Mercury's early thermal evolution and core formation in the presence of impact-generated atmosphere during accretion

Abstract

Understanding a planet's early thermal evolution and differentiation is crucial to comprehending the distribution of volatiles in its different reservoirs. Mercury is now known as a volatile rich planet. It has carbon saturated core, deeply buried volatile rich layers, a diamond layer at the core-mantle boundary and graphite floating at the crust. For carbon saturation, Mercury is believed to have accreted from Enstatite/CB chondrite rich building blocks. In the present work, we studied the early thermal evolution and core formation in the interior of Mercury by considering its accretion from water rich Enstatite chondrites prior to the dispersal of solar nebula. The heat sources for the melting and differentiation of Mercury include the decay energy of SLR ²⁶Al and the blanketing effect of the impact generated H₂O+CO+H₂ along with primordial atmosphere. The results suggest the complete core formation with lowest assumed water content in the building blocks Mercury for accretion timescales ≤1.5 Myr after the formation of CAIs. The longer accretion timescales, it needed higher abundance of water to cause significant blanketing effect at the surface. During differentiation process, the volatiles dissolved in the magma ocean under the pressure of overlying atmosphere, could partition into the core. Hence, the outcomes of present study have implications to explain the distribution of volatile in the interior of Mercury. Conversely, under the strong blanketing effect, the surface silicate could vaporize and dissolve in the steam atmosphere.