The influence of degree-1 mantle heterogeneity on the past dynamo of Mars

The hemispheric dichotomy in the crustal magnetic field of Mars may indicate that the planet’s past dynamo was influenced by a degree-1 heterogeneity on the outer boundary of its liquid metallic convecting core. Here we use numerical dynamos driven by purely volumetric internal heating with imposed degree-1 heat flux heterogeneities to study mantle control on the past dynamo of Mars. We quantify both south–north and east–west magnetic field dichotomies from time-average properties that are calculated according to two different end member crust formation scenarios. Our results indicate that a moderate heat flux anomaly may have been sufficient for obtaining the observed dichotomy. Because of the excitation of a strong equatorial upwelling in the dynamo, the efficiency of a mantle heterogeneity centered at the geographical pole in producing a south–north dichotomy is much higher than that of an heterogeneity centered at the equator in producing an east–west dichotomy. These results argue against a significant True Polar Wander event with major planet re-orientation after the cessation of the dynamo.

► We provide a plausible range for the north–south dichotomy of the Martian magnetic field. ► Numerical dynamos with a moderate amplitude Y10 pattern of core–mantle boundary heat flux can reproduce the observed dichotomy. ► Y10 is much more efficient than Y11, so an inclined degree-1 anomaly produces mostly a north–south magnetic field anomaly. ► Our results suggest that there was no significant True Polar Wander event in the history of Mars.