible and cannot reheal. In and around the craters, sediment experiences bulk failure and is pulverized or amorphized (red). #5) Melted bomb parts and sediment line the walls of craters that are up to 20 m deep under the tower and ∼3 m deep further away. Melted/plastic bomb parts and sediment (red and green) are ejected from the crater. The maximum modeled pressure = ∼5 GPa, and the maximum shock speed = ∼4200 m/s. Click the bar below for the Bulk Failure video .
https://vimeo.com/manage/videos/932493287Asteroid : 80 m wide; 13.7 Mt, velocity 17 km/s; density 1500 kg/m3; entry 90°; initial fragmentation at 72.5 km; burst height 662 m (Figures 8–14).
80-m asteroid, semiquantitative temperature (shows convection currents) before airburst. The asteroid is modeled as a deformed solid sphere but, in actuality, is a swarm of co-moving fragments and vapor. Atmospheric friction causes the swarm to assume a fragmented, pancake-like shape, although the total airburst energy remains unchanged. #1) The approximate modeled temperature in front of and in the wake of the asteroid before the airburst is ∼60,000 K. This value is consistent with studies by Silber et al. and others [99, 100, 146], who reported meteoroid temperatures ranging up to 95,000 K. Zhilyaev et al. [100] also modeled temperatures of up to 100,000 K. However, the temperature in the flow field behind the bolide may exceed that value. Field-of-view is 4000 × 2000 m. Estimated asteroid recurrence interval = ∼1700 years [40, 42], although this interval applies to all 80-m impact events, not just airbursts. Click the bar below for the Temperature video .
https://vimeo.com/manage/videos/932492911
80-m asteroid, semiquantitative temperature (shows convection currents) at 38 ms. #1) The asteroid airburst is ∼680 m wide. Maximum pressure = ∼2 GPa, maximum shock speed = 13,000 m/s, and maximum approximate temperature = 32,000 K. The model of a high-rise building is 75 × 175 m; the model of the middle building is 50 × 150 m; and the smallest building is 25 × 50 m.
80-m asteroid, semiquantitative temperature (shows convection currents) at 116 ms. #1) The asteroid airburst is ∼1,220 m wide. Maximum ground surface pressure = 15 GPa, sufficient to produce shocked quartz. The maximum shock speed = 7400 m/s. #2) The expanding airburst produces a reflection wave after striking the Earth. #3) The high-velocity, high-pressure wave melts surface sediment, and fragments create shallow craters. #4) High-temperature shockwaves at >8000 K engulf the smallest of six modeled buildings.
80-m asteroid, semiquantitative temperature (shows convection currents) at 367 ms. The maximum shock speed = <7,000 m/s, and the maximum pressure decreases to ∼8 GPa, sufficient to produce shock metamorphism. The estimated maximum temperature is ≥95,000 K, based on the existing literature [99]. #1) The asteroid airburst is >2,000 m wide. #2) The temperature at the surface of the high-rise building is sufficient to vaporize most materials. #3) The high-velocity, high-pressure wave severely damages the tallest building. #4) The high-velocity, high-pressure wave has demolished the smallest building and ejected most of its debris upwards. #5) The temperatures in some surface craters briefly exceed the melting point of quartz, producing meltglass.
80-m asteroid, visible state of materials at 10 ms. #1) The airburst is ∼265 m wide at an altitude of 662 m. Bolide fragments (red) expand outward, creating near-vacuum conditions (white, “void”), through which asteroid fragments travel faster because of the lack of atmospheric friction. Shock speed at 10 ms is 23,000 m/s, pressure = 130 GPa in the airburst, and the approximate temperature = 38,000 K. #2) The model of a high-strength high-rise building is 75 × 175 m high with a density of ∼3.3 g/cm3. #3) Model of a large, low-strength building is 50 × 150 m long at ∼1.6 g/cm3. #4) Model of a small, low-strength building is 25 × 50 m long at ∼1.6 g/cm3. The field of view is 2000 × 1000 m.
80-m asteroid, pressure at 92 ms. #1) The asteroid airburst is ∼1100 m wide, maximum shock speed = 7400 m/s, and the approximate maximum temperature is estimated as ≥95,000 K, based on the existing literature [99]. #2) The high-velocity, high-temperature, high-pressure wave melts surface sediment, and impactor fragments create shallow craters. #3) High-pressure fragments strike the Earth at 15 GPa in small areas, sufficient to produce shock metamorphism. #4) As the lateral pressure increases on the high-rise building, it produces added strain at the base of the building.
