K. Kurosawa, T. Kadono, S. Sugita, K. Shigemori, T. Sakaiya, Y. Hironaka, N. Ozaki, A. Shiroshita, Y. Cho, S. Tachibana, T. Vinci, S. Ohno, R. Kodama and T. Matsui
We conducted a spectroscopic study of shock-heated silicate (diopside) and obtained the time evolution of the spectral contents, the line widths of emission lines, and the time- and irradiance-averaged peak shock temperatures. The peak shock pressures ranged from 330 to 760 GPa. Time-resolved emission spectra indicated that the initial spectrum was blackbody radiation; the spectrum evolved to yield several ionic emission lines, which in turn evolved to yield atomic lines at the later stages. The shock-heated diopside was highly dissociated and ionized, even though it is likely to have been subjected to high-pressure conditions near the liquid-vapor phase boundary. The time evolution of the spectra, from ions to atoms, strongly suggests that electron recombination occurred in the expanding shock-induced diopside vapor. The time- and irradiance-averaged peak shock temperatures at >330 GPa were lower than the theoretical Hugoniot curve, with a constant isochoric specific heat, indicating endothermic shock-induced ionization. Thus, we conclude that electrons behave as an important energy reservoir in energy partitioning via endothermic shock-induced ionization and subsequent exothermic electron recombination. This electron behavior leads to a higher degree of vaporization after isentropic release and a lower cooling rate due to the exothermic electron recombination in expanding impact-induced silicate vapors than previously expected. These results will affect the predictions associated with hypervelocity impact events in planetary science, such as the origin of the Moon and chemical reactions and production of silicate dust particles in impact-generated silicate vapor clouds.