Tomoki Nakamura

Most carbonaceous chondrite (CC) meteorites contain abundant chondrules, millimeter-scale spherical inclusions of previously molten material. Ivuna-type carbonaceous chondrites (CIs) do not contain chondrules, nor do samples of the carbonaceous asteroid Ryugu. CIs and Ryugu share isotopic properties distinct from those of other CCs, implying that their parent bodies formed at a different time or place than those of other CCs. We applied manganese-chromium dating and oxygen isotope thermometry to Ryugu and Ivuna samples. The results indicate that the parent bodies of Ryugu and Ivuna formed less than 2 million years after the beginning of Solar System formation—before the majority of chondrules in CCs and the parent bodies of other CCs formed. This implies that Ryugu and CIs are remnants of an earlier generation of carbonaceous planetesimals.

Reflectance spectra from the surface of asteroid Ryugu, measured by the Hayabusa2 spacecraft, exhibit a metal-OH absorption at ∼2.7 μ m that is approximately half as deep as the returned samples measured in the laboratory and shifted toward longer wavelengths. These spectral differences likely result from the effects of space weathering; however, this interpretation has not been confirmed. In this study, we conducted a detailed observation of the surface of the Ryugu sample A0283, which shows reflectance spectra indicative of space weathering. Electron microscope observations on the A0283 surface identified space-weathering features (e.g., smooth and frothy layers and melt splashes) as previously reported. At these modified surfaces, the 2.7 μ m band was approximately 50% shallower than that from unweathered surfaces and the position shifted up to 8 nm longer wavelengths. Additionally, the Reststrahlen band of phyllosilicates at ∼10 μ m shifted by up to 0.8 μ m from the position of unweathered samples, and its peak broadened. In particular, the regions covered by the frothy layer exhibited significant spectral changes. The frothy layer consists of vesicular amorphous silicate, with a thickness of tens to thousands of nanometers, and exhibits a lower Mg/Fe atomic ratio than the interior. These results indicate that space weathering causes amorphization, dehydration, Mg sputtering of the asteroid surface, and consequently, modification of reflectance spectra of asteroid Ryugu. In contrast, the collected samples are dominated by newly exposed surfaces, resulting in the observed spectral differences between the asteroid surface and its returned samples.

The goal of the MMX mission is to determine the origin and the formation process of the two Martian moons (Kuramoto et al. 2022). Three years remote sensing observation of Phobos and Deimos (Nakamura et al .2021) will provide us mineralogical and compositional signatures of whole sphere of the moons and thus we will obtain strong constraints on the origin. Subsequent laboratory analysis of the returned samples from Phobos will terminate the long-standing problem of the origin of Martian moons. In some cases, we can determine the origin based on only remote sensing observation. If Phobos has a high abundance of hydrous minerals, a high hydrogen concentration, a chondritic global chemical composition, and a reflectance spectrum similar to hydrous carbonaceous meteorites, then Phobos is made of primitive C or D-type asteroidal material. In this case, Phobos could be a captured asteroid coming from beyond Mars, or if it was formed by an asteroid impacting Mars, the impact would have been weak to maintain chondiritic composition.The important scientific data to determine the origin of Phobos and Deimos are: global and local elemental abundances of Phobos (Laurence et al. 2019) with enough precision to distinguish chondritic or non-chondritic composition (including Martian components or not), the high resolution map of visible and near-infrared reflectance spectrra to explore the presence or absence of hydrated silicates on Phobos and Deimos (Barucci et al. 2021), and chemical, mineralogical and isotope composition of the returned samples to determine to determine whether Phobos is primordial solar system material or high-temperature heated processed material (Fujiya et al. 2021). If we succeed the remote sensing observations and return sample analyses, the origin and formation process of the Martian moons will be clarified within a few years from the sample return to the Earth expected in 2031. References : Barucci M.A. et al. (2021) Earth, Plan. and Space, 73, 211. Fujiya W. et al. (2021) Earth, Plan. and Space, 73, 1. Kuramoto K. et al. (2022) Earth, Plan. and Space, 74, 12.Lawrence et al. (2019) Earth Space Science, 6, 2605. Nakamura et al. (2021) Earth, Plan. and Space, 73, 227.

Millimeter-sized silicate spherules embedded in primitive meteorites, namely, “chondrules,” are the primary solid component of the early solar nebula. They exhibit distinctive solidification textures, formed through rapid cooling from a molten state. The formation conditions of these textures have primarily been inferred on the basis of dynamic crystallization experiments; however, the theoretical verification of the solidification process has been largely neglected. Here, we conducted numerical simulations of the solidification of chondrule melt and successfully reproduced a crystal growth pattern resembling a typical barred olivine chondrule texture. This pattern emerged under conditions of rapid cooling, exceeding 10 ⁴ kelvins hour ⁻¹ , which is substantially larger than those inferred experimentally. These results suggest that theories of chondrule formation in the nebula, which have been developed based on experimental results, should be reexamined.

