The planforms of low‐angle impact craters in the northern hemisphere of Mars

Abstract— 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 sin²θ dependence for the cumulative fraction of craters forming below angle θ. 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 cross‐range 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.     

The Whitecourt meteorite impact crater,Alberta, Canada

Abstract– The <1,100 yr old Whitecourt meteorite impact crater, located south of Whitecourt, Alberta, Canada, is a well‐preserved bowl‐shaped structure having a depth and diameter of approximately 6 and 36 m, respectively. There are fewer than a dozen known terrestrial sites of similar size and age. Unlike most of these sites, however, the Whitecourt crater contains nearly all of the features associated with small impact craters including meteorites, ejecta blanket, observable transient crater boundary, raised rim, and associated shock indicators. This study indicates that the crater formed from the impact of an approximately 1 m diameter type IIIAB iron meteoroid traveling east‐northeast at less than approximately 10 km s^?1, striking the surface at an angle between 40° and 55° to horizontal. It appears that the main mass survived atmospheric transit relatively intact, with fragmentation and partial melting during impact. Most meteoritic material has a jagged, shrapnel‐like morphology and is distributed downrange of the crater.     

Geological and geophysical investigation of Kamil crater,Egypt

Abstract– 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 Deep Impact oblique impact cratering experiment

The Deep Impact probe collided with 9P Tempel 1 at an angle of about 30° from the horizontal. This impact angle produced an evolving ejecta flow field very similar to much smaller scale oblique-impact experiments in porous particulate targets in the laboratory. Similar features and phenomena include a decoupled vapor/dust plume at the earliest times, a pronounced downrange bias of the ejecta, an uprange “zone of avoidance” (ZoA), heart-shaped ejecta ray system (cardioid pattern), and a conical (but asymmetric) ejecta curtain. Departures from nominal cratering evolution, however, provide clues on the nature of the impact target. These departures include: fainter than expected flash at first contact, delayed emergence of the self-luminous vapor/dust plume, uprange-directed plume, narrow early-time uprange ray followed by a late-stage uprange plume, persistence of ejecta asymmetries (and the uprange ZoA) throughout the approach sequence, emergence of a downrange ZoA at late times, detachment of uprange curved rays, very long lasting non-radial ejecta rays, and high-angle ejecta plume lasting over the entire encounter. The first second of crater formation most closely resembles the consequences of a highly porous target, while later evolution indicates that the target may be layered as well. Experiments also reveal that impacts into highly porous targets produce a vapor/dust plume directed back up the incoming trajectory. This uprange plume is attributed to cavitation within a narrow penetration funnel. The observed lateral expansion speed of the initial vapor plume downrange provides an estimate for the total vaporized mass equal to ∼5mp (projectile masses) of water ice or 6mp of CO₂. The downrange plume speed is consistent with the gas expansion added to the downrange horizontal component of the DI probe. Based on high-speed spectroscopy of experimental impacts, the observed delay in brightening of the DI plume may be the result of delayed condensation of carbon, in addition to silicates. Observed molecular species in the initial self-luminous vapor plume likely represent recombination products from completely dissociated target materials. The crater produced by the impact can be estimated from Earth-based observations of total ejected mass to be 130-220 m in diameter. This size range is consistent with a 220 m-diameter circular feature at the point of impact visible in highly processed, deconvolved HRI images. The final crater, however, may resemble an inverted sombrero-hat, with a deep central pit surrounded by a shallow excavation crater. Excavated distal material observed from the Earth was likely from the upper few meters contrasted with ballistic ejecta observed from the DI flyby, which included deep materials (10-30 m) within the diffuse plume above the crater and shallower (5-10 m) materials within the ejecta curtain.     

Crater geometry and ejecta thickness of the Martian impact crater Tooting

Abstract— We use Mars Orbiter Laser Altimeter (MOLA) topographic data and Thermal Emission Imaging System (THEMIS) visible (VIS) images to study the cavity and the ejecta blanket of a very fresh Martian impact crater ?29 km in diameter, with the provisional International Astronomical Union (IAU) name Tooting crater. This crater is very young, as demonstrated by the large depth/diameter ratio (0.065), impact melt preserved on the walls and floor, an extensive secondary crater field, and only 13 superposed impact craters (all 54 to 234 meters in diameter) on the ?8120 km² ejecta blanket. Because the pre‐impact terrain was essentially flat, we can measure the volume of the crater cavity and ejecta deposits. Tooting crater has a rim height that has >500 m variation around the rim crest and a very large central peak (1052 m high and >9 km wide). Crater cavity volume (i.e., volume below the pre‐impact terrain) is ?380 km³ the volume of materials above the pre‐impact terrain is ?425 km³. The ejecta thickness is often very thin (<20 m) throughout much of the ejecta blanket. There is a pronounced asymmetry in the ejecta blanket, suggestive of an oblique impact, which has resulted in up to ?100 m of additional ejecta thickness being deposited down‐range compared to the up‐range value at the same radial distance from the rim crest. Distal ramparts are 60 to 125 m high, comparable to the heights of ramparts measured at other multi‐layered ejecta craters. Tooting crater serves as a fresh end‐member for the large impact craters on Mars formed in volcanic materials, and as such may be useful for comparison to fresh craters in other target materials.     

