Mercury’s enigmatic magnetic field: a dynamo case against an extended basal iron-snow layer in the liquid outer core?

Mercury, the innermost planet in the solar system, remains an enigma after the wealth of measurements collected by NASA's MESSENGER mission. The state of Mercury's iron-rich core and the dynamo action that generates Mercury’s relatively weak, axially aligned, north-south asymmetric internal magnetic field is not well understood. Here we investigate the dynamo action associated with one unique possibility of Mercury's core: a double-iron-snow (DIS) dynamo with an extended, stably-stratified basal iron snow zone. This DIS structure model is one possible scenario that fits most other geophysical constraints at Mercury, including the Moment of Inertia. Our three-dimensional numerical dynamo survey varying the convective forcing (Rayleigh number), relative electrical conductivity (magnetic Prandtl number), and Brunt-Väisälä frequency revealed that although a relatively weak surface magnetic field can be achieved within this set-up, the external magnetic field remains highly dipolar and north-south symmetric under most scenarios. We hypothesize that this symmetry preference is due to the magnetic anchoring effect of the basal iron snow zone and the electromagnetic screening effect of the top iron snow zone. Our results indicate that while the existence of a top iron-snow zone (or a stably stratified layer) can lead to a weak and more axisymmetric magnetic field, the existence of an extended basal iron-snow zone would prohibit the equatorial symmetry breaking in the magnetic field observed at Mercury. Thus, our dynamo modeling results argue against the existence of an extended basal iron snow zone inside Mercury's core at present.