Clustered impacts: Experiments and implications
Clustered impacts: Experiments and implications
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DOI：
10.1029/jb090ib05p03701
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发表时间：
1985-04
影响因子：
--
通讯作者：
P. Schultz;D. Gault
中科院分区：
文献类型：
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作者：
P. Schultz;D. Gault
由射弹集群而不是单个射弹撞击可能是由几个过程造成的：大气层破裂、潮汐破裂和大型初次撞击的喷出物。已经进行了实验，以确定这种事件在各种撞击速度（15米/秒至6公里/秒）下的特征。在非常低的撞击速度（15-200米/秒）下，通过发射铝丸、钢丸、铁屑和沙子的成组射弹产生集群撞击。在中高速（0.8-6公里/秒）下，耐热玻璃球在通过铝箔或纸时在目标上方破碎，从而形成一个总半径为rc的明确的碎片簇。团簇的总半径rc与相同质量的固体撞击器的半径rs的比率提供了团簇分散的量度。为了定性地比较目标强度的微小差异的附加效应，使用了沙子和压实浮石目标。实验表明，集束射弹的撞击与单一固体的撞击之间存在明显的差异。“紧密”簇（rc/rs 20）的质量减少10倍。这种成坑效率的降低主要表现为对于疏散星团具有高达30的纵横比（直径/深度）的浅坑。团簇的大小和速度（以及撞击物和目标之间的密度和强度对比）可显著影响弹坑形态。开放的集群影响压实浮石产生一个单位，丘状的地板与初期的多环模式，而一个紧密的集群影响相同的目标产生一个中央地板土墩。对称星系团的斜向撞击形成了一系列指向上方的V形脊。脊的顶角取决于簇的分散和冲击角。对撞击后射弹材料的清点表明，射弹大部分留在表面，并从撞击方向向下散布。保留在表面上的抛射体材料的量随着撞击角度（从水平方向）的减小和目标相对于抛射体的强度的增加而增加。撞击坑和月球次级撞击坑在广泛的形态特征上具有惊人的相似性。在此基础上，在合理的喷流幕结构模型的基础上，我们认为，这些实验为了解月球大型撞击坑周围的喷流侵位提供了新的线索。当大型撞击周围的喷出物幕被视为厚的碎片墙，而成群的撞击物被视为这种幕的单元部分时，实验结果表明，即使对于大于100公里的月球陨石坑，连续的喷出物相也可能包含多达90%的原始物质。非相互作用的撞击事件除了连续的喷出物相，喷出物的影响集群提供了一个物理基础，了解各种各样的次生形态和大范围的光谱特征的主要材料在陨石坑射线。
Impact by clusters of projectiles rather than a single projectile can result from several processes: atmospheric breakup, tidal breakup, and ejecta from a large primary impact. Experiments have been performed in order to establish the characteristics of such events over a wide range of impact velocities (15 m/s to 6 km/s). At very low impact velocities (15–200 m/s), clustered impacts were produced by launching a grouped projectiles of aluminum shot, steel shot, iron filings, and sand. At moderate to high velocities (0.8–6 km/s), pyrex spheres were shattered above the target during passage through aluminum foil or paper, thereby forming a well-defined cluster of fragments of overall radius rc. The ratio of the overall radius rc of the cluster to the radius rs of a solid impactor of the same mass provides a measure of the cluster dispersion. Sand and compacted pumice targets were used in order to compare qualitatively the additional effect of slight differences in target strength. The experiments reveal marked contrasts between impacts by clusters of projectiles and impacts by a single solid body. “Tight” clusters (rc/rs 20) displace a factor of 10 less mass. This reduction in cratering efficiency is largely expressed as a shallow crater with an aspect ratio (diameter/depth) as large as 30 for open clusters. The size and velocity of the cluster (as well as the density and strength contrast between the impactor and target) can dramatically affect crater morphology. Open clusters impacting compacted pumice produce a flat, hummocky floor with an incipient multiring pattern, whereas a tight cluster impacting the same target produces a central floor mound. Oblique impacts by symmetrical clusters form a characteristic array of V-shaped ridges pointing uprange. The apex angle of the ridges depends on the cluster dispersion and impact angle. Inventories of postimpact projectile material reveal that the projectile is largely retained on the surface and spread downrange from the impact direction. The amount of projectile material retained on the surfaces increases with decreasing impact angle (from the horizontal) and with increasing strength of the target relative to the projectile. Clustered impact craters and lunar secondary craters bear striking similarities over a broad range of morphologic features. On this basis and on the basis of reasonable models of ejecta curtain structure, we suggest that these experiments provide new clues for understanding ejecta emplacement around large lunar impact craters. When the ejecta curtain around large impacts is viewed as a thick wall of debris and clustered impactors are viewed as a unit section of such a curtain, then experimental results indicate that the continuous ejecta facies even for lunar craters larger than 100 km could contain as much as 90% primary material, Such a conclusion contrasts with many existing models that derive mixing ratios implicitly based on single, noninteracting impact events. Beyond the continuous ejecta facies, impacting clusters of ejecta provide a physical basis for understanding the wide variety of secondary morphologies and the large range in spectral signatures of primary material in crater rays.