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1.
The Campo del Cielo impact structure exhibits several penetration funnels and impact craters. Here, we model the formation of these funnels with pre-impact conditions consistent with the results of meteoroid entry models. We study vertical impacts to find the dependence of funnel geometry (depth, diameter) on impact velocity and target porosity. At velocities above 1 km s−1, we observe strong deformation of the projectile and transformation of funnels into regular impact craters. We also use 3-D impact models to study oblique impacts and find that in the case of impact angles <25° to the horizon, the projectile bounces off the target. Instead of a funnel, an elongated groove forms, while the fragmented projectile escapes and moves farther downrange. At steeper impact angles, funnels form with the projectile at its tip. Early interpretations of the Campo del Cielo impact angle at 9–10° were based on (i) an oversimplified atmospheric model allowing “correct” strewn field elongation and (ii) the results of excavation in which the sloping boundary between breccia-like materials and infilling loess was interpreted as a true crater floor and its slope was equated to the impact angle. As our models show, the projectile trajectory within the target is not a straight line, and the angle to horizon changes from a steep one at the impact point to zero and then to a negative value (the projectile is moving upward). We also model two impact craters (Hoyo de la Cañada and Laguna Negra) created by high-velocity fragments to demonstrate the projectile remnants ricochet in the downrange direction.  相似文献   

2.
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.  相似文献   

3.
Linné is a simple crater, with a diameter of 2.23 km and a depth of 0.52 km, located in northwestern Mare Serenitatis. Recent high‐resolution data acquired by the Lunar Reconnaissance Orbiter Camera revealed that the shape of this impact structure is best described by an inverted truncated‐cone. We perform morphometric measurements, including slope and profile curvature, on the Digital Terrain Model of Linné, finding the possible presence of three subtle topographic steps, at the elevation of +20, ?100, and ?200 m relative to the target surface. The kink at ?100 m might be related to the interface between two different rheological layers. Using the iSALE shock physics code, we numerically model the formation of Linné crater to derive hints on the possible impact conditions and target physical properties. In the initial setup, we adopt a basaltic projectile impacting the Moon with a speed of 18 km s?1. For the local surface, we consider either one or two layers, in order to test the influence of material properties or composite rheologies on the final crater morphology. The one‐layer model shows that the largest variations in the crater shape take place when either the cohesion or the friction coefficient is varied. In particular, a cohesion of 10 kPa marks the threshold between conical‐ and parabolic‐shaped craters. The two‐layer model shows that the interface between the two layers would be exposed at the observed depth of 100 m when an intermediate value (~200 m) for the upper fractured layer is set. We have also found that the truncated‐cone morphology of Linné might originate from an incomplete collapse of the crater wall, as the breccia lens remains clustered along the crater walls, while the high‐albedo deposit on the crater floor can be interpreted as a very shallow lens of fallout breccia. The modeling analysis allows us to derive important clues on the impactor size (under the assumption of a vertical impact and collision velocity equal to the mean value), and on the approximate, large‐scale preimpact target properties. Observations suggest that these large‐scale material properties likely include some important smaller scale variations, disclosed as subtle morphological steps in the crater walls. Furthermore, the modeling results allow advancing some hypotheses on the geological evolution of the Mare Serenitatis region where Linné crater is located (unit S14). We suggest that unit S14 has a thickness of at least a few hundreds of meters up to about 400 m.  相似文献   

4.
Over the past thirty years, two bodies of literature have developed in parallel presenting mutually exclusive views of the Sun’s upper transition region. One model holds that the Sun’s upper-transition-region plasmas are confined primarily in hydrostatic funnels with a substantial backheating component. The other model holds that discrete structures, which are effectively isolated from the corona, predominate in the Sun’s upper transition region. Purveyors of the latter position have recently begun to present near-resolved observations of discrete structures. The funnel scenario, in contrast, has only been addressed by modeling unresolved upper transition region emission. To address this paradox we have constructed hydrostatic funnel models and tested them against a wider set of solar observations than previously performed. We reproduce the results of the previous analyses, yet find that the hydrostatic funnels are unable to self-consistently match the wider set of observations against which we test the models. We show that it is not possible for a class of funnels having peak temperatures in the transition region or in the corona to match the observations. We conclude that it is implausible that a class of hydrostatic funnels constitutes the dominant emitting component of the Sun’s upper-transition-region plasmas as has been suggested.  相似文献   

