Introduction
The destruction of an ancient city
Bunch et al. [1] described archeological excavations conducted since 2005 at Tall el-Hammam (“TeH”), an ancient walled city in the southern Jordan Valley northeast of the Dead Sea (Figure 1; Appendix, Site setting below). Located on what is called a “tell” or “tall” in Arabic and “tel” in Hebrew, the ruins are the stratified remains of an ancient heavily fortified urban center, now recognized as the largest continuously occupied Bronze Age city in the southern Levant [2]. Fifteen years of excavations revealed a widespread charcoal-and-ash-rich “terminal destruction layer” [3] up to 2 m thick that contained melted materials, including potsherds, mudbricks, roofing clay, and building plaster [1]. This layer also displayed peak concentrations of shock-fractured quartz, microspherules, charcoal, soot, and melted grains of platinum, iridium, nickel, zircon, and chromite [1]. For more information, see Appendix, Figures A1 and A2; Appendix, Melted materials below.
Location of Tall el-Hammam. (A) Photograph of the southern Levant, looking north, showing the Dead Sea, the site location (TeH), and nearby countries. “Dead Sea Rift” (dashed blue line) indicates a major tectonic plate boundary. The base image is from NASA’s Space Shuttle, “The Sinai Peninsula and the Dead Sea Rift.” Photograph: sts109-708-024, taken 12/16/2009, from the NASA Langley Research Center Atmospheric Science Data Center. (B) West-southwest-facing view of Tall el-Hammam showing sampling locations at the palace, temple, ring road, and wadi. The Dead Sea is faintly visible at the upper left below the arrow. Images and captions are adapted from Bunch et al. [1], usable under Creative Commons, CC by 4.0 (http://creativecommons.org/licenses/by/4.0/).
The ∼2-m-thick destruction layer also contained potsherds from thousands of pottery vessels, some displaying individually distinctive decorative patterns, facilitating their reassembly. Plotting the locations of sherds from individual vessels showed them to be oriented in an approximately SW-to-NE linear direction across the complex (Figure 2). The trails of sherds were intermixed with chaotically deposited debris, including mudbrick fragments, objects of daily life, carbonized wood beams, charred grain, bones, and limestone cobbles burned to a chalk-like consistency. Most of these materials were suspended within the destruction matrix above, not atop the Middle-Bronze-Age (MBA) floors. For more on stratigraphy and sampling sites reported in Bunch et al. [1], see Appendix, Stratigraphy of Tall el-Hammam below.
Directionality of debris across Tall el-Hammam. Color-coded arrows indicate the type and inferred direction of six categories of debris. A red dashed arrow highlights the inferred variation in the directionality of the airburst shockwave, moving from approximately SW to NE across excavations covering an area of ∼58,000 m2 (∼480 m long by up to ∼240 m wide). While the directions of most oriented materials fall within the red-shaded area, not all did. The conclusion supporting directionality is based on the following: 32 photographs and drawings of NE-oriented potsherds; 8 photographs of 7 ∼NE-oriented bones; 16 of charred grains and charcoal; 7 of plaster detritus; 4 of windblown “blow-over” deposits; and 12 captioned photographs and ∼100 observations from archaeological Season Reports and the PhD dissertation of co-author, Dr. Silvia [4]. Image direction and caption have been revised from Bunch et al. [1], usable under Creative Commons, CC by 4.0 (http://creativecommons.org/licenses/by/4.0/).
In addition, unusually high salt concentrations (up to 4 wt%) in the destruction layer produced hypersalinity across the site. Background concentrations above and below the destruction layer were typically <1 wt%. The salt appears to have inhibited local agriculture around 3600 years ago and caused an approximately 300-600-year-long abandonment of ∼120 regional settlements within a diameter of ∼26 km [1, 4].
Bunch et al. [1] concluded that the best explanation for this wide range of evidence is a cosmic airburst that destroyed the city, including the multi-story palace and temple complexes, and did significant damage to much of the massive 4-m-thick mudbrick rampart surrounding the city, especially along the SW-facing facade (see Appendix, Figures A3 and A4; Appendix, Destruction of the palace below).
Bunch et al. [1] modeled airbursts from 60-m to 75-m-wide bolides that produced near-surface airburst events. These are defined as “touch-down” or Type 2 airbursts [5–8], in which a high-temperature, high-velocity jet of vapor and fragments of the bolide reach Earth’s surface [9]. The modeled airburst generated SW-to-NE-trending high-velocity winds that demolished all the city buildings and scoured the city ruins. Bunch et al.’s models are consistent with existing literature that fragmentation during an airburst is common and that when bolide fragments from airbursts reach Earth’s surface, they can cause considerable surface damage [9, 10] (see Appendix, Fragmentation of bolides below).
Previous studies conclude that although airbursts mainly vaporize the bolide, fragments commonly reach the ground surface, and the kinetic energy of the airburst vapor jet may be high enough to produce shallow craters [7, 9, 11], along with shocked quartz, meltglass, microspherules, breccia, and other impact-related proxies. The following are some proposed examples of low-altitude Type 2 airbursts that caused extensive damage to Earth’s surface: (i) Chrudim/Pardubice in the Czech Republic [12, 13], (ii) Nalbach/Saarlouis in Germany [14–16]. (iii) Chiemgau in Germany [17–27], (iv) Niederrhein in Germany [28], (v) Franconia in Germany [29], (vi) Sachsendorf Bay in Germany [30], (vii) seven possibly related strewn fields across about half of the Czech Republic [13], (viii) a 6400-year-old strewn field in Finland [31], (ix) the Luzice melt rock and megabreccia outcrops, proposed as evidence of a low-altitude airburst [32], (x) the 20-km-diameter Kolesovice airburst crater in the Czech Republic [33], (xi) a 2600-year old strewn field in Kansas [34], (xii) a 1600-year-old airburst in Ohio that destroyed several Native American villages [35], (xiii) a 5000-year-old airburst by an iron meteorite in Poland [36], (xiv) a ∼12,500-year-old strewn field in the Atacama Desert of Chile [37–39], (xv) multiple airbursts on four continents 12,800 years ago by the Younger Dryas impact event [40–60], (xvi) a large airburst at the Dakhleh Oasis in Egypt ∼145,000 years ago [61–63], (xvii) two near-surface airbursts in Antarctica, one ∼430,000 years old and a second one ∼2.3 Myr old [7, 8], and (xiii) an airburst in the Libyan Desert ∼29 million years ago [64]. For more details, see Appendix, Airburst examples below.
The process by which airbursts can produce meltglass is described by Boslough and Crawford [65, 66] and quoted here because of its importance: “… 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.”
Tall el-Hammam may be the second oldest human settlement destroyed by a cosmic airburst/impact, after the destruction of the village at Abu Hureyra, Syria ∼12,800 years ago [46, 47, 53, 54, 67]. It may also be the earliest human settlement whose destruction by a cosmic airburst led possible eyewitnesses to construct an oral history that was written down centuries later [1]. For further discussions of this evidence, see Bunch et al. [1] and references [2–4, 68–82].
