Masanao Abe

Abstract Aqueous alteration was one of the earliest geological processes in the solar system and significantly shaped the mineralogical diversity observed in primitive minor bodies. Carbonate minerals are reliable tracers of the physicochemical conditions during alteration processes on early planetesimals. We present a comprehensive characterization of carbonates in the returned samples of asteroids Ryugu and Bennu, performed using the MicrOmega near-infrared hyperspectral microscope. We identify two major carbonate species, dolomite (CaMg(CO ₃ ) ₂ ) and breunnerite ((Mg,Fe)CO ₃ ), with similar relative abundances and mean elemental compositions on both asteroids, indicating analogous aqueous alteration pathways on their parent bodies. A general spatial separation of dolomite and breunnerite is observed, indicating that at size scales of 10–100 μ m, alteration conditions favored the precipitation of one species over the other. We interpret this as evidence for a spatially heterogeneous local water–rock ratio, suggesting that a high local water–rock ratio suppresses the formation of dolomite, instead favoring precipitation of breunnerites. Based on relative carbonate abundances, we conclude that chambers A (sampling the surface) and C (predominantly sampling the subsurface) of the Ryugu sample contain different mixtures of lithologies with different alteration degrees.

Abstract Analyzing primitive extraterrestrial samples from asteroids is key to understanding the evolution of the early solar system. The OSIRIS‐REx mission returned samples from the B‐type asteroid Bennu, providing a valuable opportunity to compare them with the Ryugu samples collected by the Hayabusa2 mission. This study examines the representativeness of a fraction of the Bennu samples, which was allocated from NASA to JAXA, by nondestructive characterization of their physical and spectral properties without atmospheric exposure. The reflectance and observed spectral features in the visible‐to‐infrared range of the Bennu sample resemble those from the spectroscopic analysis of different fractions. Additionally, we found differences in the slope of the visible range and band‐center of ~2.7 μm band between the samples and the asteroid surface, which could be explained by the degree of space weathering. A comparative analysis of the Bennu and Ryugu samples revealed spectral similarities, including absorption features indicative of Mg‐rich phyllosilicates, organics, and carbonates, without any evidence of sampling bias or terrestrial alteration. This finding can be used as a benchmark for subsequent Ryugu–Bennu comparative studies.