Ab initio study of silica and hydrated silica during giant impacts
- Monod Campus
The Moon Impact formation theory was first proposed in 1975 by Hartmann and Davis. To this day, it is still the prevalent theory to explain the unique aspects concerning the Earth-Moon system. Although well accepted in the scientific community, this theory is still subject of debate regarding the conditions of the impact and the resulting scenario. It still challenging to encompass in a single solution the problematic aspects of chemical equilibration that took place post-impact. Different models of impact have been proposed over the years in order to address the aforementioned issues. These models often rely on equations of state to describe the behavior of the materials present in the Earth and the Moon. Equations of state (EOS) describe the distribution of materials’ phases and they are used on collision models based in hydrodynamic simulations to predict the final composition. The use of precise EOS lead to more correct post-impact chemical models where phases can be estimated correctly.
The majority of minerals present on rocky bodies in the Solar System are SiO2 based and they are the building blocks of the Earth and the Moon. In terms of composition, SiO2 represents more than 44% of lunar mare basalts and highlands, and about 45% of Earth’s primitive mantle. However similar in part of their composition, Moon and Earth differ considerably when it comes to the presence of volatile elements, where H2O is arguably the most relevant. The impart of energy given by the large impact is sufficient to melt and vaporize silicate minerals. It produces a silicate atmosphere, where physicochemical properties and geochemical signatures of the resulting scenario depend on the fractionation of liquid-vapor. The presence of volatiles can change the behavior of typical equations of state of silicate materials and eventually be responsible for increasing the amount of vapor and supercritical material in such conditions.
We aim to construct equations of state for the silica-water binary system and subsequently study structural and transport properties of such system, comparing the behavior of the main elements under different conditions. We employ ab initio molecular dynamics (AIMD) methods where forces and energy are calculated with density functional theory as implemented in the VASP® package. We start by describing the behavior of pure SiO2 under temperatures from 4000 to 7000 K and densities from 0.2 to 2.33 g/cm3. We follow by inserting 9, 18, 36 and 72 H2O molecules inside the SiO2 cell to obtain four different water-silica ratios and compare changes in the thermodynamic behavior as well as potential effects in elemental structural and transport properties. We study the four systems at temperatures of 2000, 3000, 4000 and 5000 K and densities of 0.34 to 2.77 g/cm3 depending on the system. We calculate the critical point of SiO2 in between 5000 and 5500 K, from 0.6 to 0.85 g/cm3 and 0.15 to 0.25 GPa. The presence of water has a direct effect on the placement of the critical point, reducing it in more than 2000 K at specific conditions. Our results show that supercritical state materials are often underestimated in moon forming hydrodynamic models, which may impact elemental mixing in the outcome of the Giant Impact.
Thesis defense of Mrs. BRANDELLI SCHAAN Renata under the supervision of Mr. CARACAS Razvan