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

the current state of knowledge about shatter cones introduction to the special issue shatter cones are a fracture phenomenon that is exclusively associated with shock metamorphism and has also been produced in the laboratory in several shock experiments the occurrence of shatter cones is the only accepted meso to macroscopic recognition criterion for impact structures shatter cones exhibit a number of geometric characteristics orientation apical angles striation angles sizes that can be best described as varied from case to case possible links between geometric properties with impact or crater parameters have remained controversial and the lack of understanding of the mechanism of formation of shatter cones does not offer a physical framework to discuss or understand them a database of shatter cone occurrences has been produced for this introduction paper to the special issue of meteoritics and planetary science on shatter cones distribution of shatter cones with respect to crater size and lithology suggests that shatter cones do not occur in impact craters less than a few kilometers in diameter with a few currently questionable exceptions all pertinent hypotheses of formation are presented and discussed several may be discarded in light of the most recent observations the branching fracture mechanism and the interference models proposed respectively by sagy et al 2002 and baratoux and melosh 2003 require further evaluation new observations experiments or theoretical considerations presented in this special issue promise an important step forward based on a renewed effort to resolve the enigmatic origin of these important features

devolatilization or melting of carbonates at meteor crater az we have investigated the carbonates in the impact melts and in a monolithic clast of highly shocked coconino sandstone of meteor crater az to evaluate whether melting or devolatilization is the dominant response of carbonates during high speed meteorite impact both melt and clast carbonates are calcites that have identical crystal habits and that contain anomalously high sio2 and al2o3 also both calcite occurrences lack any meteoritic contamination such as fe or ni which is otherwise abundantly observed in all other impact melts and their crystallization products at meteor crater the carbon and oxygen isotope systematics for both calcite deposits suggest a low temperature environment 100 c for their precipitation from an aqueous solution consistent with caliche we furthermore subjected bulk melt beads to thermogravimetric analysis and monitored the evolving volatiles with a quadrupole mass spectrometer co2 yields were 5 wt with typical values in the 2 wt range also total co2 loss is positively correlated with h2o loss an indication that most of these volatiles derive from the secondary calcite also transparent glasses considered the most pristine impact melts yield 100 wt element totals by empa suggesting complete loss of co2 the target dolomite decomposed into mgo cao and co2 the co2 escaped and the cao and mgo combined with sio2 from coexisting quartz and feo from the impactor to produce the dominant impact melt at meteor crater although confined to meteor crater these findings are in stark contrast to osinski et al 2008 who proposed that melting of carbonates rather than devolatilization is the dominant process during hypervelocity impact into carbonate bearing targets including meteor crater

chemical projectile target interaction and liquid immiscibility in impact glass from the wabar craters saudi arabia abstract impact glasses are usually strongly affected by secondary alteration and chemical weathering thus in order to understand relevant formation processes detailed petrographic studies on unweathered impact glasses are necessary as preserved heterogeneities in quenched impact glasses may serve as a tool to better understand their genesis here we report on petrography and microchemistry of impact glasses from the wabar impact craters saudi arabia that with an age of 300 years are among the youngest terrestrial impact craters the fact that parts of the iiiab iron meteorite have survived impact and subsequent weathering is granting wabar a special role among the presently 184 confirmed terrestrial impact structures electron microprobe analysis empa and transmission electron microscopy tem obtained on the black impact melt glass variety at wabar suggest that meteoritic fe was selectively mixed with high silica target melt at high temperatures due to selective oxidation resulting in high fe ni ratios for the black melt 37 on average individual values range from 13 to 449 and low fe ni ratios for projectile droplets feni spheres with a fe ni ratio of 3 on average fe ni ratio for the meteorite is 12 the black melt shows emulsion textures that are the result of silicate liquid immiscibility liquid liquid phase separation resulted in the formation of a poorly polymerized ultrabasic melt lfe rich in divalent cations like fe 2 ca 2 or mg 2 that is dispersed in a highly polymerized high silica melt lsi matrix the typical wabar black melt emulsion displays a spheres in a matrix texture of 10 20 lfe homogeneously dispersed in the form of two sets of spheres and droplets 10 30 nm and 0 1 0 4 m in diameter in 80 90 lsi matrix plus occasionally disseminated feni spheres around large 10 m feni spheres however the typical emulsion texture changes to 21 lsi dispersed in 79 lfe this change of texture is interpreted as evidence for the transfer of meteoritic fe from the meteoritic feni spheres into the target melt due to selective oxidation of fe over ni and co variations in the bulk composition of wabar black melt largely depend on the volume ratios between immiscible ultrabasic lfe felsic lsi and remains of meteoritic feni spheres based on natural occurrences of phase separated glasses this work and literature and quenching experiments literature there is growing evidence that liquid immiscibility is a major process in the formation of glassy impactites

isotope systematics and shock wave metamorphism i u pb in zircon titanite and monazite shocked experimentally up to 59 gpa this study reports the first u pb isotope analyses on experimentally shocked zircon titanite and monazite extracted from proterozoic granitoid rocks in all three types of minerals shock waves produce drastic changes in the crystal lattices causing strong lowering of birefringence turbidization and decolorization of the individual grains moreover x ray patterns indicate transition of the crystals into polycrystalline aggregates of 10 5 mm block size precisely dated grains with concordant or nearly concordant ages were embedded in kbr and shocked at 35 47 5 and 59 gpa u pb isotope analyses on these grains show that shock metamorphism does not fractionate pb isotopes within the analytical precision of 0 1 as far as chemical fractionation is concerned there is no difference in degree of concordancy between shocked and unshocked monazite and small degrees 2 of relative upb fractionation in shocked zircon and titanite are due to time integrated pb loss and not to the shock experiment in consequence the data document that shock wave metamorphism alone does not measurably effect the u pb chronometer questioning the view that lower intercept ages of discordant u pb data reflect shock induced re equilibration of the chronometer in moderately to highly shocked rapidly cooling rocks

hydrocode simulation of the chicxulub impact event and the production of climatically active gases we constructed a numerical model of the chicxulub impact event using the chart d squared csq code coupled with the analytic equation of state aneos package in the simulations we utilized a target stratigraphy based on borehole data and employed newly developed equations of state for the materials that are believed to play a crucial role in the impact related extinction hypothesis carbonates calcite and evaporites anhydrite simulations explored the effects of different projectile sizes 10 to 30 km in diameter and porosity 0 to 50 the effect of impact speed is addressed by doing simulations of asteroid impacts vi 20 km s and comet impacts vi 50 km s the masses of climatically important species injected into the upper atmosphere by the impact increase with the energy of the impact event ranging from 350 to 3500 gt for co2 from 40 to 560 gt for s and from 200 to 1400 gt for water vapor while our results are in good agreement with those of ivanov et al 1996 our estimated co2 production is 1 to 2 orders of magnitude lower than the results of takata and ahrens 1994 indicating that the impact event enhanced the end cretaceous atmospheric co2 inventory by at most 40 consequently sulfur may have been the most important climatically active gas injected into the stratosphere the amount of s released by the impact is several orders of magnitude higher than any known volcanic eruption and with h2o is high enough to produce a sudden and significant perturbation of earth s climate