Age of the terminal destruction layer
To date the destruction layer in the palace, 20 radiocarbon dates were acquired by Bunch et al. [1] on carbonized wood (n = 11), carbonized grain (n = 3), carbonized material (n = 4), burned bone (n = 1), and organic sediment (n = 1) (Appendix, Bayesian analysis, Table A1 ). Bunch et al. [1] used the OxCal radiocarbon calibration program, version 4.4 (IntCal20 calibration curve), to determine a Bayesian-derived age for the terminal destruction layer of 3611 ± 21 cal BP (before 1950), rounded to 3600 cal BP.
Study objectives
In this study, we investigate and present new evidence for shock-fractured quartz, brecciated meltglass, and the SW-to-NE orientation of melted debris within the city. We also introduce a hydrocode-based airburst model to explain the destruction of Tall el-Hammam. Most airbursts occur at high altitudes with minimal effects on the Earth, so we chose to model a 55-m asteroid that would produce a touch-down airburst approximately 653 m above the Earth’s surface. The model tests whether such an airburst can produce the evidence observed at Tall el-Hammam. Note that the modeling parameters are not unique solutions; similar effects can be produced by numerous variations in the selected parameters (e.g., different bolide diameters, densities, velocities, and entry angles).
We explore specific questions about the touch-down airburst modeling:
Are the temperatures high enough to melt pottery and produce meltglass?
Can touch-down airburst conditions lead to the formation of microspherules?
Is the shockwave powerful enough to destroy a multi-story mudbrick palace?
Can the shockwave shatter and distribute potsherds directionally across 22 m?
Can a touch-down airburst produce impact brecciated meltglass?
Can impacts by airburst fragments produce shock metamorphism in quartz?
Are airburst pressures alone sufficiently high to produce shock metamorphism?
Results and Discussion
Oriented trails of potsherds
Bunch et al. [1] reported linear directionality at Tall el-Hammam for potsherds, charred grains, and bones but presented limited evidence to support that claim. That study attributed the directionality to airburst-related high-velocity winds, modeled at 255 m/s for a 60-m asteroid to 330 m/s for a 75-m asteroid. Our study presents additional evidence supporting this linear directionality from SW to NE across the site. The conclusion about directionality for potsherds, bones, charred grains, charcoal, plaster detritus, and blow-over deposits (Figure 2) is based on 79 site photographs and drawings and ∼100 field observations from archaeological Season Reports and the PhD dissertation of co-author Dr. Silvia [4]. See Figure 2 and Appendix, Oriented materials, Table A2 for further details.
Of the uniquely identifiable potsherds analyzed for this study, ∼187 out of 191 (98%) are oriented in groups from ∼NNE to ENE (averaging NE). Some clusters contain potsherds from multiple vessels; others are from single vessels (Figures 3–5). Additional examples of directional potsherds are shown in Appendix, Figures A5–A10. As illustrated below, widely separated potsherds from several distinctively decorated pots were found spread narrowly across up to ∼22 m of the excavations in an average SW-to-NE direction. The site locations where directional evidence was found are listed below in the Appendix, Oriented materials, Table A2 .
Directionality of potsherds from a single vessel. (A) Palace: a distinctively decorated large vessel partially reconstructed from multiple potsherds. (B) An aerial view of the excavation site shows the locations of seven clusters of potsherds from the vessel, as shown in panel A. The sherds were distributed ∼22 m across six palace walls in multiple rooms. Also, the potsherds were found on top of some foundations, suggesting the walls no longer stood when the sherds were emplaced. (C) This panel represents a side view of the occupation surface. No potsherds from this single distinctively decorated vessel were found in contact with the occupation surface. Instead, all were found “floating” ∼0.25 to 1.75 m above the floor and wall foundations within the churned-up terminal destruction layer.
Directionality of potsherds from a single vessel. (A) Palace: ∼12 reassembled sherds from a single distinctively decorated large vessel. (B) An aerial view of the excavation site shows the potsherds locations from the vessel, as shown in panel A, distributed across ∼9 m from SW to NE. They were mixed with ∼50 different vessels in three 6×6-m excavation squares and were mingled with charcoal and carbonized grain. (C) This panel represents a side view of the occupation surface. No potsherds from this single distinctively decorated vessel were found in contact with the occupation surface on which they were initially placed. Instead, all were found “floating” randomly ∼0.25 to 1.75 m above the floor within ∼1.5 m of the 2-m-thick churned-up terminal destruction layer (yellow shaded area).
Directionality of potsherds. (A) Ring road: a ∼1.2-m line of potsherds from a single pot. White arrows indicate some representative examples of directional sherds. (B) Ring road: a ∼1.2-m trail of potsherds from the single pot, curving along a NE-trending mudbrick wall. (C) Ring road: a ∼3.9-m long trail of sherds from different vessels mixed with rocks from a fallen wall. (D) Palace: a ∼1.4-m-long trail of potsherds from a single pot found on top of the foundation of a fallen wall.
Notably, none were found in contact with the original floor where they were almost certainly placed initially. Instead, they were suspended at random depths within up to 2 m of a churned-up matrix of potsherds, broken and pulverized mudbricks, meltglass, melted pottery, microspherules, and charred building materials. All potsherds from single vessels were linearly oriented approximately SW to NE (range: approximately ± 25°) (Figures 3–5). We found only a few smaller intact vessels, likely preserved due to their small size or sheltered location. Of the broken ones, we rarely found all the pieces within any given 6×6-m excavated square, i.e., at least some sherds from the same shattered vessel were typically separated by more than 6 m. As discussed in Bunch et al. [1], this situation was unique to the MBA terminal destruction layer and was not evident in any older or younger layers at Tall el-Hammam. This situation is atypical for earthquakes when vessels are locally shattered in situ and buried beneath collapsed walls and roofing material.
We drew dashed lines around the clusters and labeled the inferred direction for clarity. Also, for clarity, we often enlarged, cropped, and globally adjusted the photographs’ contrast, brightness, and sharpness; they have not been otherwise altered. Most photographs of potsherds display a north arrow, or in other cases, N-S and E-W excavation string lines are visible, allowing us to establish a north direction. In the other cases, directionality was inferred using the time of day recorded in the metadata of the original photographs with sun-shadow software (https://app.shadowmap.org/) to determine the photograph’s north compass direction, estimated to be accurate within approximately ± 25°.
To summarize, potsherds from undecorated vessels were rarely found on the original occupation floor and, instead, were found randomly distributed at varying depths within the 2-m-thick terminal destruction layer. For every distinctively decorated vessel recovered, no potsherd was found in contact with the occupation floor on which they were initially placed. Instead, all were found directionally oriented from SW to NE and “floating” ∼0.25 to 1.75 m above the floor and wall foundations within the churned-up terminal destruction layer among numerous other potsherds, broken and pulverized mudbrick, meltglass, melted pottery, microspherules, and charred building materials. Their positions and conditions are inconsistent with emplacement by earthquakes, windstorms, and warfare but are consistent with high-velocity winds produced during an airburst.
Oriented trails of charred grains and charcoal
Nearly every 6 × 6-m square excavated contained irregular streaks of charred grains and charcoal; an estimated 80% were oriented NNE to ENE, averaging ∼NE (Figure 6). Additional examples of directional charred grains/charcoal are shown in Appendix, Figures A11 and A12. Note that this material is commonly visible on the side faces of vertical excavations and on horizontal surfaces. However, directionality cannot be determined from a side view only; it must be observed from the top down during excavation or as viewed in photographs. The selected photographs below are all approximately top-down views of the ∼NE-oriented charred grains and charcoal.
