Transient Crater Growth in Low Density Targets
Transient Crater Growth in Low Density Targets
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低密度目标中的瞬态弹坑增长
DOI：
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发表时间：
2003
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通讯作者：
P. Schultz
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文献类型：
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作者：
P. Schultz
简介：最近的研究表明，撞击高度多孔的目标材料需要一个新的由压缩效应控制的陨石坑制度（1）。这种研究对于解释小天体的地质历史和即将于2005年进行的深度撞击使命都很重要。另一种观点（2）表明，即使是高度多孔的目标（密度为0.2g/cc）也会经历重力控制的生长，并且先前的研究低估了环境大气的影响，如先前所记录的（3，4）。本研究重新审视这些相互矛盾的观点方法：超高速撞击实验在美国宇航局艾姆斯垂直炮范围内进行。目标材料包括低密度的粘土（和粘土混合物）、微球、浮石和沙子。撞击器材料包括铝和铜，但主要是派热克斯玻璃，以确保完全失效，就像更高速度撞击的情况一样。高速成像（每秒500帧和6000帧）使用半空间和四分之一空间目标配置捕获挖掘过程。四分之一空间实验先前记录了大气对陨石坑挖掘的影响，并证明最终测量的陨石坑与塌陷前的挖掘陨石坑有显著差异（3，4）。结果如下：使用筛分的靶在1-g下垂直撞击半空间（正常）靶显示出与以前的报告一致的成坑效率显著降低（1）。然而，与这些研究相反的是，大量的物质从陨石坑中喷出。高速成像显示两个喷出物制度。第一个是一个长寿命的柱状羽状物，位于火山口上方，含有高角度喷出物（80-90°）。第二种是典型的向外推进的锥形喷出物幕，其喷出角（50-60°）比撞击多孔性较小的材料（浮石或沙子）产生的喷出物更大。当向外移动的帷幕离开短暂的陨石坑内部时，高角度的喷出物柱返回陨石坑，陨石坑坍塌，边缘物质返回内部。四分之一空间实验表明，三种材料的位移制度发生在超高速。穿透管在早期表征第一状态，因为靶材料在破碎射弹前面被压缩。这种管的圆柱形膨胀类似于超高速进入大气层时产生的马赫管。位移的第二阶段由高角度喷出物代表。这种高角度的形成是由于在类似深埋爆炸装药的深度处扩大空腔的结果。第三阶段的特征是柱状羽流，并持续整个陨石坑的形成过程。
Introduction: Recent studies suggest that impacts into highly porous target materials require a new cratering regime controlled by compressional effects (1). Such research is important for interpreting the geologic history of small bodies and for the upcoming Deep Impact mission in 2005. An alternative view (2) suggests that even highly porous targets (density of 0.2g/cc) undergo gravity controlled growth and that previous studies underestimated the effects of an ambient atmospheres as previously documented (3,4). The present study reexamines these conflicting views Approach: Hypervelocity impact experiments were performed at the NASA Ames Vertical Gun range. Target materials included low-density perlite (and perlite mixes), microspheres, pumice, and sand. Impactor materials included aluminum and copper but primarily pyrex in order to ensure complete failure , as would be the case for much higher velocity impacts. High-speed imaging (500 and 6000 frames per second) captured the excavation process using both half-space and quarter-space target configurations. Quarter-space experiments previously documented atmospheric effects on crater excavation and demonstrated that the final crater measured was significantly different from the pre-collapse excavation crater (3,4). Results: Vertical impacts into half-space (normal) targets at 1-g using sieved perlite targets revealed significant reduction in the cratering efficiency consistent with previous reports (1). In contrast with these studies, however, significant material was ejected from the crater. High-speed imaging revealed two ejecta regime. The first is a long-lived, pillar-like plume above the crater containing high-angle ejecta (80-90°). The second is the classic outward advancing conical ejecta curtain with higher ejection angles (50-60°) than ejecta produced by impacts into less porous materials (pumice or sand). As the outward-moving curtain leaves the transient crater interior, the high-angle ejecta pillar returns to the crater and the crater collapses returning rim material to the interior. Quarter-space experiments reveal that three material displacement regimes occur at hypervelocity. A penetration tube characterizes the first regime at early-times as target material is compressed in front of the fragmenting projectile. This tube expands cylindrically similar to the mach tube creating during hypervelocity atmospheric entry. The second stage of displacement is represented by the high-angle ejecta. Such high angles develop as the result of an expanding cavity at depth analogous deeply buried explosive charge. A pillar-like plume characterizes the third stage and lasts throughout crater formation.