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

在本研究项目中，我们主要开展了两项工作。首先是高速激光枪的研制。在这项任务中，我们成功地发射了高达~6公里/S的金属弹丸，这是典型的二级轻气炮的最大速度，这是最常用的常规发射技术之一。此外，我们还通过实验验证了发射速度与激光强度与靶柱质量之比的平方根成正比的理论定标规则。这一定标规则表明，激光能量到弹丸动能的转换很高，这使得人们预计，进一步增加激光强度将大幅提高这项技术的发射速度。与两级轻气枪不同，激光枪不会受到燃烧气体的污染。利用这一特性，我们进行了第二个任务：在开放系统中进行撞击除气实验。由于脱气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中起着重要的作用，因此本课题重点研究了碳酸盐气体的撞击脱气现象。自20世纪80年代S以来，碳酸盐脱气实验主要在美国进行，但由于实验结果大多是在封闭的体系中进行的，脱气气体与枯竭的目的岩之间容易发生反反应，因此实验结果受到了质疑。在我们的实验中，我们使用了结晶方解石靶材以及金和铜弹。初步结果表明，脱气效率在峰值压力高于33 Gpa时达到饱和，这有力地表明了在激波压力33 Gpa或更低时发生完全脱气。这个压力阈值明显低于Ahrens和他的同事获得的40-50 Gpa，这表明撞击脱气发生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.