Comet 81P/Wild 2: The size distribution of finer (sub‐10 μm) dust collected by the Stardust spacecraft

Abstract– The fluence of dust particles <10 μm in diameter was recorded by impacts on aluminum foil of the NASA Stardust spacecraft during a close flyby of comet 81P/Wild 2 in 2004. Initial interpretation of craters for impactor particle dimensions and mass was based upon laboratory experimental simulations using projectiles less than >10 μm in diameter and the resulting linear relationship of projectile to crater diameter was extrapolated to smaller sizes. We now describe a new experimental calibration program firing very small monodisperse silica projectiles (470 nm–10 μm) at approximately 6 km s^?1. The results show an unexpected departure from linear relationship between 1 and 10 μm. We collated crater measurement data and, where applicable, impactor residue data for 596 craters gathered during the postmission preliminary examination phase. Using the new calibration, we recalculate the size of the particle responsible for each crater and hence reinterpret the cometary dust size distribution. We find a greater flux of small particles than previously reported. From crater morphology and residue composition of a subset of craters, the internal structure and dimensions of the fine dust particles are inferred and a “maximum‐size” distribution for the subgrains composing aggregate particles is obtained. The size distribution of the small particles derived directly from the measured craters peaks at approximately 175 nm, but if this is corrected to allow for aggregate grains, the peak in subgrain sizes is at <100 nm.     

Micron‐scale hypervelocity impact craters: Dependence of crater ellipticity and rim morphology on impact trajectory,projectile size,velocity, and shape

The interstellar collector on NASA's Stardust mission captured many particles from sources other than the interstellar dust stream. Impact trajectory may provide a means of discriminating between these different sources, and thus identifying/eliminating candidate interstellar particles. The collector's aerogel preserved a clear record of particle impact trajectory from the inclination and direction of the resultant tracks. However, the collector also contained aluminum foils and, although impact crater studies to date suggest only the most inclined impacts (>45° from normal) produce crater morphologies that indicate trajectory (i.e., distinctly elliptical), these studies have been restricted to much larger (mm and above) scales than are relevant for Stardust (μm). It is unknown how oblique impact crater morphology varies as a function of length scale, and therefore how well Stardust craters preserve details of impactor trajectory. Here, we present data from a series of impact experiments, together with complementary hydrocode modeling, that examine how crater morphology changes with impact angles for different‐sized projectiles. We find that, for our smallest spherical projectiles (2 μm diameter), the ellipticity and rim morphology provide evidence of their inclined trajectory from as little as 15° from normal incidence. This is most likely a result of strain rate hardening in the target metal. Further experiments and models find that variation in velocity and impactor shape complicate these trends, but that rim morphology remains useful in determining impact direction (where the angle of impact is >20° from normal) and may help identify candidate interstellar particle craters on the Stardust collector.     

Hypervelocity impacts into ice‐topped layered targets: Investigating the effects of ice crust thickness and subsurface density on crater morphology

Many bodies in the outer solar system are theorized to have an ice shell with a different subsurface material below, be it chondritic, regolith, or a subsurface ocean. This layering can have a significant influence on the morphology of impact craters. Accordingly, we have undertaken laboratory hypervelocity impact experiments on a range of multilayered targets, with interiors of water, sand, and basalt. Impact experiments were undertaken using impact speeds in the range of 0.8–5.3 km s^?1, a 1.5 mm Al ball bearing projectile, and an impact incidence of 45°. The surface ice crust had a thickness between 5 and 50 mm, i.e., some 3–30 times the projectile diameter. The thickness of the ice crust as well as the nature of the subsurface layer (liquid, well consolidated, etc.) have a marked effect on the morphology of the resulting impact crater, with thicker ice producing a larger crater diameter (at a given impact velocity), and the crater diameter scaling with impact speed to the power 0.72 for semi‐infinite ice, but with 0.37 for thin ice. The density of the subsurface material changes the structure of the crater, with flat crater floors if there is a dense, well‐consolidated subsurface layer (basalt) or steep, narrow craters if there is a less cohesive subsurface (sand). The associated faulting in the ice surface is also dependent on ice thickness and the substrate material. We find that the ice layer (in impacts at 5 km s^?1) is effectively semi‐infinite if its thickness is more than 15.5 times the projectile diameter. Below this, the crater diameter is reduced by 4% for each reduction in ice layer thickness equal to the impactor diameter. Crater depth is also affected. In the ice thickness region, 7–15.5 times the projectile diameter, the crater shape in the ice is modified even when the subsurface layer is not penetrated. For ice thicknesses, <7 times the projectile diameter, the ice layer is breached, but the nature of the resulting crater depends heavily on the subsurface material. If the subsurface is noncohesive (loose) material, a crater forms in it. If it is dense, well‐consolidated basalt, no crater forms in the exposed subsurface layer.     

