Porous Targets Research Articles

The Steinheim Basin impact crater (SW-Germany) – Where are the ejecta?

The ∼24km Nördlinger Ries and the ∼3.8km Steinheim Basin in southern Germany are thought to represent a ∼14.8Ma old impact crater doublet. The complex craters of the Steinheim Basin with its crater fill deposits and the Nördlinger Ries and its voluminous impact ejecta blanket are still widely preserved. Although located in an environmental setting that presumably underwent the same erosional history as the Ries crater, field geologic studies suggest that no proximal or distal ejecta of the Steinheim impact event are presently preserved. Generally, the lack of the ejecta blanket around the crater could be explained either by intense erosion, the scarcity of outcrops, or it never formed. In contrast to the lack of ejecta, fluvial and lacustrine Middle Miocene sediments deposited prior to, synchronous with, and shortly after the impact are preserved in many places in the surroundings of to the Steinheim Basin.On low-density asteroids or planets with highly porous target rocks (⩾30–40% effective porosity), impact structures can form without significant ejecta outside the craters due to the compaction of porosity and a concordant drastic reduction of the ejecta velocity. In the Steinheim area, the target rocks comprised loose, porous Miocene sands, Upper Jurassic limestones and Middle Jurassic porous sand- and claystones. The average porosity of the entire sedimentary target suite may have reached 20–30% or even higher values assuming the existence of open karst cavities in the Upper Jurassic carbonates. Compaction of the porous target rocks, resulting in the reduction of ejected material, in combination with erosion could explain the apparent lack of impact ejecta in the wider periphery of the Steinheim impact structure. The Steinheim Basin represents the first proposed terrestrial example of an impact crater characterized by porosity-related ejecta suppression, and it is suggested that other sediment-hosted impact structures on Earth might exhibit analogous excavation-process characteristics.

Momentum Transfer in Hypervelocity Impact Experiments on Rock Targets

A special phenomenon observed in hypervelocity impacts on rock targets is the so-called momentum multiplication, i.e. the momentum transferred to the target is greater than the original momentum of the projectile. This effect is caused by ejection of debris in the direction opposite to the flight direction of the projectile. In the present study momentum multiplication was investigated as a function of target material properties and projectile velocity. Hypervelocity impact experiments on target materials with different porosities were conducted and the momentum transfer was measured using a ballistic pendulum.Low porous materials like quartzite show larger momentum multiplication than porous materials like sandstone. The smallest momentum multiplication was measured for highly porous aerated concrete. Higher projectile velocity leads to higher momentum multiplication. Furthermore, this increase is stronger for low porous materials compared with porous materials.These observations can be explained by the different ejection behavior. Low porous materials show a directional and very fast ejection whereas porous materials show a slower ejection. The highly porous material shows a diffuse ejection behavior. Furthermore, cratering efficiency is reduced in porous targets leading to a smaller amount of ejected debris. This effect is attributed to energy dissipation caused by irreversible crushing of pore space.

X-ray imaging of subsurface dynamics in high-Z materials at the Diamond Light Source

In this paper, we describe a new approach enabling study of subsurface dynamics in high-Z materials using the unique combination of high-energy synchrotron X-rays, a hybrid bunch structure, and a new dynamic loading platform. We detail the design and operation of the purpose-built, portable small bore gas-gun, which was installed on the I12 high-energy beamline at the Diamond Light Source and used to drive compression waves into solid and porous metal targets. Using a hybrid bunch structure and broadband X-ray pulses of up to 300 keV, radiographic snapshots were captured during various dynamic deformation processes in cm-scale specimens, thereby contributing to a more complete understanding of the evolution of mesoscale damage. Importantly, we highlight strategies for overcoming the challenges associated with using high-energy X-rays, and suggest areas for improvement needed to advance dynamic imaging through large-scale samples of relevance to engineering scenarios. These preliminary measurements demonstrate the feasibility of probing highly transient phenomena using the presented methodology.