Directionality of charcoal and charred grains. (A) Ring road: streaks of charred grains spanning up to 1.1 m (black dashed line with white arrows). (B) Palace: ENE-trending ∼50-cm-long streak of charred grain that spilled out of broken ENE-trending vessel. (C) Palace: ∼50-cm-long streak of charred grains among NE-trending potsherds (red dashed line). Panel C is from Bunch et al. [1], usable under Creative Commons, CC by 4.0 (http://creativecommons.org/licenses/by/4.0/). (D) Palace: a NE-trending streak of charcoal that spilled out of a broken vessel.
Oriented trails of bones
Numerous small bone fragments were found in every 6 × 6-m square across the site. Most bones were small, so we could not determine whether they were human or animal. However, large, identifiably human bones were rare (Figure 7). The count was ∼23 single human bones, for which 7 of 10 groups (70%) were oriented ∼NNE to ENE, averaging ∼NE ± 25°. The other three groups were oriented ∼NW. We infer this to be the expected orientation of bodies exposed to high-velocity winds that tend to orient long objects, such as a body, lengthwise in the direction of least resistance, which in this case is ∼SW to NE. Experts in Middle Eastern burial practices confirm that these remains were not emplaced during intentional burials [70]. Instead, all were suspended at random depths within the ∼2-m-thick destruction layer among a churned-up matrix of potsherds, broken and pulverized mudbricks, meltglass, melted pottery, microspherules, and charred building materials. In some cases, broken pieces of the same bone were found within a 6-m radius [70]. In all cases, most bones from any given skeleton were not recovered.
Directionality of bones and skeletons. (A) Ring road: the sun’s shadow is from the south (top), indicating a SW-NE direction. (B) Ring road: the edge of the N-S excavation is to the right, confirming the NE alignment of bones. (C) Palace: disarticulated skull and bones. The sun’s shadow is from the SE, upper left, confirming a SW-NE alignment. (D) Ring road: two disarticulated legs and foot bones are oriented ENE. Panels A and C are from Bunch et al. [1], usable under Creative Commons, CC by 4.0. (http://creativecommons.org/licenses/by/4.0/).
Summary of directionality
The airburst shockwave’s energy appears to have been redirected around substantial obstructions, including ramparts, the multi-story palace, foundations, and cross-walls, following the path of least resistance along walls and streets. Thus, although the directionality of debris trended typically SW-NE, the orientation occasionally varied by approximately ± 25° when the shockwave encountered obstacles.
The directionality method used in this study is similar to that used to investigate the airburst at Tunguska, where the compass directions of hundreds of trees were compiled to provide an average local direction. At Tunguska, the tree-fall pattern produced mostly straight lines radiating from ground zero, but there was substantial variation. For groups of trees at four locations, researchers measured blast directions that varied approximately ± 20-35° (Fig. 3 of Florenskiy [84]). Other researchers reported large-scale variations in average tree-fall directions varying by ± 180° from some radial lines (Figs. 4–5 of Florenskiy [84] and Fig. 1 of Longo et al. [85]). These variations at Tunguska are interpreted to have resulted from turbulent convection cells within the shockwave and from deflection by topographical features, such as hills and streambeds. This effect is similar to what is inferred from obstructions within Tall el-Hammam.
Brecciated melt rock
Breccia is a rock typically composed of angular fragments of minerals or rocks cemented together by a fine-grained matrix. Sedimentary and metamorphic processes can produce breccia, but if its matrix is melted, breccia is typically associated with high-energy, high-velocity crater-forming impact events [86–89] and touch-down airbursts [22, 32].
Dr. Ted Bunch, lead author of the previous Tall el-Hammam study [1], prepared the following analysis of the brecciated melt rock for this manuscript before he passed away. In 2010, a fine-grained brecciated melt rock measuring 19 cm wide and weighing 672 g (Figure 8) was discovered ∼9 km SW of Tall el-Hammam at the site of a small abandoned village now called Tall Mwais. Although this site remains unexcavated, numerous Middle Bronze Age (MBA) potsherds were exposed at the surface in unconsolidated sand. The melt rock was found adjacent to the foundation of an MBA stone wall and surrounded by MBA potsherds in close association. No pottery from any other archaeological period was associated with the breccia, and post-depositional disturbance at the site appeared minimal. This archaeological association suggests the breccia is contemporaneous with the MBA terminal destruction layer at Tall el-Hammam, which contains the same types of potsherds. Although the provenance of this breccia cannot be accurately established, its complex melt attributes and structure are consistent with some type of impact event during the Middle Bronze Age. Future research is needed to search for breccia and other impact-related materials in well-stratified contexts at this site and others within a few km of Tall el-Hammam.
Brecciated melt rock from Tall Mwais, ∼8.5 km SW of Tall el-Hammam. (A) Top view of the 672-g piece of breccia; (B) bottom view of the same melt rock. Note the locations of lithologies marked as #a, #b, and #c. (C) The side of the melt rock shows a glass-like melted surface and accretional feature. The green color is typical of melted quartz sand, and the white area is melted gypsum and carbonates. (D) Cut surface of melt rock slice, showing numerous vesicles formed by gas trapped as the molten rock rapidly cooled; (E) another image of the glossy, glass-like melted surface, showing various lithologies, marked as #a, #b, and #c.
Lithological and petrological analyses were performed to explore the possibility of impact-related melting. The fine-grained brecciated melt rock comprises a matrix of melted and unmelted sandy limestone mixed with rounded clasts mainly of quartz and sandstone. The breccia displays unmelted and partially melted fine-grained quartz grains, gypsum, and carbonates that are common in the region near Tall el-Hammam. For major elements, the bulk composition of the breccia averages 62.0 wt% SiO2, CaO at 10.1 wt%, and Al2O3 at 14.0 wt%.
The melt rock displays three distinct morphologies, designated as Lithologies A, B, and C. Lithology A (Figure 8B, 8E) has a fine granular texture of partially melted and fused grains, mainly quartz, that is almost entirely coated with a glossy veneer of melted SiO2. The top of the melt rock is composed almost entirely of lithology A with a thin-to-thick veneer of melted, shiny glass covering most of the upper surface and sides. The raised points of the surface are thinly covered with glass, and the lower areas are thickly covered, indicating that the glass flowed at very low viscosity. Lithology A material appears composed of partially molten debris fragments that were lightly stuck together before being firmly welded by exposure to very high temperatures. This glass has a hardness of >6.5 on the Mohs scale and is almost optically clear, consistent with the melting of orthoquartzite.
The bluish-to-greenish color of most of the glass coating likely results from mixing melted quartz with trace elements, e.g., Ni or Cu. During melting, red ferric Fe (Fe3O4) reduces to a blue-green color when combined with a small amount of Fe2O3 (<0.5 wt%). This coloration occurs only under very low oxygen fugacity (minimal O2 availability), which is rare under typical geological conditions. However, it is common in high-temperature, flash heating/melting events, including atomic detonations (trinitite), lightning strikes (fulgurites), and cosmic impacts.