Stardust interstellar dust calibration: Hydrocode modeling of impacts on Al‐1100 foil at velocities up to 300 km s−1 and validation with experimental data

Abstract– We present initial results from hydrocode modeling of impacts on Al‐1100 foils, undertaken to aid the interstellar preliminary examination (ISPE) phase for the NASA Stardust mission interstellar dust collector tray. We used Ansys’ AUTODYN to model impacts of micrometer‐scale, and smaller projectiles onto Stardust foil (100 μm thick Al‐1100) at velocities up to 300 km s^?1. It is thought that impacts onto the interstellar dust collector foils may have been made by a combination of interstellar dust particles (ISP), interplanetary dust particles (IDP) on comet, and asteroid derived orbits, β micrometeoroids, nanometer dust in the solar wind, and spacecraft derived secondary ejecta. The characteristic velocity of the potential impactors thus ranges from <<1 to a few km s^?1 (secondary ejecta), approximately 4–25 km s^?1 for ISP and IDP, up to hundreds of km s^?1 for the nanoscale dust reported by Meyer‐Vernet et al. (2009) . There are currently no extensive experimental calibrations for the higher velocity conditions, and the main focus of this work was therefore to use hydrocode models to investigate the morphometry of impact craters, as a means to determine an approximate impactor speed, and thus origin. The model was validated against existing experimental data for impact speeds up to approximately 30 km s^?1 for particles ranging in density from 2.4 kg m^?3 (glass) to 7.8 kg m^?3 (iron). Interpolation equations are given to predict the crater depth and diameter for a solid impactor with any diameter between 100 nm and 4 μm and density between 2.4 and 7.8 kg m^?3.     

SMART‐1 end of life shallow regolith impact simulations

The SMART‐1 end‐of‐life impact with the lunar surface was simulated with impacts in a two stage light‐gas gun onto inclined basalt targets with a shallow surface layer of sand. This simulated the probable impact site, where a loose regolith will have overlaid a well consolidated basaltic layer of rock. The impact angles used were at 5° and 10° from the horizontal. The impact speed was ~2 km s^?1 and the projectiles were 2.03 mm diameter aluminum spheres. The sand depth was between approximately 0.8 and 1.8 times the projectile diameter, implying a loose lunar surface regolith of similar dimensions to the SMART‐1 spacecraft. A crater in the basement rock itself was only observed in the impact at 10° incidence, and where the depth of loose surface material was less than the projectile diameter, in which case the basement rock also contained a small pit‐like crater. In all cases, the projectile ricocheted away from the impact site at a shallow angle. This implies that at the SMART‐1 impact site the crater will have a complicated structure, with exposed basement rock and some excavated rock displaced nearby, and the main spacecraft body itself will not be present at the main crater.     