Optical Properties of Silicon Nanoparticles Prepared by Laser Ablation Method in Various Media

Bright photoluminescent silicon nanoparticles were successfully fabricated from porous silicon target in air and n-hexane using 532 nm line of YAG:Nd pulsed laser. The whole procedure was carried out at room temperature, in atmosphere. The morphology and composition of as-synthesized nanoparticles were characterized by SEM and EDS measurements. Their optical properties were investigated. The PL peak position of the as-synthesized Si nanoparticle ablated in air is slightly blue-shifted in comparison with those ablated in n-hexane, while it was almost no change of peak position of Si nanoparticles fabricated from p-Si in hexane and original p-Si wafer target. In contrast to PL intensity quenching of original p-Si sample and particles produced in air, those produced in n-hexane solvent exhibited an enhancement. The presence of dangling bonds or energy transfer from excited Si particles to oxygen molecules on the surface can be the cause of the quenching effect.

Small-scale physical modeling of seismic-wave propagation using unconsolidated granular media

Laboratory physical modeling and laser-based experiments are frequently proposed to tackle theoretical and methodological issues related to seismic prospecting, e.g., when experimental validations of processing or inversion techniques are required. Lasers are mainly used to simulate typical field acquisition setups on homogeneous and consolidated materials assembled into laboratory-scale physical models (PMs) of various earth structures. We suggested the use of granular materials to study seismic-wave propagation in unconsolidated and porous media and target near-surface exploration and hydrogeologic applications. We designed and tested the reproducibility of an experimental procedure to build and probe PMs consisting of micrometric glass beads (GBs). A mechanical source and a laser-Doppler vibrometer were used to record small-scale seismic lines at the surface of three GBs models. When guided surface acoustic mode theory should prevail in such unconsolidated granular packed structure under gravity, we only considered elastic-wave propagation in stratified media to interpret recorded data. Thanks to basic seismic processing and inversion methods (first arrivals and dispersion analyses), we were able to correctly retrieve the gradients of pressure- and shear-wave velocities in our models. A 3D elastic finite difference simulation of the experiment offered, despite significant differences in terms of amplitudes, a supplementary validation of our approximation, as far as elastic properties of the medium were concerned.

Hypervelocity cratering and disruption of porous pumice targets: Implications for crater production, catastrophic disruption, and momentum transfer on porous asteroids

Most asteroids for which porosities have been inferred have porosities from 20% to>50%. To investigate the effects of target porosity on cratering, impact disruption, and momentum transfer we performed a series of 17 hypervelocity impact experiments on high-porosity (60% to 85% porous), terrestrial, pumice targets impacted at speeds ranging from 3.5 to 5.2km/s at the NASA Ames Vertical Gun Range. Eleven disruptions demonstrated that pumice targets are significantly stronger, i.e., they require more impactor kinetic energy per unit target mass to produce an equivalent disruption, than non-porous targets. The threshold collisional specific energy, QD⁎, for this pumice is ~2380J/kg, more than 60% greater than the value previously determined for ordinary chondrite meteorites having ~10% porosity, and more than three times the literature value for non-porous terrestrial basalt. As a result, in the same impactor environment non-porous asteroids, with properties similar to terrestrial basalt, and highly porous asteroids with the properties of this pumice are equally likely to be disrupted, possibly explaining the survival of asteroids with moderate or high porosity. The six cratering events produced steep-walled, roughly cylindrical craters, with depth/diameter ratios of ~1 to ~2.7, rather than the bowl-shaped craters with depth/diameter ~0.5 produced in non-porous targets. Computed microtomography shows little or no damage to the pumice outside the excavated crater volume even for impactor energies of approximately one-half QD⁎, an energy shown to produce global damage in non-porous targets. Two large, overlapping craters were produced by successive hypervelocity impacts into one pumice target, a result consistent with the interpretation of the large, overlapping craters on the asteroid 253 Mathilde being a result of its high (>50%) porosity. We measured the post-impact momentum of a pumice target, showing that the recoil from the crater ejecta exceeded the direct momentum transferred by absorption of the projectile by ~30%.

