highland contamination in lunar mare soils improved mapping with multiple end member spectral mixture analysis mesma 1 multiple end member spectral mixture analysis mesma was applied to the clementine uvvis global 1 km multispectral data set and the resulting highland material fraction image was used to investigate highland contamination of mare surfaces by impact cratering mesma decomposes each pixel with the number of end members fewer than the number of the spectral bands of clementine uvvis data this allows the use of variable end member combinations to accommodate global spectral variance a 13 end member set of lunar soil components that spans the lunar spectral diversity was selected from the uvvis data and included 1 mature and 4 fresh highland soils 4 mature and 2 fresh mare soils and 2 soils of dark mantling materials this set was applied to the lunar data through mesma and an aggregated global highland abundance was derived comparison of the highland fraction results from mesma and traditional spectral mixture analysis sma demonstrated that mesma better accommodates the compositional variation of mare soils resulting in a greater accuracy in the measurement of highland with the derived products we investigated highland contamination in mare nectaris mare fecunditatis and mare crisium these mare surfaces are proximal to the large craters theophilus taruntius langrenus and proclus and provided an opportunity to investigate highland contamination by large impacts the analyses indicated that theophilus impact resulted in highland contamination of 20 80 on most of mare nectaris while taruntius and langrenus impacts caused 5 40 contamination of mare fecunditatis the trend of minimal highland abundance in mare fecunditatis correlates poorly with basalt thickness suggesting an efficient lateral mixing due to the large craters taruntius and langrenus additionally proclus combining with other craters was responsible for highland contamination in most of the mare crisium surface and some basalt classification in this mare reflects highland contamination of the spectral properties mesma significantly reduces uncertainty in calculating global composition variations due to mixing and these analyses demonstrate that large impacts played a dominant role in delivering highland materials onto three mare surfaces

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

aptian albian palaeogeography of neo tethyan domain abstract the aptian albian interval 124 5 97 0 ma was a critical time both globally and for the tethyan domain in the tethyan domain it was the time when a united neo tethyan subduction zone became established between the future site of the alps and southeast asia and greatly accelerated the rate of north south convergence throughout the tethyan region a consequent widespread transgression on both laurasian and gondwanian margins deposited orbitolina limestones and associated clastics on wide continental shelf areas on the sea floor of the neo tethys ocean organic rich siliceous black shales radiolarites and turbidites with minor amounts of carbonates were deposited siliceous black shales accumulated under euxinic conditions of restricted small ocean basins of the neo tethyan realm and along their margins these areas received large amounts of organic debris from deltaic areas and coastal plains of the neighbouring continental shelves following the establishment of the north dipping subduction zone the laurasian passive margin was largely converted to an active one fringed by a large number of magmatic arcs that greatly influenced the sedimentation on the margin land areas and continental basins were widespread on this margin in northern tibet china and indo china this was a consequence of the late triassic to early jurassic continental collissions and accompanied intracontinental convergence that lasted well into the cretaceous a detailed aptian albian facies map and a crude palinspastic palaeogeographic map of the entire tethyan realm are prepared in order to express the contrasts in sedimentation on laurasian and gondwanian margins and to show the compatibility of the presented data with the most recent and reliable reconstructions of the major tectonic elements in tethysides

estimates of primary ejecta and local material for the orientale basin implications for the formation and ballistic sedimentation of multi ring basins abstract a clear understanding of thickness distributions of primary ejecta and local material is critical to interpreting the process of ballistic sedimentation provenances of lunar samples the evolution of the lunar surface and the origin of multi ring basins the youngest lunar multi ring basin orientale provides the best preserved structure for determining the thicknesses of primary ejecta and local material in general the primary ejecta thickness was often estimated using crater morphometry however previous methods ignored either crater erosion the crater interior geometry or both in addition ejecta deposits were taken as mostly primary ejecta and as far as we know the local material thickness had not been determined for the orientale in this work we proposed a model based on matching measurements of partially filled pre orientale craters pfpocs with the simulations of crater erosion to determine their thicknesses we provided estimates of primary ejecta thickness distribution with the thickness of 0 85 km at cordillera ring and a decay power law exponent of b 2 8 the transient crater radius of 200 km excavation volume of 2 3 10 6 km 3 primary ejecta volume of 2 8 10 6 km 3 these results suggest that previous works e g fassett et al 2011 moore et al 1974 might overestimate the primary ejecta thicknesses of orientale and the primary ejecta thickness model of pike 1974a for multi ring basins may give better estimates than the widely cited model of mcgetchin et al 1973 and the scaling law for impacts into ottawa sand housen et al 1983 structural uplift decays slower than previously thought and rim relief is mostly rim uplift for orientale the main reason for rim uplift may be the fracturing and squeezing upward of the surrounding rocks the proportion of local material to ejecta deposits increases with increasing radial distance from basin center and the thickness of local material is larger than that of primary ejecta at distance larger than certain distance 1 5 basin radius for orientale these results suggest ballistic sedimentation is important for multi ring basins and ejecta deposits can t be considered as mostly primary ejecta everywhere

modeling an exogenic origin for the equatorial ridge on iapetus abstract iapetus has a ridge along the equator that extends continuously for more than 110 in longitude parts of the ridge rise as much as 20 km above the surrounding terrains most models for the formation of this enigmatic ridge are endogenic generally requiring the formation of a fast spinning iapetus with an oblate shape due to the rotation speed many of these require specific scenarios and have constraining parameters in order to generate a ridge comparable to what is seen today an exogenic formation mechanism has also been proposed that the ridge represents the remains of an early ring system around iapetus that collapsed onto the surface thus far none of the models have conclusively identified the origin of the ridge in this study an exogenic origin for the ridge is assumed which is derived from a collapsing disk of debris around iapetus without invoking any specific model for the generation of the debris disk here we evaluate whether it is possible to generate a ridge of the size and shape as observed by simulating the impact of the collapsing debris using the cth hydrocode pi scaling calculations suggest that extremely oblique impact angles 1 10 are needed to add to ridge topography these extreme impact angles severely reduce the cratering efficiency compared to a vertical impact adding material rather than eroding it during crater formation furthermore material is likely to be excavated at low angles enhancing downrange accumulation multiple impacts from debris pieces will heighten this effect because infalling debris is predicted to impact at extremely low angles both of these effects might have contributed to ridge formation on iapetus the extreme grazing angles of the impacts modeled here decouple much of the projectile energy from the target and impact heating of the surface is not significant these models suggest that a collapsing disk of debris should have been able to build topography to create a ridge around iapetus

the importance of secondary cratering to age constraints on planetary surfaces small craters less than one kilometer diameter can be primary craters produced by impact of interplanetary debris or they can be secondary craters produced by fallback of high velocity ejecta blocks from much larger but infrequent primary impacts the prevalent assumption over recent decades has been that primaries are most abundant so most small craters are independent random events and can be used for dating however recent results from europa and mars support the early theory that distant secondaries globally dominate the number of small lunar craters this would invalidate part of production functions that have been widely used for age dating crater excavation results in higher mean ejection velocities for smaller fragments resulting in a steeper size frequency distribution for secondary craters than is produced by the same size frequency distribution of interplanetary debris this review also discusses how small craters can sometimes be used to derive meaningful age constraints

modeling the provenance of the apollo 16 regolith 1 the regolith at the apollo 16 landing site the only apollo landing site in the central lunar highlands contains material derived from a number of sources a model that accounts for the introduction of basin ejecta and mixing of the megaregolith is used to estimate the abundance of basin material in the apollo 16 regolith megaregolith mixing model estimates of the abundance of primary ejecta from the imbrium serenitatis and nectaris basins are found to be present in roughly equal 8 10 proportions additionally the presence of mare derived material in the apollo 16 regolith suggests that a significant component of the regolith 15 23 is derived from lateral transport there are inherent difficulties in directly comparing model results with ground truth at apollo 16 results suggest shallower mixing during ejecta emplacement than predicted by oberbeck et al 1975

the nature of crater rays the copernicus example crater rays are formed during a cratering event as target material is ballistically ejected to distances of many crater radii forming narrow generally high albedo approximately linear features extending outward from the crater the nature of crater rays was examined for the lunar crater copernicus using new information on the composition of surface material from near ir reflectance measurements surface roughness from radar backscatter measurements and photogeologic data from available images part of the data analysis included use of mixing models to quantify the mixing systematics observed between primary ejecta and local substrate of the ray on the basis of compositional parameters from reflectance spectra primary material from copernicus can be detected in the surface material of rays in decreasing amounts with increasing radial distance e g 20 25 primary ejecta at six crater radii for distances greater than three crater radii the proportion of local material to primary ejecta observed from these compositional reflectance data is approximately equal to that predicted by previous laboratory and ballistic studies of craters within three crater radii the compositional data indicate a higher proportion of primary ejecta than predicted for extended areas along the ray that do not contain large secondary craters the primary ejecta is intimately mixed on the granular scale with local material throughout the regolith the relatively high albedo of the rays of copernicus is due to the feldspathic composition highland of the primary ejecta in rays emplaced on a mare substrate immature local substrate is only observed in copernicus s ray at large unmantied secondary craters or other areas with sufficient topographic slope to prevent the accumulation of mature soils crater rays are filamentous generally high albedo features that emanate nearly radially from young impact structures rays are narrow in relation to the crater radius extend distally for many crater radii and constitute a distinctive albedo feature around some of the largest and freshest lunar craters visible from earth synoptic views of ray systems figure la give the strong impression that the rays comprise bright material that has been excavated from the crater cavity and been emplaced beyond the continuous ejecta deposit if this visual impression is correct then the process that forms crater rays represents a significant mechanism for the lateral transport of primary material to great distances from its point of origin and particularly for craters larger than a let tens of kilometers in diameter constitutes a fundamental process for the regional mixing of crustal materials earth based telescopic images of the moon allowed the first studies and descriptions of rays and ray systems shoemaker 1962 described the arcuate and loop shaped streaks of the copernicus ray systems and showed that these patterns could be locally resolved into individual en echelon feather shaped elements 15 20 km in length with their long axes oriented approximately radially to the crater he noted the presence of elongate depressions or gouges in the ray system ranking in diameter from 8 km down to the limits of resolution and generally located at the proximal ends of the individual ray elements shoemaker observed that some of the gouges were

