[P-PS08] Formation and evolution of planetary materials in the Solar System

[PPS08-08] ISOTOPE DISTRIBUTIONS AND ANOMALOUS MATERIALS IN RETURN SAMPLES FROM THE ASTEROID RYUGU.

*Naoya Sakamoto¹, The Hayabusa2-initial-analysis chemistry team, The Hayabusa2-initial-analysis core (1.Creative Research Institution, Hokkaido University)

Keywords:Asteroid RYUGU, Hayabusa2 mission, Isotope microscope, Isotope distribution, Light element distribution, Carbonaceous material

One of the objectives of the Hayabusa2 mission that sampled the C-type asteroid Ryugu in two touchdown sites is to clarify the origin and evolution of the pristine materials of the early Solar System, especially the origin of organic matter and water [1], two important constituents of life on Earth. Isotopic imaging allows us to observe distributions of organic matter, presolar grains, and minor elements. Here we report the preliminary results on isotope imaging of the Ryugu samples.
Polished sections, A0058-C1001 and C0002-C1001, prepared from samples from 1^st and 2^nd touchdown sites, respectively, were used. Their isotopic distributions were documented using the isotope microscope system at Hokkaido University [2]. A ~20 nA primary ¹⁶O⁻ ion beam with an impact energy of 23 keV irradiated the sample surface over 100×80 µm² through a square aperture. Other conditions were described in [3 and 4]. Direct ion images of ¹H⁺, ⁷Li⁺, ¹¹B⁺, ²³Na⁺, ²⁴Mg⁺, ²⁷Al⁺, ²⁸Si⁺, ³⁹K⁺, ⁴⁰Ca⁺, and ⁵⁶Fe⁺ were projected onto the SCAPS ion imager. Analyzed areas include 11 and 37 maps in A0058 and C0002, respectively, totaling ~384,000 µm². Images of ¹H⁻, ¹²C⁻, ¹³C⁻, ¹²C¹⁴N⁻, ¹²C¹⁵N⁻, ¹⁶O⁻, ¹⁷O⁻, ¹⁸O⁻, ¹⁹F⁻, ²⁸Si⁻, ³¹P⁻, ³²S⁻, and ³⁵Cl⁻ were collected using Cs⁺ ions at ~2 nA and 15 keV impact energy. Each map corresponds to a 50×50 µm² area on the sample surface. Analyzed areas include 132 maps in C0002, ~330,000 µm² in total.
The secondary ion imaging revealed abundant carbonaceous materials in matrices of the Ryugu samples, These include: (i) N-rich carbonaceous objects (Fig. 1a), and (ii) isotopically anomalous carbonaceous materials, such as ¹³C-rich presolar SiC (Fig. 1b) and ¹⁵N-rich carbonaceous grains (Fig. 1c). We also observed H-rich and B-rich objects scattered in the matrix (Fig. 1d). Backscattered electron (BSE) images of these objects are shown in Fig. 1 as well; their phase identification is still in progress. A surface of the A0058-C1001 section shows enrichment in sodium as indicated by yellow arrows in Fig. 1d. BSE images revealed the presence of several lithologies/clasts in samples studied − white, dark, and intermediate (Fig. 1e). The white lithology is enriched in sulfur (Fig. 1e); N-rich carbonaceous objects are found in this lithology. The dark lithology appears to be enriched in carbon, but resin contamination should be also considered. The fluorine and chlorine contents are inversely correlated in the dark and intermediate lithologies; one of the regions in the former is highly enriched in F (Fig. 1e).
The chemical distributions and the presence of isotopically anomalous materials in the Ryugu samples indicate that the asteroid Ryugu was not chemically and isotopically homogenized; some primordial materials survived during the extensive aqueous alteration it experienced. Carbon- and N-rich materials are important ingredients of life and will be further studied in detail.

References: [1] Tachibana (2021) In Sample Return Missions (Ed. A. Longobardo), Elsevier, pp 147–162. [2] Yurimoto et al. (2003) Appl. Surf. Sci. 203–204, 793–797. [3] Tagawa et al. (2021) Nat. Commun. 12, #2588. [4] Matsuda et al. (2019) Geochem. 79, 125524.

The Hayabusa2-initial-analysis chemistry team: T. Yokoyama, K. Nagashima, 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. D. 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, Y. Masuda, K. D. McKeegan, M. Morita, K. Motomura, F. Moynier, I. Nakai, 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, E. D. Young, 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.


Fig. 1 (a) Back-scattered electron (BSE) image of N-rich carbonaceous object in the C0002-C1001. (b) ¹²C⁻, ¹³C⁻ and BSE images with X-ray maps of O and Si for a presolar SiC grain (δ¹³C ~ +4,000‰), ~150 nm across. Note some bright C signals including a line at top-right corner are due to epoxy filling cracks and holes. (c) ¹²C¹⁴N⁻, ¹²C¹⁵N⁻ and BSE images of a ¹⁵N-rich carbonaceous grain (δ¹⁵N ~ +700‰). (d) BSE, ¹H⁺, ¹¹B⁺ and ²³Na⁺ images of A0058-C1001. (e) BSE image of C0002-C1001, which is composed of brecciated clasts of S-rich (white), dark and intermediate.