5.
Using one-arcsecond-slit-scan observations from the Hinode/EUV Imaging Spectrometer (EIS) on 5 February 2007, we find the plasma outflows in the open and expanding coronal funnels at the eastern boundary of AR 10940. The Doppler-velocity map of Fe?xii 195.120 Å shows the diffuse closed-loop system to be mostly red-shifted. The open arches (funnels) at the eastern boundary of AR exhibit blue-shifts with a maximum speed of about 10?–?15 km?s?1. This implies outflowing plasma through these magnetic structures. In support of these observations, we perform a 2D numerical simulation of the expanding coronal funnels by solving the set of ideal MHD equations in appropriate VAL-III C initial temperature conditions using the FLASH code. We implement a rarefied and hotter region at the footpoint of the model funnel, which results in the evolution of slow plasma perturbations propagating outward in the form of plasma flows. We conclude that the heating, which may result from magnetic reconnection, can trigger the observed plasma outflows in such coronal funnels. This can transport mass into the higher corona, giving rise to the formation of the nascent solar wind.  相似文献   

6.
Abstract– The majority of meteorite impacts occur at oblique incidence angles. However, many of the effects of obliquity on impact crater size and morphology are poorly understood. Laboratory experiments and numerical models have shown that crater size decreases with impact angle, the along‐range crater profile becomes asymmetric at low incidence angles, and below a certain threshold angle the crater planform becomes elliptical. Experimental results at approximately constant impact velocity suggest that the elliptical threshold angle depends on target material properties. Herein, we test the hypothesis that the threshold for oblique crater asymmetry depends on target material strength. Three‐dimensional numerical modeling offers a unique opportunity to study the individual effects of both impact angle and target strength; however, a systematic study of these two parameters has not previously been performed. In this work, the three‐dimensional shock physics code iSALE‐3D is validated against laboratory experiments of impacts into a strong, ductile target material. Digital elevation models of craters formed in laboratory experiments were created from stereo pairs of scanning electron microscope images, allowing the size and morphology to be directly compared with the iSALE‐3D craters. The simulated craters show excellent agreement with both the crater size and morphology of the laboratory experiments. iSALE‐3D is also used to investigate the effect of target strength on oblique incidence impact cratering. We find that the elliptical threshold angle decreases with decreasing target strength, and hence with increasing cratering efficiency. Our simulations of impacts on ductile targets also support the prediction from Chapman and McKinnon (1986) that cratering efficiency depends on only the vertical component of the velocity vector.  相似文献   

7.
We determined the shock‐darkening pressure range in ordinary chondrites using the iSALE shock physics code. We simulated planar shock waves on a mesoscale in a sample layer at different nominal pressures. Iron and troilite grains were resolved in a porous olivine matrix in the sample layer. We used equations of state (Tillotson EoS and ANEOS) and basic strength and thermal properties to describe the material phases. We used Lagrangian tracers to record the peak shock pressures in each material unit. The post‐shock temperatures (and the fractions of the tracers experiencing temperatures above the melting point) for each material were estimated after the passage of the shock wave and after the reflections of the shock at grain boundaries in the heterogeneous materials. The results showed that shock‐darkening, associated with troilite melt and the onset of olivine melt, happened between 40 and 50 GPa with 52 GPa being the pressure at which all tracers in the troilite material reach the melting point. We demonstrate the difficulties of shock heating in iron and also the importance of porosity. Material impedances, grain shapes, and the porosity models available in the iSALE code are discussed. We also discuss possible not‐shock‐related triggers for iron melt.  相似文献   

8.
The intense, super-Eddington, well-collimated radiation field generated by non-Keplerian accretion disks orbiting black holes can accelerate matter to high velocities. By using an energy balance argument which is both simple and physically appealing, we show that ordinary electron-proton plasmas can only reach relativistic gamma-factors up to 1.5. A light electron-positron plasma could be accelerated up to 5. The approach is valid if the optical depth along the accelerating funnel is significant, and the optical depth across the funnel is negligible. It is, therefore, appropriate for long, narrow funnels. Our method could easily be generalized to disk geometries other than the examples we give.  相似文献   

