IR spectra of Ryugu's anhydrous ingredients compared with primitive dust from the outer solar system
Authors: Brunetto Rosario , Nakamura Tomoki , Lantz Cateline , Fukuda Yoshino , Aléon-Toppani Alice , Dionnet Zélia ,   Hayabusa2 Stone team
Published: July 2024

Introduction: Ryugu is a second-generation C-type asteroid formed by the reassembly of fragments of a previous larger body in the main belt. While the majority of Ryugu samples returned by Hayabusa2 are composed of a lithology dominated by aqueously altered minerals, clasts of a more pristine olivine-pyroxene lithology remain in the least-altered samples [1]. These clasts are objects of prime interest for revealing the composition of the original building blocks of Ryugu&#8217;s parent asteroid and of the dust from which they formed.We used infrared hyperspectral imaging to analyze four mm-sized sections of Ryugu samples extracted from Chamber A (A0026) and Chamber C (C0002, C0023, C0025). We compare the Ryugu IR spectra to observations of asteroids, comets, meteorites and interplanetary dust particles (IDPs), to study the potential links between the original building blocks of Ryugu&#8217;s parent asteroid and objects that retained dust from the outer Solar System.Methods: We used different FTIR microscopes at the SMIS beamline of synchrotron SOLEIL [2]: (1) a synchrotron-radiation-fed microscope equipped with a large mid-IR range MCT/B-detector, (2) a far-IR bolometer-equipped microscope, (3) an imaging microscope equipped with a 128&#215;128 pixels focal plane array detector. For (1) and (2), different apertures were used from 5 to 100 &#181;m. For (3), we used a field of view of ~420 &#181;m and pixel size of 3.3 &#181;m, and several IR tiles were accumulated in mosaic IR hyperspectral images, to analyze mm-sized areas in Ryugu stones. Complementary point-to-point micro-IR measurements were performed at Tohoku University (globar source), with spots of ~100-200 &#181;m.Results and discussion: While a large fraction of the matrix of C0002 shows the 10-&#181;m feature of phyllosilicates similar to aqueously altered chondrites, very different silicate spectral features are observed at some clasts detected by Nakamura et al. [1], because of the presence of anhydrous inclusions. Most of these inclusions have a double-peak structure, due to the intimate mixture of hydrated and anhydrous silicates, in particular olivine with a band at ~880 cm-1 (11.36 &#181;m). Some of them show the signature of pure olivine, while others show a strong spectral contribution of pyroxene at ~1075 cm-1 (9.3 &#181;m). A very few grains have a large spectral profile indicating a significant contribution of amorphous phases. Several olivine grains are also observed in C0023 and C0025, in the less-altered lithologies. These features are absent from A0026.In a principal component analysis, spectra of Ryugu samples, CI chondrites and hydrated IDPs reveal a pattern that correlates with increasing alteration, from the least altered clasts of Ryugu stones, to weakly altered Ryugu clasts, and then major (most altered) lithology of Ryugu, Alais, Tagish-Lake, Orgueil, and the hydrated IDPs.Some grains enriched in amorphous silicates discovered in C0002 have IR spectra similar to D-type asteroid Hektor (a Jupiter Trojan) [3], to comet Hale-Bopp [4], and to anhydrous chondritic porous IDPs of cometary origin [5]. They can be identified with anhydrous grains rich in GEMS (Glass with Embedded Metal and Sulfides) [1], which in turn are similar in texture and composition to the GEMS found in IDPs of probably cometary origin, formed in the protoplanetary disk.Conclusions: The amorphous-rich grains in C0002 are one of the most interesting reservoirs of anhydrous &#8220;cometary-like&#8221; dust found in Ryugu. Their IR spectra suggest a possible link between at least one of the reservoirs from which Ryugu&#8217;s parent asteroid originated and the reservoir that formed comets and D-type asteroids in the outer protoplanetary disk [6].Acknowledgments: This work is part of the multi-analytical sequence of the Hayabusa2 &#8220;Stone&#8221; MIN-PET group, led by T. Nakamura. It was supported by the Centre National d&#8217;Etudes Spatiales (CNES-France, Hayabusa2 mission) and by the ANR project LARCAS (Grant ANR-22-CE49-0009-01) of the French Agence Nationale de la Recherche. The micro-spectroscopy measurements were supported by grants from Region Ile-de-France (DIM-ACAV) and SOLEIL.References: [1]&#160;Nakamura T. et al. (2022). Science doi:10.1126/science.abn8671. [2] Rubino S. et al. (2023). Earth, Planets and Space 75. doi:10.1186/s40623-022-01762-8. [3] Emery J.P. et al. (2006). Icarus, 182, 496&#8211;512. [4] Crovisier J. et al. (1997) Science, 275, 1904. [5] Brunetto R. et al. (2011). Icarus, 212, 896&#8211;910. [6] Brunetto R. et al. (2023). ApJL, 951:L33. doi:10.3847/2041-8213/acdf5c.