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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)

   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. Oxygen and Al‐Mg isotopic compositions of grossite‐bearing refractory inclusions from CO 3 chondrites

   https://doi.org/10.1111/maps.13282  

   Simon, Steven B. ; Krot, Alexander N. ; Nagashima, Kazuhide ( April 2019 , Meteoritics & Planetary Science)

   The distribution of the short‐lived radionuclide²⁶Al in the early solar system remains a major topic of investigation in planetary science. Thousands of analyses are now available but grossite‐bearing Ca‐, Al‐rich inclusions (CAIs) are underrepresented in the database. Recently found grossite‐bearing inclusions inCO3 chondrites provide an opportunity to address this matter. We determined the oxygen and magnesium isotopic compositions of individual phases of 10 grossite‐bearingCAIs in the Dominion Range (DOM) 08006 (CO3.0) andDOM08004 (CO3.1) chondrites. All minerals inDOM08006CAIs as well as hibonite, spinel, and pyroxene inDOM08004 are uniformly¹⁶O‐rich (Δ¹⁷O = −25 to −20‰) but grossite and melilite inDOM08004CAIs are not; Δ¹⁷O of grossite and melilite range from ~ −11 to ~0‰ and from ~ −23 up to ~0‰, respectively. Even within this small suite, in the two chondrites a bimodal distribution of the inferred initial²⁶Al/²⁷Al ratios (²⁶Al/²⁷Al)₀is seen, with four having (²⁶Al/²⁷Al)₀≤1.1 × 10⁻⁵and six having (²⁶Al/²⁷Al)₀≥3.7 × 10⁻⁵. Five of the²⁶Al‐richCAIs have (²⁶Al/²⁷Al)₀within error of 4.5 × 10⁻⁵; these values can probably be considered indistinguishable from the “canonical” value of 5.2 × 10⁻⁵given the uncertainty in the relative sensitivity factor for grossite measured by secondary ion mass spectrometry. We infer that the²⁶Al‐poorCAIs probably formed before the radionuclide was fully mixed into the solar nebula. All minerals in theDOM08006CAIs, as well as spinel, hibonite, and Al‐diopside in theDOM08004CAIs retained their initial oxygen isotopic compositions, indicating homogeneity of oxygen isotopic compositions in the nebular region where theCOgrossite‐bearingCAIs originated. Oxygen isotopic heterogeneity inCAIs fromDOM08004 resulted from exchange between the initially¹⁶O‐rich (Δ¹⁷O ~−24‰) melilite and grossite and¹⁶O‐poor (Δ¹⁷O ~0‰) fluid during hydrothermal alteration on theCOchondrite parent body; hibonite, spinel, and Al‐diopside avoided oxygen isotopic exchange during the alteration. Grossite and melilite that underwent oxygen isotopic exchange avoided redistribution of radiogenic²⁶Mg and preserved undisturbed internal Al‐Mg isochrons. The Δ¹⁷O of the fluid can be inferred from O‐isotopic compositions of aqueously formed fayalite and magnetite that precipitated from the fluid on theCOparent asteroid. This and previous studies suggest that O‐isotope exchange during fluid–rock interaction affected mostCAIs in CO ≥3.1 chondrites.

    

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3. Sierra Gorda 009: A new member of the metal‐rich G chondrites grouplet

   https://doi.org/10.1111/maps.13546  

   Ivanova, Marina A. ; Lorenz, Cyril A. ; Humayun, Munir ; Corrigan, Catherine M. ; Ludwig, Thomas ; Trieloff, Mario ; Righter, Kevin ; Franchi, Ian A. ; Verchovsky, Alexander B. ; Korochantseva, Ekaterina V. ; et al ( August 2020 , Meteoritics & Planetary Science)

