Effect of Re-impacting Debris on the Solidification of the Lunar Magma Ocean

TL;DR

This study models how re-impacting debris from the Moon-forming impact could have affected the cooling and solidification of the lunar magma ocean, revealing that impacts could significantly accelerate or prolong lunar crust formation.

Contribution

It introduces a thermal evolution model incorporating impact-induced hole formation and energy conversion, providing new insights into lunar magma ocean solidification dynamics.

Findings

Impact-induced holes can reduce cooling time by over 5 times.

Shock energy from impacts can extend solidification by 50% or more.

Re-impacting debris influences lunar crust formation timescales.

Abstract

Anorthosites that comprise the bulk of the lunar crust are believed to have formed during solidification of a Lunar Magma Ocean (LMO) in which these rocks would have floated to the surface. This early flotation crust would have formed a thermal blanket over the remaining LMO, prolonging solidification. Geochronology of lunar anorthosites indicates a long timescale of LMO cooling, or re-melting and re-crystallization in one or more late events. To better interpret this geochronology, we model LMO solidification in a scenario where the Moon is being continuously bombarded by returning projectiles released from the Moon-forming giant impact. More than one lunar mass of material escaped the Earth-Moon system onto heliocentric orbits following the giant impact, much of it to come back on returning orbits for a period of 100 Myr. If large enough, these projectiles would have punctured holes in the nascent floatation crust of the Moon, exposing the LMO to space and causing more rapid cooling. We model these scenarios using a thermal evolution model of the Moon that allows for production (by cratering) and evolution (solidification and infill) of holes in the flotation crust that insulates the LMO. For effective hole production, solidification of the magma ocean can be significantly expedited, decreasing the cooling time by more than a factor of 5. If hole production is inefficient, but shock conversion of projectile kinetic energy to thermal energy is efficient, then LMO solidification can be somewhat prolonged, lengthening the cooling time by 50% or more.