2016 年 50 巻 3 号 p. 135-154
It is widely believed that the Hadean Earth suffered an intense impact bombardment. Shock vaporization/subsequent chemical reaction is one of the most curious processes on the Hadean Earth because it leads to unique chemical reactions never driven under a mean field on the Earth due to the injection of reducing materials with a significant amount of kinetic energy. Here, I review a recent research development on the nature of impactors at the terminal stage of planetary accretion. After a long discussion over a few decades, a self-consistent model referred to as “a sawtooth-like timeline” has been proposed. Then, I introduce a framework to understand shock-induced chemistry based on the entropy method. Hypervelocity impact experiments in an open system would be essential to investigate the impact-generated chemical species quantitatively. Finally, I discuss a future plan to estimate the Hadean environment on Earth from an impact laboratory. An intense and impulsive perturbation due to hypervelocity impacts might cause the release of a large amount of free energy into a prebiotic field and produce unique geochemical features, such as isotopic anomalies in oxygen and sulfur in sediments, through the recovery processes. A combination analysis with the recent model for the impact bombardment and a more detailed model for impact-induced chemical reactions will allow us to re-construct the surface environment on the Hadean Earth.