Origin of Planetary Atmosphere: Theoretical Estimation of Vapor Production by Planetary Impacts

The vapor production due to planetary impacts is an important process to shape planetary atmospheres. Hyper-velocity impacts induce shock waves and their propagations irreversibly increase pressure, temperature and entropy, and, as a result, the bodies are vaporized. The adiabatic expansion following the shock wave passing decreases the pressure, and vaporization then occurs if the temperature is high enough. The entropy does not change in the adiabatic expansion. Therefore, the amount of vapor is determined by the shock increase of the entropy. We carried out iSALE simulations for such high-velocity impacts and obtained the distribution of shock entropy as a function of a radial distance, using a sophisticated solid EOS model, ANEOS. We then calculated the amount of vapor as a function of impact velocities. To understand the dependence on materials, we analytically derived the shock increase of pressure and internal energy. We could obtain analytical solutions for the shock distribution. These solutions allow us to analytically estimate the amount of vapor as a function of impact velocity. These results are useful for understanding atmospheric compositions connected to collisional history.