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

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

incompletely compacted equilibrated ordinary chondrites we document the size distributions and locations of voids present within five highly porous equilibrated ordinary chondrites using high resolution synchrotron x ray microtomography ct and helium pycnometry we found total porosities ranging from 10 to 20 within these chondrites and with ct we show that up to 64 of the void space is located within intergranular voids within the rock given the low s1 s2 shock stages of the samples and the large voids between mineral grains we conclude that these samples experienced unusually low amounts of compaction and shock loading throughout their entire post accretionary history with fe metal and fes metal abundances and grain size distributions we show that these chondrites formed naturally with greater than average porosities prior to parent body metamorphism these materials were not fluffed on their parent body by impact related regolith gardening or events caused by seismic vibrations samples of all three chemical types of ordinary chondrites ll l h are represented in this study and we conclude that incomplete compaction is common within the asteroid belt

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

ejecta thickness and structural rim uplift measurements of martian impact craters implications for the rim formation of complex impact craters the elevated rim in simple craters results from the structural uplift of preimpact target rocks and the deposition of a coherent proximal ejecta blanket at the outer edge of the transient cavity given the considerable widening of the transient cavity during crater modification and ejecta thickness distributions the cause of elevated crater rims in complex craters is less obvious the thick proximal ejecta in complex impact craters is deposited well inside the final crater rim and target thickening should rapidly diminish with increasing distance from the transient cavity rim our study of 10 complex martian impact craters ranging from 8 2 to 53 0 km in diameter demonstrates that the mean structural rim uplift at the final crater rim makes 81 of the total rim elevation while the mean ejecta thickness contributes 19 thus the structural rim uplift seems to be the dominant factor to build up the total amount of the raised crater rim of complex craters to measure the widening of the transient cavity during modification and the distance between the rim of the final crater and that of the transient cavity we constructed balanced cross section restorations to estimate the transient cavity of nine complex martian impact craters the final crater radii are 1 38 1 87 times the transient cavity radii we propose that target uplift at the position of the final crater rim was established during the excavation stage

the canyon diablo impact event projectile motion through the atmosphere meteor crater is one of the first impact structures systematically studied on earth its location in arid northern arizona has been ideal for the preservation of the structure and the surviving meteoric material the recovery of a large amount of meteoritic material in and around the crater has allowed a rough reconstruction of the impact event an iron object 50 m in diameter impacted the earths surface after breaking up in the atmosphere the details of the disruption however are still debated the final crater morphology deep bowl shaped crater rules out the formation of the crater by an open or dispersed swarm of fragments in which the ratio of swarm radius to initial projectile radius cd is larger than 3 the final crater results from the sum of the craters formed by individual fragments on the other hand the lack of significant impact melt in the crater has been used to suggest that the impactor was slowed down to 12 km s by the atmosphere implying significant fragmentation and fragments separation up to 4 initial radii this paper focuses on the problem of entry and motion through the atmosphere for a possible canyon diablo impactor as a first but necessary step for constraining the initial conditions of the impact event which created meteor crater after evaluating typical models used to investigate meteoroid disruption such as the pancake and separated fragment models we have carried out a series of hydrodynamic simulations using the 3d code sova to model the impactor flight through the atmosphere both as a continuum object and a disrupted swarm our results indicate that the most probable pre atmospheric mass of the meteor crater projectile was in the range of 4x10 8 to 1 2x10 9 kg equivalent to a sphere 4666 m in diameter during the entry process the projectile lost probably 30 to 70 of its mass mainly because of mechanical ablation and gross fragmentation even in the case of a tight swarm of particles cd 3 small fragments can separate from the crater forming swarm and land on the plains tens of km away from the crater as individual meteorites starting from an impactor pre atmospheric velocity of 18 km s which represents an average value for earth crossing asteroids we find that after disruption the most probable impact velocity at the earths surface for a tight swarm is around 15 km s or higher a highly dispersed swarm would result in a much stronger deceleration of the fragments but would produce a final crater much shallower than observed at meteor crater

the formation of peak ring basins working hypotheses and path forward in using observations to constrain models of impact basin formation abstract impact basins provide windows into the crustal structure and stratigraphy of planetary bodies however interpreting the stratigraphic origin of basin materials requires an understanding of the processes controlling basin formation and morphology peak ring basins exhibiting a rim crest and single interior ring of peaks provide important insight into the basin formation process as they are transitional between complex craters with central peaks and larger multi ring basins new image and altimetry data from the lunar reconnaissance orbiter as well as a suite of remote sensing datasets have permitted a reassessment of the origin of lunar peak ring basins we synthesize morphometric spectroscopic and gravity observations of lunar peak ring basins and describe two working hypotheses for the formation of peak rings that involve interactions between inward collapsing walls of the transient cavity and large central uplifts of the crust and mantle major facets of our observations are then compared and discussed in the context of numerical simulations of peak ring basin formation in order to plot a course for future model refinement and development

the early impact histories of meteorite parent bodies we have developed a statistical framework that uses collisional evolution models shock physics modeling and scaling laws to determine the range of plausible collisional histories for individual meteorite parent bodies it is likely that those parent bodies that were not catastrophically disrupted sustained hundreds of impacts on their surfaces compacting heating and mixing the outer layers it is highly unlikely that many parent bodies escaped without any impacts processing the outer few kilometers the first 10 20 myr were the most important time for impacts both in terms of the number of impacts and the increase of specific internal energy due to impacts the model has been applied to evaluate the proposed impact histories of several meteorite parent bodies up to 10 parent bodies that were not disrupted in the first 100 myr experienced a vaporizing collision of the type necessary to produce the metal inclusions and chondrules on the cb chondrite parent around 1 5 of bodies that were catastrophically disrupted after 12 myr sustained impacts at times that match the heating events recorded on the iab winonaite parent body more than 75 of 100 km radius parent bodies which survived past 100 myr without being disrupted sustained an impact that excavates to the depth required for mixing in the outer layers of the h chondrite parent body and to protect the magnetic field on the cv chondrite parent body the crust would have had to have been thick approximately 20 km to prevent it being punctured by impacts

chear variations within a dunite nodule as a function of fluid inclusion morphology combined laser extraction and manometric quadrupole mass spectroscopic measurements permit the simultaneous analysis of co2 he and ar in selected fluid inclusion generations trapped in a dunite nodule from reunion island variations in 40ar36ar in primary fluid inclusions 1000 8500 implies that atmospheric ar is present in all the trapped fluids possibly introduced by percolation of seawater into the magma from which the olivine crystallised later inclusion generations have lower 40ar36ar 500 3500 co23he ratios 1 9 6 109 are at the lower range of reported values from high 3he4he xenoliths 1 20 109 but show no systematic variation with inclusion generation 4he40ar values are constant 2 4 0 1 for all inclusion generations and over wide ranges in co2he 0 2 8 2 105 this is not consistent with the relative solubilities of co2 he and ar in basaltic melts but implies that co2 is determined by some other process possibly the partitioning of co2 into an immiscible phase mixing between high 3he4he high he abundance mantle primitive he mantle with a degassed and incompatible element enriched component can generate the coupled high 3he4he radiogenic sr and pb isotopic compositions of reunion lavas the range in c3he likely for the primitive he mantle 1 20 109 spans the estimated c3he of chondrites 2 109 the primitive he component cannot represent undegassed primitive mantle as the c36ar values of reunion 1 109 are higher than those of chondrites 8 107

a plagioclase rich rock measured by yutu 2 rover in von karman crater on the far side of the moon abstract the chang e 4 spacecraft successfully landed in von karman crater inside the south pole aitken basin on the lunar far side on january 3 2019 and the yutu 2 rover has performed explorations on the lunar surface for nine lunar days as of september 2019 our earlier analysis of the visible and near infrared spectrometer measurements made by the yutu 2 rover during the first two lunar days shows that the regolith of the landing site may have come from the nearby finsen crater and is dominated by plagioclase with lesser amount of mafic minerals during its third lunar day explorations the yutu 2 photographed a small piece of lunar rock and measured its reflectance spectra compared with the spectra of its surrounding regolith this rock s spectrum has deeper absorption features indicating its fresher nature to obtain the mineralogy of the rock we compared the rock s spectrum with the spectral library data of nasa s reflectance experiment laboratory of returned lunar rocks and lunar meteorites we found that this rock is also plagioclase rich with a possible plagioclase abundance of 60 80 vol a source region analysis using the moon mineralogy mapper s remote sensing observations indicates this rock was ejected from the zhinyu crater about 30 km west of the landing site rather than directly from the finsen crater numerical simulations of the zhinyu crater on the impact cratering process and ejecta thickness distribution confirmed our findings and imply that the surficial materials at the ce 4 landing site experienced a complicated evolution rather than simply retaining the pristine or primordial ejecta directly from the finsen crater

