Impact Fragmentation and the Development of the Deep Lunar Megaregolith

The megaregolith of the Moon is the upper region of the crust, which has been extensively fractured by intense impact bombardment. Little is known about the formation and evolution of the lunar megaregolith. Here we implement the Grady-Kipp model for dynamic fragmentation into the iSALE shock physics code. This implementation allows us to directly simulate tensile in situ impact fragmentation of the lunar crust. We find that fragment sizes are weakly dependent on impactor size and impact velocity. For impactors 1km in diameter or smaller, a hemispherical zone centered on the point of impact contains meter-scale fragments. For an impactor 1km in diameter this zone extends to depths of 20km. At larger impactor sizes, overburden pressure inhibits fragmentation and only a near-surface zone is fragmented. For a 10-km-diameter impactor, this surface zone extends to a depth of 20km and lateral distances 300km from the point of impact. This suggests that impactors from 1 to 10km in diameter can efficiently fragment the entire lunar crust to depths of 20km, implying that much of the modern day megaregolith can be created by single impacts rather than by multiple large impact events. Plain Language Summary The Moon's surface has experienced eons of bombardment by asteroid impacts. This bombardment is thought to have thoroughly shattered the lunar crust, the outermost layer of the Moon, to depths of 20km or more. Despite this, the processes of fracturing and fragmentation during an asteroid impact have received little attention though. In this study, we simulated asteroid impacts and tracked how impacts shatter the lunar crust. We found that impacts break the lunar crust into roughly meter-sized blocks. Impacts can break up the lunar crust down to approximately 20km, and objects greater than 1km in diameter are the most efficient at fragmenting the crust. 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. Similarly, impacts likely shattered the crust of the ancient Earth and Mars. Understanding how thoroughly fractured ancient planetary crusts are may help us determine if and when these environments became habitable.