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Shocked meteorites can be used to probe the dynamics of the early Solar system. Carbonaceous chondrites are less shocked than ordinary chondrites, regardless of the degree of aqueous alteration. Here, we show that this shock metamorphic dichotomy is a consequence of impact-driven oxidation of organics that are abundant in carbonaceous but not ordinary chondrites. Impact experiments at 3–7 km s −1 using analogs of chondrite matrices reveal evidence of local heating in the matrix up to ~2000 K. Impacts on carbonaceous asteroids cause explosive release of CO and/or CO 2 , which can efficiently remove evidence of shock. We show that highly shocked materials are lost to space from typical-sized chondrite parent bodies (~100 km in diameter), but are retained on the largest known carbonaceous asteroid, namely, (1) Ceres, due to its stronger gravity. Ceres’ surface is thus a witness plate for the ancient impact environment of carbonaceous chondrite parent bodies. Due to its strong gravity, Ceres’ surface still retains evidence of past collisions, which have been lost via impact-driven oxidation of organic material from typical C-rich asteroids, according to impact experiments

Neumann band in iron meteorites, which is deformation twins in kamacite (Fe–Ni alloy), has been known to be a characteristic texture indicating ancient collisions on parent bodies of meteorites. We conducted a series of shock recovery experiments on bcc iron with the projectile velocity at 1.5 km/s at various initial temperatures, room temperature, 670 K, and 1100 K, and conducted an annealing experiment on the shocked iron. We also conducted numerical simulations with the iSALE-2D code to investigate peak pressure and temperature distributions in the nontransparent targets. The effects of pressure and temperature on the formation and disappearance of the twins (Neumann band) were explored based on laboratory and numerical experiments. The twin was formed in the run products of the experiments conducted at room temperature and 670 K, whereas it was not observed in the run product formed by the impact at 1100 K. The present experiments combined with the numerical simulations revealed that the twin was formed by impacts with various shock pressures from 1.5–2 GPa to around 13 GPa. The twin in iron almost disappeared by annealing at 1070 K. The iron meteorites with Neumann bands were shocked at this pressure range and temperatures at least up to 670 K, and were not heated to the temperatures above 1070 K after the Neumann band formation.

Basaltic rocks occur widely on the terrestrial planets and differentiated asteroids, including the asteroid 4 Vesta. We conducted a shock recovery experiment with decaying compressive pulses on a terrestrial basalt at the Chiba Institute of Technology, Japan. The sample recorded a range of pressures, and shock physics modeling was conducted to add a pressure scale to the observed shock features. The shocked sample was examined by optical and electron microscopy, electron back‐scattered diffractometry, and Raman spectroscopy. We found that localized melting occurs at a lower pressure (∼10 GPa) than previously thought (>20 GPa). The shocked basalt near the epicenter represents “shock degree C” of a recently proposed classification scheme for basaltic eucrites and, as such, our results provide a pressure scale for the classification scheme. Finally, we estimated the total fraction of the basaltic eucrites classified as shock degree C to be ∼15% by assuming the impact velocity distribution onto Vesta. Plain Language Summary Basaltic rocks occur on numerous planetary bodies, including Mars, the Moon, and the asteroid Vesta. Shock metamorphic features in meteorites from such bodies are the ancient imprints of past impact events. We can extract information about the bombardment histories experienced by such bodies if we have an accurate method to link the degree of metamorphism to the impact conditions. Although two such methods for basaltic rocks have been published, one of these does not have a scale that relates the shock features and peak pressures. In this study, we designed an impact experiment with a terrestrial basalt sample to add a pressure scale to one of these methods. We found that basaltic materials are more easily melted than previously expected. The shock features of our shocked sample match “shock degree C.” The required pressure for producing the materials classified into this shock degree is 1–2 × 105 times greater than atmospheric pressure. Our results may provide insights into impact processes on Vesta. We estimate that the total fraction of meteorites from Vesta classified into shock degree C is ∼15%. Key Points We investigated the shock effects in basaltic rocks with impact experiments and shock physics modeling We added a pressure scale to the shock degree classification for basaltic eucrites, allowing us to link our results with the Stöffler table Localized melting occurs from 10 GPa rather than 20 GPa as previously thought

Carbonaceous asteroids, including Ryugu and Bennu, which have been explored by the Hayabusa2 and OSIRIS-REx missions, were probably important carriers of volatiles to the inner Solar System. However, Ryugu has experienced significant volatile loss, possibly from hypervelocity impact heating. Here we present impact experiments at speeds comparable to those expected in the main asteroid belt (3.7 km s −1 and 5.8 km s −1 ) and with analogue target materials. We find that loss of volatiles from the target material due to impacts is not sufficient to account for the observed volatile depletion of Ryugu. We propose that mutual collisions in the main asteroid belt are unlikely to be solely responsible for the loss of volatiles from Ryugu or its parent body. Instead, we suggest that additional processes, for example associated with the diversity in mechanisms and timing of their formation, are necessary to account for the variable volatile contents of carbonaceous asteroids.

