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Register a new userCrater-ray formation through mutual collisions of hypervelocity-impact induced ejecta particles
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We investigate the patterns observed in ejecta curtain induced by hypervelocity impact (2-6 km/s) with a variety of the size and shape of target particles. We characterize the patterns by an angle, defined as the ratio of the characteristic length of the pattern obtained by Fourier transformation to the distance from the impact point. This angle is found to be almost the same as that obtained by the reanalysis of the patterns in the previous study at lower impact velocities (Kadono et al., 2015, Icarus 250, 215-221), which are consistent with lunar crater-ray systems. Assuming that the pattern is formed by mutual collision of particles with fluctuation velocity in excavation flow, we evaluate an angle at which the pattern growth stops and show that this angle is the same in the order of magnitude as the ratio of the fluctuation velocity and the radial velocity. This relation is confirmed in the results of experiments and numerical simulations. Finally, we discuss the dependence of the patterns on impact conditions. The experiments show no dependence of the angle on impact velocity. This indicates that the ratio between the fluctuation and radial velocity components in excavation flow does not depend on impact velocity. Moreover, the independences on particle size and particle shape suggest that the angle characterizing the structure of the patterns does not depend on cohesive force. Since cohesive forces should be related with elastic properties of particles, the structure does not depend on elastic properties, though inelastic collisions are important for the persistence and contrast of the patterns.
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We explore the origin of a ~280 m wide, heavily eroded circular depression in Palm Valley, Northern Territory, Australia using gravity, morphological, and mineralogical data collected from a field survey in September 2009. From the analysis of the survey, we debate probable formation processes, namely erosion and impact, as no evidence of volcanism is found in the region or reported in the literature. We argue that the depression was not formed by erosion and consider an impact origin, although we acknowledge that diagnostics required to identify it as such (e.g. meteorite fragments, shatter cones, shocked quartz) are lacking, leaving the formation process uncertain. We encourage further discussion of the depressions origin and stress a need to develop recognition criteria that can help identify small, ancient impact craters. We also encourage systematic searches for impact craters in Central Australia as it is probable that many more remain to be discovered.
We performed numerical simulations of impact crater formation on Europa to infer the thickness and structure of its ice shell. The simulations were performed using iSALE to test both the conductive ice shell over ocean and the conductive lid over warm convective ice scenarios for a variety of conditions. The modeled crater depth-diameter is strongly dependent on thermal gradient and temperature of the warm convective ice. Our results indicate that both a fully conductive (thin) shell and a conductive-convective (thick) shell can reproduce the observed crater depth-diameter and morphologies. For the conductive ice shell over ocean, the best fit is an approximately 8 km thick conductive ice shell. Depending on the temperature (255 - 265 K) and therefore strength of warm convective ice, the thickness of the conductive ice lid is estimated at 5 - 7 km. If central features within the crater, such as pits and domes, form during crater collapse, our simulations are in better agreement with the fully conductive shell (thin shell). If central features form well after the impact, however, our simulations suggest a conductive-convective shell (thick shell) is more likely. Although our study does not provide firm conclusion regarding the thickness of Europas ice shell, our work indicates that Valhalla-class multiring basins on Europa may provide robust constraints on the thickness of Europas ice shell.
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