9.
Results of nondestructive gamma‐ray analyses of cosmogenic radionuclides (7Be, 22Na, 26Al, 46Sc, 48V, 54Mn, 56Co, 57Co, 58Co, and 60Co) in 19 fragments of the Ko?ice meteorite are presented and discussed. The activities varied mainly with position of fragments in the meteoroid body, and with fluxes of cosmic‐ray particles in the space affecting radionuclides with different half‐lives. Monte Carlo simulations of the production rates of 60Co and 26Al compared with experimental data indicate that the pre‐atmospheric radius of the meteoroid was 50 ± 5 cm. In two Ko?ice fragments, He, Ne, and Ar concentrations and isotopic compositions were also analyzed. The noble‐gas cosmic‐ray exposure age of the Ko?ice meteorite is 5–7 Myr, consistent with the conspicuous peak (or doublet peak) in the exposure age histogram of H chondrites. One sample likely contains traces of implanted solar wind Ne, suggesting that Ko?ice is a regolith breccia. The agreement between the simulated and observed 26Al activities indicate that the meteoroid was mostly irradiated by a long‐term average flux of galactic cosmic rays of 4.8 particles cm?2 s?1, whereas the short‐lived radionuclide activities are more consistent with a flux of 7.0 protons cm?2 s?1 as a result of the low solar modulation of the galactic cosmic rays during the last few years before the meteorite fall.  相似文献   

10.
Dust from comet 81P/Wild 2 was captured at high speed in silica aerogel collectors during the Stardust mission. Studies of deceleration tracks in aerogel showed that a number of cometary particles were poorly cohesive and fragmented during impact. Fragments are now scattered along the walls of impact cavities. Here, we report a transmission electron microscope study of a piece of aerogel extracted from the wall of track 10. We focused on micron‐sized secondary tracks along which fragments of a fine‐grained material are disseminated. Two populations of fragments were identified. The first is made of polycrystalline silicate assemblages (olivine, pyroxene, and spinel) that appear to be chemically related to each other. The second corresponds to silica‐rich glassy clumps characteristic of a mixture of melted cometary material and aerogel. A significant number of fragments have been found with a composition close to chondritic CI for the major elements Fe‐Mg‐S at a submicron scale. These fragments have thus never been chemically differentiated by high‐temperature processes prior to the accretion on the comet, in contrast to terminal particles, which are dominated by larger, denser, and frequently monomineralic components.  相似文献   

11.
We provide the circumstances and details of the fireball observation, search expeditions, recovery, strewn field, and physical characteristics of the Ko?ice meteorite that fell in Slovakia on February 28, 2010. The meteorite was only the 15th case of an observed bolide with a recovered mass and subsequent orbit determination. Despite multiple eyewitness reports of the bolide, only three videos from security cameras in Hungary were used for the strewn field determination and orbit computation. Multiple expeditions of professionals and individual searchers found 218 fragments with total weight of 11.3 kg. The strewn field with the size of 5 × 3 km is characterized with respect to the space distribution of the fragments, their mass and size‐frequency distribution. This work describes a catalog of 78 fragments, mass, size, volume, fusion crust, names of discoverers, geographic location, and time of discovery, which represents the most complex study of a fresh meteorite fall. From the analytical results, we classified the Ko?ice meteorite as an ordinary H5 chondrite.  相似文献   

12.
Via the potential field extrapolation of the observed photospheric magnetic field, the structure of the photospheric magnetic fields above solar quiet regions is renewed. As revealed by the result, below 20 Mm the open magnetic lines exhibit many obvious small funnel structures. These funnels expand with height and at the height of about 20 Mm they combine into large funnel structures. By a systematic study of the tendency of change of the cross section areas of funnels, it is discovered that the cross section areas of funnels in solar quiet regions expand approximately linearly. The velocity of expansion of magnetic funnels at rather low altitudes (< 20 Mm) is larger than that at high altitudes (> 20 Mm). This phenomenon has important significance for the two-dimensional numerical simulations of the origin of solar wind and the mass flow in magnetic loops. At the same time it is found that the number of closed magnetic lines decreases in the form of exponential function.  相似文献   