chemical modification of projectile residues and target material in a memin cratering experiment in the context of the memin project a hypervelocity cratering experiment has been performed using a sphere of the iron meteorite campo del cielo as projectile accelerated to 4 56 km s 1 and a block of seeberger sandstone as target material the ejecta collected in a newly designed catcher are represented by 1 weakly deformed 2 highly deformed and 3 highly shocked material the latter shows shock metamorphic features such as planar deformation features pdf in quartz formation of diaplectic quartz glass partial melting of the sandstone and partially molten projectile mixed mechanically and chemically with target melt during mixing of projectile and target melts the fe of the projectile is preferentially partitioned into target melt to a greater degree than ni and co yielding a fe ni that is generally higher than fe ni in the projectile this fractionation results from the differing siderophile properties specifically from differences in reactivity of fe ni and co with oxygen during projectile target interaction projectile matter was also detected in shocked quartz grains the average fe ni of quartz with pdf about 20 and of silica glasses about 24 are in contrast to the average sandstone ratio about 422 but resembles the fe ni ratio of the projectile about 14 we briefly discuss possible reasons of projectile melting and vaporization in the experiment in which the calculated maximum shock pressure does not exceed 55 gpa

geochemical processes between steel projectiles and silica rich targets in hypervelocity impact experiments abstract 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

impacts into quartz sand crater formation shock metamorphism and ejecta distribution in laboratory experiments and numerical models we investigated the ejection mechanics by a complementary approach of cratering experiments including the microscopic analysis of material sampled from these experiments and 2 d numerical modeling of vertical impacts the study is based on cratering experiments in quartz sand targets performed at the nasa ames vertical gun range in these experiments the preimpact location in the target and the final position of ejecta was determined by using color coded sand and a catcher system for the ejecta the results were compared with numerical simulations of the cratering and ejection process to validate the isale shock physics code in turn the models provide further details on the ejection velocities and angles we quantify the general assumption that ejecta thickness decreases with distance according to a power law and that the relative proportion of shocked material in the ejecta increase with distance we distinguish three types of shock metamorphic particles 1 melt particles 2 shock lithified aggregates and 3 shock comminuted grains the agreement between experiment and model was excellent which provides confidence that the models can predict ejection angles velocities and the degree of shock loading of material expelled from a crater accurately if impact parameters such as impact velocity impactor size and gravity are varied beyond the experimental limitations this study is relevant for a quantitative assessment of impact gardening on planetary surfaces and the evolution of regolith layers on atmosphereless bodies

laser induced melting experiments simulation of short term high temperature impact processes this study introduces an experimental approach using direct laser irradiation to simulate the virtually instantaneous melting of target rocks during meteorite impacts we aim at investigating the melting and mixing processes of projectile iron meteorite steel and target material sandstone under idealized conditions the laser experiments le were able to produce features very similar to those of impactites from meteorite craters and cratering experiments i e formation of lechatelierite partial to complete melting of sandstone and injection of projectile droplets into target melts the target and projectile melts have experienced significant chemical modifications during interaction of these coexisting melts emulsion textures observed within projectile contaminated target melts indicate phase separation of silicate melts with different chemical compositions during quenching reaction times of 0 6 to 1 4 s could be derived for element partitioning and phase separation processes by measuring time depended temperature profiles with a bolometric detector our le allow i separate melting at high temperatures to constrain primary melt heterogeneities before mixing of projectile and target ii quantification of element partitioning processes between coexisting projectile and target melts iii determination of cooling rates and iv estimation of reaction times moreover we used a thermodynamic approach to calculate the entropy gain during laser melting the entropy changes for laser melting of sandstone and iron meteorite correspond to shock pressures and particle velocities produced during the impact of an iron projectile striking a quartz target at a minimum impact velocity of 6 km s 1 inducing peak shock pressures of 100 gpa in the target

on the use of a split hopkinson pressure bar in structural geology high strain rate deformation of seeberger sandstone and carrara marble under uniaxial compression abstract there is increasing evidence that seismogenic fractures can propagate faster than the shear wave velocity of the surrounding rocks strain rates within the tip region of such super shear earthquake ruptures can reach deformation conditions similar to impact processes resulting in rock pulverization the physical response of brittle rocks at high strain rates changes dramatically with respect to quasi static conditions rocks become stiffer and their strength increases a measure for the dynamic behavior of a rock and its strain dependency is the dynamic increase factor dif which is the ratio of the dynamic compressive strength to the quasi static uniaxial compressive strength to investigate deformation in the high strain rate regime experimentally we introduce the split hopkinson pressure bar technology to the structural geology community a method that is frequently used by rock and impact engineers we measure the stress strain response of homogeneous fine grained seeberger sandstone and carrara marble in uniaxial compression at strain rates ranging from 10 1 to 10 2 s 1 with respect to tangent modulus and dynamic uniaxial compressive strength we present full stress strain response curves of seeberger sandstone and carrara marble at high strain rates and an evaluation method to determine representative rates of deformation results indicate a rate dependent elastic behavior of carrara marble where an average increase of 18 could be observed at high strain rates of about 100 s 1 dif reaches a factor of 2 2 2 4 seeberger sandstone does not have a rate dependent linear stress strain response at high strain rates its dif was found to be about 1 6 1 7 at rates of 100 s 1 the onset of dynamic behavior is accompanied with changes in the fracture pattern from single to multiple fractures to pervasive pulverization for increasing rates of deformation seismogenic shear zones and their associated fragment size spectra should be carefully revisited in the light of dynamic deformation

early fracturing and impact residue emplacement can modelling help to predict their location in major craters understanding the nature and composition of larger extraterrestrial bodies that may collide with the earth is important one source of information lies in the record of ancient impact craters some of which have yielded chemical information as to the impacting body many deeply eroded craters have no remaining melt sheet or ejecta yet may contain impactor residue within basement fractures the emplacement mechanism for fractionated siderophile residues is likely to be gaseous although melt droplets and some solid materials may survive for breccia and melt filled fractures to contain extraterrestrial material they must form very early in the impact process most current numerical models do not dwell on the formation and location of early major fractures although fractures in and around small craters on brittle glass exposed to hypervelocity impact in low earth orbit have been successfully simulated modelling of fracture development associated with larger craters may help locate impact residues and test the models themselves

structural geology of impact craters abstract the formation of impact craters is a highly dynamic and complex process that subjects the impacted target rocks to numerous types of deformation mechanisms understanding and interpreting these styles of micro meso and macroscale deformation has proved itself challenging for the field of structural geology in this paper we give an overview of the structural inventory found in craters of all size ranges on earth and look into the structures of craters on other planetary bodies structural features are discussed here that are caused by i extremely high pressures and temperatures that occur during the initial passage of the shock wave through the target rock and projectile ii the resulting flow field in the target that excavates and ejects rock materials and iii the gravitationally induced modification of the crater cavity into the final crater form a special focus is put on the effects that low angle impacting bodies have on crater formation we hope that this review will help both planetary scientists and structural geologists understand the deformation processes and resulting structures generated by meteorite impact

