gravity field of the moon from the gravity recovery and interior laboratory grail mission spacecraft to spacecraft tracking observations from the gravity recovery and interior laboratory grail have been used to construct a gravitational field of the moon to spherical harmonic degree and order 420 the grail field reveals features not previously resolved including tectonic structures volcanic landforms basin rings crater central peaks and numerous simple craters from degrees 80 through 300 over 98 of the gravitational signature is associated with topography a result that reflects the preservation of crater relief in highly fractured crust the remaining 2 represents fine details of subsurface structure not previously resolved grail elucidates the role of impact bombardment in homogenizing the distribution of shallow density anomalies on terrestrial planetary bodies

rugged crater ejecta as a guide to megaregolith thickness in the southern nearside of the moon the southern highlands of the moon comprise superposed ejecta layers individually as thick as a few kilometers from the major basins smaller 1 16 km diameter impact craters that penetrate this layered megaregolith and excavate material from depth have radar properties that provide insight into the variability of megaregolith thickness above a postulated basement of large crustal blocks we observe a significant difference in the population of radar bright craters 1 16 km and larger in diameter between regions of the southeastern near side highlands north and south of lat 48 s there are about one third more radar bright craters north of this line than to the south broadly coincident with the mapped boundary between southern deposits mapped as pre nectarian age and those of nectarian imbrian age to the north the radar bright crater population is consistent with a megaregolith thickness of 1 5 km in the north and 2 5 km in the south a difference we attribute to south pole aitken basin ejecta

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

numerical modelling of impact melt production in porous rocks abstract melting of rocks by impact is a fundamental geologic process that results from shock wave compression and subsequent release from high pressure shock melted material is found in many terrestrial craters however most impact craters in sedimentary targets which are typically porous have been reported as containing significantly less melt than craters in nonporous crystalline targets to investigate the effect of porosity on shock melting we present a numerical modelling approach that combines a porosity compaction model with an equation of state for the solid component to demonstrate that our model accurately calculates shock pressures in porous materials we compare these results with hugoniot data for porous quartzite coconino sandstone and calcite our results show that the presence of porosity significantly reduces the critical pressure required for melting which enhances melt production we show that this effect is reduced but not totally diminished by faster shock wave decay in porous materials so that all other things being equal impact melt production is greater when target porosity is higher we conducted a series of simulations of impacts into targets of different material composition quartzite calcite dunite and porosity of up to 50 and varied the impact velocity between 6 and 50 km s we found a significant increase in the production of impact melt with increasing velocity in agreement with several previous studies and quantify the net increase in impact melt due to target porosity for the nonporous case our results agree well with estimated melt volumes at terrestrial impact craters over a range in size 7 90 km in diameter in porous targets our results contradict the widely cited observation of a melt deficiency in sedimentary target craters however our model results agree well with recently revised estimates of melt volumes at terrestrial craters formed in sedimentary or mixed sedimentary and crystalline targets this further strengthens the view that substantial volumes of impact melt should be produced during impacts into porous sedimentary targets and that the apparent paucity of impact melt in some sedimentary target craters may be due to difficulties in recognizing impact melts derived from sedimentary rocks

thickness of proximal ejecta from the orientale basin from lunar orbiter laser altimeter lola data implications for multi ring basin formation 1 quantifying the ejecta distribution around large lunar basins is important to understanding the origin of basin rings the volume of the transient cavity the depth of sampling and the nature of the basin formation processes we have used newly obtained altimetry data of the moon from the lunar orbiter laser altimeter lola instrument to estimate the thickness of ejecta in the region surrounding the orientale impact basin the youngest and best preserved large basin on the moon our measurements yield ejecta thicknesses of 2900 m near the cordillera mountains the topographic rim of orientale decaying to 1k m in thickness at a range of 215 km these measurements imply a volume of ejecta in the region from the cordillera ring to a radial range of one basin diameter of 2 9 10 6 km 3 and permit the derivation of an ejecta thickness decay model which can be compared with estimates for the volume of excavation and the size of the transient cavity these data are consistent with the outer rook mountains as the approximate location of the transient cavity s rim crest and suggest a volume of 4 8 10 6 km 3 for the total amount

