Dynamics of core merging after a mega-impact with applications to Mars’ early dynamo

Abstract

A giant impact occurring within the first 500Myr of martian history may have been responsible for the dichotomy between the northern lowlands and the southern highlands and may have influenced the initiation or cessation of early and short-lived core dynamo. We hypothesize that a significant volume of metallic iron from a differentiated impactor merged with a preexisting martian core. We investigate the dynamics and thermal effects of this core merging, assuming that the impactor’s core sank as a single metallic diapir through a solid mantle. We explore the consequences of this process for dynamo action and for Mars’ magnetic field history. For large impacts (with radii larger than 100km) and plausible mantle viscosities, merging is expected to occur in less than 1Myr. Depending on the temperature-dependence of the mantle viscosity, viscous dissipation within the diapir may be very large. Where thermal mixing of the hot diapir into a preexisting core is complete, merging can increase the temperature gradient to the surrounding mantle and consequently drive a dynamo until this additional heat is transferred to the mantle, which takes on the order of 100Myr. If merging leads to strong thermal stratification in the core, however, dynamo action may be inhibited.