^1,6,7Hung Kwan Fok,^2,3,4,7Marco Pignatari,^2,7Benoît Côté,^5,1,7Reto Trappitsch
The Astrophysical Journal Letters 977, L24 Open Access Link to Article [DOI 10.3847/2041-8213/ad91ab]
¹Department of Physics, Brandeis University, Abelson-Bass-Yalem 107, Waltham, MA 02453, USA
²Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, HUN-REN, Konkoly Thege M. út 15-17, Budapest 1121, Hungary
³ CSFK, MTA Centre of Excellence, Konkoly Thege Miklós út 15-17, Budapest 1121, Hungary
⁴E. A. milne Centre for Astrophysics, University of Hull, Cottingham Road, Kingston upon Hull, HU6 7RX, UK
⁵Laboratory for Biological Geochemistry, School of Architecture, Civil & Environmental Engineering, École
Polytechnique Fédérale de Lausanne, GR C2 505, Station 2, 1015 Lausanne, Switzerland
⁶Morton K. Blaustein Department of Earth & Planetary Sciences, Johns Hopkins University, Olin Hall, 3300 San Martin Drive, Baltimore, MD 21218, USA
⁷NuGrid Collaboration (https://nugridstars.org)

Presolar grains are stardust particles that condensed in the ejecta or in the outflows of dying stars and can today be extracted from meteorites. They recorded the nucleosynthetic fingerprint of their parent stars and thus serve as valuable probes of these astrophysical sites. The most common types of presolar silicon carbide grains (called mainstream SiC grains) condensed in the outflows of asymptotic giant branch stars. Their measured silicon isotopic abundances are not significantly influenced by nucleosynthesis within the parent star but rather represent the pristine stellar composition. Silicon isotopes can thus be used as a proxy for galactic chemical evolution (GCE). However, the measured correlation of ²⁹Si/²⁸Si versus ³⁰Si/²⁸Si does not agree with any current chemical evolution model. Here, we use a Monte Carlo model to vary nuclear reaction rates within their theoretical or experimental uncertainties and process them through stellar nucleosynthesis and GCE models to study the variation of silicon isotope abundances based on these nuclear reaction rate uncertainties. We find that these uncertainties can indeed be responsible for the discrepancy between measurements and models and that the slope of the silicon isotope correlation line measured in mainstream SiC grains agrees with chemical evolution models within the nuclear reaction rate uncertainties. Our result highlights the importance of future precision reaction rate measurements for resolving the apparent data–model discrepancy.

^1,2,3Akira Tsuchiyama, ⁴Hirotaka Yamaguchi, ⁴Motohiro Ogawa, ⁵Akiko M. Nakamura, ⁶Tatsuhiro Michikami, ⁷Kentaro Uesugi
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2024.116432]
¹Chinese Academy of Sciences (CAS) Key Laboratory of Mineralogy and Metallogeny/Guangdong Provincial Key Laboratory of Mineral Physics and Materials, Guangzhou Institute of Geochemistry, CAS, 511 Kehua Street, Wushan, Tianhe District, Guangzhou 510640, China
²CAS Center for Excellence in Deep Earth Science, 511 Kehua Street, Wushan, Tianhe District, Guangzhou 510640, China
³Research Organization of Science and Technology, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu, Shiga 525-8577, Japan
⁴Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kitashirakawa-Oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan
⁵Department of Planetology, Graduate School of Science, Kobe University, Rokkodai, Nada-ku, Kobe 657-8501, Japan
⁶Faculty of Engineering, Kindai University, Hiroshima Campus, 1 Takaya Umenobe, Higashi-Hiroshima, Hiroshima 739-2116, Japan
⁷Scattering and Imaging Division, Japan Synchrotron Radiation Research Institute (JASRI/SPring-8), 1-1-1 Kouto, Sayo-Cho, Sayo-Gun, Hyogo 679-5198, Japan
Copyright Elsevier

The shape of regolith particles on airless bodies, such as the Moon and asteroids, reflects the processes that occur on their surfaces. Recent studies have shown that particles on the asteroid Ryugu tend to be angular, whereas some particles on the asteroid Itokawa are rounded, with a larger portions of lunar particles also exhibiting a rounded shape. These differences are thought to result from abrasion, but experimental studies on particle abrasion have been lacking. In this study, we performed experiments simulating the abrasion caused by impact on airless bodies using minerals, rocks, and meteorites related to the Moon and asteroids. Aggregates of particles ranging in size from 1 to 2 mm (6.5 to10 g) were subjected to oscillation in a bead-milling apparatus to assess the amount of abrasion at different oscillation rates, varying from 100 to 3000 rpm for 0.33 to 720 min. The amount of abrasion increased with time and oscillation rate, following a power-law relationship. Once the oscillation rate exceeded a certain threshold, abrasion proceeded rapidly. At rates above 1000 rpm, particles floated and rubbed against each other due to the vertical oscillation of the container, leading to significant abrasion, whereas at rates below 300 rpm, the particles were constrained by Earth’s gravity, resulting in minimal abrasion. This indicates that experiments conducted at ≥1000 rpm effectively simulated the abrasion that occurs on the Moon and asteroids. Scanning electron microscopy was used to observe the particles before and after the experiments, and X-ray microtomography was employed to track the shape changes of individual traceable particles and to measure the three-axial lengths of approximately160 particles. As abrasion progressed, some of the corners and edges of the particles were initially chipped, eventually leading to rounded corners, edges, and surfaces. This process corresponds to “adhesive wear” in tribology, which is caused by tangential relative motion between materials. In carbonaceous chondrite samples, particles tended to split along pre-existing cracks. The particles became smaller, their angularity decreased, and their sphericity increased, while the overall 3D shape of individual particles did not significantly change from their original form; however, the average three-axial ratio became more isotropic. These results indicate that the change in the average three-axial ratio of the Moon and Itokawa regolith particles can be explained by abrasion, as previously proposed. Based on the observed abrasion rates, we discuss the potential for abrasion to be caused by the impact-induced particle motion on the Moon and asteroids, considering models of regolith convection, excavation flow, and maximum acceleration. Although this discussion is rough and only semi-quantitative due to many assumptions, experimental errors, and uncertainties in the models, the results suggest that abrasion can occur on the Moon due to impact-induced particle motion, and that the abrasion observed on Itokawa particles may have occurred not on Itokawa itself, but on its parent body. Ryugu particles, in contrast, are more prone to cracking along pre-existing cracks rather than undergoing significant abrasion, and thus exhibit minimal signs of abrasion.