a new method to determine the direction of impact asymmetry of concentric impact craters as observed in the field lockne on mars in experiments and simulations most impacts occur at an angle with respect to the horizontal plane this is primarily reflected in the ejecta distribution but at very low angle structural asymmetries such as elongation of the crater and nonradial development of the central peak become apparent unfortunately impact craters with pristine ejecta layers are rare on earth and also in areas with strong past or ongoing surface erosion on other planetary bodies and the structural analysis of central peaks requires good exposures or even on site access to outcrop however target properties are known to greatly influence the shape of the crater especially the relatively common target configuration of a weaker layer covering a more rigid basement one such effect is the formation of concentric craters i e a nested deeper inner crater surrounded by a shallow outer crater here we show that with decreasing impact angle there is a downrange shift of the outer crater with respect to the nested crater we use a combination of 1 field observation and published 3 d numerical simulation of one of the best examples of a terrestrial concentric impact crater formed in a layered target with preserved ejecta layer the lockne crater sweden 2 remote sensing data for three pristine concentric impact craters on mars with preserved ejecta layers further constraining the direction of impact as well as 3 laboratory impact experiments to develop the offset in crater concentricity into a complementary method to determine the direction of impact for layered target craters with poorly preserved ejecta layers

scaling and reproducibility of craters produced at the experimental projectile impact chamber epic centro de astrobiologia 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

ejecta from experimental impact craters particle size distribution and fragmentation energy abstract the particle size distribution psd of impact crater ejecta is an important parameter that is useful for understanding the formation of natural craters the distribution of space debris the influence of impact events on climate and energy partitioning in impact events 11 impact experiments into dry and water saturated sandstone were performed and analyzed the experiments span a range of impact velocities from 2 5 to 5 3 km s 1 using projectile sizes from 2 5 to 12 mm kinetic impact energies between 874 and 80 338 j were achieved ejecta of these experiments was collected and the psd was measured and quantified with power law fits the resulting power law exponents lie between 2 54 and 2 74 our results do not show an influence of impact energy or impact velocity on the psd of impact ejecta a significant increase in the psd values was found from dry to water saturated sandstone targets we suggest that water saturation of the target has multiple effects on ejecta fragmentation a comparison of our experimental data with data from the literature shows no correlation between the target material lithology and the ejecta psd interestingly literature data for disruption experiments revealed a strong influence imparted energy density on the d values psd values were used to calculate the energy spent for target fragmentation and show that the fraction of impact energy used for comminution is in the lower single digit percentage

application of nondestructive testing methods to study the damage zone underneath impact craters of memin laboratory experiments within the framework of the multidisciplinary experimental and modeling impact research network memin research group the damage zones underneath two experimentally produced impact craters in sandstone targets were investigated using several nondestructive testing ndt methods the 20 20 20 cm sandstones were impacted by steel projectiles with a radius of 1 25 mm at approximately 5 km s 1 resulting in craters with approximately 6 cm diameter and approximately 1 cm depth ultrasound us tomography and vibrational analysis were applied before and after the impact experiments to characterize the damage zone and micro computer tomography l ct measurements were performed to visualize subsurface fractures the newly obtained experimental data can help to quantify the extent of the damage zone which extends to about 8 cm depth in the target the impacted sandstone shows a local p wave reduction of 18 below the crater floor and a general reduction in elastic moduli by between approximately 9 and approximately 18 depending on the type of elastic modulus the results contribute to a better empirical and theoretical understanding of hypervelocity events and simulations of cratering processes

sediment transportation and deposition by rain triggered lahars at merapi volcano central java indonesia abstract a lahar is a general term for a rapidly flowing mixture of rock debris and water other than normal streamflow from a volcano and refers to the moving flow located in the populated area of central java the stratovolcano merapi 2965 m is prone to lahar generation due to three main factors 1 millions of cubic meters of pyroclastic deposits are the product of frequent pyroclastic flows which have occurred on 2 to 4 year intervals 2 rainfall intensity is high often 40 mm in 2 h on average during the rainy season from november to april and 3 drainage pattern is very dense following the 22 november 1994 eruption of merapi 31 rain triggered lahar events were recorded in the boyong river between december 1994 and may 1996 on merapi s slopes instantaneous sediment concentration at any given time of the lahars varies widely over time and space lahars are transient sediment water flows whose properties are unsteady so that the sediment load fluctuates during the flow the boundary between the flow types debris flow with sediment concentration 60 volume or hyperconcentrated flow with sediment concentration ranges from 20 to 60 volume may fluctuate within the flow itself grain size distribution physical composition of sediments shear stress yield stress and water temperature play each a role on this boundary natural self damming and rapid breakout are partly responsible for the sediment variations of the flows debris flow phases at merapi typically last a few minutes to 10 min and are often restricted to the lahar front debris flow surges are sometimes preceded and always followed by longer hyperconcentrated flow phases as a result mean sediment concentration of the lahars is low commonly from 20 to 50 volume besides transient normal streamflow phases sediment concentration low sediment load and frequent transient flows in the merapi channels may result from at least three factors 1 several breaks in slope along the channel increase the deposition rate of sediment and hinder the bulking capacity of the lahars 2 source material is mainly coarse debris of merapi type block and ash flows consequently the remobilization of coarse debris is more difficult and the clast deposition is accelerated 3 variations of rainfall intensity over time and space common in tropical monsoon rainfall also influence the sediment load variations of the lahars sedimentologic analyses of the lahar deposits in the boyong river at merapi encompass clast supported and matrix supported debris flow deposits hyperconcentrated flow deposits and streamflow deposits the stratigraphic succession of massive and stratified beds observed immediately after any given lahar event in the boyong river indicates that the sediment concentration varies widely over time and space during a single lahar event sedimentation rate varies from 3 to 4 5 cm min during relatively long lived surges to as much as 20 cm min during short lived surges these results indicate that the sediment load fluctuates during lahar flow further demonstrating that lahars are transient sediment water flows with unsteady flow properties

experimental ejection angles for oblique impacts implications for the subsurface flow field a simple analytical solution for subsurface particle motions during impact cratering is useful for tracking the evolution of the transient crater shape at late times a specific example of such an analytical solution is maxwells z model which is based on a point source assumption here the parameters for this model are constrained using measured ejection angles from both vertical and oblique experimental impacts at the nasa ames vertical gun range data from experiments reveal that impacts at angles as high as 45 to the targets surface generate subsurface flow fields that are significantly different from those created by vertical impacts the initial momentum of the projectile induces a subsurface momentum driven flow field that evolves in three dimensions of space and in time to an excavation flow field during both vertical and oblique impacts a single stationary pointsource model specifically maxwells z model however is found inadequate to explain this detailed evolution of the subsurface flow field during oblique impacts because 45 is the most likely impact angle on planetary surfaces a new analytical model based on a migrating point source could prove quite useful such a model must address the effects of the subsurface flow field evolution on crater excavation ejecta deposition and transient crater morphometry

impact production of co2 by the cretaceous tertiary extinction bolide and the resultant heating of the earth evidence at the cretaceous tertiary boundary suggests that the proposed extinction bolide struck a continental or shallow marine terrane this evidence includes shocked quartz and feldspar grains found in the boundary layer inherited from a range of rock types a high 87 sr 86 sr ratio in some planktonic fossils which could reflect continental derived sr ref 5 and evidence that the platinum group element rich clay layer is underlain at some localities by a deposit of possible tsunamic origin these observations and data demonstrate that sea level at the end of the cretaceous was 150 200 m higher than at present suggesting the possibility that the extinction bolide struck a shallow marine carbonate rich sedimentary section here we show that the impact of such a bolide 5km in radius onto a carbonate rich terrane would increase the co 2 content of the atmosphere by a factor of two to ten additional dissolution of co 2 from the ocean s photic zone could release much larger quantities of co 2 the impact induced release of co 2 by itself would enhance atmospheric greenhouse heating and give rise to a worldwide increase in temperature from 2 k to 10 k for periods of 10 4 to 10 5 years