the lunar wide effects of basin ejecta distribution on the early megaregolith the lunar surface is marked by at least 43 large and ancient impact basins each of which ejected a large amount of material that modified the areas surrounding each basin we present an analysis of the effects of basin formation on the entire lunar surface using a previously defined basin ejecta model our modeling includes several simplifying assumptions in order to quantify two aspects of basin formation across the entire lunar surface 1 the cumulative amount of material distributed across the surface and 2 the depth to which that basin material created a well mixed megaregolith we find that the asymmetric distribution of large basins across the moon creates a considerable nearside farside dichotomy in both the cumulative amount of basin ejecta and the depth of the megaregolith basins significantly modified a large portion of the nearside while the farside experienced relatively small degrees of basin modification following the formation of the large south pole aitken basin the regions of the moon with differing degrees of modification by basins correspond to regions thought to contain geochemical signatures remnant of early lunar crustal processes indicating that the degree of basin modification of the surface directly influenced the distribution of the geochemical terranes observed today additionally the modification of the lunar surface by basins suggests that the provenance of lunar highland samples currently in research collections is not representative of the entire lunar crust identifying locations on the lunar surface with unique modification histories will aid in selecting locations for future sample collection

the toms canyon structure new jersey outer continental shelf a possible late eocene impact crater abstract the toms canyon structure 20 22 km wide is located on the new jersey outer continental shelf beneath 80 100 m of water and is buried by 1 km of upper eocene to holocene sedimentary strata the structure displays several characteristics typical of terrestrial impact craters flat floor upraised faulted rim brecciated sedimentary fill but several other characteristics are atypical an unusually thin ejecta blanket lack of an inner basin peak ring or central peak being nearly completely filled with breccia seismostratigraphic and biostratigraphic analyses show that the structure formed during planktonic foraminiferal biochron p15 of the early to middle late eocene the fill unit is stratigraphically correlative with impact ejecta cored nearby at deep sea drilling project dsdp site 612 and at ocean drilling program odp sites 903 and 904 22 35 km southeast of the toms canyon structure the toms canyon fill unit also correlates with the exmore breccia which fills the much larger chesapeake bay impact crater 90 km diameter 335 km to the southwest on the basis of our analyses we postulate that the toms canyon structure is an impact crater formed when a cluster of relatively small meteorites approached the target site bearing n 50 e and struck the sea floor obliquely

microimpact phenomena on australasian microtektites implications for ejecta plume characteristics and lunar surface processes abstract detailed investigations of the microimpact phenomena on australasian microtektites from four samples from the central indian basin reveal an array of features such as very low velocity captured droplets welded projectiles angular fragments and dust craters generated by projectiles defining an oblique trajectory high velocity pitless craters and the conventional hypervelocity craters with well defined central pits and radial and concentric cracks found commonly on lunar surface materials the microimpacts are a consequence of interparticle collisions within the ejecta plume as suggested by their chemistry subsequent to a major impact and therefore reveal processes inherent in an impact generated plume all the impact phenomena observed here have taken place while the targets and projectiles were in flight and are therefore secondary impacts in lunar terms however some of the resultant features are analogous to lunar micro craters attributed to primary impacts by cosmic dust therefore ballistic sedimentation on the moon is likely to contain plume collisional debris as well

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

cosmic wabi sabi tell tale morphological imperfections in impact crater shapes revealed by numerical analysis abstract geomorphological features result from a combination of factors such as material strength structure and the forces of nature an impact crater is a good example in an ideally simple situation e g a vertical impact of a spherical object into a homogeneous target the resulting crater will obtain a perfectly circular shape any deviation from the perfect circle can reveal important information about the cratering process target properties and even the surficial environment in which the impact occurred e g the presence of an atmosphere however irregularities in the crater shape are not always obvious what appears to be a perfect circle may hide subtle information undetectable to the naked eye here we present a new method of numerical aspect classification that allows even the slightest morphological irregularities to be detected our method is based on a numerical analysis of dip directions of segments within a digital terrain model dtm of the crater rim we show the successful application of our method on a crater with well known polygonal shape meteor crater usa as well as two apparently circular martian impact craters our crater aspect classification cac can be applied on any other micro to mega scale structures for which a dtm can be produced thus allowing applications to fields of science and technology within and beyond the scope of geomorphology

remote sensing and geologic studies of the schiller schickard region of the moon near infrared reflectance spectra multispectral images and photogeologic data for the schiller schickard ss region were obtained and analyzed in order to determine the composition and origin of a variety of geologic units these include light plains deposits orientale related deposits mare units and dark haloed impact craters dhcs spectral data indicate that the pre orientale highland surface was dominated by noritic anorthosite near ir spectra show that dhcs in the region have excavated ancient 3 8 ga mare basalts from beneath highland bearing material emplaced by the orientale impact ancient mare basalts were widespread in the ss region prior to the orientale event and their distribution appears to have been controlled by the presence of several old impact basins including the schiller zucchius basin and a basin previously unrecognized both near ir spectra and multispectral images indicate that light plains and other orientale related units in the ss region contain major amounts of local pre orientale mare basalt the amounts of local material in these deposits approach but seldom exceed the maximum values predicted by the local mixing hypothesis of oberbeck and co workers

geological and geophysical investigation of kamil crater egypt we detail the kamil crater egypt structure and refine the impact scenario based on the geological and geophysical data collected during our first expedition in february 2010 kamil crater is a model for terrestrial small scale hypervelocity impact craters it is an exceptionally well preserved simple crater with a diameter of 45 m depth of 10 m and rayed pattern of bright ejecta it occurs in a simple geological context flat rocky desert surface and target rocks comprising subhorizontally layered sandstones the high depth to diameter ratio of the transient crater its concave yet asymmetric bottom and the fact that kamil crater is not part of a crater field confirm that it formed by the impact of a single iron mass or a tight cluster of fragments that fragmented upon hypervelocity impact with the ground the circular crater shape and asymmetries in ejecta and shrapnel distributions coherently indicate a direction of incidence from the nw and an impact angle of approximately 30 to 45 newly identified asymmetries including the off center bottom of the transient crater floor downrange maximum overturning of target rocks along the impact direction and lower crater rim elevation downrange may be diagnostic of oblique impacts in well preserved craters geomagnetic data reveal no buried individual impactor masses 100 kg and suggest that the total mass of the buried shrapnel 100 g is approximately 1050 1700 kg based on this mass value plus that of shrapnel 10 g identified earlier on the surface during systematic search the new estimate of the minimum projectile mass is approximately 5 t

the davy crater chain implications for tidal disruption in the earth moon system and elsewhere linear crater chains on ganymede and callisto apparently provide an impact record of tidally disrupted bodies like comet shoemaker levy 9 at jupiter we propose that the davy crater chain on the earth s moon records a similar tidal disruption event in the earth moon system most lunar crater chains result from either volcanism or secondary impacts but the davy chain differs in setting and appearance from these crater chains and instead shows morphologic similarities to the jovian crater chains while the davy chain is much smaller than the jovian crater chains tidal disruption models indicate that this smaller size is consistent with the earth s lower mass the disruption models also indicate that most earth crossing comets and asteroids are moving too fast for fragment dispersion before impact to produce the observed crater chain length thus the davy and jovian crater chains apparently result from bodies with different orbital characteristics in addition the limited dispersion of high velocity fragments at the moon suggests that crater chains may not form for all disruption events or on all planetary satellites based on modeled fragment dispersions elsewhere in the solar system we suggest that similar crater chains may be present on iapetus and possibly on titan titania or oberon

discrete element modeling of subglacial sediment deformation 1 the discrete element method dem is used in this study to explore the highly nonlinear dynamics of a granular bed when exposed to stress conditions comparable to those at the bed of warm based glaciers complementary to analog experiments the numerical approach allows a detailed analysis of the material dynamics and the shear zone development during progressive shear strain the geometry of the heterogeneous stress network is visible in the form of force carrying grain bridges and adjacent volumetrically dominant inactive zones we demonstrate how the shear zone thickness and dilation depend on the level of normal overburden stress and we show how high normal stress can mobilize material to great depths the particle rotational axes tend to align with progressive shear strain with rotations both along and reverse to the shear direction the results from successive laboratory ring shear experiments on simple granular materials are compared to results from similar numerical experiments the simulated dem material and all tested laboratory materials deform by an elastoplastic rheology under the applied effective normal stress these results demonstrate that the dem is a viable alternative to continuum models for small scale analysis of sediment deformation it can be used to simulate the macromechanical behavior of simple granular sediments and it provides an opportunity to study how microstructures in subglacial sediments are formed during progressive shear strain

hidden tectonism on miranda s elsinore corona revealed by polygonal impact craters abstract we mapped the spatial distribution and orientations of polygonal impact craters pics on the classical uranian satellite miranda to gain insight into the complex tectonic history of this moon pic straight rim segments in regions without visible fractures can be used to infer sub regolith and or sub resolution fracture orientations we identified and analyzed 14 pics on miranda nine of which are located on its trailing hemisphere overprinting elsinore corona and the nearby cratered terrain these pics do not follow the e w orientations of the ridges within elsinore s bounding terrain indicating that these ridges are not underlain by deep rooted at least as deep as the crater bowl sub vertical normal or strike slip faults instead the ridges may be contractional features consisting of folds and possibly thrust faults or cryovolcanic features without fault structures furthermore the pics in this region indicate that a previously undetected nw oriented fracture system is present over much of miranda s trailing hemisphere another pic somewhat distal from elsinore on miranda s anti uranus hemisphere might be associated with the nw fractures as well additionally three pics are present proximal to verona rupes on miranda s uranus facing hemisphere these pics exhibit n s orientations consistent with the surrounding fractures that make up miranda s global rift system one other pic is proximal to arden and might be associated with the arden bounding terrain identification of the nw fracture system provides new information on miranda s complex geologic history based on cross cutting relationships with miranda s ancient subdued crater population the nw fracture system is older than elsinore however this fracture system also overprints elsinore suggesting reactivation of the nw fractures after this corona had formed along with discussion of miranda s geologic history we consider the possibility that miranda has undergone true polar wander and we compare its terrains and geologic features to similar features on the saturnian satellite enceladus

lunar megaregolith mixing by impacts spatial diffusion of basin melt and its implications for sample interpretation abstract the formation ages of lunar impact basins are critical to understanding the late accretion history of the inner solar system furthermore the correct interpretation of the provenance and isotopic dates of basin derived impact melt basin melt is essential for the calibration of lunar chronology function however abundances of basin melt in the lunar near surface are not well understood basin melt has been gardened by a long sequence of subsequent impact events altering its abundance and size distribution we developed a numerical model to investigate this process by means of the monte carlo method in a spatially resolved model the fraction of melt in ejecta was tracked globally and at the apollo 14 17 and luna 20 sampling sites and was compared with k ar age distributions of lunar impact melt breccias it was found that melt produced by the very large spa basin as well as the relatively late forming imbrium basin should be dominant in the near surface top one meter the simulation shows that the melt component at the apollo 14 17 and luna 20 sites is strongly affected by nearby mid to late forming basins imbrium melt should be abundant in apollo 14 17 samples crisium melt is the most significant component of basin sourced melt in luna 20 samples all the apollo 14 17 and luna 20 samples could include melt from serenitatis and the spa basin nectaris melt should occur in apollo 16 apollo 17 and luna 20 samples and orientale melt has no significant mixing in the apollo 14 17 and luna 20 sampling sites in general besides a prominent age peak at 3 9 ga related to the imbrium basin the model predicts pronounced abundance peaks of older basin melt 3 9 ga which tend to be absent from distributions of k ar ages of impactites from landing sites the diffusion characteristics of basin melt suggest that for future sampling aimed at collecting early basin melt the re excavation zones of late impact craters larger than tens of kilometer in diameter inside basins may provide the highest abundances of melt from early basins

