Incipient space weathering on asteroid 162173 Ryugu recorded by pyrrhotite

Regolith samples returned from asteroid 162173 Ryugu by the Hayabusa2 mission provide direct means to study how space weathering operates on the surfaces of hydrous asteroids. The mechanisms of space weathering, its effects on mineral surfaces, and the characteristic time scales on which alteration occurs are central to understanding the spectroscopic properties and the taxonomy of asteroids in the solar system. Here, we investigate the behavior of the iron monosulfides mineral pyrrhotite (Fe1−xS) at the earliest stages of space weathering. Using electron microscopy methods, we identified a partially exposed pyrrhotite crystal that morphologically shows evidence for mass loss due to exposure to solar wind ion irradiation. We find that crystallographic changes to the pyrrhotite can be related to sulfur loss from its space‐exposed surface and the diffusive redistribution of resulting excess iron into the interior of the crystal. Diffusion profiles allow us to estimate an order of magnitude of the exposure time of a few thousand years consistent with previous estimates of space exposure. During this interval, the adjacent phyllosilicates did not acquire discernable damage, suggesting that they are less susceptible to alteration by ion irradiation than pyrrhotite

Influx of nitrogen-rich material from the outer Solar System indicated by iron nitride in Ryugu samples

Large amounts of nitrogen compounds, such as ammonium salts, may be stored in icy bodies and comets, but the transport of these nitrogen-bearing solids into the near-Earth region is not well understood. Here, we report the discovery of iron nitride on magnetite grains from the surface of the near-Earth C-type carbonaceous asteroid Ryugu, suggesting inorganic nitrogen fixation. Micrometeoroid impacts and solar wind irradiation may have caused the selective loss of volatile species from major iron-bearing minerals to form the metallic iron. Iron nitride is a product of nitridation of the iron metal by impacts of micrometeoroids that have higher nitrogen contents than the CI chondrites. The impactors are probably primitive materials with origins in the nitrogen-rich reservoirs in the outer Solar System. Our observation implies that the amount of nitrogen available for planetary formation and prebiotic reactions in the inner Solar System is greater than previously recognized

Four‐dimensional‐STEM analysis of the phyllosilicate‐rich matrix of Ryugu samples

Ryugu asteroid grains brought back to the Earth by the Hayabusa2 space mission are pristine samples containing hydrated minerals and organic compounds. Here, we investigate the mineralogy of their phyllosilicate-rich matrix with four-dimensional scanning transmission electron microscopy (4D-STEM). We have identified and mapped the mineral phases at the nanometer scale (serpentine, smectite, pyrrhotite), observed the presence of Ni-bearing pyrrhotite, and identified the serpentine polymorph as lizardite, in agreement with the reported aqueous alteration history of Ryugu. Furthermore, we have mapped the d-spacings of smectite and observed a broad distribution of values, ranging from 1 to 2 nm, with an average d-spacing of 1.24 nm, indicating significant heterogeneity within the sample. Such d-spacing variability could be the result of either the presence of organic matter trapped in the interlayers or the influence of various geochemical conditions at the submicrometer scale, suggestive of a range of organic compounds and/or changes in smectite crystal chemistry

A dehydrated space-weathered skin cloaking the hydrated interior of Ryugu

Without a protective atmosphere, space-exposed surfaces of airless Solar System bodies gradually experience an alteration in composition, structure and optical properties through a collective process called space weathering. The return of samples from near-Earth asteroid (162173) Ryugu by Hayabusa2 provides the first opportunity for laboratory study of space-weathering signatures on the most abundant type of inner solar system body: a C-type asteroid, composed of materials largely unchanged since the formation of the Solar System. Weathered Ryugu grains show areas of surface amorphization and partial melting of phyllosilicates, in which reduction from Fe3+ to Fe2+ and dehydration developed. Space weathering probably contributed to dehydration by dehydroxylation of Ryugu surface phyllosilicates that had already lost interlayer water molecules and to weakening of the 2.7 µm hydroxyl (–OH) band in reflectance spectra. For C-type asteroids in general, this indicates that a weak 2.7 µm band can signify space-weathering-induced surface dehydration, rather than bulk volatile loss

Radiation Damage Behavior in Multiphase Ceramics (YSZ, Al2O3 and MgAl2O4) and the Effect of Heterointerfaces

Current technology ceramics for radiation environments rely on their intrinsic properties for damage tolerance, but the they can be further improved by designing the microstructure for use in applications for space, nuclear fission/fusion and nuclear waste environments. Interfaces are known to be sinks for point defects, which enhance radiation damage tolerance, and more disordered interfaces work better than clean interfaces. Heterointerfaces, or interfaces between dissimilar phases, are expected to have more disorder than grain boundaries in a single-phase system. In this study, radiation damage behavior of a multiphase ceramic material with heterointerfaces and submicron grains was systematically investigated and compared to single phase polycrystalline materials with grain boundaries. The multiphase composite survived a high fluence while single phase materials fractured. X-ray diffraction and transmission electron microscopy analysis showed more efficient point defect annihilation in the multiphase system than in the single-phase system. This work shows that high radiation damage tolerance can be achieved without complicated careful grain boundary modification or ultra nano grain fabrication, which is difficult to scale up and unreliable at elevated temperatures

Radiation Damage Behavior in Multiphase Ceramics (YSZ, Al2O3 and MgAl2O4) and the Effect of Heterointerfaces

Current technology ceramics for radiation environments rely on their intrinsic properties for damage tolerance, but the they can be further improved by designing the microstructure for use in applications for space, nuclear fission/fusion and nuclear waste environments. Interfaces are known to be sinks for point defects, which enhance radiation damage tolerance, and more disordered interfaces work better than clean interfaces. Heterointerfaces, or interfaces between dissimilar phases, are expected to have more disorder than grain boundaries in a single-phase system. In this study, radiation damage behavior of a multiphase ceramic material with heterointerfaces and submicron grains was systematically investigated and compared to single phase polycrystalline materials with grain boundaries. The multiphase composite survived a high fluence while single phase materials fractured. X-ray diffraction and transmission electron microscopy analysis showed more efficient point defect annihilation in the multiphase system than in the single-phase system. This work shows that high radiation damage tolerance can be achieved without complicated careful grain boundary modification or ultra nano grain fabrication, which is difficult to scale up and unreliable at elevated temperatures