The early age of the solar system has seen numerous giant impacts, on a scale energetic enough to un-make and make proto-planets, planetesimals and moons. The Earth did not escape this pattern; if early giant impacts could have formed ponds or even seas of magma, their signature cannot be directly retrieved today, but only inferred. But the last giant impact was large enough to result in a total breakup of the initial bodies and to transform the proto-Earth and the impactor, Theia, into a disk or a synestia. This object was formed of hot liquid droplets and gas. Large parts of it were probably in supercritical state. After its formation this starts to cool and condense.
The central accumulation of dense matter forms the Earth. The timescale for its accumulation is on the order of a few tens of hours. The central body is surrounded for a long time of an atmosphere. Models predict either the formation of a distinct central body and outer atmosphere or of a continuous transitional state with no sharp distinction, like a surface. The first case happens if the liquid-gas
The two scenarios have tremendous implications, for explaining the observed geochemical similarities between the Earth and the Moon. In the first case the disk must have been extremely homogenous from the beginning, for example because the impactor and the proto-Earth would have been very similar in composition. In the second case this similarity constrain is relaxed, as a persistent continuous object would be better mixed.
Choosing the right scenario depends on many things, like the energy of the impact, the masses, the angular momentum of the system. But also it largely depends on the thermodynamic relations of the different mineral components. As a function of the energy of the impact the supercritical state can be reached. And more importantly, depending on the position of the critical point the supercritical state can be maintained for a shorter or longer time. (Figure 1).
However, some of the most important thermodynamic parameters that are necessary in the modelling of such an impact are missing today: the position of the critical points, the boundaries of the critical state for multicomponent systems, the liquid-vapour equilibria, and the equation of state for both liquids and the supercritical fluids.
The estimations of the critical points currently in use have been based on extrapolations of the thermodynamic parameters from ambient conditions up to the high temperatures and low densities specific to silicate critical points. Only very recently more through approaches based on ab initio simulations have started to address simple relevant mineral systems, like silica and forsterite.
In our project we computed these missing parts of the phase diagrams for the representative systems of the protolunar disk. We have addressed a wide set of chemical compositions, both pure idealized phases, i.e. iron, feldspars, brucite, and silica, as well as a realistic average composition of the Earth’s mantle, the Bulk Silicate Earth. We have also considered a series of volatiles, like CO, CO2, He, H2O, and Zn, whose behaviour during the Impact and its aftermath we are tracking.
We obtain the liquid spinodal points from which we infer the position of the critical points. We compute the equation of states of the liquids and of the supercritical fluids, we determine the fluid structure, the chemical speciation, and we calculate the transport properties.
