Constraints on lunar regolith resurfacing from coupled modeling of stochastic gardening and neutron capture effects

Abstract

The regolith evolution of airless bodies, like the Moon, is primarily controlled by impact cratering. Since the Apollo Era, measurements of cosmic ray exposure (CRE)-induced Sm and Gd isotopes in lunar drill cores have provided insights into the secondary neutron spectra in the lunar regolith. Since the production and transport of secondary neutrons vary with the regolith's chemical composition and depth, the neutron fluence profile can be employed to track the evolution of lunar and asteroidal regolith. We developed a stochastic model that incorporates state-of-the-art cosmogenic production rate calculations for Sm and Gd isotopes in an effort to understand regolith evolution in the presence of meteoroid bombardments. By comparing the simulated depth profiles to those observed in the lunar drill cores from the Apollo 15, 16, and 17 missions, we find that the deviations from a static profile are due to continuous surface meteoroid bombardments. These bombardments result in the formation of nuclear-reworked zones near the lunar surface. Based on the surface neutron fluence of lunar rocks and regolith, our modeling shows that the regolith surface is reset by large impact-induced excavation and deposition of blanket ejecta every few hundred million years.