the mixing of lunar regolith vital updates to a canonical model abstract in this work we update the regolith mixing model presented by gault et al 1974 including new input values and reworking key parameters much as gault et al did we present a way to calculate the rate at which lunar regolith is overturned at depth the model describes a mixing front that proceeds downward from the surface following a power law function of time our most important update is the inclusion of secondary impacts our calculations show that secondaries are necessary to produce the reworking rate inferred from the depth distribution of surface correlated material in apollo cores fruchter et al 1977 morris 1978 blanford 1980 from the rate at which splotches rework the top 3 cm of regolith speyerer et al 2016 and from the rate at which diviner cold spots bandfield et al 2014 and crater rays pieters et al 1985 hawke et al 2004 werner and medvedev 2010 are reworked into background regolith overturn calculations that only consider the impact of primaries fail to describe observed reworking rates at all depths and timescales we conclude that secondary impacts dominate mixing in the top meter of lunar regolith

atmospheric effects on ejecta emplacement laboratory experiments allow the investigation of complex interactions between impacts and an atmosphere although small in scale they can provide essential first order constraints on the processes affecting late stage ballistic ejecta and styles of ejecta emplacement around much larger craters on planetary surfaces the laboratory experiments involved impacting different fine grained particulate targets under varying atmospheric pressure and density different gas compositions during crater formation ballistic ejecta form the classic cone shaped profile observed under vacuum conditions as atmospheric density increases for a given pressure however the ejecta curtain bulges at the base and pinches above this systematic change in the ejecta curtain reflects the combined effects of deceleration of ejecta smaller than a critical size and entrainment of these ejecta within atmospheric vortices created as the outward moving wall of ejecta displaces the atmosphere additionally a systematic change in emplacement style occurs as a function of atmospheric pressure largely independent of density contiguous ejecta rampart superposing ballistically emplaced deposits 0 06 to 0 3 bar ejecta flow lobes 0 3 to 0 7 bar and radial patterns 0 8 bar underlying processes controlling such systematic changes in emplacement style were revealed by observing the evolution of the ejecta curtain by changing target materials including layered targets and low density particulates by varying atmospheric density by changing impact angle and by comparing the ejecta run out distances with first order models of turbidity flows three distinct ejecta emplacement processes can be characterized ejecta ramparts result from coarser clasts sorted and driven outward by vortical winds behind the outward moving ejecta curtain this style of wind modified emplacement represents minimal ejecta entrainment and is enhanced by a bimodal size distribution in the ejecta such eddy supported flows are observed to increase in run out distance scaled to crater size with increasing atmospheric pressure by analogy with turbidity flows this scaled distance should increase as r1 2 for a given atmospheric pressure and degree of entrainment ejecta flows with much greater run out distances develop as the turbulent power in atmospheric response winds increase such flows overrun and scour the inner ejecta facies thereby producing distinct inner and outer facies the degree of ejecta entrainment depends on the dimensionless ratio of drag to gravity forces acting on individual ejecta and the intensity of the winds created by the outward moving curtain entrainment increases with increasing atmospheric density and ejection velocity crater size but decreases with ejecta density and size the intensity of curtain generated winds increases with ejection velocity crater size the dimensionless drag ratio characterizing the laboratory experiments can be applied to mars since the reduced atmospheric density is offset by the increased ejection velocities for kilometer scale events for a given crater size ejection velocity and atmospheric conditions a wide range of nonballistic ejecta emplacement styles could occur simply by varying ejecta sizes even without the presence of water alternatively the onset crater diameter for nonballistic emplacement styles can reflect the range of ejecta sizes possible from the diverse martian geologic history massive basalts to fine grained aeolian deposits scaling considerations further predict that ejecta run out distances scaled to crater size on mars should increase as r1 2 hence long run out flows dependent on crater diameter need not reflect depth to a buried reservoir of water on venus however the dense atmosphere maximizes entrainment and results in ejecta flow densities approaching a constant fraction of the atmospheric density under such conditions ejecta run out distances should decrease as r 1 2

occurrence and mechanisms of impact melt emplacement at small lunar craters abstract using observations from the lunar reconnaissance orbiter camera lroc we assess the frequency and occurrence of impact melt at simple craters less than 5 km in diameter nine hundred and fifty fresh randomly distributed impact craters were identified for study based on their maturity albedo and preservation state the occurrence frequency and distribution of impact melt deposits associated with these craters particularly ponded melt and lobate flows are diagnostic of melt emplacement mechanisms like larger craters those smaller than a few kilometers in diameter often exhibit ponded melt on the crater floor as well as lobate flows near the crater rim crest the morphologies of these deposits suggest gravity driven flow while the melt was molten impact melt deposits emplaced as veneers and sprays thin layers of ejecta that drape other crater materials indicate deposition late in the cratering process the deposits of fine sprays are particularly sensitive to degradation exterior melt deposits found near the rims of a few dozen craters are distributed asymmetrically around the crater and are rare at craters less than 2 km in diameter pre existing topography plays a role in the occurrence and distribution of these melt deposits particularly for craters smaller than 1 km in diameter but does not account for all observed asymmetries in impact melt distribution the observed relative abundance and frequency of ponded melt and flows in and around simple lunar craters increases with crater diameter as was previously predicted from models however impact melt deposits are found more commonly at simple lunar craters i e those less than a few kilometers in diameter than previously expected ponded melt deposits are observed in roughly 15 of fresh craters smaller than 300 m in diameter and 80 of fresh craters between 600 m and 5 km in diameter furthermore melt deposits are observed at roughly twice as many non mare craters than at mare craters we infer that the distributions and occurrences of impact melt are strongly influenced by impact velocity and angle target porosity pre existing topography and degradation additionally areally small and volumetrically thin melt deposits are sensitive to mixing with solid debris and or burial during the modification stage of impact cratering as well as post cratering degradation thus the production of melt at craters less than 800 m in diameter is likely greater than inferred from the present occurrence of melt deposits which is rapidly affected by ongoing degradation processes

lunar orientale impact basin secondary craters spatial distribution size frequency distribution and estimation of fragment size secondary impact craters features created by projectiles ejected from a primary impact contain important information about the primary cratering event and the nature and distribution of its ejecta the orientale impact basin d similar to 930km is the youngest and the least degraded large impact basin on the moon and has the most recognizable secondary impact craters we identified and mapped 2 728 secondary craters in the investigated area of similar to 1 66x10 7 km 2 covering an area from the rim of orientale to six radii secondary crater diameters range from similar to 2 to 27km and the median diameter decreases as distance increases secondary craters are concentrated predominantly in the northwest and southwest the ejecta deposit pattern inferred from secondary crater distribution suggests that the orientale basin was formed by an oblique impact in which the downrange direction was 240 degrees 265 degrees in azimuth and the incidence angle was steeper than 20 degrees the cumulative size frequency distribution of mapped secondary craters steepens as diameter increases and is very well approximated with a weibull distribution with an exponent 1 32 a widely used crater scaling relationship predicts that the fragments that produced the secondary craters were predominantly in similar to 0 5 2 km diameter range over the investigated area the diameter of the largest fragment however decreases with increasing distance from orientale on the basis of the diameter of the largest secondary crater of orientale and other craters and basins the largest secondary crater of the south pole aitken basin is estimated to be similar to 40km in diameter we explore the implications of these findings for the evolution of the megaregolith and future sample return missions

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

the smart 1 lunar impact the smart 1 spacecraft impacted the moon on 3rd september 2006 at a speed of 2 km s 1 and at a very shallow angle of incidence 1 the resulting impact crater is too small to be viewed from the earth accordingly the general crater size and shape have been determined here by laboratory impact experiments at the same speed and angle of incidence combined with extrapolating to the correct size scale to match the smart 1 impact this predicts a highly asymmetric crater approximately 5 5 26 m long 1 9 9 m wide 0 23 1 5 m deep and 0 71 6 9 m3 volume some of the excavated mass will have gone into crater rim walls but 0 64 6 3 m3 would have been ejecta on ballistic trajectories corresponding to a cloud of 2200 21 800 kg of lunar material moving away from the impact site the shallow messier crater on the moon is similarly asymmetric and is usually taken as arising from a highly oblique impact the light flash from the impact and the associated ejecta plume were observed from earth but the flash magnitude was not obtained so it is not possible to obtain the luminous efficiency of the impact event

the canyon diablo impact event 2 projectile fate and target melting upon impact abstract despite its centennial exploration history there are still unresolved questions about meteor crater the first recognized impact crater on earth this theoretical study addresses some of these questions by comparing model results with field and laboratory studies of meteor crater our results indicate that meteor crater was formed by a high velocity impact of a fragmented projectile ruling out a highly dispersed swarm as well as a very low impact velocity projectile fragmentation caused many fragments to fall separately from the main body of the impactor making up the bulk of the canyon diablo meteorites most of these fragments were engulfed in the expansion plume as they approached the surface without suffering high shock compression and were redistributed randomly around the crater thus the distribution of canyon diablo meteorites is not representative of projectile trajectory as is usual for impactor fragments in smaller strewn fields at least 50 of the main impactor was ejected from the crater during crater excavation and was dispersed mostly downrange of the crater as molten particles spheroids and highly shocked solid fragments shrapnel when compared with the known distribution model results suggest an impactor from the sw overall every model case produced much higher amounts of pure projectile material than observed the projectile target mixing was not considered in the models however this process could be the main sink of projectile melt as all analyzed melt particles have high concentrations of projectile material the fate of the solid projectile fragments is still not completely resolved model results suggest that the depth of melting in the target can reach the coconino sandstone formation however most of the ejected melt originates from 30 40 m depth and thus is limited to moenkopi and upper kaibab material some melt remains in the target based on the estimated volume of the breccia lens at meteor crater our models suggest at most a 2 content of melt in the breccia finally a high water table at the time of impact could have aided strong dispersion of target and projectile melt