hypervelocity impacts on dry and wet sandstone observations of ejecta dynamics and crater growth this study deals with the investigation of highly dynamic processes associated with hypervelocity impacts on porous sandstone for the impact experiments two light gas accelerators with different calibers were used capable of accelerating steel projectiles with diameters ranging from 2 5 to 12 mm to several kilometers per second the projectiles impacted on dry and water saturated seeberger sandstone targets the study includes investigations of the influence of pore water on the shape of the ejecta cloud as well as transient crater growth the results show a significant influence of pore water on ejecta behavior steeper ejecta cone angles are observed if the impacts are conducted on wet sandstones the transient crater grows at a faster rate and reaches a larger diameter if the target is water saturated in our experiments target porosity leads to smaller crater sizes compared with nonporous targets water within the pore space reduces porosity and counteracts this process power law fits were applied to the crater growth curves the results show an increase in the scaling exponent with increasing pore space saturation

shock experiments with the h6 chondrite kernouve pressure calibration of microscopic shock effects shock recovery experiments were carried out on samples of the h6 chondrite kernouve at shock pressures of 10 15 20 25 30 35 45 and 60 gpa and preheating temperatures of 293 k low temperature experiments and 920 k high temperature experiments using a calculated equation of state of kernouve pressure pulse durations of 0 3 to 1 2 s were estimated the shocked samples were investigated by optical microscopy to calibrate the various shock effects in olivine orthopyroxene oligoclase and troilite the following pressure calibration is proposed for silicates 1 undulatory extinction of olivine gpa 2 weak mosaicism of olivine from 10 15 gpa to 20 25 gpa 3 onset of strong mosaicism of olivine at 20 25 gpa 4 transformation of oligoclase to diaplectic glass completed at 25 30 gpa low temperature experiments and at 20 25 gpa high temperature experiments 5 onset of weak mosaicism in orthopyroxene at 30 35 gpa low temperature experiments and at 25 30 gpa high temperature experiments and 6 recrystallization or melting of olivine starting at 45 60 gpa low temperature experiments and at 35 45 gpa high temperature experiments and completed above 45 60 gpa in the high temperature experiments troilite displays distinct differences between the samples shocked at low and high temperatures in the low temperature experiments the following effects can be observed in troilite 1 undulatory extinction up to 25 gpa 2 twinning up to 45 gpa 3 partial recrystallization from 30 to 60 gpa and 4 complete recrystallization 35 gpa whereas in the high temperature experiments troilite shows 1 complete recrystallization from 10 up to 45 gpa and 2 melting and crystallization above 45 gpa localized shock induced melting is observed in samples shocked to pressures 15 gpa in the high temperature experiments and 30 gpa for the low temperature experiments in the form of feni metal and troilite melt injections and intergrowths and as pockets and veins of whole rock melt obviously the onset and abundance of shock induced localized melting strongly depends on the initial temperature of the sample

target projectile interaction during impact melting at kamil crater egypt abstract in small meteorite impacts the projectile may survive through fragmentation in addition it may melt and chemically and physically interact with both shocked and melted target rocks however the mixing mingling between projectile and target melts is a process still not completely understood kamil crater 45 m in diameter egypt generated by the hypervelocity impact of the gebel kamil ni rich ataxite on sandstone target allows to study the target projectile interaction in a simple and fresh geological setting we conducted a petrographic and geochemical study of macroscopic impact melt lapilli and bombs ejected from the crater which were collected during our geophysical campaign in february 2010 two types of glasses constitute the impact melt lapilli and bombs a white glass and a dark glass the white glass is mostly made of sio 2 and it is devoid of inclusions its negligible ni and co contents suggest derivation from the target rocks without interaction with the projectile 2 o 3 0 84 18 7 wt feo t 1 83 61 5 wt and nio 2 melt that eventually solidified as white glass behaved as an immiscible liquid and did not interact with the projectile during the excavation stage dark glass melt engulfed and coated the white glass melt target fragments and got stuck to iron meteorite shrapnel fragments this model could also explain the common formation of white and dark glasses in small impact craters generated by iron bodies e g wabar

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

deep impact excavating comet tempel 1 deep impact collided with comet tempel 1 excavating a crater controlled by gravity the comet s outer layer is composed of 1 to 100 micrometer fine particles with negligible strength 1000 kelvins a large increase in organic material occurred during and after the event with smaller changes in carbon dioxide relative to water on approach the spacecraft observed frequent natural outbursts a mean radius of 3 0 0 1 kilometers smooth and rough terrain scarps and impact craters a thermal map indicates a surface in equilibrium with sunlight

shock induced melting and vaporization of shatter cone surfaces evidence from the sudbury impact structure analytical scanning electron microscopy has been used to investigate the surface textures and compositions of newly exposed shatter cones from the 1 85 ga sudbury impact structure canada unusual surface microstructures are observed at the micron scale including silicate melt smears melt fibres and melt splats silicate and ni rich spherules up to 5 m in diameter adorn earlier formed surface features and we interpret these to be condensates formed due to shock induced vaporization of the shatter cone surfaces the development of striations on the shatter cones is attributed to shock related fracture and slip formation of melts and spherules indicates that the highest ranks of shock metamorphism stages iv and v were realized but only on a very localized scale shatter cone surfaces are therefore likely sites for the development of high pressure polymorphs and if the chemistry is appropriate fullerenes as such they may be equivalent to type a pseudotachylytes and shock veins in meteorites

shocked h2o ice thermal emission measurements and the criteria for phase changes during impact events 1 impact cratering events on icy planetary bodies may produce transient liquid water and vapor we present the first thermal emission measurements from shocked h2o ice and derive peak and post shock temperatures under shock pressures between 8 2 and 13 6 gpa initially 165 k ice is heated to between 673 and 1055 k in the time frame of the experiment the shocked h2o releases to the saturation vapor curve and does not achieve full decompression the temperature results are used to validate the new 5 phase h2o model equation of state eos the 5 phase eos is used to predict the critical shock pressures required to induce melting and vaporization of ice for a wide range of ambient pressures and temperatures impact events with velocities as low as 1 km s will initiate phase changes on icy surfaces thus shock induced melting and vaporization of ice is a widespread process in the solar system

anatomy of black veins in zagami clues to the formation of high pressure phases abstract black melt veins in the martian basaltic meteorite zagami were investigated using analytical transmission electron microscopy with the goal to constrain the mechanisms and ptt conditions that result in the formation of high pressure phases therein our study reveals the existence of a number of high pressure minerals stishovite k and na ca rich hollandites akimotoite silicate ilmenite as well as amorphous grains of silicate perovskite composition crystals of a ferro magnesian silicate titanite with a low ca content have been identified for the first time the veins formed by rapid shear melting and solidified in extremely short times as a consequence the vein melt could not homogeneously mix and it partially preserves the chemical heterogeneity of precursor minerals accounting for the large diversity of high pressure minerals most of the tiny phases directly crystallized from the melt under high pressure at very high temperatures silicate perovskite probably amorphized upon decompression and evolved at its grain boundaries by solid state diffusion toward the lower pressure assemblage wustite stishovite not only do these observations help to understand the formation of high pressure phases in shock events but they also provide clues to the mineralogical complexity that can be expected in planetary mantles

