Sizes of Asteroids Responsible for Large Impact Basins on the Moon during the Late Heavy Bombardment

Introduction: Scaling relations derived from laboratory experiments and dimensional analysis [1] provide first-order estimates for the diameters of objects responsible for craters on the planets. At basin scales, however, the final crater rim may not be preserved due to rim collapse. Assumptions about impactor speed, angle, and density further preclude a unique determinations. As a result, the sizes of asteroids (or comets) colliding with the Moon during the Late Heavy Bombardment (LHB) are very poorly constrained. Independent estimates for impactor sizes, however, can be determined from the pattern of ejecta generated by oblique impacts [2]. This contribution reviews this strategy and considers the implications for asteroid sizes during the Late Heavy Bombardment. Strategy: Laboratory impact experiments reveal the evolving crater shapes as a function of impact angle [3]. At planetary scales (craters and basins 30km to 300km in diameter), however, such asymmetries may be lost due to the much higher impact speeds or crater collapse, which circular-izes crater shape. Nevertheless, the distribution of ejecta around large craters clearly indicates an evolving cratering flow-field as expressed by the uprange zone of avoidance [3] or curved ray patterns [4,5]. The initial conditions leading to this asymmetry has been identified in the ejecta velocity distribution generated by impacts into particulate targets at late [6] and early times [7]. At much large scales, the Deep Impact collision captured the identical evolution of the ballistic ejecta [5]. Experiments using particulate targets, however , result in a final crater diameter 20 to 50 times the projectile size. Consequently, early asymmetries expressed by the growing crater shape (in plan) are masked as the source region becomes very small relative to the final crater. Impacts into strength-controlled targets, however, retain the footprint of the early coupling stage. As a result, the final crater diameter may be only 5-7 (vertical) down to 3-5 (oblique) times the projectile diameter. Moreover, the early-time scour pattern in the target closely resembles in-flight ejecta trajectories from impacts into particulates [6,7]. For strength-controlled targets, however, early time scours remain on the surface (Fig. 1). Some of these patterns emanate from the first point of contact and relate to processes associated with the failed projectile re-impacting the surface down-range. Conversely, scours that converge uprange relate directly to the dimensions of the projectile. The identical pattern can be documented in hydrocode models at planetary scales [8, 9]. Such observations, then, provide a possible strategy for constraining …