Molybdenum and Osmium isotopic compositions of Ryugu sample
Nao Nakanishi1,2, Akira Ishikawa2, Tetsuya Yokoyama2, and Richard J. Walker1,The Hayabusa2-initial-analysis chemistry team, The Hayabusa2-initial-analysis core.
1
Dept. of Geology, University of Maryland,USA
2
Dept. of Earth & Planetary Sciences, Tokyo Institute of Technology, JapanThe Hayabusa2 mission conducted two sampling sequences on the Cb-type asteroid Ryugu and brought 5.4 g of theasteroidal materials back to Earth. The initial analyses found that Ryugu samples are dominated by minerals that experienced
extensive aqueous alteration in the parent body and are related to CI (Ivuna-like) carbonaceous chondrites [1]. To further
investigate the origin of the materials that accreted to form Ryugu and chemical processes that affected these materials afteraccretion we examined the isotopic compositions of the siderophile elements Os and Mo. Eleven 187Os/188Os ratios wereobtained from bulk Ryugu samples (A0106-A0107, and C0108 from both sampling locations) using N-TIMS (Triton Plus) at
Tokyo Tech. A composite sample of four bulk Ryugu samples that were combined (A0106-A0107, A0106, C0108, and C0107
from both sampling locations) was dedicated for the determination of the Mo isotopic composition of the composite using MCICP-MS (Neptune Plus) at UMd.
The bulk Ryugu samples have uniform 187Os/188Os ratios, averaging
0.1264 ± 0.0005 (2SD, n = 11) (Fig. 1), which is consistent with the187
Os/188Os ratio of bulk CI chondrites (0.1265 ± 0.0001, 2SD; [2]). Thisobservation supports the main conclusion of previous studies that Ryugu ismainly composed of materials related to CI chondrites [3]. The uniform
187
Os/188Os ratio implies homogeneous distribution of elemental Re/Os
ratios of the Ryugu materials. This is likely to reflect the redistribution of
Re and Os by aqueous alteration that occurred on the Ryugu parent body.
Given the sample size for one analysis (0.4–1.1 mg), the Re/Os
homogenization would be achieved even on a small scale.
The composite bulk Ryugu sample analyzed for Mo is characterized bypositive εiMo values (εiMo = [(iMo/96Mo)sample/(iMo/96Mo)standard − 1] ×104)
for 92Mo, 94Mo, 95Mo, and 97Mo, consistent with a deficit of s-process MoFigure 1. Os isotopic composition of Ryugu
isotopes (Fig. 2). Previous studies have shown that Mo isotopic data formaterials and chondrites. Reference data are from[2]. The error bars reflect the range of isotopic
carbonaceous (CC) and non-carbonaceous (NC) meteorites define twocompositions measured for each category (Ryugu:9594
separate linear trends on the ε Mo–ε Mo diagram (e.g., [4]). The Ryugu
2SD; Chondrites: 1SD).
sample plots on the CC line (Fig. 3), which is consistent with theobservation that Ryugu samples are also characterized by CC-type ε50Ti–ε54Cr isotopic systematics [3]. The ε94Mo and ε95Mo values for thecomposite Ryugu sample are larger than those of any known bulk carbonaceous chondrites including CIs (Fig. 3).
There are at least three possible explanations for the large s-process deficits in the Ryugu sample. First, the measured Mo
isotopic composition might accurately reflect the composition of the bulk asteroid. In this case, it would suggest that Ryugu
formed from a nebular region with a Mo isotopic composition that differed from that of the limited number of CI chondrites
analyzed for Mo to date. This would be in conflict with the evidence from the lithophile Ti and Cr isotopic compositions.
Second, Mo isotopes might have been heterogeneously distributed on the asteroid Ryugu. In this case, the Mo isotopic
composition obtained from the ~70 mg of Ryugu material analyzed may not represent the bulk composition of the asteroid, andmay instead reflect the redistribution and concentration of strongly s-process depleted Mo by aqueous processes. Third isincomplete dissolution of presolar SiC grains during the acid digestion steps, given that SiC grains are resistant to dissolution
by the acid digestion techniques applied for this measurement. SiC is characterized by s-process-enriched compositions [e.g.,5]. A leachate study on Orgueil found that incomplete digestion creates anomalies in Mo isotopic composition [6]. Thus, the sprocess deficit may reflect an absence of the SiC component contribution to the bulk composition.

Figure 2. Mo isotopic composition of Ryugu andAllende analyzed in this study. The reference bulk
meteorite data are from [6-8]

Figure 3. ε95Mo-ε94Mo diagram of Ryugu materialand bulk Allende. The reference bulk meteorite data arefrom [8].

References[1] Yada T. et al. 2022. Nature Astronomy 6:214. [2] Walker R. J. et al. 2002. GCA 66:4187. [3] Yokoyama T. et al. 2022
Science abn7850. [4] Kleine, T. et al. 2020. Space Sci. Rev. 216:1. [5] Stephan T. et al. 2019. ApJ 877:101. [6] Dauphas N. etal. 2002 ApJ 569:139. [7] Burkhardt C. et al. 2011. EPSL 312:390. [8] Yokoyama, T. et al. 2019. ApJ 883:62.
The Hayabusa2-initial-analysis chemistry team: T. Yokoyama, K. Nagashima, I. Nakai, E.D. Young, Y. Abe, J. Aléon,C.M.O'D. Alexander, S. Amari, Y. Amelin, K. Bajo, M. Bizzarro, A. Bouvier, R. W. Carlson, M. Chaussidon, B.-G. Choi, N.
Dauphas, A. M. Davis, T. Di Rocco, W. Fujiya, R. Fukai, I. Gautam, M. K. Haba, Y. Hibiya, H. Hidaka, H. Homma, P. Hoppe,G.R. Huss, K. Ichida, T. Iizuka, T.R. Ireland, A. Ishikawa, M. Ito, S. Itoh, N. Kawasaki, N. T. Kita, K. Kitajima, T. Kleine, S.
Komatani, A. N. Krot, M.-C. Liu, Yuki Masuda, K.D. McKeegan, M. Morita, K. Motomura, F. Moynier, A. Nguyen, L. Nittler,M. Onose, A. Pack, C. Park, L. Piani, L. Qin, S.S. Russell, N. Sakamoto, M. Schönbächler, L. Tafla, H. Tang, K. Terada, Y.
Terada, T. Usui, S. Wada, M. Wadhwa, R.J. Walker, K. Yamashita, Q.-Z. Yin, S. Yoneda, H. Yui, A.-C. Zhang, H. Yurimoto.
The Hayabusa2-initial-analysis core: S. Tachibana, T. Nakamura, H. Naraoka, T. Noguchi, R. Okazaki, K. Sakamoto, H.
Yabuta, H. Yurimoto, Y. Tsuda, S. Watanabe.