Below this is the thickest layer, composed of material fractured or fragmented by impacts but not ejected (Heiken et al., 1991). Underneath the regolith layer in the lunar highlands lies the “large-scale ejecta layer,” which is composed of material ejected during the formation of large basins and is approximately a few kilometers thick (Heiken et al., 1991). The uppermost stratum is the regolith layer, which is meters thick and is composed of fine-grained heavily reworked material (Bart et al., 2011 Papike et al., 1982). The thoroughly fragmented upper 10 km or more of the lunar crust is known as the megaregolith (Hartmann, 1973 Heiken et al., 1991), which can be subdivided into three layers. As the Moon's ancient highlands are the best preserved and best studied example of an ancient planetary crust, our work focuses on in situ impact fragmentation of the lunar crust, making it the first study of its kind. The role of in situ fragmentation during crater formation, however, has received little attention. The volume of this fragmented material is much larger than the volume excavated by the impact (Melosh et al., 1992). The rarefaction can produce strong tensile stresses and high strain rates, triggering dynamic fragmentation (Melosh et al., 1992). This succession of shock and rarefaction sets up the flow that excavates the crater (Melosh, 1984). During a hypervelocity impact a shockwave propagates into the projectile and target and is followed by a rarefaction wave that propagates from free surfaces. Impact cratering is a ubiquitous process that dominates the evolution of ancient planetary crusts (Melosh, 1989). Understanding how thoroughly fractured ancient planetary crusts are may help us determine if and when these environments became habitable. Similarly, impacts likely shattered the crust of the ancient Earth and Mars. This implies that the fragmented bedrock can be almost exclusively created by large-scale high-speed impacts and the lunar crust was thoroughly fractured early in its history. Impacts can break up the lunar crust down to approximately 20 km, and objects greater than 1 km in diameter are the most efficient at fragmenting the crust. We found that impacts break the lunar crust into roughly meter-sized blocks. In this study, we simulated asteroid impacts and tracked how impacts shatter the lunar crust. Despite this, the processes of fracturing and fragmentation during an asteroid impact have received little attention though. This bombardment is thought to have thoroughly shattered the lunar crust, the outermost layer of the Moon, to depths of 20 km or more. The Moon's surface has experienced eons of bombardment by asteroid impacts. This suggests that impactors from 1 to 10 km in diameter can efficiently fragment the entire lunar crust to depths of ~20 km, implying that much of the modern day megaregolith can be created by single impacts rather than by multiple large impact events. For a 10-km-diameter impactor, this surface zone extends to a depth of ~20 km and lateral distances ~300 km from the point of impact. At larger impactor sizes, overburden pressure inhibits fragmentation and only a near-surface zone is fragmented. For an impactor 1 km in diameter this zone extends to depths of 20 km. For impactors 1 km in diameter or smaller, a hemispherical zone centered on the point of impact contains meter-scale fragments. We find that fragment sizes are weakly dependent on impactor size and impact velocity. This implementation allows us to directly simulate tensile in situ impact fragmentation of the lunar crust. Here we implement the Grady-Kipp model for dynamic fragmentation into the iSALE shock physics code. Little is known about the formation and evolution of the lunar megaregolith. The megaregolith of the Moon is the upper region of the crust, which has been extensively fractured by intense impact bombardment.
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