Shock-induced silicate vaporization: The role of electrons

• Published in: Journal of Geophysical Research: Planets Vol. 117; no. E4
• Main Authors: Kurosawa, Kosuke, Kadono, Toshihiko, Sugita, Seiji, Shigemori, Keisuke, Sakaiya, Tatsuhiro, Hironaka, Yoichiro, Ozaki, Norimasa, Shiroshita, Akiyuki, Cho, Yuichiro, Tachibana, Shogo, Vinci, Tommaso, Ohno, Sohsuke, Kodama, Ryosuke, Matsui, Takafumi
• Format: Journal Article
• Language: English
• Published: Washington, DC Blackwell Publishing Ltd 01.04.2012; American Geophysical Union
• Subjects: Atmospheric sciences; Comets; Earth sciences; Earth, ocean, space; electron; equations of state; Exact sciences and technology; high power laser; hypervelocity impacts; Materials science; Planetology; Planets; Plasma physics; Scientific apparatus & instruments; silicate vaporization; Space; time-resolved spectroscopy; Recombination; chain silicates; Moon; reservoirs; electrons; chemical reactions; diopside; Specific heat; pyroxene; temperature; irradiance; silicates; ions; Emission line; clinopyroxene; energy; Cooling rate; Blackbody radiation; pressure; high pressure; vapor phase; lead; Vaporization; Ionization; Line width; prediction; emission spectra
• ISSN: 0148-0227; 2169-9097; 2156-2202; 2169-9100
• DOI: 10.1029/2011JE004031

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. Key Points The world's first silicate vaporization experiments with natural silicate We found that electrons behave as an energy reservoir during impact processes The understanding of evolution of silicate vapor clouds will be revised