Microstructural and chemical features of impact melts on Ryugu particle surfaces: Records of interplanetary dust hit on asteroid Ryugu.

The Hayabusa2 spacecraft delivered samples of the carbonaceous asteroid Ryugu to Earth. Some of the sample particles show evidence of micrometeoroid impacts, which occurred on the asteroid surface. Among those, particles A0067 and A0094 have flat surfaces on which a large number of microcraters and impact melt splashes are observed. Two impact melt splashes and one microcrater were analyzed to unveil the nature of the objects that impacted the asteroid surface. The melt splashes consist mainly of Mg-Fe-rich glassy silicates and Fe-Ni sulfides. The microcrater trapped an impact melt consisting mainly of Mg-Fe-rich glassy silicate, Fe-Ni sulfides, and minor silica-rich glass. These impact melts show a single compositional trend indicating mixing of Ryugu surface materials and impactors having chondritic chemical compositions. The relict impactor in one of the melt splashes shows mineralogical similarity with anhydrous chondritic interplanetary dust particles having a probable cometary origin. The chondritic micrometeoroids probably impacted the Ryugu surface during its residence in a near-Earth orbit.

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.

Discovery of fossil asteroidal ice in primitive meteorite Acfer 094.

Carbonaceous chondrites are meteorites believed to preserve our planet's source materials, but the precise nature of these materials still remains uncertain. To uncover pristine planetary materials, we performed synchrotron radiation-based x-ray computed nanotomography of a primitive carbonaceous chondrite, Acfer 094, and found ultraporous lithology (UPL) widely distributed in a fine-grained matrix. UPLs are porous aggregates of amorphous and crystalline silicates, Fe─Ni sulfides, and organics. The porous texture must have been formed by removal of ice previously filling pore spaces, suggesting that UPLs represent fossils of primordial ice. The ice-bearing UPLs formed through sintering of fluffy icy dust aggregates around the H2O snow line in the solar nebula and were incorporated into the Acfer 094 parent body, providing new insight into asteroid formation by dust agglomeration.

Sample preparation toward seamless 3D imaging technique from micrometer to nanometer scale.

Three-dimensional (3D) imaging techniques, such as x-ray computed tomography (XCT), serial sectioning method, transmission electron microtomography (TEMT) and 3D atom probe (3DAP), provides 3D internal structures and external form of objects. In order to obtain the 3D images of one object from the synchrotron-XCT (SR-XCT), FIB-SEM serial sectioning, TEMT and 3DAP, in the present study, the common sample holder and improvement in the TEM tomography retainer were made. We report the sample holder, the TEM retainer, and the sample preparation method using focused ion beam (FIB) and show the 3D images obtained from SR-XCT, FIB-SEM and TEMT of quartz sample containing fluid inclusions.The present common sample holder was made from tungsten needle and copper pipe. The tungsten needle was made from the wire by electropolishing in aqueous ammonia and salt as molten material. A micro-sample of quartz containing fluid inclusions was picked up from the thin section using a focused ion beam (FIB) system (FEI, Quanta 200 3DS), Kyoto University. The FIB system used a Ga(+) ion gun at the condition of 30 kV and 3-65 nA. After a specific area (ca. several ten µm on a side) of the quartz was cut out to a depth of 10 - 30 µm by FIB, it was held at a tip of tungsten needle with platinum deposition (Figure 1a) [1]. Then it was observed by imaging tomography system using a Frenel zone plate at BL47XU, SPring-8, Japan [2]. The size of voxel (pixel in 3D) was 50-80 nm, which gave the effective spatial resolution of ∼200 nm. The characteristic of this method (FIB-XCT) is that the XCT sample can be exactly picked up from a specific area from thin section and bulk specimen after the observation using optical microscopy and/or scanning electron microscopy (SEM). After the FIB-XCT observation, the sample held at a tip of tungsten needle was directly inserted into the FIB-SEM system and the cross-section surface were observed by FIB-SEM. Figure 1b shows a snap shot of the cross-section surface. 3D image was reconstructed from the obtained surface image series and the spatial resolution of this 3D image is higher than XCT although it is depended on the spatial resolution of FIB and SEM. When we obtain TEMT image from the specific area after FIB-XCT and FIB-SEM, TEMT specimen is picked up during FIB-SEM processing and is attached to another tungsten needle. TEMT specimen can be also made from the remaining sample of FIB-SEM held at same holder. Then TEMT specimen was carefully milled into a sharp rod-form with annular-mask by FIB (Figure 1c). This specimen can be analysis by 3DAP. The tungsten needle with rod specimen at the top was mounted in the specimen retainer which was shaped by nipper to achieve the ± 90 ° tilt without any impedimenta and was dug the channel by femto-second laser system to fix the tungsten needle. TEMT experiments were carried out using a JEM-2100F (JEOL LTD., Japan) at Kyoto University. The acceleration voltage was 200kV. The inclusion of about 20 to 100 nm in quartz, which could not be detected by FIB-XCT, was clearly identified in a TEM and TEMT image (Figure 1d).jmicro;63/suppl_1/i24/DFU055F1F1DFU055F1Fig. 1.(a) FIB-XCT sample held at a tip of tungsten needle. (b) A snap shot of the cross-section surface by FIB-SEM at tilt = 52°. The hole observed in the center is the trace of fluid inclusion. (c) Sharp rod-formed TEMT specimen at a tip of tungsten needle. (d) Bright field TEM image of quartz TEMT specimen. The inclusion of about 50 × 100 nm (inside of dashed line circle) was observed in quartz.

Three-dimensional structure of Hayabusa samples: origin and evolution of Itokawa regolith.

Regolith particles on the asteroid Itokawa were recovered by the Hayabusa mission. Their three-dimensional (3D) structure and other properties, revealed by x-ray microtomography, provide information on regolith formation. Modal abundances of minerals, bulk density (3.4 grams per cubic centimeter), and the 3D textures indicate that the particles represent a mixture of equilibrated and less-equilibrated LL chondrite materials. Evidence for melting was not seen on any of the particles. Some particles have rounded edges. Overall, the particles' size and shape are different from those seen in particles from the lunar regolith. These features suggest that meteoroid impacts on the asteroid surface primarily form much of the regolith particle, and that seismic-induced grain motion in the smooth terrain abrades them over time.