Impact Fragmentation and the Development of the Deep Lunar Megaregolith

Abstract

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 1 km in diameter or smaller, a hemispherical zone centered on the point of impact contains meter-scale fragments. For an impactor 1 km in diameter this zone extends to depths of 20 km. 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 ~20 km and lateral distances ~300 km from the point of impact. 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.