Takafumi Matsui

Hydrogen cyanide production due to mid-size impacts in a redox-neutral N2-rich atmosphere.

Cyanide compounds are amongst the most important molecules of the origin of life. Here, we demonstrate the importance of mid-size (0.1–1xa0km in diameter) hence frequent meteoritic impacts to the cyanide inventory on the early Earth. Subsequent aerodynamic ablation and chemical reactions with the ambient atmosphere after oblique impacts were investigated by both impact and laser experiments. A polycarbonate projectile and graphite were used as laboratory analogs of meteoritic organic matter. Spectroscopic observations of impact-generated ablation vapors show that laser irradiation to graphite within an N2-rich gas can produce a thermodynamic environment similar to that produced by oblique impacts. Thus, laser ablation was used to investigate the final chemical products after this aerodynamic process. We found that a significant fraction (>0.1xa0mol%) of the vaporized carbon is converted to HCN and cyanide condensates, even when the ambient gas contains as much as a few hundred mbar of CO2. As such, the column density of cyanides after carbon-rich meteoritic impacts with diameters of 600xa0m would reach ~10xa0mol/m2 over ~102xa0km2 under early Earth conditions. Such a temporally and spatially concentrated supply of cyanides may have played an important role in the origin of life.

In-situ spectroscopic observations of silicate vaporization due to >10 km/s impacts using laser driven projectiles

[1]xa0We present the results of shock-induced silicate vaporization experiments using laser driven hypervelocity projectiles. In-situ spectroscopic observations of shock-heated quartz and diopside were conducted. We observed both atomic emission lines and blackbody continuum. Because emission lines occur only in a gas phase, this observation indicates that the incipient vaporization of silicates actually occurs at >10 km/s. We estimated the peak-shock temperatures from the blackbody spectra. The obtained results suggest that the temperature dependence of the isochoric specific heat Cv depends rather strongly on material at extremely high pressures. Such difference in Cv dependence on temperature will influence the impact vaporization efficiency. Thus, investigation on the Cv of the other major silicates is necessary for understanding impact-related phenomena. Furthermore, the observed high intensity of emission lines shows the possibility that a variety of the thermodynamic variables of expanding silicate vapor can be measured with a higher speed spectrometer.

Shock‐induced silicate vaporization: The role of electrons

[1]xa0We 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.

Shock compression response of forsterite above 250 GPa

Shocked forsterite above 250 GPa indicates incongruent crystallization of MgO, its phase transition, and remelting. Forsterite (Mg2SiO4) is one of the major planetary materials, and its behavior under extreme conditions is important to understand the interior structure of large planets, such as super-Earths, and large-scale planetary impact events. Previous shock compression measurements of forsterite indicate that it may melt below 200 GPa, but these measurements did not go beyond 200 GPa. We report the shock response of forsterite above ~250 GPa, obtained using the laser shock wave technique. We simultaneously measured the Hugoniot and temperature of shocked forsterite and interpreted the results to suggest the following: (i) incongruent crystallization of MgO at 271 to 285 GPa, (ii) phase transition of MgO at 285 to 344 GPa, and (iii) remelting above ~470 to 500 GPa. These exothermic and endothermic reactions are seen to occur under extreme conditions of pressure and temperature. They indicate complex structural and chemical changes in the system MgO-SiO2 at extreme pressures and temperatures and will affect the way we understand the interior processes of large rocky planets as well as material transformation by impacts in the formation of planetary systems.

Formation and geomorphologic history of the Lonar impact crater deduced from in situ cosmogenic 10Be and 26Al

The Lonar impact crater is one of a few craters on Earth formed directly in basalt, providing a unique opportunity to study an analog for crater degradation processes on Mars. Here we present surface 10Be and 26Al exposure dates in order to determine the age and geomorphic evolution of Lonar crater. Together with a 14C age of preimpact soil, we obtain a crater age of 37.5u2009±u20095.0 ka, which contrasts with a recently reported and apparently older 40Ar/39Ar age (570u2009±u200947 ka). This suggests that the 40Ar/39Ar age may have been affected by inherited radiogenic 40Ar (40Ar*inherited) in the impact glass. The spatial distribution of surface exposure ages of Lonar crater differs from that for Barringer crater, indicating Lonar crater rim is actively eroding. Our new chronology provides a unique opportunity to compare the geomorphological history of the two craters, which have similar ages and diameters, but are located in different climate and geologic settings.

Oxidizing Proto-atmosphere on Titan: Constraint from N2 Formation by Impact Shock

Titan is the only satellite that possesses a thick atmosphere, composed mainly of N2 and CH4. However, its origin and evolution remain largely unknown. Knowledge of the acquirement of a N2 atmosphere on Titan would provide insights into nitrogen evolution in planetary atmospheres as well as the formation of satellite systems around gas giants. Previous studies have proposed that the atmospheric N2 would have been converted from NH3 via shock heating by accreting satellitesimals in the highly reducing proto-atmosphere composed of NH3 and CH4. Nevertheless, the validity of this mechanism strongly depends on both the composition of the proto-atmosphere and kinetics of shock chemistry. Here, we show that a CO2-rich oxidizing proto-atmosphere is necessary to form N2 from NH3 efficiently by atmospheric shock heating. Efficient shock production of N2 is inhibited in a reducing protoatmosphere composed of NH3 and CH4, because CH4 plays as the coolant gas owing to its large heat capacity. Our calculations show that the amount of N2 produced in a CO2-rich proto-atmosphere could have reached ∼20 times that on the present Titan. Although further quantitative analysis are required (especially, the occurrence of catalytic reactions), our results imply that the chemical composition of satellitesimals that formed the Saturnian system is required to be oxidizing if the current atmospheric N2 is derived from the shock heating in the proto-atmosphere during accretion. This supports the formation of regular satellites in an actively supplied circumplanetary disk using CO2-rich materials originated from the solar nebula at the final stage of gas giant formation.