Lithology B (Figure 8B, 8E) occurs mainly on the basal surface of the Tall Mwais breccia and shows evidence of fracturing. It is darker than lithology A and lacks a glassy coating. This lithology is composed of a mixture of gypsum, quartz grains, and clasts of orthoquartzite sandstone, and it displays numerous large, deep, irregular vesicles, some of which are tube-like, with the remainder being bubble-like. These vesicles most likely were produced during the rapid outgassing of H2O and SOx gases, the rapid thermal decomposition of gypsum and carbonates, and the vaporization of trapped pore water. Although pure carbonates decrepitate under gradual exposure to high temperatures (i.e., break into small flakes), they can melt if exposed to temperatures greater than 1500 °C for a few seconds, followed by rapid quenching [1]. However, impure carbonates can melt at lower temperatures.
Lithology C (Figure 8B, 8E) is composed of white, partially melted glass that is fused to the larger melt rock (Figure 8C). This lithology appears as a clump of orthoquartzite sandstone that is fritted (viscous from heating but not completely melted). The material appears to have fallen onto the still-molten surface glass of lithology A with only partial melting at the margins.
In the breccia from Tall Mwais, we also observed zircon grains with multiple phases that display varying degrees of melting (Figure 9A, 9B) and in situ fracturing (Figure 9C, 9D). These breccia zircons are similar to those previously reported by Bunch et al. [1] from Tall el-Hammam in melted pottery and melted mudbricks. Additionally, one zircon shows signs of potential shock metamorphism (Figure 9E, 9F).
SEM images of melted zircon grains in melt rock from Tall Mwais. (A) A melted zircon grain from the Tall Mwais brecciated melt rock shows large vesicles produced by the melting of the grain; (B) SEM topographical image of the same grain. (C) Distorted, fractured zircon grain, most likely produced in situ by thermal or mechanical shock; (D) close-up of the vesicular surface caused by temperatures high enough to boil the zircon. (E) Possibly shock-metamorphosed zircon grain with three sets of potential shock lamellae (crystalline lattice) as indicated in the inset (F). Bohor et al. [90] showed images of impact-shocked zircons from the K-Pg impact event at 66 Ma that are morphologically indistinguishable from those at Tall Mwais.
SEM-EDS analyses confirmed that zircons in the breccia show enrichments in baddeleyite (ZrO2), displayed as bright areas distributed across the grains (Figure 9A, 9C, 9D). In several samples from the terminal destruction layer at nearby Tall el-Hammam, Bunch et al. [1] observed zircons with reduced SiO2 concentrations due to a loss of volatile SiO resulting from the dissociation of SiO2. This alteration occurs at high temperatures of ∼1676 °C, slightly below zircon’s melting point of ∼1687 °C [91], thus forming distinctive granular textures of pure ZrO2 known as baddeleyite [90]. At Tall el-Hammam, Bunch et al. reported that nearly all zircons observed on the surfaces of melted materials showed some conversion to baddeleyite, as do the zircons in the Tall Mwais breccia.
All zircon grains observed were also vesiculated, indicative of outgassing, likely caused by the dissociation of SiO2 during high-temperature melting and boiling. Vesicular (i.e., decorated) zircon grains are uncommon in nature, but they are commonly associated with cosmic impact events [1], as evidenced by vesicular zircons from the known airburst/impact at Dakhleh Oasis, Egypt, and the proposed airburst at the Younger Dryas boundary at Abu Hureyra, Syria [47]. The bubbles indicate that temperatures reached at least 1676 °C, causing the zircon to outgas and dissociate. Similar dissociated zircon grains have been found in glass and distal fallback ejecta from the ∼180-km-wide K-Pg impact crater in Mexico [92] and the 28-km-wide Mistatin Lake crater in Canada [92].
The zircon evidence suggests exposure to extreme temperatures >1676 °C for an inferred duration of less than several seconds, during which these grains began to melt, outgas, and disassociate into baddeleyite. However, because this brecciated melt rock is partially vesiculated and shows low-viscosity flow, it was most likely exposed to transient temperatures >2230 °C, the boiling point of quartz. These temperatures are within the range modeled for this experiment.
Glass-filled, shock-fractured quartz grains
Shock metamorphism in quartz
This section is adapted from West et al. [9] Multiple studies have investigated various types of impact-related shock metamorphism in quartz, including planar deformation features (PDFs) [10, 18, 93–103] and planar fractures (PFs) [10, 102, 104]. Both types of lamellae are typically parallel, planar, less than a few microns wide, spaced a few microns apart, and crystallographically controlled. These lamellae are also commonly filled with non-hydrated amorphous silica [10], considered diagnostic of impact-cratering events [105]. In contrast, natural fractures in quartz and non-impact-related tectonic deformation lamellae (DLs) are typically non-parallel, non-planar, and contain no non-hydrated amorphous silica [9, 10, 47, 95, 96, 98, 101, 102, 106–110].
Some studies of cosmic impact structures have described another type of lamellae resulting from impact shock metamorphism and given them various names, including vermicular (i.e., wormlike) microfractures [111–113], shock extension fractures (SEFs) [111, 112, 114, 115], and shock fractures [108, 116]. These shock fractures are intragranular cracks in quartz grains that are typically sub-parallel, sub-planar, greater than a few microns wide, spaced more than a few microns apart, not crystallographically controlled, and may or may not contain amorphous silica [9, 10, 17, 18, 20, 53, 117–121]. Here, we follow previous studies [9, 10, 46, 121] and adopt the term “shock fractures” to denote microfractures in quartz produced by thermal and mechanical shock. This study focuses only on the subset of shock fractures that contain amorphous silica (a term we use interchangeably with “glass”).
Origin of glass-filled shock fractures
Shock-metamorphic, glass-filled fractures differ from classical shock lamellae. Buchanan et al. [113] wrote “Vermicular quartz [i.e., glass-filled, shock-fractured quartz], which apparently is composed of near-planar lamellae of silica glass in a host of crystalline quartz, suggests either formation by melting due to extremely high ambient temperatures (∼1610 °C) or by shock melting.” Kieffer et al. [118] reported glass-filled shock-fractured quartz grains from Meteor Crater that differed from classical shock lamellae. To explain these, 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. [122] also observed that during the early stages of an impact, molten material might be jetted when the impactor contacts target rocks. Similarly, Ernstson [119, 120] observed that target rocks and grains may fracture from thermal shock and spallation (i.e., tensile fracturing), which occurs “where the expanding compressive shock front superimposes with the tensile rarefaction waves starting from reflection at the free surface of the impacted target.” [120], during which the stress on the target materials from the rarefaction wave exceeds their tensile strength and, thus, produces fractures. If the shock pressures are sufficient to melt or vaporize the target material, silica vapor or melt can be injected into the fractures.
Previous investigations of shock-fractured quartz
One previous study focused on the Trinity atomic airburst and on Meteor Crater [10]. The other two studies focused on the airburst event at the Younger Dryas boundary (YDB) at Abu Hureyra, Syria [46] and at three sites in South Carolina, Maryland, and New Jersey along the Eastern Seaboard of the USA [121]. All three studies presented evidence and a protocol for identifying glass-filled, shock-fractured quartz associated with airbursts. Their key conclusion is that quartz fractures filled with melted silica strongly indicate shock metamorphism at pressures approximately >1 GPa (= ∼10,197 kg/cm2), whether from an airburst or a typical crater-forming impact.