how clay grain coats inhibit quartz cement and preserve porosity in deeply buried sandstones observations and experiments observations and hydrothermal experiments were used to derive new information about how clay grain coats inhibit quartz cement and preserve porosity in deeply buried sandstones samples of deeply buried porous sandstones with different types of clay coats were split in two coats removed from one of each pair of splits and grain surfaces inspected with scanning electron microscopy quartz grains in a fluvial deltaic sandstone buried to 115c had no visible authigenic quartz on grain surfaces cleaned of diagenetic chlorite coats though well developed overgrowths occurred on nearby naturally uncoated grains however in similar sandstones buried to 164c quartz grain surfaces exposed by chlorite coat removal were covered with small 5 m mainly anhedral syntaxial quartz overgrowths similar overgrowths were observed under various detrital and diagenetic clay coats in porous eolian sandstones buried to temperatures up to 215c we conclude that clay coats may retard quartz nucleation at moderate temperatures but at high temperatures many coats permit quartz nucleation and preserve porosity by limiting cement growth to investigate cement growth limitation mechanisms samples with coats removed were subjected to quartz cementing conditions in a hydrothermal reactor during experiments the naturally occurring small overgrowths on clay cleaned grains coalesced and grew suggesting that clay particles in coats inhibit cement growth by forming barriers to early overgrowth coalescence although the fraction of grain surface coverage is the primary control on cement inhibition by coats cement growth interference textures vary with coat type providing a mechanism by which coat composition may be a secondary control on inhibitory effectiveness in deeply buried sandstones quartz cement can fill significant microporosity within diagenetic chlorite coats potentially affecting mechanical and petrophysical rock properties

shock experiments on pre heated α and β quartz i optical and density data abstract discs of single crystal quartz unheated and pre heated to 275 c and 540 c i e quartz and 630 c i e quartz were experimentally shocked to pressures ranging from 20 to 40 gpa with the shock front propagating parallel to either 10 1 0 or 0001 refractive indices density and the orientation of planar deformation features pdfs were determined on the recovered quartz samples refractive indices of pre heated quartz are unaffected up to 25 gpa but density starts to decrease slightly up to this pressure above 25 gpa pre heating causes drastic variations refractive indices and birefringence of quartz shocked at ambient temperature decrease continuously until complete isotropization is reached at 35 gpa in quartz shocked at 630 c refractivity drops discontinuously in the interval from 25 to 26 gpa and complete transformation to diaplectic glass is reached at 26 gpa density follows the trends demonstrated by the optical parameters with higher pre shock temperatures yielding lower density at a given shock pressure these results indicate that the threshold pressure for the onset of transformation to diaplectic quartz glass is largely temperature invariant lying at 25 gpa whereas the pressure limit for complete transformation decreases with increasing pre shock temperature from 35 to 26 gpa quartz shocked parallel to 0001 always has a higher density and refractivity than that shocked parallel to 10 1 0 indicating a significant influence of the structural anisotropy this is also evident from the distribution of pdf orientations pressures 25 gpa cause in quartz shocked parallel to 10 1 0 pdfs that are predominantly oriented parallel to 10 1 2 while quartz shocked to the same pressures but parallel to 0001 contains almost exclusively pdfs parallel to 10 1 3 pdf orientations in quartz shocked at ambient temperature parallel to 10 1 0 show the following characteristics i the frequency of 10 1 3 is high at 20 gpa declines with increasing pressure and finally is totally absent at 32 gpa ii 10 1 3 begins to occur at 25 gpa increases and finally is the only remaining orientation at 32 gpa and iii 10 1 2 and 11 2 2 are only present at 20 and 25 gpa in contrast quartz shocked at 630 c parallel to 10 1 0 displays a broad pdf distribution with indistinct maxima and 10 1 3 is totally absent the small structural difference between and quartz which is reflected in the pdf distribution should allow in principle evaluation of the pre shock temperature in naturally impacted metamorphic rocks the results substantiate that formation of shock effects in quartz is dependent on i shock pressure ii pre shock temperature and iii shock wave direction so far the latter two parameters have not been considered in the application of experimental results to impact sites

shock metamorphism of quartz in nature and experiment ii significance in geoscience the occurrence of shock metamorphosed quartz is the most common petrographic criterion for the identification of terrestrial impact structures and lithologies its utility is due to its almost ubiquitous occurrence in terrestrial rocks its overall stability and the fact that a variety of shock metamorphic effects occurring over a range of shock pressures have been well documented these shock effects have been generally duplicated in shock recovery experiments and thus serve as shock pressure barometers after reviewing the general character of shock effects in quartz the differences between experimental and natural shock events and their potential effects on the shock metamorphism of quartz are explored the short pulse lengths in experiments may account for the difficulty in synthesizing the high pressure polymorphs coesite and stishovite compared to natural occurrences in addition post shock thermal effects are possible in natural events which can affect shock altered physical properties such as refractive index and cause annealing of shock damage and recrystallization the orientations of planar microstructures however are unaffected by post impact thermal events except if quartz is recrystallized and provide the best natural shock barometer in terms of utility and occurrence the nature of planar microstructures particularly planar deformation features pdfs is discussed in some detail and a scheme of variations in orientations with shock pressure is provided the effect of post impact events on pdfs is generally limited to annealing of the original glass lamellae to produce decorated pdfs resulting from the exsolution of dissolved water during recrystallization basal 0001 pdfs differ from other pdf orientations in that they are multiple mechanical brazil twins which are difficult to detect if not partially annealed and decorated the occurrence and significance of shock metamorphosed quartz and its other phases namely coesite stishovite diaplectic glass and lechatelierite are discussed for terrestrial impact structures in both crystalline non porous and sedimentary porous targets the bulk of past studies have dealt with crystalline targets where variations in recorded shock pressure in quartz have been used to constrain aspects of the cratering process and to estimate crater dimensions at eroded structures in sedimentary targets the effect of pore space results in an inhomogeneous distribution in recorded shock pressure and temperature which requires a different classification scheme for the variation of recorded shock compared to that in crystalline targets this is discussed along with examples of variations in the relative abundances of planar microstructures and their orientations which are attributed to textural variations in sedimentary target rocks examples of the shock metamorphism of quartz in distal ejecta such as at the k t boundary and from nuclear explosions are illustrated and are equivalent to that of known impact structures except with respect to characteristics that are due to long term post shock thermal effects finally the differences between the deformation and phase transformation of quartz by shock and by endogenic tectonic and volcanic processes are discussed we confirm previous conclusions that they are completely dissimilar in character due to the vastly different physical conditions and time scales typical for shock events compared to tectonic and volcanic events well characterized and documented shock effects in quartz are unequivocal indicators of impact in the natural environment

the extra large light gas gun of the fraunhofer emi applications for impact cratering research abstract the extra large light gas gun xllgg at the fraunhofer ernst mach institut emi efringen kirchen germany is a two stage light gas gun that can accelerate projectile masses of up to 100 g up to velocities of 6 km s 1 the accelerator s set up allows various combinations of pump and launch tubes for applications in different fields of hypervelocity impact research in the framework of the memin multidisciplinary experimental and modeling impact research network program the xllgg is used for mesoscale cratering experiments with projectiles made of steel and of iron meteorites and targets consisting of sandstone and other rocks the craters produced with this equipment reach a diameter of up to 40 cm a size unique in laboratory cratering research with the implementation of neural networks the acceleration process is being optimized currently yielding peak velocities of 7 8 km s 1 for a 100 g projectile here we summarize technical aspects of the xllgg

