clustered impacts experiments and implications impact by clusters of projectiles rather than a single projectile can result from several processes atmospheric breakup tidal breakup and ejecta from a large primary impact experiments have been performed in order to establish the characteristics of such events over a wide range of impact velocities 15 m s to 6 km s at very low impact velocities 15 200 m s clustered impacts were produced by launching a grouped projectiles of aluminum shot steel shot iron filings and sand at moderate to high velocities 0 8 6 km s pyrex spheres were shattered above the target during passage through aluminum foil or paper thereby forming a well defined cluster of fragments of overall radius rc the ratio of the overall radius rc of the cluster to the radius rs of a solid impactor of the same mass provides a measure of the cluster dispersion sand and compacted pumice targets were used in order to compare qualitatively the additional effect of slight differences in target strength the experiments reveal marked contrasts between impacts by clusters of projectiles and impacts by a single solid body tight clusters rc rs 20 displace a factor of 10 less mass this reduction in cratering efficiency is largely expressed as a shallow crater with an aspect ratio diameter depth as large as 30 for open clusters the size and velocity of the cluster as well as the density and strength contrast between the impactor and target can dramatically affect crater morphology open clusters impacting compacted pumice produce a flat hummocky floor with an incipient multiring pattern whereas a tight cluster impacting the same target produces a central floor mound oblique impacts by symmetrical clusters form a characteristic array of v shaped ridges pointing uprange the apex angle of the ridges depends on the cluster dispersion and impact angle inventories of postimpact projectile material reveal that the projectile is largely retained on the surface and spread downrange from the impact direction the amount of projectile material retained on the surfaces increases with decreasing impact angle from the horizontal and with increasing strength of the target relative to the projectile clustered impact craters and lunar secondary craters bear striking similarities over a broad range of morphologic features on this basis and on the basis of reasonable models of ejecta curtain structure we suggest that these experiments provide new clues for understanding ejecta emplacement around large lunar impact craters when the ejecta curtain around large impacts is viewed as a thick wall of debris and clustered impactors are viewed as a unit section of such a curtain then experimental results indicate that the continuous ejecta facies even for lunar craters larger than 100 km could contain as much as 90 primary material such a conclusion contrasts with many existing models that derive mixing ratios implicitly based on single noninteracting impact events beyond the continuous ejecta facies impacting clusters of ejecta provide a physical basis for understanding the wide variety of secondary morphologies and the large range in spectral signatures of primary material in crater rays

isocrater impacts conditions and mantle dynamical responses for different impactor types abstract impactors of different types and sizes can produce a final crater of the same diameter on a planet under certain conditions we derive the condition for such isocrater impacts from scaling laws as well as relations that describe how the different impactors affect the interior of the target planet these relations are also valid for impacts that are too small to affect the mantle the analysis reveals that in a given isocrater impact asteroidal impactors produce anomalies in the interior of smaller spatial extent than cometary or similar impactors the differences in the interior could be useful for characterizing the projectile that formed a given crater on the basis of geophysical observations and potentially offer a possibility to help constrain the demographics of the ancient impactor population a series of numerical models of basin forming impacts on mercury venus the moon and mars illustrates the dynamical effects of the different impactor types on different planets it shows that the signature of large impacts may be preserved to the present in mars the moon and mercury where convection is less vigorous and much of the anomaly merges with the growing lid on the other hand their signature will long have been destroyed in venus whose vigorous convection and recurring lithospheric instabilities obliterate larger coherent anomalies

near encounter with asteroid 253 mathilde overview abstract on june 27 1997 the near spacecraft carried out the first ever encounter with a c type asteroid flying by 253 mathilde at a distance of 1212 km we summarize findings derived from 330 images obtained by near s msi camera which cover about 60 of the surface of the asteroid the highest resolution achieved was about 160 m pixel mathilde is a low reflectance object geometric albedo 0 047 with principal diameters of 66 48 44 km the mean radius of 26 4 1 3 km is somewhat smaller than the value of 30 km suggested by previous telescopic data mathilde s surface morphology is dominated by large craters at least four of which have diameters comparable to the radius of mathilde the two largest ishikari and karoo have diameters of 29 3 and 33 4 km respectively no evidence of layering is exposed in the crater walls but suggestions of downslope movement are present the surface density of craters in the diameter range from 0 5 to 5 km is close to equilibrium saturation a situation in which as many craters are being destroyed as are being produced observed depth to diameter ratios for craters in this size range are close to those observed on the lunar surface a disruption lifetime of about 4 billion years has been estimated for mathilde based on the mass determination obtained from doppler tracking d k yeomans et al 1997 science 278 2106 2109 and the volume derived from msi images the average density of mathilde is remarkably low 1 3 0 3 g cm 3 a value consistent with a rubble pile structure for the interior assuming that mathilde s rock type is similar to that found in cm meteorites the porosity of the interior must be some 50 shock and seismic disturbances associated with major impacts are expected to be transmitted very poorly by mathilde s underdense interior a fact which may explain the remarkable degree to which surface morphology and topography have been preserved in spite of later major collisional events except for the lower geometric albedo 0 047 0 005 the photometric properties of mathilde are closely similar to those of phobos the surface is extremely homogeneous in terms of both color or albedo specifically no color or albedo variations associated with craters have been identified