Extraterrestrial minerals on the surface of airless Solar System bodies undergo gradual alteration processes known as space weathering over long periods of time. The signatures of space weathering help us understand the phenomena occurring in the Solar System. However, meteorites rarely retain the signatures, making it impossible to study the space weathering processes precisely. Here, we examine samples retrieved from the asteroid Ryugu by the Hayabusa2 spacecraft and discover the presence of nonmagnetic framboids through electron holography measurements that can visualize magnetic flux. Magnetite particles, which normally provide a record of the nebular magnetic field, have lost their magnetic properties by reduction via a high-velocity (>5 km s –1 ) impact of a micrometeoroid with a diameter ranging from 2 to 20 μm after destruction of the parent body of Ryugu. Around these particles, thousands of metallic-iron nanoparticles with a vortex magnetic domain structure, which could have recorded a magnetic field in the impact event, are found. Through measuring the remanent magnetization of the iron nanoparticles, future studies are expected to elucidate the nature of the nebular/interplanetary magnetic fields after the termination of aqueous alteration in an asteroid. Electron holography discovered nonmagnetic framboids and many iron nanoparticles with a vortex magnetic flux formed by magnetite reduction due to a micrometeoroid impact on asteroid Ryugu, providing a new way to study the Solar System magnetic field.

The farside gravity field of the Moon is improved from the tracking data of the Selenological and Engineering Explorer (SELENE) via a relay subsatellite. The new gravity field model reveals that the farside has negative anomaly rings unlike positive anomalies on the nearside. Several basins have large central gravity highs, likely due to super-isostatic, dynamic uplift of the mantle. Other basins with highs are associated with mare fill, implying basalt eruption facilitated by developed faults. Basin topography and mantle uplift on the farside are supported by a rigid lithosphere, whereas basins on the nearside deformed substantially with eruption. Variable styles of compensation on the near- and farsides suggest that reheating and weakening of the lithosphere on the nearside was more extensive than previously considered.

Knowledge of pressure conditions is essential in understanding phase changes and chemical reactions. Although several methods have been proposed for the measurement of pressure in impact‐induced vapor plumes, they are somewhat unreliable. In this study, we developed a pressure measurement method for vapor clouds based on spectral line broadening. Under the nearest neighbor approximation, it is possible to analytically express the full width at half maximum as a function of the perturber number density. On the basis of these relations and spectroscopic constants of atomic emission lines of Fe I at 381.58 nm and Ca I at 646.26 nm, quadratic Stark broadening is the dominant broadening mechanism if the degree of ionization exceeds 1%. Resonance broadening and van der Waals broadening should be considered if the degree of ionization is less than 1%. Taking into account the appropriate broadening mechanisms, it is possible to accurately measure the pressure in vapor clouds. We conducted laser ablation experiments using the proposed pressure measurement method in combination with the Boltzmann plot method for temperature. The obtained pressures and temperatures are consistent with an adiabatic expansion. This strongly suggests that the proposed method can measure pressure of vapor clouds accurately. Because thermodynamic quantities can be determined when both pressure and temperature are known, the proposed method enables a complete thermodynamic description of vapor clouds, thereby serving as a powerful tool in investigating the thermodynamic and chemical properties of impact‐ and laser‐induced vapor clouds.

Radiation phenomena are usually observed during fracture of quartz-bearing rocks. Since quartz is a piezoelectric material, the associated electrical processes such as the electrification of fracture surface and the flight of electrons between fracture surfaces should be important for radiation during fractures. In this article, supposing that travelling electrons between crack surfaces cause the radiation, we experimentally investigate X-ray emission in a vacuum and visible-light emission in the atmosphere during rock and mineral fracture and verify the consistency of both emissions. The number of electrons in flight between surfaces during fracture that result in X-ray is estimated and the comparison with the number of photons in visible light suggests that one electron repeatedly collides with N2 molecules. The estimated number of collisions resulting in a visible-light emission is slightly less than the expected upper limit. This is reasonable because the collision would cause the light emission not always in the wavelengths of visible light. Moreover, the number of electrons resulting in X-rays is comparable with the number of electrons resulting in the emission of radio waves during fracture obtained in previous studies. Thus, we conclude that the radiations during fracture can be attributed to the flight of electrons between fracture surfaces. Finally, we evaluate the feasibility of observing the X-ray emission in planetary exploration and the radio waves and the visible light in natural earthquakes and find that these radiations are observable.

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

In order to investigate impact production of carbonaceous products by asteroids on Titan and other satellites and planets, simulation experiments were carried out using a 2-stage light gas gun. A small polycarbonate or metal bullet with about 6.5 km/s was injected into a pressurized target chamber filled with 1 atm of nitrogen gas, to collide with a ice + iron target or an iron target or a ice + hexane + iron target. After the impact, black soot including fine particles was deposited on the chamber wall. The soot was carefully collected and analyzed by High Performance Liquid Chromatography (HPLC), Fourier Transform Infrared Spectroscopy (FT-IR), and Laser Desorption Time-of-Flight Mass Spectrometry (LD-ToF-MS). As a result of the HPLC analysis, about 0.04–8 pmol of glycine, and a lesser amount of alanine were found in the samples when the ice + hexane + iron target was used. In case of the ice + iron target and the iron target, less amino acids were produced. The identification of the amino acids was also supported by FTIR and LD-ToF-MS analysis.