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

consequences of large impacts on enceladus core shape the intense activity on enceladus suggests a differentiated interior consisting of a rocky core an internal ocean and an icy mantle however topography and gravity data suggests large heterogeneity in the interior possibly including significant core topography in the present study we investigated the consequences of collisions with large impactors on the core shape we performed impact simulations using the code isale2d considering large differentiated impactors with radius ranging between 25 and 100 km and impact velocities ranging between 0 24 and 2 4 km s our simulations showed that the main controlling parameters for the post impact shape of enceladus rock core are the impactor radius and velocity and to a lesser extent the presence of an internal water ocean and the porosity and strength of the rock core for low energy impacts the impactors do not pass completely through the icy mantle subsequent sinking and spreading of the impactor rock core lead to a positive core topographic anomaly for moderately energetic impacts the impactors completely penetrate through the icy mantle inducing a negative core topography surrounded by a positive anomaly of smaller amplitude the depth and lateral extent of the excavated area is mostly determined by the impactor radius and velocity for highly energetic impacts the rocky core is strongly deformed and the full body is likely to be disrupted explaining the long wavelength irregular shape of enceladus core by impacts would imply multiple low velocity 2 4 km s collisions with deca kilometric differentiated impactors which is possible only after the lhb period

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

ries crater and suevite revisited observations and modeling part ii modeling we present the results of numerical modeling of the formation of the ries crater utilizing the two hydrocodes sova and isale these standard models allow us to reproduce crater shape size and morphology and composition and extension of the continuous ejecta blanket some of these results cannot however be readily reconciled with observations the impact plume above the crater consists mainly of molten and vaporized sedimentary rocks containing very little material in comparison with the ejecta curtain at the end of the modification stage the crater floor is covered by a thick layer of impact melt with a total volume of 6 11 km3 the thickness of true fallback material from the plume inside the crater does not exceed a couple of meters ejecta from all stratigraphic units of the target are transported ballistically no separation of sedimentary and crystalline rocks as observed between suevites and bunte breccia at ries is noted we also present numerical results quantifying the existing geological hypotheses of ries ejecta emplacement from an impact plume by melt flow or by a pyroclastic density current the results show that none of these mechanisms is consistent with physical constraints and or observations finally we suggest a new hypothesis of suevite formation and emplacement by postimpact interaction of hot impact melt with water or volatile rich sedimentary rocks

investigating the onset of multi ring impact basin formation multi ring basins represent some of the largest oldest rarest and therefore least understood impact crater structures various theories have been put forward to explain their formation there is currently however no consensus here numerical modeling is used to investigate the onset of multi ring basin formation on the moon using two thermal profiles suitable for the lunar basin forming epoch various multi ring basin formation hypotheses are discussed compared and evaluated against target deformation and strain distribution in the models as well as geological and geophysical observations the mechanism that most closely resembles the numerical models in terms of basin formation and structure as well as observations appears to be the ring tectonic theory whereby ring formation is dependent on transient cavities penetrating entirely through the moon s lithosphere into the asthenosphere below the numerical models suggest that all lunar basins larger than schrodinger 320 km diameter should be capable of forming multiple rings as their transient cavities penetrate into the asthenosphere for both thermal profiles additionally the models demonstrate that the target s thermal profile starts to influence basin formation and structure when impact energy exceeds that of the schrodinger event

formation of melt droplets melt fragments and accretionary impact lapilli during a hypervelocity impact abstract we present a model that describes the formation of melt droplets melt fragments and accretionary impact lapilli during a hypervelocity impact using the isale hydrocode coupled to the aneos equation of state for silica we create high resolution two dimensional impact models to track the motion of impact ejecta we then estimate the size of the ejecta products using simple analytical expressions and information derived from our hydrocode models ultimately our model makes predictions of how the size of the ejecta products depends on impactor size impact velocity and ejection velocity in general we find that larger impactor sizes result in larger ejecta products and higher ejection velocities result in smaller ejecta product sizes we find that a 10 km diameter impactor striking at a velocity of 20 km s creates millimeter scale melt droplets comparable to the melt droplets found in the chicxulub ejecta curtain layer our model also predicts that melt droplets melt fragments and accretionary impact lapilli should be found together in well preserved ejecta curtain layers and that all three ejecta products can form even on airless bodies that lack significant volatile content this prediction agrees with observations of ejecta from the sudbury and chicxulub impacts as well as the presence of accretionary impact lapilli in lunar breccia

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

formation of the sputnik planum basin and the thickness of pluto s subsurface ocean we simulate the formation of the large elliptical impact basin associated with pluto s sputnik planum sp informal name the location of sp suggests that it represents a large positive mass anomaly to find the conditions necessary for sp to have a positive mass anomaly we consider impacts into targets with a range of thermal states and ocean thicknesses assuming the basin evolves to its current day configuration we calculate the mass and gravity anomalies associated with sp we find that sp can only achieve a large positive mass anomaly if pluto has a more than 100 km thick salty ocean this conclusion may help us better understand the composition and thermal evolution of pluto furthermore our work supports the hypothesis that sp basin has an impact origin

formation of the orientale lunar multiring basin multiring basins large impact craters characterized by multiple concentric topographic rings dominate the stratigraphy tectonics and crustal structure of the moon using a hydrocode we simulated the formation of the orientale multiring basin producing a subsurface structure consistent with high resolution gravity data from the gravity recovery and interior laboratory grail spacecraft the simulated impact produced a transient crater 390 kilometers in diameter that was not maintained because of subsequent gravitational collapse our simulations indicate that the flow of warm weak material at depth was crucial to the formation of the basin s outer rings which are large normal faults that formed at different times during the collapse stage the key parameters controlling ring location and spacing are impactor diameter and lunar thermal gradients

post impact thermal evolution of porous planetesimals abstract impacts between planetesimals have largely been ruled out as a heat source in the early solar system by calculations that show them to be an inefficient heat source and unlikely to cause global heating however the long term localized thermal effects of impacts on planetesimals have never been fully quantified here we simulate a range of impact scenarios between planetesimals to determine the post impact thermal histories of the parent bodies and hence the importance of impact heating in the thermal evolution of planetesimals we find on a local scale that heating material to petrologic type 6 is achievable for a range of impact velocities and initial porosities and impact melting is possible in porous material at a velocity of 4 km s burial of heated impactor material beneath the impact crater is common insulating that material and allowing the parent body to retain the heat for extended periods millions of years cooling rates at 773 k are typically 1 1000 k ma matching a wide range of measurements of metallographic cooling rates from chondritic materials while the heating presented here is localized to the impact site multiple impacts over the lifetime of a parent body are likely to have occurred moreover as most meteorite samples are on the centimeter to meter scale the localized effects of impact heating cannot be ignored

the carancas meteorite impact crater peru geologic surveying and modeling of crater formation and atmospheric passage the recent carancas meteorite impact event caused a worldwide sensation an h4 5 chondrite struck the earth south of lake titicaca in peru on september 15 2007 and formed a crater 14 2 m across it is the smallest youngest and one of two eye witnessed impact crater events on earth the impact violated the hitherto existing view that stony meteorites below a size of 100 m undergo major disruption and deceleration during their passage through the atmosphere and are not capable of producing craters fragmentation occurs if the strength of the meteoroid is less than the aerodynamic stresses that occur in flight the small fragments that result from a breakup rain down at terminal velocity and are not capable of producing impact craters the carancas cratering event however demonstrates that meter sized stony meteoroids indeed can survive the atmospheric passage under specific circumstances we present results of a detailed geologic survey of the crater and its ejecta to constrain the possible range of impact parameters we carried out numerical models of crater formation with the isale hydrocode in two and three dimensions depending on the strength properties of the target the impact energies range between approximately 100 1000 mj 0 024 0 24 t tnt by modeling the atmospheric traverse we demonstrate that low cosmic velocities 12 1 4 kms 1 and shallow entry angles 20 are prerequisites to keep aerodynamic stresses low 10 mpa and thus to prevent fragmentation of stony meteoroids with standard strength properties this scenario results in a strong meteoroid deceleration a deflection of the trajectory to a steeper impact angle 40 60 and an impact velocity of 350 600 ms 1 which is insufficient to produce a shock wave and significant shock effects in target minerals aerodynamic and crater modeling are consistent with field data and our microscopic inspection however these data are in conflict with trajectories inferred from the analysis of infrasound signals