The global budget of impact-derived sediments on Venus

The observed record of impact craters on the surface of the planet Venus can be used to calculate the contribution of fine materials generated by impact processes to the global sedimentary cycle. Using various methods for the extending the population of impact craters with diameters larger than 8 km observed on the northern 25% of the Venus to the entire surface area of the planet, we have estimated how materials ejected from the integrated record of impact cratering over the past 0.5 to 1.0 æ might have been globally distributed. Relationships for computing the fraction of ejected materials from impact craters in a given size range originally developed for the Moon (and for terrestrial nuclear explosion cratering experiments) were scaled for Venus conditions, and the ejecta fragments with sizes less than 30 m were considered to represent those with the greatest potential for global transport and eventual fallout. A similar set of calculations were carried out using the observed terrestrial cratering record, corrected for the missing population of small craters and oceanic impacts that have either been eroded or are unobserved (due to water cover). Our calculations suggest that both Venus and the Earth should have experienced approximately 6000 impact events over the past 0.5 to 1 æ (in the size range from 1 km to about 180 km). The cumulative global thickness of impact-derived fine materials that could have produced from this record of impacts in this time period is most likely between 1–2 mm for Venus, and certainly no more than 6 mm (assuming an enhanced population of large 150–200 km scale impact events). For Earth, the global cumulative thickness is most likely 0.2 to 0.3 mm, and certainly no more than 2 to 3 mm. The cumulative volume of impact ejecta (independent of particle size) for Venus generated over the past 1 æ, when spread out over the global surface area to form a uniform layer, would fall between 2 and 12 meters, although 99% of this material would be deposited in the near rim ejecta blanket (from 1 to 2.3 crater radii from the rim crest), and only 0.02% would be available for global transport as dust-sized particles. Thus, our conclusion is that Venus, as with the Earth, cannot have formed a substantial impact-derived regolith layer over the past billion years of its history as is typical for smaller silicate planets such as the Moon and Mercury. This conclusion suggests that there must be other extant mechanisms for sediment formation and redistribution in the Venus environment, on the basis of Venera Lander surface panoramas which demonstrate the occurrence of local sediment accumulations.'Geology and Tectonics of Venus', special issue edited by Alexander T. Basilevsky (USSR Acad. of Sci. Moscow), James W. Head (Brown University, Providence), Gordon H. Pettengill (MIT, Cambridge, Massachusetts) and R. S. Saunders (J.P.L., Pasadena).     

A new method to determine the direction of impact: Asymmetry of concentric impact craters as observed in the field (Lockne), on Mars,in experiments,and simulations

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

Structural uplift and ejecta thickness of lunar mare craters: New insights into the formation of complex crater rims

We investigate the elevated crater rims of lunar craters. The two main contributors to this elevation are a structural uplift of the preimpact bedrock and the emplacement of ejecta on top of the crater rim. Here, we focus on five lunar complex mare craters with diameters ranging between 16 and 45 km: Bessel, Euler, Kepler, Harpalus, and Bürg. We performed 5281 measurements to calculate precise values for the structural rim uplift and the ejecta thickness at the elevated crater rim. The average structural rim uplift for these five craters amounts to S_RU = 70.6 ± 1.8%, whereas the ejecta thickness amounts to E_T = 29.4 ± 1.8% of the total crater rim elevation. Erosion is capable of modifying the ratio of ejecta thickness to structural rim uplift. However, to minimize the impact of erosion, the five investigated craters are young, pristine craters with mostly preserved ejecta blankets. To quantify how strongly craters were enlarged by crater modification processes, we reconstructed the dimensions of the transient crater. The difference between the transient crater diameter and the final crater diameter can extend up to 11 km. We propose reverse faulting and thrusting at the final crater rim to be one of the main contributing factors of forming the elevated crater rim.     