   We investigated the metal‐rich chondrite Sierra Gorda (SG) 009, a member of the new G chondrite grouplet (also including NWA 5492, GRO 95551). G chondrites contain 23% metal, very reduced silicates, and rare oxidized mineral phases (Mg‐chromite, FeO‐rich pyroxene). G chondrites are not related to CH‐CB chondrites, based on bulk O, C, and N isotopic compositions, mineralogy, and geochemistry. G chondrites have no fine‐grained matrix or matrix lumps enclosing hydrated material typical for CH‐CB chondrites. G chondrites’ average metal compositions are similar to H chondrites. Siderophile and lithophile geochemistry indicates sulfidization and fractionation of the SG 009 metal and silicates, unlike NWA 5492 and GRO 95551. The G chondrites have average O isotopic compositions Δ¹⁷O>0‰ ranging between bulk enstatite (E) and ordinary (O) chondrites. An Al‐rich chondrule from SG 009 has Δ¹⁷O>0‰ indicating some heterogeneity in oxygen isotopic composition of G chondrite components. SG 009’s bulk carbon and nitrogen isotopic compositions correspond to E and O chondrites. Neon isotopic composition reflects a mixture of cosmogenic and solar components, and cosmic ray exposure age of SG 009 is typical for O, E, and R chondrites. G chondrites are closely related to O, E, and R chondrites and may represent a unique metal‐rich parent asteroid containing primitive and fractionated material from the inner solar system. Oxidizing and reducing conditions during SG 009 formation may be connected with a chemical microenvironment and possibly could indicate that G chondrites may have formed by a planetesimal collision resulting in the lack of matrix.

    

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4. Protracted Timescales for Nebular Processing of First-formed Solids in the Solar System

   https://doi.org/10.3847/1538-4357/acb156  

   Mane, Prajkta ; Bose, Maitrayee ; Wadhwa, Meenakshi ; Defouilloy, Céline ( March 2023 , The Astrophysical Journal)

   Abstract The calcium–aluminum-rich inclusions (CAIs) from chondritic meteorites are the first solids formed in the solar system. Rim formation around CAIs marks a time period in early solar system history when CAIs existed as free-floating objects and had not yet been incorporated into their chondritic parent bodies. The chronological data on these rims are limited. As seen in the limited number of analyzed inclusions, the rims formed nearly contemporaneously (i.e., <300,000 yr after CAI formation) with the host CAIs. Here we present the relative ages of rims around two type B CAIs from NWA 8323 CV3 (oxidized) carbonaceous chondrite using the 26 Al– 26 Mg chronometer. Our data indicate that these rims formed ∼2–3 Ma after their host CAIs, most likely as a result of thermal processing in the solar nebula at that time. Our results imply that these CAIs remained as free-floating objects in the solar nebula for this duration. The formation of these rims coincides with the time interval during which the majority of chondrules formed, suggesting that some rims may have formed in transient heating events similar to those that produced most chondrules in the solar nebula. The results reported here additionally bolster recent evidence suggesting that chondritic materials accreted to form chondrite parent bodies later than the early-formed planetary embryos, and after the primary heat source, most likely 26 Al, had mostly decayed away. 

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5. 3 μm Spectroscopic Survey of Near-Earth Asteroids

   https://doi.org/10.3847/PSJ/ac8ced  

   McGraw, L. E. ; Emery, J. P. ; Thomas, C. A. ; Rivkin, A. R. ; Wigton, N. R. ; McAdam, M. ( October 2022 , The Planetary Science Journal)

   Near-Earth Asteroids (NEAs) are excellent laboratories for processes that affect airless body surfaces. S-complex (including V-type) NEAs were not expected to contain OH/H₂O on their surfaces because they formed in the anhydrous regions of the solar system and their surface temperatures are high enough to remove these volatiles. However, a 3μm feature typically indicative of OH/H₂O was identified on other seemingly dry bodies in the inner solar system, raising the question of how widespread volatiles may be on NEAs. We observed 29 NEAs using both prism (0.7–2.52μm) and LXD_short (1.67–4.2μm) modes on SpeX on NASA’s IRTF in order to accurately characterize asteroid spectral type and the 3μm region. Eight of the observed NEAs have a 3μm absorption feature at >1σ(three of which are present to >2σ), and they exhibit four identified band shape types. Possible sources for OH/H₂O on these bodies include carbonaceous chondrite impacts and/or interactions with protons implanted by solar wind. Characteristics such as composition and aphelion appear to play an important role in the delivery and/or retention of OH/H₂O, as all eight NEAs with an absorption feature are S-complex asteroids and six enter the main asteroid belt. Additionally, perihelion, size, albedo, and orbital period may play a minor role. Our observations determined that nominally anhydrous, inner solar system bodies, and therefore near-Earth space in general, contain more OH/H₂O than previously expected. The identified trends should help predict which NEAs that have not yet been observed might contain OH/H₂O on their surfaces.

    

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