13.
《Icarus》1987,72(1):95-127
The possibility that snowmelt could have provided liquid water for valley network formation early in the history of Mars is investigated using an optical-thermal model developed for dusty snowpacks at temperate latitudes. The heating of the postulated snow is assumed to be driven primarily by the absorption of solar radiation during clear sky conditions. Radiative heating rates are predicted as a function of depth and shown to be sensitive to the dust concentration and the size of the ice grains while the thermal conductivity is controlled by temperature, atmospheric pressure, and bulk density. Rates of metamorphism indicate that fresh fine-grained snow on Mars would evolve into moderately coarse snow during a single summer season. Results from global climate models are used to constrain the mean-annual surface temperatures for snow and the atmospheric exchange terms in the surface energy balance. Mean-annual temperatures within Martian snowpacks fail to reach the melting point for all atmospheric pressures below 1000 mbar despite a predicted temperature enhancement beneath the surface of the snowpacks. When seasonal and diurnal variations in the incident solar flux are included in the model, melting occurs at midday during the summer for a wide range of snow types and atmospheric pressures if the dust levels in the snow exceed 100 ppmw (parts per million by weight). The optimum dust concentration appears to be about 1000 ppmw. With this dust load, melting can occur in the upper few centimeters of a dense coarse-grained snow at atmospheric pressures as low as 7 mbar. Snowpack thickness and the thermal conductivity of the underlying substrate determine whether the generated snow-melt can penetrate to the snowpack base, survive basal ice formation, and subsequently become available for runoff. Under favorable conditions, liquid water becomes available for runoff at atmospheric pressures as low as 30 to 100 mbar if the substrate is composed of regolith, as is expected in the ancient cratered terrain of Mars.  相似文献   

14.
A model for gas outflows is proposed which simultaneously explains the correlations between the (i) equivalent widths of low-ionization and Lyα lines, (ii) outflow velocity, and (iii) star formation rate observed in Lyman break galaxies (LBGs). Our interpretation implies that LBGs host short-lived (30 ± 5 Myr) starburst episodes observed at different evolutionary phases. Initially, the starburst powers a hot wind bound by a denser cold shell, which after ≈5 Myr becomes dynamically unstable and fragments; afterwards the fragment evolution is approximately ballistic while the hot bubble continues to expand. As the fragments are gravitationally decelerated, their screening ability of the starlight decreases as the ultraviolet (UV) starburst luminosity progressively dims. LBG observations sample all these evolutionary phases. Finally, the fragments fall back on to the galaxy after ≈60 Myr. This phase cannot be easily probed as it occurs when the starburst UV luminosity has already largely faded; however, galaxies dimmer in the UV than LBGs should show infalling gas.  相似文献   

15.
Siljan, central Sweden, is the largest known impact structure in Europe. It was formed at about 380 Ma, in the late Devonian period. The structure has been heavily eroded to a level originally located underneath the crater floor, and to date, important questions about the original size and morphology of Siljan remain unanswered. Here we present the results of a shock barometry study of quartz‐bearing surface and drill core samples combined with numerical modeling using iSALE. The investigated 13 bedrock granitoid samples show that the recorded shock pressure decreases with increasing depth from 15 to 20 GPa near the (present) surface, to 10–15 GPa at 600 m depth. A best‐fit model that is consistent with observational constraints relating to the present size of the structure, the location of the downfaulted sediments, and the observed surface and vertical shock barometry profiles is presented. The best‐fit model results in a final crater (rim‐to‐rim) diameter of ~65 km. According to our simulations, the original Siljan impact structure would have been a peak‐ring crater. Siljan was formed in a mixed target of Paleozoic sedimentary rocks overlaying crystalline basement. Our modeling suggests that, at the time of impact, the sedimentary sequence was approximately 3 km thick. Since then, there has been around 4 km of erosion of the structure.  相似文献   