impact microcrater morphology on australasian microtektites scanning electron microscopy of 137 australasian microtektites and fragments from 4 sediment cores in the central indian ocean reveals more than 2000 impact generated features in the size range of 0 3 to 600 m 2 dominant crater number densities in m and sub m sizes therefore tektite producing impacts can lead to the generation of microcraters that mimic those found on lunar surface materials and for the lunar rocks to qualify as reliable cosmic dust flux detectors their tumbling histories and lunar surface orientations have to be known precisely

giordano bruno small crater populations implications for self secondary cratering abstract giordano bruno is a lunar farside copernican age crater the relatively few superposed impact craters on its floor and ejecta blanket and extensive bright rays indicate its youth high resolution lunar reconnaissance orbiter camera images reveal that the frequency and the characteristics of the cumulative size frequency distributions of the small diameter impact craters and their morphology vary across the clastic ejecta and impact melt covered surfaces crater frequencies n 10 defined as 10 m km 2 vary by a factor of 10 s 100 s n 10 2 700 across the ejecta blanket and between the ejecta and the melt deposits numerous craters on the ejecta blanket are degraded and buried by debris and impact melt indicating that these partially buried craters formed during the deposition of the ejecta and prior to or during the emplacement of the impact melt from geologic relations and crater statistics we conclude that a significant fraction of the craters observed on the ejecta blanket and the melt were formed during the cratering process itself and represent self secondaries further we conclude that these craters do not represent an extra lunar primary impact production population self secondary craters are formed by material launched into near vertical trajectories and having velocities such that their flight time is sufficiently long that the bulk of the clastic ejecta and impact melt are deposited before that material impacts the surface the presence of a significant number of self secondary craters on the ejecta makes the determination of relative and absolute age dates problematic crater counts would indicate an inappropriately old age using data for craters on the melt surfaces and for small diameter bright ejecta craters an absolute model age of 1 ma is estimated this age is considerably younger than that estimated by other studies and probably represents a maximum age

constraints on ripple migration at meridiani planum from opportunity and hirise observations of fresh craters 1 observations of fresh impact craters by the opportunity rover and in high resolution orbital images constrain the latest phase of granule ripple migration at meridiani planum to have occurred between 50 ka and 200 ka opportunity explored the fresh resolution crater cluster and concepcion crater that are superposed on and thus younger than the ripples these fresh craters have small dark pebbles scattered across their surfaces which are most likely fragments of the impactor suggesting that the dark pebbles and cobbles observed by opportunity at meridiani planum are a lag of impactor derived material either meteoritic or secondary impactors from elsewhere on mars two larger fresh rayed craters in meridiani planum bracket ripple migration secondaries from ada crater are clearly superposed on and secondaries from an unnamed 0 84 km diameter crater have been modified and overprinted by the ripples three methods were used to estimate the age of these craters and thus when the latest phase of ripple migration occurred the inactivity of the ripples over the past 50 ka at meridiani is also consistent with other evidence for the stability of the ripples the lack of observed eolian bed forms in craters that formed in the past 20 years and little evidence for much dune motion in the past 30 yr on mars observations of crater morphology and their interaction with the ripples allow the development of a general time scale for craters in meridiani planum over the past million years

cometary and meteorite swarm impact on planetary surfaces the velocity flow fields energy partitioning and ejecta distributions resulting from impact of porous fragmented icy cometary nuclei with silicate planetary surfaces at speeds from 5 to 45 km s are different than those resulting from the impact of solid ice or silicate meteorites the impact of 1 g cm 3 ice spheres onto an atmosphereless anorthosite planetary surface induces cratering flows that appear similar to those induced by normal density anorthosite meteorite impact both of these impactors lead to deep transient crater cavities for final crater diameters less than 1 to 10 km and for escape velocities 10 5 cm s moreover the fraction of internal energy partitioned into the planetary surface at the cratering site is 0 6 for both ice and anorthosite impactors at 15 km s as the assumed density of the hypothetical cometary nucleus or fragment cloud from a nucleus decreases to 0 01 g cm 3 the fraction of the impact energy partitioned into planetary surface internal energy decreases to less than 0 01 and the flow field displays a toroidal behavior in which the apparent source of the flow appears to emanate from a disc or ringlike region rather than from a single point as in the explosive cratering case the edges of the crater region are in several cases depressed and flow downward whereas the center of the crater region is uplifted moreover the resultant postimpact particle velocity flow in some cases indicates the formation of concentric ridges a central peak and a distinct absence of a deep transient cavity in contrast transient cavities are a ubiquitous feature of nearly all previous hypervelocity impact calculations the calculations of the flow fields for low density 0 01 g cm 3 impactors exhibited surface interface comet planet instabilities these are attributed to both the rayleigh taylor and helmholtz instability conditions and we believe that these occur in all flows involving volatile low density 0 01 g cm 3 projectiles it is speculated that these hydrodynamic instabilities can give rise to concentric rings in the inner crater region in large scale impacts on planetary surfaces although other mechanisms for their production may also act the ejecta mass loss versus planetary escape velocity was computed and these results imply that the critical escape velocity at which as much material is lost as is being accreted from a planet ranges from 1 2 to 2 75 km s for encounter speeds of 5 to 15 kms s with cometary impactors having a density of 0 01 to 1 g cm 3 these values compare to 0 83 and 1 5 km s for silicate impactors thereby indicating that it is more difficult for volatiles than silicates to be accreted onto objects with escape velocities similar to the moon mercury and mars for objects with escape velocities in the 0 1 to 1 km s range the accretional efficiency for silicate and various porosity ices are similar whereas for objects with escape velocities 0 1 km s the accretional efficiency of icy impactors becomes significantly lower than for silicate impactors

morphometry of impact craters on mercury from messenger altimetry and imaging abstract data acquired by the mercury laser altimeter and the mercury dual imaging system on the messenger spacecraft in orbit about mercury provide a means to measure the geometry of many of the impact craters in mercury s northern hemisphere in detail for the first time the combination of topographic and imaging data permit a systematic evaluation of impact crater morphometry on mercury a new calculation of the diameter d t at which craters transition with increasing diameter from simple to complex forms and an exploration of the role of target properties and impact velocity on final crater size and shape measurements of impact crater depth on mercury confirm results from previous studies with the exception that the depths of large complex craters are typically shallower at a given diameter than reported from mariner 10 data secondary craters on mercury are generally shallower than primary craters of the same diameter no significant differences are observed between the depths of craters within heavily cratered terrain and those of craters within smooth plains the morphological attributes of craters that reflect the transition from simple to complex craters do not appear at the same diameter instead flat floors first appear with increasing diameter in craters at the smallest diameters followed with increasing diameter by reduced crater depth and rim height and then collapse and terracing of crater walls differences reported by others in d t between mercury and mars despite the similar surface gravitational acceleration on the two bodies are confirmed in this study the variations in d t between mercury and mars cannot be adequately attributed to differences in either surface properties or mean projectile velocity

geology of lofn crater callisto lofn crater is a 180 km diameter impact structure in the southern cratered plains of callisto and is among the youngest features seen on the surface the lofn area was imaged by the galileo spacecraft at regional scale resolutions 875 m pixel which enable the general geology to be investigated the morphology of lofn crater suggests that 1 it is a class of impact structure intermediate between complex craters and palimpsests or 2 it formed by the impact of a projectile which fragmented before reaching the surface resulting in a shallow crater even for callisto the asymmetric pattern of the rim and ejecta deposits suggests that the impactor entered at a low angle from the northwest the albedo and other characteristics of the ejecta deposits from lofn also provide insight into the properties of the icy lithosphere and subsurface configuration at the time of impact the target for the lofn impact is inferred to have included layered materials associated with the adlinda multiring structure northwest of lofn and ejecta deposits from the heimdall crater area to the southeast the lofn impact might have penetrated through these materials into a viscous substrate of ductile ice or possibly liquid water this interpretation is consistent with models of the current interior of callisto based on geophysical information obtained from the galileo spacecraft

impact fragmentation of lonar crater india implications for impact cratering processes in basalt impact fragmentation is an energetic process that has affected all planetary bodies to understand its effects in basalt we studied lonar crater which is a rare terrestrial simple impact crater in basalt and analogues to kilometer scale simple craters on mars the lonar ejecta consists of basaltic fragments with sizes ranging from silt to boulder the cumulative size and mass frequency distributions of these fragments show variation of power index for different size ranges suggesting simple and complex fragmentation the general shape of the fragments is compact platy bladed and elongated with an average edge angle of 100 the size distribution of cobble to boulder sized fragments is similar to the fracture spacing distribution in the upper crater wall indicating the provenance of those large fragments its consistency with a theoretical spallation model suggests that the large fragments were ejected from near surface of the target whereas the small fragments from deeper level the petrophysical properties of the ejecta fragments reflect the geophysical heterogeneity in the target basalt that significantly reduced the original size of spall fragments the presence of fe mg phyllosilicates smectites both in the ejecta and wall indicates the role of impact in excavating the phyllosilicates from the interior of basaltic target affected by aqueous alteration the seismic images reveal a thickness variation in the ejecta blanket segregation of boulders fractures and faults in the bedrock beneath the crater rim the fracturing fragmentation and fluvial degradation of lonar crater have important implications for mars

ballistic sedimentation of impact crater ejecta implications for the provenance of lunar samples and the resurfacing effect of ejecta on the lunar surface

lunar craters with radar bright ejecta abstract a small fraction of the lunar impact craters with diameters of 1 km and greater have extensive enhanced 3 8 cm radar echoes associated with their ejecta deposits the physical properties of these ejecta deposits and the ages of the central craters have been characterized via various infrared radar and optical signatures most of these ejecta deposits are radar bright at the 3 8 cm wavelength but are not radar bright at the 70 cm wavelength some ejecta have large infrared signatures others do not although most of these ejecta have bright albedos in full moon photographs a significant fraction of the bright albedo markings do not extend beyond the crater this mix of remote sensing signatures indicates that craters with 3 8 cm radar bright halos are young and have ejecta deposits containing an excess of surface or near surface rocks relative to the surrounding terrain abundant centimeter sized rocks are inferred from the high 3 8 cm radar and infrared signatures the low 70 cm radar signatures indicate that larger blocks are much less numerous the population of craters with 3 8 cm radar bright craters on the moon is much smaller than the population of craters in a similar size range on a young mare oceanus procellarum and has a different slope this population is interpreted as a steady state population reflecting a balance between the production of fresh craters and the destruction of the high infrared and radar signatures by small scale cratering the slope of difference between visual and radar craters is attributed to more rapid destruction of the radar signatures in smaller craters relative densities of 3 8 cm radar bright craters and mare craters are estimated to be 0 04 in 0 042 0 016 at a 4 km diameter and 0 100 0 07 0 007 at a 32 km diameter assigning ages on the basis of these relative densities raises the question of whether the 4 to 32 km diameter visual crater population is truly representative of a 3 3 by age if it is and if crater rates between 3 3 by and the present have been uniform than the 3 8 cm radar crater lifetimes are estimated to be 0 13 0 04 0 25 by and 0 03 0 13 0 23 by at diameters of 4 and 32 km respectively similarly lifetimes of the infrared signatures of 4 km diameter craters may be as short as 10 7 years however some data suggest that these estimates may be in error by a factor of 5 too small comminution of blocky ejecta material and the smoothing of slopes by lunar surface processes could account for the elimination of radar signatures on these time scales and the development of a steady state crater population an altenative interpretation which we do not fsvor is that the 3 8 cm radar bright crater population is formed by a subpopulation of primary bodies or by secondary cratering