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

structural record of an oblique impact abstract most impactors strike their target at an oblique angle the common criterion for identifying craters formed by an oblique impact is the pattern of the ejecta blanket on earth however ejecta blankets are rarely preserved and other morphological structural or geophysical criteria are needed here we present structural details from the central uplift of the upheaval dome impact structure that are diagnostic of the kinematics during crater collapse and central uplift formation a characteristic imbrication of thrust slices towards the se the pattern of strata orientation within the central uplift dominant radial faults that accommodated nw se shortening and an elliptical bedding outline indicate that the displacement field during crater collapse has not been axial symmetric instead an additional lateral component roughly towards the se is preserved in the internal structure of the central uplift the structural asymmetries are largest in the core of the central uplift and disappear outwards thereby preserving the large scale circular shape of the main structural elements rim monocline ring syncline comparison with numerical models of oblique impacts suggests that the additional lateral displacement component reflects a downrange material transport during the initial stages of central uplift formation similar patterns are identified in other impact structures and may serve as general criteria for identifying the impact direction of deeply eroded impact structures in sedimentary targets

the scaling of complex craters the empirical relation between the transient crater diameter dg and final crater diameter dr of complex craters and basins is estimated using cumulative terrace widths central uplift diameters continuous ejecta radii and transient crater reconstructions determined from lunar and terrestrial impact structures the ratio dg dr is a power law function of dr decreasing uniformly from unity at the diameter of the simple complex crater morphology transition to about 0 5 for large multiring basins like imbrium on the moon the empirical constants in the dg dr relation are interpreted physically to mean that the position of the final rim relative to the transient crater and hence the extent of collapse is controlled or greatly influenced by the properties of the zone of dissociated material produced by the impact shock the continuity of the dg dr relation over the entire spectrum of morphologic types from complex craters to multiring basins implies that the rims of all these structures form in the same tectonic environment despite morphologic differences

large rock slides in impact craters on the moon and mercury abstract impact craters are the most common surface features on the moon and mercury on these two bodies we recognized and mapped large landslides on the walls of impact craters through visual inspection of high resolution imagery we compiled an inventory of 60 landslides on the moon and a second inventory of 58 landslides on mercury adopting categories used to catalog terrestrial mass movements we classified the landslides on the moon and mercury as rock slides we determined the probability density distribution of their planimetric area and we compared the distributions with similar distributions for terrestrial and martian landslides using data from the literature we found that rock slides mapped in impact craters on the moon are on average larger than analogous rock slides on mercury the relationship between the area of the individual rock slides and the area of the hosting crater suggests that rock slides on mercury initiate in smaller craters we hypothesize that the above findings are an effect of the weaker surface gravity of the moon compared to that of mercury and or an effect of the rock material properties

the effect of target properties on crater morphology comparison of central peak craters on the moon and ganymede we examine the morphology of central peak craters on the moon and ganymede in order to investigate differences in the near surface properties of these bodies we have extracted topographic profiles across craters on ganymede using galileo images and use these data to compile scaling trends comparisons between lunar and ganymede craters show that crater depth wall slope and amount of central uplift are all affected by material properties we observe no major differences between similar sized craters in the dark and bright terrain of ganymede suggesting that dark terrain does not contain enough silicate material to significantly increase the strength of the surface ice below crater diameters of 12 km central peak craters on ganymede and simple craters on the moon have similar rim heights indicating comparable amounts of rim collapse this suggests that the formation of central peaks at smaller crater diameters on ganymede than the moon is dominated by enhanced central floor uplift rather than rim collapse crater wall slope trends are similar on the moon and ganymede indicating that there is a similar trend in material weakening with increasing crater size and possibly that the mechanism of weakening during impact is analogous in icy and rocky targets we have run a suite of numerical models to simulate the formation of central peak craters on ganymede and the moon our modeling shows that the same styles of strength model can be applied to ice and rock and that the strength model parameters do not differ significantly between materials