Although similar evidence for Tall el-Hammam was presented by Bunch et al. [1], Jaret and Harris [123] contend that Bunch et al. did not follow well-established techniques and failed to provide convincing evidence of classically shocked quartz at Tall el-Hammam. Here, we explore whether the evidence suggests the formation not of classical, high-pressure shocked quartz but rather glass-filled, shock-fractured quartz, which is different. For our study, we present new evidence acquired using ten analytical techniques, as listed below. We attempted to use the universal stage, a standard technique for identifying classical shock metamorphism; this is one of the deficiencies claimed for Bunch et al. [1] by Jaret and Harris. However, we found it unsuitable because the glass-filled fractures observed at Tall el-Hammam are typically sub-parallel and sub-planar, so the fractures’ angles and inclinations could not be accurately measured with a universal stage.
Identifying shock-fractured quartz
Bunch et al. [1] and Hermes et al. [10] cited multiple studies concluding that glass-filled lamellae in quartz grains are evidence of impact-related shock metamorphism. Thus, a crucial part of this study involves identifying those quartz fractures filled with melted silica, and we report that all the quartz grains reported here contain amorphous silica within their fractures. To reach that conclusion, we used the following ten techniques:
EPI-illumination microscopy (EPI) can show whether a fracture is filled but does not show whether or not the material is amorphous.
Optical transmission microscopy (OPT) uses crossed polarizers to determine whether parts of a quartz grain are isotropic (i.e., they remain dark during rotation) and are likely to be amorphous.
Scanning electron microscopy (SEM) can determine whether fractures are filled but does not determine the material’s composition.
Energy dispersive spectroscopy (EDS) can determine the composition of any material filling the fractures, e.g., amorphous silica, hydrated silica, other minerals, or polishing compounds.
Focused ion beam milling (FIB) was used to create thin slices of quartz grains for use in the TEM to investigate crystallinity.
Transmission electron microscopy (TEM) was used to determine whether fractures are filled with material and which areas are amorphous.
Scanning transmission electron microscopy (STEM) was used to determine whether fractures are filled with material.
Selected area diffraction (SAD), fast-Fourier transform (FFT), and inverse fast-Fourier transform (IFFT) are TEM techniques used to generate diffraction patterns that show which parts of a quartz grain are amorphous.
Cathodoluminescence (CL) was used to determine which parts of a quartz grain are crystalline or amorphous. Non-luminescent (black) areas indicate the presence of amorphous silica.
Electron backscatter diffraction (EBSD) was used to determine which parts of a quartz grain are amorphous and the degree to which the crystalline lattice has been damaged by shock.
Our study explores the characteristics of the glass-filled quartz fractures observed by Bunch et al. in the terminal destruction layer in the palace and temple. Importantly, we also investigate whether the characteristics of these fractured quartz grains differ from those of classically shocked quartz. Abundant new evidence is presented in Figures 10–14 below.
Shock-fractured quartz grain from the palace. All images are from grain 7GG7-29×10. (A) Cross-polarized optical photomicrograph of the shock-fractured quartz grain. Yellow arrows indicate visible shock-fractured lamellae in this grain and the following panels. Three sets of differently oriented lamellae are apparent. Amorphous silica remains dark during rotation under crossed polars. (B) SEM image of the quartz grain. (C) Close-up SEM image showing two sets of lamellae. Note the short feather-like lamellae, indicative of low-pressure shock-fractured quartz [1, 124]. (D) Cathodoluminescence (CL) image showing oriented lamellae. Darker linear features represent fractures filled with melted silica (glass), an indicator of shock metamorphism [10, 46]. (E) Electron backscatter diffraction (EBSD) image. Dark linear features, marked by arrows, indicate lamellae filled with melted silica. The range of colors represents minor crystalline lattice dislocations caused by shock metamorphic damage to the grain [10, 46]. The legend of color-coded Miller-Bravais indices is at the upper right. (F) EBSD image of the same grain, where the two colors represent Dauphine twinning, commonly observed in shock-fractured quartz grains [10, 46]. (G) Scanning-transmission electron microscope (STEM) image (inverted color). Darker features at the arrows are sometimes bounded by lighter borders, representing sub-parallel and sub-planar shock lamellae [10, 46]. Note that glass-filled fractures are non-planar and non-parallel, unlike classical shock lamellae, and, therefore, cannot be indexed with a universal stage. (H) Transmission electron microscope (TEM) close-up image showing a lamella infilled with melted silica (glass) and bounded by open fractures (light-colored bands). (I) The selected area diffraction (SAD) pattern was acquired from the region in panel ‘H.’ The bright diffuse ring indicates the presence of melted silica within an area that includes crystalline quartz, indicated by bright spots. The outer border of the diffuse halo corresponds to the {01 ˉ1 1} diffraction line of quartz. Panels C, E, and F are from Bunch et al. [1], usable under Creative Commons, CC by 4.0 (http://creativecommons.org/licenses/by/4.0/).
Shock-fractured quartz grain from the temple complex. All images are from grain LS42K-13×12. (A) Epi-illuminated photomicrograph of shock-fractured quartz grain. Yellow arrows indicate visible shock-fractured lamellae here and in the following panels. Two sets of differently oriented lamellae are apparent. (B) EBSD-SEM image of the quartz grain. (C) Close-up SEM image of lamellae, indicative of shock-fractured quartz [1, 124]. (D) Cathodoluminescence (CL) image displaying oriented lamellae. Darker linear features represent open fractures filled with melted silica (glass), an indicator of shock metamorphism [10, 46]. (E) Electron backscatter diffraction (EBSD) image. Linear features at arrows indicate lamellae infilled with melted silica. They appear as “twins” but are not of the Dauphine variety. The wide range of colors represents crystalline lattice dislocations caused by shock metamorphic damage to the grain [10, 46]. (F) EBSD image in which the two colors represent Dauphine twinning, commonly observed in shock-fractured quartz grains [10, 46]. (G) Scanning-transmission electron microscope (STEM) image (inverted color). The darker features at the arrows, bounded in some cases by light borders, represent sub-parallel, sub-planar shock lamellae [10, 46]. Note that glass-filled fractures are non-planar and non-parallel, unlike classical shock lamellae, and, therefore, cannot be indexed with a universal stage. (H) A close-up transmission electron microscope (TEM) image shows a lamella at arrows filled with melted quartz (glass). (I) This selected area diffraction (SAD) image was acquired in the region in panel ‘H.’ The bright diffuse ring indicates the presence of melted silica within an area that includes crystalline quartz, indicated by bright spots. The outer border of the diffuse halo corresponds to the {01 ˉ1 1} diffraction line of quartz. Panels ‘A’ and ‘D’ are from Bunch et al. [1], usable under Creative Commons, CC by 4.0 (http://creativecommons.org/licenses/by/4.0/).