80-m asteroid, bulk failure at 331 ms. #1) The airburst is >2000 m wide. #2) The high-rise building shows extensive surface and interior damage but remains standing. The bottom right inset shows original building shapes and sizes. #3) The pressure wave damages the building-earth interface, melting and vaporizing some material (white). #4) The medium-sized building is heavily damaged, and some material has been ejected upwards. #5) The small building suffers more extensive damage. Asteroid fragments travel faster through the near-vacuum zone (white), so they are capable of greater damage. #6) Fragments create multiple shallow craters up to ∼5 m deep and spanning ∼1,480 m. Surface materials melt and mix with impactor vapor and fragments, reported to be less than ∼2 wt% [87, 147, 148]. #7) Earth’s surface is damaged beyond its shear strength (called bulk failure, shown in red) and becomes pulverized or amorphized (Autodyn does not determine its exact state). Bulk failure occurs at the ground impact points and as the pressure wave expands into the Earth. During the first 200 ms, maximum shock speed = 30,000 m/s; maximum semiquantitative temperature = ≥95,000 K; and maximum pressure = >200 GPa. Click the bar below for the Bulk Failure video .
https://vimeo.com/manage/videos/932493327Comet : 100 m wide; 57.9 Mt, velocity 30 km/s; density 1032 kg/m3; entry 90°; initial fragmentation at 89.2 km; burst height 203 m (Figures 15–21).
100-m comet, semiquantitative temperature (shows convection currents). The comet is modeled as a solid deformed sphere but is assumed to be a swarm of co-moving fragments and vapor. Approximately 15 ms before the airburst, when the bolide is ∼350 m above the surface, atmospheric friction causes the impactor to deform into a pancake-like shape, although the airburst energy is the same whether or not the object is fragmented. Velocity is ∼8,600 m/s, the trail is ∼300 m wide, the estimated temperature in the shock front and wake of the bolide is ∼30,000 K, and the external pressure is ∼127,000 kPa (0.127 GPa). Estimated recurrence interval = ∼2500 years [40, 42], although this interval applies to all 100-m impact events, not just airbursts. The field of view is 2000 × 1000 m. Click the bar below for the Temperature video .
https://vimeo.com/manage/videos/932493018
100-m comet, pressure at 8 ms. #1) The touch-down airburst is ∼275 m wide at an altitude of 203 m. Maximum internal pressure = 16 GPa, shock speed = 50,000 m/s, and maximum estimated temperature = 45,000 K. Click the bar below for the Pressure video .
https://vimeo.com/manage/videos/932492967
100-m comet, pressure at 27 ms. #1) The comet airburst is ∼660 m wide. The maximum shock speed = 100,000 m/s, and the maximum temperature is estimated at ≥95,000 K, based on existing literature [99]. #2) High-pressure fragments strike the surface at 6 GPa, sufficient to produce shock-fractured quartz.
100-m comet, pressure at 67 ms. #1) The comet airburst is ∼1275 m wide. Maximum shock speed = 92,000 m/s, and maximum temperature is estimated as ≥95,000 K, based on the existing literature [99]. #2) Comet fragments strike the Earth, producing craters and surface damage. #3) A high-pressure reflection wave (∼5 GPa) rebounds upward off a dense bedrock layer. This pressure is sufficient to produce shock metamorphism.
100-m comet, visible representation at 45 ms. #1) The comet airburst is ∼920 m wide. Maximum shock speed = 90,000 m/s, and maximum temperature is estimated as ≥95,000 K, based on the existing literature [99]. #2) Comet fragments (red) expand up and outward from the airburst’s center. Comet fragments travel faster through a near-vacuum (white). #3) A high-pressure wave (∼1.6 GPa) of comet fragments (red) rebounds upward as a reflection wave. This pressure is sufficient to produce shock metamorphism.
100-m comet, bulk failure at 100 ms. #1) The comet airburst is ∼1645 m wide. Maximum shock speed = 93,000 m/s, and the maximum temperature is estimated as ≥95,000 K, based on the existing literature [99]. The maximum pressure = 1.5 GPa, sufficient to produce shock metamorphism. #2) Comet fragments (green) expand outward from the airburst. #3) A near-vacuum zone (white) is produced in the center of the airburst and at some places on the ground surface. Comet fragments travel faster through the near-vacuum zone, so they are capable of greater damage. #4) Comet fragments strike the Earth, producing shallow craters up to 10 m deep, spanning 920 m. #5) Earth’s surface is heavily damaged (bulk failure) beyond its shear strength, becoming pulverized or amorphized (red). #6) Increased bulk failure occurs when the pressure wave encounters a dense bedrock layer.
