Shock physics of giant impacts: Transforming rocky planets into supercritical synestias*

Rocky planets form by a series of giant impacts with sufficient energy to vaporize the outer layers of the bodies. In many giant impacts, the colliding planets are transformed into a new type of astronomical object called a synestia, which is a body that exceeds the limit of a spheroidal shape. In most events that produce an Earth-mass body, the collision creates a supercritical synestia with an internal temperature-pressure profile that exceeds the critical point for silicates. Here, we present the results from numerical simulations of giant impacts using a forsterite equation of state for the silicate mantle. We compare our results to recently obtained critical points for silicates derived from experiments at the Sandia Z Machine and ab initial calculations. Transformation of planets into supercritical synestias affects the chemical and thermal evolution of the body. Cooling and differentiation of synestias follows a different thermodynamic path than previous models of magma oceans. We emphasize the critical need for studies of multicomponent chemical systems to understand the outcomes of giant impacts during planet formation.