Mercury: Radar images of the equatorial and midlatitude zones
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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.
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Abstract The formation and structure of the Orientale basin on the Moon has been extensively studied in the past; however, estimates of its transient crater size, excavated volume and depth, and ejecta distribution remain uncertain. Here we present a new numerical model to reinvestigate the formation and structure of Orientale basin and better constrain impact parameters such as impactor size and velocity. Unlike previous models, the observed ejecta distribution and ejecta thickness were used as the primary constraints to estimate transient crater size—the best measure of impact energy. Models were also compared to basin morphology and morphometry, and subsurface structures derived from high‐resolution remote sensing observations and gravity data, respectively. The best fit model suggests a 100 km diameter impactor with a velocity of ~12 km s −1 formed the Orientale basin on a relatively “cold” Moon. In this impact scenario the transient crater diameter is ~400 km or 460 km depending on whether the crater is defined using the diameter of the excavation zone or the diameter of the growing cavity at the time of maximum crater volume, respectively. The volume of ejecta material is ~4.70 × 10 6 km 3 , in agreement with recent estimates of the Orientale ejecta blanket thickness from remote sensing studies. The model also confirms the remote sensing spectroscopic observations that no mantle material was excavated and deposited at Orientale's rim.
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Abstract– Some fresh impact craters on Ganymede have the overall ejecta morphology similar to Martian double-layer ejecta (DLE), with the exception of the crater Nergal that is most like Martian single layer ejecta (SLE) craters (as is the terrestrial crater Lonar). Similar craters also have been identified on Europa, but no outer ejecta layer has been found on these craters. The morphometry of these craters suggests that the types of layered ejecta craters identified by Barlow et al. (2000) are fundamental. In addition, the mere existence of these craters on Ganymede and Europa suggests that an atmosphere is not required for ejecta fluidization, nor can ejecta fluidization be explained by the flow of dry ejecta. Moreover, the absence of fluidized ejecta on other icy bodies suggests that abundant volatiles in the target also may not be the sole cause of ejecta fluidization. The restriction of these craters to the grooved terrain of Ganymede and the concentration of Martian DLE craters on the northern lowlands suggests that these terrains may share key characteristics that control the development of the ejecta of these craters. In addition, average ejecta mobility (EM) ratios indicate that the ejecta of these bodies are self-similar with crater size, but are systematically smaller on Ganymede and Europa. This may be due to the effects of the abundant ice in the crusts of these satellites that results in increased ejection angle causing ejecta to impact closer to the crater and with lower horizontal velocity.
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