100-m comet, semiquantitative temperature (shows convection currents) at 130 ms. #1) The comet airburst is ∼2000 m wide. Maximum shock speed = 92,000 m/s; maximum pressure = 1.5 GPa, sufficient to produce shock metamorphism. #2) Comet fragments strike the Earth, producing a large group of mini-craters up to 10 m deep, spanning ∼1700 m. #3) The cratering spans ∼700 m at ground zero. During the first 150 ms of the 100-m comet model, the maximum shock speed = 100,000 m/s, and the maximum pressure = >200 GPa.
Comet : 140 m wide; 236 Mt, velocity 51 km/s; density 530 kg/m3; entry 90°; initial fragmentation at 108.0 km; burst height 193 m (Figures 22–28).
140-m comet, semiquantitative temperature (shows convection currents). The comet is modeled as a solid deformed sphere, but in actuality, it is assumed to be a swarm of co-moving fragments and vapor. Atmospheric friction causes the swarm to assume a pancake-like shape. Nevertheless, the airburst energy is the same. A few ms before the airburst occurs, the comet is ∼350 m above the surface. Object velocity is ∼12,000 m/s, the trail is ∼350 m wide, the approximate temperature in the shock front is ∼37,000 K, and the external pressure is ∼110,000 kPa (0.11 GPa). Estimated recurrence interval = ∼7400 years [40, 42], although this interval applies to all 140-m impact events, not just airbursts. The field of view is 2000 × 1000 m. Click the bar below for the Temperature video .
https://vimeo.com/manage/videos/932493192
140-m comet, pressure at 4 ms. #1) The touch-down airburst is ∼250 m wide at an altitude of 193 m. Maximum internal pressure = 120 GPa, shock speed = 62,000 m/s, and maximum approximate temperature = 66,000 K. Click the bar below for the Pressure video .
https://vimeo.com/manage/videos/932493075
140-m comet, shockwave speed at 30 ms. #1) The airburst is ∼740 m wide. Maximum internal pressure = 6.5 GPa and the approximate maximum temperature = ≥95,000 K, based on the previous literature [99]. #2) Comet fragments expand at ∼200,000 m/s. #3) An upward-moving reflection wave forms as comet fragments and vapor rebound off Earth’s surface.
140-m comet, visible materials at 40 ms. #1) The airburst is ∼915 m wide. Maximum internal pressure = 5.1 GPa, and the approximate maximum temperature is ≥95,000 K, based on the previous literature [99]. #2) Comet fragments expand at ∼160,000 m/s. Comet fragments travel faster through the near-vacuum zone (white), so they are capable of greater damage. #3) A reflection wave forms as comet fragments rebound off Earth’s surface, creating a near-vacuum zone. #4) Comet fragments damage Earth’s surface, creating craters up to 10 m deep. Click the bar below for the Visible Materials video .
https://vimeo.com/manage/videos/932493248
140-m comet, material bulk failure at 60 ms. #1) The airburst is ∼1190 m wide. Maximum internal pressure = 4.7 GPa, and the approximate maximum temperature is ≥95,000 K, based on the previous literature [99]. #2) Comet fragments (green in airburst) expand at ∼140,000 m/s. A near-vacuum zone (white) is produced in the center of the airburst and at the ground surface. Comet fragments (green) travel faster through the near-vacuum zone, so they are capable of greater damage. #3) Earth’s surface is heavily damaged (bulk failure), becoming pulverized or amorphized (red). #4) Comet fragments strike Earth’s surface, creating numerous shallow craters up to 10 m deep, collectively spanning 1000 m. #5) Additional bulk failure occurs when the pressure wave encounters a dense bedrock layer and reflects upward.
140-m comet, semiquantitative temperature (shows convection currents) at 60 ms. #1) The airburst is ∼1190 m wide. Maximum internal pressure = 4.7 GPa. #2) The approximate maximum temperature is ≥95,000 K, based on the previous literature [99]. These high temperatures occur at the ground surface near the fragment-generated impact craters and are sufficient to vaporize quartz. Maximum shock speed is 140,000 m/s.
