Early thermal evolution of the embryos of Earth: Role of 26Al and impact-generated steam atmosphere

Abstract

Earth is known to share a common origin with the Enstatite chondrites (dry) due to their similar isotopic composition. Recent findings have revealed that the Enstatite chondrites are not volatile free. These comprise enough hydrogen to provide sufficient water to Earth. It implies the formation of an impact generated water-vapor steam atmosphere on the embryos of Earth during accretion. Further, the new isotopic measurements and planet formation theories recommend the early formation of the embryos of Earth within the initial ~2 ​Ma of the formation of solar system. The early accretion suggests the role played by short-lived radionuclide (SLR) 26Al in large-scale heating and melting of their interior. We present the results of numerical simulations, performed probably for the first time, to understand the early thermal evolution and core-mantle differentiation of embryos of Earth (0.2ME-0.6ME) by considering the combined contribution of the heat of 26Al, accretion luminosity, and blanketing effect of the impact-generated steam atmosphere during accretion. The atmosphere was assumed to be grey, radiative, and plane-parallel. The numerical simulations were performed by assuming the accretion duration of embryos and initial water content of accreting planetesimals as a free parameter. The results indicate the formation of magma ocean at the surface of embryos during accretion. The depth of the magma ocean at surface increased with increase in the water content of the accreting planetesimals. Further, the interiors of embryo were completely differentiated within the initial ~5 ​Ma of the formation of solar system if they accreted in <1.5 ​Ma after the formation of CAIs. The massive embryos (0.4ME-0.6ME) due to high interior pressure required early differentiation (after the melting of iron or 10% melting of bulk silicates) for complete core-mantle segregation within the initial 5 ​Ma. These results were found to be consistent with the recent finding for early accretion and differentiation of the main accretion phase of Earth. The outcomes of the present study could have implications to explain the anomalously high abundance of Highly siderophile elements (HSEs) and Platinum group elements (PGEs) in Earth's mantle.