The Earth-Lunar Disk Connection: Favorable Aspects of a High-Angular Momentum Giant Impact

Abstract:

In the giant impact hypothesis, the Moon accretes from a disk around the proto-Earth. In the canonical model, the impact also sets the present-day angular momentum (AM). Alternatively, the Moon may form via a high AM giant impact and the present-day AM was established by a lunar orbital resonance.

Here we examine the dynamics of disks formed by high AM impacts. Initially, the Earth’s extended atmosphere and the inner region of the rotationally-supported disk do not intersect the liquid-vapor phase boundary and form a continuous vapor. The atmosphere has a higher angular velocity than the partially pressure-supported disk; as a result, shear instabilities lead to mixing between the atmosphere and disk and net transfer of AM from the planet to the disk. The pressure-supported atmosphere’s continuity with the inner disk limits the accretion rate onto the planet, even when the planet’s rotation rate is below the spin stability limit.

While the inner disk remains vapor, the disk’s surface cools radiatively. In the outer disk, droplets settle to the midplane and form moonlets via gravitational instabilities. We find that the potential energy reduction of the system by lunar accretion is primarily converted to heating the inner disk and atmosphere. Moonlets in the outer disk are not heated sufficiently to vaporize. Because net accretion onto the planet is limited, droplet formation in the outer disk is resupplied by pressure gradients between the inner and outer disk.

In the canonical case, the atmosphere and disk intersect the liquid-vapor boundary and do not form a continuous fluid. Also, the planet has a lower angular velocity than the disk. Then, condensates from the disk may fall freely to Earth. Thus, compared to the canonical model, high AM disks have enhanced lunar accretion efficiency. Furthermore, if mixing between the atmosphere and inner disk is efficient, then a wide range of high AM giant impacts may produce the isotopic similarity between the Earth and Moon.