Erosion of lunar surface rocks by impact processes: A synthesis. (December 2020)

• Record Type: Journal Article
• Authors: Hörz, Friedrich; Basilevsky, Alexander T.; Head, James W.; Cintala, Mark J.
• Is Part Of: Planetary and space science. Volume 194(2020)
• Publication Date: 2020-12
• Subjects: Planetary regoliths -- Crater-morphology -- Thermal fatigue -- Particle abrasion -- Collisional fragmentation -- Rock erosion; Space sciences -- Periodicals; Atmosphere, Upper -- Periodicals; Sciences spatiales -- Périodiques; Haute atmosphère -- Périodiques; 523
• DOI: 10.1016/j.pss.2020.105105
• Languages: English
• ISSNs: 0032-0633

Abstract

Abstract: 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 returned Apollo rocks and soils, it is surprising how little has been learned about the impact process from the photography 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. Highlights: Impact produces a quasi hemispherical shock wave that results in centrosymmetric craters. These craters are characterized by flat spall zones, a central zone of crushed material and a glass lined pit. Small craters (<1 μm) have glass lined pits only, large craters (>10 ​cm) consist only of spall zones. Small scale impactors accomplish rock erosion by abrasion, while energetic impacts cause collisional fragmentation of individual rocks. Collisional fragmentation erodes an order of magnitude more material than abrasion. Tell tale signs of impact include also radial and concentric fracture systems.