hydrocode simulations of chicxulub crater collapse and peak ring formation we use hydrocode modeling to investigate dynamic models for the collapse of the chicxulub impact crater our aim is to integrate the results from numerical simulations with kinematic models derived from seismic reflection and wide angle velocity data to further our understanding of the formation of large impact craters in our simulations we model the collapse of a 100 km diameter bowlshaped cavity formed in comprehensively fractured crustal material to facilitate wholesale collapse we require that the strength of the target be significantly weakened in the present model we achieve this using acoustic fluidization where strong vibrations produced by the expanding shock wave cause extreme pressure fluctuations in the target at times and positions where the overburden pressure is sufficiently counteracted the frictional resistance is reduced enabling the rock debris to flow our simulations produce a collapsed crater that contains most of the features that we observe in the seismic data at chicxulub in particular we observe a topographic peak ring formed as material that is originally part of the central uplift collapses outward and is thrust over the inwardly collapsing transient crater rim this model for peak ring generation has not been previously demonstrated by numerical simulations and predicts that the peak ring is composed of deeply derived material and that the stratigraphy within the peak ring is overturned c 2002 elsevier science usa

reconstruction of the morasko meteoroid impact insight from numerical modeling the morasko strewn field located near poznan poland comprises seven impact craters with diameters ranging from 20 to 90 m all of which were formed in glacial sediments around 5000 yr ago numerous iron meteorites have been recovered in the area and their distribution suggests a projectile with the trajectory from ne to sw similar impact events producing crater strewn fields on average happen every 500 yr and pose a serious risk for modern civilization which is why it is of utmost importance to study terrestrial strewn fields in detail in this work we investigate the morasko meteoroid passage through the atmosphere the distribution of its fragments on the ground and the process of forming individual craters by means of numerical modeling by combining atmospheric entry modeling pi group scaling of transient crater size and hydrocode simulations of impact processes we constructed a comprehensive model of the morasko strewn field formation we determined the preatmospheric parameters of the morasko meteoroid the entry mass is between 600 and 1100 tons the velocity range is between 16 and 18 km s 1 and the trajectory angle is 30 40 such entry velocities and trajectory angles do not deviate from typical values for near earth asteroids although the initial mass we determined can be considered as small our studies on velocities and masses of crater forming fragments showed that the biggest morasko crater was formed by a projectile about 1 5 m in diameter with the impact velocity 10 km s 1 environmental consequences of the morasko impact event are very localized

shock metamorphism and impact melting in small impact craters on earth evidence from kamil crater egypt kamil is a 45 m diameter impact crater identified in 2008 in southern egypt it was generated by the hypervelocity impact of the gebel kamil iron meteorite on a sedimentary target namely layered sandstones with subhorizontal bedding we have carried out a petrographic study of samples from the crater wall and ejecta deposits collected during our first geophysical campaign february 2010 in order to investigate shock effects recorded in these rocks ejecta samples reveal a wide range of shock features common in quartz rich target rocks they have been divided into two categories as a function of their abundance at thin section scale 1 pervasive shock features the most abundant including fracturing planar deformation features and impact melt lapilli and bombs and 2 localized shock features the least abundant including high pressure phases and localized impact melting in the form of intergranular melt melt veins and melt films in shatter cones in particular kamil crater is the smallest impact crater where shatter cones coesite stishovite diamond and melt veins have been reported based on experimental calibrations reported in the literature pervasive shock features suggest that the maximum shock pressure was between 30 and 60 gpa using the planar impact approximation we calculate a vertical component of the impact velocity of at least 3 5 km s 1 the wide range of shock features and their freshness make kamil a natural laboratory for studying impact cratering and shock deformation processes in small impact structures

mid sized complex crater formation in mixed crystalline sedimentary targets insight from modeling and observation large impact crater formation is an important geologic process that is not fully understood the current paradigm for impact crater formation is based on models and observations of impacts in homogeneous targets real targets are rarely uniform for example the majority of earth s surface is covered by sedimentary rocks and or a water layer the ubiquity of layering across solar system bodies makes it important to understand the effect target properties have on the cratering process to advance understanding of the mechanics of crater collapse and the effect of variations in target properties on crater formation the first bridging the gap workshop recommended that geological observation and numerical modeling focussed on mid sized 15 30 km diameter craters on earth these are large enough to be complex small enough to be mapped surveyed and modelled at high resolution and numerous enough for the effects of target properties to be potentially disentangled from the effects of other variables in this paper we compare observations and numerical models of three 18 26 km diameter craters formed in different target lithology ries germany haughton canada and el gygytgyn russia based on the first order assumption that the impact energy was the same in all three impacts we performed numerical simulations of each crater to construct a simple quantitative model for mid sized complex crater formation in a subaerial mixed crystalline sedimentary target we compared our results with interpreted geological profiles of ries and haughton based on detailed new and published geological mapping and published geophysical surveys our combined observational and numerical modeling work suggests that the major structural differences between each crater can be explained by the difference in thickness of the pre impact sedimentary cover in each case we conclude that the presence of an inner ring at ries and not at haughton is because basement rocks that are stronger than the overlying sediments are sufficiently close to the surface that they are uplifted and overturned during excavation and remain as an uplifted ring after modification and post impact erosion for constant impact energy transient and final crater diameters increase with increasing sediment thickness

estimating transient crater size using the crustal annular bulge insights from numerical modeling of lunar basin scale impacts 1 the transient crater is an important impact cratering concept its volume and diameter can be used to predict impact energy and momentum impact melt volume and maximum depth and volume of ejected material transient crater sizes are often estimated using scaling laws based on final crater rim diameters however crater rim estimates especially for lunar basins can be controversial here we use numerical modeling of lunar basin scale impacts to produce a new alternative method for estimating transient crater radius using the annular bulge of crust observed beneath most lunar basins using target thermal conditions appropriate for the lunar imbrian and nectarian periods we find this relationship to be dependent on lunar crust and upper mantle temperatures this result is potentially important when analyzing lunar basin subsurface structures inferred from the grail mission

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

geological characterization of the ina shield volcano summit pit crater on the moon evidence for extrusion of waning stage lava lake magmatic foams and anomalously young crater retention ages ina a distinctive 2 3 km d shaped depression is composed of unusual bulbous shaped mounds surrounded by optically immature hummocky blocky floor units the crisp appearance optical immaturity and low number of superposed impact craters combine to strongly suggest a geologically recent formation for ina but the specific formation mechanism remains controversial we reconfirm that ina is a summit pit crater vent on a small shield volcano 3 5 billion years old following detailed characterization we interpret the range of ina characteristics to be consistent with a two component model of origin during the waning stages of summit pit eruption activities the ina pit crater floor is interpreted to be dominated by the products of late stage low rise rate magmatic dike emplacement magma in the dike underwent significant shallow degassing and vesicle formation followed by continued degassing below the solidified and highly microvesicular and macrovesicular lava lake crust resulting in cracking of the crust and extrusion of gas rich magmatic foams onto the lava lake crust to form the mounds these unique substrate characteristics highly porous aerogel like foam mounds and floor terrains with large vesicles and void space exert important effects on subsequent impact crater characteristics and populations influencing 1 optical maturation processes 2 regolith development and 3 landscape evolution by modifying the nature and evolution of superposed impact craters and thus producing anomalously young crater retention ages accounting for these effects results in a shift of crater size frequency distribution model ages from

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

a global system of furrows on ganymede indicative of their creation in a single impact event abstract furrows are a concentric system of tectonic troughs and are the oldest recognizable surface feature on ganymede we analyzed the distribution of furrows utilizing voyager and galileo images and found that furrows over ganymede s surface are part of a global concentric circular structure if this multi ring structure is impact origin this is the largest impact structure identified so far in the solar system deviations of the shapes of the furrows from the concentricity are small everywhere which implies that the relative location of the blocks of the dark terrains over the entire surface of ganymede has not changed appreciably even during formation of the bright terrains the estimate of the impactor size is difficult but an 150 km radius impactor is consistent with the observed properties of furrows the furrow forming impact should have significant effects on the satellite s geological and internal evolution which are expected to be confirmed by future explorations of jupiter s icy moons such as the juice jupiter icy moon explorer or europa clipper mission

high velocity collisions from the lunar cataclysm recorded in asteroidal meteorites lunar samples suggest that the inner solar system was bombarded by asteroids about 4 gyr ago radiometric ages of meteorites suggest an unusual number of high velocity asteroids at this time consistent with a dynamical origin of the bombardment in which the asteroids were pushed by outer planet migration onto highly eccentric orbits