impact craters of venus a continuation of the analysis of data from the venera 15 and 16 spacecraft additional studies of venusian impact craters have been made based on an analysis of venera 15 16 radar imagery and altimetry the crater population on venus has been subdivided into groups representing different morphological classes and types the craters display the size dependent variations in morphology which are well known from other planets assuming a crater production rate based on estimates by hartmann et al 1981 their areal density indicates an age for the total population of approximately 1 b y alternative estimates based on the assumption that the recent cratering rates on the earth and venus were similar would give a significantly lower age the areal and size frequency distribution of a number of circular features of unclear origin have also been analyzed and it is proposed that some of these features may be highly degraded and modified impact craters corresponding to a population approximately 3 b y in age a suggested model of the effect of the atmosphere on the crater diameter frequency curve indicates that cometary impacts played only a minor role in forming the contemporary venus environment a comparative study of craters in the area where the arecibo earth based radar and venera 15 16 images overlap indicates that the radar dark inner zones of the craters in the high incidence angle arecibo images correspond to the flat bottoms of the craters seen in the venera images

optical maturity of ejecta from large rayed lunar craters lucey et al 2000 have developed a methodology for extracting an optical maturity parameter omat from multispectral clementine images the omat parameter characterizes the overall maturity of lunar soils and crater ejecta by changes in reflectance spectra using these omat images we surveyed large craters 20 km diameter on the moon that had previously been mapped as possessing or possibly possessing rayed ejecta we generated average radial profiles of omat values for rays of these large craters from these profiles we classified the craters into three relative age groups 1 older than copernicus inferred age of 810 myr 2 intermediate and 3 as young or younger than tycho inferred age of 109 myr we suspect that there is a bias to our classification scheme such that the omat profiles of smaller craters look like that of larger but older craters nevertheless some large craters such as eudoxus 67 km and aristillus 55 km are now known from this study to have optically mature ejecta and therefore are suspected to be older than copernicus this is consistent with an age of 1 3 gyr suggested for aristillus by ryder et al 1991 such craters were included by mcewen et al 1997 when estimating the density of craters younger than or contemporaneous with copernicus therefore the case for a modest increase in the cratering rate in the past 800 myr versus the previous 2 4 gyr indicated from that work has been weakened grier and mcewen 2001 given current constraints on dating large and recent lunar craters we cannot support or disprove the hypothesis that there has been a significant increase in the rate of large terrestrial impact events in the past 100 400 myr

the self secondary crater population of the hokusai crater on mercury whether or not self secondaries dominate small crater populations on continuous ejecta deposits and floors of fresh impact craters has long been a controversy this issue potentially affects the age determination technique using crater statistics here the self secondary crater population on the continuous ejecta deposits of the hokusai crater on mercury is unambiguously recognized superposition relationships show that this population was emplaced after both the ballistic sedimentation of excavation flows and the subsequent veneering of impact melt but it predated the settlement and solidification of melt pools on the crater floor fragments that formed self secondaries were launched via impact spallation with large angles complex craters on the moon mercury and mars probably all have formed self secondaries populations dating young craters using crater statistics on their continuous ejecta deposits can be misleading impact melt pools are less affected by self secondaries overprint by subsequent crater populations with time reduces the predominance of self secondaries

the phanerozoic impact cratering rate evidence from the farside of the moon the relatively recent 1 b y flux of asteroids and comets forming large craters on the earth and moon may be accurately recorded by craters with bright rays on the moon s farside many previously unknown farside rayed craters are clearly distinguished in the low phase angle images returned by the clementine spacecraft some large rayed craters on the lunar nearside are probably significantly older than 1 ga rays remain visible over the maria due to compositional contrasts long after soils have reached optical maturity most of the farside crust has a more homogeneous composition and only immature rays are visible the size frequency distribution of farside rayed craters is similar to that measured for eratosthenian craters up to 3 2 b y at diameters larger than 15 km the areal density of farside rayed craters matches that of a corrected tabulation of nearside copernican craters hence the presence of bright rays due to immature soils around large craters provides a consistent time stratigraphic basis for defining the base of the copernican system the density of large craters less than 3 2 b y old is 3 2 times higher than that of large farside rayed craters alone this observation can be interpreted in two ways 1 the average cratering rate has been constant over the past 3 2 b y and the base of the copernican is 1 ga or 2 the cratering rate has increased in recent geologic time and the base of the copernican is less than 1 ga we favor the latter interpretation because the rays of copernicus 800 850 m y old appear to be very close to optical maturity suggesting that the average copernican cratering rate was 35 higher than the average eratosthenian rate other lines of evidence for an increase in the phanerozoic 545 ga cratering rate are 1 the densities of small craters superimposed on copernicus and apollo landing sites 2 the rates estimated from well dated terrestrial craters 120 m y and from present day astronomical observations and 3 the proterozoic rate suggested by the crater record of australia the hypothesis most consistent with several key observations is that the cratering rate has increased by 2x during the past 300 m y

topographic constraints on impact crater morphology on venus from high resolution stereo synthetic aperture radar digital elevation models 1 high resolution digital elevation models dems derived from magellan left left stereo synthetic aperture radar data of venus for a set of impact craters ranging in rim diameter from 5 to 300 km exhibit depths broadly as expected from theory but with significant departures for both large and small craters in craters larger than 38 km diameter rim floor depth becomes independent of diameter with a mean of about 900 m most craters smaller than 18 km diameter are also found to be unexpectedly shallow we discuss the accuracy and principal sources of error in our dems and conclude that this shallowness is real this shallowness of small craters shows that midsize bolides 200 1000 m diameter fragment in venus atmosphere and disperse over an area 10 to 20 times the original bolide diameter we identify these craters as a new class of impact feature the compound crater which may correspond to the knobby base morphological class defined by others

on estimating contributions of basin ejecta to regolith deposits at lunar sites we have developed a quantitative model for predicting characteristics of ejecta deposits that result from basin sized cratering events this model is based on impact crater scaling equations housen schmitt and holsapple 1983 holsapple 1993 and the concept of ballistic sedimentation oberbeck 1975 and takes into account the size distribution of the individual fragments ejected from the primary crater using the model we can estimate for an area centered at the chosen location of interest the average distribution of thicknesses of basin ejecta deposits within the area and the fraction of primary ejecta contained within the deposits model estimates of ejecta deposit thicknesses are calibrated using those of the orientale basin moore hodges and scott 1974 and of the ries basin horz ostertag and rainey 1983 observed densities of secondary craters surrounding the imbrium and orientale basins are much lower than the modeled densities similarly crater counts for part of the northern half of the copernicus secondary cratering field are much lower than the model predicts and variation in crater densities with distance from copernicus is less than expected these results suggest that mutual obliteration erases essentially all secondary craters associated with the debris surge that arises from the impacting primary fragments during ballistic sedimentation if so a process other than ballistic sedimentation is needed to produce observable secondary craters regardless our ejecta deposit model can be useful for suggesting provenances of sampled lunar materials providing information complementary to photogeological and remote sensing interpretations and as a tool for planning rover traverses e g haskin et al 1995 2002

recent grazing impacts on the earth recorded in the rio cuarto crater field argentina the most probable angle of a meteoroid impact on a planet is 45 and an impact at 15 from the horizontal or lower is as likely as at 75 or higher 1 2 yet little direct evidence for oblique impacts exists on the earth for two reasons unless the impact angle is very low any asymmetry created during the initial transfer of energy from impactor to target is lost as the crater is formed 3 moreover the shallow craters formed by oblique impact are more easily obscured by subsequent erosion during routine flights two years ago however one of us r e l noticed an anomalous alignment of oblong rimmed depressions 4 km x 1 km on the otherwise featureless farmland of the pampas in argentina we argue here from sample analysis and by analogy with laboratory experiments that these structures resulted from a low angle impact and ricochet of a chondritic body originally 150 300 m in diameter

comparisons of fresh complex impact craters on mercury and the moon implications for controlling factors in impact excavation processes the impact cratering process is usually divided into the coupling excavation and modification stages where each stage is controlled by a combination of different factors although recognized as the main factors governing impact processes on airless bodies the relative importance of gravity target and projectile properties and impact velocity in each stage is not well understood we focus on the excavation stage to place better constraints on its controlling factors by comparing the morphology and scale of crater exterior structures for similar sized fresh complex craters on the moon and mercury we find that the ratios of continuous ejecta deposits continuous secondaries facies and the largest secondary craters on the continuous secondaries facies between same sized mercurian and lunar craters are consistent with predictions from gravity regime crater scaling laws our observations support that gravity is a major controlling factor on the excavation stage of the formation of complex impact craters on the moon and mercury on the other hand similar sized craters with identical background terrains on mercury have different spatial densities of secondaries on the continuous secondaries facies suggesting that impactor velocity may also be important during the excavation stage as larger impactor velocity may also cause greater ejection velocities moreover some craters on mercury have more circular and less clustered secondaries on the continuous secondaries facies than other craters on mercury or the moon this morphological difference appears not to have been caused by the larger surface gravity or the larger median impact velocity on mercury a possible interpretation is that at some places on mercury the target material might have unique properties causing larger ejection angles during the impact excavation stage we conclude that gravity is the major controlling factor on the impact excavation stage of complex craters while impact velocity and target properties also affect the excavation stage but to a lesser extent than gravity

