This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: a b s t r a c t Editor: T. Spohn Keywords: early dynamos growing planets meteoritic impacts numerical modeling 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.