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

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} Ω>> 1). In this regime angular momentum transport in the disk leads to steady heating unless α< (t_{cool} Ω)^{-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.

arXiv comment22 pages, 2 figures, accepted by ApJ

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• Vapor drainage in the protolunar disk as the cause for the depletion in volatile elements of the Moon
• Inefficient volatile loss from the Moon-forming disk: reconciling the giant impact hypothesis and a wet Moon
• Thermodynamics of Element Volatility and its Application to Planetary Processes
• Large planets may not form fractionally large moons
• Chemical Equilibrium Calculations for Bulk Silicate Earth Material at High Temperatures
• A Magnetized, Moon-Forming Giant Impact
• Composition, Structure and Origin of the Moon