vredefort shatter cones revisited shatter cones have been described from a number of circular and polygonal structures worldwide the origin of which has been alternatively ascribed to the impacts of large extraterrestrial projectiles or to catastrophic endogenic processes despite their association with enigmatic catastrophic processes the nature of shatter cones and the physics involved in their formation have not been comprehensively researched results of detailed field and laboratory studies of shatter cones from three areas in the collar of the vredefort dome in south africa are presented vredefort shatter cones are directly related to a widely displayed fracture phenomenon termed multiply striated joint sets msjs msjs are planar to curviplanar fractures occuring at spacings of 1 to several millimeters the joint sets have a fractal character when a new measurement protocol is used in the field involving study of all joint surfaces and all steps and striae exposed on these surfaces new information is gained on the genesis and significance of the msjs and on their relationship to striated conical fractures the internal constitution of a rock specimen with msjs was examined in detail by documenting the precise geometry of many fractures in a suite of parallel thin sections transecting the specimen the steps and striae on shatter cone surfaces have the characteristics of displacement fractures microfaults along which evidence of melting is observed shatter cone and msjs surfaces are often covered with glassy films we evaluate whether these fracture phenomena are linked to the formation of pseudotachylitic friction melt our field and petrographic observations can be interpreted as consistent with the generation of shatter cones msjs relatively late in the formation of the vredefort structure this scenario contrasts sharply with the widely held view that shatter cones are formed during the early compression phase of a shock event that affected horizontal strata

crater ejecta scaling laws fundamental forms based on dimensional analysis a model of crater ejecta is constructed using dimensional analysis and a recently developed theory of energy and momentum coupling in cratering events general relations are derived that provide a rationale for scaling laboratory measurements of ejecta to larger events specific expressions are presented for ejection velocities and ejecta blanket profiles in two limiting regimes of crater formation the so called gravity and strength regimes in the gravity regime ejecta velocities at geometrically similar launch points within craters vary as the square root of the product of crater radius and gravity this relation implies geometric similarity of ejecta blankets that is the thickness of an ejecta blanket as a function of distance from the crater center is the same for all sizes of craters if the thickness and range are expressed in terms of crater radii in the strength regime ejecta velocities are independent of crater size consequently ejecta blankets are not geometrically similar in this regime for points away from the crater rim the expressions for ejecta velocities and thickness take the form of power laws the exponents in these power laws are functions of an exponent that appears in crater radius scaling relations thus experimental studies of the dependence of crater radius on impact conditions determine scaling relations for ejecta predicted ejection velocities and ejecta blanket profiles based on measured values of are compared to existing measurements of velocities and debris profiles

quantitative analysis of impact induced seismic signals by numerical modeling we quantify the seismicity of impact events using a combined numerical and experimental approach the objectives of this work are 1 the calibration of the numerical model by utilizing real time measurements of the elastic wave velocity and pressure amplitudes in laboratory impact experiments 2 the determination of seismic parameters such as quality factor q and seismic efficiency k for materials of different porosity and water saturation by a systematic parameter study employing the calibrated numerical model by means of numerical experiments we found that the seismic efficiency k decreases slightly with porosity from k 3 4 10 3 for nonporous quartzite to k 2 6 10 3 for 25 porous sandstone if pores are completely or partly filled with water we determined a seismic efficiency of k 8 2 10 5 which is approximately two orders of magnitude lower than in the nonporous case by measuring the attenuation of the seismic wave with distance in our numerical experiments we determined the seismic quality factor q to range between 35 for the solid quartzite and 80 for the porous dry targets for water saturated target materials q is much lower 10 the obtained values are in the range of literature values translating the seismic efficiency into seismic magnitudes we show that the seismic magnitude of an impact event is about one order of magnitude smaller considering a water saturated target in comparison to a solid or porous target obtained seismic magnitudes decrease linearly with distance to the point of impact and are consistent with empirical data for distances closer to the point of impact the seismic magnitude decreases more rapidly with distance for a water saturated material compared to a dry material

interaction of aluminum projectiles with quartz sand in impact experiments formation of khatyrkite cual 2 and reduction of sio 2 to si abstract we analyzed the interaction of spherical 6 36 mm diameter cu bearing aluminum projectiles with quartz sand targets in hypervelocity impact experiments performed at nasa ames vertical gun range impact velocities and inferred peak shock pressures varied between 5 9 and 6 5 km s and 41 and 48 gpa respectively shocked particles impact melt particles coated with thin crusts of molten projectile material were recovered from the floors of the ca 33 cm diameter craters and the respective ejecta blankets through petrographic and chemical optical microscopy fe empa sem edx and xrf analysis we show that these particles have a layered structure manifested in distinct layers of decreasing shock metamorphism these can be characterized by the following physical and chemical reactions and alteration products i complete melting and subsequent recrystallization of the projectile forming a distinct crystallization texture in the fused metal crust ii projectile target mixing involving a redox reaction between cu bearing al alloy und sio 2 leading to formation of khatyrkite cual 2 al 2 o 3 melt euhedral silicon crystals and spherical droplets of silicon iii melting of quartz to lechatelierite and formation of planar deformation features in relic quartz grains and iv shock lithification of quartz grains with fracturing of grains grain boundary melting planar deformation features and complete loss of porosity to our knowledge this is the first report of khatyrkite formed experimentally in hypervelocity impact experiments these results have implications for the understanding of a similar redox reaction between al cu metal and siliceous impact melt recently postulated for the khatyrka cv3 carbonaceous chondrite moreover these results bear on the processes that lead to layers of regolith on the surfaces of planetary bodies without atmospheres such as asteroids in the main belt e g 4 vesta and on the moon specifically impacts of mm sized projectiles at velocities between 4 and 6 km s into regolith covered asteroidal surfaces in the main belt should yield similar impact melt particles that feature a continuum of shock effects i e partially to completely molten projectile remnants adhering to impact melted regolith agglomerates as well as projectile contaminated impact melts and local shock melting along grain boundaries

a crater and its ejecta an interpretation of deep impact we apply recently updated scaling laws for impact cratering and ejecta to interpret observations of the deep impact event an important question is whether the cratering event was gravity or strength dominated the answer gives important clues about the properties of the surface material of tempel 1 gravity scaling was assumed in pre event calculations and has been asserted in initial studies of the mission results because the gravity field of tempel 1 is extremely weak a gravity dominated event necessarily implies a surface with essentially zero strength the conclusion of gravity scaling was based mainly on the interpretation that the impact ejecta plume remained attached to the comet during its evolution we address that feature here and conclude that even strength dominated craters would result in a plume that appeared to remain attached to the surface we then calculate the plume characteristics from scaling laws for a variety of material types and for gravity and strength dominated cases we find that no model of cratering alone can match the reported observation of plume mass and brightness history instead comet like acceleration mechanisms such as expanding vapor clouds are required to move the ejected mass to the far field in a few hour time frame with such mechanisms and to within the large uncertainties either gravity or strength craters can provide the levels of estimated observed mass thus the observations are unlikely to answer the questions about the mechanical nature of the tempel 1 surface