140-m comet, pressure at 110 ms. #1) The airburst is ∼2000 m wide. The approximate maximum temperature is estimated at ≥95,000 K, based on the existing literature [99]. Maximum shock speed has decreased to 110,000 m/s. #2) The collective crater field is up to 10 m deep and ∼1770 m wide. #3) A high-pressure reflection wave (>1 GPa) rebounds from bedrock and is sufficient to produce shock metamorphism. #4) Fragment impacts and high-velocity shock speed eject melted sediment from the surface. During the first 150 ms of the 140-m comet model, maximum pressure = >200 GPa; maximum shock speed = 87,000 m/s; and maximum temperature is estimated as ≥95,000 K, based on the existing literature [99].
Other modeling parameters and results
The above results illustrate touch-down airbursts by three bolides: an 80-m asteroid, a 100-m comet, and a 140-m comet. To estimate how rare such events might be, we used the EIEP to explore the effects of using different bolide entry angles (90°, 60°, 45°, 30°, and 15°) while keeping density and entry velocity the same (Table 4). In addition, we selected bolide diameters that would produce nearly the same burst heights for each example. Doing so caused the bolides to vary up to ∼4× larger in diameter and up to ∼60× in energy (Mt). In another test, we retained the original diameters, densities, and entry velocities but changed the entry angle to 45°. This change in angle resulted in high-altitude airbursts rather than touch-down events.
Notably, the results of these experiments suggest that innumerable variations in entry angles and bolide diameters can produce essentially the same low-altitude burst heights of <700 m. These experiments also demonstrate that innumerable possible variations of all parameters can produce higher burst heights without producing touch-down airbursts. Thus, although touch-down airbursts are a small subset of all airbursts at all altitudes, the evidence discussed here suggests they are not statistically rare.
Discussion
Modeled results for Trinity
Trinity nuclear detonation
The Trinity nuclear detonation provides valuable observational data for further testing of the modeling accuracy.
Pressure Table 5 compares observational data for the Trinity nuclear test with those from the Autodyn model. Measurements from ground sensors distributed across the Trinity site were reported by Marley et al. [72], who observed a maximum pressure of 76,000 kPa (0.076 GPa) on the ground surface 63 m from the fireball’s center. This measurement is similar to Autodyn’s modeled ground pressure of 100,000 kPa (0.10 GPa) at ∼63 m. These values differ by only 0.76 times (Table 5).
However, the ground measurements do not account for the higher pressures on the ground surface generated by fragments of the bomb and the tower. For those conditions, Hermes et al. [73] reviewed and summarized reports that key minerals display the shock effects of the extreme pressures generated in the Trinity nuclear test: 25 to 60 GPa for vesiculated feldspar [74], 8 to 10 GPa for shocked quartz [75, 76], 7 to 10 GPa for shocked zircon [77], 8 GPa based on the fractionation of zinc [78], and 5 to 8 GPa based on quasi-crystalline minerals in trinitite [79]. Autodyn’s modeled pressure range of 5 to 60 GPa matches these minerals’ reported range of 5 to 60 GPa. These high pressures likely reflect those generated by the nuclear shockwave by impacts by bomb and tower fragments with the ground surface and are sufficiently high to produce the observed shock metamorphism in quartz [73]. Previous studies also report that the Trinity nuclear airburst produced shock-fractured quartz [73, 75, 76]. The modeled and actual pressure ranges are the same (Table 5).
Shock speed The Trinity shockwave was ∼185 m wide after 62 ms for a velocity of ∼2985 m/s [80]. The modeled value for Trinity of 1280 m/s at 64 ms when the fireball was ∼82 m wide is ∼2.3× smaller in diameter (Table 5).
Temperature Although it was impossible to model accurate temperatures in this study, some experimental data exist. For Trinity, Glasstone and Dolan [81] (their Fig. 2.123) estimated the maximum temperature as ∼300,000 °C, and Brode [82] reported that temperatures at the center of an atomic fireball can reach ∼700,000 K. Both of these temperatures fall within the range modeled by Autodyn (Table 5).
Regardless of the limitations in determining temperatures in the Trinity fireball, the reported and modeled temperatures are much higher than the vaporization temperature of quartz at 2230 to 5,000 °C [83, 84]. However, after ∼2 s, the fireball temperature abruptly fell below the boiling point of quartz at 2230 °C (Fig. 2.123 in Glasstone and Dolan [81]). We speculate that these highly transitory, extreme temperatures are sufficient to melt the surfaces of quartz grains but are unlikely to melt large grains entirely because of quartz’s efficient insulating properties.