ejecta from impact craters an important feature of impacts into solar system bodies is the fate of crater ejecta the near surface material launched during the highly dynamic crater formation process laboratory measurements of impact crater ejecta from 18 studies are summarized the data are examined and used to assess our understanding of how the ejecta velocity and mass distributions depend on the conditions of an impact event the effects of impact speed on the ejecta are reasonably well understood but the dependences on target properties such as strength and porosity are only poorly constrained a point source scaling model for the ejecta mass and velocity distributions is developed and fit to the data for several classes of materials distinguished by porosity

impact induced compositional variations on mercury abstract remote sensing data suggest mercury s surface has compositional variations spatially associated with crater and basin ejecta the so called low reflectance material lrm which has been suggested to be enriched in a subsurface native darkening agent that is excavated and redeposited onto the surface this unit may record the evidence of impact induced mixing of mercury s outer layers during its early history here we develop a fully three dimensional monte carlo model of impact cratering excavation and ejecta blanket deposition on a global scale for mercury new dynamical simulations of the early evolution of the asteroid belt hint at the presence of additional asteroids in a region interior to the present day belt known as the e belt we use monte carlo methods to show that the predicted bombardment from this population matches the observed spatial crater densities on mercury impacts large enough to pierce through the crust create surface ejecta deposits rich in mantle material later impacts onto enriched ejecta deposits redistribute mantle material away from the basins for the suggested average mercurian crustal thickness of 50 km the surface has on average 0 4 mantle material by volume the most enriched areas have 30 mantle by volume the regional coverage of impact induced compositional changes is strongly dependent on the thickness to the subsurface source because observations indicate lrm covers 15 of mercury s surface our model suggests the darkening agent is 30 km deep considering the current estimated average mercurian crustal thickness of 50 km this implies the darkening agent is likely located within a chemically distinct lower crust

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

maskelynite in asteroidal lunar and planetary basaltic meteorites an indicator of shock pressure during impact ejection from their parent bodies abstract maskelynite is a diaplectic glass that forms from plagioclase at shock pressures of 20 30 gpa depending on the ca concentration the proportion of maskelynite rich samples in a basaltic meteorite group correlates with the parent body escape velocity and serves as a shock indicator of launching conditions for eucrites basalts widely presumed to be from vesta vesc 0 36 km s 1 5 of the samples are maskelynite rich for the moon vesc 2 38 km s 1 30 of basaltic meteorites are maskelynite rich for mars vesc 5 03 km s 1 93 of basaltic meteorites are maskelynite rich in contrast literature data show that maskelynite is rare 1 among mare basalts and basaltic fragments in apollo 11 12 15 and 17 soils which were never ejected from the moon angrites are unbrecciated basaltic meteorites that are maskelynite free they were ejected at low to moderate shock pressures from an asteroid smaller than vesta because most impacts that eject materials from a large 100 km parent body are barely energetic enough to do that a collision that has little more than the threshold energy required to eject a sample from vesta will not be able to eject identical samples from the moon or mars there must have been relatively few impacts if any that launched eucrites off their parent body that also imparted shock pressures of 20 30 gpa in the ejected rocks more energetic impacts were required to launch basalts off the moon and mars on average vesta ejecta were subjected to lower shock pressures than lunar ejecta and lunar ejecta were subjected to lower shock pressures than martian ejecta h and ll ordinary chondrites have low percentages of shock stage s5 maskelynite bearing samples 1 and 4 respectively probably reflecting shock processes experienced by these rocks on their parent asteroids in contrast l chondrites have a relatively high proportion of samples containing maskelynite 11 most likely a result of catastrophic parent body disruption 470 ma ago