Impact experiments on highly porous targets: Cavity morphology and disruption thresholds in the strength regime

Small bodies were probably very porous during the formation of the solar system. To understand the evolution of such bodies, impact experiments on sintered glass-bead targets with porosities of 80%, 87%, and 94% were performed at velocities of 1.8–7.2kms−1 using various projectiles with densities ranging from 1.1gcm−3 to 4.5gcm−3. Here we report on the resulting cavity morphologies formed by these impacts, with particular attention paid to the depth from the cavity׳s entrance hole to its maximum diameter, the entrance-hole diameter, and the maximum diameter. We obtained empirical relations of the entrance-hole diameter and the maximum diameter using non-dimensional parameters for crater scaling. We also report on the targets׳ disruption thresholds, Q⁎. Each Q⁎ value is on the order of kilojoules per kilogram, which is higher than the equivalent values for pure ice targets and basalt targets determined from high-velocity impact experiments. Non-dimensional disruption thresholds, ρtQ⁎/Y, where ρt and Y are the targets׳ bulk densities and compressive strengths, respectively, are calculated for various targets including those used in this study; they are shown to be within approximately one order of magnitude for a given porosity, although the impact velocities and target sizes range from 1ms−1 to 7.2kms−1 and from 2cm to 14cm, respectively. The previous proposed strength parameter for the catastrophic disruption threshold, Πs⁎, is also calculated. It is shown to be roughly constant, irrespective of porosity if we assume that the scaling parameter μ decreases linearly with increasing porosity.

Size dependence of the disruption threshold: laboratory examination of millimeter–centimeter porous targets

The outcome of collision between small solid bodies is characterized by the threshold energy density Q⁎s, the specific energy to shatter, that is defined as the ratio of projectile kinetic energy to the target mass (or the sum of target and projectile) needed to produce the largest intact fragment that contains one half the target mass. It is indicated theoretically and by numerical simulations that the disruption threshold Q⁎s decreases with target size in strength-dominated regime. The tendency was confirmed by laboratory impact experiments using non-porous rock targets (Housen and Holsapple, 1999; Nagaoka et al., 2014).In this study, we performed low-velocity impact disruption experiments on porous gypsum targets with porosity of 65–69% and of three different sizes to examine the size dependence of the disruption threshold for porous material. The gypsum specimens were shown to have a weaker volume dependence on static tensile strength than do the non-porous rocks. The disruption threshold had also a weaker dependence on size scale as Q⁎s∝D−γ,γ≤0.25−0.26, while the previous laboratory studies showed γ=0.40 for the non-porous rocks. The measurements at low-velocity lead to a value of about 100Jkg−1 for Q⁎s which is roughly one order of magnitude lower than the value of Q⁎s for the gypsum targets of 65% porosity but impacted by projectiles with higher velocities. Such a clear dependence on the impact velocity was also shown by previous studies of gypsum targets with porosity of 50%.

Positron bunching system for producing positronium clouds into vacuum

A magnetic transport line, a magnetic field terminator and a positron buncher were designed and built to focus low-energy positron pulses from a Surko-type accumulator on a porous target. The 25 electrodes buncher, which produces a parabolic potential, was designed to implant 5 ns positron bunches with a spot of 3 mm into a target held at cryogenic temperature. These pulses will be used to obtain cooled Ps clouds into vacuum for laser excitation in spectroscopy experiments. By using high-voltage fast switches and a proper mu-metal shield the requirement to form Ps in a free (magnetic and electric) field region was satisfied compatibly with the request of injecting positrons at energies of 5-9 keV. The optical design, the electrical circuitry of the buncher and the construction solutions of the whole apparatus will be presented and explained.

