Volatile and organic-rich C-type asteroids may have been one of the main sources of Earth’s water. Our best insight into their chemistry is currently provided by carbonaceous chondritic meteorites, but the meteorite record is biased: only the strongest types survive atmospheric entry and are then modified by interaction with the terrestrial environment. Here we present the results of a detailed bulk and microanalytical study of pristine Ryugu particles, brought to Earth by the Hayabusa2 spacecraft. Ryugu particles display a close compositional match with the chemically unfractionated, but aqueously altered, CI (Ivuna-type) chondrites, which are widely used as a proxy for the bulk Solar System composition. The sample shows an intricate spatial relationship between aliphatic-rich organics and phyllosilicates and indicates maximum temperatures of ~30 °C during aqueous alteration. We find that heavy hydrogen and nitrogen abundances are consistent with an outer Solar System origin. Ryugu particles are the most uncontaminated and unfractionated extraterrestrial materials studied so far, and provide the best available match to the bulk Solar System composition.

Discovery of fossil asteroidal ice in a meteorite provides new insight into asteroid formation in the early Solar System.

Samples from asteroid Ryugu returned by the Hayabusa2 mission contain evidence of extensive alteration by aqueous uids and appear related to the CI chondrites. To understand the sources of the uid and the timing of chemical reactions occurring during the alteration processes, we investigated the oxygen, carbon, and 53 Mn-53 Cr systematics of carbonate and magnetite in two Ryugu particles. We nd that the uid was initially between 0 − 20°C and enriched in 13 C, and 17 O and 18 O, and subsequently evolved towards lighter carbon and oxygen isotopic compositions as alteration proceeded. Carbonate ages show that this uid-rock interaction took place within the rst ~ 1.4 million years of solar system history requiring early accretion and preservation of carbonaceous material, either in a planetesimal less than ~ 17 km in diameter or a larger body which was disrupted and reassembled.