Kieffer [85] analyzed shock-metamorphosed sandstone from Meteor Crater and concluded that the shock metamorphism of quartz commenced at 5.5 GPa, well below the pressures determined for Trinity. Later, Kieffer et al. [86] reported quartz grains from Meteor Crater that they considered had been shocked at <10 GPa to form shock fractures infilled with amorphous silica. To explain this, they proposed a process called “jetting,” in which molten quartz is injected under high pressure into shock-generated fractures in the grains. Wakita et al. [87] also observed that during the early stages of an impact, molten material might be jetted when the impactor contacts target rocks.
In support of jetting, Ernstson [88, 89] conducted laboratory experiments and field observations of touch-down airbursts in Germany and the Czech Republic. He found that rocks and grains may fracture due to thermal shock and spallation, during which the stress on the target materials exceeds their tensile strength. If the shock pressures are sufficient to melt or vaporize the target material, silica vapor or melt can be injected into the fractures.
This mix of melted material usually contains <1 wt% of the impactor [90], typically containing high levels of elements such as Ni, Co, Pt, and Ir. Impact jetting has also been proposed to produce melt in lightly shocked material during grain-on-grain collisions in the terrestrial target material [87]. The jetting hypothesis is consistent with observations from the Trinity nuclear detonation and should also apply to touch-down airbursts by asteroids and comets.
Table 5 shows that most measured Trinity parameters (temperature, pressure, shock speed, and fireball expansion) are reasonably similar to the modeled values (differences of ∼0.76 to 2.3 times). The Trinity comparison suggests that Autodyn’s models are also accurate enough for modeling touch-down airbursts caused by asteroids or comets.
Modeled results for asteroids and comets
Pressure
Autodyn calculated that touch-down airbursts by asteroids and comets could generate ground-surface pressures of up to 28 GPa. This result raises the question of the lower pressure limit required to produce shocked quartz. To address this, Christie et al. [91] performed laboratory experiments to generate slow-strain conditions in milled quartz cylinders. These experiments used a confining pressure of 1.5 GPa and a stress differential of up to 3.6 GPa to produce glass-filled shock lamellae.
Further laboratory experiments by Kowitz et al. investigated the shock alteration of quartz grains using a steel plate explosively driven at various pressures into cylinders of quartz-rich sandstone [92–94]. Visible shock fractures infilled with amorphous silica began to form at ∼2.5 to 5 GPa [92, 93] as glass-filled “sub-planar, intra-granular fractures [93].” These experiments indicate that glass-filled shock-generated features may form in quartz even at pressures well below those modeled by Autodyn.
Shock speed
Autodyn calculated 6300–200,000 m/s surface shock speeds for asteroids and comets. Multiple experiments support such high velocities. Stöffler et al. [95] performed experiments using NASA’s vertical gun to fire 0.3-g cylindrical Lexan projectiles vertically into quartz sand at 5.9 to 6.9 km/s, well within the range generated in Autodyn’s models. These experiments produced shock-metamorphosed quartz particles.
Ebert et al. [96] used a small sphere of meteoritic material weighing 4.12 g and fired at ∼4.56 km/s into sandstone. The ejecta fragments included shock-metamorphosed quartz grains, fully melted quartz grains, and melted sandstone, as predicted for small impactor fragments in our study.
Similarly, Boslough et al. [97] used NASA’s vertical gun to demonstrate that 32-mm-diameter impactors can produce shocked quartz. They fired spherical impactors at 4.61 km/s into unconsolidated quartz and 4.97 km/s into a sandstone block. They reported that both experiments produced significant shock metamorphism at lower velocities than the modeled airbursts reported here.
Wünnemann et al. [98] also used NASA’s vertical gun for experiments with small projectiles consisting of Lexan cylinders (1.2 g/cm3) and aluminum spheres (2.8 g/cm3) fired vertically into quartz sand with velocities ranging from 5.86 to 6.90 km/s. These experiments also employed the iSALE-2D hydrocode and found excellent agreement between the experiments and their models. Three types of impact-related particles were formed: (i) melted particles, i.e., meltglass, (ii) shock-lithified aggregates, and (iii) shock-metamorphosed grains. In addition, shocked quartz and meltglass were produced at velocities well below those modeled here by Autodyn for touch-down airbursts.