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

craters without ejecta abstract a significant portion of the solar system s population of minor bodies may be quite porous a unique aspect of crater formation in porous bodies is that large craters may form without the ejecta deposits that are associated with craters on less porous bodies in this paper laboratory experiments and scaling theories are used to identify the conditions under which ejecta deposits are suppressed the results are consistent with the interpretation that large craters on asteroid mathilde porosity 50 and saturn s moon hyperion porosity 40 apparently formed without producing significant ejecta deposits while smaller bodies do have notable regoliths

boulder stranding in ejecta launched by an impact generated seismic pulse abstract we consider how an impact generated seismic pulse affects the surface of an asteroid distant from the impact site with laboratory experiments on dry polydisperse gravel mixtures we track the trajectories of particles ejected from the surface by a single strong upward propagating pressure pulse high speed video images show that ejecta trajectories are independent of particle size and collisions primarily take place upon landing when they land particles are ballistically sorted as proposed by shinbrot et al 2017 leaving larger particles on the surface and smaller particles more widely dispersed a single strong pulse can leave previously buried boulders stranded on the surface boulder stranding due to an impact excited seismic pulse is an additional mechanism that could leave large boulders present on the surface of rubble asteroids such as 162173 ryugu 101955 bennu and 25143 itokawa

the tsiolkovskiy crater landslide the moon an lroc view abstract evidence suggests that the lobate flow feature that extends 72 km outward from the western rim of tsiolkovskiy crater is a long runout landslide this landslide exhibits three possibly four morphologically different parts likely caused by local conditions all of these plus the ejecta of tsiolkovskiy crater and its mare fill are approximately of the same crater model age i e 3 55 0 1 ga the enormous size of this landslide is unique on the moon and is a result of a combination of several geometric factors e g its location relative to fermi crater and that tsiolkovskiy crater was an oblique impact that produced an ejecta forbidden zone on its western side schultz 1976 the landslide formed in this ejecta free zone as the rim of tsiolkovskiy collapsed and its debris flowed across the relatively smooth flat floor of fermi crater in this location it could be easily identified as a landslide and not ejecta its mobility and coefficient of friction are similar to landslides in valles marineris on mars but less than wet or even dry terrestrial natural flows this suggests that the mars landslides may have been emplaced dry the high density of small craters on the landslide is likely an illusion caused by the effects of age related differences in regolith thickness on crater morphology and the presence of the abundant young circular secondary craters produce by debris ejected from distant fresh craters

evidence of rift segmentation and controls of middle to late jurassic synrift deposition in the ryggsteinen ridge area northern north sea multiple episodes of rifting and preexisting structure reactivation may result in rift segmentation which is a fundamental factor in deformation variability and sediment dispersal to understand the structural framework and basin evolution in segmented rift systems this study addresses an integrated structural and stratigraphic analysis of the middle upper jurassic sequences in the northern north sea four major fault systems were active during the jurassic oblique rifting 1 east northeast striking synthetic shear faults along preexisting caledonian structures which developed into basin wide transfer fault zones and induced first order rift segmentation each rift segment shows independent deformation 2 north south striking reactivated permian triassic faults which uplifted the footwall developing major structural highs 3 southwest northeast striking normal faults and 4 northwest southeast striking antithetic shear oblique slip faults which caused internal deformation and induced second order rift segmentation within rift segments rift segmentation led to stepovers from the viking graben to the sogn graben the entry of regional drainages was determined by first order rift segmentation prior to segmentation bathonian callovian drainage shows a northeast southwest line source nature and facies changes are quite homogenous during the oxfordian kimmeridgian segmentation took place and modified the drainage system into a point source facies show a large spectrum of variability from shallow marine to deep water turbidites over short distances furthermore drainage routes were influenced by second order rift segmentation west directed drainages deflected toward the northwest by adapting to gradient surfaces established by oblique slip faults as such the structural and stratigraphic expressions in this study provide an example of synrift deposition controlled by different orders of rift segmentation