Ejecta formation and crater development of the Mjølnir impact

Abstract— 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 Mjølnir 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.     

Variations in magnetic properties of target basalts with the direction of asteroid impact: Example from Lonar crater,India

Abstract– The Lonar crater in Maharashtra state, India, has been completely excavated on the Deccan Traps basalt (approximately 65 Ma) at approximately 570 ± 47 ka by an oblique impact of a possible chondritic asteroid that struck the preimpact target from the east at an angle of approximately 30–45^o to the horizon where the total duration of the shock event was approximately 1 s. It is shown by our early work that the distribution of ejecta and deformation of target rocks around the crater rim are symmetrical to the east–west plane of impact ( Misra et al. 2010 ). The present study shows that some of the rock magnetic properties of these shocked target basalts, e.g., low‐field anisotropy of magnetic susceptibility (AMS), natural remanent magnetization (NRM)/bulk susceptibility (χ), and high‐coercivity and high‐temperature (HC_HT) magnetization component, are also almost symmetrically oriented with reference to the plane of impact. Studies on the relative displacements of K₃ (minimum) AMS axes of shocked basalts from around the crater rim and from the adjacent target rocks to the approximately 2–3 km west of the crater center suggest that the impact stress could have branched out into the major southwestward and northwestward components in the downrange direction immediately after the impact. The biaxial distribution of AMS axes in stereographic plots for the unshocked basalts transforms mostly into triaxial distribution for the shocked basalts, although transitional type distribution also exists. The degree of anisotropy (P′) of AMS ellipsoids of the shocked basalts decreases by approximately 2% when compared with those of the unshocked target (approximately 1.03). The NRM/χ (Am^?1) values of the shocked basalts on the rim of the Lonar crater do not show much change in the uprange or downrange direction on and close to the east–west plane of impact, and the values are only approximately 1.5 times higher on average over the unshocked basalts around the crater. However, the values become approximately 1.4–16.4 times higher for the shocked basalts on the crater rim, which occur obliquely to the plane of impact. The target basalts at approximately 2–3 km west of the crater center in the downrange also show a significant increase (up to approximately 26 times higher) in NRM/χ. The majority of the shocked basalt samples (approximately 73%) from around the crater rim, in general, show a lowering of REM, except those from approximately 2–3 km west of the crater center in the downrange, where nearly half of the sample population shows a higher REM of approximately 3.63% in average. The shocked target basalts around the Lonar crater also acquired an HC_HT magnetization component due to impact. These HC_HT components are mostly oriented in the uprange direction and are symmetrically disposed about the east–west plane of impact, making an obtuse angle with the direction of impact. The low‐coercivity and low‐temperature (LC_LT) components of both the unshocked and shocked basalts are statistically identical to the present day field (PDF) direction. This could be chemical and/or viscous remanent magnetization acquired by the target basalts during the last 570 ± 47 ka, subsequent to the formation of the Lonar crater. The shocked Lonar target basalts appear to have remagnetized under high impact shock pressure and at low temperature of approximately 200–300 °C, where Ti‐rich titanomagnetite was the main magnetic remanence carrier.     

Lunar and Venusian radar bright rings

Twenty-one lunar craters have radar bright ring appearances which are analogous to eleven complete ring features in the earth-based 12.5 cm observations of Venus. Radar ring diameters and widths for the lunar and Venusian features overlap for sizes from 45 to 100 km. Radar bright areas for the lunar craters are associated with the slopes of the inner and outer rim walls, while level crater floors and level ejecta fields beyond the raised portion of the rim have average radar backscatter. We propose that the radar bright areas of the Venusian rings are also associated with the slopes on the rims of craters.The lunar craters have evolved to radar bright rings via mass wasting of crater rim walls and via post impact flooding of crater floors. Aeolian deposits of fine-grained material on Venusian crater floors may produce radar scattering effects similar to lunar crater floor flooding. These Venusian aeolian deposits may preferentially cover blocky crater floors producing a radar bright ring appearance.We propose that the Venusian features with complete bright ring appearances and sizes less than 100 km are impact craters. They have the same sizes as lunar craters and could have evolved to radar bright rings via analogous surface processes.     

