Occurrence and characterization of nanosulfide‐rich regions on asteroid Ryugu: Insights from mackinawite and pyrrhotite

Abstract Samples returned from asteroid Ryugu by the Hayabusa2 mission are dominated by fine‐grained matrix material made of phyllosilicates and nanosulfides. Here, we report the mineralogical, textural, and chemical characteristics of nanosulfide‐rich regions identified in Ryugu particles. High‐resolution scanning transmission electron microscopy and energy‐dispersive X‐ray spectroscopy reveal nanoscale heterogeneities in sulfide composition and morphology, indicating formation under variable conditions. Nanosulfide‐rich regions are dominated by the presence of mackinawite (FeS) and pyrrhotite (Fe 1‐ x S), in different proportions. Mackinawite, identified for the first time in Ryugu, occurs as well‐crystallized lamellar crystals with some areas containing greigite (Fe 3 S 4 ) and others showing signs of oxidation. In contrast, pyrrhotite appears either as euhedral nanocrystals or as structurally complex grains composed of stacked platy segments, which are characterized by numerous defects, including inclusions and planar defects. The distribution and associations of these phases are consistent with low‐temperature aqueous alteration under alkaline and reducing conditions, likely occurring in Ryugu's parent body. The presence of mackinawite implies complex thermodynamic and kinetic constraints and suggests the presence of localized fluids in which Fe concentrations exceeded those of S by an order of magnitude.