It is now possible to bring back samples from planetary bodies of the Solar System other than the moon. This research method enables a direct link between astronomical observations and meteorite analyses, which were previously disconnected. The Hayabusa, Hayabusa2, and OSIRIS REx sample return missions have provided detailed information on the composition of S-, C-, and B-type asteroids, respectively, and the processes by which they were formed. This paper reviews the results of these three asteroid sample return missions, and also introduces the next Martian moon sample return mission MMX. Although sample returns are currently achieved only from near-Earth objects in the Solar System, it is hoped that in the future it will be possible to collect samples from outer Solar System objects and even from small objects flying from outside the Solar System.

The isotopic compositions of samples returned from Cb-type asteroid Ryugu and Ivuna-type (CI) chondrites are distinct from other carbonaceous chondrites, which has led to the suggestion that Ryugu/CI chondrites formed in a different region of the accretion disk, possibly around the orbits of Uranus and Neptune. We show that, like for Fe, Ryugu and CI chondrites also have indistinguishable Ni isotope anomalies, which differ from those of other carbonaceous chondrites. We propose that this unique Fe and Ni isotopic composition reflects different accretion efficiencies of small FeNi metal grains among the carbonaceous chondrite parent bodies. The CI chondrites incorporated these grains more efficiently, possibly because they formed at the end of the disk’s lifetime, when planetesimal formation was also triggered by photoevaporation of the disk. Isotopic variations among carbonaceous chondrites may thus reflect fractionation of distinct dust components from a common reservoir, implying CI chondrites/Ryugu may have formed in the same region of the accretion disk as other carbonaceous chondrites.

Context. Ch-type asteroids are distinctive among other dark asteroids in that they exhibit deep negative polarization branches (NPBs). Nevertheless, the physical and compositional properties that cause their polarimetric distinctiveness are less investigated. Aims. We aim to investigate the polarimetric uniqueness of Ch-type asteroids by making databases of various observational quantities (i.e., spectroscopic and photometric properties as well as polarimetric ones) of dark asteroids. Methods. We conducted an intensive polarimetric survey of 52 dark asteroids (including 31 Ch-type asteroids) in the R_C band to increase the size of polarimetric samples. The observed data were compiled with previous polarimetric, spectroscopic, and photometric archival data to find their correlations. Results. We find remarkable correlations between these observed quantities, particularly the depth of NPBs and their spectroscopic features associated with the hydrated minerals. The amplitude of the opposition effect in photometric properties also shows correlations with polarimetric and spectral properties. However, these observed quantities do not show noticeable correlations with the geometric albedo, thermal inertia, and diameter of asteroids. Conclusions. Based on the observational evidence, we arrive at our conclusion that the submicrometer-sized structures (fibrous or flaky puff pastry-like structures in phyllosilicates) in the regolith particles could contribute to the distinctive NPBs of hydrated asteroids.

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.

Reproduction experiments of radial pyroxene (RP) chondrules were carried out using an Ar–H₂ or Ar gas-jet levitation system in a reducing atmosphere in order to simulate chondrule formation in the protoplanetary disk. The experiments reproduced RP-chondrule textures, consisting of sets of thin pyroxene crystals and mesostasis glass between crystals. However, iron partition coefficients between pyroxene and glassy mesostasis (D_Fe =Fe mol%_pyroxene/Fe mol%_mesostasis) in natural RP chondrules were much higher than that in the experimentally reproduced RP chondrules. The high D_Fe in natural RP chondrules suggests that iron was removed from the mesostasis melt at high temperatures after the growth of pyroxene crystals. We found that many small iron-metal inclusions had formed in the mesostasis glass, indicating that FeO in the high-temperature melt of mesostasis was reduced to metallic iron, and iron in the mesostasis diffused into the newly formed metal inclusions. The formation of the iron-metal inclusions in the mesostasis was reproduced by our experiments in a reducing atmosphere, confirming that D_Fe in natural RP chondrules increased after the growth of RP crystals. Therefore, the D_Fe of RP chondrules can be an indicator to constrain cooling rates and redox states during chondrule formation.

Polycyclic aromatic hydrocarbons (PAHs) contain ≲20% of the carbon in the interstellar medium. They are potentially produced in circumstellar environments (at temperatures ≳1000 kelvin), by (~10 kelvin) interstellar clouds, or by processing of carbon-rich dust grains. We report isotopic properties of PAHs extracted from samples of the asteroid Ryugu and the meteorite Murchison. The doubly- ¹³ C substituted compositions (Δ2× ¹³ C values) of the PAHs naphthalene, fluoranthene, and pyrene are 9 to 51‰ higher than values expected for a stochastic distribution of isotopes. The Δ2× ¹³ C values are higher than expected if the PAHs formed in a circumstellar environment, but consistent with formation in the interstellar medium. By contrast, the PAHs phenanthrene and anthracene in Ryugu samples have Δ2× ¹³ C values consistent with formation by higher-temperature reactions.