  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 (ca.
1975-2014) proposed very low or negligible impact cratering in the period from
accretion (>4.4 Ga) to about 4.0 Ga ago, followed by a 170-million-year-long
spike of cataclysmic cratering, during which most prominent multi-ring impact
basins formed at age of about 3.9 Ga. More recent dynamical models suggest very
early intense impact rates, declining throughout the period from accretion
until an age of about 3.0 Ga. These models remove the basin-forming spike. This
shift has important consequences on 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 vs. 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.