impact melt in small lunar highland craters 1 impact melt deposits have been identified in small simple impact craters within the lunar highlands such deposits are rare but have been observed in craters as small as 170 m diameter the melt occurs as well defined pools on the crater floor as well as veneers on the inner crater wall and stringers of material extending over the rim and away from the crater model calculations indicate that the amount of melt formed in craters 100 2000 m diameter would amount to a few to 106 m3 representing 1 of the crater volume thus significant visible impact melt deposits would not be expected in such small craters as most of the melt material that was formed would be ejected variations in the properties of the projectile or the target cannot account for the amount of observed melt the amount of melt produced is largely insensitive to such variations rather we suggest that these small melt containing craters represent near vertical impacts in which the axes of melting and melt motion are essentially straight down toward the base of the transient cavity for a given event energy under vertical impact conditions the volume of melt produced would be greater than in an oblique impact and the momentum of the material would be directed vertically downward with minimal lateral momentum such that most of the melt is retained within the crater interior since vertical impacts are relatively rare such small craters with visible interior melt deposits are rare while we focus here on the highlands such craters also occur on the maria

analog modeling of normal faulting above middle east domes during regional extension we study the effects of planform dome shape on fault patterns developing with and without concurrent regional extension oriented oblique to the long axis of the dome the motivation was the need to understand fault and fracture patterns in two adjacent mature hydrocarbon fields in the middle east one an elliptical dome and one an irregularly shaped dome the largest faults have throws of approximately 30 m 98 ft which is close to the resolution limit of older two dimensional seismic reflection data the known fault trends are not parallel to the highest transmissivity direction but could form compartment boundaries fault and fracture patterns developed over the modeled domes provide insight into the populations of faults and fractures that are likely to exist in the reservoirs but have been undetected because they are at or below the resolution limit of reflection seismic data major domal structural elements crestal fault systems end splay systems and radial faults are observed in modeled domes rising both with and without concurrent regional extension experimental results indicate that fault and fracture patterns are influenced by the effects of dome shape regional extension and relative timing of uplift with respect to regional extension the expression of particular sets of faults and fractures associated with concurrent doming and regional extension depends on the interaction among regional extension outer arc extension over the dome and tangential extension around the dome margins our results also indicate that the two adjacent natural domes possibly experienced different kinematic histories from those previously interpreted

terrace width variations in complex lunar craters the widths of terrace structures in complex craters on the moon are compared to existing theoretical models of their origin terrace widths in an individual crater increase monotonically outward toward the crater rim similarly the width w of the terraces lying closest to the rim of a crater of diameter d increases monotonically obeying a least squares power law relation w km 0 09d0 87 km a simple model of slumping that ignores inertial forces and assumes a constant bedrock yield strength is in good agreement with the observations

topographic characterization of lunar complex craters 1 we use lunar orbiter laser altimeter topography data to revisit the depth d diameter d and central peak height hcp diameter relationships for fresh complex lunar craters we assembled a data set of young craters with d 15 km and ensured the craters were unmodified and fresh using lunar reconnaissance orbiter wide angle camera images we used lunar orbiter laser altimeter gridded data to determine the rim to floor crater depths as well as the height of the central peak above the crater floor we established power law d d and hcp d relationships for complex craters on mare and highlands terrain our results indicate that craters on highland terrain are on average deeper and have higher central peaks than craters on mare terrain furthermore we find that the crater depths for both mare and highlands craters are significantly deeper than previously reported this likely reflects the inclusion of transitional craters and or older and or modified craters in previous work as well as the limitations of the stereophotogrammetric and shadow length data sets used in those studies there is substantial variability in the depths and the central peak heights for craters in a given diameter range we suggest that the differences in mean d and hcp as a function of crater diameter for highlands and mare craters result from differences in bulk physical properties of the terrain types whereas the variability in d and hcp at a given diameter may reflect variations in impactor properties and impact parameters