Analytical scanning and transmission electron microscopy of laboratory impacts on Stardust aluminum foils: Interpreting impact crater morphology and the composition of impact residues

Abstract— The known encounter velocity (6.1 kms^?1) and particle incidence angle (perpendicular) between the Stardust spacecraft and the dust emanating from the nucleus of comet Wild‐2 fall within a range that allows simulation in laboratory light‐gas gun (LGG) experiments designed to validate analytical methods for the interpretation of dust impacts on the aluminum foil components of the Stardust collector. Buckshot of a wide size, shape, and density range of mineral, glass, polymer, and metal grains, have been fired to impact perpendicularly on samples of Stardust Al 1100 foil, tightly wrapped onto aluminum alloy plate as an analogue of foil on the spacecraft collector. We have not yet been able to produce laboratory impacts by projectiles with weak and porous aggregate structure, as may occur in some cometary dust grains. In this report we present information on crater gross morphology and its dependence on particle size and density, the pre‐existing major‐ and trace‐element composition of the foil, geometrical issues for energy dispersive X‐ray analysis of the impact residues in scanning electron microscopes, and the modification of dust chemical composition during creation of impact craters as revealed by analytical transmission electron microscopy. Together, these observations help to underpin the interpretation of size, density, and composition for particles impacted on the Stardust aluminum foils.     

Laboratory simulation of impacts on aluminum foils of the Stardust spacecraft: Calibration of dust particle size from comet Wild‐2

Abstract— Metallic aluminum alloy foils exposed on the forward, comet‐facing surface of the aerogel tray on the Stardust spacecraft are likely to have been impacted by the same cometary particle population as the dedicated impact sensors and the aerogel collector. The ability of soft aluminum alloy to record hypervelocity impacts as bowl‐shaped craters offers an opportunistic substrate for recognition of impacts by particles of a potentially wide size range. In contrast to impact surveys conducted on samples from low Earth orbit, the simple encounter geometry for Stardust and Wild‐2, with a known and constant spacecraft‐particle relative velocity and effective surface‐perpendicular impact trajectories, permits closely comparable simulation in laboratory experiments. For a detailed calibration program, we have selected a suite of spherical glass projectiles of uniform density and hardness characteristics, with well‐documented particle size range from 10 μm to nearly 100 μm. Light gas gun buckshot firings of these particles at approximately 6 km s^?1 onto samples of the same foil as employed on Stardust have yielded large numbers of craters. Scanning electron microscopy of both projectiles and impact features has allowed construction of a calibration plot, showing a linear relationship between impacting particle size and impact crater diameter. The close match between our experimental conditions and the Stardust mission encounter parameters should provide another opportunity to measure particle size distributions and fluxes close to the nucleus of Wild‐2, independent of the active impact detector instruments aboard the Stardust spacecraft.     

Crater morphology in sandstone targets: The MEMIN impact parameter study

Abstract– Hypervelocity (2.5–7.8 km s^?1) impact experiments into sandstone were carried out to investigate the influence of projectile velocity and mass, target pore space saturation, target‐projectile density contrast, and target layer orientation on crater size and shape. Crater size increases with increasing projectile velocity and mass as well as with increasing target pore space saturation. Craters in water‐saturated porous targets are generally shallower and larger in volume and in diameter than craters from equivalent impacts into dry porous sandstone. Morphometric analyses of the resultant craters, 5–40 cm in diameter, reveal features that are characteristic of all of our experimental craters regardless of impact conditions (I) a large central depression within a fragile, light‐colored central part, and (II) an outer spallation zone with areas of incipient spallation. Two different mechanical processes, grain fragmentation and intergranular tensile fracturing, are recorded within these crater morphologies. Zone (I) approximates the shape of the transient crater formed by material compression, displacement, comminution, and excavation flow, whereas (II) is the result of intergranular tensile fracturing and spallation. The transient crater dimensions are reconstructed by fitting quadric parabolas to crater profiles from digital elevation models. The dimensions of this transient and of the final crater show the same trends: both increase in volume with increasing impact energy, and with increasing water saturation of the target pore space. The relative size of the transient crater (in percent of the final crater volume) decreases with increasing projectile mass and velocity, signifying a greater contribution of spallation on the final crater size when projectile mass and velocity are increased.     

