Sudden termination of Martian dynamo?: Implications from subcritical dynamo simulations

The crustal magnetism measured by the Mars Global Surveyor requires that Mars possessed a strong internal field generated by a core dynamo in its early history. We use a numerical model to simulate the early Martian dynamo, focusing on the minimum energy for sustaining an established dynamo. Our results show that near its end, the Martian dynamo could reverse frequently, and could be subcritical: the energy to sustain the dynamo is significantly less than that to excite the dynamo. In addition to a longer lifetime, the subcritical dynamo implies that it could be terminated suddenly with a very small perturbation and, once turned off, it could not be reactivated without substantial increase of the buoyancy force in the Martian core.