Temperature
Multiple other models and observations support the extreme temperature range for the touch-down airburst models presented here. Even for modeled high-velocity (35 km/s) 10-cm-wide meteoroids, far smaller than the bolides modeled here, Silber et al. [99] reported meteoroid temperatures ranging up to 95,000 K, both in the shock front and in the wake of the meteoroid. Also, Zhilyaev et al. [100] reported that, although there are no direct measurements of temperatures associated with a meteor trail, a model of a small 1-cm meteoroid suggested temperatures of up to 100,000 K. In the current study, Autodyn modeled estimated temperatures >95,000 K. However, these values are considered semiquantitative, so ∼95,000 K from the literature is adopted here.
Proposed past touch-down airbursts
The following are crater-forming airbursts, mostly spanning the late Pleistocene through the Holocene. Nearly all examples are proposed as low-altitude Type 2 touch-down airbursts that caused extensive damage to Earth’s surface, including forming shallow craters filled with meltglass, microspherules, and other airburst-related proxies. Although airbursts mainly vaporize the incoming bolide, fragments commonly reach Earth’s surface, and the kinetic energy of the airburst vapor jet may be high enough to produce shallow craters [6, 7, 101].
The Chrudim/Pardubice strewn field in the Czech Republic is ∼135 km long [102, 103]. This proposed Holocene-aged touch-down airburst produced extensive clusters of small craters and widely scattered superficial and near-surface occurrences of meltglass, glass-filled fractured quartz, glass-like carbon, and multiple shock effects in polymictic breccias (planar deformation features (PDFs), diaplectic glass, silica ballen structures, and meltglass).
In Saarland, a state in Germany, a touch-down airburst event produced the approximately Holocene-aged, 15-km-long Nalbach/Saarlouis strewn field, including two primary craters, one 250 m, the other 2.3 km in diameter [104–106]. Both craters feature densely extended surficial occurrences of impactites, such as strongly shocked polymictic breccias (PDFs, diaplectic glass, silica ballen structures, heat-altered (toasted) quartz), impact glasses, meltrocks, and shatter cones.
In the Chiemgau district of southeastern Germany, extensive evidence exists for a significant Holocene-age touch-down airburst event [107–116]. The Chiemgau field is approximately elliptical, covering an area of about 60 km by 30 km (1,800 km2) and containing more than 100 impact craters ranging in diameter from a few m to 1,300 m. The strewn field displays shocked quartz, feldspar and mica; glass-filled fractured quartz; microspherules; microtektites; meltglass; ballen quartz structures; heat-altered (toasted) quartz; glass-covered cobbles; shock-spallation in cobbles and quartz grains; and shatter cones.
One significant piece of evidence for a Chiemgau impact airburst event is the widespread occurrence of chiemite, a pumice-like new impact rock predominantly consisting of >90 wt% amorphous carbon, which has been reported for the first time anywhere in the Chiemgau strewn field. Chiemite contains diamond and carbynes, which require 2500–4000 °C and several GPa of pressure to form. Fossilized plant cell structures and pseudomorphs of chiemite after wood fragments indicate formation by direct shock transformation of organic material such as wood and peat to high-rank carbon by a touch-down airburst [117].
The Chiemgau event has been dated to ∼2900–2600 years ago in the Bronze Age/Celtic Era using a novel impact proxy (artifact-in-impactites), in which culturally datable artifacts are embedded in shocked impact meltrocks [111, 112].
At Tall el-Hammam in Jordan, northeast of the Dead Sea, a touch-down airburst by a 50-m to 75-m bolide has been proposed to have destroyed an ancient city ∼3600 years ago [11]. The high-temperature event produced melted pottery and mudbricks, shock-fractured quartz, microspherules, and meltglass. In addition, hypervelocity winds from the airburst were proposed to have laterally distributed potsherds, charred grains, charcoal, and bone fragments in narrow trails up to ∼22 m long from SW to NE across the demolished city.
A 50-km-wide touch-down airburst/impact event was proposed for the Niederrhein region in Germany, where there are dozens of small craters (100–200 m wide) [118]. A large mass of stony meteorites, most likely from a rubble-pile basaltic asteroid, has been recovered from 40 locations. In addition, the evidence includes polymictic breccia, diaplectic glass, shock-fractured quartz, shocked feldspar, and highly vesicular meltglass with native iron inclusions. The age of this event is unclear, but it is most likely Holocene in age and no older than the middle Pleistocene.