The effect of the oceans on the terrestrial crater size‐frequency distribution: Insight from numerical modeling

Abstract— On Earth, oceanic impacts are twice as likely to occur as continental impacts, yet the effect of the oceans has not been previously considered when estimating the terrestrial crater size‐frequency distribution. Despite recent progress in understanding the qualitative and quantitative effect of a water layer on the impact process through novel laboratory experiments, detailed numerical modeling, and interpretation of geological and geophysical data, no definitive relationship between impactor properties, water depth, and final crater diameter exists. In this paper, we determine the relationship between final (and transient) crater diameter and the ratio of water depth to impactor diameter using the results of numerical impact models. This relationship applies for normal incidence impacts of stoney asteroids into water‐covered, crystalline oceanic crust at a velocity of 15 km s^?1. We use these relationships to construct the first estimates of terrestrial crater size‐frequency distributions (over the last 100 million years) that take into account the depth‐area distribution of oceans on Earth. We find that the oceans reduce the number of craters smaller than 1 km in diameter by about two‐thirds, the number of craters ?30 km in diameter by about one‐third, and that for craters larger than ?100 km in diameter, the oceans have little effect. Above a diameter of ?12 km, more craters occur on the ocean floor than on land; below this diameter more craters form on land than in the oceans. We also estimate that there have been in the region of 150 impact events in the last 100 million years that formed an impact‐related resurge feature, or disturbance on the seafloor, instead of a crater.     

FIB‐TEM analysis of cometary material in 10 Stardust foil craters

Aluminum foils from the Stardust cometary dust collector contain impact craters formed during the spacecraft's encounter with comet 81P/Wild 2 and retain residues that are among the few unambiguously cometary samples available for laboratory study. Our study investigates four micron‐scale (1.8–5.2 μm) and six submicron (220–380 nm) diameter craters to better characterize the fine (<1 μm) component of comet Wild 2. We perform initial crater identification with scanning electron microscopy, prepare the samples for further analysis with a focused ion beam, and analyze the cross sections of the impact craters with transmission electron microscopy (TEM). All of the craters are dominated by combinations of silicate and iron sulfide residues. Two micron‐scale craters had subregions that are consistent with spinel and taenite impactors, indicating that the micron‐scale craters have a refractory component. Four submicron craters contained amorphous residue layers composed of silicate and sulfide impactors. The lack of refractory materials in the submicron craters suggests that refractory material abundances may differentiate Wild 2 dust on the scale of several hundred nanometers from larger particles on the scale of a micron. The submicron craters are enriched in moderately volatile elements (S, Zn) when normalized to Si and CI chondrite abundances, suggesting that, if these craters are representative of the Wild 2 fine component, the Wild 2 fines were not formed by high‐temperature condensation. This distinguishes the comet's fine component from the large terminal particles in Stardust aerogel tracks which mostly formed in high‐temperature events.     

The MEMIN research unit: Scaling impact cratering experiments in porous sandstones

Abstract– The MEMIN research unit (Multidisciplinary Experimental and Modeling Impact research Network) is focused on analyzing experimental impact craters and experimental cratering processes in geological materials. MEMIN is interested in understanding how porosity and pore space saturation influence the cratering process. Here, we present results of a series of impact experiments into porous wet and dry sandstone targets. Steel, iron meteorite, and aluminum projectiles ranging in size from 2.5 to 12 mm were accelerated to velocities of 2.5–7.8 km s^?1, yielding craters with diameters between 3.9 and 40 cm. Results show that the target’s porosity reduces crater volumes and cratering efficiency relative to nonporous rocks. Saturation of pore space with water to 50% and 90% increasingly counteracts the effects of porosity, leading to larger but flatter craters. Spallation becomes more dominant in larger‐scale experiments and leads to an increase in cratering efficiency with increasing projectile size for constant impact velocities. The volume of spalled material is estimated using parabolic fits to the crater morphology, yielding approximations of the transient crater volume. For impacts at the same velocity these transient craters show a constant cratering efficiency that is not affected by projectile size.     

