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

在这个研究项目中，我们进行了两项主要任务。首先是高速激光枪的发展。在这次任务中，我们成功地发射了速度高达~ 6公里/秒的金属弹丸。这个速度是典型的两级轻气炮的最大速度，是最常用的传统发射技术之一。此外，我们还通过实验验证了理论尺度规则，表明发射速度与激光强度与靶柱质量之比的平方根成正比。这一比例规律表明，激光能量到弹体动能的转换是很高的，可以预期激光强度的进一步提高将使该技术的发射速度大幅提高。激光枪不像两级光气枪，它不受燃烧气体的污染。利用这一特性，我们进行了第二个任务：在开放系统中进行冲击脱气实验。由于脱气后的CO_2作为一种主要的温室气体，可能在类地行星表面环境的演化过程中发挥了重要的作用，因此本课题将重点研究碳酸盐的撞击脱气现象。自20世纪80年代以来，碳酸盐脱气实验主要在美国进行。然而，实验结果现在受到质疑，因为它们大多是在一个封闭的系统中进行的，在这个系统中脱气气体和耗尽的目标岩石之间容易发生反反应。在我们的实验中，我们使用了结晶方解石靶和金铜弹。初步结果表明，脱气效率在峰值压力高于33GPa时达到饱和，强烈表明在冲击压力为33GPa或更低时发生完全脱气。这一压力阈值明显低于Ahrens及其同事获得的40-50 GPa，这表明撞击脱气的效率可能比以前高得多。

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.