impact cratering in sandstone the memin pilot study on the effect of pore water abstract planetary surfaces are subjected to meteorite bombardment and crater formation rocks forming these surfaces are often porous and contain fluids to understand the role of both parameters on impact cratering we conducted laboratory experiments with dry and wet sandstone blocks impacted by centimeter sized steel spheres we utilized a 40 m two stage light gas gun to achieve impact velocities of up to 5 4 km s 1 cratering efficiency ejection velocities and spall volume are enhanced if the pore space of the sandstone is filled with water in addition the crater morphologies differ substantially from wet to dry targets i e craters in wet targets are larger but shallower we report on the effects of pore water on the excavation flow field and the degree of target damage we suggest that vaporization of water upon pressure release significantly contributes to the impact process

use of surface controlled ree sectoral zoning in apatite from llallagua bolivia to determine a single crystal smnd age abstract surface structure controlled partitioning of rees between symmetrically nonequivalent crystal faces leads to sectoral zoning between the 0001 and 1010 sectors of vein filling apatites from llallagua bolivia absolute differences in the concentration of the analyzed rees between these sectors range from a factor of 9 5 in the case of la to a factor of 1 4 for yb the two sectors have different concentrations of sm and nd as well as significantly different sm nd ratios 147 sm 144 nd 0 3558 a sm nd isochron determined from petrographically constrained coeval portions of different sectors gives an age of 43 8 4 7 ma 95 confidence limits we believe that this calculated age represents the time of crystallization of the apatite the application of intracrystalline heterogeneities that formed due to differential partitioning at the crystal surface to dating of minerals is germane for any mineral in which sectoral or intrasectoral zoning is exhibited and may be used with other isotopic systems in which sufficient segregation of parent and daughter nuclides occurs and in which concentrations are sufficient for analysis of small intracrystalline samples

numerical modeling of asteroid survivability and possible scenarios for the morokweng crater forming impact the fate of the impactor is an important aspect of the impact cratering process defining impactor material as surviving if it remains solid i e does not melt or vaporize during crater formation previous numerical modeling and experiments have shown that survivability decreases with increasing impact velocity impact angle with respect to the horizontal and target density here we show that in addition to these impactor survivability depends on the porosity and shape of the impactor increasing impactor porosity decreases impactor survivability while prolate shaped polar axis equatorial axis impactors survive impact more so than spherical and oblate shaped polar axis equatorial axis impactors these results are used to produce a relatively simple equation which can be used to estimate the impactor fraction shocked to a given pressure as a function of these parameters by applying our findings to the morokweng crater forming impact we suggest impact scenarios that explain the high meteoritic content and presence of unmolten fossil meteorites within the morokweng crater in addition to previous suggestions of a low velocity and or high angled impact this work suggests that an elongated and or low porosity impactor may also help explain the anomalously high survivability of the morokweng impactor

a strain based porosity model for use in hydrocode simulations of impacts and implications for transient crater growth in porous targets numerical modelling of impact cratering has reached a high degree of sophistication however the treatment of porous materials still poses a large problem in hydrocode calculations we present a novel approach for dealing with porous compaction in numerical modelling of impact crater formation in contrast to previous attempts e g p alpha model snowplow model our model accounts for the collapse of pore space by assuming that the compaction function depends upon volumetric strain rather than pressure our new e alpha model requires only four input parameters and each has a physical meaning the model is simple and intuitive and shows good agreement with a wide variety of experimental data ranging from static compaction tests to highly dynamic impact experiments our major objective in developing the model is to investigate the effect of porosity and internal friction on transient crater formation we present preliminary numerical model results that suggest that both porosity and internal friction play an important role in limiting crater growth over a large range in gravity scaled source size

scaling of hypervelocity impact craters in ice with impact angle 1 hypervelocity oblique impacts using spherical 1 mm diameter aluminum projectiles at velocities of 5 2 0 2 km s 1 were incident at angles ranging from normal 0 to the vertical to grazing incidence 80 onto thick polycrystalline h2o ice targets at 253 k data were obtained to distinguish changes in crater size and shape the resultant craters had a deep central pit surrounded by a shallower terrace results on crater size showed that volume and crater depth had varying levels of dependence on obliquity for the full angular range and terrace depth had a dependence only at angles 45 length and width measurements held a strong dependence on obliquity only at angles 50 from the normal these results for ice show that it is hard to determine angle of impact from crater morphology although crater depth and volume do change with angle it would be hard to separate this effect from the influence of an impact of a projectile of different speed density etc which might also affect crater depth or crater depth diameter ratios only at extreme angles 70 do real differences in shape emerge for ice

