Magnetization of sinking porous diapirs in planetesimal cores

Abstract

Magnetization by core dynamo activity in large asteroids and planetesimals is suggested by the remnant magnetization of many achondritic meteorites. However, the conditions for a sustained dynamo in such small bodies is problematic for several reasons. Such bodies may not have the energy budget or size to achieve a sufficient dynamo. Moreover, given their low internal pressures, planetesimal cores likely freeze from the top down, wherein the gravitationally unstable solid in the freezing front has high viscosity and is not easily destabilized. The solid may delaminate in intermittent pulses, which can lead to ephemeral intervals of vigorous convection sufficient to drive dynamo activity, although they may be too short lived to fully magnetize the planetesimal. However, delaminated diapiric downwellings from the solidification front are likely to be partial melts or mushes and thus porous, two-phase bodies. Percolative flow or seepage through the diapir can potentially induce magnetization in the diapir quickly, possibly allowing rapid triggering of dynamo activity. We build on our recent two-phase magnetohydrodynamic theory to examine the magnetization of a porous metallic diapir sinking through a liquid metal in the presence of an imposed background field. Without seepage of liquid through the diapir (i.e., if it were rigid and impermeable), the background magnetic field is distorted by flow around the diapir, as expected. Some of the field distortion is offset with the addition of seepage through the diapir. However, fast seepage can establish a dipole moment inside the diapir, which points opposite the direction of diapiric descent. Wide-spread delamination and diapirism could therefore rapidly establish a multi-polar poloidal field whose field lines extend outside the core, thereby allowing remanent magnetization of the solid and solidifying material above it.