Physics and chemistry of impact vapor clouds: Applications to the Earth and the origin and evolution of life

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In this research project, we conducted two major tasks. The first is development of a high-speed laser gun. In this task, we succeeded in launching metal projectiles up to 〜6 km/s. This velocity is the maximum velocity of a typical two-stage light-gas gun, which is one of the most commonly used conventional launching technologies. Furthermore, we experimentally verifed the theoretical scaling rule indicating that launch velocity is proportional to the square root of the ratio of laser intensity to target column mass. This scaling rule shows that the conversion from laser energy to projectile kinetic energy is high, leading to a expectation that further increase in laser intensity will increase substantial launch velocity with this technology. A laser gun, unlike a two-stage light-gas gun, is free from possible contamination of combustion gas. Taking advantage of this property, we conducted a second task: impact degassing experiments in an open system. In this task, we focused on impact degassing phenomena of carbonates, because degassed CO_2 may have played an important role in the evolution of surface environment of terrestrial planets as a major greenhouse gas. Degassing experiments of carbonates have been conduced extensively mostly in the United States since 1980's. However, experimental results are now subject to questions because they are mostly done in a closed system, in which back reaction between degassed gas and depleted target rocks may take place easily. In our experiments, we used crystalline calcite targets and gold and cupper projectiles. Preliminary results indicate that degassing efficiency saturates at peak pressures higher than 33GPa, strongly suggesting that complete degassing occurs at shock pressure 33GPa or lower. This pressure threshold value is significantly lower than 40-50 GPa obtained by Ahrens and colleagues, suggesting that the impact degassing may occur at much higher efficiency than previously though.