ejection behavior characteristics in experimental cratering in sandstone targets within the frame of the memin research unit multidisciplinary experimental and numerical impact research network impact experiments on sandstone targets were carried out to systematically study the influence of projectile mass velocity and target water saturation on the cratering and ejection processes the projectiles were accelerated with two stage light gas guns ernst mach institute onto fine grained targets seeberger sandstone with about 23 porosity collection of the ejecta on custom designed catchers allowed determination of particle shape size distribution ejection angle and microstructures mapping of the ejecta imprints on the catcher surface enabled linking of the different patterns to ejection stages observed on high speed videos the increase in projectile mass from 0 067 to 7 1 g correlates with an increase in the total ejected mass ejecta angles however are similar in range for all experiments the increase in projectile velocity from 2 5 to 5 1 km s 1 correlates with a total ejecta mass increase as well as in an increase in comminution efficiency and a widening of the ejecta cone a higher degree of water saturation of the target yields an increase in total ejecta mass up to 400 with respect to dry targets higher ejecta velocity and a steeper cone these data in turn suggest that the reduced impedance contrast between the quartz grains of the target and the pores plays a primary role in the ejecta mass increase while vaporization of water determines the ejecta behavior concerning ejecta velocity and particle distribution

the effect of target properties on transient crater scaling for simple craters the effects of the coefficient of friction and porosity on impact cratering are not sufficiently considered in scaling laws that predict the crater size from a known impactor size velocity and mass we carried out a systematic numerical study employing more than 1000 two dimensional models of simple crater formation under lunar conditions in targets with varying properties a simple numerical setup is used where targets are approximated as granular or brecciated materials and any compression of porous materials results in permanent compaction the results are found to be consistent with impact laboratory experiments for water low strength and low porosity materials e g wet sand and sands using this assumption we found that both the friction coefficient and porosity are important for estimating transient crater diameters as is the strength term in crater scaling laws i e the effective strength the effects of porosity and friction coefficient on impact cratering were parameterized and incorporated into group scaling laws and predict transient crater diameters within an accuracy of 5 for targets with friction coefficients f 0 4 and porosities 0 30 moreover 90 crater scaling relationships are made available and can be used to estimate transient crater diameters on various terrains and geological units with different coefficient of friction porosity and cohesion the derived relationships are most robust for targets with 10 15 applicable for a lunar environment and could therefore yield significant insights into the influence of target properties on cratering statistics

snow carrots after the chelyabinsk event and model implications for highly porous solar system objects after the catastrophic disruption of the chelyabinsk meteoroid small fragments formed funnels in the snow layer covering the ground we constrain the pre impact characteristics of the fragments by simulating their atmospheric descent with the atmospheric entry model fragments resulting from catastrophic breakup may lose about 90 of their initial mass due to ablation and reach the snow vertically with a free fall velocity in the range of 30 90 m s 1 the fall time of the fragments is much longer than their cooling time and as a consequence fragments have the same temperature as the lower atmosphere i e of about 20 c then we use the shock physics code isale to model the penetration of fragments into fluffy snow the formation of a funnel and a zone of denser snow lining its walls we examine the influence of several material parameters of snow and present our best fit model by comparing funnel depth and funnel wall characteristics with observations in addition we suggest a viscous flow approximation to estimate funnel depth dependence on the meteorite mass we discuss temperature gradient metamorphism as a possible mechanism which allows to fill the funnels with denser snow and to form the observed snow carrots this natural experiment also helps us to calibrate the isale code for simulating impacts into highly porous matter in the solar system including tracks in the aerogel catchers of the stardust mission and possible impact craters on the 67p churyumov gerasimenko comet observed recently by the rosetta mission

revision and recalibration of existing shock classifications for quartzose rocks using low shock pressure 2 5 20 gpa recovery experiments and mesoscale numerical modeling a combination of shock recovery experiments and numerical modeling of shock deformation in the low shock pressure range from 2 5 to 20 gpa for two dry sandstone types of different porosity a completely water saturated sandstone and a well indurated quartzite provides new insights into strongly heterogeneous distribution of different shock features 1 for nonporous quartzo feldspathic rocks the traditional classification scheme stoffler 1984 is suitable with slight changes in pressure calibration 2 for water saturated quartzose rocks a cataclastic texture microbreccia seems to be typical for the shock pressure range up to 20 gpa this microbreccia does not show formation of pdfs but diaplectic quartz glass sio2 melt is formed at 20 gpa 1 vol 3 for porous quartzose rocks the following sequence of shock features is observed with progressive increase in shock pressure 1 crushing of pores 2 intense fracturing of quartz grains and 3 increasing formation of diaplectic quartz glass sio2 melt replacing fracturing the formation of diaplectic quartz glass sio2 melt together with sio2 high pressure phases is a continuous process that strongly depends on porosity this experimental observation is confirmed by our concomitant numerical modeling recalibration of the shock classification scheme results in a porosity versus shock pressure diagram illustrating distinct boundaries for the different shock stages

the deep impact crater on 9p tempel 1 from stardust next abstract the stardust next sdn mission returned to comet 9p tempel 1 and viewed the site of the deep impact di collision just over one comet year later comparisons between pre impact images from the its camera on the di probe and sdn images reveal a 50 m diameter crater surrounded by a low rim about 180 m in diameter the removal of a small mound uprange but offset from the trajectory from the impact site can be related to changes in the evolution of ejecta a narrow 6 gap in the ejecta curtain downrange indicates that a ridge extending from the impact facing scarp downrange interrupted the final stages of cratering in one small region together these observations indicate that the di excavation crater diameter was about 200 m 20 m a value consistent with the ejected mass derived from earth and space based observations with the assumption that this mass represents only 10 20 of the total ejected mass as a result the di crater visible today is consistent with either a larger transient crater which collapsed or a central crater of a nested crater resembling an inverted sombrero the latter alternative would be expected from a layered target a loose particulate surface about 1 2 m deep over a slightly more competent substrate

in situ measurements of impact induced pressure waves in sandstone targets in the present study we introduce an innovative method for the measurement of impact induced pressure waves within geological materials impact experiments on dry and water saturated sandstone targets were conducted at a velocity of 4600 m s using 12 mm steel projectiles to investigate amplitudes decay behavior and speed of the waves propagating through the target material for this purpose a special kind of piezoresistive sensor capable of recording transient stress pulses within solid brittle materials was developed and calibrated using a split hopkinson pressure bar experimental impact parameters projectile size and speed were kept constant and yielded reproducible signal curves in terms of rise time and peak amplitudes pressure amplitudes decreased by 3 orders of magnitude within the first 250 mm i e 42 projectile radii the attenuation for water saturated sandstone is higher compared to dry sandstone which is attributed to dissipation effects caused by relative motion between bulk material and interstitial water the proportion of the impact energy radiated as seismic energy seismic efficiency is in the order of 10 3 the present study shows the feasibility of real time measurements of waves caused by hypervelocity impacts on geological materials experiments of this kind lead to a better understanding of the processes in the crater subsurface during a hypervelocity impact

