Rheological controls on the terrestrial core formation mechanism

Abstract

Iron core differentiation of terrestrial planetary bodies is thought to have occurred simultaneously with planetary accretion. The exact mechanisms of core formation, however, remain incompletely understood. One model proposes that cores are formed from numerous smaller iron cores from predifferentiated planetesimals. To further understand this mechanism for forming Mars-and Earth-sized bodies, we present here systematic numerical simulations. Our models include a non-Newtonian temperature-, pressure-and strain rate-dependent viscoplastic rheology. Four different core formation regimes are being observed in the study, as a function of activation volume, friction angle, Peierls stress, and the initial temperature state of the body. We derive scaling laws, which show the importance of shear heating localization and plastic yielding as mechanisms to drive planetary differentiation in planetary interiors, that are in good agreement with numerical simulations. Results indicate that the effective rheology of the planetary body has a major effect on the core formation mechanism: while bodies with a weak rheology generally show a diapiric mode of core formation, the interior of planetary bodies with a stiff rheology can be fractured or displaced toward the surface. On Earth-sized protoplanets, the water content seems also to have a significant influence on the mode of core formation. Results indicate a time scale of differentiation of a few million years, significantly shorter than expected from the Stokes sinking time in a Newtonian medium. © 2009 by the American Geophysical Union.