Shock-fractured quartz grain from the palace. All images are from grain 7GG7-33×04. (A) Cross-polarized optical photomicrograph of shock-fractured quartz grain. Yellow arrows indicate visible shock-fractured lamellae here and in the following panels. Three sets of differently oriented lamellae are visible. (B) Reflected-light close-up photomicrograph. (C) Close-up cross-polarized optical photomicrograph showing three sets of shock-fractured lamellae. (D) Cathodoluminescence (CL) image showing oriented lamellae. TEM imaging shows that some black linear features represent open fractures, while others contain melted silica (glass), an indicator of thermal or mechanical shock metamorphism [10, 46]. (E) Another CL image also displays red and dark linear features indicative of melted silica [10, 46]. (F) Electron backscatter diffraction (EBSD) image. Linear features at arrows indicate lamellae infilled with melted silica. The range of colors represents crystalline lattice dislocations caused by shock metamorphic damage to the grain [10, 46]. Dark green represents minimal damage, ranging to red, indicating substantial damage. (G) TEM image. Multiple dark linear features at the arrows represent sub-parallel and sub-planar shock lamellae [10, 46]. (H) A close-up transmission electron microscope (TEM) image shows numerous vesicles infilled with melted quartz (glass). (I) This selected area diffraction (SAD) image was acquired in the region in panel ‘H’. The bright diffuse ring indicates the presence of melted silica within the large vesicle, surrounded by crystalline quartz, indicated by the bright spots. The outer border of the diffuse halo corresponds to the {01 ˉ1 1} diffraction line of quartz. Panel ‘D’ is from Bunch et al. [1], usable under Creative Commons, CC by 4.0 (http://creativecommons.org/licenses/by/4.0/).
Shock-fractured quartz grain from the palace. All images are from grain 7GG7-29×03. (A) Cross-polarized optical photomicrograph of shock-fractured quartz grain. Yellow arrows indicate visible shock-fractured lamellae here and in the following panels. One set of lamellae is visible. (B) Scanning electron microscope (SEM) image of the same grain. (C) Close-up SEM image showing shock-fractured lamellae. Note that glass-filled fractures are non-planar and non-parallel, unlike classical shock lamellae, and, therefore, cannot be indexed with a universal stage. (D) Electron backscatter diffraction (EBSD) image. Linear features at arrows indicate lamellae infilled with melted silica (glass). The range of colors represents crystalline lattice dislocations caused by shock metamorphic damage to the grain [10, 46]. (E) Another EBSD image where the two colors indicate the presence of Dauphine twinning, an indicator of shock metamorphism [10, 46]. (F) EBSD image. The range of colors represents crystalline lattice dislocations caused by shock metamorphic damage to the grain [10, 46]. Green represents minimal damage, ranging to red, signifying high damage. (G) Transmission electron microscope (TEM) image. Multiple dark linear features at the arrows represent sub-parallel and sub-planar shock lamellae [10, 46]. (H) Inverse Fast Fourier transform (IFFT) image. Dark areas represent substantial grain damage, indicative of melted silica (glass); light gray areas represent minimal to no damage to the crystalline lattice. (I) This selected area diffraction (SAD) image was acquired in the region in panel ‘H.’ The bright diffuse ring indicates the presence of melted silica within the large vesicle, surrounded by crystalline quartz, indicated by the bright spots. The outer border of the diffuse halo corresponds to the {01 ˉ1 1} diffraction line of quartz.
Multiple shock-fractured quartz grains from the palace and temple complex. (A) Electron backscatter diffraction (EBSD) image of a quartz grain 7GG7 38×10. Linear features at arrows indicate lamellae infilled with melted silica (glass). The range of colors represents crystalline lattice dislocations caused by shock metamorphic damage to the grain [10, 46]. (B) Cross-polarized optical photomicrograph of shock-fractured quartz grain 7GG7 38×-05 from the palace with one set of shock-fractured lamellae. (C) EBSD image of the same grain as in panel ‘B’ shows one set of shock-fractured lamellae. (D) EBSD image of quartz grain LS42J 33×11 from the temple complex shows one set of shock-fractured lamellae. (E) Cross-polarized optical photomicrograph of shock-fractured quartz grain LS42J 23×05 from the temple complex, showing several sets of shock-fractured lamellae. (F) EBSD image of the same grain in panel ‘E’ with two colors representing the presence of Dauphine twinning, often an indicator of shock metamorphism [10, 46]. (G) EBSD image of quartz grain 7GG7 40×-09B from the palace, with a range of colors representing crystalline lattice dislocations caused by shock metamorphic damage to the grain [10, 46]. (H) Close-up SEM image of the same grain as in panel ‘G’ with multiple dark linear features at the arrows representing sub-parallel, sub-planar shock lamellae [10, 46]. (I) EBSD image of quartz grain 7GG7 38×-06 from the palace shows two lamellae sets. The range of colors represents crystalline lattice dislocations caused by shock metamorphic damage to the grain [10, 46].
Due to the tectonic nature of Tall el Hammam’s geologic province, we considered whether the fractured quartz grains could be tectonic in origin, superficially resembling shock-fractured quartz. However, multiple investigations [9, 10, 46, 121] observed that they can be differentiated using two major distinguishing characteristics, which must co-occur: (i) Shock--fractured quartz typically displays open fractures, i.e., open gaps between the sidewalls, and at the same time, (ii) the fractures are filled with non-hydrated melted silica. These two characteristics are never observed in tectonic quartz, where fractures are closed crystalline dislocations and are not filled with melted silica. Thus, the glass-filled, shock-fractured quartz grains found at Tall el-Hammam are not tectonic in origin and, instead, are consistent with shock-fractured quartz grains previously reported in touch-down airbursts [9, 46, 121]. However, they differ from classically shocked quartz grains produced in typical cratering impacts [10], mainly because the fractures are sub-parallel and sub-planar, as observed in proposed airbursts [46, 121].
We also considered whether the amorphous silica within the fractures might be hydrated silica (SiO2·H2O), a common mineral that forms when dissolved quartz is deposited within grain fractures. To investigate this possibility, we analyzed the silica in all fractured grains and determined that none is composed of hydrated silica. The melted silica we observed has stoichiometric EDS ratios for Si:O (∼47:53 wt%) consistent with melted quartz and inconsistent with hydrated silica, which typically contains ≥60 wt% oxygen. Thus, we conclude that the melted quartz observed in some fractured quartz grains from Tall el-Hammam is best explained as resulting from a high-temperature, high-velocity airburst event.
In summary, we investigated three layers from the palace and temple: one sample in each area taken from within, immediately above, and immediately below the terminal destruction layer. We observed glass-filled, shock-fractured quartz only in the terminal destruction layer, suggesting that a high-pressure, high-temperature event occurred when that layer was deposited around 3600 years ago.
Hydrocode modeling of a 55-m asteroid
Hydrocode modeling is commonly used for impact simulations [5, 9, 11, 125–131], and specifically, Autodyn-2D, a hydrocode program from Ansys, Inc., has seen widespread use [127, 132–144]. In the current study, we first modeled the Tall el-Hammam airburst using the Earth Impact Effects Program (EIEP) developed by Marcus et al. [145] and Collins et al. [129, 130] Second, we input the EIEP results into Autodyn-2D (Ansys, Inc.), a hydrocode software program commonly used for modeling high-velocity airbursts and impacts [5, 11, 125–143]. For more information, see Appendix, Autodyn modeling, Appendix, Testing the accuracy of modeling, Appendix, Previous modeling of airbursts below.
