Hörz Friedrich, Basilevsky Alexander T, Head James W, Cintala Mark J
Jacobs-JETS, 2224 Bay Area Boulevard, Houston, TX, 77058, USA.
Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Moscow, 1199991, Russia.
Planet Space Sci. 2020 Dec;194:105105. doi: 10.1016/j.pss.2020.105105. Epub 2020 Sep 25.
This report summarizes observations of returned Apollo rocks and soils, lunar surface images, orbital observations, and experimental impacts related to the erosion and comminution of rocks exposed at the lunar surface. The objective is to develop rigorous criteria for the recognition of impact processes that assist in distinguishing "impact" from other potential erosional processes, particularly thermal fatigue, which has recently been advocated specifically for asteroids. Impact in rock is a process that is centrally to bilaterally symmetric, resulting in highly crushed, high-albedo, quasicircular depressions surrounded by volumetrically prominent spall zones. Containing central glass-lined pits in many cases, such features provide distinctive evidence of impact that is not duplicated by any other process. Additional evidence of impact can include radial fracture systems in the target that emanate from the impact point and clusters of fragments that attest to the lateral acceleration and displacement of each one. It is also important to note that impact produces a wide variety of fragment shapes that might totally overlap with those produced by thermal fatigue; we consider fragment shape to be an unreliable criterion for either process. The stochastic nature of the impact process will result in exponential survival times of surface rocks; that is, rock destruction initially is relatively efficient, but it is followed by ever increasing surface times for the last rock remnants. Thermal fatigue, however, is essentially a thermal-equilibrium process. The corresponding distribution of survival times should be much more peaked in comparison, presumably Gaussian, and diagnostically different from that due to impact. Given the abundance of evidence that has been gleaned from Apollo rocks and soils, it is surprising how little has been learned about the impact process from the of rocks and boulders taken by the astronauts on the lunar surface. This suggests that it will require rocks and soils returned from asteroids to evaluate the relative roles of thermal versus impact-triggered rock erosion, particularly when both processes are likely to be operating.
本报告总结了有关返回的阿波罗岩石和土壤、月球表面图像、轨道观测以及与月球表面暴露岩石的侵蚀和粉碎相关的实验撞击的观测结果。目的是制定严格的标准，以识别有助于区分“撞击”与其他潜在侵蚀过程（特别是热疲劳，最近专门针对小行星提出）的撞击过程。岩石中的撞击是一个从中心到两侧对称的过程，会产生高度破碎、高反照率、准圆形的凹陷，并被体积显著的剥落带包围。在许多情况下，这些凹陷包含中央玻璃衬里的坑，这些特征提供了独特的撞击证据，而任何其他过程都不会产生这种证据。撞击的其他证据可以包括从撞击点发出的目标中的径向断裂系统以及证明每个碎片横向加速和位移的碎片群。还需要注意的是，撞击会产生各种各样的碎片形状，这些形状可能与热疲劳产生的碎片形状完全重叠；我们认为碎片形状对于这两种过程来说都是不可靠的标准。撞击过程的随机性将导致表面岩石的存活时间呈指数分布；也就是说，岩石破坏最初相对有效，但随后最后残留岩石的表面时间会不断增加。然而，热疲劳本质上是一个热平衡过程。相比之下，相应的存活时间分布应该更加尖峰化，大概是高斯分布，并且在诊断上与撞击产生的分布不同。鉴于从阿波罗岩石和土壤中收集到了大量证据，令人惊讶的是，从宇航员在月球表面采集的岩石和巨石中，我们对撞击过程了解得如此之少。这表明需要从小行星返回的岩石和土壤来评估热与撞击引发的岩石侵蚀的相对作用，特别是当这两种过程可能同时发生时。