Initiation of Run-Out Flows on Venus by Oblique Impacts

A hypervelocity oblique impact results in a downrange-moving vapor cloud, a significant fraction of which is derived from the projectile. Since the vapor cloud expands to great extent and becomes very tenuous quickly on a planet with a thin or no atmosphere, it does not leave a well-defined geologic expression. The thick atmosphere of Venus, however, is sufficient to contain such a rapidly expanding vapor cloud. As a result of atmospheric interactions, impact vapor condenses and contributes to run-out flows around craters on Venus. Previous results of both laboratory experiments and simple semi-analytical calculations indicate that an impact-vapor origin can account for the morphology of run-out flows on Venus most consistently. However, the detailed dynamics and geologic record of downrange-moving impact vapor clouds in Venus's atmosphere are not understood quantitatively. To approach these problems, we carried out two-dimensional hydrocode calculations. Parametric studies of these hydrocode calculations yield simple scaling laws for both the total downrange travel distance and the final temperature of impact vapor clouds under conditions on Venus. Under typical impact conditions, impact vapor clouds travel downrange more than a crater radius prior to the completion of crater formation. Furthermore, the scaling law for the total travel distance is compared with observations for the downrange offset of the source regions of run-out flows around oblique craters. The results of this comparison suggest that energy/momentum-partitioning processes other than pure shock coupling may play important roles in hypervelocity impact at planetary scales. The results of hydrocode calculations also indicate that the terminal temperature of the impact vapor is close to the condensation temperatures of silicates, suggesting that two scenarios are possible for expected range of impact conditions: 1. Impact vapor condenses and forms run-out flows. 2. Impact vapor fails to condense and leaves no run-out flows. Consequently, natural variation in impact angle, velocity, and projectile composition may account for partial occurrence of run-out flows around impact craters on Venus.     

Bosumtwi impact structure,Ghana: Evidence for fluidized emplacement of the ejecta

The about 10.5 km diameter Bosumtwi impact crater is one of the youngest large impact structures on Earth. The crater rim is readily noticed on topographic maps or in satellite imagery. It defines a circular basin filled by water (Lake Bosumtwi) and lacustrine sediments. The morphology of this impact structure is also characterized by a circular plateau extending beyond the rim and up to 9–10 km from the center of the crater (about 2 crater radii). This feature comprises a shallow ring depression, also described as an annular moat, and a subdued circular ridge at its outer edge. The origin of this outermost feature could so far not be elucidated based on remote sensing data only. Our approach combines detailed topographic analysis, including roughness mapping, with airborne radiometric surveys (mapping near‐surface K, Th, U concentrations) and field observations. This provides evidence that the moat and outer ring are features inherited from the impact event and represent the partially eroded ejecta layer of the Bosumtwi impact structure. The characteristics of the outer ridge indicate that ejecta emplacement was not purely ballistic but requires ejecta fluidization and surface flow. The setting of Bosumtwi ejecta can therefore be considered as a terrestrial analog for rampart craters, which are common on Mars and Venus, and also found on icy bodies of the outer solar system (e.g., Ganymede, Europa, Dione, Tethys, and Charon). Future studies at Bosumtwi may therefore help to elucidate the mechanism of formation of rampart craters.     

Asymmetric signatures in simple craters as an indicator for an oblique impact direction

Abstract— In oblique impacts with an impact angle under 45°, the bilateral shape of the distal ejecta blanket is used as the strongest indicator for an impact vector. This bilateral symmetry is attenuated and is superimposed by radial symmetry towards the crater rim, which remains circular for impact angles down to 10–15°. The possibility that remnants of bilateral symmetry might still be present in the most proximal ejecta, the overturned flap and the crater rim was explored with the intention of deducing an impact vector. A model is presented that postulates bilateral patterns using proximal ejecta trajectories and predicts these patterns in the orientation of bedding planes in the crater rim. This model was successfully correlated to patterns described by radial grooves in the proximal ejecta blanket of the oblique Tooting crater on Mars. A new method was developed to detect structural asymmetries by converting bedding data into values that express the deviation from concentric strike orientation in the crater rim relative to the crater center, termed “concentric deviation.” The method was applied to field data from Wolfe Creek crater, Western Australia. Bedding in the overturned flap implies an impactor striking from the east, which refines earlier publications, while bedding from the inner rim shows a correlation with the crater rim morphology.     

