¹Brynna G. Downey, ¹Robin M. Canup
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2026.117208]
¹Solar System Science and Exploration Division, Southwest Research Institute, 1301 Walnut Street, Boulder, 80302, CO, USA
Copyright Elsevier

In the giant impact theory for the origin of the Moon, a protoplanet collided with the Earth, producing a disk of melt-vapor debris from which the Moon accreted. Simulations of canonical impacts in which the impactor is  Mars-sized produce disks with mass 1 to  (lunar masses). However, most prior models of lunar accretion require that the initial disk have mass  because of the assumed disk state, modelled processes, and initial conditions. This inconsistency has been a challenge for the canonical impact model. To bridge this gap, we (i) update a model of the disk interior to the Roche limit to treat melt and vapor as separate, vertically stratified layers, and (ii) adopt as initial conditions for the accretion model more realistic radial mass distributions for the disk based on impact simulations. We find that treating the inner melt and vapor as vertically stratified layers lowers the final Moon mass by 20% on average compared to prior work that assumed they remained well-mixed, a relatively small effect. In contrast, the initial radial mass distribution has a substantial effect. We show that for outer disk mass , which is often 60% of the total disk mass, an  Moon accretes in as little as  days and at most  months. For almost all successful cases, only 5% of the final Moon is from the inner melt and vapor layers that might have isotopically equilibrated with the Earth’s vapor atmosphere. The Moon’s rapid accretion from material originally emplaced in an outer canonical disk requires that the isotopic similarities between the Earth and Moon be inherited from similarities between Earth and the impactor Theia, rather than through disk-planet equilibration.