HOLLOW LUNAR SPHERULES AND MICROCRATERING

Since thin-walled hollow glass spherules exist in the lunar regolith and perhaps as a component of cosmic dust, laboratory simulations of impacts by and upon such spherules were done to determine identifying features of the resulting craters and perforations. The targets were soda-lime glass, stainless steel, and hollow glass beads. Craters were generated in the first two targets by the normal impact of thin-walled hollow glass spheres with masses and velocities between eight and 240 pg and 1.8 and 10 km/s, respectively. With increasing impact velocity, the crater morphology in glass progresses as follows: 1, a dent; 2, a narrow lip around the depression; and 3, spallation around the pit that may carry away all of part of the lip. The craters differ from those formed by solid spherical projectiles in that the central pit is an annular rather than a cup-shaped depression. The craters in steel display a typical outer lip and an additional concentric inner lip which is subdued to an annular mound as the impact velocity increases. In both targets, shattered remnants of the projectiles remain in the craters at low impact velocities. At higher velocities, melting of the projectile material occurs. The annular features distinguish these craters from craters generated by solid spheres or irregular projectiles', and the existence of such a crater morphology on a surface exposed to cosmic dust would indicate the presence of thin-walled hollow spherules. Contrary to common opinion, hollow spheres do not adequately simulate cratering by low density materials because of the mass distribution. Penetrations of thin-walled hollow glass beads by high velocity, solid, micrometer-size spheres are characterized by inward and outward flowing lips that show asymmetries dependent on the angle of impact. The morphology is sufficient to discriminate against other mechanisms that cause perforations in the one to 10 μm size range in hollow lunar spherules. The identifying lip may break away by fragmentation in the impact of larger size projectiles.     

Laboratory impacts into dry and wet sandstone with and without an overlying water layer: Implications for scaling laws and projectile survivability

Abstract— Scaling laws describing crater dimensions are defined in terms of projectile velocity and mass, densities of the materials involved, strength of the target, and the local gravity. Here, the additional importance of target porosity and saturation, and an overlying water layer, are considered through 15 laboratory impacts of 1 mm diameter stainless steel projectiles at 5 km s^?1 into a) an initially uncharacterized sandstone (porosity ?17%) and b) Coconino Sandstone (porosity ?23%). The higher‐porosity dry sandstone allows a crater to form with a larger diameter but smaller depth than in the lower‐porosity dry sandstone. Furthermore, for both porosities, a greater volume of material is excavated from a wet target than a dry target (by 27–30%). Comparison of our results with Pi‐scaling (dimensionless ratios of key parameters characterizing cratering data over a range of scales) suggests that porosity is important for scaling laws given that the new data lie significantly beneath the current fit for ice and rock targets on a π_v versus π₃ plot (π_v gives cratering efficiency and π₃ the influence of target strength). An overlying water layer results in a reduction of crater dimensions, with larger craters produced in the saturated targets compared to unsaturated targets. A water depth of approximately 12 times the projectile diameter is required before craters are no longer observed in the targets. Previous experimental studies have shown that this ratio varies between 10 and 20 (Gault and Sonett 1982). In our experiments ?25% of the original projectile mass survives the impact.     

Reconstruction of the Morasko meteoroid impact—Insight from numerical modeling

The Morasko strewn field located near Poznań, Poland comprises seven impact craters with diameters ranging from 20 to 90 m, all of which were formed in glacial sediments around 5000 yr ago. Numerous iron meteorites have been recovered in the area and their distribution suggests a projectile with the trajectory from NE to SW. Similar impact events producing crater strewn fields on average happen every 500 yr and pose a serious risk for modern civilization, which is why it is of utmost importance to study terrestrial strewn fields in detail. In this work, we investigate the Morasko meteoroid passage through the atmosphere, the distribution of its fragments on the ground, and the process of forming individual craters by means of numerical modeling. By combining atmospheric entry modeling, Pi‐group scaling of transient crater size and hydrocode simulations of impact processes, we constructed a comprehensive model of the Morasko strewn field formation. We determined the preatmospheric parameters of the Morasko meteoroid. The entry mass is between 600 and 1100 tons, the velocity range is between 16 and 18 km s^?1, and the trajectory angle is 30–40^°. Such entry velocities and trajectory angles do not deviate from typical values for near‐Earth asteroids, although the initial mass we determined can be considered as small. Our studies on velocities and masses of crater‐forming fragments showed that the biggest Morasko crater was formed by a projectile about 1.5 m in diameter with the impact velocity ~10 km s^?1. Environmental consequences of the Morasko impact event are very localized.     

