The Earth atmosphere‐like bulk nitrogen isotope composition obtained by stepwise combustion analyses of Ryugu return samples

The nitrogen isotope compositions of two samples returned from the asteroid Ryugu were determined using a stepwise combustion method, along with Ivuna (CI) and Y‐980115, a CI‐like Antarctic meteorite, as references. The two Ryugu samples A0105‐07 and C0106‐07 showed bulk δ 15 N values of +1.7 ± 0.5‰ and +0.2 ± 0.6‰, respectively, significantly lower than Ivuna with +36.4 ± 0.4‰, but close to Y‐980115 with +4.0 ± 0.3‰. The Ryugu samples are further characterized by C/N and 36 Ar/N ratios up to 3.4× and 4.9× the value of Ivuna, respectively. Among all Ryugu samples and CI chondrites, a positive correlation was observed between nitrogen concentrations and δ 15 N values, with samples with lower nitrogen concentrations exhibiting lower δ 15 N. This trend is explained by a two‐component mixing model. One component is present at a constant abundance among all CI‐related samples, with a δ 15 N value around 0‰ or lower. The other varies in abundance between different samples, and exhibits a δ 15 N value of +56 ± 4‰. The first 15 N‐poor endmember is seemingly tightly incorporated into a carbonaceous host phase, whereas the 15 N‐rich endmember can be mobilized and decoupled from carbon, potentially because it is in the form of ammonia. Asteroid materials with volatile compositions that are similar to those reported here for the Ryugu samples are attractive candidates for the volatile sources among Earth's building blocks.