Magneto-rotational instability in the protolunar disk

Abstract

(Abridged) We perform the first study of magnetohydrodynamic processes in the protolunar disk (PLD). With the use of published data on the chemical composition of the PLD, along with existing analytical models of the disk structure, we show that the high temperatures that were prevalent in the disk would have led to ionization of Na, K, SiO, Zn and, to a lesser extent, O$_2$. We assume that the disk has a vapor structure. The resulting ionization fractions, together with a relatively weak magnetic field, would have been sufficient to trigger the magneto-rotational instability, or MRI. We calculate the intensity of the resulting magnetohydrodynamic turbulence, as parameterized by the dimensionless ratio $\alpha$ of turbulent stresses to gas pressure, and obtain maximum values $\alpha\sim 10^{-2}$ along most of the vertical extent of the disk, and at different orbital radii. Under these conditions, turbulent mixing within the PLD due to the MRI was likely capable of transporting isotopic and chemical species efficiently. To test these results in a conservative manner, we carry out a numerical magnetohydrodynamic simulation of a small, rectangular patch of the PLD, located at 4 Earth radii ($r_{\rm E}$) from the center of the Earth, and assuming once again that the disk is completely gaseous. We obtain turbulence with an average intensity $\alpha\sim 7\times 10^{-6}$ over the course of 280 orbital periods (133 days at 4$r_{\rm{E}}$). Despite this relatively low value of $\alpha$, the effective turbulent diffusivity of a passive tracer introduced in the flow is large enough to allow the tracer to spread across a radial distance of $10r_{\rm{E}}$ in $\sim13-129$ yr, less than the estimated cooling time of the PLD of $\sim250$ yr. Further improvements to our model will need to incorporate the energy balance in the disk, a complete two-phase structure, and a more realistic equation of state.