the anatomy of a buried complex impact structure the mjolnir structure barents sea analysis of the prominent seismic disturbance at the 40 km diameter mjolnir impact structure based on an extensive seismic reflection database shows that the observed broad brimmed bowl shaped disturbance was formed as a result of the impact of an asteroid or comet during volgian berriasian time 149 141 m seismic mapping exhibits and visualizes a 850 1400 km3 disturbed volume and analysis of several structural features within the disturbance provides insight into major cratering processes such as brecciation and excavation melting gravitational collapse of the transient crater and structural uplift a transient crater of 16 km in diameter and 4 5 km in depth is determined from transient and final crater dimensions we obtain an estimate of the degree of gravitational collapse of the order of 2 5 considerably larger than the average expected values for typical terrestrial craters the extensive collapse took place by low angle decollement surfaces at the periphery and by chaotic debris mass flows toward the center furthermore we estimate the mjolnir projectile to have been 0 9 3 km in diameter and that the physical impact released energy in the range of 2 4 53 1020 j corresponding to an earthquake of magnitude 8 3 the mjolnir impact is not associated with a significant mass extinction however dissipation of the energy released during the mjolnir impact was sufficient to have caused several short term near field perturbations such as large amplitude tsunami waves affecting the barents sea region and possibly adjacent areas in the arctic

quantifying crater production and regolith overturn on the moon with temporal imaging random bombardment by comets asteroids and associated fragments form and alter the lunar regolith and other rocky surfaces the accumulation of impact craters over time is of fundamental use in evaluating the relative ages of geologic units crater counts and radiometric ages from returned samples provide constraints with which to derive absolute model ages for unsampled units on the moon and other solar system objects however although studies of existing craters and returned samples offer insight into the process of crater formation and the past cratering rate questions still remain about the present rate of crater production the effect of early stage jetting during impacts and the influence that distal ejecta have on the regolith here we use lunar reconnaissance orbiter camera lroc narrow angle camera nac temporal before and after image pairs to quantify the contemporary rate of crater production on the moon to reveal previously unknown details of impact induced jetting and to identify a secondary impact process that is rapidly churning the regolith from this temporal dataset we detected 222 new impact craters and found 33 per cent more craters with diameters of at least ten metres than predicted by the standard neukum production and chronology functions for the moon we identified broad reflectance zones associated with the new craters that we interpret as evidence of a surface bound jetting process we also observe a secondary cratering process that we estimate churns the top two centimetres of regolith on a timescale of 81 000 years more than a hundred times faster than previous models estimated from meteoritic impacts ten million years

experimentation in planetary geology the reasons for conducting experimentation are as varied as the topics that are studied whether a specific parameter must be quantified or an attempt is being made to duplicate a peculiar process all experiments are unified by a common ultimate objective to provide a tangible link between theory and observation although a suitable theory is often nonexistent at the time of the experiment experimentation in planetary geology assumes particular importance because of the factors inherent in the field of study itself often enormous spatial scales timescales rivaled only by those of cosmology and the remoteness of the objects of study in addition any geoscientist who has entertained thoughts of developing a realistic mathematical model of even the simplest of processes rapidly becomes acquainted with the intimidating complexity of geological phenomena experimentation is still in many cases the only effective means of learning more about some natural mechanisms

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

secondary crater initiated debris flow on the moon abstract in recent work radar circular polarization echo properties have been used to identify secondary craters without distinctive secondary morphologies because of the potential for this method to improve our knowledge of secondary crater populations in particular the effect of secondary populations on crater derived ages based on small craters it is important to understand the origin of radar polarization signatures associated with secondary impacts in this paper we utilize lunar reconnaissance orbiter camera photographs to examine the geomorphology of secondary craters with radar circular polarization ratio enhancements our investigation reveals evidence of dry debris flow with an impact melt component at such secondary craters we hypothesize that these debris flows were initiated by the secondary impacts themselves and that they have entrained blocky material ejected from the secondaries by transporting this blocky material downrange we propose that these debris flows rather than solely ballistic emplacement are responsible for the tail like geometries of enhanced radar circular polarization ratio associated with the secondary craters investigated in this work evidence of debris flow was observed at both clustered and isolated secondary craters suggesting that such flow may be a widespread occurrence with important implications for the mixing of primary and local material in crater rays

bunte breccia of the ries continuous deposits of large impact craters the ries crater an impact structure of 26 km diameter in south germany is the largest terrestrial crater where substantial amounts of ejecta are preserved on occasion 100 m deep further the target stratigraphy is well known and it is possible to relate specific clasts and breccia lithologies to initial target depth as a consequence the continuous deposits of the ries also known as bunte breccia may be studied with exceptional field control we report field observations and laboratory analyses obtained from 560 m core materials taken at nine different locations that range from 16 to 37 km in radial distance from the impact center the objective is to relate the ries observations to ejection and to emplacement processes of large scale planetary crater deposits the observations regarding the modal stratigraphic characteristics of the bunte breccia may be summarized as follows only 600 m some 0 7 is composed of triassic clasts originating from 300 to 600 m depths lower and middle jurassic horizons approximately 300 150 m constitute some 2 3 and upper jurassic 0 150 m makes up some 31 5 in addition the bunte breccia contains tertiary materials in the form of 1 cm clasts 29 1 and as highly comminuted fine grained matrix 10 gpa constitute 0 1 weight of the entire deposit which indicates that bunte breccia was emplaced at essentially ambient temperatures when possible the above observations are quantified via linear regressions throughout the text all of these observations are consistent with if not predicted by a ballistic emplacement scenario as postulated by oberbeck and co workers primary crater ejecta are expelled ballistically and will form secondary craters in the local substrate a mixture of primary and secondary ejecta results and combines into a highly turbulent ground hugging debris surge as the final phase of ejecta emplacement total emplacement time for the bunte breccia 200 km is estimated to be of the order of 5 min only these findings are compared with cratering theory relating to a number of ejecta thickness decay models and with the so called z model addressing material flow during various stages of crater formation qualitative to fair agreement of observations and predictions results an initial crater radius of 6 5 km an excavation depth of 1650 m an excavation volume of 136 km and an associated transient cavity volume of aproximately 230 km appear to be reasonable estimates approximately 170 km of material was involved in slumping and restoration of the transient cavity for the above radius and z 2 7 the modal composition of ries ejecta with regard to preimpact target stratigraphy indicates that materials contained in the continuous deposits of large complex planetary craters are predominantly derived from depths as small as one hundredth the apparent crater diameter a number of implications are addressed regarding remote sensing of planetary surfaces and investigation of lunar and meteoritic impact breccias

impact spallation processes on the moon a case study from the size and shape analysis of ejecta boulders and secondary craters of censorinus crater abstract impact spallation is a fundamental process responsible for formation of ejecta boulders from impact craters although theoretical spallation models were developed about three decades ago only limited geological observations have been made so far to test these models the 3 8 km censorinus crater on the moon provides an excellent opportunity for studying the impact spallation processes associated with a fresh simple crater formed by oblique impact using the lunar reconnaissance orbiter narrow angle camera images we prepared the ejecta boulder distribution map of censorinus crater and measured the boulder sizes and shapes mapping of about 242 000 ejecta boulders enabled us to document the size distribution of boulders both radial and concentric to the impact crater larger size boulders dominate the crater rim areas while they become smaller away from the crater the boulder distribution exhibits a radial asymmetry suggesting censorinus is a oblique impact in which the uprange ejecta have smaller ranges with larger concentration of boulders near the southwestern crater rim while the downrange ejecta are in general characterized by smaller boulders with high spatial dispersion the cumulative size frequency distribution csfd of boulders shows a highly variable fragmentation history in which the uprange boulders suffered more complex fragmentation the ejecta boulders also exhibit a variety of shapes that are gleaned from their axial ratios and edge angle characteristics there is a general decrease of axial ratios away from the crater rim rectangular boulders dominate the crater rim and they become more equant away from the crater in addition to the boulder sizes the boulder shape distribution also exhibits a mild asymmetry in response to the oblique impact small size fresh impact craters 84 000 craters are abundant on the censorinus ejecta and post date censorinus these craters are found in two morphologic types in which a large majority of craters have subdued ejecta rayless craters while some possess bright rayed ejecta bright rayed craters the csfd of rayless craters show a steep power law slope with a b value of 4 0 similar to the secondary craters produced by the impact of ejecta from primary craters we therefore interpret the rayless craters as the secondary craters of censorinus on the other hand the csfd of bright rayed craters have smaller power law slope b value 2 7 which is a characteristic of primary craters and thus provide 3 ma age for censorinus crater when the characteristics of censorinus boulders are compared with the theoretical spallation models that are sensitive to the petrophysical properties of the target lunar highland the models generally agree with the censorinus boulders however the observed shape and size characteristics of the censorinus boulders are found to be more complex than the theoretical spallation models the ejecta boulders suffered more complex fragmentation and asymmetric distribution in response to the oblique impact the spallation models accounting oblique impacts have not yet been developed therefore our censorinus boulder observations can be used to develop and validate the new theoretical spallation models for the effects of oblique impacts

earth based observations of radar dark crater haloes on the moon implications for regolith properties 1 earth based radar can be used in conjunction with other data sets collected remotely or in situ to characterize the physical and chemical properties of the lunar regolith we use previously existing and newly acquired earth based radar measurements at 70 cm wavelength to investigate the properties of distinctive low return haloes surrounding 39 nearside impact craters these haloes are distinct from the well known optically dark haloes associated with volcanic craters or with impacts into cryptomare deposits we examine two possible mechanisms for formation of radar dark haloes 1 excavation of high tio2 high loss tangent basalt and 2 deposition of a mantling layer produced in the impact process that is depleted in meter sized blocks comparison of the radar data with apollo and lunar orbiter images indicates a general spatial correspondence between radar dark crater haloes and radial ridged ejecta facies distal to the radar bright crater rims comparison with clementine uv vis multispectral data indicates no apparent spatial correlation between regions of enhanced tio2 content and radar dark haloes though variations in tio2 content may play a role for some mare craters they are insufficient to account for the observed offsets in radar echo power we conclude that the radar dark haloes result from a block poor ejecta deposit that disappears over time with meteoroid bombardment a more detailed and comprehensive analysis of ejecta degradation processes and rates following acquisition of additional high resolution radar data could lead to a refinement of the lunar geologic timescale

impact penetration of europa s ice crust as a mechanism for formation of chaos terrain ice thickness estimates and impactor dynamics indicate that some impacts must breach europas ice crust and outcomes of impact experiments using ice over water targets range from simple craters to chaos like destroyed zones depending on impact energy and ice competence firstorder impacts into thick ice or at low impact energy produce craters second order impacts punch through the ice making holes that resemble raft free chaos areas third order impacts into thinnest ice or at highest energy produce large irregular raft filled zones similar to platy chaos other evidence for an impact origin for chaos areas comes from the size frequency distribution of chaos craters on europa which matches the impact production functions of ganymede and callisto and from small craters around the large chaos area thera macula which decrease in average size and density per unit area as a function of distance from theras center there are no tiny chaos areas and no craters 50 km diameter this suggests that small impactors never penetrate whereas large ones berpenetrators 2 5 km diameter at average impact velocity always do existence of both craters and chaos areas in the size range 2 40 km diameter points to spatial temporal variation in crust thickness but in this size range craters are progressively outnumbered by chaos areas at larger diameters suggesting that probability of penetration increases with increasing scale of impact if chaos areas do represent impact sites then europas surface is older than previously thought the recalculated resurfacing age is 480 302 960 ma greater than prior estimates but still very young by solar system standards

