Why might planets and moons have early dynamos?

Remanent crustal magnetization of martian and lunar crustal rocks plausibly records magnetic fields related to core dynamos active during the first few hundred Myr of evolution of these bodies. Such early fields suggest that core dynamos may occur during or as a result of accretion. We investigate whether the processes governing the segregation and sinking of metallic Fe after a large impact can yield thermal conditions in the core that favor dynamo action on growing planets. Depending on the sizes of the impactor and planet, as well as the temperature-dependence of the viscosity, we identify conditions in which an early transient core dynamo is possible. We also consider the effect of a molten layer surrounding the protocore on the duration of this early dynamo. Our results suggest that dynamos can be initiated in bodies with a radius of 3500 km radius or greater under Earth-like conditions for ohmic dissipation in the core, and in smaller bodies if a less restrictive critical magnetic Reynolds number condition is applied. These dynamos may persist for several kyr to several Myr depending on the heat transfer regime at the protocore-mantle boundary. (C) 2011 Elsevier B.V. All rights reserved.