Geochemical processes between steel projectiles and silica-rich targets in hypervelocity impact experiments

The possibility of fractionation processes between projectile and target matter is critical with regard to the classification of the impactor type from geochemical analysis of impactites from natural craters. Here we present results of five hypervelocity MEMIN impact experiments (Poelchau et al., 2013) using the Cr–V–Co–Mo–W-rich steel D290-1 as projectile and two different silica-rich lithologies (Seeberger sandstone and Taunus quartzite) as target materials. Our study is focused on geochemical target–projectile interaction occurring in highly shocked and projectile-rich ejecta fragments. In all of the investigated impact experiments, whether sandstone or quartzite targets, the ejecta fragments show (i) shock-metamorphic features e.g., planar-deformation features (PDF) and the formation of silica glasses, (ii) partially melting of projectile and target, and (iii) significant mechanical and chemical mixing of the target rock with projectile material. The silica-rich target melts are strongly enriched in the “projectile tracer elements” Cr, V, and Fe, but have just minor enrichments of Co, W, and Mo. Inter-element ratios of these tracer elements within the contaminated target melts differ strongly from the original ratios in the steel. The fractionation results from differences in the reactivity of the respective elements with oxygen during interaction of the metal melt with silicate melt. Our results indicate that the principles of projectile–target interaction and associated fractionation do not depend on impact energies (at least for the selected experimental conditions) and water-saturation of the target. Partitioning of projectile tracer elements into the silicate target melt is much more enhanced in experiments with a non-porous quartzite target compared with the porous sandstone target. This is mainly the result of higher impact pressures, consequently higher temperatures and longer reaction times at high temperatures in the experiments with quartzite as target material.

Soft X-ray enhancement from porous target irradiated by high power sub-nanosecond laser beam and the effect of plasma opacity

Abstract In this paper, we investigate numerically the enhancement of the soft continuum X-ray yield emitted from porous aluminum targets irradiated by sub-nanosecond high power pulsed laser beam. The calculations were conducted for two different conditions: (i) ignoring plasma opacity, and (ii) including it in the calculations; and finally the results were compared. In the calculations, the main emission mechanisms were assumed to be the free–bound and free–free (bremsstrahlung) transitions at water windows wavelengths ranges 2.3–4.4 nm and also 12.6–14.6 nm, which is useful for lithography applications. The free–free and bound–free transitions were also considered for calculating the opacity. The results show that for both conditions, the X-ray yield rises with increasing the porosity and the increase rate is almost equal in both cases; however, the calculations are not appropriate for estimating the absolute values of the X-ray yield when the plasma opacity is ignored.

Morphology of asteroid (4179) Toutatis as imaged by Chang'E‐2 spacecraft

AbstractClose observations by the Chang'E‐2 spacecraft reveal that the surface of asteroid (4179) Toutatis is characterized by abundant impact craters with most of them being degraded by surface resetting. The less degraded large crater with a diameter of ~800 m at the south pole is estimated to be produced by an impactor with a diameter of ~50 m from strength crater scaling relations. From the analysis of large‐impact events on highly porous targets, we argue that Toutatis is likely a rubble‐pile body and its two lobes are contact binaries. The fact that Toutatis suffered plenty of impacts with seismic shaking resetting the initial surface features but not resulting in catastrophic disruption is probably because of the material's high attenuation of shock wave.

Hypervelocity impacts on asteroids and momentum transfer I. Numerical simulations using porous targets