detection of mg spinel bearing central peaks using m 3 images implications for the petrogenesis of mg spinel abstract mg spinel bearing lithologies lacking abundant mafic materials have been discovered with images acquired by the moon mineralogy mapper m3 aboard chandrayaan 1 we conducted a systematic screening of lunar crater central peaks for the presence of mg spinel to address its distribution and petrogenesis 38 mg spinel bearing crater central peaks were identified in this study out of 166 craters investigated the results suggest that mg spinel is common in the lunar crust and appears to be extensive in the middle part of the lunar crust underneath procellarum kreep terrane pkt mg spinel neither exclusively originated from deep layers 10 km nor necessarily coexist with the appearance of olivine or pyroxene 15 mg spinel bearing central peaks originated from depth less than 10 km nine investigated central peaks only contain mg spinel and plagioclase without any detectable mafic materials all those observations imply that the origin of mg spinel is possibly related to mg suite plutonism and assimilation between high mg magma with anorthositic crust the extensive distribution and mg suite related petrogenesis indicates that mg spinel bearing lithologies might represent a new member of mg suite rocks

an atmospheric blast thermal model for the formation of high latitude pedestal craters although tenuous the atmosphere of mars affects the evolution of impact generated vapor early time vapor from a vertical impact expands symmetrically directly transferring a small percentage of the initial kinetic energy of impact to the atmosphere this energy in turn induces a hemispherical shock wave that propagates outward as an intense airblast due to high speed expansion of vapor followed by a thermal pulse of extreme atmospheric temperatures from thermal energy of expansion this study models the atmospheric response to such early time energy coupling using the cth hydrocode written at sandia national laboratories results show that the surface surrounding a 10 km diameter crater 6 km apparent diameter on mars will be subjected to intense winds 200 m s and extreme atmospheric temperatures these elevated temperatures are sufficient to melt subsurface volatiles at a depth of several centimeters for an ice rich substrate ensuing surface signatures extend to distal locations 4 apparent crater diameters for a case of 0 1 energy coupling and include striations thermally armored surfaces and or ejecta pedestals all of which are exhibited surrounding the freshest high latitude craters on mars the combined effects of the atmospheric blast and thermal pulse resulting in the generation of a crater centered erosion resistant armored surface thus provide a new very plausible formation model for high latitude martian pedestal craters

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

polygonal impact craters on dione evidence for tectonic structures outside the wispy terrain abstract plan view impact crater geometries can be indicative of pre impact structures within the target material impact events that occur on a pre fractured surface generate craters exhibiting one or more straight rim segments termed polygonal impact craters pics impact craters that appear to be pics are abundant on the surface of saturn s icy satellite dione both within the wispy terrain a region with large visible fractures and also outside the wispy terrain the non wispy terrain where less evidence for fracturing has been observed in the non wispy terrain subtle lineaments are hypothesized to be ne sw nw se and e w trending fractures suggesting that tectonism may have been an important process in this terrain results of previous studies have shown that pic straight rim segment azimuths pic azimuths commonly parallel pre impact fracture azimuths although disagreements about this relationship exist in the literature we investigated the hypothesis that fractures either subtle or nonvisible with available spacecraft images are present within dione s non wispy terrain our first step was to assess the relationship between pics and pre existing fracture azimuths in the wispy terrain our results from this initial assessment show a parallel relationship between pic azimuths and fracture azimuths based on this correlation in the wispy terrain we find it likely that this relationship would hold true in the non wispy terrain if pics are present we tested for pics using crater rim azimuth data collected from randomly distributed study locations throughout the non wispy terrain from these data we identified widespread pics in this terrain which supports the hypothesis that subtle fractures are also present analysis of the pic azimuth data yield a pattern for these inferred fractures across dione s surface that is consistent with the hypothesized global deformation that would result from a combination of satellite despinning and volume expansion our results provide evidence for these previously hypothesized events in dione s history and demonstrate that mapping pics and their azimuths is a useful tool for investigating subtle fractures on solar system bodies

size frequency distribution of different secondary crater populations 1 equilibrium caused by secondary impacts accumulation of impact craters is the major reason causing equilibrium of crater populations on airless planetary surfaces besides primary craters the effect of widespread secondaries on the equilibrium of local crater populations is little studied here the different secondary crater populations formed by the hokusai crater on mercury are systematically studied and they are compared with those on the moon to investigate their contribution to the evolution of local crater populations self secondaries cause equilibrium on continuous ejecta deposits in a short time and the equilibrium crater population has a differential sfd slope of about 3 background secondaries are abundant on mercury and equilibrium caused by a combination of primaries and potential background secondaries follows the same pattern on the moon and mercury the spatial dispersion of fragments that form both near field and distant secondaries is the major factor affecting the degree of mutual destruction and thus the final crater sfd some clustered distant secondaries on mercury are likely formed by individual fragments considering their large spatial dispersion and identical morphology with same sized primaries and the sfd rollovers of these secondaries possibly reflect the inherent sfd rollovers of the impact fragments near field secondaries and many other distant secondaries have morphology and spatial distribution that are consistent with being formed by clustered fragments and mutual destruction of secondaries may be the major reason causing the observed sfd rollovers heterogeneous secondary impacts are a potential explanation for both different crater densities within the equilibrium diameter range and different regolith thicknesses on coeval surfaces

geology and distribution of impact craters on venus what are they telling us magellan has revealed an ensemble of impact craters on venus that is unique in many important ways we have compiled a data base describing the 842 craters on 89 of venus surface mapped through orbit 2578 the craters range in diameter from 1 5 to 280 km we have studied the distribution size density morphology geology and associated surface properties of these craters both in the aggregate and for some craters in greater detail we find that 1 the spatial distribution of craters is highly uniform 2 the size density distribution of large craters diameters 35 km is similar to the young crater populations on other terrestrial planets but at a much lower density that indicates an average age of only about 0 5 ga based on the estimated population of venus crossing asteroids 3 unlike the case on other planets the density of small craters diameters 35 km declines rapidly with decreasing diameters because of atmospheric filtering 4 the spectrum of crater modification differs greatly from that on other planets 62 of all craters are pristine only 4 are embayed by lavas and the remainder are affected by tectonism but none are severely and progressively depleted as extrapolated from the size density distribution of larger craters 5 large craters have a progression of morphologies generally similar to those on other planets but small craters are typically irregular or multiple rather than bowl shaped 6 diffuse radar bright or dark features surround some craters and 367 similar diffuse splotches with no central crater are observed and 7 other crater features unique to venus include radar bright or dark parabolic arcs opening westward and extensive outflows originating in crater ejecta the first three of these observations are entirely unexpected we interpret them as indicating that the planet s cratering record was erased by a global resurfacing event or events the latest ending about 0 5 ga after which volcanic activity declined but did not cease entirely since the last resurfacing event a maximum of 10 of the planet has been resurfaced and only about 4 of the craters have been obliterated convective thermal evolution models support this interpretation arkani hamed and toksoz 1984 observations 3 7 confirm quantitatively the expectation that the dense atmosphere of venus has strongly affected the production of craters large impactors have been relatively unaffected intermediate sized ones have been fragmented and have produced overlapping or multiple craters a narrow size range has produced shock induced splotches but no craters and the smallest bodies have had no observable effect on the surface the number of craters eliminated by the atmospheric filter is enormous about 98 of the craters between 2 and 35 km in diameter that magellan might have observed on a hypothetical airless venus unique crater related features such as parabolas and outflow deposits demonstrate the roles of venus high atmospheric density and temperature in modifying the crater formation process finally heavily fractured craters and lava embayed craters are found to have higher than average densities along the major fracture belts and rifted uplands connecting aphrodite terra and atla beta themis and phoebe regiones these craters thus provide physical evidence for recent volcanic and tectonic activity at a low level

mg spinel lithology a new rock type on the lunar farside 1 high resolution compositional data from moon mineralogy mapper m3 for the moscoviense region on the lunar farside reveal three unusual but distinctive rock types along the inner basin ring these are designated oos since they are dominated by high concentrations of orthopyroxene olivine and mg rich spinel respectively the oos occur as small areas each a few kilometers in size that are widely separated within the highly feldspathic setting of the basin rim although the abundance of plagioclase is not well constrained within the oos the mafic mineral content is exceptionally high and two of the rock types could approach pyroxenite and harzburgite in composition the third is a new rock type identified on the moon that is dominated by mg rich spinel with no other mafic minerals detectable 5 pyroxene olivine all oos surfaces are old and undisturbed since basin formation they are effectively invisible in image data and are only recognized by their distinctive composition identified spectroscopically the origin of these unusual lithologies appears to be linked to one or more magmatic intrusions into the lower crust perhaps near the crust mantle interface processes such as fractional crystallization and gravity settling within such intrusions may provide a mechanism for concentrating the mafic components within zones several kilometers in dimension the oos are embedded within highly anorthositic material from the lunar crust they may thus be near contemporaneous with crustal products from the cooling magma ocean

motion of a fragmented meteoroid through the planetary atmosphere meteor observations and crater field investigations support the view that some meteoroids undergo fragmentation into a finite number of splinters which move a long distance without further disruption the motion of these fragments cannot be described in the frame of liquid like models the primary purpose of this study is to achieve some success in improving the alternative model of separate fragments which is based on studying the motion of a finite number of fragments interacting with each other through the air pressure the three dimensional 3 d numerical technique is elaborated for the direct modeling of the flow around several arbitrarily arranged fragments the model allows us to calculate the ablation mass rate and aerodynamic loading for each fragment the new approach hybrid model based on the simultaneous solution of the 3 d hydrodynamic equations for the airflow and the differential equations for the motion of discrete particles is proposed to study the debris cloud evolution taking into account successive fragmentation the simplified version of this model was applied to the sikhote alin meteorite shower benesov bolide and small meteoroids impacts against the martian surface