As discussed above, a large linear strewn field of ∼12,500-year-old meltglass has been reported in a 75-km-long narrow corridor of the Atacama Desert in Chile that some have attributed to a low-angle, touch-down airburst [16–18]. The studies reported twisted and folded silicate glasses up to 4 m in diameter and up to 15–25 cm thick [19] that contain meteoritic grains [18] and shocked quartz grains [19]. Boslough et al. [20] attribute the 75-km-long field of meltglass to “low-altitude airbursts from six fragments of a single 120-m diameter comet [that] generated separate airbursts.”
At the 12,800-year-old Younger Dryas boundary, numerous studies have reported peak abundances of shock-fractured quartz, meltglass, microspherules, soot and charcoal from impact-related biomass burning, and platinum-iridium anomalies [21–23, 26, 65, 119–133]. These unusual materials are suggested to have resulted from multiple hemispheric airbursts, some high-altitude and some as touch-downs, caused by the Earth’s collision with dust and fragments in the tail of a comet [129, 134, 135].
In another example of a proposed airburst, Osinski et al. and others [12–14] reported large quantities of meltglass produced ∼145,000 years ago at the Dakhleh Oasis in Egypt. The glass deposits were separated by >40 km and spanned ∼400 km2. The strewn field contains melted silica (lechatelierite, which forms at >1700 °C), shock-fractured quartz, and microspherules [12, 15].
Van Ginneken et al. [3, 7] describe two near-surface airbursts in Antarctica, one ∼430,000 years old and a second one ∼2.3 Myr old, the oldest ever discovered, in which high-pressure, high-velocity, high-temperature jets intersected the surface [3, 136, 137]. In the younger event, the touch-down airburst distributed melted microspherules across a radius of up to ∼1400 km (2 × 106 km2) [136].
The Libyan Desert glass field formed ∼29 million years ago extends across ∼6500 km2 of the desert in Egypt [29] and is estimated to contain 1400 tonnes of meltglass [30], microspherules [31, 32], and shocked quartz [33, 34]. Because no crater has been discovered, some have proposed that a low-altitude airburst produced the field [5, 38]. Other studies interpret the presence of shocked quartz [33, 34] as evidence that it could not have been an airburst and that a typical impact crater exists but remains undiscovered. However, based on evidence from nuclear detonations, Hermes et al. [73] suggested that bolide airbursts can produce glass-filled, shock-fractured quartz without requiring a typical crater-forming impact event.
Other proposed Holocene-aged airbursts have been described, including the following: (i) More than a dozen small craters, including one 300 m in diameter, are part of the Holocene-age Lower Franconia strewn field in Germany [138]. (ii) The Pleistocene/Holocene-aged ∼25-km-wide Sachsendorf Bay structure is a proposed airburst/impact crater in northeastern Germany [139]. (iii) The Holocene-aged 20-km-diameter Kolesovice airburst crater in the Czech Republic [140]. (iv) In Finland, a 6400-year-old strewn field contains 33 craters up to 100 m diameter [141].
Possible recurrence intervals for airbursts
The recurrence interval of touch-down airbursts is unknown and currently unquantifiable because none have been observed by scientists in the last few hundred years. The online EIEP [39, 40] provides the following approximations with significant uncertainties: an 80-m asteroid impact event with a wide range of parameters (density, entry angle, and velocity) may recur globally every 1700 years; a 100-m comet every 2500 years; and a 140-m comet every 7400 years. However, these estimates are for all impact events, including high-altitude and touch-down airbursts. Thus, touch-down airbursts are less frequent than these intervals suggest.
As discussed in the previous section above, impact researchers have proposed at least eight touch-down airbursts within the last ∼11,700 years across Western Europe (2.2 million km2): (i) Chrudim/Pardubice in the Czech Republic [102, 103], (ii) Nalbach/Saarlouis in Germany [104–106]. (iii) Chiemgau in Germany [107–116, 142], (iv) Niederrhein in Germany [118], (v) Franconia in Germany [138]. (vi) Sachsendorf Bay in Germany [139], (vii) Seven possibly related strewn fields across about half of the Czech Republic [103], and (viii) a 6400-year-old strewn field in Finland [141]. These eight events suggest one touch-down airburst recurrence approximately every 1500 years across Western Europe. However, because so little is known about touch-down airbursts, this estimate has significant uncertainties, and more research is required to determine an accurate recurrence rate.
