¹Justin I. Simon,^1,2D. Kent Ross,^1,3Ann N. Nguyen,⁴Steven B. Simon,¹Scott Messenger
The Astrophysical Journal, Letters 884, L29 Link to Article [DOI
https://doi.org/10.3847/2041-8213/ab43e4]
¹Astromaterials Research and Exploration Science, NASA Johnson Space Center, Houston, TX 77058, USA
²University of Texas at El Paso/Jacobs-JETS, Houston, TX 77058, USA
³Jacobs-JETS, Houston, TX 77058, USA
⁴Institute of Meteoritics, University of New Mexico, Albuquerque, NM 87131, USA

A spinel-rich, layered calcium- aluminum-rich spherule from the MIL 090019 CO3 chondrite contains a spinel core with a relatively ¹⁶O-rich (Δ¹⁷O ~ −18‰) and mass-fractionated oxygen isotopic composition surrounded by minerals, including spinel, that are relatively ¹⁶O-poor (Δ¹⁷O ~ −7‰), which are in turn surrounded by layers of ¹⁶O-enriched silicates (Δ¹⁷O ~ −17‰). Inclusions with refractory mineral assemblages such as this one are proposed to record inner nebula processes during the earliest epoch of solar nebula evolution. Mineralogical and textural analyses indicate that this primordial particle formed by high-temperature gas–solid reactions, partial melting, evaporation, and condensation. The radially distributed oxygen isotopic heterogeneity measured among multiple occurrences of several minerals, including spinel, requires the existence of ¹⁶O-poor gas at the beginning of solar system formation, 10⁵ yr earlier than it can be produced by photochemical self-shielding in the solar nebula and introduced to the inner protoplanetary disk.

¹J. J. Bernal,²P. Haenecour,³J. Howe,^2,4T. J. Zega,⁵S. Amari,^1,6,7L. M. Ziurys
The Astrophysical Journal, Letters 883, L43 Link to Article [DOI
https://doi.org/10.3847/2041-8213/ab4206]
¹Department of Chemistry and Biochemistry, University of Arizona, Tucson, AZ 85721, USA
²Lunar and Planetary Laboratory, University of Arizona, 1629 E. University Boulevard, Tucson, AZ 85721, USA
³Department of Materials Science and Engineering, and Department of Chemical Engineering and Applied Chemistry, University of Toronto, Canada
⁴Department of Materials Science and Engineering, University of Arizona, USA
⁵Physics Department and McDonnell Center for the Space Sciences, Washington University in St. Louis, 1 Brookings Drive, St. Louis, MO 63130, USA
⁶Department of Astronomy, Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
⁷Arizona Radio Observatory, Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA

We have conducted laboratory experiments with analog crystalline silicon carbide (SiC) grains using transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS). The 3C polytype of SiC was used—the type commonly produced in the envelopes of asymptotic giant branch (AGB) stars. We rapidly heated small (~50 nm) synthetic SiC crystals under vacuum to ~1300 K and bombarded them with 150 keV Xe ions. TEM imaging and EELS spectroscopic mapping show that such heating and bombardment leaches silicon from the SiC surface, creating layered graphitic sheets. Surface defects in the crystals were found to distort the six-membered rings characteristic of graphite, creating hemispherical structures with diameters matching that of C₆₀. Such nonplanar features require the formation of five-membered rings. We also identified a circumstellar grain, preserved inside the Murchison meteorite, that contains the remnant of an SiC core almost fully encased by graphite, contradicting long-standing thermodynamic predictions of material condensation. Our combined laboratory data suggest that C₆₀ can undergo facile formation from shock heating and ion bombardment of circumstellar SiC grains. Such heating/bombardment could occur in the protoplanetary nebula phase, accounting for the observation of C₆₀ in these objects, in planetary nebulae (PNs) and other interstellar sources receiving PN ejecta. The synthesis of C₆₀ in astronomical sources poses challenges, as the assembly of 60 pure carbon atoms in an H-rich environment is difficult. The formation of C₆₀ from the surface decomposition of SiC grains is a viable mechanism that could readily occur in the heterogeneous, hydrogen-dominated gas of evolved circumstellar shells.