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

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

the planforms of low angle impact craters in the northern hemisphere of mars we have surveyed martian impact craters greater than 5 km in diameter using viking and thermal emission imaging system themis imagery to evaluate how the planform of the rim and ejecta changes with decreasing impact angle we infer the impact angles at which the changes occur by assuming a sin2 dependence for the cumulative fraction of craters forming below angle at impact angles less than 40 from horizontal the ejecta become offset downrange relative to the crater rim as the impact angle decreases to less than 20 the ejecta begin to concentrate in the crossrange direction and a forbidden zone that is void of ejecta develops in the uprange direction at angles less than 10 a butterfly ejecta pattern is generated by the presence of downrange and uprange forbidden zones and the rim planform becomes elliptical with the major axis oriented along the projectile s direction of travel the uprange forbidden zone appears as a v curving outward from the rim but the downrange forbidden zone is a straight edged wedge although fresh martian craters greater than 5 km in diameter have ramparts indicative of surface ejecta flow the ejecta planforms and the angles at which they occur are very similar to those for lunar craters and laboratory impacts conducted in a dry vacuum the planforms are different from those for venusian craters and experimental impacts in a dense atmosphere we interpret our results to indicate that martian ejecta are first emplaced predominantly ballistically and then experience modest surface flow

cratering history and lunar chronology the moon is exceptional and important because it is the only planetary body besides the earth for which we have both a detailed stratigraphic history and datable rock samples that can be related to specific geomorphologic units fig 5 1 the moon has preserved much of its magmatic and impact record of at least the last 4 billion years while its endogenic history is of great interest for the fundamentals of planetary interiors and surfaces the moon has become a calibration plate for the cratering record of the earth moon system and by extrapolation of the entire inner solar system if one assumes a heliocentric origin for impactor populations these populations range from asteroids through long and short period comets to interplanetary dust and cover a size range from hundreds of kilometers to micrometers figure 5 1 telescopic view of the nearside of the earth s moon with landing sites of the apollo and luna missions this chapter reviews the presently available data sets in support of this paradigmatic assumption as follows 1 the phenomenology of lunar impact craters 2 the terrestrial record of the impact cratering process and the interpretation of terrestrial impactites as far as this ground truth is relevant for the interpretation of lunar impact craters and datable lunar impact breccias and melt rocks 3 the theory and numerical simulation of the cratering process and the characteristics of the earth moon crossing population of impactors asteroids and comets 4 the principles of relative age dating of lunar surface units and the general lunar stratigraphy 5 the stratigraphic significance and ages of lunar samples impactites and basalts and based on this data set the absolute ages of lunar surface units 6 the cratering rate of the moon as a function of time and 7 the time calibration of this cratering rate based on the

origin of the outer layer of martian low aspect ratio layered ejecta craters low aspect ratio layered ejecta larle craters are one of the most enigmatic types of martian layered ejecta craters we propose that the extensive outer layer of these craters is produced through the same base surge mechanism as that which produced the base surge deposits generated by near surface buried nuclear and high explosive detonations however the larle layers have higher aspect ratios compared with base surge deposits from explosion craters a result of differences in thicknesses of these layers this characteristics is probably caused by the addition of large amounts of small particles of dust and ice derived from climate related mantles of snow ice and dust in the areas where larle craters form these deposits are likely to be quickly stabilized order of a few days to a few years from eolian erosion by formation of duricrust produced by diffusion of water vapor out of the deposits

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

impacts into coarse grained spheres at moderate impact velocities implications for cratering on asteroids and planets abstract the surfaces of many planets and asteroids contain coarsely fragmental material generated by impacts or other geologic processes the presence of such pre existing structures may affect subsequent impacts particularly when the width of the shock is comparable to or smaller than the size of pre existing structures reasonable theoretical predictions and low speed

variation in ejecta size with ejection velocity a knowledge of the size velocity distribution of ejecta from impact craters is important for a number of processes that occur or may have occurred in the solar system such as planetary accretion collisional evolution of asteroids the ejection of meteorites from parent bodies larger than asteroids such as the lunar meteorites and the snc meteorites which may be from mars and massive biological extinctions on the earth in order to estimate the size velocity distribution of impact crater ejecta the sizes and ranges from the primary crater were measured for the secondary craters of twelve large craters on mercury the moon and mars the ballistic equation for spherical bodies was used to convert the ranges to velocities and the velocities and crater sizes were used in the appropriate schmidt holsapple scaling relation to estimate ejecta sizes assuming that d v lines were fitted to the log maximum size vs log ejection velocity using the least squares method to determine the velocity exponent certain problems with data collection such as partial burial of some craters and interference from other craters and topographic features made it impossible to determine a simple unique relation between size and velocity the velocity exponents are generally between 1 and 3 with an average value of approximately 1 9 the complicating effect of secondaries produced by clusters of impactors implies that the calculated lines are probably steeper than the slopes of the true fragment size velocity distributions

lunar impact basins new data for the western limb and far side orientale and south pole aitken basins from the first galileo flyby compositional aspects of impact basin materials can be analyzed using multispectral image data acquired by the galileo solid state imaging ssi experiment during the december 1990 lunar encounter these data provide important information on the spectral properties of the western lunar limb and parts of the far side the ssi images cover the wavelength range 0 4 1 0 m allowing measurement of spectral slope and estimation of the strength of the 1 m absorption due to iron in the mafic minerals olivine and pyroxene among deposits of the 930 km diameter orientale basin exterior ejecta comprising the hevelius formation is relatively homogeneous and spectrally similar to mature apollo 16 soils suggesting an upper crustal source the centrally located maunder formation is distinct from the younger mare basalts but comparable to the hevelius formation in its spectral reflectance properties supporting an interpretation as basin impact melt the montes rook formation located in an annulus between the maunder and the hevelius shows a slightly stronger mafic absorption and may be the deepest crustal material excavated the distal orientale deposits show local mafic enhancements in the schiller schickard and mendel rydberg regions interpreted to represent pre orientale mare deposits or cryptomaria intermixed with overlying basin ejecta in this case maria of sizes comparable to those presently observed were widespread in this region before the orientale impact mixing model analyses are consistent with the ballistic erosion and sedimentation model for ejecta emplacement in the distal regions beyond the continuous ejecta deposit on the southern lunar farside a huge area with an enhanced mafic absorption corresponds to the interior and rim of the pre nectarian south pole aitken impact basin 2000 2500 km in diameter the anomaly is interpreted to be due to several factors including excavation into the more mafic lower crust and the presence of extensive early volcanic fill cryptomare similar to that seen in ancient basins such as smythii and australe these results show that although basin forming events are an important factor in producing lateral heterogeneities in crustal composition and in modifying preexisting deposits such as cryptomaria the majority of material in even the largest basins was excavated from crustal levels our results suggest a gradational vertical crustal stratigraphy consisting of an uppermost mixed crustal layer of anorthosite basin ejecta and cryptomaria deposits generally corresponding to the megaregolith an upper crustal layer of anorthosite and a lower more noritic layer many of the basic questions remaining from this study could be addressed by global high resolution geochemical and mineralogical data obtained by polar orbiting spacecraft

the origin of lunar crater rays abstract lunar rays are filamentous high albedo deposits occurring radial or subradial to impact craters the nature and origin of lunar rays have long been the subjects of major controversies we have determined the origin of selected lunar ray segments utilizing earth based spectral and radar data as well as feo tio 2 and optical maturity maps produced from clementine uvvis images these include rays associated with tycho olbers a lichtenberg and the messier crater complex it was found that lunar rays are bright because of compositional contrast with the surrounding terrain the presence of immature material or some combination of the two mature compositional rays such as those exhibited by lichtenberg crater are due entirely to the contrast in albedo between ray material containing highlands rich primary ejecta and the adjacent dark mare surfaces immaturity rays are bright due to the presence of fresh high albedo material this fresh debris was produced by one or more of the following 1 the emplacement of immature primary ejecta 2 the deposition of immature local material from secondary craters 3 the action of debris surges downrange of secondary clusters and 4 the presence of immature interior walls of secondary impact craters both composition and state of maturity play a role in producing a third combination class of lunar rays the working distinction between the eratosthenian and copernican systems is that copernican craters still have visible rays whereas eratosthenian aged craters do not compositional rays can persist far longer than 1 1 ga the currently accepted age of the copernican eratosthenian boundary hence the mere presence of rays is not a reliable indication of crater age the optical maturity parameter should be used to define the copernican eratosthenian boundary the time required for an immature surface to reach the optical maturity index saturation point could be defined as the copernican period

effect of planetary rotation on distal tektite deposition on mars 1 the rapid rotation of mars creates a significant pseudoforce known as the coriolis force that greatly modifies the flight paths and subsequent deposition of distal ejecta ejecta with launch velocities of 3 km s and greater an accurate depiction of the effects of the coriolis force requires the integration of the coriolis terms directly into a series of spherical ballistic equations applied within a rotating reference frame the resulting landing positions of radially ejected particles tend to form distinct wrapping patterns that are focused to specific areas in the hemisphere opposite from the source crater around the pole for high latitude craters 45 in latitude and above and in an equatorial band for low latitude craters below 45 latitude consequently particular locations on the surface receive deposits in stages direct delivery of ejecta followed by deposits of higher velocity ejecta this staged deposition leads to regions of enhanced ejecta accumulations where meters of material collect millimeters would be predicted in nonrotational radial decay models thus high latitude craters could supply a considerable amount of distal products tektites and melts to polar locations perhaps contributing to the dark circumpolar materials

the shape and appearance of craters formed by oblique impact on the moon and venus we surveyed the impact crater populations of venus and the moon dry targets with and without an atmosphere to characterize how the 3 dimensional shape of a crater and the appearance of the ejecta blanket varies with impact angle an empirical estimate of the impact angle below which particular phenomena occur was inferred from the cumulative percentage of impact craters exhibiting different traits the results of the surveys were mostly consistent with predictions from experimental work assuming a sin2 dependence for the cumulative fraction of craters forming below angle on the moon the following transitions occur 45 degrees the ejecta blanket becomes asymmetric 25 degrees a forbidden zone develops in the uprange portion of the ejecta blanket and the crater rim is depressed in that direction 15 degrees the rim becomes saddle shaped 10 degrees the rim becomes elongated in the direction of impact and the ejecta forms a butterfly pattern on venus the atmosphere causes asymmetries in the ejecta blanket to occur at higher impact angles the transitions on venus are 55 degrees the ejecta becomes heavily concentrated downrange 40 degrees a notch in the ejecta that extends to the rim appears and as impact angle decreases the notch develops into a larger forbidden zone 10 degrees a fly wing pattern develops where material is ejected in the crossrange direction but gets swept downrange no relationship between location or shape of the central structure and impact angle was observed on either planet no uprange steepening and no variation in internal slope or crater depth could be associated with impact angle on the moon for both planets as the impact angle decreases from vertical first the uprange and then the downrange rim decreases in elevation while the remainder of the rim stays at a constant elevation for craters on venus 15 km in diameter a variety of crater shapes are observed because meteoroid fragment dispersal is a significant fraction of crater diameter the longer path length for oblique impacts causes a correlation of clustered impact effects with oblique impact effects one consequence of this correlation is a shallowing of the crater with decreasing impact angle for small craters

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