ages of mare basalts on the lunar nearside the chronology of lunar volcanism is based on radiometric ages determined from apollo and luna landing site samples regional stratigraphic relationships and crater degradation and size frequency distribution data for units largely defined prior to the end of the apollo program here we report on new crater size frequency distribution data for 139 spectrally and morphologically defined basalt units which are exposed in six nearside impact basins australe tranquillitatis humboldtianum humorum serenitatis and imbrium crater size frequency distribution measurements are a statistically robust and accurate method to derive absolute model ages of unsampled regions of the moon compared to crater degradation ages crater size frequency ages performed on spectrally defined units offer significant improvements in accuracy our investigation showed that 1 in the investigated basins lunar volcanism was active for at least 1 5 2 0 b y starting at about 3 9 4 0 b y and ceasing at 2 0 b y 2 most basalts erupted during the late imbrian period at about 3 6 3 8 b y 3 significantly fewer basalts were emplaced during the eratosthenian period 4 basalts of copernican age were not found in any of the investigated basins 5 lunar basin filling volcanism probably started within 100 m y after the formation of the individual basins we also assessed the relationship between impact basin age and the history of mare basalt emplacement in these basins we found that 1 in all pre nectarian basins australe and tranquillitatis as well as in the humboldtianum basin which is of nectarian age the distribution of surface ages is clearly dominated by only a single peak in the number of erupted units at 3 6 3 8 b y 2 in the younger basins humorum serenitatis and imbrium a second peak at 3 3 3 5 b y is observed 3 basalt eruptions younger than 2 6 b y occur only intermittently and 4 in the youngest basins serenitatis and imbrium we see an extended period of active basin filling volcanism 1 5 1 6 b y which is 500 m y longer than in the australe and humorum and even 1 0 b y longer than in tranquillitatis and humboldtianum

measurement and analysis of lunar basin depths from clementine altimetry altimetric profiles from the clementine lidar are used to calculate the depths of 29 large craters and basins on the moon plotting the depths of the best preserved structures together with values for simple and complex craters measured in pre clementine studies reveals an inflection in the depth diameter d d curve in addition to the one revealed by pre clementine data this inflection occurs in the diameter range that corresponds to the morphologic transition from complex crater to basin the best empirical power law fit for basin depths is log10 d 0 41 log10 d 0 57 this relationship is characterized by a lower slope than that for complex craters demonstrating that this morphologic transition corresponds to a further decrease in the depth of an impact structure relative to its diameter with increasing size qualitative consideration of possible causes for the second inflection leads to the conclusion that it is most likely a consequence of a short term modification mechanism that influences fundamental crater morphology such as the increasing influence of gravity with diameter thicknesses of maria in the major basins are calculated by assuming that their unfilled depths would follow thed drelation results are compared with previous estimates and yield thicknesses that are generally greater than those determined by studies of flooded craters and less than those obtained from analysis of gravity

magnetic measurements on la soufriere volcano guadeloupe lesser antilles 1976 1984 a re examination of the volcanomagnetic effects observed during the volcanic crisis of 1976 1977 abstract during the seismovolcanic crisis of 1976 1977 at la soufriere on guadeloupe a magnetic network of 12 reference markers was set up measurements of the intensity of the earth s magnetic field carried out up to once a day at each marker showed volcano magnetic variations of several nanoteslas nt the variations at certain markers were more or less concealed by transient magnetic variations due to anomalies in conductivity as early as 1978 measurements were resumed and a telemetering network was coupled with the network of reference markers to which 4 new markers were added a detailed study of conductivity anomalies was carried out on the entire volcano contrasts in conductivity linked to the existence of a superficial conducting surface on a ssw nne axis located south of the volcano caused a great lack of homogeneity in the field variations measured at the surface variations greater than about 10 nt appeared in the difference in intensity of the earth s magnetic field between two stations no long term magnetic variation was observed between 1978 and 1984 on the network of markers the accuracy of measurements of volcanic effects was at best 2 nt measurements carried out on the telemetering network during the night refined these results since their accuracy was 1 nt the only significant volcanic crisis between 1978 and 1984 5 7 january 1981 seems to be observed by telemetering stations all the measurements carried out in periods of volcanic inactivity make it possible to re examine the crisis of 1976 1977 though volcanomagnetic effects over short periods cannot be accurately determined variations with a time constant of several weeks were present over the entire volcano these variations were as high as 7 8 nt in remote stations and they can be linked to the three major phases of eruptive activity at la soufriere during the crisis of 1976 1977