Survival of refractory presolar grain analogs during Stardust‐like impact into Al foils: Implications for Wild 2 presolar grain abundances and study of the cometary fine fraction

We present results of FIB–TEM studies of 12 Stardust analog Al foil craters which were created by firing refractory Si and Ti carbide and nitride grains into Al foils at 6.05 km s^?1 with a light‐gas gun to simulate capture of cometary grains by the Stardust mission. These foils were prepared primarily to understand the low presolar grain abundances (both SiC and silicates) measured by SIMS in Stardust Al foil samples. Our results demonstrate the intact survival of submicron SiC, TiC, TiN, and less‐refractory Si₃N₄ grains. In small (<2 μm) craters that are formed by single grain impacts, the entire impacting crystalline grain is often preserved intact with minimal modification. While they also survive in crystalline form, grains at the bottom of larger craters (>5 μm) are typically fragmented and are somewhat flattened in the direction of impact due to partial melting and/or plastic deformation. The low presolar grain abundance estimates derived from SIMS measurements of large craters (mostly >50 μm) likely result from greater modification of these impactors (i.e., melting and isotopic dilution), due to higher peak temperatures/pressures in these crater impacts. The better survivability of grains in smaller craters suggests that more accurate presolar grain estimates may be achievable through measurement of such craters. It also suggests small craters can provide a complementary method of study of the Wild 2 fine fraction, especially for refractory CAI‐like minerals.     

Mid‐sized complex crater formation in mixed crystalline‐sedimentary targets: Insight from modeling and observation

Abstract— Large impact crater formation is an important geologic process that is not fully understood. The current paradigm for impact crater formation is based on models and observations of impacts in homogeneous targets. Real targets are rarely uniform; for example, the majority of Earth's surface is covered by sedimentary rocks and/or a water layer. The ubiquity of layering across solar system bodies makes it important to understand the effect target properties have on the cratering process. To advance understanding of the mechanics of crater collapse, and the effect of variations in target properties on crater formation, the first “Bridging the Gap” workshop recommended that geological observation and numerical modeling focussed on mid‐sized (15–30 km diameter) craters on Earth. These are large enough to be complex; small enough to be mapped, surveyed and modelled at high resolution; and numerous enough for the effects of target properties to be potentially disentangled from the effects of other variables. In this paper, we compare observations and numerical models of three 18–26 km diameter craters formed in different target lithology: Ries, Germany; Haughton, Canada; and El'gygytgyn, Russia. Based on the first‐order assumption that the impact energy was the same in all three impacts we performed numerical simulations of each crater to construct a simple quantitative model for mid‐sized complex crater formation in a subaerial, mixed crystalline‐sedimentary target. We compared our results with interpreted geological profiles of Ries and Haughton, based on detailed new and published geological mapping and published geophysical surveys. Our combined observational and numerical modeling work suggests that the major structural differences between each crater can be explained by the difference in thickness of the pre‐impact sedimentary cover in each case. We conclude that the presence of an inner ring at Ries, and not at Haughton, is because basement rocks that are stronger than the overlying sediments are sufficiently close to the surface that they are uplifted and overturned during excavation and remain as an uplifted ring after modification and post‐impact erosion. For constant impact energy, transient and final crater diameters increase with increasing sediment thickness.     

Investigating target versus impactor influences on Martian crater morphology at the simple‐complex transition

Comparing craters of identical diameter on a planet is an empirical method of studying the effects of different target and impactor properties while holding total impact energy nearly constant. We have analyzed the Martian crater population within a narrow diameter range (7 km < crater diameter < 9 km) at the simple‐complex crater transition using three approaches. We looked for correlations of morphology with surface geology using a global crater database and global geologic map. We examined selected regions in detail with high‐resolution images to further understand the relationship between crater morphology and bulk target properties. Finally, we examined craters in close proximity to each other in order to hold target properties constant, so that we could isolate impactor effects on crater morphology. We found a strong correlation between target properties and interior crater morphology, and we found little evidence that impactor properties (other than impact angle) affect crater appearance. Central uplift and wall slumping are enhanced for less consolidated targets. Layered targets affected both the excavation and modification stages of complex crater formation; the resulting craters have pseudoterraces, flat floors, and central pits.     

