arXiv subjects

Kosuke Kurosawa

Publications and source records attributed to Kosuke Kurosawa.

25 records · Page 2Linked to original sources

arXiv2018

Effects of friction and plastic deformation in shock-comminuted damaged rocks on impact heating

Hypervelocity impacts cause significant heating of planetary bodies. Such events are recorded by a reset of 40Ar-36Ar ages and/or impact melts. Here, we investigate the influence of friction and plastic deformation in shock-generated comminuted rocks on the degree of impact heating using the iSALE shock-physics code. We demonstrate that conversion from kinetic to internal energy in the targets with strength occurs during pressure release, and additional heating becomes significant for low-velocity impacts (<10 km/s). This additional heat reduces the impact-velocity thresholds required to heat the targets with the 0.1 projectile mass to temperatures for the onset of Ar loss and melting from 8 and 10 km/s, respectively, for strengthless rocks to 2 and 6 km/s for typical rocks. Our results suggest that the impact conditions required to produce the unique features caused by impact heating span a much wider range than previously thought.

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arXiv2017

Increase in cratering efficiency with target curvature in strength-controlled craters

Impact-cratering processes on small bodies are thought to be mainly controlled by the local material strength because of their low surface gravity, and craters that are as large as the parent bodies should be affected by the target curvature. Although cratering processes on planar surfaces in the strength-controlled regime have been studied extensively, the mechanism by which target curvature affects the cratering processes remains unclear. Herein, we report on a series of impact experiments that used spherical targets with various diameters. The resultant craters consisted of a deep circular pit and an irregular-shaped spall region around the pit, which is consistent with the features reported in a number of previous cratering experiments on planar surfaces. However, the volume and radius of the craters increased with the normalized curvature. The results indicate that the increase in the spall-region volume and radius mainly contributes to the increase in the whole crater volume and radius, although the volume, depth, and radius of pits remain constant with curvature. The results of our model indicate that the geometric effect due to curvature (i.e., whereby the distance from the equivalent center to the target free surface is shorter for higher curvature values) contributes to increases in the cratering efficiency. Our results suggest that the impactors that produce the largest craters (basins) on some asteroids are thus smaller than what is estimated by current scaling laws, which do not take into account the curvature effects.

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arXiv2017

Hydrocode modeling of the spallation process during hypervelocity impacts: Implications for the ejection of Martian meteorites

Hypervelocity ejection of material by impact spallation is considered a plausible mechanism for material exchange between two planetary bodies. We have modeled the spallation process during vertical impacts over a range of impact velocities from 6 to 21 km/s using both grid- and particle-based hydrocode models. The Tillotson equations of state, which are able to treat the nonlinear dependence of density on pressure and thermal pressure in the strongly shocked matter, were used to study the hydrodynamic and thermodynamic response after impacts. The effects of material strength and gravitational acceleration were not considered. A two-dimensional time-dependent pressure field within a 1.5-fold projectile radius from the impact point was investigated in cylindrical coordinates to address the generation of spalled material. A resolution test was also performed to reject ejected materials with peak pressures that were too low due to artificial viscosity. The relationship between ejection velocity veject and peak pressure Ppeak was also derived. Our approach shows that late stage acceleration in an ejecta curtain occurs due to the compressible nature of the ejecta, resulting in an ejection velocity that can be higher than the ideal maximum of the resultant particle velocity after passage of a shock wave. We also calculate the ejecta mass that can escape from a planet like Mars (i.e., veject higher than 5 km/s) that matches the petrographic constraints from Martian meteorites, and which occurs when Ppeak from 30-50 GPa. Although the mass of such ejecta is limited to from 0.1-1 percent of the projectile mass in vertical impacts, this is sufficient for spallation to have been a plausible mechanism for the ejection of Martian meteorites. Finally, we propose that impact spallation is a plausible mechanism for the generation of tektites.

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arXiv2017

A Lower Limit of Atmospheric Pressure on Early Mars Inferred from Nitrogen and Argon Isotopic Compositions

