In December 2020, the Hayabusa2 mission of the Japan Aerospace Exploration Agency (JAXA) returned samples from the near-Earth, C-type asteroid (162173) Ryugu. C-type asteroids have spectral properties similar to carbonaceous chondrite (CC) meteorites, which are thought to represent primitive, volatile- and organic-rich planetesimals that formed early in solar system history. The return of samples from Ryugu provides our first opportunity to study such materials unaffected by residence on Earth.
Analyses of these samples have shown that they resemble the CI type of carbonaceous chondrites, which are thought to provide the best estimate of the starting composition of the solar system. Most CI chondrites, however, experienced aqueous alteration and did not preserve their primary mineralogy. Thus, an exciting discovery is that Ryugu samples contain anhydrous minerals, such as olivine, which are remnants of primary assemblages. Recent work by Noriyuki Kawasaki and an international team focused on the oxygen isotope compositions of these preserved grains. Oxygen has three stable isotopes (16O, 17O, and 18O), and ratios of these isotopes can fingerprint the origin of meteorites or their components.
Results of this work showed a distinct separation between 16O-rich and 16O-poor compositions among the anhydrous phases. The 16O-rich compositions correlate with minerals seen in refractory inclusions in other CCs, while the 16O-poor compositions correlate with assemblages seen in chondrules. Using the inferred ratio of refractory to chondrule-derived material, the authors speculate that chondrule-forming events were less common in the accretion regions of Ryugu and CI than those of other meteorite types. Numerous arguments indicate that CCs formed in the outer solar system. The authors suggest that the 16O-rich materials in Ryugu and CI formed in the high-temperature, innermost region of the protoplanetary disk and were transported outward. A similar distribution of oxygen isotope compositions has been found in particles collected from comet 81P/Wild2 by NASA’s Stardust mission. This work suggests that Ryugu and CI chondrites formed in a similar region as 81P/Wild2. READ MORE