The temperatures modeled in this study are not fully quantitative because of the inherent difficulties in accurately determining extreme temperatures under highly chaotic conditions. The Autodyn program can account for the kinetic energy, strain energy, and contact energy but does not calculate how some parameters affect temperature (e.g., plasma chemistry/physics and thermal radiation), potentially leading to modeled temperatures that are too high. Thus, Autodyn’s calculated temperatures here should be considered semiquantitative.
Modeling limitations
Regarding the EIEP, Marcus et al. [145] emphasized that the results are estimates based on the currently limited understanding of the impact process, and the results have significant uncertainties. Similarly, the Autodyn models have high uncertainties, given that airbursts are highly complex events with multiple variables that are difficult to model. Despite these limitations, hydrocode models are widely used to explore and better understand airburst conditions.
The parameters used for modeling the airburst at Tall el-Hammam are only one set of numerous conceivable scenarios. Our use of this specific model of a 55-m asteroid does not imply that these parameters accurately describe what happened at Tall el-Hammam around 3600 years ago; instead, it is just one set of circumstances under which the city’s destruction could have occurred.
The tests of the modeled results produced by West et al. [9] with Autodyn for Trinity and the EIEP for the airbursts at Tunguska and Chelyabinsk (Appendix, Tunguska, Table A3 and Appendix, Chelyabinsk, Table A4) are reasonably close to actual values. These results provide confidence that our model for an airburst at Tall el-Hammam is also reasonably accurate.
Asteroid airburst model for Tall el-Hammam
Modeling parameters: 55-m-wide asteroid, airburst energy: 3.68 Mt, entry velocity: 11 km/s, density: 2920 kg/m3; entry angle: 90°; initial breakup height: 47.9 km; burst height: 653 m. The touch-down airburst produces an airburst with a TNT-equivalent energy of 3.68 megatons, equaling more than 160 Hiroshima-sized nuclear bombs. We modeled values for pressure, semiquantitative temperature, shock speed, bulk material failure, and visual materials (Figures 15–20). The scale of the occupation mound (comprising the upper and lower tall) is approximately accurate horizontally but vertically exaggerated about two times for better visibility. The Temple, Palace, and other structures are also vertically exaggerated about two times. It is important to note that these buildings are for representational purposes only and are not accurately positioned where they would have been.
Modeled airburst at 20 ms. A) Visible materials. The airburst radius extends to ∼250 m. Asteroid fragments (depicted in black) expand along with the airburst and 2) produce a near-vacuum zone within the airburst (“void,” depicted in white). B) Within the airburst, the semiquantitative temperature is >30,000 K; pressure reaches ∼30 GPa (≥300,000 kg/cm2), and the shockwave speed is ∼18 km/s. Several possibilities might explain the lack of vaporization at high temperatures. (i) A large proportion of the bolide is vaporized, but the temperatures are so transient (<2 s) that there is insufficient time for the complete melting of all bolide fragments. (ii) Some fragments are pushed out at the leading edge of the high-temperature wave and, therefore, are not exposed to the highest temperatures. (iii) Some fragments travel within the near-vacuum (#2 in panel A) behind the shock front and are protected from the highest temperatures.
Modeled airburst at 83 ms. (A) Bulk material failure (defined as the point at which strain on a material exceeds a specified pressure value in the tensile direction). A near-vacuum zone within the airburst (“void” in legend) is depicted in white. (1) Fragments of the asteroid disperse in all directions. (2) These fragments induce bulk material failure and amorphization upon impact with the Earth’s surface. The high-velocity fragment impacts result in small, shallow impact craters and generate pressures up to 14 GPa, sufficient to produce shocked quartz. (B) (1) The airburst radius expands to ∼700 m. (2) At the ground surface, semiquantitative temperatures (total energy, including thermal and radiant) reach exceed ∼95,000 K [149, 150], sufficient to produce meltglass and spherules from the condensation of melted and vaporized matter. The shock speed reaches ∼16 km/s, far higher than that formed by any terrestrial mechanism. For comparison, the most powerful known tornadoes have wind speeds of ∼0.16 km/s, 1000 times slower than that modeled here. Field of view = 1500 m wide × 750 m high.
Modeled airburst at 121 ms. (A) (1) The airburst radius expands to ∼950 m. (2) The airburst initiates the destruction of buildings on the lower tall, including the temple complex (largest building on the left), exerting pressures of up to 11 GPa, sufficient to produce shocked and shock-fractured quartz. Semiquantitative temperatures briefly exceed ∼95,000 K [149, 150], sufficient to melt and vaporize mudbrick walls. (B) (1) Shock speeds reach up to 15 km/s across the lower tall, sufficient to pulverize mudbrick buildings. (2) The shockwave strikes the ground and rebounds upward at up to 15 km/s, resulting in what is commonly referred to as a reflection wave.
Modeled airburst at 150 ms. (A) Visible materials. (1) The central region of the airburst experiences near-vacuum conditions (“void,” depicted in white). (2) On the lower tall, asteroid fragments (depicted in black) have almost destroyed the temple complex (to the right of the middle) and other buildings. Other fragments are approaching the palace complex on the upper tall. Pressures exceed 8 GPa within the airburst and 1 GPa on the ground. (3) Asteroid fragments (depicted in black) strike the ground, creating small, shallow craters. The airburst ejecting these fragments sometimes produces near-vacuum conditions (depicted in white). Such shallow craters would likely be rapidly erased by wind and water erosion after a short time. (B) (1) The airburst radius expands to 1090 m. Shock speeds reach up to 15 km/s. (2) Semiquantitative temperatures in portions of the reflection wave exceed ∼95,000 K [149, 150], both at the ground surface and in the atmosphere.
Modeled airburst at 250 ms. (A) (1) Asteroid fragments (depicted in black) and the shockwave begin to demolish the palace and other structures on the upper tall. The surface pressures in some areas reach 2 GPa. In some areas, semiquantitative temperatures exceed ∼95,000 K [149, 150], sufficient to melt mudbrick and pottery. (2) On the lower tall, the temple complex and surrounding buildings are almost entirely demolished after 250 ms, approximately the duration of an average eye blink. Destruction debris is ejected at velocities of up to 10 km/s and moves left from SW to NE, a direction reported by Bunch et al. [1] to have been observed throughout excavations across the entire site. (3) Numerous asteroid fragments continue to strike the surface, creating localized near-vacuum conditions and producing small, shallow craters. (B) (1) The airburst radius widens to nearly 1500 m wide after 250 ms. (2) The side of the palace facing the airburst is subjected to temperatures briefly exceeding 70,000 K. (3) Portions of the lower tall experience temperatures exceeding ∼95,000 K for only a few ms [149, 150]. (4) Impact craters SW of the lower tall also experience temperatures of >95,000 K [149, 150].
Modeled airburst at 500 ms. (A) (1) The northeastern-most buildings on the upper tall are demolished, and the resulting debris is propelled into the air at velocities of up to 10 km/s and then blown to the NE off the top of the tall [1]. (2, 3) The palace, temple, and other upper and lower tall structures are reduced to rubble that is propelled into the air at velocities of up to 10 km/s, producing near-vacuum conditions (depicted in white) underneath. (4) Numerous small, shallow craters are distributed across the plain SW of the lower tall. In certain areas, ground pressures exceed two GPa. (B) (1) After 500 ms, semiquantitative temperatures near the palace briefly exceed ∼95,000 K [149, 150], (2) as do portions of the lower tall, the plain to the SW, and the atmosphere above the city. Melting of sediment and mudbrick occurs discontinuously across the ground surface, but because exposure is brief (less than a few seconds), the melting does not extend deeper than a few cm. However, after a few seconds, temperatures drop below 1600 K (1327° C), approximately the melting point of quartz. Below this threshold, meltglass is unlikely to form.
