Modeling the Formation of the Lunar Upper Megaregolith Layer

Abstract

Abstract In this work, we divide the classic “lunar megaregolith” layer into three distinct regions: (1) a Surficial Regolith layer, about 5–20 m in depth, consisting of loose, unconsolidated fines and breccia, and characterized by frequent overturn and comminution caused by small impacts; (2) an Upper Megaregolith layer, about 1–3 km in depth, consisting of depositional layers of brecciated and/or melted material, and characterized by the transport and deposition of material via either transient crater gravitational collapse or impact ejecta ballistic sedimentation; and (3) a Lower Megaregolith layer, about 20–25 km in depth, consisting of bedrock that has been fractured in place, and characterized by a fracture-density and fragment-size distribution that decreases rapidly with increasing depth. The objective of this study is to model the formation of the lunar Upper Megaregolith layer, the least well characterized of these three layers, using modern scaling relationships and a three-dimensional terrain, Monte Carlo cratering model. We first developed a model impactor population that accurately reproduces the Lunar Highlands crater population, which is assumed to originate in the Main Asteroid Belt. We then applied this impactor population in multiple full-scale lunar surface simulations, producing an Upper Megaregolith depth of 1.4 ± 1.0 km at the point of best χ 2 fit between model and actual crater counts. This Upper Megaregolith layer consists of ∼60% crater collapse deposits and ∼40% impact ejecta deposits. We find that a total delivered impactor mass of 3.72 ± 1.14 × 1019 kg, or 0.0506 ± 0.0156 lunar weight percent (wt%), is required to reproduce the observed Lunar Highlands cratering record.