a model for the dynamics of crater centered intrusion application to lunar floor fractured craters lunar floor fractured craters are a class of craters modified by post impact mechanisms they are defined by distinctive shallow floors that are convex or plate like sometimes with a wide floor moat bordering the wall region radial concentric and polygonal floor fractures suggest an endogenous process of modification two mechanisms have been proposed to account for such deformations viscous relaxation and spreading of a magma intrusion at depth below the crater to test the second assumption and bring more constraints on the intrusion process we develop a model for the dynamics of magma spreading below an elastic overlying layer with a crater like topography as predicted in earlier more qualitative studies the increase in lithostatic pressure at the crater wall zone prevents the intrusion from spreading laterally leading to the thickening of the intrusion additionally our model shows that the final crater floor appearance after the uplift which can be convex or flat with or without a circular moat bordering the wall zone depends on the elastic thickness of the layer overlying the intrusion and on the crater size our model provides a simple formula to derive the elastic thickness of the overlying layer hence a minimum estimate for the intrusion depth finally our model suggests that crust redistribution by cratering must have controlled magma ascent below most of these craters

inversion of crater morphometric data to gain insight on the cratering process in recent years morphometric data for venus and several outer planet satellites have been collected so we now have observational data of complex craters formed in a large range of target properties we present general inversion techniques that can utilize the morphometric data to quantitatively test various models of complex crater formation the morphometric data we use in this paper are depth of a complex crater the diameter at which the depth diameter ratio changes and onset diameters for central peaks terraces and peak rings we tested the roles of impactor velocities and hydrostatic pressure vs crustal strength and we tested the specific models of acoustic fluidization melosh 1982 and nonproportional growth schultz 1988 neither the acoustic fluidization model nor the nonproportional growth in their published formulations are able to successfully reproduce the data no dependence on impactor velocity is evident from our inversions most of the morphometric data is consistent with a linear dependence on the ratio of crustal strength to hydrostatic pressure on a planet or the factor c p g

rock types of south pole aitken basin and extent of basaltic volcanism the enormous pre nectarian south pole aitken spa basin represents a geophysically and compositionally unique region on the moon we present and analyze the mineralogical diversity across this basin and discuss the implications for basin evolution rock types are derived from clementine multispectral data based on diagnostic characteristics of ferrous absorptions in fresh materials individual areas are characterized as noritic dominated by low ca pyroxene gabbroic basaltic dominated by high ca pyroxene feldspathic 3 6 feo and olivine gabbro dominated by high ca pyroxene and olivine the anorthositic crust has effectively been removed from the interior of the basin the style of volcanism within the basin extends over several 100 myr and includes mare basalt and pyroclastic deposits several areas of ancient pre orientale volcanism or cryptomaria have also been identified the nonmare mafic lithology that occurs across the basin is shown to be noritic in composition and is pervasive laterally and vertically we interpret this to represent impact melt breccia deposits derived from the lower crust a few localized areas are identified within the basin that contain more diverse lithologies gabbro olivine gabbro some of which may represent material from the deepest part of the lower crust and perhaps uppermost mantle involved in the spa event

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

the origin of lunar concentric craters abstract lunar concentric craters are a unique class of impact craters because the interior of the craters contains a concentric ridge but their formation mechanism is unknown in order to determine the origin of concentric craters we examined multiple working hypotheses which include eight impact related and endogenic processes we analyzed data sets that originated from instruments onboard clementine kaguya and the lunar reconnaissance orbiter to characterize the morphology spatial distribution composition and absolute model ages of 114 concentric craters concentric craters contain five key properties 1 a concentric ridge 2 anomalously shallow floors 3 their occurrence is concentrated near mare margins and in mare pond regions 4 the concentric ridge composition is similar to the surrounding area and 5 concentric crater ages are eratosthenian and older these five key properties served as constraints for testing impact related and endogenic mechanisms of formation we find that most impact related hypotheses cannot explain the spatial and age distribution of concentric craters as for endogenic hypotheses we deduce that igneous intrusions are the likely mechanism that formed concentric craters because of the close relationship between concentric craters and floor fractured craters and the concentration of both features near mare highland boundaries and in mare ponds furthermore we observe that floor fractured craters are common at crater diameters 15 km whereas concentric craters are common at crater diameters 15 km produce floor fractured craters

