Freezing, phase segregation and mushy magnetohydrodynamics in planetesimal cores

Core freezing and compositional convection are likely important drivers for dynamo activity in large terrestrial bodies such as Earth. The solidification of compositional mixtures, like iron and sulfur, generates partial melt mush zones at the freezing front, which eject chemically buoyant or heavy liquid that drives convection. For planetesimals in the asteroid belt, conditions for generating the necessary buoyancy flux to induce a dynamo are harder to achieve, even for rapidly cooling bodies like the putatively exposed core of the asteroid 16 Psyche. Nevertheless, evidence for magnetization of achondrite meteorites is abundant, suggesting that many planetesimal cores were magnetized. Small bodies cooling rapidly in low gravity likely spend much of their evolution with a large poorly compacted partial melt mush zone. The magnetohydrodynamic behavior of a mush zone can induce magnetization of the melt and solid phases via phase separation; conversely magnetism can impose extra forces on phases in the mush zone. To this end, we have developed a new two-phase magnetohydrodynamic theory for percolation and compaction of a mush zone. Magnetic induction by phase separation is most significant after extensive mixing of liquid and solid phases, caused by vigorous stirring due to, for example, rapid rotation of a non-spherical shell, gravitational tides, or impacts with other bodies (such as a mantle-stripping event associated with Psyche), all of which are conceivably common in the asteroid belt, especially in the early solar system. Subsequent gravitational phase separation can induce magnetization, especially in the liquid phase. In particular, magnetic field variances can be orders of magnitude larger than an imposed background field during the first free segregation of the well-mixed slurry. As the phases separate toward the top or bottom, the solid phase compacts, and segregation slows, the magnetic field variance diminishes. However, solitary waves in the mostly-solid compacting region cause large magnetic induction in the liquid, in the form of strongly magnetized wave packets that can be trapped in the solid. Thus, phase separation, segregation and compaction potentially induce large magnetic field anomalies eventually frozen into the solid layer.