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1. Chondrule-like objects and Ca-Al-rich inclusions in Ryugu may potentially be the oldest Solar System materials

   https://doi.org/10.1038/s41467-023-36268-8  

   Nakashima, Daisuke; Nakamura, Tomoki; Zhang, Mingming; Kita, Noriko T.; Mikouchi, Takashi; Yoshida, Hideto; Enokido, Yuma; Morita, Tomoyo; Kikuiri, Mizuha; Amano, Kana; et al (February 2023, Nature Communications)

   Abstract Chondrule-like objects and Ca-Al-rich inclusions (CAIs) are discovered in the retuned samples from asteroid Ryugu. Here we report results of oxygen isotope, mineralogical, and compositional analysis of the chondrule-like objects and CAIs. Three chondrule-like objects dominated by Mg-rich olivine are¹⁶O-rich and -poor with Δ¹⁷O (=δ¹⁷O – 0.52 × δ¹⁸O) values of ~ –23‰ and ~ –3‰, resembling what has been proposed as early generations of chondrules. The¹⁶O-rich objects are likely to be melted amoeboid olivine aggregates that escaped from incorporation into¹⁶O-poor chondrule precursor dust. Two CAIs composed of refractory minerals are¹⁶O-rich with Δ¹⁷O of ~ –23‰ and possibly as old as the oldest CAIs. The discovered objects (<30 µm) are as small as those from comets, suggesting radial transport favoring smaller objects from the inner solar nebula to the formation location of the Ryugu original parent body, which is farther from the Sun and scarce in chondrules. The transported objects may have been mostly destroyed during aqueous alteration in the Ryugu parent body. 

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2. Modeling Chondrule Dust Rim Growth with Ellipsoidal Monomers

   https://doi.org/10.3847/1538-4357/acc6c9  

   Xiang, C.; Carballido, A.; Matthews, L. S.; Hyde, T. W. (June 2023, The Astrophysical Journal)

   Abstract Fine-grained dust rims (FGRs) surrounding chondrules in carbonaceous chondrites encode important information about early processes in the solar nebula. Here, we investigate the effect of the nebular environment on FGR porosity, dust size distribution, and grain alignment, comparing the results for rims comprised of ellipsoidal and spherical grains. We conduct numerical simulations in which FGRs grow by collisions between dust particles and chondrules in both neutral and ionized turbulent gas. The resultant rim morphology is related to the ratioϵof the electrostatic potential energy at the collision point to the relative kinetic energy between colliding particles. In general, largeϵleads to a large rim porosity, large rim grain size, and low growth rate. Dust rims comprised of ellipsoidal monomers initially grow faster in thickness than rims comprised of spherical monomers, due to their higher porosity. As the rims grow and obtain a greater electrostatic potential, repulsion becomes dominant, and this effect is reversed. Grain size coarsening toward the outer regions of the rims is observed for low- and high-ϵregimes, and is more pronounced in the ellipsoidal case, while for the medium-ϵregime, small monomers tend to be captured in the middle of the rims. In neutral environments, ellipsoidal grains have random orientations within the rim, while in charged environments ellipsoidal grains tend to align with maximum axial alignment forϵ= 0.15. The characterization of these FGR features provides a means to relate laboratory measurements of chondrite samples to the formation environment of the parent bodies. 

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3. Hydration of Nebular Minerals through the Implantation–Diffusion Process

   https://doi.org/10.3847/1538-4357/abf839  

   Jin, Ziliang; Bose, Maitrayee (June 2021, The Astrophysical Journal)

   Abstract Recent studies have detected structurally bound water in the refractory silicate minerals present in ordinary and enstatite chondrite meteorites. The mechanism for the incorporation of the hydrogen is not well defined. In this paper we quantitatively examine a two-fold process involving the implantation and diffusion of nebular hydrogen ions that is responsible for the hydration of the chondritic minerals. Our simulations show that depending on critical parameters, including the flux of the protons in nebular plasma, retention coefficient, temperature of the silicate minerals, and desorption rate of implanted hydrogen, the implantation of low-energy hydrogen ions can result in equivalent water contents of ∼0.1 wt% in chondritic silicates within 10 years. Thus, this novel mechanism operating in the nebula at 10 −3 bar pressure and <650 K temperatures can efficiently hydrate the free-floating chondritic minerals prior to the rapid formation of planetesimals inside the snow line, and agree well with the wet accretion scenario for the inner solar system objects. 

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4. Formation and evolution of carbonaceous asteroid Ryugu: Direct evidence from returned samples

   https://doi.org/10.1126/science.abn8671  

   Nakamura, T.; Matsumoto, M.; Amano, K.; Enokido, Y.; Zolensky, M. E.; Mikouchi, T.; Genda, H.; Tanaka, S.; Zolotov, M. Y.; Kurosawa, K.; et al (September 2022, Science)

   Samples of the carbonaceous asteroid Ryugu were brought to Earth by the Hayabusa2 spacecraft. We analyzed seventeen Ryugu samples measuring 1-8 mm. CO 2 -bearing water inclusions are present within a pyrrhotite crystal, indicating that Ryugu’s parent asteroid formed in the outer Solar System. The samples contain low abundances of materials that formed at high temperatures, such as chondrules and Ca, Al-rich inclusions. The samples are rich in phyllosilicates and carbonates, which formed by aqueous alteration reactions at low temperature, high pH, and water/rock ratios < 1 (by mass). Less altered fragments contain olivine, pyroxene, amorphous silicates, calcite, and phosphide. Numerical simulations, based on the mineralogical and physical properties of the samples, indicate Ryugu’s parent body formed ~ 2 million years after the beginning of Solar System formation. 

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5. Late Pebble Accretion of Comet 81P/Wild 2 Nucleus: Evidence from a Plagioclase-bearing Chondrule Fragment, Pyxie

   https://doi.org/10.3847/1538-4357/ad55cc  

   Zhang, Mingming; Zolensky, Michael_E; Fukuda, Kohei; Nakashima, Daisuke; Weisberg, Michael_K; Kita, Noriko_T (August 2024, The Astrophysical Journal)

   Abstract Comet 81P/Wild 2 is a ∼4.5 km-sized primordial object that almost has not been modified by internal heating by²⁶Al decay. Its nucleus could have been formed by hierarchical agglomeration or gravitational collapse of pebble swarms concentrated by streaming instability. To shed light on the cometesimal formation mechanism from laboratory sample analysis, we reexamined the²⁶Al–²⁶Mg isotope systematics of the plagioclase-bearing fragment, Pyxie (from Wild 2 track 81), with significantly improved analytical precision. The revised upper limit of the initial (²⁶Al/²⁷Al)₀of Pyxie is ≤1.5 × 10⁻⁶, 2 times smaller than those estimated from other Wild 2 fragments. Assuming homogenous distribution of²⁶Al in the early solar system, the minimum crystallization age of Pyxie is estimated to be >3.6 Ma after calcium–aluminum-rich inclusions. Additional petrologic examination demonstrated that it is a chondrule fragment formed in disk environments enriched in moderately volatile elements comparable to the Si-rich rim of CR chondrules before accreting by comet Wild 2. The late accretion of the Wild 2 nucleus with most silicates likely from a common source are not favored by the hierarchical agglomeration model that considers early and continuous accretion. Instead, the results are more in line with comet formation by gentle gravitational collapse of pebbles when the²⁶Al abundance is extremely low (²⁶Al/²⁷Al ≤ 1.5 × 10⁻⁶) before gas dispersal. 

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