insights into the morphology of the serra da cangalha impact structure from geophysical modeling forward modeling is commonly applied to gravity field data of impact structures to determine the main gravity anomaly sources in this context we have developed 2 5 d gravity models of the serra da cangalha impact structure for the purpose of investigating geological bodies structures underneath the crater interpretation of the models was supported by ground magnetic data acquired along profiles as well as by high resolution aeromagnetic data ground magnetic data reveal the presence of short wavelength anomalies probably related to shallow magnetic sources that could have been emplaced during the cratering process aeromagnetic data show that the basement underneath the crater occurs at an average depth of about 1 9 km whereas in the region beneath the central uplift it is raised to 0 5 1 km below the current surface these depths are also supported by 2 5 d gravity models showing a gentle relief for the basement beneath the central uplift area geophysical data were used to provide further constraints for numeral modeling of crater formation that provided important information on the structural modification that affected the rocks underneath the crater as well as on shock induced modifications of target rocks the results showed that the morphology is consistent with the current observations of the crater and that serra da cangalha was formed by a meteorite of approximately 1 4 km diameter striking at 12 km s 1

antipodal terrains created by the rheasilvia basin forming impact on asteroid 4 vesta 1 the rheasilvia impact on asteroid 4 vesta may have been sufficiently large to create disrupted terrains at the impact antipode this paper investigates the amount of deformation expected at the rheasilvia antipode using numerical models of sufficient resolution to directly observe terrain modification and material displacements following the arrival of impact stresses we find that the magnitude and mode of deformation expected at the impact antipode is strongly dependent on both the sound speed and porosity of vesta s mantle as well as the strength of the vestan core in the case of low mantle porosities and high core strengths we predict the existence of a topographic high a peak caused by the collection of spalled and uplifted material at the antipode observations by nasa s dawn spacecraft cannot provide definite evidence that large amounts of deformation occurred at the rheasilvia antipode largely due to the presence of younger large impact craters in the region however a deficiency of small craters near the antipodal point suggests that some degree of deformation did occur

constraining the size of the south pole aitken basin impact abstract the south pole aitken spa basin is the largest and oldest definitive impact structure on the moon to understand how this immense basin formed we conducted a suite of spa scale numerical impact simulations varying impactor size impact velocity and lithospheric thermal gradient we compared our model results to observational spa basin data to constrain a best fit scenario for the spa basin forming impact our results show that the excavation depth to diameter ratio for spa scale impacts is constant for all impact scenarios and is consistent with analytical and geological estimates of excavation depth in smaller craters suggesting that spa scale impacts follow proportional scaling steep near surface thermal gradients and high internal temperatures greatly affected the basin forming process basin structure and impact generated melt volume in agreement with previous numerical studies of spa scale impacts crustal material is entirely removed from the basin center which is instead occupied by a large melt pool of predominantly mantle composition differentiation of the melt pool is needed to be consistent with observational data assuming differentiation of the thick impact generated melt sheet occurred and using observational basin data as constraints we find the best fit impact scenario for the formation of the south pole aitken basin to be an impact with an energy of 4 1026 j our specific model considered an impactor 170 km in diameter striking at 10 km s

the contamination of the surface of vesta by impacts and the delivery of the dark material abstract the dawn spacecraft recently observed the presence of dark material which in turn proved to be associated with the presence of oh and h rich material on the surface of vesta the source of this dark material has been almost unanimously identified with the low albedo asteroids likely analogous to the carbonaceous chondrites found on earth that impacted on vesta over its lifetime however it is still a matter of debate whether the delivery of the dark material is associated with a few large impact events to micrometeorites or to the continuous secular flux of impactors on vesta the continuous flux scenario in particular predicts that a significant fraction of the exogenous material accreted by vesta should be due to non dark impactors likely analogous to ordinary chondrites which instead represent only a minor contaminant in the howardite eucrite diogenite meteorites in this work we explored the continuous flux scenario and its implications for the composition of the vestan regolith taking advantage of the data from the dawn mission and the howardite eucrite diogenite meteorites to constrain the contamination history of vesta we developed a model for the delivery of the exogenous material to vesta and verified how the results it supplies are sensitive to the different parameters we consider we calibrated the flux of impactors predicted by our model with the number of dark craters observed inside the rheasilvia basin and we tested the assumptions on the impact conditions by studying the formation of cornelia crater and of its dark deposits with a hydrocode simulation we used our calibrated model to show that the stochastic events scenario and the micrometeoritic flux scenario are just natural consequences of the continuous flux scenario we then used the model to estimate the amounts of dark and hydroxylate materials that were delivered on vesta since the late heavy bombardment and we showed how our results match well with the values estimated by the dawn mission we finally used our model to assess the amount of fe and siderophile elements that the continuous flux of impactors would mix in the vestan regolith concerning the siderophile elements we focused our attention on the role of ni the results we obtained are in agreement with the data available on the fe and ni content of the howardite eucrite diogenite meteorites and can be used as a reference frame in future studies of the data from the dawn mission and of the howardite eucrite diogenite meteorites our model cannot yet provide an answer to the conundrum of the fate of the missing non carbonaceous contaminants but we discuss some possible reasons for this discrepancy with the otherwise coherent picture described by our results

ina pit crater on the moon extrusion of waning stage lava lake magmatic foam results in extremely young crater retention ages the enigmatic ina feature on the moon was recently interpreted to represent extrusive basaltic volcanic activity within the past 100 m y of lunar history an extremely young age for volcanism on the moon ina is a 2 3 km d shaped depression that consists of a host of unusual bleb like mounds surrounded by a relatively optically fresh hummocky and blocky floor documentation of magmatic volcanic processes from shield volcano summit pit craters in hawai i and new insights into shield building and dike evolution processes on the moon provide important perspectives on the origin of ina we show that the size location morphology topography and optical maturity of ina are consistent with an origin as a subsided summit pit crater lava lake on top of a broad 22 km diameter 3 5 b y old shield volcano new theoretical treatments of lunar shield building magmatic dike events predict that waning stage summit activity was characterized by the production of magmatic foam in the dike and lake the final stages of dike stress relaxation and closure cause the magmatic foam to extrude to the surface through cracks in the lava lake crust to produce the mounds the high porosity of the extruded foams 75 altered the nature of subsequent impact craters the aerogel effect causing them to be significantly smaller in diameter which could bias the crater derived model ages accounting for this effect allows for significantly older model ages to 3 5 b y contemporaneous with the underlying shield volcano thus extremely young volcanic eruptions are not required to explain the unusual nature of ina

the role of impact bombardment history in lunar evolution abstract the lunar surface features diverse impact structures originating from its early bombardment the largest among them are the lunar basins basin forming impacts delivered large amounts of energy to the target and expelled lots of material that deposited as an insulating blanket in the vicinity of the impact here we investigate how such processes may have altered the lunar evolution we combine lunar basin chronologies with numerical models of basin formation and 3d thermochemical mantle convection and analyse the role of single generic impacts resulting in basins with varying diameter formation time location and ejecta properties the direct effects of a single impact are enhanced melt generation as well as thermal and heat flux anomalies but these are limited to 100 myr following the impact we use these insights in multi impact scenarios more relevant for the moon which lead to a widespread ejecta blanket and make impact induced effects more substantial lunar contraction history may be altered by the impact bombardment in favour of an early expansion phase as suggested by recent observations moreover imprints of the early bombardment may be kept in the thermal and compositional state of the moon s interior until modern times these can be as large as those induced by uncertainties in bulk lunar heat content if surface insulation due to ejecta is efficient in this case model predicted present day thermal profiles match independent constraints better if the bulk moon is not significantly enriched in refractory elements compared to earth

movement of finite amplitude sediment accumulations the movement of finite amplitude sediment accumulations is studied using a simple advection diffusion relation derived from the sediment continuity equation and using some heuristic reasoning the movement of a finite amplitude sediment accumulation is found to be strongly diffusive with a small advection component due to the increase in transport rate of the sediment accumulation relative to the transport rate of the original bed material a semianalytical solution to the advection diffusion equation is found and the equation is applied to two laboratory experiments the equation is found to predict the general movement of finite amplitude sediment accumulations with a minimal number of parameters

numerical modelling of generation propagation and run up of tsunamis caused by oceanic impacts model strategy and technical solutions summary hypervelocity impacts of asteroids in marine environments produce tsunami waves independent of the water depth and the diameter of the projectile however the characteristics of the induced waves are affected by these parameters we present a model consisting of the wellknown sale impact model and a non linear wave propagation model to study the generation and subsequent spread out of the initial wave pattern caused by the strike of an asteroid or comet in the ocean the numerical simulation of oceanic impacts requires some changes and extensions to the original sale code especially the handling of different materials water and solid rocks is crucial as they are involved in the cratering process for the simulation of the propagation of tsunami waves that are generated by the impact process we use a newly developed wave propagation model which is based on the non linear shallow water theory with boundary conditions derived from the impact model the run up of the tsunami wave on the coastline is implemented as a special case of reflection and is realized by the wellestablished most code besides the model description we exemplify the capability of our modelling scheme by the simulation of the strike of an asteroid 800 m in diameter on a 5000 m deep ocean at 10 2 km s 1 the subsequent propagation of the induced tsunami waves over an artificial bathymetry and the run up of the wave on the coast