numerical modeling of impact induced modifications of the deep sea floor abstract the mechanics of oceanic impact events differ in several respects from the process that accompanies the strike of an asteroid on land in order to explore the cratering process on a water covered target a series of 2d hydrocode simulations for impact velocities of 10 15 20 and 30 km s and water depth diameter ratios between 5 and 7 have been carried out in particular these simulations were aimed to examine the effects of the asteroid on the deep sea floor the interaction of the impacting body and the water column causes loss of the body s kinetic energy both through oceanic drag and through mass loss due to hydrodynamic heating prior to hitting the ocean bottom regardless of whether the impactor actually hits the ocean floor the shock wave generated in the water column by the impact of the body on the water surface will leave behind unique traces in the marine sediments for example an asteroid of 1 km in diameter arriving at the ocean surface with a velocity of 20 km s induces a shock pressure of 6 gpa in the oceanic crust at a water depth of 5 km the passage of the shock wave along the water ocean bottom interface causes significant disturbance in the sediment layer moreover the collapse of the transient cavity that results from the passage of the projectile through the water column causes a strong surge of water that generates a displacement and redeposition of sediments on the ocean floor in order to examine sediment transport processes we introduce computationally a large amount of massless particles within the projectile and the target material and follow the movement of each individual particle tracer particles represent only forced material movement and do not account for any frictional forces thus comparisions with real sediment disturbances may be qualitative only for an impact velocity of 20 km s and a water depth diameter ratio of 5 this procedure reveals significant particle movements within a distance of 15 km from the center of the impact the only known deep sea impact structure so far the eltanin impact in the bellingshausen sea antarctica is characterized by a zone of chaotically mixed sediments that most probably originated from impact induced turbulent water currents based on the present observations which do not allow the identification of an impact structure at the ocean bottom our model yields estimates of a projectile diameter of 1 km and an impact velocity of 20 km s

laboratory investigations of marine impact events factors influencing crater formation and projectile survivability given that the earths surface is covered in around two thirds water the majority of impact events should have occurred in marine environments however with the presence of a water layer crater formation may be prohibited indeed formation is greatly controlled by the water depth to projectile diameter ratio as discussed in this paper previous work has shown that the underlying target material also influences crater formation e g gault and sonett 1982 baldwin et al 2007 in addition to the above parameters we also show the influence of impact angle impact velocity and projectile density for a variety of water depths on crater formation and projectile survivability the limiting ratio of water depth to projectile diameter on cratering represents the point at which the projectile is significantly slowed by transit through the water layer to reduce the impact energy to that which prohibits cratering we therefore study the velocity decay produced by a water layer using laboratory analytical and numerical modelling techniques and determine the peak pressures endured by the projectile for an impact into a water depth five times the projectile diameter the velocity of the projectile is found to be reduced to 26 32 its original value for deep water impacts we find that up to 60 of the original mass of the projectile survives in an oblique impact where survivability is defined as the solid or melted mass fraction of the projectile that could be collected after impact

impact of a cosmic body into earth s ocean and the generation of large tsunami waves insight from numerical modeling 1 the strike of a cosmic body into a marine environment differs in several respects from impact on land oceans cover approximately 70 of the earth s surface implying not only that oceanic impact is a very likely scenario for future impacts but also that most impacts in earth s history must have happened in marine environments therefore the study of oceanic impact is imperative in two respects 1 to quantify the hazard posed by future oceanic impacts including the potential threat of large impact generated tsunami like waves and 2 to reconstruct earth s impact record by accounting for the large number of potentially undiscovered crater structures in the ocean crust reconstruction of the impact record is of crucial importance both for assessing the frequency of collision events in the past and for better predicting the probability of future impact we summarize the advances in the study of oceanic impact over the last decades and focus in particular on how numerical models have improved our understanding of cratering in the oceanic environment and the generation of waves by impact we focus on insight gleaned from numerical modeling studies into the deceleration of the projectile by the water cratering of the ocean floor the late stage modification of the crater due to gravitational collapse and water resurge furthermore we discuss the generation and propagation of large tsunami like waves as a result of a strike of a cosmic body in marine environments