Visible materials Video (Click URL below or copy and paste to play). Video 1 is a depiction of the condition of visible materials during the first 500 ms. It is important to note that the impactor fragments (depicted in black) strike the Earth’s surface during this interval, forming small shallow craters and destroying all the buildings. A near-vacuum zone forms within the airburst (“void,” depicted in white). Note that the video’s duration of 20 s is ∼25 times slower than the actual duration of the airburst, during which every mudbrick structure in the city is demolished at extreme pressures, temperatures, and wind speeds, and much of the building material is vaporized.
Temperature Video (Click URL below or copy and paste to play). Video 2 is a depiction of the airburst’s semiquantitative temperatures during the initial 500 ms. Semiquantitative temperatures exceed ∼95,000 K, consistent with modeling estimates from Silber et al. [149] and Zhilyaev et al. [150]. After a few seconds, temperatures fall below most minerals’ melting point. Note that the video’s duration of 19 s is ∼26 times slower than the actual 500-ms duration of the airburst. Also, note that the ground temperature remains ambient and does not change significantly.
Future research
We suggest future investigations into whether the airburst at Tall el-Hammam may have been part of a larger cyclical bombardment rather than just an isolated event. Dating to approximately 4200-4000 cal BP, close to the age of the event at Tall el-Hammam, widespread evidence for airbursts has been reported from Syria, Sumatra, and deep-sea cores from the Mediterranean, Caspian Sea, and Indian and Austral Oceans by Courty et al. [151–154]. In addition, an airburst of similar age (∼3600-3900 cal BP) has been proposed in Kentucky by Tankersley and Meyers [155]. The hypothesis of a cyclical bombardment episode is consistent with the proposal by Napier for a multi-century-long encounter ∼3500-4000 years ago with the remnants of the Taurid meteor stream resulting from the hierarchical disintegration of a large comet called a centaur [54, 60]. Notably, Napier’s hypothesis of cyclical encounters includes a previous one called the Younger Dryas impact event ∼12,800 year ago. If correct, then the Tall el-Hammam episode resulted from a collision with disintegrated remnants of the of the same large comet.
Potential causes of the destruction at Tall el-Hammam
Ten possible causes could have produced the twelve destruction characteristics observed at Tall el-Hammam by Bunch et al. [1] and this study (Table 1). Floods, sandstorms, and tornadoes could have accounted for the linear directional suspension of bones, pottery, and debris in the destruction layer but not the widespread burning, melted pottery, microspherules, melted minerals, and shock-fractured quartz. Earthquakes, human activities (e.g., smelting, pottery-making), city fires, volcanic eruptions, and warfare could have accounted for the burning and melting of some materials but not the directionality and matrix suspension of bones, pottery, and debris. Of all the events, only lightning and an airburst could have created the shock-fractured quartz and high-temperature melted minerals, but lightning could not have accounted for the directionality and matrix suspension of bones, pottery, grain, and debris. Thus, although some non-airburst causes can account for some evidence (25% to 58%), only an airburst can account for all twelve observed destruction characteristics at Tall el-Hammam.
Potential explanations for the destruction characteristics observed at Tall el-Hammam.
| Destruction characteristics | Flood | Sandstorm | Tornado | Earthquake | Humans | City fire | Fire/eruption | Warfare | Lightning | Airburst |
|---|---|---|---|---|---|---|---|---|---|---|
| Pottery suspended above floor | Y | Y | Y | - | - | - | - | - | - | Y |
| Potsherds across multiple rooms | Y | Y | Y | - | - | - | - | - | - | Y |
| Bones suspended above floor | Y | Y | Y | - | - | - | - | - | - | Y |
| Debris moved atop foundations | Y | Y | Y | - | - | - | - | - | - | Y |
| Directionality of pottery. bones | Y | Y | Y | - | - | - | - | - | - | Y |
| Citywide burning | - | - | - | Y | Y | Y | Y | Y | Y | Y |
| Ash/charcoal deposition | - | - | - | Y | Y | Y | Y | Y | Y | Y |
| Bones burned | - | - | - | Y | Y | Y | Y | Y | Y | Y |
| Melted pottery. mudbricks, plaster | - | - | - | - | Y | Y | Y | Y | Y | Y |
| Melted Si-Fe-Ca microspherules | - | - | - | - | Y | Y | Y | Y | Y | Y |
| Melted chromite, zircon. quartz | - | - | - | - | - | - | - | - | Y | Y |
| Shock-fractured quartz | - | - | - | - | - | - | - | - | Y | Y |
| # compared to Impact | 5 of 12 | 5 of 12 | 5 of 12 | 3 of 12 | 5 of 12 | 5 of 12 | 5 of 12 | 5 of 12 | 7 of 12 | 12 of 12 |
| % compared to Impact | 42% | 42% | 42% | 25% | 42% | 42% | 42% | 42% | 58% | 100% |
“Humans” refers to human activities, such as smelting and pottery-making. “Fire/eruption” refers to volcanism.
In 1976, George Box wrote, “All models are wrong, some are useful. [156]” Regarding the utility of the model proposed here for a touch-down airburst, it is only one possibility among many. The model almost certainly does not precisely describe what happened at Tall el-Hammam around 3600 years ago – it is unlikely that any model can do so. This limitation is because temperature, pressure, and shock speed interactions in an airburst are so complex that dynamical modeling can only approximate the original event. Even so, the hydrocode model presented here is useful because the high-temperature, high-pressure results are consistent with the evidence observed at Tall el-Hammam, including shock-fractured quartz, meltglass, melted pottery, and microspherules.
Conclusions
Our analyses of glass-filled, shock-fractured quartz grains from the terminal destruction layer used ten different advanced techniques to further test the previous conclusions of Bunch et al. [1] about the occurrence of shock metamorphism at Tall el-Hammam. Our observations counter the conclusions of Jaret and Harris [123] that there is no convincing evidence for shock metamorphism at Tall el-Hammam. However, we agree that the grains are not classical high-pressure shock lamellae but, instead, are glass-filled shock fractures resulting from a lower grade of shock metamorphism. We also describe new evidence for meltglass breccia and present further support for the unusual SW-to-NE directionality of shattered potsherds randomly distributed throughout the 2-m-thick terminal destruction layer at Tall el-Hammam. This contribution introduces a computer hydrocode model for the airburst of a 55-meter asteroid above a city, showing that a touch-down airburst can intersect the Earth’s surface at extreme temperatures and pressures. The modeled airburst shatters the bolide into numerous small fragments that impact Earth’s surface at high velocities, producing small, shallow, ephemeral craters. The observed evidence is consistent with an airburst at Tall el-Hammam that produced the conditions necessary to melt pottery, produce meltglass and microspherules, form breccia, demolish the city’s buildings, disperse the debris directionality across tens of meters, and generate shock metamorphism via the shockwave and crater formation. We argue, therefore, that a cosmic touch-down airburst is the only plausible explanation for the evidence at Tall el-Hammam.
