Effects of early intense bombardment on megaregolith evolution and on lunar (and planetary) surface samples

Abstract

Impact rates in the first 500 Myr of the solar system are critical to an understanding of lunar geological history, but they have been controversial. The widely accepted, post‐Apollo paradigm of early lunar impact cratering (about 1975–2014) proposed very low or negligible impact cratering in the period from accretion (>4.4 Ga) to ~4.0 Ga ago, followed by an ~170 million year long spike of cataclysmic cratering, during which most prominent multi‐ring impact basins formed at age ~3.9 Ga. More recent dynamical models suggest very early intense impact rates, declining throughout the period from accretion until an age of ~3.0 Ga. These models remove the basin‐forming spike. This shift has important consequences vis‐à‐vis megaregolith evolution and properties of rock samples that can be collected on the lunar surface today. We adopt the Morbidelli et al. (2018) “accretion tail” model of early intense bombardment, declining as a function of time. We find effects differing from the previous models: early crater saturation and supersaturation; disturbance of magma ocean solidification; deep early megaregolith; and erosive destruction of the earliest multi‐ring basins, their impact melts, and their ejecta blankets. Our results explain observations such as differences in numbers of early lunar impact melts versus numbers of early igneous crustal rocks, highland breccias containing impact melts as old as 4.35 Ga, absence of a 170 Myr long spike in impact melt ages at 3.9 Ga among lunar and asteroidal meteorites, and GRAIL observations of lunar crustal structure.

3D-simulation of lunar megaregolith evolution: Quantitative constraints on spatial variation and size of fragment

“…Although this process has been less constrained (l B , l B0 ), the dominant fragment size is found to be invariably in kilometer scale. The uncertainty of the basin inventory is another key factor that limits our knowledge of the megaregolith (Hartmann, 2019;Hartmann and Morbidelli, 2020). With the completing of the basin list, especially the old structure-erased ones, the estimated upper megaregolith would increase by several kilometers, and the deep crust of more regions would be damaged by the autochthonous fragmentation.…”

“…Numerous large impacts occurred during the early bombardment period, shattering and fracturing the lunar crust, producing a globally fragmented layer known as megaregolith (Hartmann, 1973(Hartmann, , 2003(Hartmann, , 2019Hartmann and Morbidelli, 2020;Hörz et al, 1991). Understanding how the megaregolith evolved is important for understanding the lunar thermal evolution history since the crust became more fractured with the cumulative impact cratering influencing the thermal conductivity (Warren and Rasmussen, 1987).…”

“…The lunar surface is covered with fine-grained regolith, and surface rocks even of meter-scale are typically eroded within a few hundred Ma. To collect lunar rock samples, we are dependent on excavation processes in the megaregolith (Hartmann and Morbidelli, 2020). The distribution of fragment size is also essential for the interpretation of remote sensing data.…”