Effect of Reimpacting Debris on the Solidification of the Lunar Magma Ocean

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 remelting and recrystallization 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.

A long-lived magma ocean on a young Moon

“…Similar to previous studies on LMO solidification ( 2 , 3 ), we consider the crystallization of an initially 1000-km-deep LMO. An initially whole-mantle LMO has also been proposed ( 19 ).…”

“…However, once buoyant plagioclase started to crystallize, it floated upward and formed an insulating lid ( 4 ), which markedly slowed down the cooling of the lunar magma ocean (LMO). As a result, the last ~20% of the LMO is thought to have crystallized on a longer time scale of 10 million years (Ma) ( 2 ) to 30 Ma ( 3 ). It has been proposed that tidal dissipation in the crust ( 5 , 6 ) or the magma ocean itself ( 7 ) may prolong the duration of the LMO solidification.…”

“…Thus, tidal dissipation does not seem to have had a strong influence on the LMO’s lifetime. Last, impacts of leftover debris from the Moon-forming event onto the lunar crust may have influenced the duration of LMO crystallization in two ways ( 3 ): Holes punctured in the crust may have accelerated cooling, and deposition of the impactors’ kinetic energy may have slowed it down. The net result of these two opposing effects is not well constrained, but the most recent results indicate that holes in the crust are short-lived and, therefore, did not significantly enhance the cooling of the LMO ( 8 ).…”

The New Moon: Major Advances in Lunar Science Enabled by Compositional Remote Sensing from Recent Missions

“…Vast spread in the radiometric ages of the anorthositic rocks (ferroan anorthosites) has had been another key challenge for the magma ocean hypothesis and taken at face value, requires a long time for magma ocean solidification, necessitating mechanisms for sustaining the magma ocean or raising the possibility of primary crust formation through other mechanisms (e.g., [124,125]). Beyond radiogenic heating from short-lived radioactive isotopes, tidal heating from the Earth, insulation by floatation crust and energy from impacts have been suggested although the latter could also blast away portions of the insulating layer, expediting magma ocean solidification (e.g., [126,127]). The large spread in the radiometric ages of ferroan anorthosites could also possibly be explained by the origin of at least some of them through recycling and secondary crystallization.…”

Collision Chains among the Terrestrial Planets. III. Formation of the Moon

“…A molten Moon-sized silicate body radiating at the liquidus will solidify in <1 Myr, except that it forms an insulating crust that regulates deeper cooling (Solomon 1977), leading to an estimated ∼10-100 Myr solidification timescale, absent other factors. If solidification is relatively rapid (e.g., Elkins-Tanton et al 2011;Perera et al 2018) then geochemical closure might be close to the time of the giant impact. Otherwise, LMO solidification might have been delayed by radioactive elements (incompatibles) concentrated in the residual melt or KREEP and by tidal friction during the Moon's orbital evolution, starting from a few radii away.…”