Sohsuke Ohno

Sulfur chemistry in laser-simulated impact vapor clouds: implications for the K/T impact event

Abstract One of the most promising mechanisms for the mass extinction at the K/T boundary event is blockage of sunlight by sulfuric acid aerosol, which is induced by impact vaporization of sulfate in evaporite deposits around the K/T impact site. One of the advantages of this hypotheses is that it may cause an impact winter much longer than that by silicate dust and soot due to a global wildfire. However, the residence time of sulfuric acid aerosol in the stratosphere depends strongly on the ratio of SO 2 /SO 3 in the K/T impact vapor. If SO 3 was dominant, the blockage of sunlight by the sulfuric acid aerosol would not last longer than that by silicate dust and soot. The chemical reaction of sulfur oxides in an impact vapor cloud has not been studied extensively before. This study carries out chemical equilibrium calculations, kinetic model calculations, and laser irradiation experiments with a quadrupole mass spectrometer to estimate the SO 2 /SO 3 ratio in the K/T impact vapor cloud. The results strongly suggest that most of sulfur oxides in the K/T impact vapor cloud may have been SO 3 , not SO 2 . The sulfuric acid aerosol may not have been able to block the sunlight for a long time.

Rotational-Temperature Measurements of Chemically Reacting CN Using Band-Tail Spectra

The vibrational state of chemically reacting CN radicals does not necessarily have a Boltzmann distribution because it may be influenced by the chemical reaction leading to the formation of the CN radicals. Here, we develop a new method to measure the rotational temperature of chemically reacting nonequilibrium cyanide radicals using the band tails of their emission spectra. Because of the very short relaxation time scales, both the translational and rotational states reach a thermal equilibrium even when the vibrational state does not have a Boltzmann distribution. The method proposed in this study has two advantages. First, it is not sensitively affected by self-absorption. Second, it does not require as high a wavelength resolution as other methods because it uses the overall shape of the tail part of the CN emission bands. Thus, our method is more suitable for high-speed temperature measurements, where a high-wavelength-resolution measurement is difficult to obtain. To investigate the validity of our method, we carried out laser-ablation experiments within an N 2 - H 2 O - CO 2 - Ar gas mixture using graphite targets and measured the rotational temperature of laser-induced CN and C 2 radicals using the proposed method. The rotational temperatures exhibit reasonable trends as functions of time and beam cross sections, strongly suggesting that our method is useful for translational-rotational-temperature estimation of chemically reacting nonequilibrium CN radicals.

Time-resolved spectroscopic observations of shockinduced silicate ionization

We conducted time-resolved spectroscopic observations of shock-heated quartz and forsterite using a high-power laser. The results revealed that ionization occur easily under shockinduced warm dense conditions. We compare the obtained temperatures on the Hugoniot with a few theories. The comparison suggests that the contribution of shock-induced ionization to the isochoric specific heat during shock compression is ~1 kb/atom for quartz and forsterite. Shock-induced ionization and subsequent electron recombination leads to a larger amount of vapor and may lead to dynamical and chemical evolution of silicate vapor clouds different from our current understandings.

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.

Flyer acceleration by high-power laser and impact experiments at velocities higher than 10 km/s

Flyer acceleration technique using high-power lasers has several advantages such as the achieved velocities higher than 10 km/s and non-contamination to the products generated by impacts. In this study, we show that a high-power laser can achieve impact velocities higher than 10 km/s up to 60 km/s using sheet flyers with a diameter of ~ 0.5 mm and a thickness of ~ 30 - 50 im and spherical projectiles with a diameter of 0.1 - 0.3 mm. We discuss the difference between sheet and spherical flyers in the energy transfer efficiency from laser energy to flyer kinetic energy.

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...

Direct measurement of chemical composition of SOx in impact vapor using a laser gun

The final chemical composition of vapor clouds created by the impacts higher than 10 km/s is important to investigate the evolution of the planetary surface environment and life. However, no previous experimental study has observed directly it because of experimental difficulties. In this study, we conducted hypervelocity impact experiments using a laser gun and measured the chemical compositions of the impact-generated sulfuric oxides directly. The result clearly shows that the sulfur oxides released by hypervelocity impacts are dominated by SO3, not SO2.