impact experiments in low temperature ice new results of low velocity impact experiments in cubic and cylindrical 20 cm water ice targets initially at 257 and 81 k are reported impact velocities and impact energies vary between 0 1 and 0 64 km sec and 10 9 and 10 10 ergs respectively observed crater diameters range from 7 to 15 cm and are two to three times larger than values found for equal energy impacts in basaltic targets crater dimensions in ice targets increase slightly with increasing target temperatures crater volumes of strength controlled ice craters are about 10 to 100 times larger than those observed for craters in crystalline rocks based on similarity analysis general scaling laws for strength controlled crater formation are derived and are applied to crater formation on the icy galilean and saturnian satellites this analysis indicates that surface ages based on impact crater statistics on an icy crust will appear greater than those for a silicate crust which experienced the same impact history the greater ejecta volume for cratering in ice versus cratering in silicate targets leads to accelerated regolith production on an icy planet

the memin research unit scaling impact cratering experiments in porous sandstones 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

laboratory impacts into dry and wet sandstone with and without an overlying water layer implications for scaling laws and projectile survivability 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 v versus 3 plot v gives cratering efficiency and 3 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

shocked quartz grains from the malingen structure sweden evidence for a twin crater of the lockne impact structure the malingen structure in sweden has for a long time been suspected to be the result of an impact however no hard evidence i e shock metamorphic features or traces of the impactor has so far been presented here we show that quartz grains displaying planar deformation features pdfs oriented along crystallographic planes typical for shock metamorphism are present in drill core samples from the structure the shocked material was recovered from basement breccias below the sediment infill and the distribution of the orientation of the shock produced pdfs indicates that the studied material experienced low shock pressures based on our findings we can exclude that the material is transported from the nearby lockne impact structure which means that the malingen structure is a separate impact structure the seventh confirmed impact structure in sweden furthermore sedimentological and biostratigraphic aspects of the deposits that fill the depression at malingen are very similar to features at the lockne impact structure this implies a coeval formation age and thus also the confirmation of the first known marine target doublet impact craters on earth i e the lockne malingen pair

physical dynamics and biological implications of cyclone noah in the lee of hawai i during e flux i abstract quasi synoptic observations of the horizontal and vertical structure of a cold core cyclonic mesoscale eddy feature cyclone noah were conducted in the lee of hawai i from november 4 22 2004 as part of the e flux interdisciplinary collaborative research program cyclone noah appears to have spun up to the southwest of the alenuihaha channel between maui and hawai i as a result of strong and persistent northeasterly trade winds through the channel shipboard hydrographic surveys 2 5 months later suggest that noah weakened and was in a hypothesized spin down phase of its life cycle although the initial surface expression of noah was limited in scale to 40 km in diameter and as evidenced by surface temperatures 2 3 c cooler than the surrounding waters depth profiles revealed a fully developed semi elliptical shallow feature 200 m 144 km long and 90 km wide based on sigma t 23 kg m 3 with tangential speeds of 40 80 cm s 1 and substantial isopycnal doming potential vorticity distribution of noah suggests that radial horizontal flow of the core water was inhibited from the surface to depths of 75 m with high vorticity confined above the sigma t 23 5 kg m 3 isopycnal surface upward displacements of isopycnal surfaces in the eddy s center 50 m were congruent with enhanced pigment concentrations 0 50 mg m 3 comparisons of the results obtained for e flux i noah and e flux iii opal suggest that translation characteristics of cyclonic hawaiian lee eddies may be important in establishing the biogeochemical and biological responses of the oligotrophic ocean to cyclonic eddies

shock metamorphism on the ocean floor numerical simulations abstract impact craters are unknown in deep ocean basins important reasons for this are that the ocean crust is younger than continents and the impacts are weakened by the water column laboratory experiments can show specific features of water impacts however impact velocities are too small

hypervelocity impact fragmentation of basalt and shale projectiles abstract results are presented for the fragmentation of projectiles in laboratory experiments 1 5 mm cubes and spheres of basalt and shale were impacted onto water at normal incidence and speeds from 0 39 to 6 13 km s 1 corresponding to peak shock pressures 0 7 32 gpa projectile fragments were collected and measured over 100 000 fragments in some impacts at sizes down to 10 m power laws were fitted to the cumulative fragment size distributions and the evolution of the exponent vs impact speed and peak shock pressure found the gradient of each of these power laws increased with increasing impact speed peak shock pressure the percentage of the projectiles recovered in the impacts was found and used to estimate projectile remnant survival in different solar system impact scenarios at the mean impact speed appropriate to that scenario for pluto the moon and in the asteroid belt approximately 55 40 and 15 respectively of an impactor could survive and be recovered at an impact site finally the catastrophic disruption energy densities of basalt and shale were measured and found to be 24 104 j kg 1 and 9 104 j kg 1 respectively a factor of 2 5 difference these corresponded to peak shock pressures of 1 to 1 5 gpa basalt and 0 8 gpa shale this is for near normal incidence impacts where tensile strength is dominant for shallow angle impacts we suggest shear effects dominate resulting in lower critical energy densities and peak shock pressures we also determine a method to ascertain information about fragment sizes in solar system impact events using a known size of impactor the results are used to predict projectile fragments sizes for the veneneia and rheasilvia crater forming impacts on vesta and similar impacts on ceres