Geology of the Selk crater region on Titan from Cassini VIMS observations

Observations of Titan obtained by the Cassini Visual and Infrared Mapping Spectrometer (VIMS) have revealed Selk crater, a geologically young, bright-rimmed, impact crater located ∼800 km north-northwest of the Huygens landing site. The crater rim-crest diameter is ∼90 km; its floor diameter is ∼60 km. A central pit/peak, 20-30 km in diameter, is seen; the ratio of the size of this feature to the crater diameter is consistent with similarly sized craters on Ganymede and Callisto, all of which are dome craters. The VIMS data, unfortunately, are not of sufficient resolution to detect such a dome. The inner rim of Selk crater is fluted, probably by eolian erosion, while the outer flank and presumed ejecta blanket appear dissected by drainages (particularly to the east), likely the result of fluvial erosion. Terracing is observed on the northern and western walls of Selk crater within a 10-15 km wide terrace zone identified in VIMS data; the terrace zone is bright in SAR data, consistent with it being a rough surface. The terrace zone is slightly wider than those observed on Ganymede and Callisto and may reflect differences in thermal structure and/or composition of the lithosphere. The polygonal appearance of the crater likely results from two preexisting planes of weakness (oriented at azimuths of 21° and 122° east of north). A unit of generally bright terrain that exhibits similar infrared-color variation and contrast to Selk crater extends east-southeast from the crater several hundred kilometers. We informally refer to this terrain as the Selk “bench.” Both Selk and the bench are surrounded by the infrared-dark Belet dune field. Hypotheses for the genesis of the optically bright terrain of the bench include: wind shadowing in the lee of Selk crater preventing the encroachment of dunes, impact-induced cryovolcanism, flow of a fluidized-ejecta blanket (similar to the bright crater outflows observed on Venus), and erosion of a streamlined upland formed in the lee of Selk crater by fluid flow. Vestigial circular outlines in this feature just east of Selk’s ejecta blanket suggest that this might be a remnant of an ancient, cratered crust. Evidently the southern margin of the feature has sufficient relief to prevent the encroachment of dunes from the Belet dune field. We conclude that this feature either represents a relatively high-viscosity, fluidized-ejecta flow (a class intermediate to ejecta blankets and long venusian-style ejecta flows) or a streamlined upland remnant that formed downstream from the crater by erosive fluid flow from the west-northwest.     

The Characteristics of Polygonal Impact Craters on Venus

Polygonal impact craters (PICs) are craters whose shape in plan view is more or less angular instead of being circular or ellipsoidal. This type of craters are present and often common on the Moon, Mercury, Mars and several asteroids and icy moons and after the careful analysis we found on Venus 131 impact craters, which show at least two straight rim segments. This survey proves that there are polygonal impact craters on Venus and they may provide a good tool to analyse the properties of the planet’s surface/crust/lithosphere as well as the impact process itself. This study also collaborates our previous results, that PICs are not an anomaly among craters, but an integral part of all impact craters regardless of their size or environment. We compared the polygonal impact craters to “normal”-shaped craters by using different characteristics (diameter, altitude, geologic setting, morphologic class, floor reflectance, degradation stage, and wall terracing). It turned out that the smaller crater sizes favor the formation of straight rim segments, but otherwise these craters show similar characteristics to other craters. Our study also shows that there are regions where the straight segments of the crater rims most clearly follow the orientations of the dominant tectonic features of the area. Thus, the orientations of crater walls reflect–at least in some places–the local tectonics and zones of weakness also on Venus and could thus tell us about the directions and distributions of fractures or other zones of weakness in the crust.     

Morphology of Lonar Crater,India: Comparisons and implications

Lonar Crater is a young meteorite impact crater emplaced in Deccan basalt. Data from 5 drillholes, a gravity network, and field mapping are used to reconstruct its original dimensions, delineate the nature of the pre-impact target rocks, and interpret the emplacement mode of the ejecta. Our estimates of the pre-erosion dimensions are: average diameter of 1710 m; average rim height of 40 m (30–35 m of rim rock uplift, 5–10 m of ejected debris); depth of 230–245 m (from rim crest to crater floor). The crater's circularity index is 0.9 and is unlikely to have been lower in the past. There are minor irregularities in the original crater floor (present sediment-breccia boundary) possibly due to incipient rebound effects. A continuous ejecta blanket extends an average of 1410 m beyond the pre-erosion rim crest.In general, fresh terrestrial craters, less than 10 km in diameter, have smaller depth/diameter and larger rim height/diameter ratios than their lunar counterparts. Both ratios are intermediate for Mercurian craters, suggesting that crater shape is gravity dependent, all else being equal. Lonar demonstrates that all else is not always equal. Its depth/diameter ratio is normal but, because of less rim rock uplift, its rim height/diameter ratio is much smaller than both fresh terrestrial and lunar impact craters. The target rock column at Lonar consists of one or more layers of weathered, soft basalt capped by fresh, dense flows. Plastic deformation and/or compaction of this lower, incompetent material probably absorbed much of the energy normally available in the cratering process for rim rock uplift.A variety of features within the ejecta blanket and the immediately underlying substrate, plus the broad extent of the blanket boundaries, suggest that a fluidized debris surge was the dominant mechanism of ejecta transportation and deposition at Lonar. In these aspects, Lonar should be a good analog for the fluidized craters of Mars.     