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

shock events in the solar system the message from minerals in terrestrial planets and asteroids impacts are central to the origin and evolution of planets of the solar system the shapes of craters which can reach up to 1 000 km in diameter on the moon provide critical information on the large scale dynamics of the impact and related shock minerals formed at high pressure and temperature found in shocked terrestrial rocks and meteorites give additional and complementary insights on the shock process at a smaller scale typically from a few micrometers to a few millimeters local flaws in rocks such as voids and mineral interfaces are the preferential sites for the formation of high pressure melts and minerals calculations based on the physics of shocks and the thermodynamics and kinetics of mineral transformations provide orders of magnitude for the duration transient pressure and prevailing temperature conditions of shock events case studies on shocked terrestrial and extraterrestrial materials illustrate the links between these parameters and impact duration many of the high pressure min

a formation mechanism for concentric ridges in ejecta surrounding impact craters in a layer of fine glass beads abstract ejecta patterns are experimentally examined around craters formed in a layer of glass beads by vertical impacts at low velocities the diameters of the constituent glass beads of three different targets range 53 63 m 90 106 m and 355 500 m the impact velocities and ambient pressures range from a few to 240 m s 1 and from 500 pa to the atmospheric pressure respectively various ejecta patterns are observed around craters and are classified into two major classes based on whether they have concentric ridges or not we propose a possible formation model for the ridges in which the wake created by a projectile as it passes through the atmosphere causes the crater rim to collapse the model can explain the observation that the degree of collapse of the resultant crater rim depends on the impact velocity and ambient pressure using the ratio between the hydrodynamic drag of the airflow induced by the wake and the gravitational force of the degraded part of the rim we calculate the critical conditions of the impact velocity and ambient pressure necessary for the wake to erode the rim the conditions turn out to be roughly consistent with the boundary between the two morphological classes as a result it is possible that the projectile wake triggers the collapse of the crater rim leading to a ground hugging flow that settles to form the distal ridge observed in this study this mechanism may play a role in producing ejecta morphologies on planetary bodies with atmosphere

origin of the pluto charon system constraints from the new horizons flyby abstract new horizon s accurate determination of the sizes and densities of pluto and charon now permit precise internal models of both bodies to be constructed assuming differentiated rock ice structures we find that pluto is close to 2 3 solar composition anhydrous rock by mass and charon 3 5 solar composition anhydrous rock by mass pluto and charon are closer to each other in density than to other large 1000 km diameter kuiper belt bodies despite this we show that neither the possible presence of an ocean under pluto s water ice shell and no ocean within charon nor enhanced porosity at depth in charon s icy crust compared with that of pluto are sufficient to make pluto and charon s rock mass fractions match all four small satellites styx nix kerberos hydra appear much icier in comparison with either pluto or charon in terms of a giant impact origin both these inferences are most consistent with the relatively slow collision of partly differentiated precursor bodies canup astrophys j 141 35 2011 this is in turn consistent with dynamical conditions in the ancestral kuiper belt but implies that the impact precursors themselves accreted relatively late and slowly to limit 26 al and accretional heating the iciness of the small satellites is not consistent with direct formation of the pluto charon system from a streaming instability in the solar nebula followed by prompt collapse of gravitationally bound pebble piles a proposed formation mechanism for kuiper belt binaries nesvorn et al astron j 140 785 793 2010 growth of pluto scale bodies by accretion of pebbles in the ancestral kuiper belt is not ruled out however and may be needed to prevent the precursor bodies from fully differentiating due to buried accretional heat prior to the charon forming impact

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

physical properties of basalt ejecta boulders at lonar crater india insights into the target heterogeneity and impact spallation processes in basalt with application to mars

impact experiments i ejecta velocity distributions and related results from regolith targets velocity distributions are determined for ejecta from 14 experimental impacts into regolithlike powders in near vacuum conditions at velocities from 5 to 2321 m sec of the two powders the finer produces slower ejecta ejecta include conical sheets with ray producing jets and in the fastest impacts at vimp 700 m sec high speed vertical plumes of uncertain nature velocities in the conical sheets and jets increase with impact velocity sect 6 ejecta velocities also increase as impact energy and crater size increase a suggested method of estimating ejecta velocity distributions in large scale impacts involves homologous scaling according to r rcrater where r is radial distances from the crater sect 7 the data are consistent with holsapple schmidt scaling relationships sect 8 the fraction of initial total impact energy partitioned into ejecta kinetic energy increases from around 0 1 for the slow impacts to around 10 for the fast impacts with the main increase probably at the onset of the hypervelocity impact regime sect 9 crater shapes are discussed including an example of a possible frozen transient cavity sect 10 ejecta blanket thickness distributions as a function of r vary with target material and impact speed but the results measured for hypervelocity impacts agree with published experimental and theoretical values sect 11 the low ejecta velocities for powder targets relative to rock targets together with the paucity of powder ejecta in low speed impacts 1 projectile mass for vimp 10 m sec enhance early planetary accretion effeciency beyond that in some earlier theoretical models 100 efficient accretion is found for certain primordial conditions sect 12