16.
Abstract— Over the last few decades, rapid improvement of computer capabilities has allowed impact cratering to be modeled with increasing complexity and realism, and has paved the way for a new era of numerical modeling of the impact process, including full, three‐dimensional (3D) simulations. When properly benchmarked and validated against observation, computer models offer a powerful tool for understanding the mechanics of impact crater formation. This work presents results from the first phase of a project to benchmark and validate shock codes. A variety of 2D and 3D codes were used in this study, from commercial products like AUTODYN, to codes developed within the scientific community like SOVA, SPH, ZEUS‐MP, iSALE, and codes developed at U.S. National Laboratories like CTH, SAGE/RAGE, and ALE3D. Benchmark calculations of shock wave propagation in aluminum‐on‐aluminum impacts were performed to examine the agreement between codes for simple idealized problems. The benchmark simulations show that variability in code results is to be expected due to differences in the underlying solution algorithm of each code, artificial stability parameters, spatial and temporal resolution, and material models. Overall, the inter‐code variability in peak shock pressure as a function of distance is around 10 to 20%. In general, if the impactor is resolved by at least 20 cells across its radius, the underestimation of peak shock pressure due to spatial resolution is less than 10%. In addition to the benchmark tests, three validation tests were performed to examine the ability of the codes to reproduce the time evolution of crater radius and depth observed in vertical laboratory impacts in water and two well‐characterized aluminum alloys. Results from these calculations are in good agreement with experiments. There appears to be a general tendency of shock physics codes to underestimate the radius of the forming crater. Overall, the discrepancy between the model and experiment results is between 10 and 20%, similar to the inter‐code variability.  相似文献   

17.
Abstract— The production of 3He, 21Ne, and 22Ne in meteoroids of various sizes and in the lunar surface was investigated. The LAHET code system, a purely physical model for calculating cosmic‐ray particle fluxes, was used to simulate cosmic‐ray particle interactions with extraterrestrial matter. We discuss the depth and size dependence of the shielding parameter 22Ne/21Ne, which is used for reconstruction of pre‐atmospheric sizes, depth, and exposure histories. The 22Ne/21Ne ratio decreases with increasing depth or pre‐atmospheric size but then increases with depth in very large objects. This increase with depth in the 22Ne/21Ne ratio means that this ratio is a poor indicator of shielding in some large objects. The dependence of 3He/21Ne as function of 22Ne/21Ne was also calculated, and differences between the calculations and the Bern line are discussed.  相似文献   

18.
The Whitecourt meteorite impact crater, Alberta, Canada is a rare example of a well‐preserved small impact structure, with which thousands of meteorite fragments are associated. As such, this crater represents a unique opportunity to investigate the effect of a low‐energy impact event on an impacting iron bolide. Excellent documentation of meteorite fragment locations and characteristics has generated a detailed distribution map of both shrapnel and regmaglypted meteorite types. The meteorites' distribution, and internal and external characteristics support a low‐altitude breakup of the impactor which caused atmospherically ablated (regmaglypted) meteorites to fall close to the crater and avoid impact‐related deformation. In contrast, shrapnel fragments sustained deformation at macro‐ and microscales resulting from the catastrophic disruption of the impactor. The impactor was significantly fragmented along pre‐existing planes of weakness, including kamacite lamellae and inclusions, resulting in a bias toward low‐mass (<100 g) fragments. Meteorite mineralogy was investigated and the accessory minerals were found to be dominated by sulfides and phosphides with rare carlsbergite, consistent with other low‐Ni IIIAB iron meteorites. Considerations of the total mass of meteoritic material recovered at the site relative to the probable fraction of the impactor that was preserved based on modeling suggests that the crater was formed by a higher velocity, lower mass impactor than previously inferred.  相似文献   

19.
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.  相似文献   

20.
Asteroids and comets 10–100 m in size that collide with Earth disrupt dramatically in the atmosphere with an explosive transfer of energy, caused by extreme air drag. Such airbursts produce a strong blastwave that radiates from the meteoroid's trajectory and can cause damage on the surface. An established technique for predicting airburst blastwave damage is to treat the airburst as a static source of energy and to extrapolate empirical results of nuclear explosion tests using an energy‐based scaling approach. Here we compare this approach to two more complex models using the iSALE shock physics code. We consider a moving‐source airburst model where the meteoroid's energy is partitioned as two‐thirds internal energy and one‐third kinetic energy at the burst altitude, and a model in which energy is deposited into the atmosphere along the meteoroid's trajectory based on the pancake model of meteoroid disruption. To justify use of the pancake model, we show that it provides a good fit to the inferred energy release of the 2013 Chelyabinsk fireball. Predicted overpressures from all three models are broadly consistent at radial distances from ground zero that exceed three times the burst height. At smaller radial distances, the moving‐source model predicts overpressures two times greater than the static‐source model, whereas the cylindrical line‐source model based on the pancake model predicts overpressures two times lower than the static‐source model. Given other uncertainties associated with airblast damage predictions, the static‐source approach provides an adequate approximation of the azimuthally averaged airblast for probabilistic hazard assessment.  相似文献   

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