Shock‐induced silicate vaporization: The role of electrons

Abstract

[1] 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.

Investigation of the possible effects of comet Encke's meteoroid stream on the Ca exosphere of Mercury

“…Using diopside (CaMgSi 2 O 6 ) as a target, these authors observed the change in emission spectrum from strong blackbody radiation into a number of ionic emission lines, e.g., O + , Ca + , Mg + , and Si + . There are two noteworthy points related to the research by Kurosawa et al []: first, the temperature of the shock‐induced cloud was measured to be ranging between 15,000 K and 27,000 K, consistent with the Ca exosphere temperature derived from ground‐based spectroscopy; second, although the observed wavelength range covers the regions for molecular band emission of CaO and MgO, these species were not observed. This experimental fact, if applied to the Mercury case, could further sustain the MIV process being a likely process for producing the observed energetic Ca exosphere and the comet Encke stream particles being the main agent of the exosphere's seasonal variation.…”

“…These assumptions may be conservative since experimental works provide evidence of different abundant dissociation products. In particular, Kurosawa et al [] conducted shock‐compression experiments using a high power laser and carried out in situ spectroscopic observations of silicate vaporization from the end of compression phase to adiabatic expansion to understand energy partitioning process during hypervelocity impacts. Using diopside (CaMgSi 2 O 6 ) as a target, these authors observed the change in emission spectrum from strong blackbody radiation into a number of ionic emission lines, e.g., O + , Ca + , Mg + , and Si + .…”

“…We note that in the current work we assumed a conservative source rate for the MIV‐derived exospheric populations. Although our knowledge on the details of this mechanism is currently limited, experiments by Kurosawa et al [] provide evidence that a significant amount of energetic atomic Ca can be released through the dissociative ionization mechanism upon high‐velocity impacts. If this is the case of Mercury, our model results would further support the MIV process being the culprit for producing the observed energetic Ca exosphere and the comet Encke stream particles being the main agent of the exosphere's seasonal variation.…”

Correlating laser‐generated melts with impact‐generated melts: An integrated thermodynamic‐petrologic approach

“…Pierazzo et al, 2005] to compute the change in entropy ΔS as a function of particle velocity u p and shock pressure P. We then used the ΔS values obtained from ANEOS to correlate the laser-generated melts with impact-generated melts and to find specific impact scenarios that would yield impact melts similar to the laser-generated melts. We are aware that compared to more recent data [e.g., Kraus et al, 2012Kraus et al, , 2015Kurosawa et al, 2012], the used ANEOS versions fail to accurately describe the shock states at pressures higher than those required for incipient vaporization. However, given the fact that we focus on impact melts and considering the widespread use of these ANEOS versions, we are certain that ANEOS describes the material behavior in the shock pressure range of interest sufficiently accurate.…”

Implantation of Martian Materials in the Inner Solar System by a Mega Impact on Mars

“…For a huge impact (>100 km in radius), the effects of material strength are negligible (Jutzi et al 2010;. Note that M-ANEOS may underestimate the melting/vaporization fraction for a high pressure regime (>100 GPa where impact velocity of >8 km s −1 ; Kurosawa et al 2012;Kraus et al 2012), but our nominal case of 6 km s −1 impact (see below) would correctly predict the melting fraction.…”