Ryugu's nucleosynthetic heritage from the outskirts of the Solar System.

Little is known about the origin of the spectral diversity of asteroids and what it says about conditions in the protoplanetary disk. Here, we show that samples returned from Cb-type asteroid Ryugu have Fe isotopic anomalies indistinguishable from Ivuna-type (CI) chondrites, which are distinct from all other carbonaceous chondrites. Iron isotopes, therefore, demonstrate that Ryugu and CI chondrites formed in a reservoir that was different from the source regions of other carbonaceous asteroids. Growth and migration of the giant planets destabilized nearby planetesimals and ejected some inward to be implanted into the Main Belt. In this framework, most carbonaceous chondrites may have originated from regions around the birthplaces of Jupiter and Saturn, while the distinct isotopic composition of CI chondrites and Ryugu may reflect their formation further away in the disk, owing their presence in the inner Solar System to excitation by Uranus and Neptune.

Imprint of chondrule formation on the K and Rb isotopic compositions of carbonaceous meteorites.

Chondrites display isotopic variations for moderately volatile elements, the origin of which is uncertain and could have involved evaporation/condensation processes in the protoplanetary disk, incomplete mixing of the products of stellar nucleosynthesis, or aqueous alteration on parent bodies. Here, we report high-precision K and Rb isotopic data of carbonaceous chondrites, providing new insights into the cause of these isotopic variations. We find that the K and Rb isotopic compositions of carbonaceous chondrites correlate with their abundance depletions, the fractions of matrix material, and previously measured Te and Zn isotopic compositions. These correlations are best explained by the variable contribution of chondrules that experienced incomplete condensation from a supersaturated medium. From the data, we calculate an average chondrule cooling rate of ~560 ± 180 K/hour, which agrees with values constrained from chondrule textures and could be produced in shocks induced by nebular gravitational instability or motion of large planetesimals through the nebula.

Non-natural ruthenium isotope ratios of the undeclared 2017 atmospheric release consistent with civilian nuclear activities.

Understanding the circumstances of the undeclared 2017 nuclear release of ruthenium that led to widespread detections of the radioisotope 106Ru in the Eurasian region, and whether it derives from a civilian or military source, is of major importance for society and future improvements in nuclear safety. Until now, the released nuclear material has merely been studied by analyzing short-lived radioisotopes. Here, we report precise measurements of the stable isotopic composition of ruthenium captured in air filters before, during, and after the nuclear release, and find that the ruthenium collected during the period of the 2017 nuclear release has a non-natural isotopic composition. By comparing our results with ruthenium isotopic compositions of spent nuclear fuels, we show that the release is consistent with the isotopic fingerprints of a civilian Russian water-water energetic reactor (VVER) fuel at the end of its lifetime, and is not related to the production of plutonium for nuclear weapons.