unmelted meteoritic debris collected from eltanin ejecta in polarstern cores from expedition ant xii 4 abstract a total of 1 7 g of unmelted meteorite particles have been recovered from fs polarstern piston cores collected on expedition ant xii 4 that contain ejecta from the eltanin impact event most of the mass 1 2 g is a large single specimen that is a polymict breccia similar in mineralogy and chemistry to howardites or the silicate fraction of mesosiderites most of the remaining mass is in several large individual pieces 20 75 mg each that are polymict breccias fragments dominated by pyroxene and an igneous rock fragment the latter has highly fractionated ree similar to those reported in mafic clasts from mesosiderites other types of specimens identified include fragments dominated by maskelynite or olivine these pieces of the projectile probably survived impact by being blown off the back surface of the eltanin asteroid during its impact into the bellingshausen sea

discovery of a 25 cm asteroid clast in the giant morokweng impact crater south africa meteorites provide a sample of solar system bodies and so constrain the types of objects that have collided with earth over time meteorites analysed to date however are unlikely to be representative of the entire population and it is also possible that changes in their nature have occurred with time large objects are widely believed to be completely melted or vaporized during high angle impact with the earth consequently identification of large impactors relies on indirect chemical tracers notably the platinum group elements here we report the discovery of a large 25 cm unaltered fossil meteorite and several smaller fragments within the impact melt of the giant 70 km diameter 145 myr old morokweng crater south africa the large fragment clast resembles an ll6 chondrite breccia but contains anomalously iron rich silicates fe ni sulphides and no troilite or metal it has chondritic chromium isotope ratios and identical platinum group element ratios to the bulk impact melt these features allow the unambiguous characterization of an impactor at a large crater furthermore the unusual composition of the meteorite suggests that the morokweng asteroid incorporated part of the ll chondrite parent body not represented by objects at present reaching the earth

velocity scaling of impact craters in water ice over the range 1 to 7 3 km s 1 the impact cratering behavior of polycrystalline ice at a temperature of 259 3 k has been investigated by 16 impacts with 1 mm aluminum 2017 alloy spheres density 2790 kg m 3 over the velocity range 1 to 7 3 km s 1 using a two stage light gas gun crater cross sectional profiles and diameter depth and volume data are presented it is found that there is no significant difference in power law energy exponents for scaling of crater diameter and volume between these data and previous low velocity 1 km s 1 ice impact data dimensionless pi group scaling was also found to scale the data in a way which can be readily combined with earlier lower velocity data and data at a lower ice temperature

the preservation of fossil biomarkers during meteorite impact events experimental evidence from biomarker rich projectiles and target rocks a devonian siltstone from orkney scotland shows survival of biomarkers in high velocity impact experiments the biomarkers were detected in ejecta fragments from experiments involving normal incidence of steel projectiles at 5 6 km s 1 and in projectile fragments from impact experiments into sand and water at 2 5 km s 1 the associated peak shock pressures were calculated to be in the range of 110 147 gpa for impacts of the steel projectiles into the siltstone target and hydrocode simulations are used to show the variation of peak pressure with depth in the target and throughout the finite volume projectiles thermally sensitive biomarker ratios including ratios of hopanoids and steranes and the methylphenanthrene ratio showed an increase in thermal maturity in the ejecta and especially the projectile fragments measurement of absolute concentrations of selected biomarkers indicates that changes in biomarker ratios reflect synthesis of new material rather than selective destruction their presence in ejecta and projectile fragments suggests that fossil biomarkers may survive hypervelocity impacts and that experiments using biomarker rich rock have high potential for testing survival of organic matter in a range of impact scenarios

the effect of the oceans on the terrestrial crater size frequency distribution insight from numerical modeling 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

comment on determining chondritic impactor size from the marine osmium isotope record paquay et al reports 11 april 2008 p 214 reported that osmium isotope ratios in marine sediments can be used to determine the size of a chondritic impactor their assumptions on the fate of an impacting projectile may need to be reassessed however because only a small unpredictable fraction of the impactor ends up dissolved in seawater