A hemispherical dynamo model: Implications for the Martian crustal magnetization

In 1999 the Mars Global Surveyor detected a strong but very heterogeneous crustal magnetization mainly localized in the southern hemisphere. Their magnetization dichotomy may have either an external or an internal origin. In the first scenario, the Martian crust was fully magnetized by a dipolar dynamo induced in the Martian liquid core. After the core dynamo cessation, the crust was demagnetized by volcanoes, impacts or any other resurfacing event distributed not homogeneously over the surface. The internal origin, which is investigated here, relies on a per se hemispherical internal magnetric field. For this, we rely on that Mars never developed an inner core. The planets ancient dynamo was thus exclusively driven by secular cooling and radiogenic heating. Due to the small planetary size, the core mantle boundary (CMB) heat flux may be not as homogeneous, as in e.g. Earth. Mantle convection in smaller planets is thought to develope larger scales, maybe even a huge single-plume structure. Giant impacts might have played a crucial role in the thermal history of Mars, hence they are heating mainly one hemisphere. Giant plumes and major impact events would both cause a hemispherical CMB heat flux pattern. Therefore, we model the ancient Martian dynamo as rotating, convecting and conducting fluid heated by an internal heat source and contained in a spherical shell, where the CMB heat flux is perturbed by a sinusoidal anomaly. Compared to the classical columnar convection, we find a drastically different flow pattern. There meridional circulation seeking to equilibrate the heat difference between both hemispheres is diverted into two counterdirected cells of axisymmetric zonal flows (thermal winds) by the strong Coriolis force. Convective plumes are confined to the region of high heat flux in the vicinity of the southern pole. Core convection is thus dominated by equatorially antisymmetric and axisymmetric (EAA) modes. In the columnar regime, poloidal and toroidal magnetic fields are produced by helical motions in the individual columns via an α -mechanism. When the EAA mode dominates, the toroidal field is mainly induced by an ω -effect associated to the shear region between the thermal wind cells. Poloidal field induction is confined to the southern plumes and much less efficient. The dynamo character therefore changes from an α ²-type in columnar convection to an α ω -type in the EAA convection. We identify dynamical properties, such as morphology and equatorial asymmetry of the convective flow and the magnetic field as a function of the relative perturbation amplitude and its orientation with respect to the axis of rotation. Averaging in time and extrapolation of the magnetic field to the planetary surface yields constraints on the magnetisation scenarios.