the first memin shock recovery experiments at low shock pressure 5 12 5 gpa with dry porous sandstone as part of the memin research program this project is focused on shock deformation experimentally generated in dry porous seeberger sandstone in the low shock pressure range from 5 to 12 5 gpa special attention is paid to the influence of porosity on progressive shock metamorphism shock recovery experiments were carried out with a high explosive set up that generates a planar shock wave and using the shock impedance method cylinders of sandstone of average grain size of 0 17 mm and porosity of about 19 vol and containing some 96 wt sio2 were shock deformed shock effects induced with increasing shock pressure include 1 already at 5 gpa the entire pore space is closed quartz grains show undulatory extinction on average 134 fractures per mm are observed dark vesicular melt glass of the composition of the montmorillonitic phyllosilicate component of this sandstone occurs at an average amount of 1 6 vol 2 at 7 5 gpa quartz grains show weak but prominent mosaicism and the number of fractures increases to 171 per millimeter two additional kinds of melt both based on phyllosilicate precursor could be observed a light colored vesicular melt and a melt containing large iron particles the total amount of melt all types increased in this experiment to 2 4 vol raman spectroscopy confirmed the presence of shock deformed quartz grains near the surface 3 at 10 and 12 5 gpa quartz grains also show weak but prominent mosaicism the number of fractures per mm has reached a plateau value of approximately 200 and the total amount of the different melt types has increased to 4 8 vol diaplectic quartz glass could be observed locally near the impacted surface in addition local shock effects most likely caused by multiple shock wave reflections at sandstone container interfaces occur throughout the sample cylinders and include locally enhanced formation of pdf as well as shear zones associated with cataclastic microbreccia diaplectic quartz glass and sio2 melt overall findings from these first experiments have demonstrated that characteristic shock effects diagnostic for the confirmation of impact structures and suitable for shock pressure calibration are rare so far they are restricted to the limited formation of pdf and diaplectic quartz glass at shock pressures of 10 gpa and above

microscopic evidence of stishovite generated in low pressure shock experiments on porous sandstone constraints on its genesis it has been almost exactly half a century since the first synthesis of stishovite in shock experiments on quartz was reported but its formation conditions during shock is still under debate here we present direct transmission electron microscopic observation of stishovite within material recovered from high explosive shock experiments on porous sandstone shocked at 7 5 and 12 5 gpa our observations allow for new conclusions on the genesis of stishovite in a close to nature environment the formation of stishovite in short time shock experiments proves that its crystallization is ultrafast 1 s crystals were found only embedded in amorphous veins indicating homogeneous nucleation crystallization from melt rather than from glass can be concluded from the observation of roundish defect free crystals up to 150 nm in diameter embedded in nondensified glass the formation of stishovite at 7 5 gpa is in accordance with the phase diagram of silica if rapid undercooling is present that becomes only possible by the existence of small hot spots in an otherwise cold material which is supported by transient heat calculation the absence of coesite at 7 5 gpa suggests kinetic hindrance of its crystallization from melt and thus smaller critical cooling rates compared to stishovite where critical cooling rates are estimated to be as large as 1011 k s 1 while the amorphous veins containing stishovite represent unambiguously hot spots no associated pressure amplification could be verified within these veins the rapid liquidus crystallization of stishovite only in hot spots generated in porous material is an alternative formation mechanism to the widely accepted theory of solid solid transition from quartz to stishovite and might represent the more general mechanism occurring in nature for low shock pressure events

crater morphology in sandstone targets the memin impact parameter study 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

diaplectic quartz glass and sio2 melt experimentally generated at only 5 gpa shock pressure in porous sandstone laboratory observations and meso scale numerical modeling abstract a combination of shock recovery experiments and numerical modeling of shock deformation in the low pressure range from 2 5 to 17 5 gpa in dry porous seeberger sandstone provides new significant insights with respect to the heterogeneous nature of shock distribution in such important upper crustal material for which to date no pressure calibrated scheme for shock metamorphism exists we found that pores are already completely closed at 2 5 gpa shock pressure whole quartz grains or parts of them are transformed to diaplectic quartz glass and or sio 2 melt starting already at 5 gpa whereas these effects are not observed below shock pressures of 30 35 and 45 gpa respectively in shock experiments with quartz single crystals the appearance of diaplectic glass or melt is not restricted to the zone directly below the impacted surface but is related to the occurrence of pores in a much broader zone the combined amount of these phases increases distinctly with increasing shock pressure from 0 03 vol at 5 gpa to 80 vol at 17 5 gpa in accordance with a previous shock classification for silica phases in naturally shocked coconino sandstone from meteor crater that was based on varied slopes of the coconino sandstone hugoniot curve our observations allow us to construct a shock pressure classification for porous sandstone consistent with shock stages 1b 4 of the progressive shock metamorphism classification of kieffer 1971 numerical modeling at the meso scale provides the explanation for the discrepancy of shock deformation in porous material and single crystal quartz in keeping with our experimental results it confirms that pore space is completely collapsed at low nominal pressure and demonstrates that pore space collapse results in localized pressure amplification that can exceed 4 times the initial pressure this provides an explanation for the formation of diaplectic quartz glass and lechatelierite as observed in the low shock pressure experiments the numerical models predict an amount of sio 2 melt similar to that observed in the shock experiments this also shows that numerical models are essential to provide information beyond experimental capabilities

non intrusive measurements of crater growth abstract an experimental technique to measure crater growth is presented whereby a high speed video captures profiles of a crater forming after impact obtained using a vertical laser sheet centered on the impact point unlike previous so called quarter space experiments where projectiles were launched along a transparent plexiglas sheet so that growth of half a crater could be viewed the use of the laser sheet permits viewing changes in crater shape without any physical interference to the cratering process this technique indicates that for low velocity impacts 300 m s into 220 m glass beads that are without cohesion and where the projectile is not disrupted craters initially grow somewhat proportionally but that later their depths remain essentially constant while their diameters continue to expand in addition these experiments indicate that as the impact velocity increases the rate of growth and the transient depth to diameter ratio at the end of ejecta excavation decreases these last two observations are probably due to the large time of penetration of the projectile which becomes a significant fraction of the time of crater formation this is contrary to the expectations for the scaling rules which assumes a point source very high curtain angles 45 are also seen and could be due to the low friction angle of the target significant crater modification which is rarely seen in quarter space experiments is also observed and appears to be controlled by the dynamic angle of repose of the target these latter observations indicate that differences in target friction angles may need to be considered when determining near rim ejecta mass distributions and large scale crater modification processes on the planets

laboratory investigations of impact generated plasma the production of magnetic fields by hypervelocity meteoroid impact has been proposed to explain the presence of paleomagnetic fields in some lunar samples as well as on the lunar surface impact generated magnetic fields also may be significant for the paleomagnetic record on a variety of cratered surfaces in the solar system such as the moon mercury phobos and asteroids previous experiments with the two stage hydrogen light gas gun at the nasa ames vertical gun range demonstrated that hypervelocity impacts can produce impact generated magnetic fields by the expansion of an impact derived ionized vapor cloud impact generated plasma here we further characterize impact generated magnetic fields and plasma with hypervelocity impact experiments at the nasa ames vertical gun spherical projectiles 0 32 cm fe cu 0 64 cm al nylon with velocities from 5 2 to 6 0 km s were impacted at impact angles of 15 30 60 and 90 measured from horizontal into powdered dolomite silica sand and aluminum plate three sets of experiments using search coils as magnetic detectors characterized impact generated magnetic fields as functions of ambient field strength and orientation projectile target composition and impact angle experiments using langmuir probes indicated a charged particle density between 109 and 1011 ions el cm 3 and an electron temperature 4500 k of the impact generated plasma the inferred source of impact induced magnetic fields these new results demonstrate that impact generated magnetic fields at the laboratory scale exhibit spatial and temporal complexity dependent on impact angle velocity and projectile target composition thereby suggesting that crater related paleomagnetism associated with this mechanism should exhibit similar complexity with spatial wavelengths on the order of a fraction of the crater radius