Transient crater growth in granular targets: An experimental study of low velocity impacts into glass sphere targets

We experimentally studied the formation and collapse processes of transient craters. Polycarbonate projectiles with mass of 0.49 g were impacted into the soda-lime glass sphere target (mean diameters of glass spheres are ∼36, 72, and 220 μm, respectively) using a single-stage light-gas gun. Impact velocity ranged from 11 to 329 m s⁻¹. We found that the transient crater collapses even at laboratory scales. The shape (diameter and depth) of the transient crater differs from that of the final crater. The depth-rim diameter ratios of the final and transient craters are 0.11-0.14 and 0.26-0.27, respectively. The rim diameter of both the transient and final crater depends on target material properties; however, the ratio of final to transient crater diameter does not. This suggests that target material properties affect the formation process of transient craters even in the gravity regime, and must be taken into account when scaling experimental results to planetary scales. By observing impacts into glass sphere targets, we show that although the early stage of the excavation flow does not depend on the target material properties, the radial expansion of the cavity after the end of vertical expansion does. This suggests that the effect of target material properties is specifically important in the later part of the crater excavation and collapse.     

Resurge gullies and “inverted sombrero” morphology,Flynn Creek impact structure,Tennessee

The Flynn Creek impact structure is an approximately 3.8 km diameter, marine‐target impact structure, which is located in north central Tennessee, USA. The target stratigraphy consists of several hundreds of meters of Ordovician carbonate strata, specifically Knox Group through Catheys‐Leipers Formation. Like other, similarly sized marine‐target impact craters, Flynn Creek's crater moat‐filling deposits include, in stratigraphic order, gravity‐driven slump material, aqueous resurge deposits, and secular (postimpact) aqueous settling deposits. In the present study, we show that Flynn Creek also possesses previously undescribed erosional resurge gullies and an annular, sloping surface that comprises an outer crater rim surrounding an inner, nested bowl‐shaped crater, thus forming a concentric crater structure. Considering this morphology, the Flynn Creek impact structure has a crater shape that has been referred to at other craters as an “inverted sombrero.” In this paper, we describe the annular rim and the inner crater at Flynn Creek using geographic information system technology. We relate these geomorphic features to the marine environment of crater formation, and compare the Flynn Creek impact structure with other marine‐target impact structures having similar features.     

Scaling and reproducibility of craters produced at the Experimental Projectile Impact Chamber (EPIC), Centro de Astrobiología,Spain

The Experimental Projectile Impact Chamber (EPIC) is a specially designed facility for the study of processes related to wet‐target (e.g., “marine”) impacts. It consists of a 7 m wide, funnel‐shaped test bed, and a 20.5 mm caliber compressed N₂ gas gun. The target can be unconsolidated or liquid. The gas gun can launch 20 mm projectiles of various solid materials under ambient atmospheric pressure and at various angles from the horizontal. To test the functionality and quality of obtained results by EPIC, impacts were performed into dry beach sand targets with two different projectile materials; ceramic Al₂O₃ (max. velocity 290 m s^?1) and Delrin (max. velocity 410 m s^?1); 23 shots used a quarter‐space setting (19 normal, 4 at 53° from horizontal) and 14 were in a half‐space setting (13 normal, 1 at 53°). The experiments were compared with numerical simulations using the iSALE code. Differences were seen between the nondisruptive Al₂O₃ (ceramic) and the disruptive Delrin (polymer) projectiles in transient crater development. All final crater dimensions, when plotted in scaled form, agree reasonably well with the results of other studies of impacts into granular materials. We also successfully validated numerical models of vertical and oblique impacts in sand against the experimental results, as well as demonstrated that the EPIC quarter‐space experiments are a reasonable approximation for half‐space experiments. Altogether, the combined evaluation of experiments and numerical simulations support the usefulness of the EPIC in impact cratering studies.