growth of planetary crusts the planets and satellites of the solar system show much diversity but most have formed crusts which differ substantially from their bulk compositions three principal types of crust may be broadly distinguished primary formed after accretional heating e g lunar highlands secondary formed following partial melting in planetary mantles e g lunar maria terrestrial oceanic crust and tertiary formed by processing of secondary crusts of which the continental crust of the earth is the only identifiable example crustal growth before 3800 m y was complicated by the heavy planetesimal bombardment active erosion on the earth is likely to have destroyed the brecciated rubble resulting from over 200 mare orientale scale events in this period the mercurian crust is probably primary perhaps analogous to the lunar highlands in contrast the observable martian and venusian crusts are secondary dominated by basaltic volcanism the icy crusts of the satellites of the outer planets are mixtures of both primary and secondary crusts with complex histories ganymede is of special interest in displaying a secondary grooved terrain water ice crust which has split an older more heavily cratered possibly primary crust slight expansion has resulted probably from polymorphic transitions from high density ice viii to low density ice i the primary lunar crust 12 of lunar volume grew in 108 years secondary crusts derived by partial melting of mantles grow at a much slower rate total production of terrestrial oceanic crust over 4000 m y is only 2 of planetary volume whereas terrestrial continental crustal growth is even slower producing over the same period a volume only about 0 33 that of the earth crustal growth on the other planets appears to be an irreversible process without evidence of recycling no evidence for massive planetary expansion as required by expanding earth hypotheses is apparent on the other planets or satellites of the solar system

ejecta formation and crater development of the mjolnir impact crater ejecta correlation is an important element in the analysis of crater formation and its influence on the geological evolution in this study both the ejecta distribution and the internal crater development of the jurassic cretaceous mjolnir crater 40 km in diameter located in the barents sea are investigated through numerical simulations the simulations show a highly asymmetrical ejecta distribution and underscore the importance of a layer of surface water in ejecta distribution as expected the ejecta asymmetry increases as the angle of impact decreases the simulation also displays an uneven aerial distribution of ejecta the generation of the central high is a crucial part of crater formation in this study peak generation is shown to have a skewed development from approximately 50 90 sec after impact when the peak reaches its maximum height of 1 1 5 km during this stage the peak crest is moved about 5 km from an uprange to a downrange position ending with a final central position which has a symmetrical appearance that contrasts with its asymmetrical development

gullies and landslides on the moon evidence for dry granular flows 1 high resolution images from chandrayaan 1 terrain mapping camera and lunar reconnaissance orbiter camera reveal landslides and gully formation on the interior wall of a 7 km diameter simple crater emplaced in schrodinger basin on the farside of the moon these features occur on the steep upper crater wall where the slope is 35 the gullies show a typical alcove channel fan morphology some gullies incise bedrock where impact related faults are present slope failure along the concentric faults also led to formation of landslides dark slope streaks are abundant at the bright gully regions especially near the fan and channel deposits spectral characteristics inferred from data obtained by hyperspectral imager and moon mineralogy mapper on board chandrayaan 1 show that the gullies and landslides are characterized by high optical immaturity and devoid of prominent spectral absorption features related to water or hydroxyl molecules suggesting youthful dry granular flows mass movements on the crater wall led to the formation of arcuate ridges and ponding of fine grained sediments on the crater floor runout flows from small impact craters on the slopes indicate that impact induced seismic shaking was responsible for the downslope mass movements crater size frequency distributions suggest a minimum age of 18 2 ma for the gullies and 2 ma for the landslides while age of the host crater ejecta was inferred to be about 175 ma the gullies and landslides also occur on the interior wall of other impact craters elsewhere on the moon and probably formed by similar processes

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

deep impact craters in the isidis and southwestern utopia planitia regions of mars high target material strength as a possible cause using themis moc and mola data we havefound 51 craters in the diameter range 6 11 8 km withinsouthwestern utopia planitia and isidis planitia that aresignificantly deeper than typical fresh craters in the northernlowlands of mars the restricted geographic distribution ofthese craters their simple morphology and data fromimpact and explosion crater studies suggest that unusuallystrong target materials as much as a factor of 2 greater thanaverage materials in the martian lowlands are the cause ofthe excessive crater depth we propose that the greater targetmaterial strength acts to delay gravity dominated collapse tolarger crater sizes furthermore we suggest that a regional olivine rich mafic to untramafic rock unit identified by tesand themis is a reasonable candidate for these strongmaterials the unit is exposed on the southern edge of isidisbasin and in crater ejecta within the basin and forms layersthat dip toward the isidis basin center