numerical modeling of the ejecta distribution and formation of the orientale basin on the moon the formation and structure of the orientale basin on the moon has been extensively studied in the past however estimates of its transient crater size excavated volume and depth and ejecta distribution remain uncertain here we present a new numerical model to reinvestigate the formation and structure of orientale basin and better constrain impact parameters such as impactor size and velocity unlike previous models the observed ejecta distribution and ejecta thickness were used as the primary constraints to estimate transient crater size the best measure of impact energy models were also compared to basin morphology and morphometry and subsurface structures derived from high resolution remote sensing observations and gravity data respectively the best fit model suggests a 100 km diameter impactor with a velocity of 12 km s 1 formed the orientale basin on a relatively cold moon in this impact scenario the transient crater diameter is 400 km or 460 km depending on whether the crater is defined using the diameter of the excavation zone or the diameter of the growing cavity at the time of maximum crater volume respectively the volume of ejecta material is 4 70 106 km3 in agreement with recent estimates of the orientale ejecta blanket thickness from remote sensing studies the model also confirms the remote sensing spectroscopic observations that no mantle material was excavated and deposited at orientale s rim

re examining the main asteroid belt as the primary source of ancient lunar craters abstract it has been hypothesized that the impactors that created the majority of the observable craters on the ancient lunar highlands were derived from the main asteroid belt in such a way that preserved their size frequency distribution strom r g malhotra r ito t yoshida f kring d a 2005 science 309 1847 1850 a more limited version of this hypothesis dubbed the e belt hypothesis postulates that a destabilized contiguous inner extension of the main asteroid belt produced a bombardment limited to those craters younger than nectaris basin bottke w f vokrouhlick d minton d nesvorn d morbidelli a brasser r simonson b levison h f 2012 nature 485 78 81 we investigate these hypotheses with a monte carlo code called the cratered terrain evolution model ctem which models the topography of a terrain that has experienced bombardment due to an input impactor population we detail our effort to calibrate the code with a human crater counter we also take advantage of recent advances in understanding the scaling relationships between impactor size d i and final crater size d c for basin sized impact craters d c 300 km in order to use large impact basins as a constraint on the ancient impactor population of the moon we find that matching the observed number of lunar highlands craters with d c 100 km requires that the total number of impacting asteroids with d i 10 km be no fewer than 4 10 6 km 2 however this required mass of impactors has d c 64 km craters while not producing a crater larger than imbrium as required by the e belt hypothesis these results suggest that the lunar highlands were unlikely to have been bombarded by a population whose size frequency distribution resembles that of the currently observed main asteroid belt we suggest that the population of impactors that cratered the lunar highlands had a somewhat similar size frequency distribution as the modern main asteroid belt reflecting a similar rocky composition and collisional history but had a smaller ratio of objects capable of producing megabasins compared to objects capable of producing 100 km craters we estimate that the impactor population required to match the lunar highlands had n 5 5 km n 70 km 630 while the modern main asteroid belt ratio is n 5 5 km n 70 km 100

depth of origin of the peak inner ring in lunar impact basins numerical modeling of the peak ring basin formation showed that the peak ring forms from the material that is part of the central uplift outwardly thrust over the inwardly collapsing transient crater rim simulations of the lunar basin formation showed that the peak or inner ring in peak ring or multiring basins respectively is composed of the overturned crust and deep seated material possibly from the upper mantle numerical impact simulations were used to trace the depth of origin of material exposed within the peak or inner ring we estimate the scaling trends between basin size and the depth of origin of material exposed within the ring we also report on the likely crust mantle and projectile abundances exposed within the ring quantifying the excavation depths during the formation of the peak or inner ring provides a step toward understanding the lunar crust and mantle stratigraphy

lagrangian based simulations of hypervelocity impact experiments on mars regolith proxy most of the surface of mars is covered with unconsolidated rocky material known as regolith high fidelity models of the dynamics of impacts in such material are needed to help with the interpretation of seismic signals that are now recorded by seis the seismometer of insight we developed a numerical model for impacts on regolith using the novel hybrid optimization software suite hoss which is a lagrangian code mixing finite and discrete element formulations we use data from hypervelocity impact experiments performed on pumice sand at the nasa ames vertical gun range to identify and calibrate key model parameters the model provides insight into the plastic elastic transition observed in the data and it also demonstrates that gravity plays a key role in the material response waveforms for receivers situated vertically below the impact point are correctly modeled while more research is needed to explain the shallow receivers signals

complex crater formation insights from combining observations of shock pressure distribution with numerical models at the west clearwater lake impact structure large impact structures have complex morphologies with zones of structural uplift that can be expressed topographically as central peaks and or peak rings internal to the crater rim the formation of these structures requires transient strength reduction in the target material and one of the proposed mechanisms to explain this behavior is acoustic fluidization here samples of shock metamorphosed quartz bearing lithologies at the west clearwater lake impact structure canada are used to estimate the maximum recorded shock pressures in three dimensions across the crater these measurements demonstrate that the currently observed distribution of shock metamorphism is strongly controlled by the formation of the structural uplift the distribution of peak shock pressures together with apparent crater morphology and geological observations is compared with numerical impact simulations to constrain parameters used in the block model implementation of acoustic fluidization the numerical simulations produce craters that are consistent with morphological and geological observations the results show that the regeneration of acoustic energy must be an important feature of acoustic fluidization in crater collapse and should be included in future implementations based on the comparison between observational data and impact simulations we conclude that the west clearwater lake structure had an original rim final crater diameter of 35 40 km and has since experienced up to 2 km of differential erosion

basin formation on mercury caloris and the origin of its low reflectance material abstract the 1500 km diameter caloris basin is the largest most well preserved impact structure on mercury the interior of caloris contains an enigmatic geological unit at depth subsequently excavated by smaller impacts low reflectance material lrm lrm is interpreted to be the original basin floor material and be composed of melted and re crystallized mantle material in this work a comprehensive numerical modeling study of caloris basin formation is undertaken to provide insight into basin formation on mercury which may differ to that on the moon and to address the origin of the lrm analysis of the modeled basin attributes demonstrates that the caloris impact would have penetrated to depths approaching the core mantle boundary excavating mantle material to the surface and producing melt volumes on the order of 10 7 km 3 such a melt volume could differentiate forming a layer buried beneath later volcanism that could be the lrm in agreement with previous spectroscopic analyses furthermore lrm deposits at other smaller basins on mercury may also represent impact melted mantle material a comparison to lunar basin counterparts demonstrates that basin formation on mercury is similar to that on the moon this observation suggests that the observed difference in the number of large basins between the two bodies fewer on mercury is due to processes that modify basins i e greater impact melt volume and more extensive volcanism on mercury rather than first order impact process differences the greater average impact velocity and gravity exert a slight effect on some basin properties including an increased excavation depth to transient crater diameter ratio on mercury

geomorphic analysis of small rayed craters on mars examining primary versus secondary impacts 1 twenty confirmed impacts over a 7 year time period on mars were qualitatively and statistically compared to 287 secondary craters believed to originate from zunil an 500 ka 10 km diameter primary crater our goal was to establish criteria to distinguish secondaries from primaries in the general crater population on the basis of their horizontal planforms recent primary impacts have extensive air blast zones distal ray systems 100 crater radii r and ephemeral ejecta recent primaries formed clusters of craters from atmospheric fragmentation of the meteoroid body secondary craters have ejecta blankets with shorter rays that are consistent with emplacement by low impact velocities near 1 km s the mean extent of the continuous ejecta blankets was less distal for secondaries 5 38 1 57r versus primaries 18 07 7 01r though primary ejecta were less fractal fractal dimension index fdi 1 30 and more circular on average circularity ratio cr 0 55 0 25 versus 0 27 0 13 for secondaries crater rims were remarkably circular primaries cr 0 97 0 02 secondaries at 0 94 0 05 though secondaries have the lowest values cr 0 9 secondary crater rims were elongated toward or orthogonal to their primary of origin uprange source directions for most secondaries determined by ejecta planform and crater rim ellipticity point toward zunil although contamination from other primaries is considered in some areas ejecta blanket discrepancies between recent primaries and zunil secondaries are attributable to differences in impact velocity and retention age after removal of the ejecta blanket crater rims are generally not diagnostic for determining crater origin fragmentation of primaries may play some role in steepening the size frequency distribution of crater diameters in the 5 m d 30 m range