Oxidation of carbon compounds by silica-derived oxygen within impact-induced vapor plumes

Impact-induced vapor plumes produce a variety of chemical species, which may play an important role in the evolution of planetary surface environments. In most previous theoretical studies on chemical reactions within impact-induced vapor plumes, only volatile components are considered. Chemical reactions between silicates and volatile components have been neglected. In particular, silica (SiO2) is important because it is the dominant component of silicates. Reactions between silica and carbon under static and carbon-rich “metallurgic” conditions (C/SiO2 ≫ 1) are known to occur to produce CO and SiC. Actual impact vapor plumes, however, cool dynamically and have carbon-poor “meteoritic” composition (C/SiO2 ≪ 1). Reactions under such conditions have not been investigated, and final products in such reaction systems are not known well. Although CO and SiO are thermodynamically stable at high temperatures under carbon-poor conditions, C and SiO2 are stable at low temperatures. Thus, CO may not be able to survive the rapidly cooling process of vapor plumes. In this study, we conduct laser pulse vaporization (LPV) experiments and thermodynamic calculations to examine whether interactions between carbon and silica occur in rapidly cooling vapor plumes with meteoritic chemical compositions. The experimental results indicate that even in rapidly cooling vapor plumes with meteoritic compounds are rather efficiently oxidized by silica-derived oxygen and that substantial amounts of both CO2 and CO are produced. The calculation results also suggest that those oxidation reactions seen in LPV experiments might occur in planetary-scale vapor plumes regardless of impact velocity as long as silicates vaporize.

Gas recovery experiments to determine the degree of shock-induced devolatilization of calcite

Shock-induced devolatilization of volatile-bearing minerals has played an important role in the formation of the atmosphere and evolution of surface environments of terrestrial planets. The dependence of the degree of devolatilization on ambient pressure has not been investigated in detail before, even though ambient pressure dramatically affects the degree of devolatilization. In this study, we conducted shock recovery experiments on calcite (CaCO3) using newly designed sample containers for released gas analysis, and assessed the dependence of the degree of devolatilization on the partial pressure of CO2. Our results clearly show that the degree of devolatilization increases as the sample container volume increases and the initial mass of calcite decreases.

IMPACT EXPERIMENTS WITH PROJECTILES AT VELOCITIES HIGHER THAN 10 KM/S

Impact velocity of meteorites on planetary and satellite surfaces at the final stage of planetary accretion becomes more than 10 km/s. The impacts with velocities higher than 10 km/s generate very large craters and a large amount of silicate vapor, melt, and fast ejecta, and would make great effects on the planetary surface environments. However, the details of the effects by such impacts on the environments have not been understood well yet. The reasons are probably that macroscopic (>∼0.1 mm) projectiles are not easily accelerated to more than 10 km/s in laboratories. This makes it difficult to investigate experimentally the impact phenomenon with impact velocities higher than 10 km/s. In this paper, we demonstrate that higher impact velocities than 10 km/s can be achieved using projectiles with a diameter of 0.1–0.3 mm: we accelerate glass and aluminum projectiles using a high‐power laser, GEKKO XII—HIPER. The projectiles are collided into LiF targets. We observe some lines of Li gas using a time‐resol...

Dynamics of hypervelocity jetting during oblique impacts of spherical projectiles investigated via ultrafast imaging: ULTRAFAST IMAGING OF IMPACT JETTING

A series of hypervelocity impact experiments was conducted in a new laboratory at Planetary Exploration Research Center of Chiba Institute of Technology (Japan). We present the results of high-speed imaging observations of impact jetting during blunt-body penetration under oblique impacts. The observations were sampled at a frame rate of 100u2009nsu2009frame−1, which is much shorter than the characteristic time of projectile penetration under our experimental conditions. The maximum jet velocity was obtained as a function of both impact velocity and the contrast of shock impedance between a projectile and target, enabling us to test theoretical models of impact jetting during oblique impacts of spherical projectiles. We find that the jet velocities measured in this study are much slower than the prediction by the standard theory based on the previous experimental/theoretical results of collisions between two metal plates. A decaying shock pressure during blunt-body penetration is a possible origin of the discrepancy. We also present a new formulation of the jet velocity with the equations of state for realistic materials. The particle velocities of ejected materials from a free surface are calculated using the Riemann invariant along the isentropes and the Tillotson equations of state in this study. Based on the extremely high velocity of the jet, we point out that impact jetting might contribute to chemistry near the ground surface of planets/satellites with a thick atmosphere, such as Titan.