In this paper, we investigate numerically the momentum transferred by impacts of small (artificial) projectiles on asteroids. The study of the momentum transfer efficiency as a function of impact conditions and of the internal structure of an asteroid is crucial for performance assessment of the kinetic impactor concept of deflecting an asteroid from its trajectory. The momentum transfer is characterized by the so-called momentum multiplication factor β, which has been introduced to define the momentum imparted to an asteroid in terms of the momentum of the impactor. Here we present results of code calculations of the β factor for porous targets, in which porosity takes the form of microporosity and/or macroporosity. The results of our study using a large range of impact conditions indicate that the momentum multiplication factor β is small for porous targets even for very high impact velocities (β<2 for vimp⩽15km/s), which is consistent with published scaling laws and results of laboratory experiments (Holsapple, K.A., Housen, K.R. [2012]. Icarus 221, 875–887; Holsapple, K.A., Housen, K.R. [2013]. Proceedings of the IAA Planetary Defense Conference 2013, Flagstaff, USA). It is found that both porosity and strength can have a large effect on the amount of transferred momentum and on the scaling of β with impact velocity. On the other hand, the macroporous inhomogeneities considered here do not have a significant effect on β.

Numerical study of heavy-ion stopping in foam targets with one-dimensional subcell-scale hydrodynamic motions

Heavy-ion stopping in foam targets with subcell-scale hydro motions was numerically investigated in relation to ion-driven warm dense matter experiments. To simulate porous foam targets, we employed a simple 1D periodic multilayer model consisting of thin solid slabs and gaps between them. The averaged pore diameter and cell-wall thickness of the foam were represented by the gap width between the slabs and the slab thickness, respectively. The density- and temperature-dependent projectile stopping cross-sections were evaluated using a binary encounter model taking into account the electronic state of target atoms during heating and expansion. We employed a combination of 11Na projectiles and subrange 13Al foam targets with ρ=0.05ρsolid. The hydrodynamic motion of the target was calculated with a 1D code. During homogenization, hot dense spots appeared at the original gap positions, owing to stagnation of the jets. As a result, even after the pores were filled with blow-off materials, the initial inhomogeneity was not completely smeared out, and the total energy loss was still not equal to that in the homogeneous equivalent, especially for large pore sizes.

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.

Impact on porous targets: Penetration, crater formation, target compaction, and ejection

Using a granular-mechanics code, we study the impact of a sphere into a porous adhesive granular target, consisting of monodisperse silica grains. The model includes elastic repulsive, adhesive, and dissipative forces, as well as sliding, rolling, and twisting friction. Impact velocities of up to 30 m/s and target filling factors (densities) between 19% and 35% have been systematically studied. We find that the projectile is stopped by an effective drag force which is proportional to the square of its velocity. Target adhesion influences projectile stopping only below a critical velocity, which increases with adhesion. The penetration depth depends approximately logarithmically on the impact velocity and is inversely proportional to the target density. The excavated crater is of conical form and is surrounded by a compaction zone whose width increases but whose maximum value decreases with increasing target density. Grain ejection increases in proportion with impactor velocity. Grains are ejected which have originally been buried to a depth of 8R(grain) below the surface; the angular distribution favors oblique ejection with a maximum around 45°. The velocity distribution of ejected grains features a broad low-velocity maximum around 0.5-1 m/s but exhibits a high-velocity tail up to ~15% of the projectile impact velocity.

Shock experiments in range of 10–45 GPa with small multidomain magnetite in porous targets

Abstract– Physical properties of multidomain magnetite‐bearing porous pellets shocked up to 45 GPa were measured. The results show general magnetic softening as a result of shock. However, a relative magnetic hardening trend and slight magnetic susceptibility decrease is observed with increasing pressure among shocked samples. Initially, the shock also seems to cause a slight decrease in porosity, but at higher shock pressures macroscopic porosity increases progressively in our pellets. The microscopic porosity remains almost unchanged. Since our samples have distinctly higher initial porosity compared with samples used in previous studies, our results may be representative for impacts into highly porous magnetite‐bearing sedimentary or volcanic rocks and are relevant to impacts into such target rocks on Earth and Mars.