We examine the history of the loss and replenishment of the Martian atmosphere using elemental and isotopic compositions of nitrogen and noble gases. The evolution of the atmosphere is calculated by taking into consideration various processes: impact erosion and replenishment by asteroids and comets, atmospheric escape induced by solar radiation and wind, volcanic degassing, and gas deposition by interplanetary dust particles. Our model reproduces the elemental and isotopic compositions of N and noble gases (except for Xe) in the Martian atmosphere, as inferred from exploration missions and analyses of Martian meteorites. Other processes such as ionization-induced fractionation, which are not included in our model, are likely to make a large contribution in producing the current Xe isotope composition. Since intense impacts during the heavy bombardment period greatly affect the atmospheric mass, the atmospheric pressure evolves stochastically. Whereas a dense atmosphere preserves primitive isotopic compositions, a thin atmosphere on early Mars is severely influenced by stochastic impact events and following escape-induced fractionation. The onset of fractionation following the decrease in atmospheric pressure is explained by shorter timescales of isotopic fractionation under a lower atmospheric pressure. The comparison of our numerical results with the less fractionated N ($^15$N/$^14$N) and Ar ($^38$Ar/$^36$Ar) isotope compositions of the ancient atmosphere recorded in the Martian meteorite Allan Hills 84001 provides a lower limit of the atmospheric pressure in 4 Ga to preserve the primitive isotopic compositions. We conclude that the atmospheric pressure was higher than approximately 0.5 bar at 4 Ga.

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arXiv2016

Recovery of entire shocked samples in a range of pressure from ~100 GPa to Hugoniot Elastic Limit

We carried out laser shock experiments and wholly recovered shocked olivine and quartz samples. We investigated the petrographic features based on optical micrographs of sliced samples and found that each recovered sample comprises three regions, I (optically dark), II (opaque) and III (transparent). Scanning electron microscopy combined with electron back-scattered diffraction shows that there are no crystal features in the region I; the materials in the region I have once melted. Moreover, numerical calculations performed with the iSALE shock physics code suggest that the boundary between regions II and III corresponds to Hugoniot Elastic Limit (HEL). Thus, we succeeded in the recovery of the entire shocked samples experienced over a wide range of pressures from HEL (~ 10 GPa) to melting pressure (~ 100 GPa) in a hierarchical order.

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arXiv2015

Impact-driven planetary desiccation: The origin of the dry Venus

The fate of surface water on Venus is one of the most important outstanding problems in comparative planetology. Here a new concept is proposed to explain water removal on a steam-covered proto Venus, referred to as impact-driven planetary desiccation. Since a steam atmosphere is photochemically unstable, water vapor dissociates into hydrogen and oxygen. Then, hydrogen escapes easily into space through hydrodynamic escape driven by strong extreme ultraviolet radiation from the young Sun. The focus is on the intense impact bombardment during the terminal stage of planetary accretion as generators of a significant amount of reducing agent. The fine-grained ejecta remove the residual oxygen, the counter part of escaped hydrogen, via the oxidation of iron-bearing rocks in a hot atmosphere. Thus, hypervelocity impacts cause net desiccation of the planetary surface. I constructed a stochastic cratering model using a Monte Carlo approach to investigate the cumulative mass of nonoxidized, ejected rocks due to the intense impact bombardment. It is shown that a thick steam atmosphere with a mass equivalent to that of the terrestrial oceans would be removed. The cumulative mass of rocky ejecta released into the atmosphere reaches 1 wt% of the host planet, which is 10000 times of the current mass of the Earths atmosphere. These results strongly suggest that chemical reactions between such large amounts of ejecta and planetary atmospheres are among the key factors required to understand atmospheric mass and its composition, not only in the Solar System but also in extrasolar systems.

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arXiv2015

Planetary and meteoritic Mg/Si and d30Si variations inherited from solar nebula chemistry

The bulk chemical compositions of planets are uncertain, even for major elements such as Mg and Si. This is due to the fact that the samples available for study all originate from relatively shallow depths. Comparison of the stable isotope compositions of planets and meteorites can help overcome this limitation. Specifically, the non-chondritic Si isotope composition of the Earth's mantle was interpreted to reflect the presence of Si in the core, which can also explain its low density relative to pure Fe-Ni alloy. However, we have found that angrite meteorites display a heavy Si isotope composition similar to the lunar and terrestrial mantles. Because core formation in the angrite parent-body (APB) occurred under oxidizing conditions at relatively low pressure and temperature, significant incorporation of Si in the core is ruled out as an explanation for this heavy Si isotope signature. Instead, we show that equilibrium isotopic fractionation between gaseous SiO and solid forsterite at 1370 K in the solar nebula could have produced the observed Si isotope variations. Nebular fractionation of forsterite should be accompanied by correlated variations between the Si isotopic composition and Mg/Si ratio following a slope of 1, which is observed in meteorites. Consideration of this nebular process leads to a revised Si concentration in the Earth's core of 3.6 (+6.0/-3.6) wt% and provides estimates of Mg/Si ratios of bulk planetary bodies.

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