A history of mild shocks experienced by the regolith particles on hydrated asteroid Ryugu

Abstract

Micrometeorites, a possible major source of Earth’s water, are thought to form from explosive dispersal of hydrated chondritic materials during impact events on their parental asteroids. However, this provenance and formation mechanism have yet to be directly confirmed using asteroid returned samples. Here, we report evidence of mild shock metamorphism in the surface particles of asteroid Ryugu based on electron microscopy. All particles are dominated by phyllosilicates but lack dehydration textures, which are indicative of shock-heating temperatures below ~500 °C. Microfault-like textures associated with extensively shock-deformed framboidal magnetites and a high-pressure polymorph of Fe–Cr–sulfide have been identified. These findings indicate that the average peak pressure was ~2 GPa. The vast majority of ejecta formed during impact on Ryugu-like asteroids would be hydrated materials, larger than a millimetre, originating far from the impact point. These characteristics are inconsistent with current micrometeorite production models, and consequently, a new formation mechanism is required.

Mineralogical approach on laboratory weathering of uncontaminated Ryugu particles: Comparison with Orgueil and perspective for storage and analysis

“…Previously, ferrihydrite was thought to be produced by aqueous alteration in the CI parent body (Bullock et al, 2005;Tomeoka & Buseck, 1988). However, many recent studies reported that ferrihydrite is not present in fresh CI-like Ryugu particles (Ito et al, 2022;Nakamura, Kobayashi, et al, 2022;Nakamura, Matsumoto, et al, 2022;Tomioka et al, 2023;Yamaguchi et al, 2023;this study). In contrast, ferrihydrite is a common low-crystalline phase in terrestrial environments (e.g., Bigham et al, 1996).…”

“…The first-year analytical campaign of Ryugu particles, including mineralogy, petrology, and oxygen isotopes, confirmed the link between Ryugu particles and CI chondrites (e.g., Greenwood et al, 2022;Ito et al, 2022;Liu et al, 2022;McCain et al, 2023;Nakamura, Kobayashi, et al, 2022;Nakamura, Matsumoto, et al, 2022;Tomioka et al, 2023;Yamaguchi et al, 2023;Yokoyama et al, 2022). Because Ryugu particles are among the most uncontaminated solar system materials available for laboratory analysis on Earth (e.g., Ito et al, 2022;Yokoyama et al, 2022), they should be an excellent experimental example for tracking terrestrial weathering products (minerals) and the abundances of water over time under controlled conditions (i.e., temperature, humidity, and duration).…”

Nonmagnetic framboid and associated iron nanoparticles with a space-weathered feature from asteroid Ryugu

“…The pseudo-magnetites observed in the present study were not likely produced during the sampling operation by Hayabusa2 for the following reasons. The upper limit of the peak pressure experienced by Ryugu particles during sampling operations of Hayabusa2, in which a tantalum projectile was accelerated to 300 ± 30 m s −1 in the sampler horn 33 , was estimated to be only 1.3 GPa 34 . These events were not sufficient to heat the Ryugu particles to 300 K 8 .…”

The Formation of a Rubble Pile Asteroid: Insights from the Asteroid Ryugu

“…Although a large number of impact craters were observed on the surface of current Ryugu [4,16], no evidence of such extreme conditions has been recognized, and low-temperature products such as phyllosilicates and OM are abundantly preserved in the Ryugu samples investigated so far [19,20,23,27,30]. The recently discovered microfault-like textures and the high-pressure polymorph of Fe-Cr sulfide in the Ryugu particles indicated that asteroid Ryugu has experienced mild shocks with the average peak pressure of <2 GPa [54], but this is unlikely to support an impact capable of shattering the Ryugu progenitor body. Alternatively, the high microporosity of the progenitor body, similar to that of a yarn ball, which consists mainly of phyllosilicates interconnected with nm-to µm-sized voids, might have precluded any record of intense impacts on Ryugu.…”