In situ flash X-ray observation of projectile penetration processes and crater cavity growth in porous gypsum target analogous to low-density asteroids

Recent studies of impact craters formed on low-density asteroids led to the proposal of a new crater formation mechanism dominated by pore collapse and compaction. Thus, it is important to study the crater formation process associated with the projectile penetration on porous cohesive targets. Laboratory impact experiments were conducted for a porous gypsum target with porosity of 50%, and flash X-rays were used to visualize the interior of the target for in situ observation of crater formation and projectile penetration. Spherical projectiles made of three different materials, stainless steel, aluminum, and nylon were impacted at 1.9–2.4km/s (low-velocity impact) and 5.6–6.4km/s (high-velocity impact) by using a two-stage light-gas gun. Two imaging plates were used to take two X-ray images at a different delay time from the impact moment for one shot. Two types of crater cavity shape were found on the porous gypsum target, that is, penetration holes or hemispherical cavities, depending on the projectile size and density, and the impact velocity. The drag coefficient of a projectile was determined by measuring the penetration depth changing with time, and we found that it was closely related to the crater cavity shape: it was about 0.9 for a penetration hole, while it was 2.3–3.9 for a hemispherical cavity. This large value for a hemispherical cavity could have been caused by the deformation or the disruption of the projectile. The cratering efficiency, ρtVcr(t)/mp, was found to have a power law relationship to the scaling time for crater growth, πt=vit/rp, where vi is the impact velocity, rp is the projectile radius, and t is the time after the impact, and all data for stainless steel and aluminum projectiles merged completely and could be fitted by a power-law equation of ρtVcr(t)/mp=2.69×10-1πt1.10. Furthermore, the scaled crater volume, πV=Vcr_finalρt/mp, where Vcr_final is the final crater cavity volume, ρt is the target density, and mp is the projectile mass, was successfully fitted by a power law equation when another scaling parameter was used for the crater formation in strength regime, πY=Yt/ρtvi2, where Yt is the target material strength, as follows: πV=1.69×10-1πY-0.51. As a result, the crater formed on porous gypsum was revealed to be more than one order of magnitude smaller than that formed on basalt. Based on our experimental results, which visualize how crater cavities on porous cohesive materials grow with projectile penetration, we are able to discuss compression and excavation processes during crater formation quantitatively. This observation enables us to investigate and revise numerical models and crater scaling laws for high-velocity impacts into porous cohesive materials.

Diagnostics of jet structures formed in the plasma corona during the irradiation of porous targets by intense laser pulses

Jet structures formed during laser irradiation of porous targets with an average density of ρ = 1−30 mg/cm3 were studied experimentally by using the diagnostic complex of the Mishen facility. To study complicated plasma structures, the experimental data were processed using specially elaborated mathematical methods. The probability of the emergence of jet plasma structures in plane open-pore triacetate cellulose targets was studied as a function of the parameter ρd, where ρ is the average mass density and d is the target thickness. Analysis of the experimental results and their comparison with the existing data on the jet structures formed during laser irradiation of solid-density targets allowed the authors to reveal the characteristic features and mechanisms of the development of large-scale plasma jets.

Experimental study on collisional disruption of highly porous icy bodies

Knowing the collisional process among small porous icy bodies in the outer solar system is a key to understanding the formation of EKBOs and the evolution of icy planetesimals. Impact experiments of sintered porous ice spheres with 40%, 50%, 60% and 70% porosity were conducted by using three types of projectiles at the impact velocity from 2.4 to 489m/s, and we studied the effects of porosity on the collisional processes. Projectile sticking occurred at the impact velocity higher than 44m/s for 60% porosity targets and higher than 13m/s for 70% porosity targets. The antipodal velocity of the porous ice target increased with the increase of energy density, Q, and it increased slightly with the increase of porosity, although it was exceptionally high in cases when the projectile penetrated the target. The shattering strength of porous ice targets was found to decrease from 100 to 31J/kg with the increase of porosity from 40% to 70%. The cumulative fragment mass distribution was found to depend on the energy density and the target porosity, and the slopes of the distribution in the small fragment region were almost flat for more porous targets. We reanalyzed the cumulative fragment mass distribution and first obtained the empirical equation showing the fragment mass distribution of porous ice targets as a function of the energy density and the porosity.