subsurface morphology and scaling of lunar impact basins impact bombardment during the first billion years after the formation of the moon produced at least several tens of basins the gravity recovery and interior laboratory grail mission mapped the gravity field of these impact structures at significantly higher spatial resolution than previous missions allowing for detailed subsurface and morphological analyses to be made across the entire globe grail derived crustal thickness maps were used to define the regions of crustal thinning observed in centers of lunar impact basins which represents a less unambiguous measure of a basin size than those based on topographic features the formation of lunar impact basins was modeled numerically by using the isale 2d hydrocode with a large range of impact and target conditions typical for the first billion years of lunar evolution in the investigated range of impactor and target conditions the target temperature had the dominant effect on the basin subsurface morphology model results were also used to update current impact scaling relationships applicable to the lunar setting based on assumed target temperature our new temperature dependent impact scaling relationships provide estimates of impact conditions and transient crater diameters for the majority of impact basins mapped by grail as the formation of lunar impact basins is associated with the first approximately 700 myr of the solar system evolution when the impact flux was considerably larger than the present day our revised impact scaling relationships can aid further analyses and understanding of the extent of impact bombardment on the moon and terrestrial planets in the early solar system

numerical modeling of the formation and structure of the orientale impact basin 1 the orientale impact basin is the youngest and best preserved lunar multi ring basin and has thus been the focus of studies investigating basin forming processes and final structures a consensus about how multi ring basins form however remains elusive here we numerically model the orientale basin forming impact with the aim of resolving some of the uncertainties associated with this basin by using two thermal profiles estimating lunar conditions at the time of orientale s formation and constraining the numerical models with crustal structures inferred from gravity data we provide estimates for orientale s impact energy 2 9 1025 j impactor size 50 80 km diameter transient crater size 320 480 km excavation depth 40 55 km and impact melt volume 106 km3 we also analyze the distribution and deformation of target material and compare our model results and orientale observations with the chicxulub crater to investigate similarities between these two impact structures

hydrocode simulation of ganymede and europa cratering trends how thick is europa s crust abstract one of the continuing debates of outer solar system research centers on the thickness of europa s ice crust as it affects both the habitability and accessibility of its sub surface ocean here we use hydrocode modeling of the impact process in layered ice and water targets and comparison to europan cratering trends and galileo derived topographic profiles to investigate the crustal thickness full or partial penetration of the ice crust by an impactor occurred in simulations in which the ice thickness was less than 14 times the projectile radius craters produced in these thin shell simulations were consistently smaller than for larger ice thicknesses which will complicate inference of large impactor population sizes simulations in which the resultant crater was 3 times the ice layer thickness resulted in summit pit morphology this work supports that summit pit craters noted on both rocky and icy bodies can be created by the presence of a weaker layer at depth we suggest that floor pits seen only on ice rich bodies require a different formation mechanism to summit pits pristine craters formed in a target with high heat flow were shallower than for the same impact into a target of lesser heat flow suggesting that the starting crater morphology for viscous relaxation isostatic readjustments and erosion rate studies is different for craters formed in times of different heat flow we find that the crater depth diameter trend of europa can only be recreated when simulating impact into an upper brittle ice layer of 7 km depth with a corresponding geothermal gradient of 0 025 k m as this ice thickness estimate is below 10 km results from this work suggest that convective overturn of the surface ice may occur or have occurred on europa making the development of indigenous life a possibility

compaction as the origin of the unusual craters on the asteroid mathilde the asteroid mathilde has suffered at least five giant impacts previous studies suggest that mathilde s giant craters should be surrounded by blankets of ejecta that are kilometres deep1 2 yet the craters show no evidence of filling by material excavated during later nearby impacts1 3 computer simulations of impacts have been used to suggest that the absence of ejecta arises because the impact energy is deposited in a small volume due to mathilde s unusually high porosity4 which produces ejecta velocities so high that nearly all of the material escapes mathilde s gravitational field5 here we report laboratory measurements of high velocity impacts into porous material which support an alternative explanation3 the crater is formed mainly by compaction not excavation the small amount of ejecta lofted in our experiments have velocities sufficiently low that nearly all of the material is redeposited within the crater bowl the crater itself results from material being compressed rather than ejected this type of cratering implies that highly porous asteroids are minor contributors of meteorites because essentially no material escapes the asteroids

the lawn hill annulus an ordovician meteorite impact into water saturated dolomite the lawn hill impact structure lhis is located 250 km n of mt isa in nw queensland australia and is marked by a highly deformed dolomite annulus with an outer diameter of similar to 18 km overlying low metamorphic grade siltstone sandstone and shale along the ne margin of the georgina basin this study provides detailed field observations from sections of the lawn hill annulus and adjacent areas that demonstrate a clear link between the deformation of the dolomite and the lawn hill impact ar 40 ar 39 dating of impact related melt particles provides a time of impact in the ordovician 472 8 ma when the georgina basin was an active depocenter the timing and stratigraphic thickness of the dolomite sequence in the annulus suggest that there was possibly up to 300 m of additional sedimentary rocks on top of the currently exposed thorntonia limestone at the time of impact the exposed annulus is remarkably well preserved with preservation attributed to postimpact sedimentation the lhis has an atypical crater morphology with no central uplift the heterogeneous target materials at lawn hill were probably low strength porous and water saturated with all three properties affecting the crater morphology the water saturated nature of the carbonate unit at the time of impact is thought to have influenced the highly brecciated nature of the annulus and restricted melt production the impact timing raises the possibility that the lawn hill structure may be a member of a group of impacts resulting from an asteroid breakup that occurred in the mid ordovician 470 6 ma

impact induced porosity and microfracturing at the chicxulub impact structure porosity and its distribution in impact craters has an important effect on the petrophysical properties of impactites seismic wave speeds and reflectivity rock permeability strength and density these properties are important for the identification of potential craters and the understanding of the process and consequences of cratering the chicxulub impact structure recently drilled by the joint international ocean discovery program and international continental scientific drilling program expedition 364 provides a unique opportunity to compare direct observations of impactites with geophysical observations and models here we combine small scale petrographic and petrophysical measurements with larger scale geophysical measurements and numerical simulations of the chicxulub impact structure our aim is to assess the cause of unusually high porosities within the chicxulub peak ring and the capability of numerical impact simulations to predict the gravity signature and the distribution and texture of porosity within craters we show that high porosities within the chicxulub peak ring are primarily caused by shock induced microfracturing these fractures have preferred orientations which can be predicted by considering the orientations of principal stresses during shock and subsequent deformation during peak ring formation our results demonstrate that numerical impact simulations implementing the dynamic collapse model of peak ring formation can accurately predict the distribution and orientation of impact induced microfractures in large craters which plays an important role in the geophysical signature of impact structures plain language summary the chicxulub crater mexico is widely known for its association with the extinction of the nonavian dinosaurs at the end of the cretaceous period the crater was first identified due to its gravitational and magnetic anomalies potential impact structures are often identified in part on the basis of geophysical anomalies most commonly including a circular gravity low gravity is slightly weaker at craters because the impact cratering process removes mass from the impact site in this study we examine the cause of the chicxulub gravity anomaly by combining observations from recent drilling of the crater geophysical data measured across the crater and numerical impact simulations we demonstrate that porosity in rocks beneath the crater floor is primarily accommodated by fracturing during the impact cratering process that the orientation of those fractures are consistent with predictions from numerical impact simulations and that impact induced porosity is one of the primary causes of gravity anomalies in large impact craters

the formation of the baptistina family by catastrophic disruption porous versus non porous parent body in this paper we present numerical simulations aimed at reproducing the baptistina family based on its properties estimated by observations a previous study by bottke et al 2007 indicated that this family is probably at the origin of the k t impactor is linked to the cm meteorites and was produced by the disruption of a parent body 170 km in size due to the head on impact of a projectile 60 km in size at 3 km s 1 this estimate was based on simulations of fragmentation of non porous materials while the family was assumed to be of c taxonomic type which is generally interpreted as being formed from a porous body using both a model of fragmentation of non porous materials and a model that we developed recently for porous ones we performed numerical simulations of disruptions aimed at reproducing this family and at analyzing the differences in the outcome between those two models our results show that a reasonable match to the estimated size distribution of the real family is produced from the disruption of a porous parent body by the head on impact of a projectile 54 km in size at 3 km s 1 thus our simulations with a model consistent with the assumed dark type of the family requires a smaller projectile than previously estimated but the difference remains small enough to not affect the proposed scenario of this family history we then find that the break up of a porous body leads to different outcomes than the disruption of a non porous one the real properties of the baptistina family still contain large uncertainties and it remains possible that its formation did not involve the proposed impact conditions however the simulations presented here already show some range of outcomes and once the real properties are better constrained it will be easy to check whether one of them provides a good match