Potential hazards of airbursts
In July 2023, the near miss of asteroid 2023 NT1, a 30–60 m asteroid, was only discovered two days after it had passed Earth [143, 144]. The closest approach of 2023 NT1 was only about one-quarter of the Earth-Moon distance and had a potential impact energy of 10 Mt [143, 144], >600 times the energy of the Trinity atomic detonation. If this asteroid’s orbit had been slightly different, causing it to collide with Earth, it would have been large enough to have produced a significant touch-down airburst. This recent event is a reminder that even smaller objects pose a significant threat to humans and highlights the importance of understanding the magnitude of the threat posed by small, difficult-to-detect bolides that produce touch-down airbursts over a modern city.
Proposed scenario for touch-down airbursts and their potential effects
Several essential points characterize touch-down airbursts compared to typical crater-forming impact events. This comparison is based on both independent evidence cited herein and our experimental Autodyn and EIEP models:
Airbursts are far more common than typical crater-forming impact events.
Bolides only slightly smaller than those that produce typical impact craters can produce touch-down airbursts.
Fragments from many airbursts reach Earth’s surface, typically at terminal velocities.
Touch-down airbursts can create relatively shallow craters.
High temperatures and pressures from airbursts can produce meltglass and melted microspherules, as with typical crater-forming impact events.
Surface collisions by airburst fragments can produce shock metamorphism in various minerals and rocks (PDFs, PFs, and glass-filled shock-fractured minerals), along with ballen structures and shatter cones.
Based on the above, we suggest the following scenario for touch-down airbursts and the features they produce. This scenario is similar to but not identical to that proposed by Boslough and Crawford [4].
Touch-down or Type 2 airbursts occur at altitudes sufficiently low for the hypervelocity jet to intersect Earth’s surface.
Within the first few ms, high temperatures produce in situ meltglass.
Surficial material drawn into the plume produces Si-rich microspherules (mostly from bulk sediment but also from the impactor) and Fe-rich microspherules (mostly from magnetite grains in the bulk sediment but also from Fe in the impactor).
Bolide fragments can strike Earth’s surface at hypervelocities, producing small, ephemeral craters, often coalescing into one large, shallow crater.
Bolide fragments can produce pressures high enough to produce well-known shock effects in multiple minerals but, at reduced pressures, also fracture quartz and other mineral grains through fragment-to-grain, grain-to-grain collisions, and intra-grain spallation.
Furthermore, classically shocked quartz grains can form with parallel, planar lamellae.
In addition, extreme temperatures can produce thermal shock fractures in quartz without collisions.
The high temperatures (≥2,230 K) boil and vaporize some quartz grains and partially vaporize the surfaces of others. Then, low-viscosity quartz liquid and vapor are injected (jetted) into fractures, producing glass-filled, shock-fractured quartz.
Well-studied crater-forming impacts with large single impactors like Meteor Crater are known to have produced abundant meltglass, microspherules, and shock-metamorphosed grains, the latter of which have long been considered an essential diagnostic proxy for confirming an impact event, making shock metamorphism a ‘gold standard’ for confirmation. However, our modeling results reveal the potential formation of shock-metamorphosed grains during touch-down airbursts, both from the high-temperature, high-pressure shockwave and from hypervelocity bolide fragments that strike the ground. However, metamorphosed grains appear at lower abundances in touch-down airbursts, making such events challenging to identify in the geologic record. In addition, any shallow craters that form can be easily and rapidly eroded. Nevertheless, even comparatively low concentrations of shock-metamorphosed quartz grains should be satisfactory for positively identifying potential touch-down airbursts when coupled with the presence of high-temperature meltglass and microspherules. Furthermore, our study suggests that the presence of shock-metamorphosed grains, meltglass, microspherules, and other impact-related proxies eliminates the previously accepted requirement for an impact crater. Instead, these proxies may have formed in a touch-down airburst.
Conclusions
This study’s model of touch-down airbursts by comets and asteroids revealed pressures, temperatures, and shock speeds high enough to produce shock metamorphism, high-temperature meltglass, and microspherules. If these airbursts occur close enough to the Earth’s surface, they can produce multiple shallow craters with scaled-down effects comparable to typical crater-forming impacts. However, such shallow airburst craters are likely geologically ephemeral features easily obscured by sedimentation from wind and water. Without a visible crater, these proxies should be useful in identifying candidate sites of previously unrecognized touch-down airbursts. The evidence suggests that such airbursts are geologically more common than typical large crater-forming events. The airburst-generated extreme temperatures, pressures, and wind velocities are potentially highly destructive, suggesting a pressing need for further research.