A hot big bang theory: magnetic fields and the early evolution of the protolunar disk

TL;DR

This paper investigates the early evolution of the protolunar disk, emphasizing magnetic coupling, turbulent heating, and outflows, which influence the disk's thermal state, angular momentum, and material mixing during Moon formation.

Contribution

It introduces a model where magnetic turbulence causes a hot, thick, and magnetized disk phase, affecting the disk's cooling, mixing, and mass loss processes in lunar formation.

Findings

Disk opacity leads to inefficient cooling (t_cool * Omega >> 1).

Magnetic coupling induces a hot, turbulent disk phase.

Bipolar outflows can remove mass and angular momentum.

Abstract

The leading theory for the formation of the Earth's moon invokes a collision between a Mars-sized body and the proto-Earth to produce a disk of orbiting material that later condenses to form the Moon. Here we study the early evolution of the protolunar disk. First, we show that the disk opacity is large and cooling is therefore inefficient (t_{cool} \Omega >> 1). In this regime angular momentum transport in the disk leads to steady heating unless \alpha < (t_{cool} \Omega)^{-1} << 1. Following earlier work by Charnoz and Michaut, and Carballido et al., we show that once the disk is completely vaporized it is well coupled to the magnetic field. We consider a scenario in which turbulence driven by magnetic fields leads to a brief, hot phase where the disk is geometrically thick, wit h strong turbulent mixing. The disk cools by spreading until it decouples from the field. We point out that approximately half the accretion energy is dissipated in the boundary layer where the disk meets the Earth's surface. This creates high entropy material close to the Earth, driving convection and mixing. Finally, a hot, magnetized disk could drive bipolar outflows that remove mass and angular momentum from the Earth-Moon system.