Martian Interior Structures and the Termination of Martian Dynamo

Several recent research results on the Martian crustal magnetic anomalies and the giant impacts on the surface provide new insights on the Martian dynamo and the likely correlation of its timing with the formation of the giant impact basins on the Mars: the Martian dynamo could be subcritical during this period, i.e. it can be terminated by small perturbations (≤ 1%) to the heat flow (the Rayleigh number) across the CMB and, once terminated, it could not be reactivated unless substantial increases in the heat flow (e.g. 25%). The giant impacts during the early to mid-Noachian period could be sufficient to provide the perturbations to turn off the Martian dynamo. Could Martian interior structures introduce complexities to this scenario? The numerical simulation of the subcritical dynamo is carried out with a finite solid inner core of a radius approximately one third of the core- mantle boundary radius and with a uniform heat flux across the CMB. It is very likely that the heat flux is heterogeneous due to geophysical processes outside the CMB. The inner core size is unknown even if it was formed in the dynamo era. What are the consequences of these complications, in particular the inner core dimension, to the Martian dynamo and its termination? We continue our numerical simulations with three different core sizes, focusing on exciting dynamos from purely convecting states and termination of an existing dynamo. Our initial results show that the critical Rayleigh number for the dynamo onset increases when the inner core size is reduced. This could imply a larger subcritical domain, provided that the critical Rayleigh numbers for the termination of the dynamos do not increase equally fast. In addition, numerical results suggest multiple dynamo states near the onset of the dynamo, which could bring multiple subcritical branches of the subcritical Martian dynamos.