Radial variation of lunar crater rim topography

The variation of rim topography as a function of range from the crater rim has been determined for a group of morphologically fresh lunar craters (D = 10–140 km) using the recent series of Lunar Topographic Orthophotomaps. The rate at which exterior crater topography converges with the surrounding surface is highly variable along different radial directions at individual craters as well as between different craters. At several craters, oblique impact appears to have contributed to azimuthal elevation/range variations. The topographic expression of a crater above the surrounding surface typically decreases to one-tenth of the estimated rim height at a range of 1.3R–1.7R, well within the rough-textured ejecta deposit surrounding the crater. Comparisons with terrestrial craters suggest that the topographic crater rim is predominantly a structural feature. In most craters large portions of the hummocky facies and virtually all of the radial facies, in spite of their rough appearance and local topographic variations, provide no significant net topographic addition to the preexisting surface. The extreme variability of crater rim topography strongly suggests that ejecta thicknesses are highly variable and that a unique power-law expression cannot truly represent the radial variation of ejecta deposit thickness.     

Generation and emplacement of fine-grained ejecta in planetary impacts

We report here on a survey of distal fine-grained ejecta deposits on the Moon, Mars, and Venus. On all three planets, fine-grained ejecta form circular haloes that extend beyond the continuous ejecta and other types of distal deposits such as run-out lobes or ramparts. Using Earth-based radar images, we find that lunar fine-grained ejecta haloes represent meters-thick deposits with abrupt margins, and are depleted in rocks ?1 cm in diameter. Martian haloes show low nighttime thermal IR temperatures and thermal inertia, indicating the presence of fine particles estimated to range from ∼10 μm to 10 mm. Using the large sample sizes afforded by global datasets for Venus and Mars, and a complete nearside radar map for the Moon, we establish statistically robust scaling relationships between crater radius R and fine-grained ejecta run-out r^* for all three planets. On the Moon, r^* ∼ R^−0.18 for craters 5-640 km in diameter. For Venus, radar-dark haloes are larger than those on the Moon, but scale as r^* ∼ R^−0.49, consistent with ejecta entrainment in Venus’ dense atmosphere. On Mars, fine-ejecta haloes are larger than lunar haloes for a given crater size, indicating entrainment of ejecta by the atmosphere or vaporized subsurface volatiles, but scale as R^−0.13, similar to the ballistic lunar scaling. Ejecta suspension in vortices generated by passage of the ejecta curtain is predicted to result in ejecta run-out that scales with crater size as R^1/2, and the wind speeds so generated may be insufficient to transport particles at the larger end of the calculated range. The observed scaling and morphology of the low-temperature haloes leads us rather to favor winds generated by early-stage vapor plume expansion as the emplacement mechanism for low-temperature halo materials.     

Morphology and geometry of the distal ramparts of Martian impact craters

Abstract— We used Mars Orbiter Laser Altimeter (MOLA), Thermal Emission Imaging System visible light (THEMIS VIS), and Mars Orbiter Camera (MOC) data to identify and characterize the morphology and geometry of the distal ramparts surrounding Martian craters. Such information is valuable for investigating the ejecta emplacement process, as well as searching for spatial variations in ejecta characteristics that may be due to target material properties and/or latitude, altitude, or temporal variations in the climate. We find no systematic trend in rampart height that would indicate regional variations in target properties for 54 ramparts at 37 different craters 5.7–35.9 km in diameter between 52.3°S to 47.6°N. Rampart heights for multi‐lobe and single‐lobe ejecta are each normally distributed with a common standard deviation, but statistically distinct mean values. Ramparts range in height from 20–180 m, are not symmetric, are typically steeper on their distal sides, and may be as much as ?4 km wide. The ejecta blanket proximal to parent crater from the rampart may be very thin (<5 m). A detailed analysis of two craters, Toconao crater (21°S, 285°E) (28 measurements), and an unnamed crater within Chryse Planitia (28.4°N, 319.6°E) (20 measurements), reveals that ejecta runout distance increases with an increase in height between the crater rim and the rampart, but that rampart height is not correlated with ejecta runout distance or the thickness of the ejecta blanket.