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.