Age and Formation Mechanism for an Innermost Inner Core

Abstract:

The concept of a magma ocean existing while a rocky planet accretes is useful for modeling its geochemical evolution, and is particularly successful in explaining moderately siderophile element concentrations relative to chondrites in the Earth's mantle. Here we examine the physical consequences of the existence of a magma ocean covering an Earth-like planet during accretion using continuous and stochastic accretion scenarios from N-body Grand Tack-style simulations. We find that release of gravitational potential energy is the dominant control on the temperature of the growing core inside the planet and leads to thermal stratification of the core, resistant to convection. Temperatures at the center of the accreting planet are low enough that the increasing silicate overburden pressure leads to freezing. An inner core develops early in all of the Earth accretion histories explored so far and persists beyond the end of accretion, controlled by the heating time the whole core which can be lengthy. Thus recent seismic studies supporting the existence of an innermost inner core may be observing a relic from the very earliest stage of Earth's accretion history. The composition of this part of the core may reflect the composition of the core at its earliest accretion stage rather than of the present core, yielding a different form and intensity of anisotropy.