Comparative Mineralogy of Returned Samples from the Carbonaceous Asteroids Ryugu (Hayabusa2) and Bennu (OSIRIS-REx)

The Hayabusa2 mission returned approximately 5.4 g of sample from the Cb-type asteroid (162173) Ryugu in 2020, whereas the OSIRIS-REx mission returned 121.6 g of material from the B-type asteroid (101955) Bennu in 2023 under contamination-controlled curation conditions, enabling direct mineralogical comparison of returned samples from two carbonaceous asteroids. This review synthesizes published mineralogical analyses of the two samples with emphasis on six major mineral groups: phyllosilicates, carbonates, sulfides, oxides, phosphates, and anhydrous primary silicates. The two samples share key mineralogical features, including a serpentine–saponite-dominated phyllosilicate matrix, low-temperature aqueous alteration, dolomite-dominated carbonates, and pyrrhotite–pentlandite sulfide assemblages, suggesting that their parent bodies are closely related to CI-like carbonaceous materials. In contrast, Bennu preserves a late-stage evaporite assemblage comprising Mg–Na phosphates, Na-bearing carbonates, sulfates, chlorides, and fluorides, and also contains abundant ammonia and nitrogen-rich soluble organic matter. This evaporite assemblage provides direct mineralogical evidence that alkaline brines in Bennu’s parent body underwent late-stage evaporation and concentration, preserving information that is difficult to retain in meteorite samples owing to terrestrial exposure and alteration. Bennu samples also record space-weathering products, including Fe nitride and impact-melt deposits, indicating that space weathering on carbonaceous asteroid surfaces may differ from models developed for the Moon and S-type asteroids. The contrasting spectral-slope trends observed for Ryugu and Bennu further suggest that carbonaceous-body space weathering varies with composition, grain size, and surface-exposure history. The comparative mineralogical synthesis presented here provides a basis for target selection, sampling strategy, and post-return curation design for future near-Earth asteroid sample-return missions that Korea may undertake.