investigating the lake bosumtwi impact structure insight from numerical modeling 1 the current topography of the lake bosumtwi crater and some of its structural dimensions have been determined by geophysical methods we combine these data with sophisticated numerical models to evaluate the cratering process itself for example melt and tektite generation as well as to test the modeling code the geophysical maps show some asymmetry in plan view with the main anomaly north of the crater center the simulations of the early stage show asymmetric patterns only in ejecta and tektite distributions while the late stage is modeled for the vertical impact without any asymmetry we estimate the projectile size from scaling laws and then varying material properties reproduce a crater which is similar to the bosumtwi but too deep bulking allows us to reconcile differences between the model results and the observed topography shock melt estimates are in good agreement with the bosumtwi magnetic signature modeled distribution of tektites assumes an impact angle of 30 45 and an impact direction from the n ne the combination of numerical models and field evidence not only provides necessary information for upcoming scientific drilling of the structure but also suggests interesting and well suited drill sites besides the central uplift and the annular moat with a suggested thick breccia cover drilling at the location of the geophysical anomalies and comparison of downrange and transversal locations will provide new insight into preimpact and impact induced asymmetries

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

excavation of the lunar mantle by basin forming impact events on the moon abstract global maps of crustal thickness on the moon derived from gravity measurements obtained by nasa s gravity recovery and interior laboratory grail mission have shown that the lunar crust is thinner than previously thought hyperspectral data obtained by the kaguya mission have also documented areas rich in olivine that have been interpreted as material excavated from the mantle by some of the largest lunar impact events numerical simulations were performed with the isale 2d hydrocode to investigate the conditions under which mantle material may have been excavated during large impact events and where such material should be found the results show that excavation of the mantle could have occurred during formation of the several largest impact basins on the nearside hemisphere as well as the moscoviense basin on the farside hemisphere even though large areas in the central portions of these basins were later covered by mare basaltic lava flows surficial lunar mantle deposits are predicted in areas external to these maria our results support the interpretation that the high olivine abundances detected by the kaguya spacecraft could indeed be derived from the lunar mantle

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

low energy deposition in the chicxulub crater during the impact to post impact transition article i nfo the crater fill units that were deposited during the terminal stages of the formation of the shallow marine chicxulub crater in northern yucatan were interpreted by some authors as products of a catastrophic resurge of water this interpretation contrasts markedly with the dominant sand grain size b300 m and the abundant small scale current cross bedding structures indicative of low energy deposition to solve this discrepancy we applied first principles of physical sedimentology in an analysis of grain size and sedimentary structures to determine current velocities and minimum water depths required to generate the observed sedimentary bedforms we combine this approach with numerical modeling of crater formation in order to explore the dimensions of the ejecta ring wall being formed by a chicxulub impactor in water depths of 1000 m we find that the ejecta wall reaches heights in the order of 1200 m above sea floor and 200 m above sea level it thus prevented the resurge of water into the crater cavity and consequently also the subsequent formation of a collapse wave the ring wall significantly retarded the refilling of the crater cavity with seawater and refilling took place by permeation through and localized erosion of the ring wall this resulted in low energy currents with velocities between 1 5 and 0 18 m s 1 slowly reentering the crater and depositing the cm scale low energy laminated and crossbedded sandy terminal impact deposits

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

dependence of secondary crater characteristics on downrange distance high resolution morphometry and simulations on average secondary impact craters are expected to deepen and become more symmetric as impact velocity vi increases with downrange distance l we have used high resolution topography 1 2 m pixel to characterize the morphometry of secondary craters as a function of l for several well preserved primary craters on mars the secondaries in this study n 2 643 span a range of diameters 25 m d 400 m and estimated impact velocities 0 4 km s vi 2 km s the range of diameter normalized rim to floor depth d d broadens and reaches a ceiling of d d 0 22 at l 280 km vi 1 1 2 km s whereas average rim height shows little dependence on vi for the largest craters h d 0 02 d 60 m populations of secondaries that express the following morphometric asymmetries are confined to regions of differing radial extent planform elongations l 110 160 km taller downrange rims l 280 km and cavities that are deeper uprange l 450 500 km populations of secondaries with lopsided ejecta were found to extend to at least l 700 km impact hydrocode simulations with isale 2d for strong intact projectile and target materials predict a ceiling for d d vs l whose trend is consistent with our measurements this study illuminates the morphometric transition from subsonic to hypervelocity cratering and describes the initial state of secondary crater populations this has applications to understanding the chronology of planetary surfaces and the long term evolution of small crater populations

modeling damage and deformation in impact simulations numerical modeling is a powerful tool for investigating the formation of large impact craters but is one that must be validated with observational evidence quantitative analysis of damage and deformation in the target surrounding an impact event provides a promising means of validation for numerical models of terrestrial impact craters particularly in cases where the final pristine crater morphology is ambiguous or unknown in this paper we discuss the aspects of the behavior of brittle materials important for the accurate simulation of damage and deformation surrounding an impact event and the care required to interpret the results we demonstrate this with an example simulation of an impact into a terrestrial granite target that produces a 10 km diameter transient crater the results of the simulation are shown in terms of damage a scalar quantity that reflects the totality of fragmentation and plastic strain both total plastic strain the accumulated amount of permanent shear deformation regardless of the sense of shear and net plastic strain the amount of permanent shear deformation where the sense of shear is accounted for damage and plastic strain are both greatest close to the impact site and decline with radial distance however the reversal in flow patterns from the downward and outward excavation flow to the inward and upward collapse flow implies that net plastic strains may be significantly lower than total plastic strains plastic strain in brittle rocks is very heterogeneous however continuum modeling requires that the deformation of the target during an impact event be described in terms of an average strain that applies over a large volume of rock large compared to the spacing between individual zones of sliding this paper demonstrates that model predictions of smooth average strain are entirely consistent with an actual strain concentrated along very narrow zones furthermore we suggest that model predictions of total accumulated strain should correlate with observable variations in bulk density and seismic velocity

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

effects of moon s thermal state on the impact basin ejecta distribution we investigate how different temperature gradients of the moon affect the ejection of lithic and molten materials for impact basin several hundred kilometers in diameter to quantify the thickness and melt content of ejecta blanket as a function of radial distance we find by means of numerical modeling that the ejecta thickness and melt content similar to the basin formation is sensitive to the thermal properties of the target for two similar impact scenarios the ejecta thickness with radial distance is proportional to a power law but for a warm target it declines faster than for a cold target in addition the impact on the warm target produces more molten ejecta than in the case of the cold target the thermal effects on the ejecta thickness distribution can be testified by the topographic variations around imbrium and orientale basins which were thought to be formed on a warm and cold moon respectively our study demonstrates that the thermal effect needs to be taken into account to estimate the ejecta thickness distribution for large scale impact basins on airless planetary surfaces

the role of asteroid strength porosity and internal friction in impact momentum transfer abstract earth is continually impacted by very small asteroids and debris and a larger object though uncommon could produce a severe natural hazard during impact crater formation the ballistic ejection of material out of the crater is a major process which holds significance for an impact study into the deflection of asteroids in this study we numerically simulate impacts into low gravity strength dominated asteroid surfaces using the isale shock physics code and consider the double asteroid redirection test dart mission as a case study we find that target cohesion initial porosity and internal friction coefficient greatly influence ejecta mass velocity launch position distributions and hence the amount by which an asteroid can be deflected our results show that as the cohesion is decreased the ratio of ejected momentum to impactor momentum 1 increases 1 also increases as the initial porosity and internal friction coefficient of the asteroid surface decrease using nominal impactor parameters and reasonable estimates for the material properties of the didymos binary asteroid the dart target our simulations show that the ejecta produced from the impact can enhance the deflection by a factor of 2 to 4 we use numerical impact simulations that replicate conditions in several laboratory experiments to demonstrate that our approach to quantify ejecta properties is consistent with impact experiments in analogous materials finally we investigate the self consistency between the crater size and ejection speed scaling relationships previously derived from the point source approximation for impacts into the same target material

