Integrative Approaches to Building Planet Earth: New Wide-Ranging Equations of State

Understanding the final accretion of terrestrial planets via giant impacts requires self-consistent equations of state (EOS) that are robust over a vast range of pressures and temperatures. During the giant impact stage of planet formation, rocky planets are melted and partially vaporized. However, most EOS models fail to reproduce experimental constraints on the thermodynamic properties of the major minerals over the required phase space. Developing accurate equations of state (EOS) is a major challenge, and uncertainties and errors in EOS models limit our ability to investigate planetary accretion. Model development requires integrating static and dynamic laboratory measurements and first-principles calculations of material properties into multi-phase EOS models.

Here, we present an updated version of the widely-used ANEOS model that includes a user-defined heat capacity limit in the thermal free energy term. Our revised model for forsterite (Mg2SiO4), a common proxy for the mantles of rocky planets, provides a better EOS fit over most of the phase space of giant impacts. In addition, we provide an updated ANEOS parameter set for iron. The new models provide greater fidelity in the calculated pressures and temperatures, which are necessary to study chemical processes. We discuss the implications of the revised EOS for the amounts of impact-induced melting and vaporization during planetary accretion and the onset of miscibility between iron and silicates in Super-Earths.

This work is supported by the Sandia Z Fundamental Science Program; DOE-NNSA DE-NA0002937; DOE-NNSA DE-NA0003842; NASA NNX15AH54G; NASA NNX16AP35H; UCOP LFR-17-449059. SNL is managed by NTESS under DOE-NSSA contract DE-NA0003525. Prepared by LLNL under Contract DE-AC52-07NA27344.