an endogenic origin of cerean organics abstract localized deposits of aliphatic organic rich material have been found on the surface of dwarf planet ceres and based on mineralogical context it has been suggested that these organics formed within ceres de sanctis et al 2017 however no obvious source and exposure mechanism can be inferred from the geological settings of the organic rich terrains the alternative hypothesis is that the organics were delivered from an exogenic source via impact pieters et al 2018 we numerically simulate the thermal degradation of aliphatic organics delivered to ceres during hypervelocity impacts as well as the thermal degradation and excavation of organics from the subsurface our models show that exogenic delivery of aliphatic organics is inefficient as most of the spectral signature of the organic species would be thermally degraded and diluted by mixing with target material in the ejecta blanket of a given crater instead our models suggest that the formation of ernutet crater excavated a subsurface reservoir of aliphatic organics from a depth of 3 6 km bringing it close to the surface where it could be subsequently exhumed by small craters and landslides compatible with observations

regolith mixing by impacts lateral diffusion of basin melt abstract impact cratering has been the primary process to alter the distribution of lunar highland material since the formation of a crust this impact history is recorded in the radiogenic clocks of impact melts which are accessible for study on lunar samples and meteorites however primary impact melt is exposed to a long time gardening process i e re melting excavation burial and re excavation by subsequent impacts resulting in a complex spatial distribution of materials representing specific impact events to investigate the diffusion behavior of impact melt a model tracing the evolving distribution of melt laterally and with depth was built using a monte carlo approach given scaling laws concerning melt production and ejecta distribution the size frequency distribution of impact craters and the rate function for crater formation we examine the evolution of melt component occurrence of different ages three mid to late forming basins serenitatis crisium and imbrium are chosen as a case study for the diffusion of melt from major basin forming events the survival probability of basin melt occurrence at the apollo and luna sampling spots is derived it is expected to find abundant imbrium and crisium melt at the apollo and luna sampling sites consistent with the k ar radiometric dates of highland samples whereas the older serenitatis melt was subjected to the later long term gardening strongly influenced by later local impacts and thus is less abundant understanding the diffusion of impact melt is helpful for interpretation of radiometric ages of lunar samples and can be used to predict the distribution of differently aged melts at future landing sampling sites such as the chinese chang e 4 ce 4

three dimensional petrography of metal phases in equilibrated l chondrites effects of shock loading and dynamic compaction abstract to examine the role of impacts in the evolution of asteroids as seen through their chondritic offspring we have performed a quantitative three dimensional 3d study of metal grains in a suite of increasingly shocked l chondrites with synchrotron x ray microtomography xmt our data allow rigorous quantification of size number distributions and collective morphology of fe ni metal phases in chondritic meteorites at the resolution of our xmt measurements 8 4 17 9 m voxel the number of metal particles increase with higher degrees of petrographically identified shock loading indicating a coalescing of fe ni metal at or below this scale our results demonstrate that collective degrees of metal grain preferred orientation increase with greater degrees of impact related compaction and shock loading ductile metal grains in l chondrites begin to show foliation at peak shock pressures

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

the role of substrate characteristics in producing anomalously young crater retention ages in volcanic deposits on the moon morphology topography subresolution roughness and mode of emplacement of the sosigenes lunar irregular mare patch lunar irregular mare patches imps comprise dozens of small distinctive and enigmatic lunar mare features characterized by their irregular shapes well preserved state of relief apparent optical immaturity and few superposed impact craters imps are interpreted to have been formed or modified geologically very recently 100 ma braden et al 2014 however their apparent relatively recent formation modification dates and emplacement mechanisms are debated we focus in detail on one of the major imps sosigenes located in western mare tranquillitatis and dated by braden et al 2014 at 18 ma the sosigenes imp occurs on the floor of an elongate pit crater interpreted to represent the surface manifestation of magmatic dike propagation from the lunar mantle during the mare basalt emplacement era billions of years ago the floor of the pit crater is characterized by three morphologic units typical of several other imps i e 1 bulbous mounds 5 10 m higher than the adjacent floor units with unusually young crater retention ages meters thick regolith and slightly smaller subresolution roughness than typical mature lunar regolith 2 a lower hummocky unit mantled by a very thin regolith and significantly smaller subresolution roughness and 3 a lower blocky unit composed of fresh boulder fields with individual meter scale boulders and rough subresolution surface texture using new volcanological interpretations for the ascent and eruption of magma in dikes and dike degassing and extrusion behavior in the final stages of dike closure we interpret the three units to be related to the late stage behavior of an ancient dike emplacement event following the initial dike emplacement and collapse of the pit crater the floor of the pit crater was flooded by the latest stage magma the low rise rate of the magma in the terminal stages of the dike emplacement event favored flooding of the pit crater floor to form a lava lake and co gas bubble coalescence initiated a strombolian phase disrupting the cooling lava lake surface this phase produced a very rough and highly porous with both vesicularity and macroporosity lava lake surface as the lake surface cooled in the terminal stage of the eruption dike closure with no addition of magma from depth caused the last magma reaching shallow levels to produce viscous magmatic foam due to h2 o gas exsolution this magmatic foam was extruded through cracks in the lava lake crust to produce the bulbous mounds we interpret all of these activities to have taken place in the terminal stages of the dike emplacement event billions of years ago we attribute the unusual physical properties of the mounds and floor units anomalously young ages unusual morphology relative immaturity and blockiness to be due to the unusual physical properties of the substrate produced during the waning stages of a dike emplacement event in a pit crater the unique physical properties of the mounds magmatic foams and hummocky units small vesicles and large void space altered the nature of subsequent impact cratering regolith development and landscape evolution inhibiting the typical formation and evolution of superposed impact craters and maintaining the morphologic crispness and optical immaturity accounting for the effects of the reduced diameter of craters formed in magmatic foams results in a shift of the crater size frequency distribution age from 100 myr to billions of years contemporaneous with the surrounding ancient mare basalts we conclude that extremely young mare basalt eruptions and resulting modification of lunar thermal evolution models to account for the apparent young ages of the imps are not required we suggest that other imp occurrences both those associated with pit craters atop dikes and those linked to fissure eruptions in the lunar maria may have had similar ancient origins

an international and multidisciplinary drilling project into a young complex impact structure the 2004 icdp bosumtwi crater drilling project an overview the bosumtwi impact crater in ghana arguably the best preserved complex young impact structure known on earth displays a pronounced rim and is almost completely filled by lake bosumtwi a hydrologically closed basin it is the source crater of the ivory coast tektites the structure was excavated in 2 1 2 2 gyr old metasediments and metavolcanics of the birimian supergroup a drilling project was conceived that would combine two major scientific interests in this crater 1 to obtain a complete paleoenvironmental record from the time of crater formation about one million years ago at a near equatorial location in africa for which very few data are available so far and 2 to obtain a complete record of impactites at the central uplift and in the crater moat for ground truthing and comparison with other structures within the framework of an international and multidisciplinary drilling project led by the international continental scientific drilling program icdp 16 drill cores were obtained from june to october 2004 at six locations within lake bosumtwi which is 8 5 km in diameter the 14 sediment cores are currently being investigated for paleoenvironmental indicators the two impactite cores lb 07a and lb 08a were drilled into the deepest section of the annular moat 540 m and the flank of the central uplift 450 m respectively they are the main subject of this special issue of meteoritics planetary science which represents the first detailed presentations of results from the deep drilling into the bosumtwi impactite sequence drilling progressed in both cases through the impact breccia layer into fractured bedrock lb 07a comprises lithic in the uppermost part and suevitic impact breccias with appreciable amounts of impact melt fragments the lithic clast content is dominated by graywacke besides various metapelites quartzite and a carbonate target component shock deformation in the form of quartz grains with planar microdeformations is abundant first chemical results indicate a number of suevite samples that are strongly enriched in siderophile elements and mg but the presence of a definite meteoritic component in these samples cannot be confirmed due to high indigenous values core lb 08a comprises suevitic breccia in the uppermost part followed with depth by a thick sequence of graywacke dominated metasediment with suevite and a few granitoid dike intercalations it is assumed that the metasediment package represents bedrock intersected in the

spherule layers crater scaling laws and the population of ancient terrestrial impactors abstract ancient layers of impact spherules provide a record of earth s early bombardment history here we compare different bombardment histories to the spherule layer record and show that 3 2 3 5 ga the flux of large impactors 10 100 km in diameter was likely 20 40 times higher than today the e belt model of early solar system dynamics suggests that an increased impactor flux during the archean is the result of the destabilization of an inward extension of the main asteroid belt bottke et al 2012 here we find that the nominal flux predicted by the e belt model is 7 19 times too low to explain the spherule layer record moreover rather than making most lunar basins younger than 4 1 gyr old the nominal e belt model coupled with a corrected crater diameter scaling law only produces two lunar basins larger than 300 km in diameter we also show that the spherule layer record when coupled with the lunar cratering record and careful consideration of crater scaling laws can constrain the size distribution of ancient terrestrial impactors the preferred population is main belt like up to 50 km in diameter transitioning to a steep distribution going to larger sizes